Ring-closing oleflin metathesis of 3,3 '-(D-glycopyranosylidene)bis(1-propene) compounds as a route to anomeric spiro sugars

New peracetylated 3,3 '-(D-glycopyranosylidene)bis(1-propene), prepared fro m peracetylated anomeric sugar dihalides and allyltributyltin in excess und er UV irradiation conditions (AIBN, ca. 35 degreesC), lent themselves to ri ng-closing olefin metathesis in the presence of Grubbs' catalyst (6 mol %) to afford the corresponding peracetylated spiro[1,5-anhydro-D-glycopyranosi tol-1,4 ' -cyclopent-1 ' -enes] (D-gluco: 81%, D-Manno: 89%, D-galacto: 72% isolated yield), which could be deacetylated quantitatively. The bis(allyl ation) reaction (D-gluco: 40%, D-manno: 34%, D-galacto: 24% isolated yield) was in competition with radical-induced rearrangement and elimination reac tions. The last process, found to be favored at higher temperature (80 degr eesC), opens an easy route to sugar dienes difficult to prepare otherwise.