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Preparation of methyl 3(S)-hydroxy-4-oxopentanoate-4-ethylenedithioacetal by bakerís yeast reduction of the corresponding ketone; enantiomeric pure beta-hydroxy-ester

SyntheticPage 205 (2002)
Submitted 17th Oct 2002, published 21st Oct 2002

Carlos Afonso (cma@dq.fct.unl.pt),
A contribution from the Carlos A. M. Afonso, C. D. Maycock Group, New University of Lisbon, Portugal


Reaction Scheme

Chemicals Used
Bakerís yeast (from local bakery), sucrose (local supermarket), celite (BDH), methyl 3,4-dioxopentanoate-4-ethylenedithioacetal ([SyntheticPage 204] ), absolute ethanol (Merck), acetyl chloride (Merck), ammonia.

Procedure
Preparation of methyl 3(S)-hydroxy-4-oxopentanoate-4-ethylenedithioacetal (2): Fresh baker's yeast (404 g) was suspended in a stirred solution of sucrose (424 g) in tap water (1009 ml). The mixture was allowed to stand at 21 °C during 1hr (gas evolution). Methyl 3,4-dioxopentanoate-4-ethylenedithioacetal (1) (10.00 g, 45.39 mmol) was added and stirring was continued for 48 hours. The mixture was then saturated with NaCl and filtered through celite. The solid was washed with ethyl acetate (2 x 500 ml) and the aqueous filtrate was extracted with diethyl ether (4 x 500 ml). The combined organic phases were dried (MgSO4), filtered, evaporated under vacuum and the crude product was chromatographed on a silica gel column (eluents: petroleum ether 40/60 : dichloromethane from 8:2 to 1:1) to afford, in order of elution, starting material (1) (4.20 g, 42 % recovery) and product (2) (4.82 g 48 %). The product was obtained as a clear colourless oil and the ee determined as described at the end of this section. Preparation of 3(S)-hydroxy-4-oxopentanamide-4-ethylenedithioacetal (4): To a solution of methyl 3(S)-hydroxy-4-oxopentanoate-4-ethylenedithioacetal (2) (1.268 g, 5.70 mmol, e.e. 95 %) in anhydrous methanol (40 ml) at -60 °C was passed ammonia to give a total volume of 55 ml and the mixture was closed and allowed to stand at room temperature for 3 days (caution, see authorís comments). After this time the vessel was opened and the ammonia was allowed to slowly disperse. The mixture was then evaporated to dryness. The residue was purified by chromatography on a silica gel column (eluents: petroleum ether 40/60 : ethyl acetate from 1:1 to 0:1) to afford (4) (1.135 g, 96 %) as a solid (Rf = 0.36 (ethyl acetate)). Enantiomerically pure amide was obtained after two crystallizations (petroleum ether 40/60 : ethyl acetate) m.p. 118.5-119 °C; alfa 25D = -41.1° (c 1.0 ethanol). Conversion of 4 into 3: To recrystallised amide (4) (3.738 g, 18.03 mmol) dissolved in absolute ethanol (75 ml) was added acetyl chloride (5.1 ml, 72.1 mmol) and heated at reflux for 18 h under a N2 atmosphere. After allowing the reaction mixture to cool to room temperature the mixture was then neutralised by addition of NaHCO3, and the ethanol then evaporated. To the residue was added diethyl ether (50 ml), water (50 ml) and the aqueous phase was extracted with diethyl ether (3 x 50 ml). The combined organic phases were dried (MgSO4), filtered, evaporated to dryness, and chromatographed on a silica gel column (eluents: petroleum ether 40/60 : dichloromethane from 1:1 to 0:1) to afford (3) (3.397 g, 80 %) as a clear colourless oil, alfa25D = -35.2 (c 1.2, CHCl3), e.e. > 97 % determined as described below. Note: none of the other enantiomer was observed by 1H NMR. Determination of ee: The ee of the transesterification product, ethyl 3(S)-hydroxy-4-oxopentanoate-4-ethylenedithioacetal (3) was determined by 1H NMR(300 MHz, 0.035 M in CDCl3) on basis of signals for (R) and (S) of C-5 Me (respectively 2.38 and 2.63 ppm) and OCH2CH3 (respectively 1.41 and 1.29 ppm) in the presence of 0.64:1 molar ratio Eu(tfc)3/substrate. Preparation of (3) by transesterification of (2): To (0.062 g, 0.28 mmol) of (2) in absolute ethanol (1 ml) and acetyl chloride (0.04 ml) were heated at reflux for 3 hr, usual work-up and silica gel column (eluents: petroleum ether 40/60 : dichloromethane 1:1)) to give the product (3) (0.055g, 83%).

Author's Comments
The absolute configuration of the reduced product (2) was assigned by converting the ethyl ester (3) into known ethyl 3(S)-hydroxypentanoate by reducing the 1,3-dithiolane functional group to the corresponding methylene group using Raney® nickel protocol. The simple protocol presented here allows the preparation of a potential useful starting chiral compound, due to the presence of the hydroxyl, ester and protected carbonyl functional groups which can be further manipulated. This protocol have been repeated several times. Below are presented more specific comments of each step. STEP 1; Bakersí yeast reduction: The protocol presented here corresponds to the optimized conditions. Initially, the reduction was tested for the corresponding ethyl ester of (1) (ethyl 3,4-dioxopentanoate-4-ethylenedithioacetal (5)), but lower yields and lower optical purity were observed, (23 oC, 48 h, [substrate (5)] = 40 mM, yield = 12 %, ee = 85 %). Using the corresponding ethyl ester and diethoxy ketal (ethyl 4-diethoxy-3-oxopentanoate (6)) no reduction product was detected and the substrate was recovered in 53 %, even using automatic control of pH = 6-7 (KH2PO4/K2HPO4 buffer and automatic pH controller). For the methyl ester substrate (1) (48 h, [substrate (1)] = 45 mM, yeast = 0.4 g/l) lower yields and minor reduction erosion of ee was observed using higher operation temperature: 21 oC (48 %, ee 95%); 23 oC (38 %, ee 92%); 25 oC (24 %, ee not determined); 27 oC (26 %, ee 91%). Using bakersí yeast immobilized in sodium alginate (23 oC, 72 h, [substrate (1)] = 45 mM, immobilized yeast = 0.3 g/l) gave lower convertion and optical purity (yield = 19%, ee = 87 %). The pH of the reaction media was folllowed during the yeast reduction and was observed that during the first 5 hours of operation the pH dropped from pH = 7 to pH = 3.5. Even using KH2PO4/K2HPO4 buffer the pH still dropped to pH = 4.3. However in both cases (without and with buffer) the pH recovered after 48 h of operation respectivelly to pH = 4.5 and pH = 5.0. STEP 2: Amide formation: Caution!! for this step we used a cylinder glass flask (diam. = 5 cm, height = 12 cm) with a teflon stopper (Quickfit, Rotaflow) made by our glassblower. Using this apparatus we have carried out this procedure more than 10 times without any explosion. However, this step should be performed with special care due to the ammonia pressure at room temperature. The crystallization of amide (4) was performed four times in order to improve the ee. We observed after each recrystallisation the following optical shift: first, -41.1 ±0.1 (c, 0.9, EtOH); second -41.1 ±0.1 (c, 1.0, EtOH); third -41.5 ±0.1 (c, 0.9, EtOH); fourth -41.2 ±0.1 (c, 0.9, EtOH). We recovered the following yields after each recrystallisation: first, 86%; second 97%; third 98%; fourth 96%). From these results we can see that a single recrystallisation can be used to prepare optically pure amide (4). The mother liqueur obtained from the first crystallization presented an ee of 65 % (determined by conversion to the corresponding ethyl ester (3)) which confirms the effective increase of the optical purity by crystallization. The increase of the optical purity of beta-hydroxy-esters by crystallization of the corresponding amide were also observed for the following substrates: 3(S)-hydroxy-butanamide (initial ee of 90%; after two crystallizations from n-hexane/EtOC gave ee > 97 %); 3(S)-hydroxy-6-heptenamide (initial ee of 63 Ė 68%; after three crystallizations from n-hexane/EtOC gave ee > 97 %); 3(R)-hydroxy-6-heptenamide (initial ee of 83%; after three crystallizations from n-hexane/EtOC gave ee > 97 %). STEP 3; Conversion of amide (4) to the ethyl ester (3): From the conversion of the amide (4) to the ethyl ester (3) under acidic conditions we also isolated the side product 3-methyl-2-ethoxycarbonylmethyl-5,5-dihydro-1,4-dithiine in 13 % due to the occurrence of well documented 1,2-sulphur migration process (Afonso, C. A. M.; Barros, M. T.; Godinho, L. S.; Maycock, C. D. Synthesis 1991, 575 and references cited therein).

Data
Methyl 3(S)-hydroxy-4-oxopentanoate-4-ethylenedithioacetal (2): 1H NMR(60 MHz, CCl4) d: 1.71(3H, s), 2.57(1H, d J=9 Hz), 2.68(1H, d J=5 Hz), 3.20(1H, bl), 3.31(4H, s), 3.70(3H, s), 4.10, (1H, dd J=5, 9 Hz); IR (film): 3500(br, OH), 2960, 2940, 2870, 1745(C=O), 1445, 1380, 1290, 1175, 1065, 995, 890, 895, 860, 760 cm-1. Ethyl 3(S)-hydroxy-4-oxopentanoate-4-ethylenedithioacetal (3): 1H NMR(300 MHz, CDCl3) d: 1.28(3H, t J=7.4 Hz, OCH2CH3), 1.76(3H, s, H-5), 2.55(1H, dd J=16.1, 10.3 Hz, H-2), 2.83(1H, d J=16.1 Hz, H-2), 3.20(1H, br, OH), 3.28-3.36(4H, m, S(CH2)2S), 4.15-4.22(1H, CHOH, masked by q of OEt), 4.18(2H, q J=7.4 Hz, OCH2Me); IR (film): 3480(OH), 2980, 2940, 2880, 1730(C=O), 1450, 1380, 1285, 1180, 1100, 1045, 950, 910, 890, 855, 760, cm-1. 3(S)-Hydroxy-4-oxopentanamide-4-ethylenedithioacetal (4): 1H NMR(300 MHz, CDCl3)d: 1.75(3H, s, H-5), 2.52(1H, d J=9 Hz, H-2), 2.65(1H, d J=3 Hz, H-2), 3.35(4H, s, S(CH2)2S), 3.40(3H, br, OH and NH2), 4.12(1H, dd J=3, 9 Hz, CHOH); IR (nujol): 3390(NH and OH), 3350(NH and OH), 3290(NH and OH), 3240(NH and OH), 3180(NH and OH), 2720, 1660(C=O), 1610(C=C), 1305, 1275, 1245, 1190, 1160, 1100, 1090, 1055, 1025, 975, 885, 875, 850, 760, 720, cm-1; MS m/e: 207(M+), 189(M+-H2O), 171, 156, 145, 119(C4H7S2+, base), 97; Anal. Calcd for C7H13NO2S2: C 40.56, H 6.32, N 6.76%; Found:C 40.56, H 6.41, N 6.65%.


Lead Reference
Afonso, C. A. M.; Barros, M. T.; Godinho, L. S.; Maycock, C. D. Tetrahedron 1993, 4283.

Other References
H. G. Davies, R. H. Green, D. R. Kelly, S. M. Roberts Biotransformations in Preparative Organic Chemistry: The Use of Isolated Enzymes and Whole Cell Systems, Academic Press, 1989.

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