5-AMINOTHIAZOLIUM SALTS AS POTENTIAL CYCLIC AZOMETHINE YLIDES - BASE-INDUCED INTRAMOLECULAR CYCLOADDITION REACTIONS OF N-(O-ALLYLPHENYL)-SUBSTITUTED AND N-(O-(ALLYLOXY)PHENYL)-SUBSTITUTED DERIVATIVES

A series of 5-aminothiazolium chlorides (1) bearing tethered benzene r ing on N-3, C-4 or the exocyclic nitrogen atom are prepared by a three -component methodology and subjected to basic treatment. The initially generated mesoionic thiazoles 2 undergo internal 1,3-dipolar cycloadd ition across the pendant olefin when the 2-allylphenyl group is connec ted to the endocyclic N-3. The reaction leads to the formation of orig inal N-bridged thiazoloquinolines as a mixture of two regioisomers 3,4 which are readily separated by chromatography. The structural assignm ents of the cycloadducts are deduced from their spectroscopic NMR prop erties and unequivocally established by an X-ray diffraction analysis. Intramolecular sequence also occurs using the 2-(allyloxy)phenyl subs tituent on the same position to give a single regioisomeric 1,4-methan othiazolobenzoxazepine (7). On the contrary. hydrolysis and ring-openi ng or oxidation and rearrangement of the mesoionic intermediates are t he exclusive base-promoted conversions of other thiazolium salts 1.