Microbial reduction of 2-chloro-3-aryl-3-oxopropionic acid esters

Touahcdron Letters. Vol. 33, No. 48. pp. 7337-7340.1992 Priitcd in Great Britain

Microbial

oo40-4039192 $5.00 + .OO

Pag-

Reduction

2-Chloro-3-aryl-3-oxopropionic

PAW L&l

of Acid

Esters

Odile Cabon, Marc LarchevCque+ Labofato~deChimiede I’Ecole NommleSupcriem,Unit6 Propre CNRS, 24, rue Lhomond, 75231-

Pat-is C&X

05. France

Didier Buisson and Robert Azerad+ Latwatohe de Cbimie et Bicchimie Phmnacologique.s et Toxicologiqw.

Associi au CNR!? , Universitc RDesmes,

45, rue des Saints-P&es, 75270 - Pads Cedex 06, France

Keywords: 2-chloro-3-kmsters;

2.3-epoxyesters; microbial ntdwtions; Baker’s yeast; Mucor racenwsus; Rhobonh

glurinis.

Abstmet: 2-Chloro-3-aryl-3-oxopropiDnic acid esters are reduced by microorganisms to syn- &or anti-2-chloro-3hydroxy-3-atylpropionates, in competition with dechlorination anddecarbo@ation reactions.Usingselected strains, it is possible to obtain enantiomerically pure chlorohydroxyesters, which were converted to the corresponding phenyl&idic esters with high stereospccificiry.

The microbial reduction’ of racemic a-alkyl @ketoestcrs has been shown to frequently produce a single stereomer of the corresponding a-alkyl P-hydroxyester’, as a consequence of the fast epimerisation of the oxoester through enol formation, combined with an enantioselective and stereospecific reduction. As a continuation of our work, we have been interested in examining the potential diastereo- and enantioselective microbial reduction of a-chloro-&ketoesters. in order to obtain u-functional&d asymmetric compounds of high synthetic valud. This could be effected either through direct nucleophilic substitution of the chlorine atom, leading for example to nitrogen a-substituted p-hydroxy acids, such as 3-phenylserine (1) isomers4, or through conversion to the corresponding 2.3-epoxyesters of various relative and absolute configurations5, which constitute valuable synthons or intermediates for the preparation of a number of optically active compounds of biological interest, such as the N-benzoyl-2R,3S-3-phenyl isoserine (2) side chain of taxo16, or 2S,fS-Diltiazem (3)‘.

Little data concerning the microbial reduction of 2-chloro-3-oxoesters to 2-chloro-3-hydroxyester isomers is available. Baker’s yeast has been used to reduce (&)-ethyl 2-chloro-3-oxobutanoate to a 1: 1 mixture of synand anti- chlorohydroxyesters8, but an estimation of the stereochemical purity of the products was not directly available, as their corresponding derived epoxyacid derivatives were crystallized as brucine salts. A recent patent9 has claimed the preparation of all four stereomeric ethyl 2-chloro-3-hydroxy-3-(4’-methoxy)phenyl 7337

7338

propionates by the reduction of the corresponding (f)-2-chloro-3-oxoester using various microorganisms, bacteria, yeasts, or fungi; the effective yields of the reduced products were generally poor, indicating low reduction rates and/or undesiredconversion to other products. We report here our investigations of the stereospecifii reduction of ethyl 2-chloro-3-oxo-3-arylpropionates 4a-b by baker’s yeast and several fungal strains, the configuration of the resulting chlorohydroxyesters and the stereochemistry of their conversion to the corresponding asymmetric phenylglycidic esters. Until now, such compounds have most often been obtained by resolution methods’. Incubation of 4a or 4b with baker’s yeast afforded very low yields of chlorohydroxyesters (Table 1). At low concentrations, the substrates were entirely converted to the corresponding ethyl 3-hydroxy-3-aryl propionates 7, probably due to an enzymatic dechlorination reaction1o . However, at higher concentrations, the rate of thii reaction was strongly decreased and significant amounts of chlorohydroxyesters could be detected. A preliminary incubation of yeast with 2 g.L-* of ethyl 2-chloro-3-oxobutanoate, which inactivates the dechlorination reaction1 ‘, allowed a nomal reduction of 4a (1.5 g.L-1) yielding a 6:4 mixture of syn- (5a) and anti- (6a) chlorohydroxyesters12 (77% yield), however with low optical purities13 (68% and 30% e.e.respectively). Table 1. Bioconversion of Ethyl 2-Chloro-3-aryl-3-oxopropionates 4a and 4b by Baker’s Yeast

incubation substrate 4a

(R=H) 4b (R=oMe)

(p.L-1) 1 5 10 1

(hzs) 7 48 144

v synd (%)

conversion 96 100 60 23

24

(%)

4 3

95

(& 100 30 7

17 6

-

95

Other strains were tested for their ability to reduce the same cblorooxoesters. Some of the results (Table 2) show that most of these microorganisms afforded high yields of chlorohydroxyesters as mixtures of syn- and anti-isomers with moderate to high optical purities, as well as small amounts of the dechlorinated ester 7; a chlommethyl aryl-carbinol 8 was also found14, sometimes in significant amounts. Table 2. Reduction Products of Ethyl 2-Chloro-3-aryl-3-oxopropionatcs 4a-b by Various Microorganism@ 6

5

substrate microorganism 4a Rhoabtarula glutinis b (R=H) Rhizopus arrhizus C 1, I,

II II 4b

Rhizopus arrhizus d Mucor plumbeus e Mucor racemosus f Sporotrichum exile 8 Rhizopus arrhizus C

a&mated by GC. brmu

~-1091. cATE

% 6

(e.e.%) -

% 89

(e.e.8) (95)

23 20 38 73 79 50

(82) (98) (96) (95) (99)

26

(92)

36 7 6

N) (71) (74)

2

2 51 77

10

9

4

15 15

10

39

11145.d ATCC24563.eCBS 110-16.flocal strain.8 QM 1250.

7339

5 6 8 9 Mucor racemosus and Rhodotorula glutinis were selected for preparative reductions. 4a (2 g), as a solution in 10 ml of Tween 80-EtOH (2:8 voYvo1).was incubated for 24 hours at 27“C with washed mycelium’5 of Mracemosus in 0.2 M pH 6.0 potassium phosphate buffer (2 L). The products were extracted and separated by silicagel chromatography to give 0.9 g of the syn-(2S,3R)-chlorohydroxyester Sa. [aID= - 3 (c 1.7, CHCl3) (96% e.e.). Using R.glutinis, 4a (1 g). added to the culture medium (1 L)l’, was completely reduced in 2 hours to the anti-(2R,3R)-2-chloro-3-hydroxyester 6a, [u]D= - 42 (c 1.5. CHC13) (0.55 g, 95% e-e.). The stereospecific conversion of chlorohydroxyesters to phenylglycidic esters is not a straightforward reaction. It is commonly acknowledged that the cis-epoxyester (from the syn-chlorohydroxyester) is more difficult to obtain than the tram-epoxyester (frpm the anti-chlorohydroxyester). Shown, in Table 3, are some results obtained from the syn-chlorohydroxyester with various alkaline reagents. From this data, it is clear that the best conditions for cis-epoxide formation are the use of K$O@MF-water. while, in contrast, the use of NaOEt in ethanol results in a nearly exclusive formation of the trans-phenylglycidate. This last outcome has been claimed to be the result of a retroaldolisation reaction and experiments supporting this mechanism have been reported16 . Such a reaction may be a major drawback in our attempts to obtain enantiospecific conversions. However, the use of enantiomerically pure chlorohydroxyesters affords a mechanistic probe which was not available in the previous work.

Table 3: Relative Configurations of 23-Epoxyesters Obtained from the syn-2-Chloro-3-hydroxy3-phenylpropionic Acid Ethyl Ester 5a * Using Various Alkaline Reagents. Reaction time Chlorohydroxyester 2,3-Epoxyesters 9a (hours) recovered (96)” cis (%)b truns (%)b 6c 94 6 2oc 32 47 21 2d 8 92 24e 100 7oc 23 11 66 48c 74 5 21 * all reactions were performed on a 8~2syn/unrichlorohydroxyester mixture,exceptfor entries1 and 2 wherea >95% puresynesterwasused.b determined by CKt9. C reaction at WC. d OT. e refluxing temperature. Reagent K$X$DMF-water6 NaI-M-IMpT’e NaOEt/EtOH Ag$Xiimethoxyethane’7 CSF/TI-IF’~ KF/18-crown-6/CH3CN’*

When the syn (2S,3R)-chlorohydroxyester 5a (96% e.e.) was treated with K$Og/DMF-water, a 94:6 mixture of cis:trans phenylglycidate was obtained, the cis compound being the (2R3R)-enantiomer (>95% e.eJU). The same chlorohydroxyester treated with NaOEt in ethanol afforded an 8:92 mixture of the cis:trans epoxyester, the tram (2S,3R)-isomer being >95% enantiomerically pure21. When the optically pure anti(2R,3R)-chlorohydroxyester 6a was treated in similar conditions, the optically pure tram-(2S,3R)phenylglycidate was obtained in both cases. Some of these results are not compatible with a mechanism involving a retroaldolisation reaction, which would have given a mixture of racemic epoxyesters from the optically pure syn-chlorohydroxyester. Another mechanism, involving an epimerisation of the C-2 atom of the chlorohydroxyester, competitively with an SN2 ring closure, is more credible. Isomerisation at the epoxyester level has been excluded on the basis of the literature data=. This work has shown that it is possible, using selected microorganisms, to obtain mainly or exclusively nearly optically pure syn- and anti-(3R)-2-chloro-3-hydroxy-3-phenyl propionates and, subsequently. the (3R)cis and tram isomers of a phenylglycidic ester. A cis-(2R,3R)-2,3-epoxy-3-phenyl propionic acid ester was previously used as an intermediate in the synthesis of the tax01 side-chai#.

7340

Work is in progress to obtain the corresponding (3s) esters by similar methods. We thankJ.-L.Seris and J.-A.Laffitte (GRL. La@ for tinancial assistanceand continuous encouragementin this work. References and notes 1. Sih, C. J. ; Chen, C.-S. Angew.Chem.In~.Ed.Engl. 1984.23, 570-578. For recent reviews about baker’s yeast reduction of fi-ketoesters, see Servi, S. Synthesis 1990, 1-25 or Csuk, R. ; Gl&nzer, B. I. ChemRev. 1991, 91. 49-97. 2. Deal. B. S.; Ridley, D. D. ; Simpson, G. W. Aust.J.Chem 1976.29.2459-2467. For a compilation of recent references about the stereochemistry of the reduction of cr-alkyl P-ketoesters, see Vanmiddlesworth, F. ; Sih. C. J. Biocurulysis 1987, I, 117-127 3 Corey, E. J. ; Choi. S. Tetrahedron Lat. 1991,32, 2857-2860. 4 HBnig. H.; Seufer-Wasserthal, P. ; Weber, H. Tetrahedron 1990,46,3841-3850. 5 Petit, Y.; Sanner, C. ; Larchevhue, M. Synthesis 1988, 538-540. 6 Denis, J.-N.; Correa, A. ; Greene, A. J.Org.Chem. 1490,55, 1957-1959. 7 Schwartz, A.; Madan, P. B.; Mohacsi, E.; O’Brien, J. P.; Todaro, L. J. ; Coffen, D. L. J.Org.Chem 1992.57, 85 l-856. Akita, H.; Matsukura, H. ; &hi.

; 10

I1 12

13

14

15 16 17 18 19 20 21 22 23

T. Tetrahedron Lat. 1986.27.5397-5400. Shibatani. T. 1991, Eur.Pat.91103837.0. A hypothetic mechanism for the dehydrochlorination of chlorohydroxyesters leading to a reducible pketoester enolate was eliminated on the basis of the high stability of 2-chloro-3-atyl-3-hydroxypropionic acid esters in the presence of yeast in the reduction conditions. Thus a mechanism for a direct enzymatic dechlorination reaction of the chloro-oxoesters must be found. Buisson, D. ; Azerad, R. unpublished results. Analytical separation and quantification of diastereomeric chlorohydroxyesters was effected on EtOAc extracts of incubation aliquots by GC on a DBwax capillary column.(30 m) at 22O’C. The relative configuration of syn- and anti-isomers was determined by IH-NMR spectroscopy of purified samples. 6 ppm, J Hz (CDC13, 250 MHz):syn @a), 1.12 (3H. t, J= 7.3, CH3), 2.94 (lH, d. J= 3.6, OH), 4.10 (2H. q, J= 7.3, CH2). 4.43 (lH, d, J= 6.5, CHCI), 5.12 (lH, dd, J= 6.5 and 3.6, CHOH), 7.32-7.37 (5H, m, ArH); anti (6a). 1.25 (3H, t, J= 7.3, CH3), 2.99 (lH, d, J= 5.1, OH), 4.23 (2H, q. J= 7.3, CH2), 4.36 (lH, d, J= 8.0, CHCl), 5.03 (IH, dd, J= 8.0 and 5.1, CHOH), 7.33-7.39 (5H, m. ArH). 96 E.e. were determined by GC after derivatization with S-acetyllactic chloride 23; absolute configur@ons were determined in two steps: i) determination of relative configuration of pure diastereomers ; ii) analytical separation of (S)-(0-acetyl)lactyl esters by GC on BP10 capillary column at 180-2OO’C (1‘Cumin) and determination of absolute configuration after catalytic hydrogenation (Pd/C, NEptOH) to known ethyl (S)-3-hydroxy-3-phenyl propionate; found [a]$O= -44 (c 1.5. CHCl3). lit.-39.8 . Incubation, in the same conditions used for microbial reduction, with either a 7:3 mixture of syn and anti chlorohydroxyesters, ethyl 3-hydroxy-3-phenylpropionate, or several obvious intermediates, has shown that the decarboxylation products originated from the enzymatic hydrolysis of the (chloro)hydroxyesters, followed by reoxidation to the oxoacid, decarboxylation, and partial or complete reduction of the resulting (chloro)acetophenone. Buisson, D.; Azerad, R.; Sanner, C. ; Larchevque, M. Biocutulysis 1990.3. 85-93. Seyden-Penne, J.; Roux-Schmitt, M. C. ; Roux, A. Tetrahedron 1970,26,2649-2656. McLure, J. J.Org.Chem. 1%7,32, 3888-3894. Mukaiyama. T.; Haga, T. ; Iwasawa, N. Chem Lett. 1982, 1601-1604. Analytical separation and quantification of diastereomeric cis and puns-epoxyesters was effected by GC on a SE30 capillary column at 18O’C. A small amount of epim@ation (~5%) was observed in such conditions. IH-NMR spectra were in agreement with the literature * . [a]~~~= +25 (c 1.18, CHC13). % E.e. determined by HPLC on a Chiralpack AD (250 x 4.6 mm) column, 95:5 hexane-isopropanol ,0.5 mYmin.(elution times: 2R,3R, 11.8 min.; 2S,3S. 12.7 min.). [a]D20= +152 (c 1.3, CHC13). 8 E.e. determined by GC on a XE-60-polysiloxane-S-valine-S-aphenylethylamide Chrompack capillary column (50 m) operated with helium at 13O’C (retention times: 2S,3R, 30.8 min.; 2R,3S, 31.3 min.). Bachelor, F. W. ; Bansal, R. K. J.Org.Chem. 1969.34, 3600-3604. Mosandl, A.; Gessner, M.; Gunther, C.; Deger, W. ; Singer, G. J.High Resolurion Chromutogr. Chromutogr.Commun.

1987.10,

67-70.

24 Manzocchi, A.; Casati, R.; Fiecchi, A. ; Santaniello, E. J.Chem.Soc.Perkin Trans.1 1987, 2753-2757. (Received in France 27 July 1992)