Stereoselective intramolecular hetero Diels–Alder reactions of 1-oxa-1,3-butadienes: a novel approach for the synthesis of complex annulated uracils

The intramolecular hetero Diels–Alder reactions of 1-oxa-1,3-butadienes 4, obtained from salicylaldehyde 1 via O-allylation followed by Knoevenagel condensation with barbituric acids 3 in the presence of hydrochloric acid as catalyst, affords the tetracyclic uracil derivatives 5 and 6 in a stereoselective manner and high overall yields.