Substituent effect on the diastereoselectivity in the sulfa-Staudinger cycloaddition

Abstract

The substituent effect on the diastereoselectivity in sulfa-Staudinger cycloadditions has been investigated. The diastereoselectivity is controlled by the competition between the direct conrotatory ring closure of the thiazabutadiene-type zwitterionic intermediates, generated from sulfonyl chlorides and imines, and the isomerization of their iminium moiety. The direct ring closure generates cis-β-sultams, while the isomerization and subsequent ring closure deliver trans-β-sultams. The C-electron-withdrawing substituents of imines reduce the bond order of imines and their iminium moiety, favoring the isomerization of the thiazabutadiene-type intermediates, resulting in the increase of trans-selectivity. Otherwise, the cis-selectivity is predominant for imines with electron-donating N- and C-substituents. The trans-β-sultams are afforded favorably, or even exclusively in some cases, as the effect of the electron-withdrawing substituents of sulfonyl chlorides, because the substituents decelerate the direct ring closure obviously. The N- and C-substituent effects of imines are opposite to those in the Staudinger ketene–imine cycloadditions, while the substituent effect of sulfonyl chlorides is accordant to the ketene-substituent effect in the Staudinger cycloadditions. The solvent and temperature do not affect the diastereoselectivity obviously. The current results provide useful information on controlling the diastereoselectivity in the synthesis of trans-β-sultams via sulfa-Staudinger cycloaddition.