A novel gem-difluoromethylenated castanospermine analogue B was designed and synthesized, starting from 3-bromo-3,3-difluoropropene and L-()-malic acid. The key steps involve substitution cyclization reaction and RCM reaction to construct the aza fused bicyclic framework. A novel gem-difluoromethylenated castanospermine analogue B was designed and synthesized, starting from 3-bromo-3,3-difluoropropene and L - () - malic acid. The key steps involve substitution cyclization reaction and RCM reaction to construct the aza fused bicyclic framework .