Heterocycles by cycloaddition. Part 6. Cycloaddition–double fragmentation reactions of mesoionic compounds with an oxabicyclo[2.2.1]heptadiene: synthesis of five- and six-membered heterocycles

Abstract

The reaction of dimethyl 7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate (1) with some five-membered mesoionic compounds [(2) and (10)] gave five-membered aromatic heterocycles [(6)–(9) and (12)] by thermal double fragmentation of the initially formed cycloadducts [(3), (4), and (11)]. The periselectivity of the cyclo-addition varies according to the mesoionic compound employed. With a dithiolone (10a) and a thiazolone (10b), the cycloadducts (11) were isolable, and the corresponding S-oxides (13) were prepared. Pyrolysis of the S-oxides gave a thiopyranone (14a) and a pyridone (14c).