Chiral Phosphoric Acid-Catalyzed Enantioselective Formal [4+2] Cycloaddition Between Dienecarbamates and 2-Benzothioazolimines

An enantioselective chiral phosphoric acid catalyzed formal [4 + 2] cycloaddition between 2-benzothi azolimine s and N-H-1,3 -dienecarbamates is described. A divergence in reaction pathways was observed depending on the dienes employed. The reaction performed with 4-substituted dienes produced benzothiazolopyrimidines as major product in yields ranging from 42 to 67%, as single diastereoisomer and with enantioselectivity between 93 and 99%. The same reaction performed with 3-substituted dienes, however, gave highly enantioenriched 1,2,3,4-tetrahydroquinolines as the major products albeit with moderate diastereoselectivity.