Treatment of o-nitrostyrenes with aqueous TiCl3 solution at room temperature afforded indoles through a formal reductive C(sp(2) )-H amination process. A range of functions such as halides (Cl, Br), carbonyl (ester, carbamate), cyano, hydroxy, and amino groups were tolerated. From β,β-disubstituted o-nitrostyrenes, 2,3-disubstituted indoles were formed by a domino reduction/cyclization/migration process. Mild conditions, simple experimental procedure, ready accessibility of the starting materials and good to excellent yields characterize the present transformation. The methodology was used as a key step in a concise synthesis of rizatriptan and a formal total synthesis of aspidospermidine.
The reaction of allyl ethyl carbonates with isocyanides in the presence of a catalytic amount of Pd(OAc) provided ketenimines through β-hydride elimination of the allyl imidoylpalladium intermediates. The insertion of the isocyanide into the π-allyl Pd complex proceeded via an unusual η -allyl Pd species. The resulting ketenimines were hydrolyzed to β,γ-unsaturated carboxamides during purification by flash column chromatography on silica gel or converted in situ into 1,5-disubstituted tetrazoles by [3+2] cycloaddition with hydrazoic acid or trimethylsilyl azide.
Treatment of o-nitrostyrenes with aqueous TiCl 3 solution at room temperature afforded indoles through af ormal reductive C(sp 2 )-H amination process.Arange of functions such as halides (Cl, Br), carbonyl (ester,carbamate), cyano,h ydroxy,a nd amino groups were tolerated. From b,bdisubstituted o-nitrostyrenes,2 ,3-disubstituted indoles were formed by ad omino reduction/cyclization/migration process. Mild conditions,s imple experimental procedure,r eady accessibility of the starting materials and good to excellent yields characterize the present transformation. The methodology was used as ak ey step in ac oncise synthesis of rizatriptan and aformal total synthesis of aspidospermidine.
The palladium-catalyzed reaction of α-haloketones with isocyanides afforded α-oxo-ketenimines through β-hydride elimination of the β-oxo-imidoyl palladium intermediates. Reaction of these relatively stable α-oxo-ketenimines with nucleophiles such as hydrazines, hydrazoic acid, amines, and Grignard reagent afforded pyrazoles, tetrazole, β-keto amidines, and enaminone, respectively, with high chemoselectivity. Whereas amines attack exclusively on the ketenimine functions, the formal [3+2] cycloaddition between N-monosubstituted hydrazines and α-oxo-ketenimines was initiated by nucleophilic addition to the carbonyl group.
The first total synthesis of (+)-lophirone H (1) and its pentamethyl ether 29, featuring an oxonium-Prins cyclization/benzylic cation trapping reaction, is described.
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