Synthesis of Cyclopenta[b]indoles via Sc(III)-Catalyzed Annulation of Vinyl Diazoacetates with Indole-Derived Unsaturated Imines

Scandium Triflate Catalyzed Cycloadditions of Vinyl Diazo Compounds and In Situ Formed Naphthoquinone Methides

Vinyl diazo carbonyl compounds have received great attention and are widely employed in the cycloadditions of in situ formed reactive intermediates. Metal carbenes are predominantly involved in cycloadditions, and transformations of vinyl diazo compounds that do not proceed via the metal carbene pathway have been seldom reported. Herein, scandium-catalyzed cycloadditions of vinyl diazo compounds and in situ formed 2-naphthoquinone-8-methides are achieved, and naphthalene-fused polycyclic products were obtained in up to 88% yield. In the transformation, the nucleophilic conjugate addition of vinyl diazo compounds to in situ formed 2-naphthoquinone-8-methides generates vinyl diazonium intermediates, which undergo an intramolecular Friedel-Crafts reaction and intramolecular transesterification to yield the final product.

Cu-catalyzed borylative cyclization of N-(o-alkynylaryl)imines

We report herein the first copper-catalyzed intramolecular borylative cyclization of alkynes with imines, providing efficient access to alkenyl boronate-containing indolines. Preliminary mechanistic studies suggested a nucleophilic addition process for the alkenyl copper-trapping step, in which the nucleophilicity of alkenyl copper was largely enhanced by the formation of boronate complexes.

Stereoselective (3 + 3)-Cycloannulation of Carbonyl Ylides and (Z)-3-Benzylidene-3H-indoles Enabled by Cooperative Rh/Chiral Phosphoric Acid Catalysis

A diastereo- and enantioselective (3 + 3)-cycloannulation of in situ generated carbonyl ylides and (Z)-3-benzylidene-3H-indoles enabled by cooperative Rh/chiral phosphoric acid catalysis is reported. This study is the first to describe an enantioselective synthesis of cyclohepta[b]indole with an oxa-bridged motif. The scope of the reaction is broad, and the oxa-bridged cyclohepta[b]indole products were obtained in moderate to good yields and with excellent diastereoselectivities and good to high enantioselectivities.

Sc(OTf)3-Catalyzed Precise Construction of Medium-Sized [4.2.1]-Oxabridged Scaffolds

In this study, the readily available and inexpensive Sc(OTf)3 was utilized to activate ortho-benzoyl diazo compounds to in situ generate highly reactive cyclic 1,3-dipoles, which underwent a regioselective [4 + 3] cycloaddition with dioxopyrrolidines, yielding the [4.2.1]-oxabridged scaffolds bearing multiple contiguous quaternary carbon centers with high diastereoselectivities. The method not only exhibited significant economic and operational advantages but also demonstrated practical value through gram-scale synthesis and derivatization experiments.

Enantioselective Cycloaddition of in Situ Formed aza-Dienes and Vinyl Diazo Compounds for the Synthesis of Optically Enriched and Diazo Containing Tetrahydropyridazine

A copper catalyzed enantioselective formal aza-Diels-Alder reaction of in situ formed 1,2-diaza-1,3-dienes from α-halohydrazones and vinyl diazo compounds was described. The protocol provides a variety of optically enriched diazo-containing tetrahydropyridazines in moderate yields and with up to excellent enantioselectivities. The present methodologies utilize chiral oxazolines as the chiral ligands for asymmetric catalysis and feature mild reaction conditions, readily available substrates, and broad substrate scope.

Brønsted Acid-Catalyzed Dehydrative Nazarov-type Cyclization of CF3-Substituted 3-Indolylallyl Alcohols: Divergent Synthesis of 1-Trifluoromethylated Cyclopenta[b]indoles

An expedient and efficient synthetic method for the divergent synthesis of 1-trifluoromethylated cyclopenta[b]indoles that relies on Brønsted acid-catalyzed dehydrative Nazarov-type cyclization of CF3-substituted 3-indolylallyl alcohols is described. Two classes of 1-trifluoromethylated cyclopenta[b]indoles can be easily accessed simply by changing the NH-protecting group of indoles. With arylsulfonyl protected 3-indolylallyl alcohols as starting materials, the reaction provided the arylsulfonyl protected 1-trifluoromethylated cyclopenta[b]indoles in good to excellent yields, whereas pivaloyl (Piv) protected substrates led to the formation of NH-free 1-trifluoromethylated cyclolopenta[b]indoles with another alkenyl isomer. This protocol features tunable chemoselectivity, operational simplicity, excellent functional group compatibility, and mild metal-free conditions.

Molecular-iodine catalyzed selective construction of cyclopenta[b]indoles from indoles and acetone: a green gateway to indole-fused cycles

Molecular-iodine catalyzed access to an important class of bio-relevant indole derivatives, cyclopenta[b]indoles, has been achieved via a cascade addition/intramolecular cyclization reaction of indoles and acetone. Explorations of diverse substitution patterns revealed an essential substrate-control in the reaction. The high-density electronic core of indole is pivotal in favouring the formation of indolyl-cyclopenta[b]indole derivatives; in contrast, the electron deficiency of the core hindered the cyclization process, directing the formation of bis(indolyl)propanes. Investigations on the mechanistic pathway revealed that bis(indolyl)alkanes were the intermediates for the addition-cyclization process. This simple experimental method provides sustainable synthetic access to cyclopentannulated indoles.

Benchtop nickel-catalyzed reductive coupling of aldehydes with alkynes and ynamides

We demonstrate the potential of Ni(COD)(DQ), a bench-stable Ni0 complex, as a catalyst for the reductive coupling of aldehydes with alkynes and ynamides, providing silylated allyl alcohols with excellent yields and regioselectivities. Mass spectrometric identification of the intermediates and DFT studies supported the proposed mechanism.