Production of optically pure molecules by asymmetric reductions with yeasts

1. Raffaella Gandolfi1, Isabella Rimoldi2, Diego Romano3, Francesco Molinari3 . UNIVERSITA’ degli STUDI di MILANO 1Dipartimento di Scienze Molecolari Applicate ai Biosistemi, sez. Chimica Organica “A. Marchesini”, Via Venezian 21, 20133 Milano; 2Dipartimento di Chimica Inorganica Metallorganica e Analitica “L. Malatesta”, Via Venezian 21,20133 Milano, 3DISTAM-Sez. Microbiologia Industriale, Via Mangiagalli 25,, 20133 Milano. The preparation of optically pure compounds has became a major objective in the transformation industry. Many pharmaceuticals, agrochemicals, flavours, fragrances and food additives are now prepared as single enantiomers/stereoisomers. Asymmetric bioreductions catalysed by microbial dehydrogenases play a pivotal role in the field of stereoselective biocatalysis since they asymetrically transform prostereogenic carbonyls into stereo-defined secondary alcohols. Biocatalytic reductions are, therefore, an attractive alternative to metal-catalysed hydrogenation. CASE 1: Enantioselective reduction of b-ketoaminoesters to obtain a-hydroxy-b-aminoacids[1] (building blocks for the synthesis of unnatural peptides, azetidinones and b-lactams): a comparison between yeasts and metal-catalysed hydrogenation. Production of optically pure molecules by asymmetric reductions with yeasts It is possible to obtain high enantiomeric and diastereoisomeric excesses with both the methods of reduction of (1). Table 1 reports the result obtained after optimization of the biotransformation Table 1: Reduction of the (1) under optimized conditions. By catalytic reductions it is possible to obtain 4 different diastereoisomers. The results show that, in most cases, only one diasteisomer is obtained. This depend on different factors: the hindrance of substituents, the stereo-electronic properties of the molecules and the presence of keto-enolic equilibrium. The last one allow to obtain high diastereoselectivity end enantioselectivity due to the presence either of a selective enoyl reductase or of a carbonyl reductase which takes a kinetic resolution of the substrate. CASE 2:Enantioselective reduction of tetrasubstituted double bonds to achieve 2R,3S phenyl isoserine (side chain of paclitaxel) Drugs starting from plant have always been an important role in the treatment of an enormous type of pathology for example cancer. Certainly the most important natural anticancer agent in use is Paclitaxel. For the asymmetric synthesis of lateral chain different strategies were developed. We focused our attention on the asymmetric reduction of 3-benzoylamino-3-phenyl-(ethyl, 2-oxalyl) propenoic acid ethyl ester (1), reported in the Bristol-Mayer Squibb’s patent. The asymmetric reduction of this particular tetra substituted alkene would allow to decrease the number of steps and to lead directly the right conformation of N-benzoyl-3-phenylisoserine (3). The catalytic reduction will realized with two different approaches: chemocatalysis and biocatalysis in order to compare but also combine both the methodologies. The particular double bond of substrate (1) can be compare to a protected enol. Using the whole cells, the product (4) can be obtained by two ways: a direct reduction of double bond and subsequently spontaneous hydrolysis or by the production of the intermediate (3) (obtained by hydrolysis) where the reduction is carried out on carbonyl group. 1 e.d. % of syn diastereisomers. 2 e.e. % of diastereisomer (2R3S) CASE 3: Regio and enantioselective reduction of 1,2- diaryl-1,2-diketones (benzils) to obtain the corresponding benzoins with dehydrogenases from Pichia glucozyma; optically pure benzoins are useful building block in the synthesis of different pharmaceuticals (i.e. Olivomycin, Kurosoin). [2] Reduction of symmetric 1,2-diaryl-ethanediones catalysed by lyophilised cells (20g/L) of Pichia glucozyma CBS 5766 The reduction of 1a and 1b could lead to two pairs of enantiomers The bioreduction of different diketones is a clean methodology to obtain enantiomerically pure benzoins. The advantages of using lyophilized cells to catalyse the reduction of benzil and benzil derivatives in mild reaction conditions are shown reaching very high yields and enantiomerically excess. This protocols represents a potential useful tool for a green and sustainable synthesis of this kind of compounds. References [1] Gandolfi R., Cesarotti E., Molinari F., Romano D., Rimoldi I,. Tetrahedron: Asymmetry 2009, 20, 411-414. [2] Hoyas P., Sansottera G., Fernandez M., Molinari F., Sinisterra J.V., Alcantara A.R., Tetrahedron2008, 64, 7929-7936