SULFENYLATION USING SULFOXIDES - INTRAMOLECULAR CYCLIZATION OF 2-ACYLPYRROLES AND 3-ACYLPYRROLES

AlCl3-catalyzed acylation of N-(phenylsulfonyl)pyrrole with 2-(ethylthio)benzoyl chloride followed by hydrolysis provided 2-(2-(ethylthio)benzoyl)pyrrole (3). Compound 3 was also available from ethyl 2-(ethylthio)benzoate and pyrrolylmagnesium chloride (Scheme 2). Oxidation(NaIO4) gave the corresponding sulfoxide 4 which when refluxed 6 h inp-xylene (bp 138 degrees C) gave predominately the C-3 cyclization product 11 (51%) along with the rearrangement product 12 (17%). Conducting the thermal reaction in the presence of DMAP in refluxing toluene gave mainly the N-1 cyclization product 13 (46%) also accompanied by the rearrangement product 12 (20%). Formation of a 2H-pyrrole spirocyclic intermediate from thermally promoted intramolecular ipso-sulfenylation in 4 is suggested to account for the formation of 12. The N-methyl derivative of 4 (i.e. 6) produced 9 and 10 after 9 h at 138 degrees C; in the presence of DMAP only 9 is produced after 18 h at 138 degrees C. The 3-acyl sulfoxide 8 cyclized to 17 during 6.5 h at 138 degrees C. These results indicate that although sulfoxides are useful for intramolecular sulfenylation reactions, care must be taken in assigning structures to the product(s) because rearrangement may take place during and/or after the initial cyclization reaction. Addition of a basic amine to the reaction mixture may prevent acid-catalyzed rearrangements of the initially formed products.