Patent ID: 8778636

Claim:
A process for preparing pimecrolimus, comprising the following steps: a) selective enzymatic acylation of the hydroxyl in position 33 of ascomycin with lipase from Candida antartica , in the presence of an acylating agent and an organic solvent; b) conversion of the 33-acylated derivative this obtained to the corresponding 24-tert-butyl-dimethyl-silyl either, with a silylating agent in the presence of an organic base; c) enzymatic removal with lipase from Candida antartica of the acyl group in position 33 of the compound prepared in step b), in the presence an organic solvent and a C 1 -C 8 primary aliphatic alcohol, to obtain 24-tert-butyl-dimethyl-silyl ether of ascomycin, wherein the hydroxyl group in position 33 of the 24-tert-butyl-dimethyl-silyl ether of ascomycin obtained in step c) is substituted with a chlorine atom to obtain a 24-tert-butyldimethylsilyl-33-epi-chloro ascomycin derivative, followed by removal of the tert-butyl-dimethyl-silyl ether in position 24.