Abstract On irradiation (λ= 366 nm), the 4-thia-2-cyclopentenone 3a behaves in complete analogy to the oxa-enone 3c undergoing regio-and stereospecific cyclodimerization, regiospecific cycloaddition with 2-methylpropene and cycloaddition with 2, 3-dimethyl-2- butene to afford cyclobutane derivatives. In contrast, the 4-aza-2-cyclopentenone 3b does not undergo the above-mentioned reactions but only slow photodecomposition.
