One-pot synthesis of enantiomerically pure 1, 2-diols: asymmetric reduction of aromatic α-oxoaldehydes catalysed by Candida parapsilosis ATCC 7330

Tetrahedron: Asymmetry 22 (2011) 2156–2160

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One-pot synthesis of enantiomerically pure 1, 2-diols: asymmetric reduction of aromatic a-oxoaldehydes catalysed by Candida parapsilosis ATCC 7330 Pula Mahajabeen a, Anju Chadha a,b,⇑ a b

Laboratory of Bioorganic Chemistry, Department of Biotechnology, Indian Institute of Technology Madras, Chennai 600 036, India National Center for Catalysis Research, Indian Institute of Technology Madras, Chennai 600 036, India

a r t i c l e

i n f o

Article history: Received 15 November 2011 Accepted 15 December 2011 Available online 16 January 2012

a b s t r a c t A facile and simple one-pot method was developed to produce a series of optically active (S)-1-phenyl1,2-ethanediols with good yields (up to 70%) and high enantiomeric excess (>99%) via asymmetric reduction of various substituted aromatic a-oxoaldehydes using Candida parapsilosis ATCC 7330. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Enantiomerically pure forms of vicinal diols are valuable chiral building blocks for the synthesis of pharmaceuticals and agrochemicals.1 In particular, enantiomerically pure substituted 1-phenyl1,2-ethanediols are the precursors of Fluoxetine, an anti-depressant, together with Tembamide and Aegeline, which show hypoglycemic activity.2 These diols can also be readily transformed into chiral epoxides,3 aziridines and amino alcohols.4 Enantiomerically pure (S)-1-phenyl-1,2-ethanediol has been used in stereoselective polymerizations to produce chiral biphosphones and also as a chiral initiator.5 Numerous chemical methods have been reported for the synthesis of enantiomerically pure 1,2-diols.6 The reduction of aoxoaldehydes is one of the methods used to prepare these diols and the only chemical method, which uses a-and b-oxoaldehydes as starting materials for the synthesis of 1,2-diols using TiCl3 (4 equiv)/NH3 as the catalyst for the reduction to give racemic phenyl-1,2-ethanediol in an 80% yield.7 Enantiomerically pure 1,2-diols can also be prepared by several biocatalytic methods. The biocatalytic hydrolysis of epoxides using enantioselective and enantioconvergent epoxide hydrolases is one of the most commonly used methods for the synthesis of enantiopure diols.8 The enantioselective hydrolysis of racemic epoxides using a variety of epoxide hydrolases from different microbes has been reported with a maximum yield of 50% and moderate to good enantiomeric excess (ee).9 A major drawback of these enantioselective epoxide hydrolases is the low theoretical yield of 50%. The enantioconvergent hydrolysis of racemic epoxides using epoxide hydrolases from Solanum tuberosum gave the corresponding (R)-diol with 97% ee and an 88% yield,10 whereas a combination of bacterial and marine fish epoxide hydrolases gave the enantiopure (R)-phe⇑ Corresponding author. Tel.: +91 44 2257 4106; fax: +91 44 2257 4102. E-mail address: [email protected] (A. Chadha). 0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetasy.2011.12.008

nyl-1,2-ethane diol with 90% ee and an 94% yield.11 Other biocatalytic methods include microbial stereoinversion,12 the kinetic resolution of vicinal diols by using lipases13 [which requires an additional hydrolysis step to obtain the enantiopure (S)-diol], asymmetric dihydroxylation of styrene using dioxygenases14 and microbial reduction of a-hydroxy and a-acetoxy ketones to give enantiomerically pure 1,2-diols.24d The asymmetric reduction of a-hydroxy ketones was reported by Tsujigami et al. using the microorganism Yamadazyma farinosa IFO 10896 in an anti-Prelog manner to give (S)-1-phenyl-1,2-ethane diol in high yield and with >99% ee, but the reaction time of the asymmetric reduction was quite long (24 h).15 Recently, Xu et al. reported an anti-Prelog reduction of 2hydroxyacetophenone to the (S)-1-phenyl-1,2-ethanediol using an alcohol dehydrogenase from Candida parapsilosis CCTCC M203011 with an enantiomeric purity of >99%.16 They also explored the coenzyme specificity and enantioselectivity of the enzyme using site directed mutagenesis to produce (R)-1-phenyl-1,2-ethanediol with an enantiomeric purity of 51.8% and yield of 37.9%.17 Biocatalytic methods, which use a-oxoaldehydes as starting materials for the asymmetric reduction, report the use of a methylglyoxal reductase from Saccharomyces cerevisiae, which catalyses the selective reduction of methylglyoxal to lactaldehyde although the stereochemistry of the product was not mentioned.18 More recently, two different enzymes, the aldoketo reductase from Escherichia coli and the alcohol dehydrogenase from Lactobacillus brevis, were used for the asymmetric reduction of phenylglyoxal to (S)-1phenyl-1,2-ethanediol19 but the total reaction time for the two-step process was 22 h, in addition to the fact that expensive cofactors were used. Hence, a one pot reduction of a-oxoaldehydes to prepare a series of enantiomerically pure 1,2-diols without the need for the addition of cofactors is highly desirable. Whole cells of Candida parapsilosis ATCC 7330 are effective for the deracemization of aand b-hydroxy esters,20a–c propargylic esters,20d allylic alcohols20e and the resolution of amino acids.20f This biocatalyst is also very

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useful for the asymmetric reduction of alkyl-2-oxo-4-arylbutanoates, alkyl-2-oxo-4-arylbut-3-enoates21a and aryl imines21b to yield an array of chiral synthons. Extending the scope of the substrate for the biocatalyst C. parapsilosis ATCC 7330, allowed the synthesis of a series of enantiomerically pure (S)-1-phenyl-1,2-ethanediols in good yields and with high enantiomeric excess using various substituted aromatic a-oxoaldehydes as starting materials is reported herein. These aromatic a-oxoaldehydes contain both aldehyde and keto functionalities, and are therefore interesting molecules to study the selectivity and specificity of the biocatalyst Candida parapsilosis ATCC 7330 towards asymmetric reductions.

O

OH Candida parapsilosis ATCC 7330

H

OH (S)

25 °C, 3-7 h

O R

R 1a-1i

2a-2i Yield: 43-70% ee: 92->99%

R = H, p-CH3, p-OCH3, p-NO2, p-F, p-Cl, p-Br, m-NO2, m-OCH3, *o-Cl, o-CH3 *(R)-enantiomer was obtained as the major enantiomer Scheme 1. Asymmetric reduction of aromatic a-oxoaldehydes 1a–1i using the whole cells of Candida parapsilosis ATCC 7330.

2. Results and discussion A series of substituted aromatic a-oxoaldehydes 1a–1k were synthesised according to the reported methods,22 and 2-oxo-2phenylacetaldehyde (phenylglyoxal) 1a was used as a standard substrate for the optimisation of the biocatalytic asymmetric reduction reaction. Phenylglyoxal, when incubated with the whole cells of C. parapsilosis ATCC 7330 resulted in complete conversion to enantiomerically pure (S)-1-phenyl-1,2-ethanediol with a yield of 70% and ee of 99% in 3 h at 25 °C (Scheme 1). The time course of the reaction was monitored by HPLC using a reverse phase column and revealed an intermediate at 10 min which was identified as 2-hydroxy-1-phenylethanone.23 This indicates that the biocatalyst C. parapsilosis ATCC 7330 first selectively reduces the aldehyde group of phenylglyoxal followed by the reduction of the keto group. The bioreduction of phenylglyoxal to enantiomerically pure (S)-1-phenyl-1,2-ethanediol using whole cells of C. parapsilosis ATCC 7330 involves a one-pot, two-step process via the formation of 2-hydroxy-1-phenylethanone (Scheme 2) as an intermediate. It is noteworthy that in the present study, the reduction of phenylglyoxal was completed in 3 h as compared to the multienzymatic preparation of (S)-1-phenyl-1,2-ethanediol reported by Gennaro

O

OH Candida parapsilosis ATCC 7330

H

OH (S)

25 °C, 3 h

O

10

m

in

O OH

2-hydroxy-1-phenylethanone Scheme 2. Intermediate in the asymmetric reduction of aromatic a-oxoaldehydes.

et al., which requires 22 h for completion of the reaction.19 As can be seen in Table 1, various a-oxoaldehydes were reduced by whole cells of C. parapsilosis ATCC 7330 to give enantiomerically

Table 1 Asymmetric reduction of various substituted phenylglyoxals using whole cells of Candida parapsilosis ATCC 7330 Entry

Product

eea (%)

Yieldb (%)

Reaction time (h)

½a30 D Values

Abs configc

99

70

3

+66.9 (c 1.0, CHCl3)

(S)

>99

68

3

+68.5 (c 1.12, CHCl3)

(S)

>99

56

3

+60.3 (c 0.50, CHCl3)

(S)

90

63

5

+46.0 (c 1.0, CHCl3)

(S)

92

43

3

+16.5 (c 1.0, MeOH)

(S)

OH OH

2a

OH OH 2b

H3C OH OH 2c

H3CO OH OH 2d

OCH3 OH OH 2e

O2N (continued on next page)

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Table 1 (continued) Entry

Product

eea (%)

Yieldb (%)

Reaction time (h)

½a30 D Values

Abs configc

97

43

3

+24.0 (c 1.5, MeOH)

(S)

97

47

7

+62.8 (c 1.0, CHCl3)

(S)

99

47

3

+52.0 (c 1.60, CHCl3)

(S)

62

39

24

43.92 (c 1.90, EtOH)

(R)

>99

54

3

+43.4 (c 1.0, CHCl3)

(S)

OH OH 2f

NO2 OH OH

2g

F OH OH

2h

Cl OH OH

2i

Cl OH OH

2j

Br a b c

Determined by chiral HPLC. Isolated yield. Absolute configuration was determined by comparison of the specific rotation with the literature.24

pure (S)-1-phenyl-1,2-ethanediol in good yields (up to 70%) and high enantiomeric excess (>99%). Substrates with different parasubstituents (Table 1, 2b, 2c, 2e, 2g, 2h and 2j) on the aromatic ring seem to affect the yields of C. parapsilosis ATCC 7330 mediated asymmetric reduction when compared to the unsubstituted phenylglyoxal (Table 1, 2a). The electron donating groups like p-CH3 (Table 1, 2b), p-OCH3 (Table 1, 2c), p-Cl (Table 1, 2h) and p-Br (Table 1, 2j) lower the yield (47–68%) without changing the ee (99%) or the reaction time (3 h). The p-F (Table 1, 2g) substituted a-oxoaldehyde gave a lower yield of 47% and ee of 97% even after increasing the reaction time to 7 h, Finally, the electron-withdrawing groups such as p-NO2 (Table 1, 2e) decreases both yield (43%) and ee (92%) but maintains the reaction time (3 h). Substrates with different meta-substituents such as m-OCH3 and m-NO2 (Table 1, 2d and 2f) show low yields (43–63%) and ee (90–97%). The reaction time was maintained (3 h) in the case of 2f whereas it increased (7 h) with 2d. The absolute configurations for all the aforementioned products were assigned to be (S) by comparing their specific rotation values with those reported in the literature.24 An ortho-chloro substituent on the aromatic ring of an a-oxoaldehyde, when incubated with C. parapsilosis ATCC 7330, resulted in (R)-1-(2-chlorophenyl)ethane-1,2-diol (Table 1, 2i) with a chemical yield of 39% and ee of 62% in 24 h. In addition to the diol, the aldehyde reduced intermediate 1-(2-chlorophenyl)-2-hydroxyethanone was also isolated in a 55% yield. Conversely meta- and parachloro substituted a-oxoaldehydes gave the corresponding (S)-diol as the product. A methyl substituent at the ortho-position of the aromatic ring of an a-oxoaldehyde gave 1-(2-methylphenyl)-2hydroxyethanone with an isolated yield of 85%, which is the aldehyde reduced product; even after increasing the reaction time to 72 h, the corresponding diol was not obtained. Therefore, these results suggest that the position and the electronic nature (electron withdrawing or electron donating) of the substituent are the key parameters that influence the yield, enantioselectivity and config-

uration of the product obtained in the asymmetric reduction of aoxoaldehydes using whole cells of C. parapsilosis ATCC 7330. 3. Conclusion We have reported a one-pot, two-step synthesis of various optically active 1-phenyl-1,2-ethanediols via the asymmetric reduction of various substituted aromatic a-oxoaldehydes using the whole cells of Candida parapsilosis ATCC 7330 in moderate to good yields (up to 70%) and with high enantiomeric excess (>99%) under mild reaction conditions (25 °C and pH 6.8) without the need for additional cofactors. The position of the substituent on the aromatic ring of the a-oxoaldehydes has a large influence on the ee and the configuration of the product. A substituent at an m- and p-positions of the a-oxoaldehydes gave the corresponding enantiomerically pure (S)-1-phenyl-1,2-ethanediols, whereas chloro substitution at the o-position leads to the (R)-1-(2-chlorophenyl)ethane-1,2-diol. 4. Experimental 4.1. General methods Candida parapsilosis ATCC 7330 was purchased from ATCC, Manassas, VA 20108, USA. All chemicals used for the media preparation were purchased locally. All substrates were synthesised using the reported method.22 1H and 13C NMR spectra were recorded in CDCl3 solution on a Bruker AVANCE III 500 MHz spectrometer operating at 500 MHz and 125 MHz. Chemical shifts are expressed in ppm values using TMS as an internal standard. Infrared spectra were recorded on a Shimadzu IR 470 Instrument. Mass spectra were recorded on a Q TOF micro mass spectrometer. The enantiomeric excess (ee) was determined by HPLC using a chiral column on a Jasco PU-1580 liquid chromatograph equipped with PDA detector. The chiral columns used were Chiralcel OD-H,

P. Mahajabeen, A. Chadha / Tetrahedron: Asymmetry 22 (2011) 2156–2160

Chiralcel OB-H, Chiralpak AD-H (Daicel, 4.6  250 mm) and Lux 5l Cellulose-2 (Phenomenex, 250  4.6 mm) chiral columns. The solvents used were a hexane/isopropanol mixture at a flow rate of 1 mL min1 and the absorbance was monitored using a PDA detector at 254 nm. Optical rotations were determined on Rudolph, Autopol IV digital polarimeter. TLC was carried out using Kieselgel 60 F254 aluminium sheets (Merck 1.05554). 4.2. Synthesis of substrates 4.2.1. Synthesis of various substituted phenylglyoxals22 In a 100 mL round bottomed flask were added 1.11 g (0.1 mol) of SeO2, 12 mL of 1,4-dioxane and 4 mL of water, fitted with a reflux condenser. The mixture was heated to 50–55 °C and stirred until the solid dissolved. This was followed by the addition of substituted acetophenones (0.1 mol equiv), and refluxing of the mixture with stirring was continued. The colour of the solution turned orange, which changed to red and then to deep red within half an hour. After about 2 h, the solution becomes clear and a small amount of precipitation of selenium was observed. The solution was refluxed for a further 8 h and the completion of the reaction was monitored by TLC. The hot solution was decanted from the precipitated selenium and filtered through Celite. The solvent from the filtrate was removed under vacuum and the residual substituted phenylglyoxal was diluted with ethyl acetate, washed with water, and then saturated NaHCO3, dried over Na2SO4 and purified by silica gel column chromatography (hexane–acetone/95:05) to give the pure phenylglyoxal. The yields of the pure substituted a-oxoaldehydes (a yellow liquid) ranged from 71% to 79%. 4.2.2. Synthesis of racemic 1-phenyl-1,2-ethanediol All racemic 1-phenyl-1,2-ethanediols were prepared in quantitative yields by the reduction of the corresponding substituted phenylglyoxal (1 mmol) with NaBH4 (2 mmol) in ethanol (7 mL) at 0 °C to room temperature for 1 h. After completion of the reaction the excess ethanol was removed. The reaction mixture was hydrolysed with dilute HCl and the mixture was extracted with ethyl acetate. The organic layer was dried, concentrated and purified by column chromatography. 4.3. Growth conditions of Candida parapsilosis ATCC 733025 Candida parapsilosis ATCC 7330 was grown in a yeast malt broth medium (50 mL) in 250 mL Erlenmeyer flasks incubated at 25 °C, 200 rpm. The cells were harvested by centrifuging the 14th h culture broth at 10,000 rpm for 10 min at 4 °C and subsequently washed with distilled water. The process was repeated twice, and the wet cells were used for the biotransformation. In total, 1 g wet cells were obtained per 50 mL medium in a 250 mL Erlenmeyer flask. 4.4. Asymmetric reduction of phenylglyoxal using the whole cells of Candida parapsilosis ATCC 7330 To a 150 mL conical flask containing 10 g of wet cells of Candida parapsilosis ATCC 7330 suspended in 30 mL of water, 60 mg (0.4 mmol) of phenylglyoxal were dissolved in 1.5 mL of ethanol as the co-solvent and incubated at 25 °C, 200 rpm for 3 h in a water bath shaker. After completion of the reaction, the reaction mixture was centrifuged at 10,000 rpm for 10 min. The product was isolated with ethyl acetate and the organic layer was dried over anhydrous sodium sulfate. The solvent was removed by rotary evaporator and the enantiomerically pure (S)-1-phenyl-1,2-ethanediol 2a was obtained as a colourless solid after purification with silica gel column chromatography using chloroform/methanol (99:01) as the mobile phase. The ee was found to be 99%, as determined using HPLC. The yield of the isolated product was 70%. Spectroscopic data

2159

were identical to those reported in the literature.24f The rest of the aoxoaldehydes 1b–1k were also used as substrates in the same manner (Scheme 1, Table 1) for the asymmetric reduction. Control experiments were performed in parallel without the whole cells under identical conditions. 4.4.1. (S)-1-Phenyl-1,2-ethanediol 2a Colourless solid, mp 64–66 °C; ½a30 D ¼ þ66:9 (c 1.0, CHCl3) 99% ee, {lit.24a ½a20 D ¼ þ54:9 (c 1.0, CHCl3) 97% ee}. Spectroscopic data were consistent with that reported previously.24f The enantiomeric excess was determined by HPLC analysis using Chiracel OB-H column (eluent, hexane/2-propanol = 90:10, flow rate: 0.5 mL/min) tR = 15.10 min [minor, (R)-isomer]; tR = 19.22 min [major, (S)isomer]. 4.4.2. (S)-1-(4-Methylphenyl)-1,2-ethanediol-2b Colourless solid, mp 66–67 °C; ½a30 D ¼ þ68:5 (c 1.12, CHCl3) >99% ee, {lit.24b ½a22 D ¼ þ69:2 (c 1.10, CHCl3) 99% ee}. Spectroscopic data were consistent with that reported previously.24c The enantiomeric excess was determined by HPLC analysis using Chiracel OBH column (eluent, hexane/2-propanol = 90:10, flow rate: 0.5 mL/ min) tR = 15.80 min [minor, (R)-isomer]; tR = 18.26 min [major, (S)-isomer]. 4.4.3. (S)-1-(4-Methoxyphenyl)-1,2-ethanediol-2c Colourless solid, mp 75–77 °C; ½a30 D ¼ þ60:3 (c 0.50, CHCl3) >99% ee, {lit.24c ½a22 ¼ þ51:6 (c 0.49, CHCl 3) 66% ee}. Spectroscopic D data were consistent with that reported previously.24c The enantiomeric excess was determined by HPLC analysis using Chiracel ODH column (eluent, hexane/2-propanol = 90:10, flow rate: 0.5 mL/ min) tR = 26.05 min [minor, (R)-isomer]; tR = 31.53 min [major, (S)-isomer]. 4.4.4. (S)-1-(3-Methoxyphenyl)-1,2-ethanediol-2d 24e Yellow oil; ½a30 D ¼ þ46:0 (c 1.0, CHCl3) 90% ee, {lit. ½a22 ¼ þ71:8 (c 0.5, CHCl ) >99.5% ee}. Spectroscopic data were 3 D consistent with that reported previously.24e The enantiomeric excess was determined by HPLC analysis using Chiralpak AD-H column (eluent, hexane/2-propanol = 95:05, flow rate: 1 mL/min) tR = 25.25 min [major, (S)-isomer]; tR = 28.58 min [minor, (R)isomer]. 4.4.5. (S)-1-(4-Nitrophenyl)-1,2-ethanediol-2e Colourless solid, mp 87–88 °C; ½a30 D ¼ þ16:5 (c 1.0, MeOH) 92% ee, {lit.24d ½a21 ¼ þ23 (c 1.0, MeOH) 100% ee}. Spectroscopic data D were consistent with that reported previously.24d The enantiomeric excess was determined by HPLC analysis using Chiracel OD-H column (eluent, hexane/2-propanol = 98:02, flow rate: 0.8 mL/min) tR = 38.90 min [minor, (R)-isomer]; tR = 43.62 min [major, (S)-isomer]. 4.4.6. (S)-1-(3-Nitrophenyl)-1,2-ethanediol-2f Colourless solid, mp 72–74 °C; ½a30 D ¼ þ24:0 (c 1.5, MeOH) 97% ee, {lit.24d ½a22 D ¼ þ26 (c 1.5, MeOH) 98.5% ee}. Spectroscopic data were consistent with that reported previously.24d The enantiomeric excess was determined by HPLC analysis using Chiracel OD-H column (eluent, hexane/2-propanol = 95:05, flow rate: 0.5 mL/min) tR = 62.60 min [minor, (R)-isomer]; tR = 69.92 min [major, (S)isomer]. 4.4.7. (S)-1-(4-Fluorophenyl)-1,2-ethanediol-2g Colourless solid, mp 75–76 °C; ½a30 D ¼ þ62:8 (c 1.0, CHCl3) 97% ee, {lit.9a ½a20 ¼ 49:2 (c 1.01, CHCl ) 78% ee for (R)}. Spectroscopic data 3 D were consistent with that reported previously.24g The enantiomeric excess was determined by HPLC analysis using Phenomenox Lux-2 chiral column (eluent, hexane/2-propanol = 95:05, flow rate:

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0.5 mL/min) tR = 21.18 min [minor, (R)-isomer]; tR = 23.71 min [major, (S)-isomer]. 4.4.8. (S)-1-(4-Chlorophenyl)-1,2-ethanediol-2h Colourless solid, mp 79–80 °C; ½a30 D ¼ þ52:0 (c 1.60, CHCl3) 99% ee, {lit.24c ½a23 ¼ þ37:0 (c 0.82, CHCl 3) 60% ee}. Spectroscopic data D were consistent with that reported previously.24c The enantiomeric excess was determined by HPLC analysis using Chiracel OD-H column (eluent, hexane/2-propanol = 98:02, flow rate: 1 mL/min) tR = 48.20 min [minor, (R)-isomer]; tR = 54.44 min [major, (S)isomer]. 4.4.9. (R)-1-(2-Chlorophenyl)-1,2-ethanediol-2i Colourless solid, mp 98–99 °C; ½a30 D ¼ 43:9 (c 1.90, EtOH) 62% ee, {lit.24d ½a23 D ¼ 50 (c 2.0, EtOH) 60% ee}. Spectroscopic data were consistent with that reported previously.9b The enantiomeric excess was determined by HPLC analysis using Phenomenox Lux-2 chiral column (eluent, hexane/2-propanol = 95:05, flow rate: 1 mL/ min) tR = 18.97 min [major, (R)-isomer]; tR = 24.05 min [minor, (S)isomer]. 4.4.10. (S)-1-(4-Bromophenyl)-1,2-ethanediol-2j Colourless solid, mp 101–102 °C; ½a30 D ¼ þ43:4 (c 1.0, CHCl3), >99% ee, {lit.9a ½a20 ¼ 37:2 (c 1.03, CHCl 3) 72% ee for (R)}. SpectroD scopic data were consistent with that reported previously.24b The enantiomeric excess was determined by HPLC analysis using Chiracel OD-H column (eluent, hexane/2-propanol = 98:02, flow rate: 0.8 mL/min) tR = 68.40 min [minor, (R)-isomer]; tR = 77.26 min [major, (S)-isomer]. 4.4.11. 1-(2-Chlorophenyl)-2-hydroxyethanone Spectroscopic data were consistent with previously.26

that

reported

4.4.12. 1-(2-Methylphenyl)-2-hydroxyethanone Spectroscopic data were consistent with previously.26

that

reported

Acknowledgements One of the authors (Pula Mahajabeen) gratefully acknowledges the Council of Scientific and Industrial research (CSIR, India) for the fellowship. We thank the Sophisticated Analytical Instrumentation Facility (SAIF) and Department of Chemistry, IIT Madras for the NMR and Mass analysis. References 1. (a) Chouhan, G.; Kamal, A. Tetrahedron Lett. 2004, 45, 8801–8805; (b) Hasegawa, J.; Ogura, M.; Tsuda, S.; Maemoto, S.; Kutsuki, H.; Ohashi, T. Agric. Biol. Chem. 1990, 54, 1819–1827. 2. Sadyandy, R.; Fernandes, R. A.; Kumar, P. Arkivoc 2005, 3, 36–43.

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