Metal- and Catalyst-Free One-Pot Cascade Coupling of α-Enolic Dithioesters with in situ Generated 4-Chloro-3-formylcoumarin: Access to Thioxothiopyrano[3,2-c]chromen-5(2H)-ones

Abstract

An efficient and viable one-pot protocol for the synthesis of a specific class of 2-thioxothiopyrano[3,2-c] chromen-5(2H)-ones has been devised by the cross-coupling of 4-hydroxycoumarin and α-enolic dithioesters under metal- and additive-free conditions in open air. The reaction proceeds via in situ generation of 4-chloro-3-formylcoumarin followed by consecutive Michael-type addition/intramolecular cyclization/ elimination cascade, enabling the creation of thiopyran-2-thione ring over coumarin framework through successive formation of C−C and C−S bonds. Remarkably, the benign conditions, atom-economy, and quantifying forbearance of a wide horizon of functional groups are added characteristics to this strategy. (Figure presented.). © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim