Mass Spectra of New Heterocycles: XXXII. Electron Ionization Fragmentation of 5,6-Dihydropyridin-2(1H)-ones Derived from 2,3-Dihydropyridines

The electron ionization fragmentation of previously unavailable 3,6-substituted 5,6-dihydropyridin-2(1H)-ones synthesized from allenic and acetylenic carbanions, isothiocyanates, and methyl iodide has been studied for the first time. All the studied compounds form the molecular ion M+•, the fragmentation pathway of which depends on the substituents in both position 3 and position 6. The fragmentation of the molecular ion of 3-methoxy-5,6-dihydropyridin-2(1H)-one is initiated at the radical site, leading to the loss of a radical from position 6 and the localization of the positive charge on the endocyclic nitrogen atom. For 3-phenyl-5,6-dihydropyridin-2(1H)-one, the primary fragmentation pathway involves the formation of an odd-electron ion with m/z 144 (Irel 100%) via the loss of an HNCO molecule from the M+• ion or of an MeCH=NH molecule from the isomerized molecular ion M1+•. The fragmentation of the molecular ion of 3-methyl-1-azaspiro[5.5]undec-3-en-2-one is characterized by the formation of the radical ions [M – HNCO]+• (Irel 100%) and [M – C4H8]+• (Irel 92%). Experimental evidence has been obtained for the formation of the precursors of 5,6-dihydropyridin-2(1H)-ones―2-(methylsulfanyl)-1,2,5,6-tetrahydropyridin-2-ols, which undergo heteroring cleavage under electron ionization.