Synthesis of both possible isomers of the northwest quadrant of altromycin B

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-Backlund 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.