Intramolecular capture of Pummerer rearrangement intermediates. IV. Preparation of pyrrolo[2,1-b][1,3]benzothiazin-9-ones via intramolecular sulfenylation of an N-acylpyrrole

Pyrrolo[2,1-6][1,3]benzothiazin-9-one (6a) was synthesized in 45% overall yield from thiosalicylic acid in six steps. The title compound (88%) and its 1-trifluoroacetyl (83%) and 1-formyl derivatives were synthesized via intramolecular reaction of 1-(2-ethylsulfinyl) benzoylpyrrole (5) by thermal cyclization or treatment with trifluoroacetic anhydride or DMF/POCl3, respectively. The key step in each case is formation of the heterocycle by sulfoxide activation followed by C-S bond formation via attack of sulfur. Reaction of 6a with common electrophiles (trifluoroacetic anhydride (86%), POCl3/DMF (75%), and acetyl nitrate) indicates that C-1 is the predominant site of electrophilic substitution.