title: Intramolecular 1,3-dipolar cycloadditions of dihydroimidazolium ylides: synthesis of pyrrolo[1,2,3-de]quinoxalines and imidazo[1,2-a]indoles.
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.
authors.affiliation: Chemistry Department, The Open University, Walton Hall, Milton Keynes, UK.
authors.forename: Pedro M J
authors.initials: PM
authors.lastname: Lory
authors.forename: Raymond C F
authors.initials: RC
authors.lastname: Jones
authors.forename: James N
authors.initials: JN
authors.lastname: Iley
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Coles
authors.forename: Michael B
authors.initials: MB
authors.lastname: Hursthouse