Cyclobutanol: fragmentation ratios for the singlet and triplet excited states in the type II photochemistry of some α-alkylated cyclohexanones

Abstract

In the Type II photochemistry of the conformationally fixed cyclohexanones, cis-4-t-butyl-2-n-propylcyclohexanone (1), 17β-acetoxy-2α-n-propyl-5α-oestran-3-one (6), 2α-n-propyl-5α-cholestan-3-one (9), and 17α-acetoxy-4α-n-propyl-5α-oestran-3-one (12), there is in each case a higher cyclobutanol : fragmentation ratio than that usually found for open-chain ketones. The effect, which is probably a consequence of the fragmenting bond being held at an angle unfavourable for continuous overlap, is more marked in the triplet state reaction, where the ratio for (1) is 73 : 27. In both states the effect is in fact quite small, probably because the reactions are very exothermic. In the case of the 4α-n-propyl steroid (12), the cyclobutanol : fragmentation ratio is larger for both the singlet and the triplet states than in the other cases, as a result of the presence of the ring B residue adjacent to the propyl group. Type I products from the steroidal ketones were not detected, and the Type I product from cis-4-t-butyl-2-n-propylcyclohexanone (1) is very largely of the ester and not the aldehyde kind. 2-Cyclohexylcyclohexanone (14) gives a high cyclobutanol : fragmentation ratio in both the triplet (ca. 80 : 20) and singlet states (64 : 36).