Organocatalytic Asymmetric Domino Knoevenagel/Diels–Alder Reactions: A Bioorganic Approach to the Diastereospecific and Enantioselective Construction of Highly Substituted Spiro[5,5]undecane-1,5,9-triones

Enantioselective Synthesis of Octahydrofuranoindole Core of Aspidosperma Alkaloids via a Diels-Alder/Reduction/Fluoroetherification Reaction Sequence

Herein, we disclose the enantioselective synthesis of novel tricyclic fluorooctahydrofuranoindole spirooxindoles bearing five contiguous stereocenters via an organocatalytic sequential Diels-Alder/Reduction/Fluoroetherifiction reaction strategy. The potential of the developed approach was witnessed by generating vast examples (up to 20 examples) of library molecules embedding natural product core with good yields and phenomenal diastereo- and enantioselectivities (up to 77 % overall yield, up to 99 % ee and 10 : 1 dr). The synthetic utility of our protocol was further demonstrated by synthesizing tricyclic iodooctahydroindole spirooxindole framework through sequential Diels-Alder/reduction/iodoetherification reaction in 65 % overall yield and excellent stereoselectivity (99 % ee and 4 : 1 dr).

Trienamine catalysed unprecedented remote olefin E/Z isomerisation/[4 + 2]-cycloaddition reaction to access spirooxindole hexahydroindoles

Herein, for the first time, we report the asymmetric synthesis of an unexpected stereoisomer of spirohexahydroindole via a trienamine-catalysed remote olefin E/Z isomerisation/[4 + 2]-cycloaddition reaction. The reaction afforded a vast library of aesthetically pleasing spirooxindole hexahydroindole scaffolds with exceptional enantio- and diastereo-selectivities (up to 95% yield, 99% ee and >99 : 1 dr). In addition, we demonstrated the synthetic transformation of enantiomerically pure spirooxindole hexahydroindoles to synthesize alkyl homologated spirooxindole hexahydroindole and fluoro-pyranooctahydroindole moieties with four and seven contiguous stereocenters, respectively, in excellent yield and selectivities. We have also demonstrated the evidence for the proposed pathway through NMR investigations.

Asymmetric synthesis of the perhydroepoxyethanoindole core via sequential [4 + 2]-addition/reduction/fluoroannulation reactions

Herein, we present the sequential aminocatalytic [4 + 2]-addition/reduction and fluoroannulation reactions to afford a novel class of bridged fluoro-perhydroepoxyethanoindole spiropyrazolone and fluoro-perhydroepoxyethanoindole spirooxindole moieties with six contiguous stereocenters. An array of perhydroepoxyethanoindole core derivatives (up to 31 examples) mimicking aspidosperma alkaloids were obtained with moderate to good yields and excellent enantio- and diastereo-selectivities (up to 69% overall yield, up to 99.9% ee and up to >20 : 1 dr). Furthermore, we have also disclosed the synthesis of the unexpected tribromo derivative of hexahydroepoxyethanoindole spiropyrazolone in a moderate yield with excellent selectivity by employing the developed protocol in sequential bromoannulation reactions.

Organocatalytic Enantioselective Diels-Alder Reaction of 2-Trifluoroacetamido-1,3-dienes with α,β-Unsaturated Ketones

We report herein the first examples of chiral phosphoric acid-catalyzed enantioselective Diels-Alder reactions between 2-trifluoroacetamido-1,3-dienes 1 and α,β-unsaturated carbonyl compounds 2. Polysubstituted 1-acetamido cyclohexenes 3 were formed in high yields with excellent diastereo- and enantioselectivities. The reaction proceeds through a stepwise process as shown by deuterium labelling experiments. A catalytic enantioselective three-component reaction of 1, 2 and ortho-hydroxybenzhydryl alcohols 4 was subsequently developed furnishing the densely functionalized hexahydroxanthenes 5 in a highly stereoselective manner. This multicomponent reaction generates four chemical bonds with concurrent creation of five contiguous stereocenters.

Donor-Acceptor Cyclopropanes: Activation Enabled by a Single, Vinylogous Acceptor

A novel class of highly activated donor-acceptor cyclopropanes bearing only a single, vinylogous acceptor is presented. These strained moieties readily undergo cycloadditions with aldehydes, ketones, thioketones, nitriles, naphth-2-ols and various other substrates to yield the corresponding carbo- and heterocycles. Diastereocontrol can be achieved through the choice of catalyst (Brønsted or Lewis acid). The formation of tetrahydrofurans was shown to be highly enantiospecific when chiral cyclopropanes are employed. A series of mechanistic and kinetic experiments was conducted to elucidate a plausible catalytic cycle and to rationalize the stereochemical outcome.

Aminocatalytic asymmetric [4 + 2]-annulation to access functionally rich hexahydrospiroindole pyrazolones

Herein, we report the [4 + 2]-annulation of in situ generated trienamine from 2-(E)-benzylidine-3-pyrrolidinyl acraldehyde with pyrazolone olefins for the first time. It is a robust protocol for the synthesis of biologically appealing functionally rich hexahydrospiroindole pyrazolones which is reflected in excellent yields, stereoselectivity, operational simplicity and broad substrate scope as demonstrated in this paper. Furthermore, we have also explored the synthetic applications of optically pure hexahydrospiroindole pyrazolone to construct biologically important alkylated hexahydrospiroindole pyrazolone and octahydrospiroindole pyrazolone with good yields and high selectivity.

Metal-Free Construction of Fused Pyrimidines via Consecutive C-C and C-N Bond Formation in Water

A facile and efficient protocol has been developed for mild construction of fused pyrimidines via l-proline-catalyzed reaction of 4-hydroxy coumarins, aldehydes, and 2-aminobenzothiazoles/urea. The reaction has been carried out rapidly and efficiently in water under mild and metal-free conditions. Current etiquette has efficiently synthesized the heterocycles and avoids the use of hazardous solvents over conventional organic solvents. A plausible reaction mechanism has been established in this study. This study represents the first case in which l-proline as a homogeneous catalyst has been explored in the synthesis of fused pyrimidines in water in view of simple procedure and acceptable efficiency. This method gives the target product in excellent yield with ease of workup.

Co-catalysis between DABCO and a Brønsted acid in the catalytic [4 + 2] annulation of isatin with but-3-yn-2-one and mechanistic investigations

Catalytic amounts of the base catalyst DABCO in cooperation with a proton source afford the [4 + 2] cycloadducts of isatins with but-3-yn-2-one in moderate to good yields. Moreover, related plausible mechanisms have been proposed in detail based on control and deuterium labeling experiments.

Highly Efficient and Stereoselective Construction of Bispirooxindole Derivatives via a Three-Component 1,3-Dipolar Cycloaddition Reaction

A highly regio- and stereoselective synthesis of bispirooxindoles by 1,3-dipolar cycloaddition of in situ generated azomethine ylides from isatin and proline to different electron-deficient alkenes has been developed. The synthesis affords the desired bispiro scaffold compounds in excellent yields with high regioselectivity under mild conditions. The stereochemistry was determined by single-crystal X-ray analysis.

Efficient synthesis of functionalized benzo[b][1,8]naphthyridine derivatives via three-component reaction catalyzed by L-proline

A facile and efficient one-pot procedure for the preparation of functionalized benzo[b][1,8]naphthyridine derivatives by three-component reaction of 2-chloroquinoline-3-carbaldehyde, 1,3-dicarbonyl compounds, and enaminones catalyzed by l-proline is described. This new protocol has the advantages of environmental friendliness, good yields, and convenient operation.

l-Proline mediated synthesis of quinoxalines; evaluation of cytotoxic and antimicrobial activity

A simple, greener and highly efficient method for the synthesis of functionalized quinoxalines has been developed employing l-proline as a catalyst in water. The newly synthesized quinoxaline–sulphonamide conjugates exhibited significant cytotoxic and antimicrobial activities.

l-Proline-catalysed three-component Cascade Reaction for the Facile Synthesis of 3,4-dihydro-2H-pyrano[3,2-c]quinolin-2,5(6H)-dione Derivatives

A series of 4-aryl-3,4-dihydro-2 H-pyrano[3,2- c]quinolin-2,5(6 H)-dione derivatives was synthesised by the three-component reaction of aromatic aldehyde, quinoline-2,4-diol and Meldrum's acid catalysed by l-proline. This protocol has the advantages of easy work-up, mild reaction conditions and high yields.