Stereoselective synthesis of fully protected (S)-1,7-dioxaspiro[5,5]undec-4-ene derivatives of Sugars

Perbenzylated 1,7-dioxaspiro[5,5]undec-4-ene derivatives of sugars are obtained in three steps, starting from fully protected glycono-1,5-lactones. The procedure is based on the attack of a lithiated dithiinyl reagent (6) on the starting δ-glyconolactone. The C-glycosidation leads to the sole thermodynamically more stable α-hemiacetal derivative, the spirocyclization of which is then accomplished with BF3·Et2O. The fully protected unsaturated spiroacetals can be smoothly desulfurized for the purpose of any further elaborations of the free double bond.