Thallium(III) nitrate oxidation of chromens: cis-dihydroxylation and dialkoxylation

Abstract

The oxidation of the chromens (6) and (10) with thallium(III) nitrate is described. In the case of the 2-methyl-2H-chromen (6) oxidation in aqueous dioxan gave a single cis-1,2-diol (7), unexpectedly arising from initial thalliation syn to the 2-methyl. The product stereochemistry was determined by single-crystal X-ray analysis, using direct methods. Rationalisation is proposed. With the 2,2-dimethylchromen (10), oxidation in methanol gave only the cis-dimethoxychroman (11); similar reactions were observed in other hydroxylic solvents. In ethane- 1,2-diol the fused dioxan (18) was obtained, but in poor yield.