Citraconic acid and its anhydride-based hetero-Diels–Alder reactions in the synthesis of new thiopyrano[2,3-d][1,3]thiazole derivatives

Novel rel-(5R,5aR,11bS)-5-methyl-2,6-dioxo-3,5a,6,11b-tetrahydro-2H,5H-chromeno[4΄,3΄:4,5]thiopyrano[2,3-d][1,3]thiazole-5-carboxylic acids were synthesized in 63–72% yields via tandem acylation-hetero-Diels–Alder reaction of 5-(2-hydroxybenzylidene)-4-thioxo-2-thiazolidinones with citraconic acid and its anhydride. The regio- and stereochemistry of the process was confirmed by NMR spectral data and a single-crystal X-ray diffraction analysis.