A New Synthesis of Thioflavanones from Thiosalicylic Acid

摘要

The thioflavanones (2-phenylthiochroman-4-ones),the thio analogues of flavanones,are an important class of heterocycles and serve as precursors of biologically active benzothiazepins and thiochroman-4-one 1,1-dioxides.The synthesis of thiochroman-4-ones has been generally accomplished by the intramolecular Friedel-Crafts acylation of 3-arylthiopropanoic acid derivatives with H2SO4 or Lewis acids such as SnCl4 and Bi(NTf2)3.However,this method is not suitable for the synthesis of thioflavanones with 2-substituted phenyl groups.The direct condensation of thiophenol with alpha,beta-unsaturated acids proceeds at high temperature to give thiochroman-4-ones in low to moderate yields with side products such as the corresponding disulfides and enol thioethers.The construction of thio-chroman-4-one rings is also performed by the acyl radical cyclization of 2-allylthiotriphenylhydrazides at high temperature in multiple steps from thiosalicylic acid.Alternatively,thioflavanones are synthesized by the catalytic hydrogen-ation of thioflavones,derived from the condensation of thiophenols and beta-keto esters or the intramolecular Wittig cyclization of salicilate thioesters,with H2/Pd-C,but the desired products are obtained in low yields with side products.