A Short, Protecting Group-Free Total Synthesis of Bruceollines D, E, and J

Iron-Mediated Hydrogen Atom Transfer Radical Cyclization of Alkenyl Indoles and Pyrroles Gives Their Fused Derivatives: Total Synthesis of Bruceolline E and H

The iron-mediated hydrogen atom transfer (HAT) reaction is efficaciously employed for the synthesis of dihydropyrroloindoles and dihydropyrrolizines via 5-exo-trig radical cyclization where indoles and pyrroles are used as an acceptor. This radical approach has also been extended for the synthesis of tetrahydrocyclopenta[b]indolones via the Baldwin-disfavored 5-endo-trig cyclization pathway. The formal synthesis of bruceolline J and the total synthesis of bruceollines E and H have been expeditiously carried out by employing the former strategy.

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.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Catalytic asymmetric indolization by a desymmetrizing [3 + 2] annulation strategy

A new catalytic asymmetric indolization reaction by a desymmetrizing [3 + 2] annulation strategy is developed. The reaction proceeds via a rhodium-catalyzed enantioposition-selective addition/5-exo-trig cyclization/dehydration cascade between ortho-amino arylboronic acids and 2,2-disubstituted cyclopentene-1,3-diones to produce N-unprotected cyclopenta[b]indoles bearing an all-carbon quaternary stereocenter in high yields with good enantioselectivities. A quantitative structure-selectivity relationship (QSSR) model was established to identify the optimal chiral ligand, which effectively controlled the formation of the stereocenter away from the reaction site. Density functional theory (DFT) calculations, non-covalent interaction analysis, and Eyring analysis were performed to understand the key reaction step and the function of the ligand.

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

A Lewis acid Sc(OTf)₃-catalyzed annulation reaction of vinyl diazoacetates with in situ formed α,β-unsaturated imines for the preparation of indole-fused tricyclic rings has been reported. This strategy involves a sequential addition/rebound addition process of vinyl diazoacetates and an in situ dedinitrogenation. This annulation protocol features low-cost catalysts, mild reaction conditions, and facilely prepared substrates.

Phosphine-catalysed denitrative rearomatising (3 + 2) annulation of α,β-ynones and 3-nitroindoles

We describe a metal-free strategy to access various α-arylidene cyclopenta[b]indoles via phosphine-catalysed (3 + 2) annulation of α,β-ynones and 3-nitroindoles. For the first time, the rearomatisation of the indole nucleus was observed in such an annulative transformation. The method was extended to the synthesis of an antimalarial natural product, bruceolline E.

Enantioselective Domino Reaction of 3-Vinylindole and p-Quinone Methides Enabled by Chiral Imidodiphosphoric Acids: Asymmetric Synthesis of Multisubstituted 3-Indolyl Cyclopenta[b]indoles

The development of a stereoselective method for the rapid assembly of structurally complex molecules remains fascinating and challenging in synthetic organic chemistry. Here, we report an enantioselective domino reaction between 3-vinylindole and p-quinone methide for the preparation of 3-indolyl cyclopenta[b]indoles containing multiple chiral centers. Chiral imidodiphosphoric acids enable this cascade asymmetric process, delivering a series of products with excellent yields (≤99%), enantioselectivities (≤99%), and diastereoselectivities (≤20:1 dr).

Catalytic Enantioselective Desymmetrizing Fischer Indolization through Dynamic Kinetic Resolution

The first catalytic enantioselective Fischer indolization of prochiral diketones containing enantiotopic carbonyl groups is developed and shown to proceed through dynamic kinetic resolution (DKR). Catalyzed by the combination of a spirocyclic chiral phosphoric acid and ZnCl₂ (Lewis acid assisted Brønsted acid), this direct approach combines 2,2-disubstituted cyclopentane-1,3-diones with N-protected phenylhydrazines to furnish cyclopenta[b]indole derivatives containing an all-carbon quaternary stereocenter with good to excellent enantioselectivities.

Mechanism and enantioselectivity of the asymmetric [3+2]-annulation between N-methylindole and enoldiazoacetamide catalyzed by prolinate-coordinated dirhodium: A theoretical study

The mechanistic study of Rh₂(S-MSP)₄-catalyzed asymmetric [3 + 2]-annulation of N-methylindole and enoldiazoacetamide carbenoid to produce C2,C3-fused indole skeleton has been carried out by combining the density functional theory and semi-empirical method with ONIOM methodology. The mechanism and origin of enantioselectivity were disclosed by our calculations. The cycloaddition process was initiated by vinylogous attack of indole on the enol terminus of carbenoid, which was considered to be the enantio-controlling step. Followed by ring closure step where C-Rh bond adds to the iminium, adduct was afforded enantioselectively. The rate-controlling step, depending on the pathways the reaction proceeds along, could be either one of the two steps. Moreover, we elucidated the effect of weak interaction on the enantioselectivity using the independent gradient model analysis and provided possible insight into reaction design. The catalytic cycle was also kinetically interpreted in a non-conventional way, which is the energetic span model, realized by the AUTOF program.

From synthetic control to natural products: a focus on N-heterocycles

Natural products containing a N-heterocycle motif are widespread in nature and medicinal plants, in particular, have proved to be a source of almost unlimited N-derived structures with high molecular diversity. Because of their intrinsic potential for use in both biomedical and agricultural applications, there is a general need for new compounds and for the synthesis of 'natural-inspired' analogues. Importantly, transition of a natural product from discovery to a 'market lead' is associated with an increasingly challenging demand for more of the compound, which cannot be met by isolation from natural plant sources, often due to low extraction yields and uneven availability of the plant source itself. Synthesis remains the most reliable approach to provide valuable products for the market. In this review, a comprehensive overview of our contribution to synthetic access to N-derived natural products is given. Major strengths of the proposed methodologies are discussed critically. © 2019 Society of Chemical Industry.

Synthesis of Four Illudalane Sesquiterpenes Utilizing a One-Pot Diels-Alder/Oxidative Aromatization Sequence

The concise, divergent total syntheses of four illudalane sesquiterpenes using an indanone as the key intermediate are reported. The key elements in these total syntheses, which involve only four to six operational steps, consist of a Suzuki cross-coupling and a one-pot Diels-Alder/oxidative aromatization reaction.

Total Synthesis of Bruceolline I

The first total synthesis of the natural product bruceolline I, isolated in small quantities from the ethanol extract of Brucea mollis stems, was achieved in 29% yield over nine steps and with high enantiomeric purity (>98%). The key step of the process is the tandem gold-catalyzed rearrangement/Nazarov reaction of a propargylic acetate derivative. This synthesis provides a sufficient amount of synthesized bruceolline I for further bioassays.

Facile Installation of 2-Reverse Prenyl Functionality into Indoles by a Tandem N-Alkylation-Aza-Cope Rearrangement Reaction and Its Application in Synthesis

An unprecedented tandem N-alkylation-ionic aza-Cope (or Claisen) rearrangement-hydrolysis reaction of readily available indolyl bromides with enamines is described. Due to the complicated nature of the two processes, an operationally simple N-alkylation and subsequent microwave-irradiated ionic aza-Cope rearrangement-hydrolysis process has been uncovered. The tandem reaction serves as a powerful approach to the preparation of synthetically and biologically important, but challenging, 2-reverse quaternary-centered prenylated indoles with high efficiency. Notably, unusual nonaromatic 3-methylene-2,3-dihydro-1H-indole architectures, instead of aromatic indoles, are produced. Furthermore, the aza-Cope rearrangement reaction proceeds highly regioselectively to give the quaternary-centered reverse prenyl functionality, which often produces a mixture of two regioisomers by reported methods. The synthetic value of the resulting nonaromatic 3-methylene-2,3-dihydro-1H-indole architectures has been demonstrated as versatile building blocks in the efficient synthesis of structurally diverse 2-reverse prenylated indoles, such as indolines, indole-fused sultams and lactams, and the natural product bruceolline D.

Simple indole alkaloids and those with a nonrearranged monoterpenoid unit

This review summarizes the isolation, structure determination, total syntheses and biological activities of simple indole alkaloids and those with a nonrearranged monoterpenoid unit, with literature coverage from 2012 to 2013.

Concise asymmetric total synthesis of bruceolline J

Concise biomimetic and asymmetric approach involving Sharpless asymmetric dihydroxylation and Lewis acid catalysed cyclopenta[b]annulation as key steps to synthesize (+)-bruceolline J.

Gold(I)-catalyzed 1,2-acyloxy migration/[3+2] cycloaddition of 1,6-diynes with an ynamide propargyl ester moiety: highly efficient synthesis of functionalized cyclopenta[b]indoles

A gold-catalyzed cycloisomerization of 1,6-diynes containing an ynamide propargyl ester or carbonate moiety has been developed that provides an attractive route to a diverse-substituted 3-acyloxy-1,4-dihydrocyclopenta[b]indoles. Mechanistic studies indicate that the reaction likely proceeds through a competitive 1,2-OAc migration followed by [3+2] cycloaddition of the vinyl gold-carbenoid intermediate with the pendant triple bond. The synthetic utility of the obtained cyclopenta[b]indole products was demonstrated by their efficient transformations by deprotection or double-bond isomerization reactions.