Sulfenylation Using Sulfoxides. Intramolecular Cyclization of 2- and 3-Acylpyrroles

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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 in p-xylene (bp 138 °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 °C; in the presence of DMAP only 9 is produced after 18 h at 138 °C. The 3-acyl sulfoxide 8 cyclized to 17 during 6.5 h at 138 °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. © 1994, American Chemical Society. All rights reserved.

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Journal of Organic Chemistry