Reaction of 2,5-bis-trifluoromethyl-1,3,4-oxadiazole with 7-oxanorbornenes revisited: Experimental and quantum-chemical study of reaction stereoselectivity

The stereochemical outcome of reaction of 2,5-bis-trifluoromethyl-1,3,4-oxadiazole with 7-oxanorbornenes under various conditions was investigated. For the first time, microwave irradiation in carrying this type of reactions was used, resulting in comparable yields in significantly shorter reaction times. Regardless on substrate, in all reactions mixtures of the two isomeric O3-[3]polynorbornanes, bent and linear were obtained, with slight preference for bent structure. In some cases, retro Diels–Alder fragmentation was observed resulting in formation of isobenzofuran species. Reaction mechanism was also studied computationally (RHF/6-31G* method), and the origin of stereoselectivity explained by repulsive lone pair interactions between oxygen bridges in the transition state of the 1,3-dipolar addition.