Intramolecular Capture of Pummerer Reaction Intermediates by an Aromatic Nucleophile : Selective Construction of 1, 4-Benzothiazine and Indole Ring Systems

The simple alkyl sulfoxide 6 carrying two aromatic nucleophiles, when treated with trifluoroacetic anhydride at room temperature (Pummerer reaction conditions), underwent an intramolecular aromatic sulfenylation of the 6-exo-tet process in an exclusive manner to yield two regioisomeric 1, 4-benzothiazine derivatives, 8 and 9. On the other hand, a similar reaction of the α-acyl sulfoxide 7, possessing identical aromatic nucleophiles, caused an intramolecular aromatic alkylation of the 5-exo-trig process to produce the 3-oxo-indole derivative 14 in a quantitative yield. These results demonstrate that the construction of 1, 4-benzothiazine and indole ring systems can be achieved in a selective manner by proper choice of the sulfoxide side chain.