Photochemical ring contraction of 1-aryl-1,4-dihydropyrazine to 1-aryl-1H-imidazole

The photochemical properties of 1-aryl-1,4-dihydropyrazines, including their UV–vis absorption, photostability and photoreaction, are investigated. The photostability of the 1-aryl-1,4-dihydropyrazines is studied in conventional solvents, and the results demonstrated that the 1-aryl-1,4-dihydropyrazines are unstable under irradiation with UV light. The 1-aryl-1,4-dihydropyrazines undergo photochemical ring contractions to 1-aryl-1H-imidazoles, as determined by 1H NMR, 13C NMR, HRMS, and single crystal X-ray diffraction analyses. The photochemical ring contraction is proposed to occur through a 6 π-electron cyclisation and a [1 + 2] cycloreversion.