Intramolecular 1,3-dipolar cycloadditions of dihydroimidazolium ylides: synthesis of pyrrolo[1,2,3-de]quinoxalines and imidazo[1,2-a]indoles

Lory, Pedro M.J.; Jones, Raymond C.F.; Iley, James N.; Coles, Simon J. and Hursthouse, Michael B. (2006). Intramolecular 1,3-dipolar cycloadditions of dihydroimidazolium ylides: synthesis of pyrrolo[1,2,3-de]quinoxalines and imidazo[1,2-a]indoles. Organic and Biomolecular Chemistry, 4 pp. 3155–3165.

DOI: https://doi.org/10.1039/b605458g

Abstract

N-Alkylation of 4,5-dihydroimidazoles with alkene-containing bromomethyl ketones and treatment of
the so-formed 4,5-dihydroimidazolium ions with DBU gives rise to an intramolecular 1,3-dipolar
cycloaddition reaction that affords (via a reaction cascade involving eliminative ring-opening,
recyclisation and prototropic tautomerism) unexpected hexahydropyrrolo[1,2,3-de]quinoxaline
products. Steric bulk in both the dihydroimidazole and the dipolarophile allows isolation of an
imidazo[1,2-a]indole, the initial product of cycloaddition. When the bromomethyl ketone contains no
other functionality, or when cycloaddition is inhibited due to steric constraints, the dihydroimidazolium
ion undergoes ring-opening hydrolysis followed by recyclization of the exposed amino ketone to afford
either 3-alkyl-1-formylpiperazine-2-ones or 3-aryl-1-formyl-1,4,5,6-tetrahydropyrazines.

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