Competition among 1,2- and 1,3-acyl shifts, and reduction reactions, in the UV irradiation of cyclopent-2-enones bearing a C-3 terminal-alkyne chain

UV irradiation in MeCN of 3-(pent-4′-ynyl)cyclopent-2-enone 13 (prepared from 3-iodocyclopent-2-enone 12 and the cuprate obtained from 5-iodopent-1-yne 10a) gave mainly the hydrogen-addition product of the primary photocycloadduct 15, tricyclo[5.3.0.03,7]decan-4-one 18a (56%), besides minor 16 (22%), 19 (12%), 17 (8%) and traces of 15 (2%). In acetone as solvent, formation of 18a remained the main route owing, presumably, to strain release on hydrogen addition in the primary photocycloadduct 15. In CD 3CN as solvent, photoconversion of 13 was slower, deuterium was incorporated at the tertiary bridgeheads in the reduction product 18b, and the product balance was slightly in favour of the 1,2-acyl-shift product 16. This situation was not substantially ameliorated even in the typically reduction-hindering solvent hexadeuterioacetone. UV irradiation of the side-chain homologous 3-(hex-5′-ynyl)cyclopent-2-enone 14 in MeCN led to mainly the products of [2 + 2] photocycloaddition (tricyclo[5.4.0.0 3,7]undec-10-en-7-one 20) and 1,3-acyl-shift (tricyclo[4.3.2.0 1,6]undec-10-en-7-one 21), albeit in modest yields, 12 and 24%, respectively.