Oxidative electro-organic synthesis of dimeric hexahydropyrrolo-[2,3-b]indole alkaloids involving PCET: total synthesis of (±)-folicanthine

Highly Diastereoselective Synthesis of Oxindole-Bearing Pyrroloindoline via Gold Catalyzed Carbophilic Carbene Transfer Reaction

A new type of gold catalyzed intermolecular dearomative cyclization reaction between diazo oxindole and tryptamine has been developed by utilizing the combination of L3AuCl/Ag3PO4 (L3=hexamethyl phosphane triamine) as the catalyst. This method allows for the facile preparation of a series of C-3-oxindole fused pyrroloindoline products in moderate to good yields with high diastereoselectivities. A plausible reaction mechanism, involving a cascade process of regioselective nucleophilic addition of the carbophilic gold carbenoid intermediate onto tryptamine, followed by an intramolecular cyclization, has been proposed, in which, the high diastereoselectivity is attributed to a preferred transition state. Moreover, three oxindole-bearing indolo[2,3-b] quinoline derivatives were synthesized through dearomative cyclization of homotryptamine derivative 4a with the respective 3-diazoindolin-2-one substrates using a similar strategy. A gram-scale experiment successfully yielded 1.95 g of 3 aa in 85 % yield with a diastereomeric ratio (dr) of 10.4/1, ultimately enabling a five-step synthesis of the natural product (±)-folicanthine.

Synthetic Approaches to Dimeric and Oligomeric Hexahydropyrrolo[2,3-b]indole Alkaloids

Natural product synthesis has been the prime focus for the development of new chemical transformations and the drug discovery. The dimeric and oligomeric hexahydropyrrolo[2,3-b]indole alkaloids represent a architecturally intriguing class of cyclotryptamine alkaloids. These alkaloids share contiguous stereogenic centers with vicinal all-carbon quaternary stereogenic centers. Members of this class of alkaloids are reported to show interesting biological activities. Therefore, there are tremendous synthetic interests not only because of their striking architecture but also due to their important biological activities. In this feature article we have discussed the detailed biogenetic hypothesis of these alkaloids and various elegant strategies from different groups, including our contributions in this area. Special emphasis is given for the synthetic approaches to dimeric, trimeric and tetrameric hexahydropyrrolo[2,3-b]indole alkaloids.

Homodimerization of 3-substituted-2-oxindoles for the construction of vicinal all-carbon quaternary centers: chemical, photochemical and electrochemical approaches

Advancements in organic synthesis are revolutionizing the synthesis of complex natural products, which are essential in biomedical research and drug discovery due to their intricate structures. Natural products such as chimonanthine, folicanthine, calycanthine, psychotriadine, etc., with vicinal all-carbon quaternary stereocenters, are particularly significant for their strong binding properties and biological activities. One common feature of these natural products is the presence of dimeric 3-substituted-2-oxindoles having vicinal all-carbon quaternary stereocenters. This review focuses on the chemical, photochemical, and electrochemical approaches for the homodimerization of 3-substituted-2-oxindoles employed by different researchers, with a strong focus on the mechanistic details of proton-coupled electron transfer (PCET). The article also demonstrates that PCET facilitates the reduction of kinetic barriers through the formation of low-energy intermediates and the expansion of synthetic possibilities. Furthermore, natural product syntheses (folicanthine and chimonanthine) from dimeric 3-substituted-2-oxindoles are discussed. Chemical syntheses are time-consuming and, even more importantly, generate significant waste due to the use of metal-based oxidants and catalysts. In this regard, electrochemical synthesis methods offer promising solutions by avoiding the use of chemical oxidants and metal catalysts, thus minimizing environmental impact. The article also outlines the advantages and disadvantages of different synthesis methods and proposes a new direction for future research in this field.

Regioselective Electrochemical Construction of Csp2-Csp2 Linkage at C5-C5' Position of 2-Oxindoles via an Intermolecular Anodic Dehydrogenative Coupling

Applying electricity as a reagent in synthetic organic chemistry has attracted particular attention from synthetic chemists worldwide as an environmentally benign and cost-effective technique. Herein, we report the construction of the Csp2-Csp2 linkage at the C5-C5' position of 2-oxindole utilizing electricity as the traceless oxidant in an anodic dehydrogenative homo-coupling process. A variety of 3,3-disubstituted-2-oxindoles were subjected to dimerization, achieving yields of up to 70 % through controlled potential electrolysis at an applied potential of 1.5 V versus Ag/Ag+ nonaqueous reference electrode. This electro-synthetic approach facilitates the specific assembly of C5-C5' (para-para coupled) dimer of 3,3-disubstituted-2-oxindole without the necessity of any external oxidants or additives and DFT (Density Functional Theory) calculations provided confirmation of this pronounced regioselectivity. Furthermore, validation through control experiments and voltammetric analyses substantiated the manifestation of radical-radical coupling (or biradical pathway) for the dimerization process.

Electrochemically enabled (3+2) cycloaddition of unbiased alkenes and β-dicarbonyls

A (3+2) cycloaddition between unbiased alkenes and 1,3-dicarbonyls is accomplished by judicious choice of electrode material and electrocatalyst to access dihydrofuran derivatives. A fluorinated porous carbon electrode with appropriate thickness governs unprecedented reactivity. This methodology eliminates the necessity for any stabilizing group within the alkene substrate. This is a rare example of the annulation of unbiased internal and terminal alkenes with cyclic and acyclic β-dicarbonyls.

Concise total syntheses of bis(cyclotryptamine) alkaloids via thio-urea catalyzed one-pot sequential Michael addition

Naturally occurring bis(cyclotryptamine) alkaloids feature vicinal all-carbon quaternary stereocenters with an elongated labile C-3a-C-3a' Sigma bond with impressive biological activities. In this report, we have developed a thio-urea catalyzed one-pot sequential Michael addition of bis-oxindole onto selenone to access enantioenriched dimeric 2-oxindoles with vicinal quaternary stereogenic centers at the pseudobenzylic position (up to 96% ee and >20 : 1 dr). This strategy has been successfully applied for the total syntheses of either enantiomers of chimonanthine, folicanthine, and calycanthine.

Electrocatalysis as a key strategy for the total synthesis of natural products

Electrochemical strategies have been a powerful approach for the synthesis of valuable intermediates, in particular heterocyclic motifs. Because of the mild nature, a wide range of nonclassical bond disconnections have been achieved via in situ-generated radical intermediates in a highly efficient manner. In particular, anodic electrochemical oxidative strategies have been utilized for the total synthesis of many structurally intriguing natural products. In this review article, we have discussed a number of total syntheses of structurally intriguing alkaloids and terpenoids in which electrochemical processes play an important role as a key methodology.