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

Abstract

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 hydrogenaddition product of the primary photocycloadduct 15, tricyclo[5.3.0.0^3,7]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 CD₃CN 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 off 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.