Abstract: The stereoselective reaction was studied of ampholenic aldehyde with singlet oxygen which was produced by hypocrellin A as a sensitizer with a high pressure sodium lamp. An "ene" reaction process was postulated in accordance with the analysis of the products composition and distribution. The predominant product, α-(2,2-dimethyl-3-methylene-4-hydroxy-l-cyclopentayl) acetaldehyde, was formed stereoselectively as singlet oxygen attacked C₄-position of double bond of cyclopentenyl on Ⅲ-face. The control stereoselective factors include molecular conformation, steric hindrance of substituted methyl groups and the axil orientation of allylic hydrogen. The eclipsed conformation between carbonyl double bond and single bond of C₁-cyclo-pentenyl ring as a key factor was demonstrated.

Key words: photooxidation, stereoselesivity, molecular conformation, singlet oxygen, campholenic aldehyde