The photo-Nazarov cyclisation of 1-cyclohexenylphenyl-methanone revisited II: Reaction between primary and secondary key intermediates

Trans-1-cyclohexenyl -phenyl-methanone (2) and enol 4, both key intermediat es in the title reaction, react with each other in a Michael-type addition to form predominantly enol 10. This enol, kinetically stable but tao reacti ve to be isolated, reveals its presence in the irradiated solutions by form ation of the isomeric ketone 11 on acid catalysis, and by formation of the oxidation product 9 on exposure to atmospheric oxygen. In the absence of ac id, formation of 10 competes significantly with the title reaction of cis-1 -cyclohexenyl-phenyl-methanone (1). In a secondary photoreaction of 10, 1,6 -hydrogen abstraction by the excited carbonyl group and cyclisation afford 13 and 14. Enols 4, 10, and 13, in striking contrast to enol ethers and to thermodynamically stable enols, are unstable towards atmospheric oxygen. Th us, 4 antoxidises to form five compounds(Ox-1 through Ox-5), 10 to form 9, and 13 to form 15. (C) 2001 Elsevier Science B.V. All rights reserved.