Synthesis and Photochemistry of 5,5-Dimethyl-1H-pyrrol-2(5H)-one and of Some N-Substituted Derivatives

In two steps, 5,5-dimethyl-1H-pyrrol-2(5H)-one ( 3a ) was prepared from 5,5-dimethylpyrrolidine-2,4-dione ( = dimethyltetramic acid; 4 ) in 71% overall yield (Scheme 1) and further converted to N-substituted derivatives 3b–f via acylation, alkylation, or methoxycarbonylation of its anion (Scheme 2). The substituents on the N-atom exert a strong influence on the photochemical reactivity (2 + 2] photocycloaddition to 2,3-dimethylbut-2-ene, photocyclodimerisation, photoreduction) of these aza-enones 3 (Scheme 3). In general, N-alkyl compounds react much slower and with less efficiency than either the (N-unsubstituted) title compound 3a or its N-acetyl and N-(methoxycarbonyl) derivatives 3e and 3f , respectively. These compounds behave similarly to the corresponding lactone, 5,5-dimethyl-2(5H)-furanone, studied previously.