1,3-Dipolar Cycloadditions of New Mesoionic Compounds. Synthesis of 1H-Pyrrolo[1,2-c]thiazoles, Pyrrolizines and 5,6,7,8-Tetrahydroindolizines

We studied the 1,3-dipolar cycloaddition reactions between alkyne dipolarophiles and the new mesoionic compounds 2H,5H,7H-thiazolo[4,3-b]oxazol-2-one (12), 2H,5H,7H-pyrrolo[2,1-b]oxazol-2-one (13) and 2H,5H,7H-oxazolo[3,2-a]pyridin-2-one (14). These 1,3-dipoles were prepared in situ by means of cyclodehydration with acetic anhydride of the corresponding α-substituted 4-oxo-3-thiazolidine- (9), 2-oxo-1-pyrrolidine- (10) and 2-oxo-1-piperidineacetic acids (11). The cycloaddition reactions with alkyne dipolarophiles afforded single 1H-pyrrolo[1,2-c]thiazole, pyrrolizine and 5,6,7,8-tetrahydroindolizine derivatives, or a mixture of the two possible regioisomers, depending on whether symmetrical or unsymmetrical alkynes.