Formal fusion of a pyrrole ring onto 2-pyridyl and 2-pyrimidyl cations: one-step gas-phase synthesis of indolizine and its derivatives |
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Authors: | Sparrapan R Mendes M A Carvalho M Eberlin M N |
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Institution: | Institute of Chemistry, State University of Campinas, SP, Brazil. |
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Abstract: | Two ortho-hetarynium ions, the 2-pyridyl and 2-pyrimidyl cations, react promptly with 1,3-dienes in the gas phase by annulation, formally by fusion, onto the ions of a pyrrole ring. This novel reaction proceeds through an initial polar 4 + 2+] cycloaddition across the Ctriple bond]N+ bond, followed by fast ring opening, a 1,4-H] shift, and finally a recyclization that results in a contraction of a six- to a five-membered ring and dissociation by the loss of a methyl radical. For the 2-pyridyl cation, this reaction yields ionized indolizines (pyrrolo1,2-a]pyridines), while for the 2-pyrimidyl cation, it gives ionized pyrrolo1,2-a]pyrimidines. The annulation reaction, performed in the rf-only collision quadrupole of a pentaquadrupole (QqQqQ) mass spectrometer, occurs readily with both 1,3-butadiene and isoprene, and is thermodynamically and kinetically favored as predicted by ab initio calculations. Ortho-hetarynium ions and 1,3-dienes provide, therefore, the two building blocks for the efficient one-step gas-phase synthesis of ionized bicyclic pyrrolo1,2-a]pyridine (indolizine) and pyrrolo1,2-a]pyrimidine, as well as their analogues and derivatives. |
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