D-Glucosamine trimethylene dithioacetal derivatives: formation of six- and seven-membered ring amino carbasugars. Synthesis of (–)-calystegine B₃,

By virtue of carefully chosen protecting groups, D-glucosamine trimethylene dithioacetal derivatives were successfully oxidized to the corresponding 6-aldehydes. This methodology reverses the donor and acceptor position on a normal open chain sugar and changes the relative position of the N-substituent. From the 6-aldehydes, heptose epoxide derivatives were prepared by a Corey–Chaykovsky reaction, and cyclized by the Corey–Seebach method. Depending on the designed protecting groups, the orthogonally protected six- and seven-membered ring amino carbasugars can be produced selectively and efficiently. (–)-Calystegine B₃ was synthesized from one of those products with high yield. This is the first anionic cyclization pathway to calystegine type structures.