Abstract
We present a modular, synthetic entry to polysubstituted pyrroles employing readily available 2,5-dihydrothiophenes. Ring-opening of the heterocycle provides access to a panel of 1,3-dienes which undergo pyrrole formation in the presence of inexpensive chloramine-T trihydrate. The transformation is conducted in an open flask and proceeds at ambient temperatures (23 °C) in nondry solvents. A careful adjustment of the electronics and sterics of the 1,3-diene precursor allows for the isolation of key intermediates. DFT studies identified a reaction mechanism that features a 6π-electrocyclization of a sulfilimine intermediate followed by spontaneous ring-contraction to reveal the pyrrole skeleton.
Highlights
Ring-opening of the heterocycle provides access to a panel of 1,3-dienes which undergo pyrrole formation in the presence of inexpensive chloramine-T trihydrate
With the establishment of 1,3-dipoles by Huisgen, cycloaddition reactions became available as a robust method to synthesize a variety of five-membered heterocycles.[7]
During our studies to convert readily available 2,5dihydrothiophenes 413 into tetrasubstituted furans 6, we found an unprecedented 6π-electrocyclic ring-opening as part of the reaction mechanism (Scheme 1B).[14]
Summary
Ring-opening of the heterocycle provides access to a panel of 1,3-dienes which undergo pyrrole formation in the presence of inexpensive chloramine-T trihydrate. Employing Sharpless’ conditions for the synthesis of N-tosyl sulfilimines (chloramine-T trihydrate, acetonitrile, 23 °C),[17] we observed rapid conversion of 1,3-diene 5a to pyrrole 9a in 53% yield (Scheme 2, entry 1).
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