STEREOSELECTIVITY IN PHOTOCYCLOADDITION REACTIONS .5. PHOTOREACTION OF 2-CYCLOPENTEN-1-ONE WITH ENOL ETHERS

The regio- and stereoselectivity of [2 + 2] photocycloaddition reactions between 2-cyclopenten-1-one (1) and the electron-rich alkenes: ethyl, isopropyl, and phenyl vinyl ether (2-4) as well as 2,3-dihydrofuran (10) were examined. In all cases the annulated cyclobutene derivatives 5-7 and 11 were isolated in yields between 70 and 99%. The regioisomeric HT/HH products were formed in a ratio of 3:1 to 4:1. For the determination of the product stereochemistry, INADEQUATE and H-1, H-1-decoupling experiments were performed. Computer-assisted investigations on the molecular geometry and H-1, H-1-coupling constants were performed by using semiempirical methods (AM1, FINITE). The HT regioisomers were formed with moderate endo diastereoselectivty, whereas in the case of the HH regioisomers both diastereomers (exo/endo) were formed in equal amounts. Photoreaction with 2,3-dihydrofuran (10) led to almost exclusive formation of exo-diastereomeric HH and HT regioisomers 11. The observed stereoselectivity is rationalized by the optimal geometry of the intermediate triplet 1,4-biradicals during intersystem crossing.