Diels–Alder reactions of chromone-3-carboxaldehydes with ortho-benzoquinodimethane. New synthesis of benzo[b]xanthones

An efficient new route to the benzo[b]xanthone system has been developed and applied to the synthesis of several new derivatives. The cycloaddition reactions of chromone-3-carboxaldehydes , reacting as dienophiles, with ortho-benzoquinodimethane gave a diastereomeric mixture of cycloadducts and . The formation of these compounds results from the Diels–Alder reactions of and followed by the in situ deformylation. The oxidation of adducts and with dimethyl sulfoxide in the presence of iodine gave the novel benzo[b]xanthones in good yields.