0040-403981 $3.00+.00 Pcrgnmon Rcssplc ENZYME-CATALYZED SYNTHESIS OF (R)-KETONE-CYANOHYDRINS AND THEIR HYDROLYSIS TO (R)-a-HYDROXY-a-METHYL-CARBOXYLIC ACIDS' Franz Effenberger*, Brigitte Hdrsch2a, Franz Weingart2b, Thomas Ziegler and Stefan Ki.ihner2= Institut fiir Organische Chemie, Universittit Stuttgart Pfaffenwaldring 55, 7000 Stuttgart 80, W.Germany Summarv: (R)-Ketone-cyanohydrins (R)-2 are obtained with high enantioselecti- vity from aliphatic ketones 1 and HCN in organic solvents using (R)-oxynitri- lase (EC 4.1.2.10) as catalyst. Acid catalyzed hydrolysis of the cyanohydrins (R)-2 affords the corresponding (R)-a-hydroxy-a-methyl-carboxylic acids (R)-3 without measurable racemization. Very few optically active cyanohydrins, derived from ketones, are descri- bed in the literature. High diastereoselectivity was observed for the opti- cally induced addition of HCN to 17-ketone steroids3 as well as for the addi- tion of ally1 trimethyl silane to optically active acyl cyanides'. Ohta et al 5 have reported, in a series of papers, on the enantioselective saponifica- tion of racemic ketone cyanohydrins with yeast cells of Pichia miso, with on- ly moderate chemical yields, however. Recently, oxynitrilase-catalyzed enantioselective addition of hydrocyanic acid to aldehydes and ketones, respectively, in aqueous media was described in a patent6. The only ketone mentioned in this patent was 3-methyl-cyclohe- xanone which was converted to the corresponding cyanohydrin without determi- nation of its optical purity6. We have demonstrated7 that enantioselectivity may be increased signifi- cantly if the (R)- or (S)-oxynitrilase catalyzed addition of HCN to aldehydes is carried out in an organid solvent instead of water . We now report on the enantioselective addition of HCN to ketones 1, also in organic solvents and employing (R)-oxynitrilase (EC 4.1.2.10) as catalyst. The (R) -cyanohydrins ith good chemical yields and in h igh optical purity8 (see (R)-2 are obtained w Table 1). 2605 2606 0 ,E + HCN R ’ CH, 1 @);OXY, YH H,O/H+ YH nitrhse R / C<;;H3 -=?+ R / “(;,““j (R>- 2 (R)-3 2 Table 1 (R)-Oxynitrilase Catalyzed Synthesis of (R)-Cyanohydrins (R)-2 and (R)-a-Hydroxy Carboxylic Acids (R)-3 from Ketones 1 1 react.cond. (R)-2 (R)-3 R time temp. yield ee yield ee h "C 3 0 % % % ethyl 41 0 80 76 85 76 n-propyl 43 0 70 97 64 99 n-butyl 42 0 90 98 72 98 n-pentyl 72 20 88 98 78 98 i-propyl 41 0 54 90 76 90 2-methylpropyl 40 0 57 98 89 96 3-methylbutyl 100 20 64 98 88 99 2-propenyl 16 0 68 94 3-butenyl 24 20 80 97 3-chloropropyl 120 0 87 84 The enantiomeric purity of the ketone cyanohydrins (R)-2 cannot be deter- mined from optical rotation. Since most of the data are not found in the li- terature, and since the optical rotation values are very small as a rule, an NMR spectroscopic determination with shift reagents of the respective ketone cyanohydrin acetates is not sufficiently precise 5 . We have therefore conver- ted the cyanohydrins (R)-2 to diastereomeric esters with Masher's Acid', and determined the ee-values by gaschromatography of the diastereoisomers. For some cases, the cyanohydrins were acetylated and analyzed on a chiral cyclo- dextrin columnlo. 2607 Optically active tert iary a-hydroxy ac ids are useful starting materials or synthetic intermediates for many chiral natural products, e.g. insect phero- moneslla, synthetic prostaglandin analogues llb , a-tocopherolllc. The synthe- sis of stereochemically pure tertiary acids is rather difficult and time con- suming. From the many methods, described for the preparation of tertiary Q- hydroxy acids12, the alkylation of dioxolanes derived from chiral a-hydroxy carboxylic acids and pival aldehyde13 as well as the enantioselective hydro- lysis of racemic a-methyl-a-benzyloxy carboxylates with a lipase14 yield acids with the highest optical purity. We have obtained the important a-hydroxy-a-methyl carboxylic acids (R)-3 by simply hydrolysing the easily accessible ketone cyanohydrins (R)-2 with concentrated acid (see Table 1). The reaction sequence from the starting ke- tone 1 to the carboxylic acid (R)-3 can be carried out in one pot, without isolation of the cyanohydrins (R)-215. Hydrolysis proceeds without measurable racemization as shown by comparison of the ee-values of the cyanohydrins (R)-2 and the hydroxy carboxylic acids (R)-3 (Table 1). With diazo methane the acids (R)-3 are converted to the corresponding methyl carboxylates and their optical purity is determined by gaschromatography on a chiral cyclodex- trin column. The proof of the (R)-configuration of the cyano hydrins (R)-2 and the hydroxy acids (R)-3 was given by comparison of the optical rotation value of the (R)-a-hydroxy-a-methyl-butanoic acid (R)-3 (R = ethyl) with li- terature data16. Acknowledgement: This work was generously supported by the Bundesministerium fur Forschung und Technologie and the Fonds der Chemischen Industrie. References 1) 2) 3) 4) 5) 6) 7) Enzyme-catalyzed Reactions, Part 9. - Part 8: F. Effenberger, B. Gutte- rer, Th. Ziegler, Liebiss Ann.Chem., 1991, a) B. Horsch, Dissertation Univ. Stuttgart 1990. - b) F. Weingart, & schunssoraktikum Univ. Stuttgart 1989. - c) S. Kiihner, Forschunosorakti- kum Univ. Stuttgart 1990. a) A. Ercoli, P. de Ruggieri, J.Am.Chem.Soc. 75, 650 (1953) - b) H. Kuhl, H.-D. Taubert, Steroids, 28, 89 (1976). - c) D.A. Livingston, J.E. Pet- re, C.L. Bergh, J.Am.Chem.Soc. 112, 6449 (1990) M.T. Reetz, K. Kessler, A. Jung, Ancfew.Chem.Int.Ed.Enql. 24, 989 (1985). a) H. Ohta, Y. Kimura, Y. Sugano, Tetrahedron Lett. 29, 6957 (1988) - H. Ohta, Y. Kimura, Y. Sugano, T. Sugai, Tetrahedron 45, 5469 (1989) U. Niedermeyer, U. Kragl, M.R. Kula, C. Wandrey, K. Makryaleas, K.H. Drauz, Europ.Pat.Anm.Nr. 0 326 063 A2 (23.01.89), Chem.Abstr. 112 (1990) 234 012 p. 4 a) F. Effenberger, Th. Ziegler, S. Forster,~w.Chem.Int.Ed.Enol. 26 458 (1987). - b) F. Effenberger, B. Horsch, S. Forster,Th. Ziegler, Tet- rahedron Lett. 31, 1249 (1990). 2608 8) 9) 10) 11) 12) 13) 14) 15) 16) (Rl-a-Hvdroxv-a-Methyl Alkane Nitriles 2: A solution of (R)-oxynitrilase (EC 4.1.2.10, 100 ~1, 1000 U/mol) was dropped on Avicel cellulose (1.5 g, soaked in 0.02 M sodium acetate buffer (pH 4.5)). Diisopropyl ether (20 ml) was added, followed by 5 mmol ketone 1 and 300 ~1 hydrocyanic acid, and the reaction mixture was stirred at either 0°C or room temperature (Table 1). The catalyst was filtered off, washed with diisopropyl ether, and the combined filtrates were concentrated to give (R)-2. Example: From 0.57 g n-pentyl-methyl k tone, 0.62 g (R)-a-hydroxy-a-methyl heptane ni- trile was obtained, [o;'i =M;gh;r (; ir1.12, chloroform). J.A. Dale, D.L. Dull, . . cr.Chem. P. Fischer,R. Aichholz, U. Bolz, M.Juza, S. 34, 2534 (1969) Krimmer, Anqew.Chem.Int.Ed. Enol. 29, 427 (1990). a) K. Mori, Tetrahedron 45, 3233 (1989). - b) J.S. Bindra "The Synthesis of Prostaglandins", in "The Total Synthesis of Natural Products"; John Wiley & Sons, N.Y. 1981, Vol. 4, p. 353. - c) T.Harada, T. Hayashiya, I. Wada, N. Iwa-ake, A. Oku, J.Am.Chem.Soc. 109, 527 (1987) and referen- ces cited therein. a) A.I. Meyers, J. Slade, J.Orq.Chem. 45, 2785 (1980) - b) X.C. He, E.L. Eliel, Tetrahedron 43, 4979 (1987). - c) G. Boireau, A. Deberley, D. Abenhaim, Tetrahedron 45, 5837 (1989) - d) S.S. Jew, S. Terashima, K. Koga, Tetrahedron 35, 2337 (1979) D. Seebach, R. Naef, G. Calderari, Tetrahedron 40, 1313 (1984) T. Sugai, H. Kakeya, H. Ohta, J.Ors.Chem. 55, 4643 (1990) JR)-a-Hvdroxv-a-Methyl-Carboxvlic Acids 3: A solution of the crude pro- duct (R)-2, prepared from 2 mmol ketone 1, as described in ref.8 was stirred in cont. HCl (5 ml) 16h at room temperature, followed by heating under reflux for 5h. The mixture was poured into water, extracted with diethyl ether, and the organic layers were dried. The residue, after fil- tration and concentration was crystallized from n-hexane. Example: From crude (R)-a-hydroxy-a-methyl heptane nitrile, prepared from 0.23 g, n- 0.25 g (R)-a-hydroxy-a-methyl-heptanoic acid was (R)-a-Hydroxy-uzte:hyl-butanoic acid, [a]'f: = -5" (c = 0.34, chloro- form); Ref. A. Kjaer, Ac~~]cse,.~~,~d_(~~ ;;,;h:;;z:prm): B.W. Christensen, (Receivedin Germany7 February 1991)