Mass Spectra of New Heterocycles. XXXI. Electron Impact Fragmentation of Functionalized 2,3-Dihydropyridines

Fragmentation of previously unknown 5,6-di-, 2,5,6-tri-, and 2,2,5,6-tetrasubstituted 2,3-dihydro­pyridines synthesized from allenic and acetylenic carbanions, isothiocyanates, and alkylating agents under electron ionization (70 eV) has been studied for the first time. Analysis of the electron impact mass spectra of the synthesized compounds has revealed their key fragmentation patterns. All compounds form detectable molecular ions whose stability and decomposition pathways are largely determined by the nature and position of substituents in the heterocycle. 2-R2-5-methoxy-6-(methylsulfanyl)-2,3-dihydropyridines (2-R1 = H, 2-R2 ≠ OMe) typically produce [M – Me]+, [M – SH]+, and [M – R2]+ ions. When a bulky substituent is present at position 2 or 5 (2-R2 ≠ Me or 5-OAlk > OMe), elimination of the Alk radical ([M – Alk]+) and alkene ([M – CnH2n]+·) become competing processes. The main fragmentation pathway of the molecular ion of 5-alkoxy-2-methoxy 6-(methylsulfanyl)-2,3-dihydropyridines [Alk = Et, Bu, EtOCH(Me)] involves expulsion of methanol molecule from the 2-position. In the fragmentation of 2,5-dimethoxy-6-(methylsulfanyl)-2,3-dihydropyridine, a competition between elimination of Me and OMe radicals from the molecular ion is observed instead of the formation of [M – MeOH]+· ion. The molecular ions of compounds with an acetal substituent at C5 generally decompose according to mechanisms typical of acetals, including rearrangements similar to the McLafferty rearrangement. Decomposition of the heterocycle through expulsion of MeSCN molecule is observed for 2,2-dimethyl-5,6-bis(methylsulfanyl)-2,3-dihydropyridine. Fragmentation of the molecular ion of 2-unsubstituted 5,6-bis(methylsulfanyl)-2,3-dihydropyridine leads to the formation of [M – Me]+ and [M – SMe]+. No decomposition products of the dihydropyridine ring, including [M – MeSCN]+· ion, were identified in the mass spectrum of this compound. 5-Phenyl- and 5-(1-methyl-1H-pyrrol-2-yl)-6-(methyl­sulfanyl)-2,3-dihydropyridines under electron impact give rise to the most stable molecular ions (Irel 93–100%) which then decompose along 4 (2-R2 = Me) and 7 pathways (2-R2 = CH2=CHOCH2). Aromatization of 2-unsubstituted and 2-monosubstituted 2,3-dihydropyridinov at elevated temperature (during chromato­graphic sample injection) and/or under electron ionization has been revealed and confirmed experimentally. The aromatization process involves elimination of hydrogen or methanol molecule (from positions 2 and 5), as well as of molecules derived from the C2-substituent, with the formation of 2,3-di-, 2,6-di-, and 2,3,6-tri­substituted pyridines.