Tripyranoside Precursors for Ansamycins. Pyranosidic Homologation. 6.

Article abstract of DOI:10.1021/jo00229a014

The primary and secondary alcohols of the dipyranoses 6 and 9a are used as implements for elaborating the upper pyranoside rings of the tripyranoses 27 and 28, respectively.Compounds 27 contains seven of the eight chiral centers of rifamycin S, while 28 fixes seven of the nine chiral centers of streptovaricin A.For the synthetic procedures, the primary alcohol is oxidized to an aldehyde, which is subjected to olefination with a phosphorane of the type PH3P=CHCH(OR)2.Treatment with pyridinium p-toluenesulfonate, specifically, in the presence of an alcohol causes cycliz ation with secondary the alcohol, resulting in a hexenepyranoside, which is glycositated in situ by the alcohol.The resulting hexenepyranoside is then epoxidized, and for this process, a new procedure was developed that involves hydroxylation with osmium tetroxide and treatment of the cis diol so formed with phosgene iminium chloride (Viehe's salt> to give a chloro carbamate, which then reacts with methoxide to give the desired epoxide.Opening of the ring with dimethylmagnesium then completes the requirements for the upper ring.