Indi an Journal of Chemis try Vol. 4 1B, May 2002, pp. 1032- 1038

Microwave promoted selective preparation of acetals and esters from aldehydes

Ruli Boraht , Dipok J Kalita!! & Jadab C Sarma*

Organic Chemistry Di vision, Regional Research Laboratory, Jorha t 785 006, Assam, Indi a

Received 23 November 2000; accepted (revised) 25 October 2001

A new selecti ve method of acetali zati on o f aldehydes and cyclic ketones wi th I ,2-diols or alcohols catalyzed by iodine under microwave irradiation is reported. Depend ing upon the reacti on conditions furth er oxidati on of the ary lidene aceta Is takes pl ace in the sys tem leading to the formation of products like iodoester 2 and hydroxy esters 3, 4 and 5. Both unsubstituted (la) and substituted arylidene acetal with electron re leasing group (ld) give hi gh yie ld of iodoester 2 (70 -80%) whereas the arylidene acetal substituted with an electron withdrawing group such as N02 (lb) gives a low y ield of the corresponding iodoester (2b, 25 %).

Microwave assisted organic synthesis is a fast developing area in synthetic organic chemjstryl . The basis of this synthetic technique is the empirical observation that some organic reactions proceed much faster and with higher yields under microwave inadiation compared to conventional heating2

Although different hypotheses have been proposed to account for the effect of microwave on organic reactions/compounds) the reason for the dramatic acceleration effect is thought to be instantaneous super heating of the reaction medium. Regardless of the exact origin of the microwave effect, it is found to be extremely efficient and applicable to a very broad range of practical syntheses4 Utilizing microwave irradiation several reactions of synthetic importance such as alkylation ,S condensation,6 halogenation7 and oxidation8 have been reported in literature recently. In two recent reports Moghaddam et al. 9 and Perio et al. 10 have mentioned the use of microwave irradiation in the catalytic protection of carbonyl groups as the 1,3-dioxolane using TsOH or FeCI) as catalyst. Even another very recent report described the use of microwave inadiation in combinatorial synthesis. II Herein we wish to present a detailed report on the catalytic acetalization method 12 promoted by microwave inadiation using iodine as a catalyst. While exploring our interest on iodine as an effective catalyst for various reactions like acetylation\), and l,l-diacetate formation from aldehydes,14 we tried to

t Present address: Department of Polymer Chemi stry, Tezpur Uni versity, Tezpur, Assam, Ind ia. !! Present address: Oil Indi a Limited, Duli aj an, Assam, Indi a

examine the efficacy of the same catalyst for acetalization of aldehydes and ketones. As the conventional method of refluxing the substrate, ethane-l ,2-diol and iodine in THF fai led to give any product, we tried the same reaction under microwave inadiation to get an excellent yield of the acetal in just 1 rrunute. A blank reaction of an aldehyde and ethane-l,2-diol under microwave ilTadiation for 10 min did not yield any acetal and the reactants were recovered thus confirrrung the importance of iodine as a catalyst.

Results and Discussion When an aldehyde or a ketone is irradiated under

microwave with ethane-l,2-diol and iodine in THF, the corresponding acetals are formed in high yields (Table I). It is observed that carbonyl groups are most commonly protected as their cyclic acetals viz. 1,3-dioxolanes and 1,3-dioxanes. They are generally prepared by reaction of the carbonyl group with ethane-l ,2-diol or propane-l ,3-diol in the presence of an acid catalyst. IS.16 Cramarossa et al. 17 have reported the use of AIFe-Pillared montmorillonite catalyst for this purpose.

In addition to its use for the carbonyl group protection during a reaction sequence, acetals are also reported to have many practical applications l8.

The reaction was found to be selective for aldehydes and cyclic ketones. Both aldehydes and cyclic ketones reacted very fast and efficiently while the acyclic ones remained unchanged under the same reaction conditions. This gives an added selectivity to differentially protect the cyclic keto group leaving the

BORAH el al. : SELECTIV E PREPARATION OF ACETALS AND ESTERS FROM ALDEHYDES 1033

o ~O~I

N o ~O~OH

N

R

(1 )

a, R = H b, R = N0 2 c, R = CI d, R = OMe

o

R

(2)

~O~O~OH N

(4)

R

(3)

o ~o~O~O~OH

RN (5)

Table 1- Microwave irradiation of aldehydes/ketones with ethane- I ,2-diol

SI. No. Substrate Power (watt ) Ti me (min) Yield (%)

I Benzaldehyde 180 I 85

2 p-Ni trobenzaldehyde 90 2 83

3 p-Ch lorobenzaldehyde 180 90

4 An isaldehyde 180 I 87

5 Glutaraldehyde 360 2 55

6 Cyclohexanone 90 92

7 Methyl cyclohexanone 90 85

8 Cholestanone 90 98

9 Tetrahydrocarvone 90 I 98

10 2-B utanone 90 2 No reaction

11 Diacetone alcohol 180 2 Decomposed

12 Pregnenolone acetate 180 2 No reaction

360 3 No reaction

a All the products were characterized by spectroscopic (IR, NMR, MS) analyses and also by comparison with authentic samples.

acyclic one free for further manipulation. Further selectivity was observed between saturated and cx,~­unsaturated aldehyde and ketone also. Carvone under similar reaction conditions yielded carvacrol whereas 16-dehydropregnenolone acetate remained unchanged. In an isolated example cyclohexanone was mixed with 2-butanone (l: 1) and subjected to the same reaction conditions only to get carbonyl protection in case of cyclohexanone and substrate recovery in the case of 2-butanone. Even in the case of 25% water solution of glutaraldehyde both the carbonyl groups were protected giving a moderate yield of the product. When ethane-l,2-diol was replaced with other alcohols like methanol or ethanol , the aromatic aldehydes yielded the corresponding acetals in good yield.

When the same reaction was carried out with changing parameters, different results were observed. For example, irradiation of benzaldehyde, ethane-l ,2-diol and iodine (as catalyst) in THF under microwave for 1 min with power setting at 90 watt yielded only 60% of the benzylidene acetal 1a and the rest was unreacted starting material. Gradual increase of reaction time or microwave power level of the oven resulted in the steady increase of product up to 85% (isolated) at power 180 watt and reaction time 1 minute. Further increase of reaction time or power level started giving a side product 2a which became the main product with 80% isolated yield at power 360 watt and time 2 min. Similar was the case for other aromatic aldehydes also. All these observations are tabulated in Table II.

1034 INDI AN J. CHEM. , SEC B, MA Y 2002

Table II - Microwavc irrad iation of aldchydes (or ketones) with ethane- I ,2-d iol at varying parameters i. c. power and time period

SI. No. Substrate

Benzaldchyde

2 Benza ldchyde

3 Benzaldehyde

4 Benzaldehyde

5 Benzaldehyde

6 la

7 la (wi thout 12)

8 p-Nitrobenzaldehyde

9 p-Chlorobcnzaldehyde

10 Anisaldehyde

II Anisaldehyde

12 Glutaraldehyde acetal

13 Cyclohexanone acetal

14 Cholestanone acetal

From the Table II it is clear that, benzaldehyde first forms the benzylidene acetal 1a which then gets oxidized by iodine to the corresponding iodoester 2a through a mechanism si milar to the one proposed by Seely et al l9

. The proposition was further confirmed when benzylidene acetal 1a on irradiation under mkrowave in the presence of ethane-I ,2-diol and iodine yielded the iodoester 2a (entry 6, Table II). Microwave irradiation of 1a and ethane-I,2-diol without iodine did not give the oxidation product 2a and the starting material was recovered. The identity of the iodoester 2a was further confirmed by comparison with authentic sample prepared by reaction of benzoyl chloride with ethane-I,2-diol in pyridine followed by iodination of the product 3a with chlorotrimethylsilane and sod ium iodide.

The importance of the transformation of benzylidene acetal into hydroxy (or halo) ester may be judged from its wide variety of applications in synthetic organic chemistry as well as in carbohydrate chemistry. Beau et al. 20 utilized NBS oxidation method to convert a benzylidene acetal into bromobenzoate in the synthetic sequence for the total sy nthesis of pseudomonic acid . Hanessian et al.21 had also applied the same method in the synthetic study of boromycin. Its application in sugar chemistry was well documented by Collins et al.22 and Hanessian et al. 23 Few earlier reports had also described the oxidation of benzylidene acetal to give ester. Deslongchamps et al. 24 reported the oxidation of

Power Time Product(s) yield (watt) (min) (%)

90 I la (60)

90 2 la (80)

180 I la (85)

90 3 la (70), 2a (20)

360 2 la (20), 2a (80)

360 2 2a (75)

360 3 No reacti on

180 2 1 b (72), 2b (25)

180 3 1 c (70), 2c (25)

180 2 Id (20), 2d (70)

360 2 2d (20), 4d (49).

Sd (12)

360 3 No reaction

180 3 Oecomposi tion

360 3 No reaction

benzylidene acetal with ozone to get the corresponding hydroxy ester. In another report Oikawa et al.25 mentioned the use of DDQ for the same conversion, but the method had a limitation that it took place only in case of highly electron donating groups. The hydroxy esters prepared by this method might find use in macrolide synthesis.26 Seeley et al. 19

had used this orthoester oxidation process for stereospecific synthesis of cis- and trans- epoxide whereas Willi ams et al.27 utilized the method for stereocontrolled synthesis of substituted tetrahydrofurans. Both the groups used NBS as the oxidizing agent for the benzylidene acetal.

In the present experiment it was further observed that only aromatic aldehydes and their acetals were oxidized to iodoesters. Acetals of cyclic ketones either remained unchanged (entry 14,Table II) or got decomposed (entry 13,Table II) and the acetal of aliphatic aldehyde (entry 12, Table II) remained unchanged under similar reaction conditions. In case of aromatic aldehydes the substi tuents had some effect on the reaction speed and product yield,especially in the oxidation stage. In the acetalization stage the substituents seemed to have no appreciable effect; but in the oxidation stage of acetal to iodoester the electron releasing substituents such as OMe group in the aromatic ring enhanced the reaction speed as well as the yield. This behaviour was similar to that observed in the case of DDQ oxidation.25 The electron withdrawing groups such as N02 in the ring

BORAH et al.: SELECTIVE PREPARATION OF ACETALS AND ESTERS FROM ALDEHYDES 1035

seemed to resist the oxidation and as a result the yield of the iodoester was found less under similar or even harsher reaction conditions. In case of anisaldehyde and benzaldehyde further condensation of ethane -1 ,2-diol with the ester took place giving rise to the products 4 and 5, respectively.

This current method of preparation of ester from aldehyde via its acetal is a one pot clean reaction , which can be controlled at will (by changing the reaction parameters) to get either the acetal or the ester.

Experimental Section General. Melting points were determined on a

'Buchi' oil heating apparatus by capillary method and are uncorrected. NMR spectra were recorded on either Varian 360L (60 MHz) or Bruker Avance DPX-300 (300 KHz) using TMS as internal standard. IR spectra in CHCh on Perkin Elmer 237B grating IR spectrophotometer; and mass spectra on a Finigan­mat 'Incos-50' GC-MS instrument. CHN analyses were done on Perkin Elmer Series 11-2400 analyser. Domestic microwave oven model MS-283 MC (multiwave) havi ng microwave frequency 2450 MHz and manufactured by LG Electronics was used.

General procedure for preparation of acetal 1. To a solution of 2 mmoles of aldehyde (or ketone) in 4 mL of THF in a SO mL conical flask was added 12 mmoles of ethane-I ,2-diol fo llowed by 0 .2 mmole of iodine. The conical flask fitted with a small funnel, was placed on the turntable in a conventional microwave oven (kitchen type). The mixture was irradiated for a specified time at a particul ar power setting as mentioned in Table I for each substrate, respectively. On completion the mixture was diluted with ch loroform, washed with a dilute solution of sodium thiosulfate followed by water. The organic layer was dried over anhydrous sodium su lfate and evaporated under reduced pressure to get the desired product (Table I). Characterization of the products was made by spectroscopic means as well as by direct comparison with authentic samples prepared through known route IS. Some spectroscopic data for the hitherto unknown acetals are mentioned below.

Benzylidene acetal l a28: IH NMR (60 MHz,

CCI4) : 8 7.00 (s,5 H), 5A3 (s, IH), 3.65 (s, 4H); IR : 2950, 2850, 1600, 1510, 1100cm-l

.

4-Nitrobenzylidene acetal Ib: mp 80-82°C; I H NMR (60 MHz, CCI4): 8 7.90 (d, J=9Hz, 2H), 7.30 (d, J=9Hz, 2H), 5_55 (s, I H), 3.85 (s, 4H); IR: 2940,

2860, 1610, 1525, 1100 cm-I. Anal.Calcd for C9H9N04 : C,55 .38, H,4.65, N,7 .18. Found C,55.36; H,4A9; N,7.11 %.

4-Chlorobenzylidene acetal lc: IH NMR (60 MHz, CCI4) : 87.07 (s, 4H), 5A7 (s, IH), 3.74 (s, 4H); IR: 2950,2860, 1600, 1490, 1100 cm- I; MS (EI) mlz: 184 (M+, 2), 183 (3), ISS (15), 149 (35), 140 (35), 135 (42),121 (60), 119 (40), 110 (55), 105 (70),76 (100).

4-MethoxybenzyJidene acetal Id: IH NMR (60 MHz, CCI4): 8 TAO (d, J=8 .5 Hz, 2H), 6.60 (d, J=8.5 Hz, 2H), 3.60 (s, 4H), 3.55 (s, 3H); IR: 2950, 2875, 1680, 1600, 1580, ISIS , 1275, 1180 em-I; MS (EI) mlz: 180 (M+, 2), 179 (3), 165 (SO) , 151 (35), 136 (40), 135 (25), 121 (55), 107 (SO), 92 (100), 77 (75), 65 (55) .

Glutaraldehyde acetal: IH NMR (60 MHz, CCI4) :

84.60 (br s, 2H), 3.70 (br s, 8H), 1.50 (br s, 6H); IR: 2915,2850,1420,1160,1050 cm-I; MS (EI) mlz: 188 (M+, 2), 159 (5) , 130 (5), 115 (45) , 87 (l00).

CycIohexanone acetaf8: IH NMR (60 MHz, CCI4): 8 3.66 (s, 4H), lAO (s, lOH); IR: 2890, 2840, 1620, 1570, 1450, 1380,1290,1050 cm- I.

MethylcycIohexanone acetal: IH NMR (60 MHz, CCI4) : 8 3.73 (s, 4H), 1.80 to 1.10 (m, 9H), 0.86 (d, J=6 Hz, 3H); IR: 2900, 2840, 1620, 1575, 1450, 1375 , 1290, 1075, 1050 cm-I; MS (EI) mlz: 156 (M+, 1), 141 (6), 128 (15), 114 (20),83 (100),58 (60).

Tetrahydrocarvone acetal: IH NMR (60 MHz, CCI4) : 8 3.76 (s, 4H), 2.30 to 1.00 (m, 9H), 0.83 (overlapping signals 9H); IR: 2930, 2890, 2840, 1640, 1570, 1490, 1450, 1390, 1250, 1160 cm-I; MS (EI) mlz: 198 (M+, 3), 183 (IS), ISS (55), 143 (65), 140 (40), 136 (100), 125 (30), 97 (SO) .

Cholestanone acetal: mp 101°C; IH NMR (60 MHz, CCI4): 83.76 (s, 4H), 2.10 to OAO (m, 46H); IR: 2880, 2840, 1460, 1375, 1115, 1040 cm- I; MS (EI) mlz: 430 (M+, 4), 415 (10),402 (10), 400 (IS), 357 (25), 345 (20),267 (100), 143 (60),98 (80) .

Irradiation of benzaldehyde with ethane-l,2-diol. To a solution of 1 mmole (106 mg) of benzaldehyde in 2 mL of THF and 6 mmoles (372 mg) of ethane-I ,2-diol was added 0.1 mmole (25 mg) of iod ine. The mixture was irradiated in a microwave oven in the same manner as stated above for a specified time and power as indicated in the Tables I and II. Usual work up followed by purification on preparative TLC (l :20-EtOAc : Hexane) yielded a mixture of products depending upon the reaction conditions. When the microwave power was set at

1036 INDIAN 1. CHEM., SEC B, MAY 2002

180 watt and the reaction time was 1 min., one major product la was isolated in 85% (127 mg) yield after purification by preparative TLC. When the microwave power was set at 360 watt and time was 2 min, the major product isolated in 80% (221 mg) yield was 2a . IH NMR (300 MHz, COCI)): 8 7.80 (dd, J=7 & 2.5 Hz, 2H); 7.20 (m, 3H); 4.40 (dd, J=6 & 7.5 Hz, 2H); 3.25 (dd, J=6 & 7.5 Hz, 2H); IR: 2900,1710,1450,1275,1125 cm· l

; MS (El) mlz: 277 (M++1, 100),276 (M+, 25), 154 (20), 150 (65), 149 (100), 105 (72); 13C NMR (75 MHz, COCI3): 8 166.75 , 133.53,130. 11,128.83, 128.18,67.12 , 1.10.

Preparation of l-iodoethyl benzoate 2a from benzoyl chloride. To a solution of 278 mg (2 mmoles) of benzoyl chloride in 186 mg (3 mmol es) of ethane-I,2-diol was added 0.1 8 mL (2.2 mmoles) of pyridine at r.t. under stirring. After 3 hr of stirring the reaction mixture was diluted with chloroform (50 mL) and washed with 3x50 mL of water. The organic layer was dried over anhydrous sodium sulfate and evaporated under reduced pressure to get 295 mg of I -hydroxyethyl benzoate 3a in almost pure form. IH NMR (300 MHz, COCI3) of 3a: 8 8.05 (m, 2H, AA'XX' pattern), 7.42 to 7.58 (m, 3H), 4.46 (splitted t, 2H), 3.95 (splitted t, 2H); IR: 3400, 2940, 1710, 1600, 1460, 1290, 1140 cm· l

; 13C NMR (75 MHz, COCl3): 8 166.75 , 133.52, 130.06, 128.78, 128.15, 67.03, 61.80. 3a acetate showed IH NMR (60 MHz, CCI4) peaks at 8 8.05 (splitted d, J= 8 Hz, 2H), 7.57 (spl itted t, 1=8 Hz, IH), 7.42 (splitted t, 1=8 Hz, 2H), 4.41 (m, 2H), 4.32 (m, 2H), 2.09 (s, 3H); IR: 1740, 1715, 1600, 1455, 1390, 1290, 1250, 1130 cm· l

; MS (EI) m/z: 208 (M+, 5), 171 (18), 149 (60), 122 (10), 105 (100), 87 (95), 77 (90). The hydroxyethyl benzoate 3a in aceton itrile was then treated with trimethylchlorosilane (0.25 mL) and sodium iodide (300 mg) at r.t. for overnight. Usual work up and purification by preparative TLC (I :20-EtOAc: Hexane) yie lded 466 mg of the des ired ester 2a.

Irradiation of la with iodine. To a solution of 0.67 mmole (100 mg) of la in 1.5 mL of THF was added 4 mmoles of ethane- I ,2-diol and 0.07 mmole of iodine. The mix ture was irradiated in a microwave oven for 2 min with power setting at 360 watt. TLC observation showed more than 75% conversion of the starting compound to iodoester 2a . The mi xture was diluted with ch loroform and washed with a dilute solution of sodium thiosul fate followed by water. The organic layer was dried over anhydrous sodi um su lfate and evaporated under reduced pressure to give

a residue which on purification by preparative TLC (I :30-EtOAc:Hexane) yielded the iodoester 2a in 75% yield (138 mg).

Irradiation of la without iodine. A blank reaction of 0.67 mmole of la and 4 mmoles of ethane-l ,2-diol in 1.5 mL of THF was irradiated in a microwave oven for 3 min with power setting at 360 watt. This reaction did not yield any product and the substrate was recovered by work up in the usual way.

Irradiation of 2-butanone with ethane-l,2-diol. To a solution of 2 mmoles of 2-butanone in 4 mmoles of ethane-I ,2-diol was added 0.2 mmole of iodine and the mixture was irradiated in a microwave oven for 2 min with power setting at 90 watt. Monitoring on TLC did not show any indication of product formation and the reactant was recovered.

Irradiation of 4-nitrobenzaJdehyde with ethane-1,2-diol. A so lution of 2 mmoles (302 mg) of 4-nitrobenzaldehyde in 4 mL of THF was irradi ated in a microwave oven for 2 minutes at power 180 watt as per the general procedure mentioned above to get 72% of aceta l Ib and 25 % of iodoester 2b after purification by preparative TLC (I :5-EtOAc:Hexane), mp of 2b, 70 - 71 DC; I H NMR (300 MHz, COCb): 8 8.30 (m, AA'XX' pattern , 4H), 4 .63 (t, 1=6.5 Hz, 2H), 3.46 (t , 1=6.5 Hz, 2H); IR: 29 15, 1720, 1610, 15 30,1 355,1290,1120 cm" ; MS (EI) mlz: 321 (M+, 2), 276 (2), 194 (100) , 154(60), 150 (65), 148 (22), 120 (30), 104 (50), 92 (35), 83 (55), 76 (30); I3C NMR (75 MHz, COCl3): 8166.23 , 135.1Q 131.09, 123 .83, 65.83, 0.02. Anal. Calcd for C9HgN041: C, 33.67; H, 2.SI; N, 4.36. Found: C, 33.83; H, 2.49 ; N, 4 .2 1%.

Irradiation of 4-chlorobenzaldehyde with ethane-l,2-diol. A solution of 28 1 mg (2 mmoles) of 4-chlorobenzaldehyde in 4 mL of THF was irradiated fo r 3 minutes in a microwave oven at power 180 watt as per the general procedure menti oned above to get 70% of acetal Ie and 25% of iodoester 2e after purification by preparative TLC (I :S -EtOAc:Hexane). IH NMR (300 MHz, COCI3) of 2e: 8 7.99 (d, 1=8.5 Hz, 2H), 7.42 (d, 1=8.5 Hz, 2H), 4 .57 (t, 1=6.7 Hz, 2H); 3.42 (t, J=6.7 Hz, 2H) ;IR: 2925, 1715 , 1590, 1490, 14 15, 1280, 1115 cm· l

; MS (EI) mlz: 312 (M++2, 1), 310 (M+,3), 183 (50), 156(40), 155 (25), 139 (100), 127(15), 111 (50), 75(30); 13C NMR (75MHz,COCb): 8 165 .22, 139.94, 13 1.33, 129.02, 128.27, 126.06,65.27,0.59.

Irradiation of 4-methoxybenzaldehyde with ethane-l,2-diol. A so luti on of 272 mg (2 mmoles) of 4- methoxybenzaldehyde in 4 mL of THF was

80RAH et al. : SELECTIVE PREPARATION OF ACETALS AND ESTERS FROM ALDEHYDES 1037

irradiated for 2 minutes in a microwave oven at power 180 watt as per the general procedure mentioned above. Usual work up and purification by preparative TLC (1: 15- EtOAc:Hexanes) yielded 20% of anisaldehyde acetal Id and 70% of iodoester 2d. Even two other products 4d and sd were isolated in 49% (119 mg) and 12% (31 mg) yields, respectively on irradiation of the mixture for 2 minutes at power setting 360 watt. 2d: IH NMR (300 MHz, COCl3): 8 8.01 (d, 1=8.8 Hz, 2H), 6.92 (d, 1=8.8 Hz, 2H), 4 .53 (t, splitted, 1=6.7 Hz, 2H); 3.85 (s, 3H), 3.41 (t, spl itted, 1=6.7 Hz, 2H); IR : 2900, 1710,1610, 1515 , 1460, 1385,1275,1190 cm· l

; MS (EI) mlz: 306 (M+, 11 ), 179 (19), ISS (11), 152 (100), 135 (95), 127 (8), 107 (11) , 92 (31), 77 (26), 64 (13); 13C NMR (75 MHz,COCl3): 8 166.01, 164.03, 132.20, 122.46, 114.13 , 65.04, 55 .86, 1.20. 4d: IH NMR (300 MHz, COCl3): 88.01 (d, 1=8.8 Hz, 2H), 6.92 (d, 1=8.8 Hz, 2H), 4.45 (m, AA'XX' pattern, 2H), 3.62 to 4.07 (overlapping m, 9H), 2.69 (br s, IH); IR: 3400,2940,2850,1710,1610, 1520, 1470, 1275, 1195 cm· l

; I3C NMR (75 MHz, COCl3): 8 166.75 , 163.83, 132 .09, 122.72, 114.02, 72.89, 69.67,64.16,62.05,55.81. 4d acetate: IH NMR (60 MHz, CCI4): 87.90 (splitted d, 1=8.5 Hz, 2H), 6.80 (splitted d, 1=8.5 Hz, 2H), 4.40 (m, 2H), 4.20 (m, 2H), 3.96 (s, 3H), 3.75 (m, 4H), 2.10 (s, 3H); IR: 2940,2870,1730,1700,1600,1510,1455,1375, 1270, 1190 em-I; MS (EI) mlz: 282 (M+, 5), 238 (20), 211 (5), 196 (3), 179 (5), 165 (5), 152 (35), 135 (100), 107 (18), 92 (44), 87 (SO), 86 (25), 77 (35). sd: IH NMR (300 MHz, COCI3:8 8.01 (sp litted d, 1=8.8 Hz, 2H), 6.90 (splitted d, 1=8.8 Hz, 2H), 4.46 (m, 2H), 3.27 to 3.86 (overlapping m, 13H), 2.40 (br s, IH); IR: 3450, 2875, 1705, 1605, 1515 , 1275, 1185, 11 20 cm· l

; I3C NMR (75 MHz, COCl3): 8 166.74, 163.84, 132 .1 3, 122.80, 114.02, 72.95, 71.09, 70.76, 69.75, 64.10, 62.16, 55 .83 . MS (as acetate, EI) mlz : 326 (M+, 5), 282 (5), 267 ( 10), 255 (5), 238 (l0), 210 (5), 196 (5), 152 (25), 135 (100), ] 07 (30), 92 (25), 87 (35), 86 (12), 77 (20).

General method of acetylation of hydroxyesters 3, 4 and 5. A solution of the hydroxyester in 0.5 mL of pyridine and I mL of acetic anhydride was left at r.t. for overnight. It was then diluted with 100 mL of water and extracted with 3x30 mL of chloroform. The organic layer was washed with water and dried over anhydrous sodium sulfate and evaporated at reduced pressure to get the desired acetate in almost pure form.

Acknowledgement Authors thank Dr J S Sandhu, Director for

providing the facilities and the DST, New Delhi for financial assistance. Thanks are also due to Dr M J Bordoloi for recording 300 MHz NMR spectra and Dr N C Barua for constant encouragement and helpful discussions.

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