Synthesis and Reactivity of 5,8-Dihydroxythioflavanone Derivatives

The synthesis of substituted chalcones, thioflavanones, and thioflavones hydroxylated in both A and B rings is described. Acetoquinone (1) was transformed into 2,5-dihydroxy-6-(p-methoxybenzylmercapto)acetophenone (2) and subsequently into its 2,5-dimethoxy (3) and 2,5-dibenzyloxy (4) derivatives. Compounds 3 and 4 were condensed with suitable benzaldehydes to give chalcones 5−10. The thiol group of the chalcones was deprotected by a cleavage of the p-methoxybenzyl protecting group using a new, mild method (silver nitrate in boiling ethanol, 2 h), and the products were cyclized to thioflavanones (15−20). Dehydrogenation of the thioflavanones gave related thioflavones (25−27). Deprotection of methoxy and benzyloxy groups is also described.