CHEMOENZYMATIC APPROACHES TO THE PREPARATION OF BIOLOGICALLY ACTIVE COMPOUNDS

Biologically active compounds are usually polyfunctional molecules bearing one or more stereocenters. Enzyme catalyzed transformations are well suitable for their preparation since they act in a very mild and selective manner. The study of three selected biocatalyzed transformations has been the aim of my PhD researches. The first project, regards the development of an advantageous and mild method to selectively transform the 24-hydroxyl function of ascomycin, a 23-membered macrolactam, bearing two secondary alcoholic moieties bounded at position 24 and 32 of its macrocycle. This selective protection is required to transform ascomycin into pimecrolimus, its 32-epi chlorine analogue, an immunomodulator useful for the treatment of inflammatory skin diseases. The advantage offered by enzymes, showed to be not only their ability to regioselective discriminate between the two secondary hydroxyl groups, but also, their chemoselectivity, i.e. the capacity to leave intact the other functional groups present in the structure of ascomycin. Among enzymes to screen for the desired transformation we selected lipases, taking into account their well know regioselectivity in the acylation of secondary alcohols and their efficiency to transform lipofilic compounds. The second topic regards the clopidogrel, a thienopyridine compound that inhibits platelet aggregation by selectively binding to adenylate cyclase-coupled ADP receptors. It is indicated for the reduction of atherosclerotic events. Its structure contains a stereocenter: since only the S(+) isomer is biologically active, the compound must be prepared in the enantiomerically pure form. Although it is known and used by several years, usually this target is achieved by means of time consuming and expensive fractional crystallization of the unnatural aminoacid easily recognizable in its structure, 2-chloro-phenylglycine. The aim of my work was to find an enzymatic approach to achieve the suitable resolution with a low cost and a good optical purity overcoming the disadvantages of fractional crystallization. Different kinds of hydrolases were screened on several amides and esters derivatives of 2-chloro-phenylglycine or of a more advanced intermediate of the preparation. The third project, realized in Canada at Université Laval, deals with the resolution of the enantiomers of erhytro-p-nitrophenylserine, a non proteogenic aminoacid bearing the opposite stereochemistry of the antibiotic chloramphenicol, the D(-)-threo-2-di-chloroacetamido-p-nitrophenylserinol. The obtained optically pure aminoacids, by means of penicillin acylase, after their reduction to aminoalcohols were then N-acylated with the proper chiral L-methioninesulfone. The diastereoisomeric amides were tested as inhibitors for amidotransferase, an enzyme absent in humans and present in some archea, organella and bacterials, and then an ideal target for new antibacterial compounds.