One step preparation of 33-epichloro-33-desoxyascomycin (33-epi-33-chloro-FR 520)

Disclosed is a one-step process for the production of 33-epichloro-33-desoxyascomycin (33-epi-33-chloro-FR 520) (formula I) from ascomycin (FR520), said process comprising reacting ascomycin with an appropriate chlorinating agent (such as dichlorotriphenylphosphorane) in an organic solvent, in the presence of a base (such as pyridine), and isolating a compound of formula I obtained from the reaction mixture, wherein said process does not use protecting groups. Also disclosed is a process wherein the chlorinating agent is provided by treating triphenylphosphine with N-chlorosuccinimide in the organic solvent (selected from aromatic hydrocarbons, ethers and nitriles). Additionally a process wherein the chlorinating agent is provided by treating triphenylphosphine with a chlorinated alkane (such as carbon tetrachloride) is further disclosed.