3,4-Annulated Indoles via Tandem Cyclopropane Ring-Opening/Conia-ene and Michael Addition/Conia-ene Reactions

NHC-Catalyzed Umpolung Driven Intramolecular Cyclizations for the Generation of 3,4-Cycloheptanone Annulated Tetracyclic Indole Derivatives

In this Letter, we disclose the development and exploitation of umpolung reactivity of N-heterocyclic carbene (NHC) organocatalysts in generating tetracyclic indole derivatives with the introduction of a cycloheptane ring forming event. The NHC-catalyzed intramolecular vinylogous Stetter, Stetter, benzoin, and formal cross-dehydrogenative coupling transformations have been executed, enabling the construction of 3,4-cycloheptannulated indole derivatives in good to excellent yields. The developed protocols utilize an inexpensive catalyst and feature operational simplicity, atom economy, gram-scale syntheses, and postsynthetic availability.

Chemoselectivity Switch between Enantioselective [2,3]-Wittig Rearrangement and Conia-Ene-Type Reactions of Propargyloxyoxindoles

Despite the existence of three competing reactions for propargyloxyoxindoles, we report a chemoselectivity switch between enantioselective propargyl [2,3]-Wittig rearrangement and Conia-ene-type reactions, with suppression of the [1,2]-Wittig-type rearrangement. Using C1-symmetric imidazolidine-pyrroloimidazolone pyridine as the ligand and Ni(acac)2 as the Lewis acid, diverse 3-hydroxy 3-substituted oxindoles containing allenyl groups were obtained in up to 98 % yield and 99 % ee via asymmetric propargyl [2,3]-Wittig rearrangement. In the presence of AgOTf-Duanphos, chiral spiro dihydrofuran oxindoles were given in up to 98 % yield and 91 % ee through a Conia-ene-type reaction.

A Synthetic Route to Tetrahydro-1H-azepino[4,3,2-cd]indoles via Ring-Opening Cyclization of Activated Azetidines with 4-Bromoindole: Toward a Vasopressin V2 Receptor Antagonist

A simple one-pot, two-step strategy for the synthesis of tetrahydro-1H-azepino[4,3,2-cd]indoles via Lewis acid-catalyzed SN2-type ring opening of activated azetidines with 4-bromoindole, followed by a Pd-catalyzed intramolecular C-N cyclization reaction, with good to excellent yields is described. Utilizing this protocol, the vasopressin V2 receptor antagonist precursor has been synthesized easily. Enantioenriched tetrahydro-1H-azepino[4,3,2-cd]indoles were obtained by starting from enantiopure azetidine.

Sn(OTf)2-Catalyzed (3 + 2) Cycloaddition/Sulfur Rearrangement Reaction of Donor-Acceptor Cyclopropanes with Indoline-2-thiones

The (3 + 2) cycloaddition/sulfur rearrangement reaction of donor-acceptor cyclopropanes bearing a single keto acceptor with indoline-2-thiones has been realized. Under the catalysis of Sn(OTf)2, a series of functionalized 3-indolyl-4,5-dihydrothiophenes were synthesized with moderate to excellent yields.

AlCl3-mediated ring-opening reactions of indoline-2-thiones with acyl cyclopropanes, bi-cyclopropanes and spirocyclic cyclopropanes

AlCl3-mediated nucleophilic ring-opening reactions of indoline-2-thiones with various acyl cyclopropanes, bi-cyclopropanes and spirocyclic cyclopropanes were investigated. A series of ketones functionalized with indolylthio groups were synthesized in yields ranging from moderate to good. Moreover, chemical transformations of 4-indolylthio butan-1-ones to dihydro-2H-thiepino[2,3-b]indoles and sulfone were carried out to further expand both synthetic utility and structural complexity.

Synthetic Study of Dragmacidin E: Enantioselective Construction of the Seven-Membered Ring-Fused Indole Skeleton with Contiguous Stereocenters

We developed an enantioselective synthetic method of constructing a seven-membered ring-fused indole skeleton with contiguous stereocenters for the synthesis of dragmacidin E. Introduction of chirality at the benzylic position was achieved by Ir-catalyzed asymmetric hydrogenation. After construction of the tricyclic molecular framework using Pd-catalyzed cascade cyclization, the tetrasubstituted carbon center was created using the Ag nitrene-mediated C-H amination reaction. The developed method provided access to the functionalized seven-membered ring-fused indole skeleton with a hydroxymethyl branch in the tetrasubstituted carbon.