A Photochemical Ring Contraction of an Imino Lactam

Article abstract of DOI:10.1021/jo00137a022

The photochemical rearrangement of three tetrahydro-2-pyrazinones in aqueous medium to imidazol-5-ones is described. 1,2,5,6-Tetrahydro-3,5,5-trimethyl-2-pyrazinone (4) gives 1,2,2,4-tetramethyl-3-imidazolin-5-one (7), 1,2,5,6-tetrahydro-3,6,6-trimethyl-2-pyrazinone (5) gives 4-methyl-5-imidazolidinone (9), and 1,2,5,6-tetrahydro-1,3,5,5-tetramethyl-2-pyrazinone (6) gives 1,4-dimethyl-5-imidazolidinone (8).A mechanism involving enediimine and isoimidazole intermediates is proposed in analogy with the mechanism for rearrangement of 5,6-dihydropyrazines to imidazoles.