Inverse stereoselectivity in the nucleophilic attack on five-membered ring oxocarbenium ions. Application to the total synthesis of 7-epi-(+)-goniofufurone
A highlystereoselective nucleophilic substitution at the anomeric position of 1,2-O-isopropylidene furanose derivatives was employed for the synthesis of 7-epi-(+)-goniofufurone and two of its stereoisomers. According to Woerpel's model, the stereoselectivity depends essentially on stereoelectronic factors that lead to a preferred nucleophilic attack on the inside face of the five-membered ring oxocarbenium