A commercial phosphorus-based reagent (P-(NMe2)3) mediates umpolung alkylation of methyl aroylformates with benzylic and allylic bromides, leading to either Barbier-type addition or ylide-free olefination products upon workup. The reaction sequence is initiated by a two-electron redox addition of the tricoordinate phosphorus reagent with an α-keto ester compound (Kukhtin–Ramirez addition). A mechanistic rationale is offered for the chemoselectivity upon which the success of this nonmetal mediated C–C bond forming strategy is based.
A reductive homocondensation of E-benzylidenepyruvate esters mediated by P(NMe2)3 is described. The transformation, initiated by the Kukhtin-Ramirez addition of the phosphorus reagent to the vinyl-substituted α-dicarbonyl substrate, proceeds via a resonance delocalized oxyphosphonium dienolate intermediate to provide access to diverse oxygenated heterocycles as a function of the substituent.
Owing to the unique chemical structure and photophysical properties of azulene, azulene derivatives show great promise for a wide range of prospective technical applications, such as molecular switching, sensors, solar cells, and biological activities. In addition to functionalization of the existed azulene cores, direct construction of azulenes via condensation with odd‐membered carbocycles such as tropolone, 2‐oxazulanone, cyclopentadiene, and 6‐aminofulvene is another classical method for obtaining azulene derivatives. Although alkynes are widely explored for construction of benzene rings, several unconventional cyclization reactions with alkynes produce azulene, pseudoazulene, or azulenone skeletons. This minireview summarizes these interesting and practical cyclization with a classification based on the function of alkyne in the azulene formation process, aiming at capturing the attention of the scientific community to make efforts in the development of such nonalternant cyclization reactions of alkynes.
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