Interrupted Pummerer Rearrangement: Capture of Tricoordinate Sulfur Species Generated under Pummerer Rearrangement Conditions

A new approach to fused N,S-heterocycles is described. Treatment of N-(2-(alkylsulfinyl)phenyl)pyrroles (5) under conditions which typically promote reaction at the carbon α to the sulfoxide group (i.e., the TFAA-initiated Pummerer Rearrangement) produces selectively either pyrrolo[2,1-b]benzothiazole (9) or 1-(trifluoroacetyl)- pyrrolo[2,1-b]benzothiazole (7). It is suggested the process occurs by reaction of the pyrrole nucleus at sulfur of the corresponding O-trifluoroacetylated sulfoxide 1 producing an intermediate S-alkylpyrrolo[2,1-b]benzothiazolium salt 3. Nucleophilic displacement of the S-alkyl substituent by the trifluoroacetate counterion liberates pyrrolo[2,1-b]benzothiazole, which may undergo trifluoracetylation in the presence of excess TFAA. This approach to sulfur activation for intramolecular cyclizations is superior to other methods (usually involving positive halogen and a sulfide) since polyhalogenation and the instability of derivative halopyrroles are avoided. © 1992, American Chemical Society. All rights reserved