Generation and trapping of 4-methylene-5-(bromomethylene)-4,5-dihydrothiazole with dienophiles

4-(Bromomethyl)-5-(dibromomethyl)thiazole (1) was prepared in good yields by bromination of 4,5-dimethylthiazole with 3.3 equiv of NBS in the presence of AIBN. Treatment of 1 with sodium iodide led to a thiazole o-quinodimethane 2 which was trapped in situ with dienophiles such as N-phenylmaleimide, DMAD, or acrylate derivatives. From the latter, 6-substituted-4,5-dihydrobenzothiazoles 7 are selectively formed. Anthra[2,3-b]thiazole-4,5-diones 13-15 were obtained from naphthoquinones. With 2- or 3-bromonaphthoquinones (11 or 12), the cycloadditions were found highly regioselective. Structural assignment of the regioisomers was made by a 2D H-1-C-13 HMBC technique performed on the aromatized cycloadduct 15b. Calculations of HOMO and LUMO frontier orbital coefficients by the semiempirical PM3 method show that the regiochemistry observed in the cycloadditions of 2 toward acrylate dienophiles or naphthoquinones 11 and 12 did not agree with the corresponding values.