APPARENT BREAKING OF THE RULES OF WOODWAR D-HOFFMANN - MECHANISM AND STEREOCHEMISTRY OF PHOTOCYCLIZATION REACTION OF N-ARYLENAMINONES

Photocyclization of N-arylenaminones obtained from cyclohexane-1,3-dio ne led stereoselectively to trans-hexahydrocarbazol-4-ones 2 which are the kinetic product of a conrotatory electrocyclization reaction, in agreement with Woodward-Hoffmann rules. On the other hand, photocycliz ation of the N-arylenaminone derived from cyclopentane-1,3-dione affor ded cis-cyclopent[b]indol-3-one 5 in apparent contradiction with Woodw ard-Hoffmann rules. A theoritical study of the reaction mechanism usin g AM1 calculations has shown the photoelectrocyclization mechanism to be favoured compared with other possible mechanisms as photochemical ( [pi(s)(2) + pi(s)(2)] photocycloaddition) or thermal ones. Stereoselec tive formation of the cis compound could then be explained by a thermo dynamic control of the dark reaction occuring after the photocyclizati on process which respects the orbital symmetry rules.