Synthesis of 1,3-Benzothiazines by Catalytic Generation of Cesium Acetylide

Significance

Reported is the unprecedented in situ generation of terminal acetylides using catalytic cesium fluoride which undergoes reaction with cyclic sulfenamides 1 to afford 1,3-benzothiazines 3. Instead of the generation of a sulfenyl fluoride intermediate by the ring-opening reaction of 1 (V. Reboul and co-workers J. Org. Chem. 2009, 3936), the present method involves fluorine anion mediated terminal deprotonation of the alkynoate (pK_a  ∪ 18.8). Subsequent reaction of the generated acetylide anion at sulfur of the sulfenamide 1 followed by ring-opening intramolecular Michael addition affords the (Z)-1,3-benzathiazine 3. The stereochemistry of the major diastereoisomer was assigned by X-ray crystallog-raphic analysis. When using 2 with R^³ = Ts the reaction is stereoselective and, in some cases, inversed (E-isomer).