Strategies for the Synthesis of Taxol

Mohima Begum Roomi Chowdhury

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Strategies for the Synthesis of Taxol

by


Abstract

Paclitaxel, known as Taxol (commercially known as Taxol®)

[1] is one of the most revered anti-cancer drugs to be

developed. After its discovery in the 1960’s, it has occupied the minds, laboratories and countless journals of

scientists all over the world. The story of Taxol dominated many different forums; from the science community

and politics to environmental and wildlife media. With the problem of cancer being ever-evident, and the

extraction of naturally occurring Taxol proving inefficient, insufficient and unsustainable, the issue of developing

the compound came in to the limelight. There was no doubt that the obtention of the then potential

chemotherapeutic drug, Taxol, ought to be pursued – prompting the necessity for chemical synthesis.

It took around three decades from the discovery of Taxol and the elucidation of its structure to the completion of

the total synthesis – with many semi-syntheses and syntheses of various analogues along the way. This long

process of the drug’s development highlights the difficulties faced by the many scientists who have attempted to

find a viable synthetic route – whether the partial or total chemical synthesis, the biosynthesis, synthesis from

cell tissue culture, or any of the other attempted methods.

When developing a viable strategy to fully synthesise Taxol, how and when to implement each part of the

structure – the rings and the regio- and stereospecific functional groups – was carefully planned, and many routes

were possible. Discussion and analysis of the various strategies for the total synthesis of Taxol will be carried

out. Each of the methods has advantages and disadvantages – which will be discussed and analysed.


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Table of Contents

1. Introduction

2. Total synthesis of Taxol

a. Introduction

b. Holton group

i. Introduction

ii. Strategy

iii. Results and discussion

c. Nicolau group

i. Introduction

ii. Strategy


d. Danishefsky group

i. Introduction

ii. Strategy


e. Wender group

i. Introduction

ii. Strategy


f. Kuwajima group

i. Introduction

ii. Strategy


g. Mukaiyama group

i. Introduction

ii. Strategy


3. Discussion and conclusion

a. Comparison of strategies


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*This is an estimate and is rounded to the closest 10.

Data collated from: Cancer Facts & Figures 2005, 2006,

2007, 2008, 2009 and 2010, American Cancer Society.

1. Introduction

The problem of cancer and its incurability has encouraged

much research in various different fields for many years. With a

general increase in the number of people being newly diagnosed

with cancer from year to year [4]

(see table), research will need to

continue and one avenue of interest will be nature and the products

it has to offer. The propensity of nature to supply useful medicines

has been known since ancient times. Nowadays with funding from

public and private sectors in countries such as the USA, nature’s true

potential has been and can be further exploited.

Many natural products have been discovered with potential

for pharmacological and therapeutic use, for instance as anti-cancer agents [10]

.

Amongst them are Epothilone B, which was found in ‘Sorangium cellulose’, a

myxobacterium (soil-dwelling bacterium); and Podophyllotoxin, which was extracted

from the roots of the ‘American Mayapple’ (Podophyllum peltatum) and various other

species of the Podophyllum genus. Camptothecin is another natural anti-cancer agent,

isolated by Wall and Wani from the bark and stem of the ‘Happy Tree’ (Camptotheca

acuminata). Other naturally occurring compounds which possess cytotoxic properties (ability to kill cancer cells)

include Vinblastine, Roscovitine, Silvestrol [5]

and many others.

Some of these compounds are already used clinically; others are at the stage of

clinical and preclinical development and of course many others are yet to be discovered.

One particular compound which was found to have cytotoxic properties in the 1960’s

gained vast interest worldwide when extraction from natural sources proved inefficient,

insufficient and unsustainable.

Paclitaxel (known as Taxol) is a complex functionalised diterpene. Diterpenes are organic compounds

comprising of four isoprene units and are produced by many plants. In the 17 years that Taxol®

has been on the

market, it has been one of the best-selling anti-cancer drugs worldwide – some have dubbed it “the best-selling

anti-cancer drug ever” [3]

. Since its introduction to the pharmaceutical market, it has been used by millions of

cancer sufferers all over the world.

Year Number of new cases of

cancer in the US*

2005 1,372,910

2006 1,399,720

2007 1,444,920

2008 1,437,180

2009 1,479,350

2010 1,529,560


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The structure of Paclitaxel (Taxol) elucidated by M. E. Wall and

M. C. Wani in 1971.

Taxol was the first phytochemical (a biologically active compound found in plants) drug product to gain

approval from the US Food and Drug Administration (FDA) in more than 25 years [3]

. In search of natural

compounds with potential medicinal uses, samples of tree bark, leaves, seeds, shrubbery and the like were

collected from many trees and plants, including the Pacific Yew tree (Taxus brevifolia); a slow-growing

evergreen tree, native to the Pacific Northwest of America. When the collection was sent for routine tests for

biological activity, the Yew tree samples showed an ability to kill cancer cells; hence attempts to isolate the active

substance began [16]

.

The cell-killing essence was first isolated, and its

structure was elucidated by Wall and Wani in 1971 [1]

.

Wall named the compound ‘Taxol’ as it comes from the

genus ‘Taxus’ and contains the alcohol functional group.

The confirmed structure shows that Taxol comprises of

the taxane ring system (a 15-membered tricyclic ring

system), a rare four-membered oxetane ring and an ester

side chain [2]

.

Extraction and structure elucidation of the active

compound took several years as Wall and Wani were

faced with many problems – including running out of

material regularly, poor yields (of less than 1%) and dated structure-elucidating lab equipment. The technique

used to characterise Taxol’s structure was the ‘heavy atom method’ (also known as the Patterson technique) [1] [12]

which is one of two X-ray crystallographic methods. The theory behind the method is that the heavy atoms

impose their diffraction pattern on the rest of the molecule; this is then analysed by X-ray Diffraction

Crystallography [13]

.

As well as problems with characterising the structure of Taxol, many problems were encountered with

developing a viable and sufficient route to obtaining the active cytotoxic product. As mentioned, the extraction

from the Yew tree bark, for instance, provided abysmal yields – sacrificing a 100 year old tree afforded 300kg of

bark which provided about one dose of Taxol (around 300mg) [2]

. As the necessity for a viable synthetic route

became apparent, many scientists showed interest in Taxol and attempts were made to synthesise the cytotoxic

compound by means of total and semi-syntheses; as well as other methods, such as cell tissue culture studies.

Chemists at the forefront of research into the synthesis of Taxol included R. A. Holton, K. C. Nicolau, S J.

Danishefsky, P. A. Wender, P. Potier, F. Guéritte-Voegelein, D. Guérnard, Kuwajima, Mukaiyama, D. G. I.

Kingston and many others.


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The challenge to artificially synthesise the Taxol, dominated many scientific studies – from chemical

synthesis and biosynthesis to botanical and cell culture studies. Also, an avenue that required studying was the

mode of action by which Taxol killed cancer cells. It was first thought that Taxol killed cancer cells by inhibiting

microtubule assembly – hence interrupting the ability of cells to divide. However, in 1979, cell culture tests

carried out by Susan B. Horwitz, a pharmacologist based in New York, confirmed that in fact Taxol worked by a

completely unique mechanism that had never been seen before [2]

.

The mode of action by which Taxol works was found to be opposite to that predicted whereby Taxol

actually stimulates the growth of microtubules. These microtubules would usually form at the beginning of cell

division and go on to break down and form tubulin (building blocks of the cell structure) and hence cancer cells

would divide – but Taxol stops this process from occurring. When Taxol is present, formation of microtubules

goes in to overdrive and with the process being irreversible, the microtubules are unable to disassemble. This

leads to mitotic arrest; hence cancer cells collapse and die (by apoptosis) [2] [7]

. This newly-discovered means of

cytotoxicity has since been found in the modes of action of many other naturally occurring products, such as

Epothilones, Eleutherobin, Discodermolide and others.

With such an effective novel mode of action, it was undeniable that an alternative route to produce Taxol

was necessary and many scientists around the world engaged in the challenge. This resulted in many semi-

syntheses from Taxol precursors, studies on cell cultures (investigations of bacteria present in various Taxus

species), and perhaps most interestingly, routes of total synthesis – first achieved simultaneously by Holton and

Nicolau groups in 1994. The challenge of synthesising Taxol from scratch was fraught with drawbacks; from the

difficult tricyclic carbon framework to the complex stereochemistry of the molecule (Taxol has 11 chiral centers

and hence 2048 diastereomeric isomers) [8]

. Alterations in the configurations were found to lead to important

changes in the biological activity; hence it was essential to ensure the correct configurations were achieved.

Various synthetic strategies for the total synthesis of Taxol will be investigated. The route of total

synthesis carried out by Holton, Nicolau, Danishefsky, Wender, Kuwajima and Mukaiyama and their groups will

each be discussed and compared.


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2. Total Synthesis of Taxol

From its discovery in the 1960’s, Taxol was recognised as a vitally important compound which

unfortunately could not be sustained naturally. This led to the need for a viable synthetic route to make Taxol

artificially. By the 1980’s, more than 30 research groups were working towards becoming the first ones to

achieve the total synthesis of Taxol. In 1994, Holton and Nicolau groups simultaneously published different

routes for the total synthesis. Other total syntheses have since been achieved by the groups of Danishefsky,

Wender, Kuwajima and Mukaiyama.

The taxane ring system posed many problems when synthetic strategies were designed, for instance, the

rigid arched structure of the whole molecule made functionalisation very difficult. Also, the specific regio- and

stereochemistry of the molecule required careful planning of which reagents and conditions to use for each step,

as well as what order to carry out the functional group additions; using conformational control elements to direct

functionality stereoselectively. Each of the groups adopted unique strategies where the construction of rings and

functionalisation around the periphery were carried out in a different sequence. A common theme in all the

strategies was that the addition of the side chain at C-13 was carried out at the end; all but one group used

Ojima’s β-lactam as the side chain precursor.

Ojima developed the synthesis of many β-lactams, including (±)-cis-1-benzoyl-3-

triethylsilyloxy-4-azetidin-2-one, which was used as the side chain for Taxol. These can be

synthesised using the chiral lithium ester enolate – imine cyclocondensation strategy [ref 14

in Ojima paper]; which gives a high yield in just three steps with an enantiomeric excess of

almost 100%

Each strategy for the total synthesis of Taxol, from commercially available and/or naturally abundant

starting materials, has limitations and many different problems were faced by each of the chemists aiming to

achieve an efficient and practical route to artificial Taxol. Many drawbacks were experienced due to the complex

and intricate configuration of the Taxol molecule – it was found that even slight modifications of just one

functional group could change the effectiveness of Taxol as a cytotoxic compound.


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Holton’s total synthesis of Taxol

Holton and his group followed a linear synthesis strategy whereby the functionalised AB-ring system was

first formed, followed by cyclisation to obtain the C-ring and then instalment of the oxetane ring. The synthesis

of Taxol was completed by the addition of the side chain at C-13 using Ojima’s β-lactam. The starting material

used was (-)-camphor (to obtain (-)-Taxol), which is readily available and abundant from a number of sources,

such as, the distillation of turpentine oil. To synthesise (+)-Taxol, (-)-patchino was used as the precursor.

The naturally abundant (-)-camphor was converted to diol (1) which is

the starting point of Holton’s strategy. This diol can also be obtained from β-

patchoulene oxide (patchino), which is a commercially available natural

product and can also be obtained from patchouli alcohol.

A major advantage of using this compound (1) as the precursor is that it

already contains 15 of the 20 carbon atoms which make up the taxane ring framework.

Holton achieved the total synthesis of Taxol in 1994; however along the way, major

milestones were achieved. For example, in 1984 Holton and his group completed the

synthesis of the taxane ring system, using patchoulene oxide as the starting material and then went on to

successfully synthesise the compound Taxusin in 1988.

The synthesis of Taxusin, along with various other molecules with

the bicyclo[5.3.1] skeleton, was achieved using this “epoxy alcohol

fragmentation” strategy (fig ?). This fragmentation is one of the first steps in

Holton’s total synthesis of Taxol.

Synthetic route

The functionalised AB-ring system (16) was formed in 12

steps from diol (1), which is an intermediate in the preparation of

taxusin and can be synthesised from (-)-camphor or β-patchoulene

oxide. As shown in figure ?, (1) can be obtained from β-patchoulene

oxide: by firstly refluxing β-patchoulene oxide (A) with tert-

butyllithium, enol (B) is obtained. Following epoxidation using t-

BuOOH and Ti(iPrO)4, the diol (1) is obtained by epoxy alcohol rearrangement using boron trifluoride and

trifluoro-methanesulfonic acid. In order to obtain the correct enantiomeric series, Holton used (-)-camphor as the

CH3CH3

CH3CH3

OHOH

(1)


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starting material for the synthesis of (-)-Taxol. The reaction

scheme below outlines the transformations carried out to

obtain the functionalised AB-ring system of Taxol.

Once (-)-camphor was converted to diol (1),

silylation using triethylsilylchloride (TESCl) gave (2)

which then underwent epoxy alcohol fragmentation via the

epoxy alcohol intermediate (3) to give the bicyclo[5.3.1]

skeleton of the AB-ring system (4). Treatment with

TBSOTf to protect the C-13 hydroxyl group was followed

by the diastereoselective aldol condensation of the

magnesium enolate of (5) to give ethyl carbonate (6), after

direct protection using phosgene and ethanol.

Hydroxylation at C-2 was then achieved using LDA

and (R)- or (S)-camphorsulfonyl oxaziridine to convert (-)-

or (+)-(6), respectively, to hydroxy carbonate (7).

Reduction of (7) using Red-Al gave the non-isolated 2, 3,

7-triol intermediate, which was treated with phosgene

and pyridine to afford carbonate (8), which underwent

Swern oxidation using dimethylsulfoxide, oxalyl

chloride and triethylamine to give ketone (9). Then

using lithium tetramethylpiperidide (LTMP), a Chan

rearrangement was carried out to give hydroxy lactone

(10) with a C-3 α-OH group. The conformational

alignment of this hydroxyl group allowed for its

reductive removal using samarium diiodide (SmI2).

Enol (11) was treated with silica gel and trans-

and cis-lactones, (12) and (13) respectively, were

obtained in a 1:6 mixture. A process of recycling was

used where trans-lactone (12) was treated with tBuOK in

THF and quenched with acetic acid to give back enol

(11), which in turn was treated with silica gel again –

this achieved 91% yield of the cis-lactone (13). The cis-


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lactone (13) was then treated with LTMP to deprotonate

at C-1, rather than C-3 surprisingly.

Then addition of (±)-camphorsulfonyl oxaziridine

gave (14) and (15) both containing the C-1 β-hydroxyl

group. Reduction of (15) using Red-Al in THF resulted

in the formation of trans-C1/C2 diol and some cis-C1/C2

diol; which was worked up under basic conditions to

give the desired trans-diol. The trans-diol containing the

C-2 α-hydroxyl group was then treated with phosgene to

obtain carbonate (16).

Ozonolysis of the terminal olefin of lactone (16),

followed by oxidation using potassium permanganate

and esterification using diazomethane afforded methyl

ester (17) and Dieckmann cyclisation resulted in the

formation of enol ester (18). Before

decarbomethoxylation was carried out, the hydroxyl

gro

up

at C-7 was protected using p-toluenesulfonic acid and 2-

methoxypropene. The MOP-protected enol ester (19) was

treated with PhSK and dimethylformamide to give ketone

(20). A more robust protecting group at C-7 was employed

by treating (20) with benzyloxymethyl (BOM) chloride and

the next step was used to introduce a TMS-ether group at

C-5, via an enol ether intermediate, using LDA and

trimethylsilyl chloride. Oxidation using mCPBA then gave

ketone (22).

An α-methyl group was added at C-4 using a methyl

Grignard reagent in methylene chloride and tertiary alcohol

(23) was then treated with Burgess’ reagent, followed by an

acidic workup to achieve elimination to give allylic alcohol

(24). Oxetanol (29) was obtained via two different routes.


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Labelled AB-ring of taxol, with B-ring in the chair-chair conformation

Osmylation of (24), followed by treatment with tosyl chloride gave diol tosylate (27); subsequent cyclisation

using base DBU provided oxetanol (29). The second route involved initial protection of the α-hydroxy group at

C-5 using mesyl chloride, followed by osmylation to give the diol mesylate (28); subsequent cyclisation using

diisopropylethylamine gave oxetanol (29).

Acetylation at C-4 using DMAP, acetic anhydride and pyridine, followed by removal of the triethylsilyl

protecting group at C-10 using HF-pyridine and MeCN

gave C-10 hydroxy oxetane (30). Treatment with

phenyllithium gave the C-2 benzoate; then oxidation at C-

10 was achieved using N-methylmorpholine-N-oxide

(NMO) and TPAP. Oxidation at C-9 using tBuOK and

benzeneseleninic anhydride was followed by direct

acetylation using tBuOK again, then acetic anhydride,

pyridine and DMAP to achieve direct acetylation at C-10,

affording 7-BOM-13-TBS baccatin III (32).

In preparation for the addition of the side chain, the

C-13 hydroxy group was deprotected using TASF. 7-BOM

baccatin III (33) was then treated with lithium

bis(trimethylsilyl)amide followed by (±)-cis-1-benzoyl-3-

triethylsilyloxy-4-azetidin-2-one (β-lactam) and acetic

acid. Taxol was then obtained after deprotection of the

silyl and BOM groups using HF-pyridine and

hydrogenolysis (H2, Pd/C and ethanol), respectively.

Results and Discussion

By controlling the conformation of the

bicyclo[5.3.1] ring system, functionality was

achieved at C-1, C-2, C-3, C-7 and C-8 in 12

steps from diol (1) to obtain carbonate (16) in

40% overall yield. Having successfully

carried out the formation of the AB-ring

system, completion of the C ring was


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achieved by a Dieckmann cyclisation (via a protocol pioneered by Holton himself). Instalment of the oxetane

ring, final functionalisations and addition of the side chain at C-13 afforded synthetic Taxol; with an overall yield

from diol (1) of ca. 4-5%.

Problems and limitations were encountered during this challenging synthesis; for instance, conventional

chemical transformations were at times unsuccessful and required the use of protecting groups to avoid unwanted

reactions – making the synthesis longer with protection and deprotection steps. Deprotonation at C-8α, for

instance, was unsuccessful at first and required C-10α protection with a silyloxy group. Furthermore, a large

epimerisable C-3α substituent had to be introduced to favour the boat-chair conformation to carry out the

formation of the C-2α alcohol, so subsequent epimerisation could return the molecule to the chair-chair

conformation for further functionalization.

The C-7 stereocenter was introduced early on, in the absence of the C-9 carbonyl group, to avoid

epimerisation. Conformational control of the 8-membered ring to shift groups around the periphery to equatorial

positions, hence minimising non-bonding interactions, was used throughout to direct functional group additions

selectively. Due to the C-4 carbonyl group being very hindered, formation of the acetoxy oxetane (30) was

difficult as addition of C-20 failed with most nucleophilic reagents with the carbonate being present. This was

resolved by introducing a robust protecting group at C-7, addition of C-20 from the α-face and indirect

reconfiguration to achieve the C-20β component via formation of an allylic alcohol intermediate (26).

Holton achieved this unique synthetic route by utilising some reactions that were still in development.

The Chan rearrangement, for example, was used for the first time in a cyclic system and it was found to be a very

stereoselective method. The Dieckmann cyclisation carried out to convert methyl ester (17) to enol ester (18)

followed a protocol which was developed by Holton.

Addition of the ester side chain at C-13 was carried out using the (+)-lactam with (-)-(33) and (+)-lactam

was used with (+)-(33) to achieve the desired stereochemistry of the final product. This step proceeded with no

problems. The removal of the TBS group, prior to addition of the β-lactam, almost posed a problem as the α-face

of the molecule was very crowded; the deprotecting agents, HF-pyridine proved ineffective and Holton then

found that TASF worked effectively to cleave the TBS group. Holton’s synthesis required 41 steps and an overall

yield of ~4-5% was achieved.


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Nicolau’s total synthesis of Taxol

Nicolau and his group followed a convergent strategy whereby rings A and C were first prepared, then

merged to form the B ring. Once the ABC-tricyclic system was obtained, the oxetane ring was installed and the

various substituents were added around the peripheries. The side chain was then added at C-13 by esterification,

after oxygenation, and the total synthesis of Taxol was completed.

The two fragments, rings A and C – which were prepared by Diels-Alder reactions using simple

precursors – were coupled by a Shapiro reaction and a McMurry pinacol coupling.

Before Nicolau and his group published their route for the total synthesis of Taxol in 1994, they made a

number of breakthroughs along the way. The total synthesis was gradually achieved by successfully synthesising

the A- and CD-ring systems in 1992, and going on to form the ABC-taxoid ring system in 1993. Complete

functionalisation of the tetracyclic framework and the addition of the side chain at C-13, hence total synthesis of

Taxol, were then achieved in 1994.

Synthetic route

The fully functionalised A-ring system was

achieved by Nicolau in 1992. The synthesis was started by

heating diene (1) at 135°C with 2-chloroacrylonitrile (2)

for 96 hours in a sealed tube. This Diels-Alder reaction

resulted in the formation of adduct (3), which was

converted to ketoacetate (4) using potassium hydroxide

and t-butanol, followed by acetic anhydride and DMAP. In

order to couple to the CD-synthon, hydrazone functionality was introduced at C-1 using 2,4,6-

triisopropylbenzenesulfonylhydrazide in methanol, affording (6).

Preparation of the CD ring system involved an initial Diels-Alder reaction between dienophile (7) and 3-

hydroxy-2-pyrone (8). Dienophile (7) was prepared in four steps from

allyl alcohol: (i) silylation using tBuPh2SiCl-imidazole, (ii) ozonolysis,

(iii) condensation using Ph3P=CH(Me)-CO2Et and finally (iv)

desilylation using n

Bu4NF. The Diels-Alder reaction was made

intramolecular using phenylboronic acid which carried the reaction

through an intermediate shown in fig. ?. This cycloaddition reaction


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achieved the correct regiochemistry to give compound (9), which

was transformed to the requisite C-ring aldehyde (14) through a

number of steps.

Firstly, the hydroxyl groups were protected using TBSOTf

and the ester group was selectively reduced with LiAlH4 to give

alcohol (10). The next step was used to add the benzyl group at

the C-7 and to protect the primary alcohol at C-9. This was

carried out by deprotecting the hydroxyl group at C-7 using an

acid catalyst, followed by selective TPS-protection at C-9 and

then addition of the benzyl group at the secondary alcohol.

Compound (11) was obtained and treated with LiAlH4, which

reductively opened the lactone and concomitantly deprotected the

hydroxyl group at C-4 to give triol (12). Treatment with 2,2-

dimethoxypropane afforded acetonide (13) which was then

oxidised using TPAP in the presence of N-methylmorpholine-N-

oxide (NMO) to give the desired aldehyde (14).

Coupling of the two fragments (6) and (14) to form

the complete ABC framework of Taxol (19). Fragments (6)

and (14) were coupled using n-butyllithium via a Shapiro

reaction whereby the hydrazone precursor (6) was treated

with nBuLi to form a vinyllithium intermediate (fig ?). This

carbanionic species then attacked the carbonyl of the

aldehyde (14) to give a single disastereoisomer of

compound (15). Epoxidation was carried out at the C1,C14

double bond using tert-butyl hydroperoxide and VO(acac)2

to give epoxide (16). Regioselective opening of the

epoxide was achieved using LiAlH4 to obtain tran-1,2-diol

(17). This was then converted to a cyclic carbonate in

preparation for later transformations. Phosgene and KH

were used to achieve this cyclisation and dialdehyde (18)

was obtained by desilylation followed by oxidation using

TBAF and TPAP (respectively).


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Dialdehyde (18) was the appropriate structure to undergo

the McMurry pinacol coupling using titanium trichloride and a

reducing agent (zinc-copper alloy on this case). This achieved

cyclisation to form the ABC-taxoid ring system (19) which then

underwent further functionalisations. Acetylation of the C-10

hydroxyl group using acetic anhydride and oxidation of the C-9

hydroxyl group using TPAP afforded (20), which then underwent

hydroboration using BH3-THF. Treatment with basic hydrogen

peroxide was then carried out and a mixture of regioisomers of

the hydroxyl group at C-5 were obtained. The acetonide group

was removed using acid and the subsequent triol (21) was

acetylated at the C-20 hydroxyl group to afford (22).

The benzyl protecting group at C-7 was replaced with a

triethylsilyl group and selective monodeacetylation was carried

out using potassium carbonate in methanol to afford triol (23).

The next step achieved construction of the oxetane ring through silylation of the primary alcohol and triflation of

th secondary alcohol, followed by treatment with mild acid. This gave oxetanol (24) which was subsequently

acetylated at the tertiary C-4 hydroxyl group to give oxetane (25), ready for the final transformations.

In order to obtain Taxol, enantiomerically pure compound (25) was used. Treatment with excess

phenyllithium regioselectively opened the cyclic carbonate and hydroxy benzoate functionality was achieved at

C-2. This was followed by the introduction of a carbonyl group at C-13 using PCC-Na(OAc) and benzene. The

C-13 carbonyl group was then stereospecifically reduced to an alcohol using excess NaBH4. Esterification and

hence addition of the side chain at C-13 was carried out using NaN(SiMe3)2 and Ojima’s β-lactam. Taxol was

then obtained by deprotecting the C-7 and C-2’ hydroxyl groups, using HF-pyridine.


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In Nicolau’s paper published in 1992, the A-ring synthesis was continued from the ketoacetate (4) to

achieve functionality at the Taxol C-13 position. However in Nicolau’s first total synthesis, published in 1994,

the A-ring synthon used was hydrazone (6) where the hydroxyl group at C-13 is introduced much later. Also, in

Nicolau’s paper published in 1993, describing the synthesis of the ABC-taxoid ring system, the protecting group

employed at the Taxol C-10 position was methoxyethoxy methyl (MEM) ether, whereas in the total synthesis,

Nicolau used the TBS group. These slight modifications made throughout the path towards total synthesis of

Taxol and the fact that many different fragments which could be used highlight the complexity of the molecule.

Similar alterations were made en route to the C- and CD-ring synthons. Nicolau presented a completed

oxetane system in the precursor synthesised in his 1992 paper, whereas an acetonide was actually used as the C-

ring fragment in the total synthesis. This could be due to the sensitivity of the oxetane ring to the transformations

carried out when coupling the two fragments.

Throughout the synthesis, mixtures of stereoisomers were achieved and chromatographic separation was

used to obtain the desired isomeric forms. For instance, when forming the triol prior to oxetane formation, a

mixture of regioisomeric alcohols was afforded, which were then separated using a silica column to obtain the

wanted regioisomer.

Danishefsky’s total synthesis of Taxol

Danishefsky and his group focused on the synthesis of baccatin III, and

between 1992 and 1996 gradually developed the various fragments to build

together the taxane tetracyclic core. A convergent strategy was used where the A-

and C-ring fragments were first prepared and then coupled, forming the B-ring

by a Heck reaction cyclisation. The oxetane ring was introduced at an early

stage, before the coupling of the A- and C-rings was carried out.

The Wieland-Miescher ketone was utilised as the predominant starting

material and as fig ? shows, it provides the structure of the C-ring with easy

functionalisation at C-7 and the angular methyl group (C-19) at C-8. It was a

suitable starting compound as it was easy to prepare by L-Proline-induced aldolisation of a trione which is

prochiral; hence a chiral compound is formed in one step from an achiral compound. (Tetrahedron Letters 41

(2000) 6951-6954)


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The A-ring fragment was formed from trimethylcyclohexane-1,2-diol; which can be prepared from

readily the available compounds 2-methyl-3-pentanone and acryloyl chloride. (Hargreaves et al J. Chem. SOC.

(C), 1968, 2599-2603, also quote J. Org. Chem 1992 57 4043-4047, use scheme 1?). It was found that

lithiation could be carried out on the A-ring fragment (at the Taxol C-1 position), which allowed for coupling to

the aldehyde group introduced at the CD-fragment (at the Taxol C-2 position). The ACD-structure then required

cyclisation to form the B-ring, which was carried out using a Heck reaction. This was followed by appropriate

functionalisations of the tetracyclic framework, including the hydroxyl group at C-13 to obtain baccatin III;

which was then converted to Taxol using Ojima’s β-lactam.

Synthetic route

The starting point for the A-ring fragment, 2,2,4-trimethylcyclohexane-1,3-dione, was prepared from 2-

methyl-3-pentanone and acryloyl chloride (Hargreaves). Monohydrazone (2A) was prepared from (1A) by

reaction with hydrazine. Treatment with iodine converted (2A) to iododienone (4A) in a

Barton reaction [ref] via the monoene

iodide intermediate (3A). This was then

treated with TMSCN and catalytic

potassium cyanide to obtain racemic

cyanohydrin (5A). The hypothetical target molecule (7)

required a metal at the Taxol C-1 position and this was

achieved by lithiation of (5A) to give the A ring fragment (6).

To obtain the CD synthon, the Wieland-Miescher ketone was

treated with sodium borohydride which reduced the carbonyl

group at C-7 to a hydroxyl group. The deconjugated ketal (2C)

was then obtained by treatment with acetic anhydride and

DMAP, followed by ethane-1,2-diol and naphthalenesulfonic

acid, then sodium methoxide. The C-7 hydroxyl group was

then protected using TBSOTf and functionality at C-3 was

attempted. Using diborane followed by hydrogen peroxide,

oxidation to an alcohol was carried out. Then oxidation with

potassium dichromate converted the alcohol to ketone (3C)

after treatment with sodium methoxide.

CH3 O

CH3

CH3

OH

M(7)


CHEM3004

17

The next target was to install the oxetane ring via a triol intermediate (5C). A methylene group was first

introduced at the C-4 position and two different routes were investigated from (3C) to (4C). One route was

proven inefficient as many intermediates were formed. This route involved triflation at C-4 followed by

conversion from the vinyl triflate to an α,β-unsaturated ester. The second route, however, proved more efficient

(Scheme ?). Treatment of (3C) with sulfonium ylide Me3S+I- and KHMDS resulted in a spiroepoxide

intermediate which was then converted to allylic alcohol (4C) by Lewis-acid-induced epoxide ring opening using

aluminium isopropoxide and toluene. Osmylation of the allylic alcohol (4C) using osmium tetroxide and N-

methylmorpholine-N-oxide (NMO) gave triol (5C).

Due to the hindered β-face, osmylation was

expected to be stereospecific, however around 15%

of the unexpected triol was also formed; the

mixture of triols was easily separated and the target

triol (5C) was obtained.

The next step involved the actual formation

of the oxetane ring. This was carried out by: (i)

silylation of the primary alcohol using

trimethylsilyl chloride, (ii) activation of the

secondary alcohol by triflation and finally (iii)

refluxing with ethylene glycol to give oxetane (6C).

Once the oxetane ring was obtained, the tertiary

hydroxyl group at C-4 needed to be protected by a

group which would be removable later and also

distinguishable from the other hydroxyl groups. A

benzyl protecting group was installed using benzyl

bromide, sodium hydride and tetrabutylammonium

iodide (TBAI). This benzyl ether was then treated

with p-toluenesulfonic acid to cleave the ketal and

obtain ketone (7C). Many different degradation options were looked at in an attempt to form a suitable CD

fragment which would be coupled with the lithiated cyanohydrin ring A synthon (6). Scheme ? outlines the

transformations chosen to convert (7C) to the CD fragment (15); this route was used due to its scaleability.


CHEM3004

18

Ketone (7C) was first treated with TMSOTf

to give silyl enol ether (8C), which was then treated

with 3,3-dimethyl dioxirane followed by

camphorsulfonic acid to achieve hydroxylation at C-

10 to give (9C). Then, lead tetraacetate was used to

fragment the ring which resulted in formation of

(10C) with a methyl ester at C-2 and an aldehyde at

C-10. The aldehyde was converted to the dimethyl

acetal using methanol and 2,4,6-collidine p-

toluenesulfonate (CPTS) and (11C) was obtained.

The methyl ester at c-2 was then reduced to a

carbinol (12C) using lithium aluminium hydride. Then, to prepare the C-2 position for oxidation to an alkene,

carbinol (12C) was treated with o-nitophenylselenium cyanide and tributylphosphine to give o-nitrophenyl

selenide (13C). This then underwent oxidation using hydrogen peroxide to obtain alkene (14C). Finally,

ozonolysis of the alkene afforded aldehyde (15) which was used as the CD synthon.

The ACD ring structure was then formed

by coupling the two prepared fragments (6) and

(15). This was started by treating the lithiated

cyanohydrin (6) with THF then adding the

aldehyde (15), which formed the new C1-C2

bond. Treatment with TBAF deprotected the

ketone at C-11 to give the first ACD compound

(16), which was further functionalised and

prepared for ring closure to form the B ring.

Epoxidation of (16) with mCBPA gave epoxide

(17) which underwent hydrogenation and a

hydroxyl group was introduced at C-1, hence diol

(18) was formed. The C-1 and C-2 hydroxyl

groups were then protected as a cyclic carbonate

(19) using carbonyl diimidazole and sodium

hydride.


CHEM3004

19

Conjugated reduction of the enone (19) using

organoborane L-selectride was then carried out to

obtain ketone (20). In preparation for the cyclisation

by the Heck reaction, triflation at C-11 gave vinyl

triflate (21) which was hydrolysed using pyridinium

p-toluenesulfonate (PPTS) to cleave the dimethyl

acetal, giving aldehyde (22). Then, following a Wittig

olefination, alkene (23) was formed and the

palladium-catalysed Heck reaction was carried out to

form the C10-C11 bond, and hence the tetracyclic

(24) was obtained.

The next focus became functionality at various

positions and it was found, by model probes, that the

TBS group at C-7 would be too difficult to remove after

further functionalisation around the tetracycle.

Therefore using TBAF and then TESOTf, the C-7

hydroxyl group was deprotected and reprotected

(respectively). The triethylsilyl ether was then treated

with mCPBA to give epoxide (25). Deprotection and

acetylation at C-4 gave compound (26) which was then

treated with phenyllithium to cleave the cyclic

carbonate and hence benzoate functionality was

achieved. Osmylation of (27), followed by treatment

with lead tetraacetate gave ketone (29) via osmate ester

(28). The epoxide oxygen was then removed using

samarium diiodide and acetic anhydride to give (30)

with the desired alkene functionality at C11-C12.

Ketone (30) was then treated with potassium tert-

butoxide and phenylseleninic anhydride followed by


CHEM3004

20

acetic anhydride to achieve the desired functionalities at C-9 and C-10 to give acetoxy ketone (31).

The final task was to functionalise C-13 and add the side chain. This was carried out by introducing a

carbonyl group at C-13 using pyridinum

chlorochromate (PCC) via an allylic

oxidation to give (32). Sodium

borohydride was then used to reduce the

ketone to an enol (33). This could then be

deprotected using HF-pyridine to give

Baccatin III or reacted with Ojima’s β-

lactam to add the side chain at C-13. Once

the side chain was added, desilylation was

carried out using HF-pyridine and

synthetic Taxol was obtained.


The (+)-Wieland-Miescher ketone was used as the starting material for the C-ring as it supplies a simple

route to a single optically active Taxol enantiomer, because of its single chiral centre. It also contains a C-7

carbonyl group which made functionalisation easy; furthermore the C-19 angular methyl group was already

present, saving further instalment steps.

Addition of the sulphur ylide to ketone (3C) to form the spiroepoxide intermediate was effectively carried

out via the Johnson-Corey-Chaykovsky reaction; this allowed for addition of C-20 prior to formation of the

oxetane ring. Another prominent reaction carried out in Danishefsky’s sequence was the Heck reaction where a

palladium catalyst induced the cyclisation to form the B-ring of the taxane skeleton.

Careful selection of protecting groups was vital throughout the synthesis as it was important to choose

effective, but cleaveable groups. For instance, many protecting groups were investigated for the protection of the

C-4 α-hydroxy group of compound (6C) and it was found that the most effective, and removeable, was the

benzyl protecting group; added using benzyl bromide, sodium hydride and tetrabutylammonium iodide (TBAI).

Limitations to this strategy include the vast number of intermediates which were generated, making the

synthesis less than optimal. Furthermore, one of the major problems encountered was the failure to protect some

groups in the presence of others – hence steric crowding hindered effective protections to be carried out; which at

times left the development of the strategy at a standstill.


CHEM3004

21

Wender’s total synthesis of Taxol

Wender and his group developed their synthetic strategy from the naturally abundant and inexpensive

compound, pinene; which is one of the most widely available natural products, found from various sources, such

as pine trees and industrial solvents like turpentine. In 1992, Wender successfully synthesised the ABC tricyclic

taxane framework by convergence of A and C ring precursors. Pinene was used as the building block for the A

ring fragment and proved to be an effective starting material as it possesses 10 of the 20 carbon atoms of the

Taxol system and could supply the correct chirality for the Taxol core, hence a concise sequence could be

achieved. It was anticipated, at the time, that a total synthesis of Taxol could be achieved by the end of 1992, but

changes in strategy and various different routes had to be investigated before a successful total synthesis was

achieved in 1997.

A linear strategy was eventually adopted where the air-

oxidation product of pinene, verbenone, provided the A-ring

fragment and using readily available compounds, a 6-

membered seco-B ring was formed. Then fragmentation gave

the AB-bicyclic precursor which was elaborated into the ABC-

tricyclic core. Functionalisations were carried out along the way and instalment of the oxetane ring and the side

chain completed the synthesis of Taxol.

Synthetic route

Pinene (1) was oxidised in air using a

cobalt catalyst to give verbenone (2). Using

potassium tert-butoxide, the dienolate of

verbenone was formed and underwent alkylation

with 1-bromo-3-methyl-2-butene. Selective

ozonolysis at the more electron-rich alkene gave

aldehyde (3); which underwent photorearrangement to achieve the correct connectivity of the A ring. The lithium

salt of ethyl propiolate was then added to the C-9 carbonyl group of aldehyde (4) and following protection with

trimethylsilyl chloride, adduct (5) was formed. Then, the conjugate addition of Me2CuLi served two purposes:

addition of the C-8 methyl group and formation of a carbanion at C-3 which effected cyclisation to form

tricarbocycle (6).


CHEM3004

22

With (6) being conformationally rigid

and due to its stereochemistry, various

functionalisations were carried out before the

taxane B ring was completed. The C-9

hydroxyl group was first oxidised to ketone (7)

using N-methylmorpholine-N-oxide (NMO)

with catalytic RuCl2(PPh3)3; although

oxidation could also have been carried out

with Dess-Martin periodinane. The

stereocontrolled introduction of the C-10 hydroxyl group was then achieved by deprotonating (7) with KHMDS

and then adding Davis’ oxaziridine to give α-hydroxyketone (8); which then underwent reduction using LiAlH4

to obtain tetraol (9). Acetonide (10) was then

formed by treating (9) with PPTS and 2-

methoxypropene after TBSCl and imidazole.

The next step achieved the AB ring

system via epoxy alcohol fragmentation using

mCPBA to first form the epoxide and then

fragmentation was prompted using the catalyst

DABCO. The hydroxyl group introduced at C-13

was directly protected with the triisopropylsilyl

(TIPS) group and the AB ring system (11) was

ready for functionalisation. Triol (12) was

obtained by a multi-process step where ketone

(11) was treated with potassium tert-butoxide and

triethyl phosphite in an oxygen atmosphere;

ammonium chloride and methanol were used in

situ to remove the TBS group and sodium

borohydride was utilised to reduce the C-2

ketone. Then, using Crabtree’s catalyst, the C3-

C8 alkene was hydrogenated and a cyclic

carbonate was formed using triphosgene, after

the C-4 hydroxyl group was protected using


CHEM3004

23

TMSCl and pyridine; hence (13) was obtained. The C-4 center was then oxidised to an aldehyde using PCC, in

preparation for rings C and D.

Aldehyde (14), the fully functionalised taxane AB system, was then transformed into the tricyclic ABC

system starting with the addition of a CH2 unit (homologation) at C-4 using

methoxymethylenetriphenylphosphine (Ph3PC(H)OMe). Using hydrochloric acid and sodium iodide, the

acetonide and enol ether groups were hydrolysed, leading to aldehyde (15). The hydroxyl group at C-9 was

protected using triethylsilyl chloride and the C- 10 hydroxyl group was oxidised to a ketone. Using

dimethylmethylideneammonium iodide (Eschemoser’s salt) and excess triethylamine, C-20 was introduced to

give enal (16). Then, Grignard addition of allyl magnesium bromide, in the presence of zinc dichloride, added the

remaining carbon atoms needed to complete the taxane system. Protection of the C-5 hydroxy group using

benzyloxymethyl chloride (BOMCl) gave ether (17), with the cyclic carbonate unaffected by the Grignard

reagent, thanks to the presence of ZnCl2. The next step involved deprotection using ammonium fluoride and

acetylation at C-9, and also the cleavage of the cyclic carbonate and formation of the C-2 benzoate using

phenyllithium; to give acetate (18). In order to

functionalise C-9 and C-10 correctly, (18) was

treated with a guanidinium base to achieve

transposition. Acetoxyketone (19) then

underwent ozonolysis of the terminal alkene to

give aldehyde (20); which was then treated with

excess DMAP to effect aldol cyclisation.

Tricyclic (21) was obtained after protection of

the C-7 hydroxyl group using 2,2,2-

trichloroethyl chloroformate (TrocCl).

Instalment of the oxetane ring was then

started by removal of the BOM group using

sodium iodide and hydrochloric acid, and

leaving group (bromide) was to be added at C-5.

Alcohol (22) was mesylated and (23) was then

treated with lithium bromide to give bromide

(24). Osmylation then introduced oxygen at C-4

and C-20, and instead of direct oxetane

formation, the C-2 benzoate group migrated to


CHEM3004

24

the C-20 hydroxyl group. To continue towards cyclisation of the D ring, Wender used imidazole to complete the

migration of the benzoate group, triphosgene was then used to form a cyclic carbonate at C1 and C2 and

potassium cyanide was used to remove the benzoate group – giving diol bromide (25). N,N-

diisopropylethylamine (Hünig’s base) was used to form the oxetane ring and following acetylation using acetic

anhydride and DMAP, tetracycle (26) was obtained. TASF was then used to remove the C-13 protecting group

and using phenyllithium, the C-2 benzoate was re-established. Hence a mixture of baccatin III (27) and 10-

deacetylbaccatin III (28) was obtained. 10-DAB (28) could be acetylated using acetic anhydride and DMAP to

give baccatin III.

Finally, the addition of the side chain at C-13 was carried out according to Ojima’s method whereby

baccatin III (27) was converted to Taxol in three steps. Sodium hydride was used to form the sodium salt at C-13,

followed by addition of Ojima’s β-lactam to give Taxol, after deprotection at C-2’ using dilute hydrochloric acid.


Taxol was synthesised in the correct enantiomeric form in 37 steps from verbenone; making Wender’s

strategy of total synthesis of Taxol the shortest and most concise to be reported. One of the unique factors of

Wender’s strategy is the use of a photorearrangement [ref 11 from Wender 2755] to achieve the correct carbon

connectivity which then accommodated the subsequent fragmentation and formation of the AB-ring system. The

rigidity of the tricarbocycle was used advantageously to selectively introduce functionality.

As with the other strategies, results from previous studies allowed for methods and conditions to be

designed appropriately. For instance, Wender found that aldol cyclisation could not be carried out on

ketoaldehyde (20) with a cyclic carbonate present at C1-C2, hence cleavage of the cyclic carbonate and

concomitant formation of the hydroxybenzoate was carried out before aldolisation.

Other problems experienced during this strategy include the attempts made o form the oxetane ring. It

was found that by placing a leaving group at C-20 did not achieve cyclisation, so a different approach was tried,

where a leaving group was placed at C-5 and was attacked by the nucleophilic C-20 hydroxyl group. This

approach successfully formed the oxetane ring.


CHEM3004

25

Kuwajima’s total synthesis of Taxol

Kuwajima and his group achieved the enantioselective total synthesis of (-)-Taxol in 1998 via a

convergent strategy. The optically active A-ring fragment was prepared from linear starting materials and two

different attempts were made for the C-ring fragment. Either a cyclohexadiene derivative or an aromatic

fragment was used as the starting point for the C-ring synthon. The diene route gave unwanted polymerisation

by-products during a number of transformations, so the aromatic route proved more efficient and scaleable.

Rings A and C were coupled to achieve the tricarbocycle, functional group manipulations were carried out and

the oxetane ring was then formed. (-)-Taxol was obtained after addition of the C-13 side chain using Ojima’s β-

lactam.

Introduction of the C-19 methyl group proved interesting in Kuwajima’s synthesis as two routes were

attempted. In the first, Kuwajima tried to utilise a method used in his synthesis of (+)-taxusin [ref 8b 2000 paper]

involving cyclopropanation of the C3-C8 double bond. This route was inefficient as it required many protecting

group exchanges along the way. The second – more viable option – was the conjugate addition approach;

introducing the C-19 methyl group using a cyano group. Another interesting feature of Kuwajima’s synthesis was

the Birch reduction of the aryl C-ring to obtain the diene-type C-ring precursor used in the conjugate addition

approach.

Hence, Kuwajima’s synthesis involved preparation of the A-ring which was coupled to a readily available

aromatic C-ring and then cyclisation of the B-ring, induced by stannic chloride, formed the ABC-tricarbocycle.

Birch reduction of the C-ring was then carried out and the precursor for the conjugate addition of the C-19

methyl group was prepared. Following conjugate addition, functional group manipulations were carried out and

then the C-ring was dihydroxylated, ready for the formation of the oxetane ring. The addition of the side chain at

C-13 and final deprotections completed the synthesis of Taxol.


CHEM3004

26

Synthetic route

Lithiated propargyl ether was added to propanal,

followed by hydrogenation using Lindlar’s catalyst to give

alkene (3). Ketone (4) was obtained by Swern oxidation

followed by the conjugate addition of isobutyric ester enolate.

Keto ester (5) was then treated with potassium tert-butoxide

and a Claisen-like cyclisation occurred followed by the

formation of the pivaloate using pivaloyl chloride. Aldehyde

(7) was obtained by removing the THP group and carrying out

Swern oxidation using oxalyl chloride, dimethylsulfoxide and

triethylamine and then treatment with TIPSOTf, DBU and

DMAP gave the silyl enol ether (8). Then the chiral ligand

DHQ-PHN was employed in Sharpless’s asymmetric

dihydroxylation to form α-hydroxy aldehyde (9), but recovery

proved

difficult.

N,N’-diethylmethylenediamine was added to crude (9) in

hot benzene and purification by a silica gel column gave

pure monomeric (9). The pivaloyl protecting group was

replaced with a TIPS group and following a silica gel

column, enone (10) was obtained. Dienol silyl ether (11)

(the A-ring fragment) was then formed, as a mixture of the

geometrical isomers, via a Peterson olefination by treating

(10) with N,N’-diethylmethylenediamine, then

PhSCH(Li)TMS followed by a silica gel column.

The A-ring fragment (11) was then coupled to the lithiated

C-ring fragment, 2-bromobenzaldehyde dibenzylacetal

(12), in the presence of magnesium (II) ion, in a chelation-

controlled addition to give adduct (13) as a single isomer.

The C1,C2-diol was protected as a borate ester (14) and

the B-ring was formed by cyclisation induced by tin


CHEM3004

27

tetrachloride to give tricarbocycle (15) after deprotection

of the borate ester using pinacol and DMAP. Next, the aryl

ketone (18) was prepared to undergo Birch reduction of the

C-ring. Conversion of (15) to (18) involved reduction of

the C-13 carbonyl group using DIBAL, then TBS-

protection of C-2 and C-13, followed by cleavage of the

benzyl and phenylthio groups and finally Swern oxidation to attain a C-9 carbonyl group. Aryl ketone (18)

underwent Birch reduction using a potassium/ammonia electride

salt and a highly-substituted alcohol, 2,2,4-trimethyl-3-isopropyl-

3-pentanol, as the proton source to give the desired diene (19).

Unwanted C-9 reduction product (20) was recycled back to

(18) via Swern oxidation and further Birch reduction afforded

diene (19). The C1,C2-diol was then protected with a benzylidene

group, following deprotection using TBAF. The C-9 carbonyl was

then reduced to an alcohol using sodium borohydride and allylic

alcohol (23) was obtained after a Rose bengal-catalysed photo-

oxidation step.

Triol (23) was used as the precursor for the conjugate

addition of the C-19 methyl group using a cyano group. Before diethylaluminium cyanide was added, enone (24)

was formed by protecting the C7,C9-diol with a p-methoxybenzilydene group and oxidising the C-2 hydroxyl

group using Dess-Martin preiodinane. Hence, treatment

of enone (24) with Et2AlCN gave enol (25) which was

then protected at C-2 to give enol ether (26).

Transforming the cyano group to a methyl group went

via a C-19 alcohol following treatment with DIBAL and

LiAlH4. Attempts to introduce a leaving group failed and

cyclopropane derivative (28) was formed. On treating

(28) with acid, cyclopropyl ketone (29) was obtained and

then treatment with samarium diiodide and TBAF gave

stable enol (30) containing the C-19 β-methyl group.

Using sodium methoxide and methanol, enol (30) was

converted to ketone (31) with a C-3 α-proton.


CHEM3004

28

Next, functional group manipulations

were carried out, starting with protection of the

C13-OH with a TBS group. This step had to be

conducted via a boron ester of the C7,C9-diol

to ensure selective protection at C-13. The

boron ester was cleaved using hydrogen

peroxide to give (32) which was selectively

oxidised at C-9 using Dess-Martin periodinane.

Ketone (33) was obtained after the C-7

hydroxyl group was protected as the MOP-

ether. The next target was to add the final

taxane carbon atom (C-20) at C-4 via an enol

triflate (34). This was formed by treatment

with KHMDS to give the C-4 enolate which was

quenched using Tf2NPh. Using the Grignard reagent

TMSCH2MgCl, in the presence of catalyst Pd(PPh3)4,

enol triflate (34) was converted to (35). Treatment with

N-Chlorosuccinimide gave alkene (36) containing an α-

chloride group at C-5, ready to be displaced later on

formation of the oxetane ring.

Functionality at C-10 was then introduced to aid

dihydroxylation of the C4-C20 alkene. Lithium

diisopropylamine was used to form the C9-C10 enolate

which was then oxidised to an alcohol using

MoO5·pyr·HMPA (MoOPh), to give (37α), after

acetylation. The C-10 α-OAc group was then isomerised

using DBN to attain (37β) containing the C-10 β-

acetoxy group. Osmylation was then carried out and the

desired C4,C20-diol (38) was obtained and the oxetane

ring was formed by DBU-induced cyclisation.


CHEM3004

29

Acetylation of the C-4 α-hydroxyl group could not be achieved using acetic anhydride and DMAP, so protecting

group exchanges were made. The C1,C2-benzyl protecting group was changed to a cyclic carbonate to sterically

aid acetylation of C-4. A more robust TES-group was installed at C-7 and the C1,C2-benzylidene group was

removed using H2 and Pd(OH)2 and triphosgene was used to form the cyclic carbonate. Acetylation then gave

(41), which was treated with phenyllithium to add the α-benzoate functionality at C-2. Then (42) was obtained by

replacing the C7-TES group with a Troc-group to avoid deprotection in the next step, when TASF was used to

cleave the TBS group at C-13. To complete the enantioselective total synthesis of (-)-Taxol, the ester side chain

was added at C-13, using Ojima’s β-lactam, and deprotection at C-7 and C-2’ was carried out.



CHEM3004

30

Mukaiyama’s total synthesis of Taxol

Mukaiyama and his group published their ‘Asymmetric Total synthesis of Taxol®’ in 1999, where they

adopted a convergent strategy starting with the formation of the cyclooctane B-ring from optically active linear

precursors. Taxol synthesis was achieved via BC to ABC to ABCD ring construction. Where the first five total

syntheses utilised Ojima’s β-lactam to add the C-13 ester side chain, Mukaiyama prepared his own by an

enantioselective aldol reaction.

- Two different B-ring structures tried: one cyclised to BC, the other didn’t

- Two routes to pre-precursor

- Why Si bulky groups used? To sterically position the alkyl chain???


CHEM3004

31

Synthetic route

The synthesis was started

with the construction of the B-

ring; which required the

preparation of an optically pure

linear precursor from two

subunits: ketene silyl acetal (4)

and an optically active aldehyde

(9). The latter was prepared in two

ways: the first involved the

conversion of commercially

available methyl 3-hydroxy-2,2-

dimethylpropionate (1) to

aldehyde (9) via aldehyde (7) and

required seven steps. The second route involved the conversion of L-serine to the same aldehyde (9) via the

protected dihydroxyaldehyde (11) and comprised of five steps.

Propionate (1) was converted to a dimethyl acetal (2) via a Swern oxidation, followed by reduction of the

ester using LiAlH4 giving aldehyde (3) after a subsequent Swern oxidation. Then the asymmetric addition of

aldehyde (3) to ketene silyl acetal (4) using a chiral promoter consisting of tin(II) triflate and a chiral diamine

afforded the optically active ester adduct (5). This aldol-adduct was then protected at the secondary alcohol with

a PMB group and reduction with LiAlH4 gave alcohol (7) as the seprataed single stereisomer. On protecting the

primary alcohol with a TBS group and cleaving the diacetal group using acetic acid, chiral aldehyde (9) was

obtained.

Preparation of this aldehyde (9)

from L-serine occurred via dihydroxyester

(10) which was reduced to aldehyde (11)

by treatment with DIBAL, following

protection of the primary and secondary

alcohols with TBS and benzyl groups,

respectively. Then, the lithium enolate of

methyl isobutyrate was reacted with


CHEM3004

32

aldehyde (11) via a stereoselective

aldol reaction to give aldol product

(12). This was subsequently

converted to the desired aldehyde (9)

by protection of the hydroxyl group

with a PMB group, followed by

reduction with DIBAL and a Swern

oxidation.

An aldol reaction between chiral

aldehyde (9) and ketene silyl acetal (4)

in the presence of MgBr2·OEt2 afforded

adduct (14), which was treated with

TBSOTf to protect the hydroxyl group

to give (15). Reduction of the ester

function using DIBAL and successive

Swern oxidation provided aldehyde

(17), which was then treated with

methylmagnesium bromide to give the

B-ring precursor (18) after a Swern

oxidation.

The optically pure methyl ketone

(18) was transformed into the C8-

methyl 8-membered B-ring (23) in five steps. The

first two steps achieved the formation of the

brominated silyl ether (20) via TMS enol ether

(19), by treatment with LHMDS and TMSCl,

followed by N-bromosuccinimide to give (20).

The α-position (20) was then methylated using

LHMDS and methyl iodide, giving intermediate

(21) after deprotection of the TBS group using

acid, followed by Swern oxidation. Cyclisation to

form the B-ring was achieved by an


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intramolecular aldol reaction induced by excess

samarium diiodide. The B-ring fragment (23) was

obtained following acetylation and treatment with

DBU. The addition of the C-ring was achieved by

Michael addition of a cuprate reagent made in situ from

tBuLi, copper cyanide and TES-protected 2-bromo-5-

penten-1-ol. Adduct (24β) was then deprotected using

acid followed by TPAP and cyclisation of the

corresponding ketone (25β) to form the C-ring was

carried out via an intramolecular aldol reaction using

sodium methoxide to give (26α). Epimerisation of the

diastereomer containing a C-7 α-hydroxyl group was

achieved using sodium methoxide to give the target β-

alcohol (26β) which was then used as the ABC-

tricarbocycle precursor.

Construction of the A-ring was started by

protecting the C7,C9-diol with isopropylidene acetal

after reducing the ketone using AlH3 in tolune.

Deprotection of the PMB group followed by oxidation

using PDC gave the conformationally-rigid C-1 ketone

(28), which then underwent alkylation using

homoallyllithium reagent in benzene. Deprotection of

the C-11 hydroxyl group using TBAF then gave cis-diol

(29) which was then treated with three different

dialkylsilyl compounds, adding to the C1,C11-diol to

give (30a-c). Methyllithium was then used to cleave the

Si-O bond at C-11 (restoring the C-11 hydroxyl group)

to form the tetravalent trialkylsilyl protecting group at

C-1, sterically positioning the α-alkyl chain at C-1 for

cyclisation. Then using TPAP and N-

methylmorpholine-N-oxide, the C-11 alcohol group was oxidised to form ketones (32a-c). Wacker oxidation

using PdCl2 was then used to oxygenate the C-12 position forming diketones (33a-c), which then underwent


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titanium-mediated intramolecular pinacol

coupling using TiCl2 and LiAlH4, to form the

ABC-ring structure (34a-c). The target

pentaol (35), possessing the exact

stereochemistry of baccatin III (and hence

Taxol) was then formed by deprotection of the

benzyl and alkylsilyl groups using Na/NH3

and TBAF, respectively.

Pentaol (35) required final

functionalisations and the addition of the

oxetane ring, in order to become baccatin III,

the precursor of Taxol. First the C1,C2-diol

was protected using

bis(trichloromethyl)carbonate and acetylation

of the C-10 hydroxyl group using acetic

anhydride and DMAP, to give (36). The

acetonide group was then cleaved using 3M

hydrochloric acid, followed by TES-

protection at C-7 and oxidation using TPAP

and NMO to form the C-9 ketone (37). Alkene

functionality at C11-C12 was then achieved

by treatment with thiocarbonyl diimidazole

(TCDI) and DMAP, followed by

desulfurisation using trimethylphosphite, to

give compound (38). Functionality was then

introduced at C-13 by oxygenation using PCC

and NaOAc to give and enone which was

reduced using K-Selectride to give α-alcohol

(39) after protection with TESOTf. Then in

preparation for oxetane ring formation, a

bromine leaving group was introduced at C-5

via allylic bromination using CuBR and CH3CN and PhCO3tBu, followed by CuBr and Ch3CN, giving (41).


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35

Osmylation was then used to introduce the dihydroxy group at C-4 and C-20 from the α-face, to give diol

bromide (42). Oxetane ring closure was induced by DBU and oxetanol (43) was obtained after acetylation.

Baccatin III was then formed by treatment of (43) with phenyllithium to benzoylate the C-2 position and

cleavage of protecting groups using HF-pyridine.

Unlike the first five strategies

discussed, where Ojima’s β-lactam was

utilised as the C-13 side chain

precursor, Mukaiyama prepared his

own from benzaldehyde and TMS-

protected S-ethyl benzyloxy-

ethanethioate (44). The

enantioselective aldol reaction was

carried out using a chiral promoter

prepared from Sn(OTf)2, a chiral

diamine and nBu2Sn(OAc)2 to give

adduct (45). The next step replaced the

β-hydroxyl group with an amine group

via a Mitsunobu reaction using

hydrogen azide, triphenylphosphine

and diethyl azodicarboxylate (DEAD),

followed by reduction of the resulting

azide using triphenylphosphine to give amine (46). Benzoyl chloride and DMAP were then utilised to give amide

(47) which was treated with aqueous silver nitrate to hydrolyse the thiol ester, affording the desired side chain

(2R,3S)-3-Benzoylamino-2-benzyloxy-3-phenylpropionic acid (48). Other side chains were also synthesised by

modifying protecting groups and chiral centers to investigate the effect on reactivity when attaching the side

chain to 7-TES baccatin III to obtain Taxol; the attachment of side chain (48) to 7-TES baccatin III was the next

step.

Using a mixture of O,O’-di(2-pyridyl) thio carbonate (DPTC) and DMAP, dehydration condensation of

side chain (48) and 7-TES baccatin III was carried out to give ester (51). Then, to obtain a good yield, this

procedure was repeated three more times after 7-TES baccatin III was recovered from the resulting mixture using

a silica gel column. Ester (51) was then converted to Taxol by deprotecting the benzyl and silyl groups using

palladium hydroxide on carbon (in a hydrogen atmosphere) and HF-pyridine, respectively. When modified side


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chains (49) and (50) were used, the dehydration condensation step did not need to be repeated, however in the

case of (49), the isopropylidene protecting group could not be deprotected. Mukaiyama’s synthesis involved a

number of

enantioselective aldol

reactions using chiral

precursors and by

careful

implementation of

functionality as the

four rings were

gradually constructed

afforded the desired

end product, Taxol.


Discussion and Conclusion

Each group demonstrated a unique strategy for the synthesis of Taxol from commercially available and/or

naturally abundant starting materials. Holton’s linear synthesis strategy was based on controlling the

conformation of the B-ring which was formed after epoxy alcohol fragmentation of a bicyclo precursor,

developed from camphor or β-patchoulene oxide. By using control elements (substituents around the ring),

conformation of the B-ring – hence whether the chair-chair, chair-boat, boat-chair or boat-boat conformation was

used – functionalisations were carried out in a regio- and stereospecific manner. Such control elements were used

in the strategies developed by others as well.

Nicolau’s convergent strategy, where the A- and C-ring fragments were coupled by a Shapiro reaction and

then a McMurry pinacol coupling, utilised the Diels-Alder reaction to form the starting points for the two

fragments. One major drawback which resulted in low yields was the need for enantiomer separation a number of

times in Nicolau’s synthesis. However and advantage of using a convergent strategy as opposed to a linear


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strategy is that derivatisations could be carried out later on in the synthesis; hence minimising the effort required

to retain one functionality whilst installing another.

Another convergent strategy was carried out by Danishefsky, who used the Wieland-Miescher ketone as

the precursor for the C-ring and used a lithiated cyanohydrin as the A-ring synthon. Coupling of the A- and C-

rings were carried out

Documents

Strategies for the Synthesis of Taxol