Intramolecular 1,3-dipolar Cycloreversion - A Key Step in Novel Thermal Isomerisations of Cyclobutane Di-(carbomethoxy) Triazolines

Thermal isomerisation of the strained cyclobutane diester triazoline led to the formation of the product possessing a novel 1,2,7-triaza-[3.3.0]octa-2-ene ring system incorporated in a norbornane framework. Experimental evidence and quantum chemical calculations (DFT) have been used to support a postulated reaction mechanism involving as the first step, a rare example of intramolecular 1,3-dipolar cycloreversion. Subsequently, several steps involving 1,3-dipolar ring closure, [1,3]hydrogen-shifts and an intramolecular addition are postulated leading to the observed product of this deep-seated isomerization. The influence of changing substituents on the product outcome of this novel reaction cascade was also studied.