Diastereoselective Desymmetrization of Prochiral Cyclopentenediones via Cycloaddition Reaction with N-Phenacylbenzothiazolium Bromides.

A metal-free highly diastereoselctive [3 + 2] cycloaddition reaction has been developed between N-phenacylbenzothiazolium bromides and prochiral cyclopentene-1,3-diones. The active 1,3 dipole benzothiazolium N-phenacylide was generated in situ with the treatment of DIPEA, and the corresponding cycloaddition products were obtained in excellent yields under mild reaction conditions. The scope of the reaction is quite broad, tolerating a variety of aryl and heteroaromatic groups. A catalytic asymmetric approach was also studied preliminarily, and moderate enantioselectivity was achieved.