COMPETITION AMONG 1,2-ACYL 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 (prepar ed from 3-iodocyclopent-2-enone 12 and the cuprate obtained from 5-iod opent-1-yne 10a) gave mainly the hydrogen-addition product of the prim ary 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 a cetone as solvent, formation of 18a remained the main route owing; pre sumably, to strain release on hydrogen addition in the primary photocy cloadduct 15. In CD3CN as solvent, photoconversion of 13 was slower, d euterium 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 ameliora ted even in the typically reduction-hindering solvent hexadeuterioacet one. UV irradiation of the side-chain homologous 3-(hex-5'-ynyl)cyclop ent-2-enone 14 in MeCN led to mainly the products of [2 + 2] photocycl oaddition (tricyclo[5.4.0.0(3,7)]undec-10-en-7-one 20) and 1,3-acyl-sh ift (tricyclo[4.3.2.0(1,6)]undec-10-en-7-one 21), albeit in modest yie lds, 12 and 24%, respectively.