Synthesis,Reactivity Studies,and X-ray Crystal Structure of (11R)-25-O-Deacetyl-11-deoxo-11-hydroxy-21,23-O-isopropylidenerifamycin S

The protected intermediate (11R)-25-O-deacetyl-11-deoxo-11-hydroxy-21,23-O-isopropylidenerifamycin S ( 7 ) has been synthesized starting from rifamicin S ( 2 ; Scheme 2), the former being a potential substrate for the preparation of new types of rifamicin-S derivatives modified at C(11) and/or C(25). The reactivity of 7 toward acylations has been studied under both base- and acid-catalyzed conditions. The compound either did not react or nr underwent unexpected reactions, and no acylation products could be isolated. The X-ray crystal structure of 7 reveals that both OH groups at C(11) and C(25) are hindered, and this is probably the reason, why other take place faster than acylation.