Support studies for formulating the nine contiguous chiral centres of streptovaricin A ansa chain

Abstract

The tripyranoside (1b), which had been previously prepared as a key intermediate for the synthesis of the Streptovaricin A ansa chain, lacks synthons for the C-24-CO₂Me and C-28 tertiary alcohol of the antibiotic [C-6 and C-10, respectively of (1b)]. The ‘upper’ acetal ring of (1b) has been cleaved selectively, an exocyclic methylene is developed at C-6, and a hydroboration sequence has been used to install an axial hydroxymethyl group as the synthon for the CO₂Me group. In order to provide a self-consistent means of structure verification, both epimers of the C-10 tertiary alcohol have been prepared by utilizing empirically determined strategies for acyclic stereoselection.