US Patent 7279571 Methods of preparing pimecrolimus

The present invention provides a process for the preparation of pimecrolimus from ascomycin in which ascomycin is reacted with a conversion reagent that converts ascomycin to its activated derivative at C-32. The activated ascomycin is then reacted with chloride ion. The process of the invention requires fewer process steps than prior art processes, and does not require the protection of the ascomycin C-24 hydroxyl group or the purification of the activated ascomycin derivative.