NEW ASPECTS OF THE 1,3-DIPOLAR CYCLOADDITION OF THIAZOLIUM N-IMINES WITH DIMETHYL ACETYLENEDICARBOXYLATE (DMAD)

The reaction of thiazolium N-imines with DMAD in methanol was studied. 4-Methylthiazolium N-imine (1a) reacted with DMAD to give three products, dimethyl [2-(3', 4'-bismethoxycarbonyl-1-pyrazolyl)-propenylthio] fumarate (4, 1 : 2 adduct)1b), dimethyl 2, 3-bis [2'-(3", 4"-bismethoxycarbonyl-1"-pyrazolyl)-propenylthio] succinate (5, 2 : 3 adduct), and 3-methyl-6-methoxycarbonyl-8-oxidothiazolo [3, 2-b] pyridazinium (6a). The structure of 6a was established by X-ray analysis and infrared (IR) spectral data. In a similar reaction with 4, 5-dimethylthiazolium N-imine (1b), 2, 3-dimethyl-6, 7-bismethoxycarbonyl-7, 7a-dihydro-4H-thiazolo [3, 2-b] pyrazole (2'b, 1 : 1 adduct) was isolated, and this compound, when heated in ethanol, underwent ring expansion to be converted into an 8-oxidothiazolo [3, 2-b] pyridazinium derivative (6b), an analog of 6a.