Methylsulfide-chlorine complex was prepared by adding oxalyl chloride (0.32 g) into a solution of dimethylsulfoxide (0.44 g) in 4 mL of methylene chloride and stirred at -70° C. for 30 minutes. The solution of the complex was slowly added to a stirred solution of ascomycin (1.6 g) in 5 mL of methylene chloride at -70° C. After stirring for 30 minutes, triethylamine (1.4 g) was added and the mixture was allowed to stir for an additional 30 minutes at -70° C. The reaction mixture was then allowed to warm to room temperature followed by additional stirring for 1 hour. The reaction mixture was diluted with 100 mL of ether, washed with 1N-HCl aq. solution (30 mL×2), brine (30 mL), dried over magnesium sulfate anhydrous, and the solvent removed in vacuo. The crude product was purified on 70 gram of silica gel, eluting with ether to obtain 0.95 g of the title compound.
