Origins of Stereocontrol in the [2 + 2] Cycloaddition between Achiral Ketenes and Chiral α-Alkoxy Aldehydes. A Pericyclic Alternative to the Aldol Reaction

Ab Initio calculations predict that the thermal [2 + 2] cycloaddition reaction between C 2 v -symmetric ketenes and enantiopure aldehydes takes place with high enough stereocontrol for preparative purposes. The sense of induction is predicted to be non-Felkin. It is also found that the [2 + 2] cycloaddition involving nonactivated ketenes is facilitated by using 5 M solutions of lithium perchlorate in diethyl ether. It is found that both the purely thermal and lithium-assisted [2 + 2] cycloadditions result in the same type of stereocontrol. This method constitutes a general route for the synthesis of homochiral 2-oxetanones and related compounds, thus providing a pericyclic alternative to the aldol reaction.