Stereoselective Construction of Benzo‐Fused Tetrahydropyrrole Thiazines From Saccharin

Five Gabriel–Colman‐derived 3‐acyl substituted benzo e 1,2thiazine 1,1‐dioxides were converted into the corresponding benzo e pyrrolo1,2‐ b 1,2thiazine 5,5‐dioxides via base‐promoted alkylation with 1,3‐dibromopropane. Thereafter, diastereo‐divergent conversion of their 10‐keto functionality is detailed. Hydrogenation (H 2 and Pd/C), or reduction with NaBH 4 , led to selective generation of the cis ‐β‐hydroxy acyl diastereomers, whereas treatment with acidic zinc gave trans ‐β‐hydroxy acyl products. Finally, as a means to access 2‐benzyl‐substituted pyrrolidine, lithium‐liquid ammonia‐mediated reductive sulfonyl excision of both types of 10‐hydroxy benzo e pyrrolo1,2‐ b 1,2thiazine 5,5‐dioxide diastereomer was examined. Use of excess lithium led to reduction of both the sulfonyl group and the benzylic alcohol. Conversely, use of less lithium efficiently removed the sulfonyl group but left the benzylic alcohol group intact.