Brønsted Acid-Mediated Annulation of α-Oxo Ketene Dithioacetals with Pyrroles: Efficient Synthesis of Structurally Diverse Cyclopenta[b]pyrroles.

Abstract

Brønsted acid-mediated annulation of internal olefins α-oxo ketene dithioacetals to pyrroles was efficiently achieved to afford cyclopenta[b]pyrroles. A pair of Brønsted acids with acid strengths, that is, trifluoroacetic acid, and para-toluenesulfonic acid hydrate, were applied to promote the annulation reactions. The resultant products were readily oxidized to sulfones by meta-chloroperoxybenzoic acid. Subsequent treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene gave desulfurized terminal olefins or [2+2] cycloaddition products from the desulfurized olefin intermediates. The present protocol provides facile access to structurally diverse cyclopenta[b]pyrrole derivatives under mild conditions.