Methylation of rifamycin S (1) with MeI in CH₃CN or dimethylformamide (DMF) in the presence of Ag₂O afforded 8-O-methyl-N-methylrifamycin S (3). Demethylation of 3 with MgI₂·ether, followed by treatment with L-ascorbic acid and MnO₂, led to N-methylrifamycin S (4). Rifamycin S imino methylethers 5 and 6 gave the corresponding oxazolorifamycin derivatives 7 and 8 on reduction with L-ascorbic acid.