title: Formal [2 + 2 + 2] cycloaddition strategy based on an intramolecular propargylic ene reaction/Diels-Alder cycloaddition cascade.
abstract: A formal, metal-free, [2 + 2 + 2] cycloaddition strategy is described based on a cascade of two pericyclic processes. The first step involves an intramolecular propargylic ene reaction of a 1,6-diyne to generate a vinylallene, which then reacts in an inter- or intramolecular Diels-Alder reaction with an alkenyl or alkynyl dienophile. Reactions involving unsymmetrical alkenyl and alkynyl dienophiles proceed with good to excellent regioselectivity, and the diastereoselectivity in the Diels-Alder step is also high, with endo cycloadducts produced as the exclusive products of the reaction. In the case of alkynyl dienophiles, [4 + 2] cycloaddition initially generates an isotoluene-type intermediate that isomerizes to the isolated aromatic product upon exposure to a catalytic amount of DBU at room temperature. The mechanism of several earlier fully intramolecular related transformations have been shown to involve an analogous process rather than the diradical-mediated pathways proposed previously.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Julia M
authors.initials: JM
authors.lastname: Robinson
authors.forename: Takeo
authors.initials: T
authors.lastname: Sakai
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Okano
authors.forename: Takafumi
authors.initials: T
authors.lastname: Kitawaki
authors.forename: Rick L
authors.initials: RL
authors.lastname: Danheiser
mesh_terms: D000475:Alkenes / Q000737:chemistry; D000480:Alkynes / Q000737:chemistry*; D003500:Cyclization; D013237:Stereoisomerism; D013379:Substrate Specificity
chemical_list: D000475:Alkenes; D000480:Alkynes