3 Photo Chemical Rearrangement Reactions

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3.1 Photorearrangements

3 Photochemical Rearrangement Reactions

in Synthetic Organic Chemistry


This chapter has to do with different types of photoisomerization reactions.

This includes the formation of consti tut ional , configurational , confor-

mat ion al and valence isomers .

3.1.1 E - - Z Isomerization of Aikenes

On e imp orta nt aspect o f the pho tochem istry o f alkenes 301,302) is the E -- Z

isomerization aro un d the C -- C dou ble bon d 303). This is also valid for

cycloalken es with the obvious exception of cyclopropene s 304a,h) whichexhibit a dist inct photochemical behaviour, and cyclobutenes and cyclo-

pentenes w here the r ing is to r igid to al low sufficient twist ing of the d oub le

bond.

The photochemical isomerization of E-st i lbenes has been applied in

the p repa ration of phenan threnes, as Z-st i lbenes underg o electrocyclic r ing

closure (cf . chapter 3.1.3) to dihydrophenanthrenes which in turn are easi ly

oxidize d to p he na nth ren es (3.1) 305). This sequen ce has also b een em ploy ed in

the synthesis o f ben zoq uino lines 306) or of b enzo quino lizines (3.2) 307)

lfcr" h, l crH h,p F-C6~4 p, C6Hz,

J~J

, s

(3.1)

26



3 Photochemical Rearrangement Reactions in Synthetic Organic Chemistry

h~'. ® .o i r ~ - ( 3 . 2 )

C

General ly for s imple alkenes the E- isomer is thermod ynam ical ly mo re s table

than the Z- isomer . Photoch emical isomer izat ion is thus a p owerful metho d

to ob tain Z-alk enes in go od yields, as sho wn for 13-ionol (3.3) 3oa).

OH

H

(3.3)

Another impor tant feature of this react ion is the synthes is of highly

s t rained E-cycloalkenes . Thus E,Z-cycloocta- l ,3-diene has been obtained

from the Z,Z -co m po un d in sensit ized irradiat io n 3o9) (3.4), while E-cyclo-octene w as obtain ed from the Z-isome r by direct i rradiat ion (3.5) 31o). The

synthesis of a trans d ou bly b ridge d olefin has also b een re po rted (3.6) 311).

(3.4)

185nrn (3.5)

(CHz)~~CH2)8v {C~CH2)8 (3.6)

The E- isomers o f cycloheptenes and cyclohexenes are usual ly not s table

enough to be isolated; nevertheless they can be trapped by alcohols , as for

cy clo he pte no ne 312) (3.7), o r o bserve d sp ectrosco pically as for ph eny lcyclo -

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3 . 1 P h o t o r e a r r a n g e m e n t s

hexene. This lat ter olefin also undergoes photoaddit ion to alcohols , but

via electron transfer from the alkene to the excited sensit izer , the result ing

radical cat ion then reacting with the solvent (3.8)313).

o o o

(3.7)

~C6H5 ~C6H5hv •

Sens, --- -I- Se ns .

(3.8)

3.1.2 Electrocycl ic Reactions of Conjugated Dienes and Trienes

Elect rocycl ic react ions were f ir st descr ibed by W ood wa rd and Ho ffman nin the ir classic series of articles. One very interesting aspect of such rea ctions

is, that for a given conjugated polyene photochem ical t ransformat ion leads

to the opposi te s tereochemical outcom e than the therma l one 3x4)

R

h~,

R

R

/

(3.9)

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3 Photochemica l Rearrangement Reac t ions in Synthe t ic Organic Chemis t ry

The ph otoe qu i l ibr iu m between 1,3-cyclohexadienes and 1,3,5-hexatr ienes

315,316) is the key step in the synthesis o f vitam in D , as sh ow n in t he for-

ma t io n of v i tamin D3 (R = C8H17) via a [1,7]s igmatropic H-shif t f romprevi ta min D whic h is obta ine d by i r radia t ing pr ovi ta min D (3.9) 317)

The r ing c losure of 1 ,3-butadienes to cyc lobutenes has been ut i l ized for

synthe t ic pu rpose s as in the prep ara t ion of cyc lobuten e i tse l f 3x8) (3.10),

of a bic yclo hep teno ne 319) (3.11), of a bicyclic la ctam 32°) (3.12) or of

dip hen ylb icyc lohe pten e (3.13) 321).

hv ~ ~ (3.10 )

0

hv ~ ~ 0oceone (3.11)

hv .~ ~ 00

I HH

(3.12)

6H5 ~ ~6H5,v ~ (3.1r3)

3.1.3 Di-pi-methane Rearrangement of 1,4-Dienes

1,4-Dienes can be conver ted photochemical ly to vinyl cyc lopropanes .

This r eac tion , a l so n ame d the Z im me rm an rea r rangem ent 322), i s com m on

to m olecules in which two n moie t ies a re subs t i tuents on a "m eth an e"

car bo n 323). Th e skeletal chan ges follo w the m ech anisti c sequence in (3.14).

(3.14)

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3,1 Photorearrangements

Ear ly appl ica t ions of t h is reac t ion a re f oun d in t he conve rs ion of ba r re lene

to semibul lva lene a24) (3.15) or in the prep ara t io n of an azabul lva lene (3.16)

32s) In a s imi la r reac t ion azabarbara lene has been prepared f rom aza-bicyc lono nat r ien e 326)

(3.15)

CH30

acetone

CH 0

The d i -n-me thane rea r rangement i s a l so a convenien t way of ob t a in ing

polycycl ic fused r ing sys tems as i l lus t ra ted in the synthesi s of a t r icyc lo-un de can e (3.17)3z7). In the i r rad ia t ion o f dihy dro t r iquin acen e the init ia l

bonding scheme i s ident ica l as in (3 .14) but ul t imate cyc lopropane for-

m at io n i s hind ered by s t ruc tura l reaso ns (3 .18) 328)

6H:v

(3.17)

c l c e t o n e(3.18)

O ne o f the C - - C doub l e bonds c a n a l so be pa r t o f a n e none s y s te m a s

show n in t he syn thes is o f 2 -cyc lopropyl -2-cyc lopentenones . Such a photo-

product was then conver ted to the sesqui te rpene taylor ione (3 .19)329)

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hv ~ R

. /. /

(3.19)

3.1.4 Photorearrangementof Arenes

Benzene i s com mo nly used a s so lvent in ph otochem ica l reac t ions ; never -

the le s s one has to t ake in to cons ide ra t ion tha t a renes do unde rgo photo-

rearr ang em ents 33°a'b). Benzene isom ers as De wa r benz ene 331) (3.20) or

benzvalene 332) (3.21) hav e been prep ared by photoly s is o f benzenes .

F F

hvneat ~ F

F F FF F

F

(3.20)

~v = ~ (3.21)25/-,nm60 °

3.1.5 Photorearrangementof =,~ and ll,7-UnsaturatedKetones

Enon es und e rgo a va r ie ty of pho torea r rang emen ts 333). In cont ra s t to

~,13-unsaturated keto nes wh ich reac t in m an y d ifferent ways, 13,7-unsaturatedke tones unde rgo the oxadi -n-me thane rea r rangement in sens i t i zed i r r a -

d ia t ions ; in com ple te ana logy the produc ts form ed a re cyc lopropyl ke tones334--336)

31



3.t Photorearrangements

Irrad iatio n o f 4,4-dialkyl-2-cyclohexenon es afford s bicyclo[3.1.0]hexanones

in a so ca l led " l um ike ton e" rearra ngem ent (3.22) 337). The reac t ion proceeds

with invers ion of conf igura t ion on the disubs t i tuted C-a tom (3.23)338) .

t. BuOH 0

o

(3.22)

0 C6Hs

EtOH "H

C6H 5

R-(+) (-)

(3.23)

2-cyclopenteno nes ma y isomerize to 3-cyclopentenones as shown for the

prop el lane molecule in (3.24). Cleavage of the cyclobutane C -- C bo nd

13 to the carb on yl gr ou p affo rds a d iradical with an allylic centre, the

recom bina t ion then giving a rearranged t r icyc lound ecenon e s39). Th e arrow s

in (3.24) indica te the c leaved and the newly formed C -- C bond .

oII

(3.24)

Cyc lopentenyl ke tones can unde rgo photo a ry la t ion when i r rad ia ted in the

pre senc e o f a Lew is acid cataly st (3.25) 340)

0 0

(3.25)

32




Th e b icycloo ctadien one in (3.26) is con verte d into a cyc lopropy l keten e ~ia

a [3,3]sigmatropic rearra nge me nt ~1).

0

II coo.

hv HzO(3.26)

As m en tion ed earlier (cf. cha pte r 2.1.1), 13,y-enones un de rgo an 1,3 acyl

shif t from the Sl-state. In sensit ized reactions they rearrange to cyclopropyl

ketones in the so termed oxadi-pi -methane rear rangemen t , as i llus trated for

the n aph thale no ne in (3.27) 342)

o

(3.27)

The analogous rear rangem ent of the bicyclooctenone in (3.28) gives

a pro duc t which serves as a sy nthon for polycyclopentano id terpenes and

pro stac yc lins 343a,b).

hi.' ~ ~ 0 (3.28)

3.1.6 Photorearrangement of 2,5-Cyclohexadienones

Th e rearra nge me nt o f cross-conjugated cyclohe xadien ones to bicyclo[3.1.0]-hex-3-en-2-ones was one of the f i rs t photoche mical react ions to b e thoro ughly

studied fo r m echanist ic and synthetic purpo ses 3,#). Th e examples o utl ined

below are the c onve rsion o f the lactone ~-san tonin to lumisa ntonin 345) (3.29)

33




and the synthesis of a precursor of the spirocyclic sesquiterpene 13-vetivone(3.30) 346)

" o ~ : (3.29)

O .. . . . .

hv

(3.30)

In acetic acid the interm ediate zwitterion is trapped by acetate affording

cyclopentenones with a n acylated hyd roxymethylene group on C-4. Use

of this sequence has been made of in the synthesis of the sesquiterpenescy clo co lor en on e 3477 (3.31) an d a-c ad ino l (3.32) 348)

x°°i h vCH3COOHH ~ COCH3

(3.31)

34



3 PhotochemicalRearrangementReactions n SyntheticOrganicChemistry

~ _ hvCH3COOH

0

~ OCH3

\ OH

(3.32)

3.1.7 Photorearrangement of Heterocycles

The re are obvious ly no c om mo n mechan is t ic features for the ph otorearrange-

me nt of heterocycles in general . Y et three-m em bere d r ings 349) and f ive-

membere~l r ing heterocycles 350) exhibit a quite part icu lar behav iour. This

topic has also been reviewe d in a hetero cyc lic che mistr y series 351). In this

chap ter only som e selected reactions o f prepa rative interest wil l be outl ined.

Oxon in has been o btained in the d i rect or sensi tized i r radiation of cyclo-

octatetraeneoxide (3.33) 352).

~ O hv ~ ~ ) (3.33)

Dih ydro pyra zines can be co nverted in goo d y ields to imidazo les (3.34) 353)

H5C6

E~OHH5C N H5C

(3.34)

1,2-Diazepines are formed in almost quanti tat ive yield in the irradiat ion

o f pyr id in iu m ylide s (3.35) 354).

0

(3.35)

The 1,3-dipolar intermediate obtained in the r ing opening react ion of

3H-azir ines via cleavage of the C -- C bon d has been t rapped to give oxa-

zo line s 35s) (3.36) a nd py rro lin es (3.37) 356)

35



3 . 1 P ho to r e a r r a nge m e n t s

H~C6E) ( 1 ) /

C6H5- C = N - - ~

0//CF3C OCH 3

H5C6 CF3

" ~ - - -~ OC H3N O

(3.36)

N

(3.37)

Electrocyclic ring opening of the aziridine in (3.38) affords aza[17]annu-tene 357)

lCOOet

h~, ~ N - - C O O e t (3.38)

3.1.8 Photocyclization of Enamides

The photocyclizat ion of enamides to quinolines or isoquinolines has become

an im po rta nt reactio n in the synthesis of alkaloid s 219,358). It has recently

been ap plied in the preparatio n of the isoquinoline alkaloid polycarpine 359).

The principle of the reaction is demons trated in the preparat ion of dihydro-

qu ino line s 36o) (3.39) an d of spirocy clohex aneiso quino line derivatives (3.40)

361). In each case the electrocyclic ring closure product undergoes a subse-

que nt 1,5-hydrogen shift.

o e oI I IH H H

N~'R C6H~z "~R

o o

(3.39)

(3.40)

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3 Photo Chemical Rearrangement Reactions