A STUDY OF SOLVENT EFFECT ON PHOTOCHEMICALLY INDUCED REACTIONS BETWEEN PYRIDINEDICARBONITRILES AND ALKENES - AN EASY APPROACH TO THE SYNTHESIS OF CYCLOPENTA[B OR C]PYRIDINES

Photochemically induced reactions of pyridinedicarbonitriles and alken es show an interesting dependence on solvent polarity. In non-polar so lvents ipso-substitution of the cyano groups in positions alpha or gam ma to the heterocyclic nitrogen occurs to a larger extent, while in po lar solvents the reaction provides a path to the formation of a new ri ng between the carbon atom of one of the cyano groups and a ring posit ion, forming a cyclopenta[b or c]pyridine derivative (pyrindine). Stud ies on the multiplicity of the excited state controlling the reaction show that the singlet state is involved in the ipso-substitution, whil e the triplet state controls the formation of the pyrindine. An explan ation for the solvent effect is given in terms of shift of the excited states with the solvent used. Theoretical calculations justify the po sition of the cyclisation, although no correlation was found for the r egioisomers ratio. This reaction represents an effective entry to the biologically interesting pyrindine systems.