Synthetic and computational investigations into reaction pathways towards complex heterocyclic ring systems

Abstract

Tetrazocines are eight-membered rings possessing four nitrogen atoms. The development of new synthetic routes to novel heterocyclic systems has remained a critical branch of synthetic organic chemistry due to potential applications in biomimetics and pharmaceuticals, such as dyes and explosives. 1,2,3,5-tetrazocines were originally proposed as an intermediate in the formation of 1,3a,6,6atetrahydroimidazo[4,5-c]pyrazole (a promising anti-fungal agent) from the irradiation of 2,3a,4,6a-tetraphenyl-3,3a,4,5,6,6a hexahydropyrrolo-[2,3-d]-1,2,3-triazole. One of our main aims was to synthesise novel 1,2,3,5-tetrazocines that possessed three phenyl and one methyl group(s) attached to the eight-membered ring, as previously synthesised 1,2,3,5-tetrazocines are of the tetra-phenyl variety. Adopting the established synthetic route in the pursuit of novel 1,2,3,5-tetrazocines proved difficult however, as the asymmetric nature of the 1,2,3,5-tetrazocine precursors (dihydrazones) resulted in altered reactivity as compared with their symmetric equivalents, this finding was rationalised using distributed multipole analysis. Ab initio calculations (B3LYP/6-31G(d)) indicate that for an underivatised 1,2,3,5- tetrazocine, the planar (aromatic) structure is the most stable, though this is not observed experimentally, presumably due to steric repulsions between the large aromatic substituents often used to stabilise these molecules. Excluding the planar geometry, we found that there is a significant thermodynamic stabilisation of 1,2,3,5- tetrazocine over the 1,3a,6,6a-tetrahydroimidazo[4,5-c]pyrazole, providing a novel route to heterocyclic systems