APPENDIX

CONVENIENT PROCEDURE FOR THE SYNTHESIS OF METHYL1 ALLYLI BENZYL ETHERS FROM CORRESPONDING ALCOHOLS AND PHENOLS

1. Synthesis of methyl ethers

Conversion of the alcohol functionality to its ether (Williamson synthesis) is a

commonly employed reaction both in synthetic organic and bioorganic chemistry.

Methyl ether functionality is widespread among natural products, synthetic drugs and

intermediates. This hnctionality is commonly generated In the beginning of a synthetic

sequence since it is stable to a reasonably strong ncldic and basic condition. For the

same reason, alcohols are protected as their methyl ethers, later to be de-protected with

stronger Lewis acids.' While the amino acid rnethionine is the chief source of methyl

group in the biological conversions, in organrc synthetic transformations several one

carbon sources such as dimethyl su~fa te ,~ diazomethane,' dimethyl phosphite,4

trimethyloxonium tetrafluoroborate5 and methyl trifluoromethanesulfonate6 are among

others, which have been routinely employed for methyl ether generation. In 1979,

Johnston and Rose reported that methyl iodide in combination with powdered KOH as

the base and dimethyl sulfoxide as the solvent is a suitable combination for generation

of the methyl ethers7 However, this procedure requires dry DMSO, which is a high

boiling and hygroscopic solvent. In addition, the procedure also requires usual work-up

and extraction of the product from the reaction mixture. In connection with other

synthetic efforts: we encountered a situation where a seemingly straightfonvard

protection of hydroxyl group ran into difficulties. While exploring this reaction we

observed that methyl ethers could be conveniently prepared with methyl iodide with

KOH (pellet) as the base without use of any extra solvent.

Results of the methyl ether preparation from a vanety of hydroxy substrates 1-2 and

5-7 are gathered in the Table I . It is evident from the table that the reaction is quite

general and the yields are good. The procedure was applied for the transformation of

primary (1-2) and secondary (5-7) alcohols to the respective methyl ethers. In addition

to the alcohols 1-2 and 5-7, Reddy and Mehta successhlly transformed five more

alcohols (3-4 and 8-10) to their corresponding ethers.' Thus, generality of the procedure

is established.

The best ratio of substrate to methyl iodide for generation of methyl ethers was 1:4.

The reaction can be carried out on large scale ( I mol) and excess methyl iodide after

completion of the reaction can be recovered through simple distillation. The efficacy of

the reaction was explored by employing phase transfer catalysts such as tetra-n-

butylammonium iodide, benzyltriethylarnmonium bromide, 18-crown-6 and

polyethylene glycol for the transformation of I-decanol to its methyl ether. However.

we observed that the PTCs did not show any beneficla1 effects such as reduction in

reaction period or increase in yields,

2. Synthesis of ally11 henzyl ethers

The allyl and benzyl groups are commonly employed for the protection of alcohol

and phenol moieties for ease of synthesis and convenient de-protection. Allyl and

benzyl ethers are also intermediates in sigmatropic rearrangement reactions such as

Claisen and Cope rearrangements. Allyl ethers can be prepared from corresponding

alcohols using several reagents such as allyl bromide,'' allyl carbonate," ally1 ethyl

carbonateI2 etc. Similarly, benzyl ethers can be prepared using benzyl bromide,'' benzyl

iodide,14 phenyl diazomethane,ls etc. Among the above reagents ally1 bromide and

benzyl bromide in combination with a base in a suitable solvent medium are frequently

employed for the generation of ally1 and benzyl ethers respectively. The protection of

alcohols1 phenols could be carried out conveniently and efficiently with allyl and benzyl

bromides, with desktop potassium hydroxide pellets without the use of any solvent.

Even though benzyl protection on a carbohydrate substrate was previously carried out

with benzyl chloride in the presence of potassium hydroxide ~ e l l e t s , ' ~ thc scope of the

reaction was not fully explored. In 1998, Bogdal and coworkers reported solvent-free

allyl and benzyl ether preparation using a combination of potassium carbonate and

potassium hydroxide bases in the presence of tetra-n-butylammonium bromide under

microwave irradiation." However, the reaction requires drastic conditions such as high

temperature. Moreover, their studies were limited to preparation of ethers from

alcohols.

To prepare ally1 ethers, an alcohol/phenol and ally1 bromide were stirred with

desktop potassium hydroxide pellets in the presence of 5 mole percent of tetra-n-

butylammonium iodide (TBAI), a phase transfer catalyst (PTC), for specified time at

room temperamre (Table 2) for completion of the reaction. Excess allyl bromide was

removed through distillation followed by purification through chromatography on short

column of silica gel. Similarly, benzyl ethers were prepared from alcohols and benzyl

bromide (Table 3).

It is evident from the table 2 and table 3 that in general, allyl ether formation was

Caster and better yields could be realized in comparison with henzyl ethers. Examples

gathered in the table show that a wide range of substrates displaying different structural

features was readily converted to ally1 ethers. The reaction with allyl bromide could be

performed on molar scale conveniently and excess reagent can be recovered through

simple distillation. The efficacy of the allyl protection of cholesterol in the presence of

different PTCs such as TBAI, benzyltriethylammonium bromide and 18-crown-6 and

found that in the presence of PTC particularly TBAI, time taken for completion of the

allyl protection was about three times less than the reaction without PTC. However, for

benzyl ether preparation no such beneficial effect of the PTC was observed.

Interestingly, benzylation of menthol did not take place under present conditions. We

found that in a competitive experiment, when cholesterol and menthol mixture was

subjected to benzylation cholesterol was completely converted to its benzyl ether in 16

h where was unreacted menthol was recovered, indicating selectivity due to subtle

changes in steric environment. Phenolic hydroxy group was also found to be completely

inert for benzylation under present conditions.

Table 1 Conversion of alcohols 1-10 to methyl ethers 11-20

Table 2 Conversion of alcohols 1-2,s-7,21-25 to ally1 ethers 26-35

ROH

CH3(CH2)~CH20H 1

C6H5CH20H 2

2 1

HO 22

HO

6

Ally1 ether

CH3(CH2)8CH20CH2CH=CHz 26

C6HSCH20Cf42CH=CHZ 27

t12C=HCtl;CO'-cr( -

28

H2C=HCH2C0 29 a:C,CH=CH1 30

94

95

89

85

Time (h)

2.5

4.5

5 0

5 5

5.5

HO

7 32

Yield (%)

95

96

96

93

90

H2C=HCH2C0

31

C6H50H

24

H,C.?-C~H~OH

25

98

C6H50CH2CH=CH2

34

H3C-4-C6H40CH2CH=CH2

35

14

15

Table 3 Conversion of alcohols 1-2,6-7, 21-23 to benzyl ethers 36-42

ROH - ROCHzC6H5

R O H

CH3(CH2),CH20H

1

C6HjCH20H 2

21

fi HO 22

HO

Allyl ether

CH3(CH2)8CH20CH2C6H5

36

C,jH5CH20CI-12C6H5 37

92

94

Time (h)

18

35

6 40

HO

Yield (%)

94

8 1

7

23

9 1

93

86

38

C6H5H$0 J-c-( 39

20

41

42

20

3. Experimental section

General

For general details about experimental conditions see Chapter-I. Methyl iodide

(Lobah Ally1 bromide (Fluka) and henzyl bromide (E. Merck) were freshly distilled

before use. The product ethers were characterized by comparing spectral data of known

compounds described in the literature or procured from commercial sources

Representative procedure for the synthesis of methyl ethers, conversion of 1-

decanol (1) to its methyl ether (11): A mixture of I-decanol 1 (158 mg, I mmol),

methyl iodide (0.26 mL, 4 mmol) and KOH pellet (1 pellet, .s 120mg, 2 mmol) was

stirred at room temperature for 14 h. The completion of the reaction was monitored by

TLC and then loaded on a pad of silica gel (100-200 mesh) and eluted with 5% EtOAc-

hexanes to yield I-methoxydecane 11 (95%) as colorless oil." Rl= 0.42 (5% EtOAc-

hexanes); IR (neat) 11 19, 1462, 2855, 2926 cm"; 'H NMR (200 MHz, CDCI,) 6 0.9 (t,

J=7 .2Hz,3H) , 1.3(brs, 14H), 1.6(m,2H),3.3(~,3H),3.32(t,J=7.2Hz,2H)ppm.

Benzyl methyl ether (12): Following the general procedure described above, benzyl

alcohol 2 (215 mg, 2 mmol), methyl iodide (1.14 g, 8 mmol) was converted to benzyl

methyl ether 12 (75%) as colorless oi1.I9 Ri= 0.45 (5% EtOAc-hexanes); IR (neat) 700,

740, 1100, 1385, 1435, 1600,2850,2950,3050 cm"; 'H NMR (200 MHz, CDC13) 6 3.5

(s, 3H), 4.48 (s, 2H), 7.36 (br s, 5H) ppm.

(-)-Menthy1 methyl ether (15): Following the general procedure described above,

menthol 5 (156 mg, 1 mmol) and methyl iodide (568 mg, 4 mmol) was converted to its

methyl ether 15 (85%) as colorless oil. RI= 0.44 (5% EtOAc-hexanes); IR (neat) 11 10,

11 15, 1445, 1460, 2990 cm.'; 'H NMR (200 MHz, CDCI,) 6 0.9- 1.5 (m. 16H), 0.8 (br

d, 3H), 0.9 (br d, 3H), 0.95 (br d, 3H), 2.85 (m, IH), 3.28 (s, 3H) ppm.

Z~Methoxycholest-5-ene (16): Following thc general procedure descnbed above,

cholesterol 6 (200 mg, 0.5 mmol) and methyl iodide (284 mg, 2 mmol) was converted

to 16 (95%) yield as colorless solid. R(= 0.41 (5% EtOAc-hexanes); mp 82-83 "C (lit.*'

84.5 'C); [uID -42 (lit. 45.8'); IR (neat) 1620. 2890, 2990 cm-I; 'H NMR (200 MHz.

CDCI,) 6 0.65 (s, 3H), 0.78-0.92 (series of d, 9H), 0.95 (s, 3H), 0.8-2.25 (series of m,

28H), 2.29-3.22 (m, IH), 3.31 (s, 3H). 5.33 ( d , J = 3 Hz, 1H).

f 0-Meth~~-1 ,2:5 ,6-di -* isopropyl ideneglnose (17): Following the general

pmedure described above, glucose-diacetonide 7 (137 mg, 0.5 mmol) and methyl

iodide (284 mg, 2 mmol) was converted to its methyl ether 17 (95%) yield as colodess

syrup. Rr = 0.42 (5% EtOAc-hexanes); [a10 -31 O (lit. -32 O); IR (neat) 1370, 1455,

2890, 2990 cm"; 'H NMR (200 MHz, CDCI,) S 1.32 (s, 3H), 1.42 (s, 3H), 1.43 (s, 3H),

1.5 (s, 3H), 3.45 (s, 3H), 3.77 (d, J = 3 Hz, IH), 3.9-4.2 (m, 3H), 4.24-4.35 (m, IH), 4.6

(d, J = 3.5 Hz, IH), 5.9 (d, J = 3.5 Hz, IH).

Representative procedure for the synthesis of allyl -ethers, synthesis of allyl decyl

ether (26)

A mixture of I-decanol 1 (166 mg, 1.05 mmol), ally1 bromide (484 mg, 4.04 mmol),

KOH (1 pellet, =I20 my, 2 mmol) and TBAl (5 mol %) was stirred at room temperature

for 16 h. After completion of the reaction (2.5 h), the mixture excluding the KOH pellet

was distilled under reduced pressure to remove excess ally1 bromide and then loaded on

a pad of silica gel (100-200 mesh, 1x5 cm column) and eluted with 5% EtOAc-hexanes

to yield allyl decyl ether 26" (95%). Rf= 0.43 (5% EtOAc-hexanes); IR (neat) 1100,

1660 cm?; 'H NMR (400 MHz, CDCI,) 60.89 (t, J = 13.2 Hz, 3H), 1.26 (broad s, 14H),

1.6 (m, 2H), 3.41 (t, J = 6.88 Hz, 2H), 3.97 (d, J = 5.37 Hz, 2H), 5.15-5.24 (m, 2H),

5.87-5.95 (m, IH).

Allyl henzyl ether (27). Following the general procedure described above, benzyl

alcohol 2 (108 mg, I mmol) and ally1 bromide (484 mg, 4 mmol) was converted to allyl

benzyl ether 27" (96%). Rf= 0.53 (5% EtOAc-hexanes); IR (neat) 1630, 1090 cm.'; 'H

NMR (60 MHz, CDCI,/CCb, 1:l) 6 4.04 (d, J = 1.46 Hz, 2H), 4.5 (s, 2H), 5.17- 5.35

(m, 2H), 5.9-6.0 (m, IH), 7.34 (m, 5H).

Allyl geranyl ether (28). Following the general procedure described above, geraniol21

(154 mg, I mmol) and ally1 bromide (484 mg, 4 mmol) were converted to its ally1 ether

28 (96%). R, = 0.54 (5% EtOAc-hexanes); IR (neat) 1660 cm-I; 'H NMR (60 MHz,

CDCI3/CCl4, 1.1) 6 1.64 (s, 6H), 1.72 (s, 3H), 2.1 (br s, 4H), 3.97 (br s, 4H), 5.1-5.2 (m.

3H), 5.4 (m, IH), 5.6-5.7 (m, IH).

Ally! neryl ether (29). Following the general procedure described above, nerol 22 (154

mg, I -01) and allyl bromide (484 mg, 4 mmol) were convened to its ally1 ether ~ 9 ~ '

(93%). R, = 0.51 (5% EtOAc-hexanes); IR (neat) 1660 cm-'; 'H NMR (60 MHz,

CDC13/CC4, ]:I) 6 1.74 (s. 3H), 1.67 (s, 3H), 1.6 (s, 3H), 2.07 (s, 4H), 3.93-3.96 (m,

4H), 5.09-5.28 (m, 2H), 5.36 (t, J = 6.85 Hz, 2H), 5.85-5.94 (m, 1H).

Ally1 menthyl ether (30). Following the general procedure described above, menthol 5

(156 mg, 1 mmol) and ally1 bromide (484 mg, 4 mmol) were converted to allyl menthyl

ether 302' (90%), RI= 0.45 (5% EtOAc-hexanes); [a ]~" = -53.13', IR (neat) 1170, 1630

cm". 'H NMR (400 MHz, CDCI,) G 0.77 (d, J = 6.84 Hz, 3H), 0.89 (d, J = 7.33 Hz,

3H), 0 91 (d, J = 6.84 Hz, 3H), 1.22-2.25 (m, 6H), 3.05 (m, IH), 3.85-4.15 (m, 3H),

5.1 1 - 5.3 (m, 2H), 5.87-5.96 (m, IH).

Allyl cholesteryl ether (31). Following the general procedure described above,

cholesterol 6 (194 mg, 0.5 mmol) and allyl bromide (242 mg, 2 mmol) were converted

to allyl cholesteryl ether 31L4 (96%). RI= 0.42 (5% EtOAc-hexanes); mp 68 'C; [ale = -

21.6". IR (neat) 1138, 1647 cm"; 'H NMR (400 MHz, CDCI,) 60.63 (s, 3H), 0.75-0.95

(br d, 9H), 0.98 (s, 3H), 0.86-2.28 (m, 28H). 2.3-3.43 (m, IH), 4.03 (d, J = 5.78 Hz,

2H), 5.06-5.23 (m, 2H), 5.35 (d, J = 24.2 Hz, IH), 5.76- 6.06 (m, IH).

3-0-allyl-1,2:5,&di-O-isopropylideneglucofuranose (32). Following the general

procedure described above, glucose diacetonide 7 (260 mg, I mmol) and allyl bromide

(484 mg, 4 mmol) were converted to its allyl ether 3225 (94%) as colorless oil. Rf= 0.44

(5% EtOAc-hexanes); [ a ] ~ = -25.76'; IR (neat) 1160, 1640 cm"; 'H NMR (400 MHz,

CDCI,) 6 1.31 (s, 3H), 1.35 (s, 3H), 1.42 (s, 3H), 1.5 (s, 3H), 3.83 (d, J = 9.28 Hz, IH),

3.94-4.29 ( m, 3H), 4.29-4.34 (m, lH), 4.55 (d, J = 3.4 Hz,2H), 5.23-5.34 (m, 2H), 5.89

(d, J = 3.9Hz,2H), 5.83- 5.89(m, IH).

3-0-allyl-1,2:4,5-di-O-isopropylidenefructopyranose (33). Following the general

procedure described above, fructose diacetonide 23 (260 mg. 1 mmol) and allyl bromide

(484 mg, 4 mmol) were converted to the corresponding allyl ether 33 (95%) as a

colorless oil in 3 h. R f = 0.41 (5% EtOAc-hexanes); [ a ] ~ = -105.36; IR (neat) 1647,

1084 cm-I; 'H NMR (400 MHz, CDCI,) G 1.36 (s, 3H), 1.41 (s, 3H), 1.49 (s, 3H), 1.53

(s, 3H), 3.44 (d, J = 6.8 Hz, IH), 3.92 (s, 2H), 3.94-4.48 (m, 6H), 5.08-5.32 (m,2H),

5.74-5.98 (m, IH).

Allyl phenyl ether (34). Following the general procedure described above, phenol 24

(14 mg. 1 mmol) and allyl bromide (484 mg, 4 mmol) were converted to its allyl ether

34 (89°io) as colorless oil." RR,= 0.51 (5% EtOAc-hexanes); IR (neat) 1630 cm", 'H

NMR (60 MHz, CDCldccb, !:I) 6 4.53 (m, ZH), 5.22-5.44 (m, 2H), 5.89-6.11 (m,

IH), 7.27 (m, 5H).

Ally] bmethylphenyl ether (35). Following the general procedure described above, 4-

methylphenol 25 (108 mg, I mmol) and ally1 bromide (484 mg, 4 m o l ) were

converted to its ally1 ether 35" (85%) as colorless oil. R f = 0.48 (5% EtOAc-hexanes);

IR (neat) 1100, 1600 cm-I; 'H NMR (400 MHz, CDCI3) S 2.26 (s, 3H), 4.51 (d, J =

1.48Hz, 2H), 5.28-5.42 (m, 2H), 6.01-6.08 (m, lH), 6.81 (d, J = 8.79 Hz, 2H), 7.08 (d, J

= 7.81 Hz, 2H).

Representative procedure for the preparation of benzyl ethers, benzyl decyl ether

(36). A mixture of I-decanol 1 (158 mg, I mmol) and benzyl bromide (0.36 mL, 3

mmol) and KOH (1 pellet, -120 mg, 2mrnol) was stirred at room temperature for 18 h.

The crude m~xture was loaded on a pad of sil~ca gel (100-200 mesh, 1x5 cm column)

and eluted with 2% EtOAc-hexanes to yield benzyl decyl ether 36 (1 12 mg, 94%). RI=

0.46 (5% EtOAc-hexanes); 1R (neat) 1028, 1103, 1204, 1362, 1454, 1495, 1707, 2855,

2926, 3030, 3067 cm"; 'H NMR (400 MHz, CDCln) S 0.9 (t, J = 12.4 Hz, 3H), 1.3 (br

s, 14 H), 1.5-1.64 (m, 2H), 3.46 (t, J = 6.41 Hz, ZH), 4.51 (s, 2H), 7.36 (s, SH).

Dibenzyl ether (37). Following the general procedure described above, benzyl alcohol

2 (108 mg, 1 m o l ) and benzyl bromide (5 13 mg, 3 mmol) were converted to dibenzyl

ether 37 (81%).~' R f = 0.42 (5% EtOAc-hexanes); IR (neat) 1549, 2858, 3033 cm-I; 'H

NMR (400 MHz, CDC13) 64.56 (s, 4H), 7.4 (s, 10H).

Benzyl geranyl ether (38). Following the general procedure described above, gcraniol

21 (154 mg, I mmol) and benzyl bromide (513 mg, 3 mmol) were converted to its

benzyl ether 3829 (91%). R,= 0.48 (5% EtOAc-hexanes); IR (neat) 1595, 1640 cm-I; 'H

NMR (400 MHz, CDC13) S 1.62 (s, 6H), 1.65 (s, 3H), 2.01 (m, 4H), 3.92 (d, J = 7.OHz,

2H), 4.40 (s, 2H), 5.03 (s, IH), 5.33 (br t, J = 7.0 Hz, IH), 7.20 (s, 5H).

Benzyl neryl ether (39). Following the general procedure described above, nerol 22

(1 54 mg, I mmol) and benzyl bromide (5 13 mg, 3 mmol) were convened to its benzyl

ether 39 (91%). Ri= 0.45 (5% EtOAc-hexanes); IR (neat) 1600, 1640 cm"; 'H NMR

(400 MHz, CDCI,) S 1.58 (s, 3H), 1.67 (s, 3H), 1.76 (br s, 3H), 2.03 (br d, J = 3Hz,

4 ~ ) , 3.94 (d. 2H. J = 6H). 4.42 (s, 2H), 5.21 (m. 2H), 7.25 (s, 5H).

Benzyl cholesteryl ether (40). Following the general procedure described above,

cholesterol 6 (100 mg, 0.26 mmol) and benzyl bromide (133 mg, 0.78 -01) were

converted to its benzyl ether 40" (86%). Rr= 0.47 (5% EtOAc-hexanes); [a],, = -5.1 lo;

'H NMR (400 MHz, CDCIJ) 60.66 (s, 3H), 0.77-0.93 (br d, 9H), 1.0 (s, 3H), 0.77-2.07

(m, 28H), 2.2-3.3 (m,lH),4.54(s, 2H), 5.35 (d,J=6.66Hz,lH), 7.3 (m, 5H).

3-0-benzyl-1,2:5,6-di-0-isopropylideneglranose (41). Following the general

procedure described above, glucose diacetonide 7 (250 mg, 0.96 mmol) and benzyl

bromide (492 mg, 2.88 mmol) were converted to its benzyl ether 41" (92%) as

colorless syrup. Rr= 0.43 (5% EtOAc-hexanes); [ale = -23.33; IR (neat) 675, 763, 863,

1025, 1078, 1219, 1374, 1428, 1596, 2361, 3026 cm"; 'H NMR (400 MHz, CDCI,) S

1.31 (s, 3H). 1.37 (s, 3H), 1.43 (s, 3H), 1.49 (s, 3H), 3.7 (m, IH), 3.97-4.21 (m, 3H),

4.27-4.42(m. IH) ,4 .55(~ ,2H) ,4 .65(d , . J=4 .88Hz , IH), 5.9(d,J=3.61 Hz, lH),7.4

im, 5H).

3-0-Ben~l-1,2-4,5-di-0-isopropylidenefructopyranose (42). Following the general

procedure described above, fructose diacetonide 23 (200 mg, 0.77 mmol) and benzyl

bromide (395 rng, 2.31 mmol) were converted to its benzyl ether 4212 (94%) as

colorless syrup. RI= 0.42 (5% EtOAc-hexanes); [ a ] ~ = -92.91, IR (neat): 776, 897,978,

1024, 1085, 11 19, 1219, 1327, 1381, 1428, 1461, 1595,2361,2892, 2939, 3026 cm",

'H NMR (400 MHz, CDCI,) 6 1.39 (s, 3H), 1.43 (s, 3H), 1.50 (s, 3H), 1.55 (s, 3H), 3.49

(d, J = 7.23 Hz, IH), 4.07 (s, 2H), 3.86-4.25 (m, 3H), 4.4 (t, J = 12.78 Hz, IH), 4.7 (d, J

= 11.98Hz, IH),5.0(d, J = 11.97Hz, IH),7.34(m,SH).

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