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.

(1 7.111.92)In two steps, 5,Sdimethyl-lH-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 viu acylation, alkylation, or methoxycarbonylation of its anion (Scheme 2). The substitutents on the N-atom exert a strong influence on the photochemical reactivity ([2 + 21 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( SH)-furanone, studied previously. Introduction. ~ In the course of our investigations on the photochemistry of cyclic a$-unsaturated carbonyl compounds [l][2], we had emphasized on the synthesis of several heteropentacyclic enones such as 1 and 2 with a geminal dimethyl group adjacent to the heteroatom to prevent tautomerization to a 2-or 3-hydroxy-substituted heterocycle. We had shown that both 3(2H)-and 2(5H)-furanones l a and 2a and 3(2H)-thiophenone l b exhibit photochemistry typical for cyclopent-2-enone itself [3-51 and that 2(5H)-thiophenones, e.g. 2b, behave differently in undergoing light-induced ring-opening reactions in hydroylic solvents [6] [7]. Finally, while lH-pyrro1-3(2H)-one l c is photoinert [8], the corresponding N-(ethoxycarbonyl) derivative Id again exhibits typical enone behavior [9]. We now report on a new -large-scale -synthesis of the 'missing link' in this