Catalytic enantioselective desymmetrization of meso-aziridines.

As a type of readily available small strained-ring heterocycle, meso-aziridines may undergo catalytic desymmetrizing transformations to empower the rapid construction of diverse nitrogen-containing structures bearing contiguous stereocenters, which have great relevance in natural product synthesis, drug development and the design and synthesis of chiral catalysts/ligands for asymmetric catalysis. This review outlines the advances achieved in the catalytic asymmetric desymmetrization of meso aziridines and highlights some promising avenues for further work in this realm.

Lanthanide-Catalyzed Tandem Addition of Amines to Cyanoalkenes: Synthesis of Cyclic Amidines.

A tandem insertion of aliphatic nitriles and unactivated alkenes to the N-H bond of secondary aliphatic amines catalyzed by simple trialkyl rare-earth metal complexes was disclosed. This reaction provides a highly atom-economic and stereoselective way to a range of cyclic amidines under mild reaction conditions.

Sc(OTf)3-Catalyzed Chemodivergent Annulations of γ-Butyrolactone-Fused Cyclopropanes with Anthranils.

The Lewis acid-catalyzed annulations between anthranils and γ-butyrolactone-fused donor-acceptor cyclopropanes have been developed. Depending on the anthranils used, such annulations proceeded in a chemodivergent way to produce either bridged cyclic products via [4 + 3] annulation or γ-butyrolactone-fused tetrahydroquinoline products via a cascade process in moderate to high yields. A probable mechanism for the two reaction pathways has been proposed. The reaction could be performed on a gram-scale, and the products could be elaborated to other useful cyclic structures bearing multiple contiguous stereogenic centers.

Lewis Acid Catalyzed Dynamic Kinetic Asymmetric Transformation of Racemic N-Sulfonylaziridines.

The first Lewis acid catalyzed stereoconvergent transformation of racemic 2-(hetero)aryl- N-sulfonylaziridines via C-N bond cleavage with nucleophiles is presented. This includes the [3 + 2] annulations with (hetero)aromatic aldehydes and 1,3-disubstituted indoles, asymmetric Friedel-Crafts type reaction with electron-rich (hetero)arenes, and asymmetric aminolysis with amines, providing facile access to chiral 1,3-isoxazolidines, pyrroloindolines, 2-(hetero)arylphenethylamines, and vicinal diamines. This method features a simple and cheaply available complex of Cu(I)-chiral BINAP catalyst, excellent yield and high diastereo- and enantioselectivities, and mild reaction conditions. A mechanism involving type I dynamic kinetic asymmetric transformations (DyKATs) of the racemic aziridines is proposed based on the results of control experiments.

FeCl3-Promoted [3 + 2] Annulations of γ-Butyrolactone Fused Cyclopropanes with Heterocumulenes.

The [3 + 2] annulations of γ-butyrolactone fused donor-acceptor (D-A) cyclopropanes with aromatic isothiocyanates and dialkyl carbodiimides promoted by FeCl3 is reported. A series of bicyclic/polycyclic γ-butyrolactone fused thioimidates and γ-butyrolactone fused amidines containing four contiguous stereogenic centers were obtained in excellent yields as single stereoisomers.

Stereocontrolled Synthesis of trans/ cis-2,3-Disubstituted Cyclopropane-1,1-diesters and Applications in the Syntheses of Furanolignans.

A new Michael addition/intramolecular alkylation sequence of ( Z)-3-(2-bromo-3-arylacryloyl)oxazolidin-2-ones and malonates was developed. By a simple switch of the reaction conditions including the base promoter, solvent, and reaction temperature, both of the cis- and trans-isomers of a series of oxazolidinone-containing 2,3-disubstituted cyclopropane-1,1-diesters could be obtained in good-to-excellent yields and with an excellent diastereoselectivity. The utility of the cyclopropane products was demonstrated in the diastereoselective syntheses of (±)-urinaligran and a stereoisomer of (±)-virgatusin involving the AlCl3-promoted [3+2] annulation with veraldehyde or piperonal as the key step.

Lewis acid-catalyzed enantiospecific [3 + 2] annulations of γ-butyrolactone fused cyclopropanes with aromatic aldehydes: synthesis of chiral furanolignans.

An enantiospecific [3 + 2] annulation of γ-butyrolactone fused cyclopropanes with aromatic aldehydes was realized under Lewis acid catalysis. This method provides facile access to a series of chiral furanolignan derivatives bearing multiple contiguous stereogenic centers in good-to-excellent yields, exclusive diastereoselectivities and excellent enantiopurities under mild reaction conditions. Elaboration work on the product of this reaction delivers stereoisomeric analogues of (+)-virgatusin and suggests a structural revision might be necessary for a previously reported isolated natural product.

Synthesis of Chiral Vicinal Diamines by Silver(I)-Catalyzed Enantioselective Aminolysis of N-Tosylaziridines.

The kinetic resolution of 2-aryl-N-tosylaziridines and the asymmetric desymmetrization of meso-N-tosylaziridines by ring openings with various primary and secondary anilines, and aliphatic amines as nucleophile have been realized by using a single silver(I)/chiral diphosphine complex as catalyst for the first time. The simple starting materials, broad scope, and easy scalability render this protocol a practical way to chiral vicinal diamine derivatives.

[3 + 2]-Annulations of N-alkyl-3-substituted indoles with quinone monoketals catalysed by Brønsted acids.

An organocatalytic dearomative [3 + 2]-annulation of N-alkyl-3-alkylindoles with quinone monoketals is developed. The reaction provides a mild and straightforward way to various benzofuro[2,3-b]indolines of potential biological and pharmaceutical interest in moderate to good yields. Moreover, when 3-phenylindole, a problematic substrate in previous relevant studies, was used as the substrate under the otherwise same reaction conditions, a novel 1,2-shift of the phenyl group occurred followed by aromatization to provide 2,3-diaryl indoles useful for cancer therapy studies in moderate yields.

Asymmetric α-amination of 3-substituted oxindoles using chiral bifunctional phosphine catalysts.

A highly enantioselective α-amination of 3-substituted oxindoles with azodicarboxylates catalyzed by amino acids-derived chiral phosphine catalysts is reported. The corresponding products containing a tetrasubstituted carbon center attached to a nitrogen atom at the C-3 position of the oxindole were obtained in high yields and with up to 98% ee.

Copper(I)-Catalyzed Kinetic Resolution of N-Sulfonylaziridines with Indoles: Efficient Construction of Pyrroloindolines.

The first Lewis acid catalyzed [3 + 2] annulation of indoles and 2-aryl-N-tosylaziridines was realized by using copper(I)/chiral diphosphine complexes as a catalyst. With this method, a variety of uniquely substituted chiral pyrroloindolines bearing multiple contiguous stereogenic centers were facilely accessed in a straightforward, high-yielding, and highly stereoselective way under mild conditions.

Asymmetric dual-reagent catalysis: Mannich-type reactions catalyzed by ion pair.

The combination of a new bifunctional phosphine and an acrylate generate a zwitterion in situ and it serves as an efficient catalyst for asymmetric reactions through a homogeneous ion-pairing mode. This new catalytic system has been successfully applied to Mannich-type reactions to give excellent results and it demonstrates a broad substrate scope. Such reactivity is not accessible with general organophosphine catalytic modes. Preliminary investigations into the mechanism are also presented.

Synthesis of polysubstituted pyrroles via [3 + 2]-annulation of aziridines and ß-nitroalkenes under aerobic conditions.

Polysubstituted pyrroles were regioselectively synthesized in moderate to good yields via the copper acetate-catalyzed [3 + 2] annulation reaction of readily accessible aziridines and nitroalkenes. This reaction was proposed to proceed through a key azomethine ylide intermediate generated by selective C-C bond cleavage of the aziridine followed by annulation with nitroalkenes under aerobic conditions.

Phosphine catalyst-controlled cycloaddition or dienylation reactions of trifluoromethyl aryl ketones with bis-substituted allenoates.

A chemoselective phosphine-catalyzed cycloaddition or dienylation reaction between trifluoromethyl-substituted ketones and bis-substituted allenoates was described. Under the catalysis of triarylphosphine, the reaction gave a range of trifluoromethylated tetrahydrofurans with broad substrate tolerance and good to excellent stereoselectivity, while the use of trialkylphosphine switched the reaction pathway to furnish CF3-substituted dienyl tertiary alcohols chemoselectively. Moreover, a preliminary study on the asymmetric version of the reaction was also performed, which represents the first example of a phosphine-catalyzed asymmetric reaction between allenoates and carbonyl compounds.

cis-2,3-Disubstituted cyclopropane 1,1-diesters in [3 + 2] annulations with aldehydes: highly diastereoselective construction of densely substituted tetrahydrofurans.

A series of cis-2,3-disubstituted cyclopropane 1,1-diesters were examined in the AlCl3-promoted [3 + 2]-annulations with aldehydes. In this reaction, these cis-cyclopropanes displayed reactivities starkly different from their trans counterparts in terms of the high chemical yields (up to 98%) and provided the desired annulation products with excellent diastereomeric purity. This protocol provides a facile and highly stereoselective way to construct synthetically useful pentasubstituted tetrahydrofurans not easily accessible using other methods.

AlCl3-promoted [3 + 2] annulation of cis-2,3-disubstituted cyclopropane 1,1-diesters with isothiocyanates: stereoselective synthesis of densely substituted 2-iminodihydrothiophenes.

cis-2,3-Disubstituted cyclopropane 1,1-diesters were found to be much more reactive than their corresponding trans-isomers in the AlCl3-promoted [3 + 2]-annulations with isothiocyanates. The reaction with the cis-cyclopropanes proceeded to completion within minutes, providing a variety of densely substituted diastereomerically pure 2-iminodihydrothiophenes in moderate to excellent yields.

Lewis Acid-Catalyzed Annulations of Geminally Disubstituted Cyclopropanes with Aldehydes or 1,3,5-Triazinanes.

Under the catalysis of simple Lewis acid catalysts, 2-(1-aroylcyclopropyl)malonates demonstrated unique reactivities in annulation reactions with aryl/alkyl aldehydes, paraformaldehyde, and 1,3,5-triazinanes. Three types of structurally diverse cyclic products that are otherwise not easy to obtain were generated in moderate to good yields and excellent diastereoselectivities. Possible reaction pathways leading to these products were proposed on the basis of the results of control experiments.

Pd(II)/Brønsted acid catalyzed enantioselective allylic C-H activation for the synthesis of spirocyclic rings.

A Pd(II)/Brønsted acid catalyzed migratory ring expansion for the synthesis of indene derivatives possessing a stereogenic spirocyclic carbon center was developed. This transformation is believed to mechanistically proceed via enantioselective allylic C-H activation with concomitant semipinacol ring expansion to the nascent π-allylpalladium species. Enantioselectivities as high as 98% ee were attainable.

Highly enantioselective [4 + 2] cycloadditions of allenoates and dual activated olefins catalyzed by N-acyl aminophosphines.

An asymmetric organocatalytic [4 + 2] cycloaddition between α-substituted allenoates and dual activated olefins using bifunctional N-acyl aminophosphine catalysts is described. The use of 2-cyano acrylate derived olefins led to the first successful incorporation of an electrophile derived from an aliphatic aldehyde into this reaction.

Indolyl-based Copper(I) Complex-Catalyzed Intermolecular Trifluoromethylazolation of Alkenes via Radical Process.

Herein, we synthesized and characterized a binuclear copper(I) complex supported by the indolyl-based ligand. Employing this complex as catalyst, we have developed a three-component intermolecular trifluoromethylazolation of alkenes to deliver various trifluoromethylated azole derivatives. The method features exclusive chemo- and regioselectivity, a broad scope of alkenes and oxazoles, thiazoles, and good tolerance of functional groups under mild conditions. Preliminary mechanistic studies support a radical process for the transformation.