Knoevenagel condensation of aromatic bisulfite adducts with 2,4-thiazolidinedione in the presence of Lewis acid catalysts

Abstract

Several commercial Lewis acids, specifically TiCl4, were able to promote the condensation reaction between aromatic bisulfite adducts and 2,4-thiazolidinedione to produce 5-arylidene derivatives. The product distribution was not affected by the equivalents of TiCl4 used, but was dependent on temperature and the nature of the solvent medium. In all cases, the reaction required activation of the aromatic bisulfite adducts by Lewis acids, followed by the loss of SO2 and subsequent regeneration of the parent aldehyde. The in situ formed aldehyde ultimately underwent acid-catalyzed carbon–carbon bond formation to give 5-arylidene derivatives. The reaction with TiCl4 afforded the products from aromatic and aliphatic bisulfite adducts in moderate to good yields. When hydrated Lewis acids were used, in addition to 5-arylidenes, the in situ formed aldehyde underwent a disproportionation reaction to give a carboxylic acid as a by-product.