Stereodivergent synthesis of diastereoisomeric carba analogs of glycal-derived vinyl epoxides: a new access to carbasugars.

A convenient method for the stereoselective synthesis of diasteroisomeric vinyl epoxides (-)-2α and (-)-2β, the carba analogs of D-galactal and D-allal-derived vinyl epoxides 1α and 1β, has been elaborated starting from tri-O-acetyl-D-glucal. The key step of this synthesis is an application of the known Claisen thermal rearrangement of a glucal derivative, the vinyl allyl ether (+)-3b, which allows to switch the glycal structure into the corresponding carba analog scaffold. Epoxides (-)-2α and (-)-2β derive from the same synthetic intermediate, the trans diol (+)-5.