Insights into the β-sultam ring formation in the sulfa-Staudinger cycloadditions.

The reaction of imines with sulfonyl chlorides or even direct sulfenes to form β-sultams is herein named as sulfa-Staudinger cycloaddition. The β-sultam formation is proposed to follow a stepwise mechanism of sulfonylation, deprotonation, and conrotatory ring closure with 2,3-thiazabutadiene-type zwitterionic intermediates as key intermediates. Cyclic (Z)-imines give rise to trans-β-sultams exclusively, suggesting that the intermediates generated from linear (E)-imines undergo a conrotatory ring closure directly to afford cis-β-sultams. Meanwhile, their iminium isomers lead to trans-β-sultams via the conrotatory ring closure.