On ultraviolet irradiation in toluene, cyclopent-2-en-one (1a) and 3-methylcyclopent-2-en-one (1b) undergo dimerisation (→ 3a, b, 4b), whereas 3-t-butyl-cyclopent-2-en-one (1e) and the bicyclo [3.2.1]octenone 2a only react with the solvent (→ 5e–7e, 8–10). The 3-ethyl-(1c) and 3-isopropyl-cyclopent-2-en-ones (1d) afford by dimerisation products 3c, d, 4c and by reaction with the solvent 5c, d, 6c, d, 7d. The β-methoxy-enones 1f and 2b are unreactive under comparable irradiation conditions. The head-to-head cyclobutane dimer 3b (HH) by separate irradiation at < 3400 Å in toluene, is reconverted to 1b. By similar irradiation each of the head-to-tail dimers 3b (anti- and syn-HT) and also the unsaturated dimer 4b give a new isomer of unknown structure besides small amounts of monomer 1b and relatively large amounts of insoluble material.

The photoreactions of 1b-e can be quenched by naphthalene. Stern-Volmer plots for the quenching of the dimers of 1b, 1c, and 1d, and of 3-t-butylcyclopentanone (5e) and the dihydro dimer 7e are linear and within each experiment, the slopes satisfactorily coincide, whereas the slope for the α-benzyl ketone 6c is distinctly greater than that for the dimers 3c. A similar differentiation, although less pronounced, is found between the benzyl ketones 6d and 6e and the respective accompanying products.