The key feature in the synthesis of topicdy-active carbonic anhydrase inhibitor MK-0507 (13b) is a Ritter reaction that exhibits an unexpected tendency to proceed with retention of chirality. This phenomenon was further studied on model compounds free from potential diastereomeric effects. A mechanism involving traneannular stabilization of the sp2- hybridized center by sulfone oxygen is proposed with the net result of double inversion. A ...