Altromycin B

Tungsten-catalyzed cycloisomerization of alkynyl alcohols including 8 provides only the endocyclic enol ether (11) as a key intermediate for the branched C-glycoside substructure (2) of altromycin B. A sequence of Stille cross-coupling reaction and regio- and stereoselective functional group transformations affords each C13-diastereomer of the branched C-arylglycoside (2a and 2b). [reaction: see text]

The synthesis of the northwest quadrant of Altromycin B is described. The preparation of the two epimers at the quaternary carbon of the 6-deoxy-C-altrose moiety in the northwest quadrant is accomplished starting from d-glucose. A key step of our synthetic sequence is the formation of the C-glycoside linkage via the Ramberg-Bäcklund reaction. Two different routes are explored, which differ mainly on the timing of the conversion of glucose to altrose, either before or after the preparation of the C-glycoside. The conformation behavior of variously substituted C-altropyranoside rings is also discussed.