Hydrogen-Bonding-Promoted Cascade Rearrangement Involving the Enlargement of Two Rings: Efficient Access to Polycyclic Quinoline Derivatives

An efficient cascade reaction of tryptamine-derived isocyanides with C,N-cyclic azomethine imines is described. The polycyclic pyrrolo[2,3-c]quinoline derivatives, which benefited from rearrangement process driven by hydrogen bonding, could be directly assembled in moderate to good yields (40–87 %) under metal-free and mild conditions. This transformation involved four new heterocyclic rings formations and uniquely, ring opening of indole as well as ring expansion of C,N-cyclic azomethine imine. Both experimental and DFT studies provided guidance on the in-depth insight into the reaction pathways and hydrogen bonding was identified to lower the free energy barrier in transition states. This work constitutes a rare example of tryptamine-derived isocyanide-based cascade reactions, and potentially could be a powerful synthetic strategy for accessing polycyclic analogues involved in natural products.     

The Lewis acid-catalyzed [3+1+1] cycloaddition of azomethine ylides with isocyanides

A Lewis acid-catalyzed [3+1+1] cycloaddition reaction between aliphatic isocyanides and azomethine ylides generated in situ from aziridines, leading to pyrrolidine derivatives, has been developed. This reaction proceeds smoothly under mild conditions and can also be modified by employing aromatic isocyanides to generate four-membered heterocycles, azetidines, through a [3+1] cycloaddition reaction.     

Generation and metathesis of azomethine imines in reaction of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes with het(aryl)methylidenemalononitriles

A new metathesis reaction of azomethine imines is found. Catalytic or thermal diaziridine ring opening of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes leads to azomethine imines reacting further with het(aryl)methylidenemalononitriles to give in situ new azomethine imines inaccessible by common synthetic methods. New azomethine imines are detected as pyrazolines formed via a 1,4-H shift and trapped by the [3+2] cycloaddition with various dipolarophiles to yield 1,5-diazabicyclo[3.3.0]octane derivatives bearing pharmacophoric heterocycles, e.g. furan, nitrofuran, thiophene, and indole. The best results are achieved in the Et₂O·BF₃-catalyzed reactions in ionic liquids.     

Organocatalyzed cycloaddition reactions of azomethine imines—A lookback

The versatile and convergent nature of 1,3-dipolar cycloaddition reactions has made them an indispensable tool in organic chemistry for synthesizing five-membered heterocycles. Among the various dipoles, azomethine imines (AMI) constitute a versatile class, increasingly used to synthesize biologically relevant heterocycles. The organocatalytic cycloaddition reactions of azomethine imines are relatively unexplored compared to the corresponding transition metal-catalyzed reactions. This review highlights the unique organocatalytic cycloaddition reactions of AMI with different dipolarophiles. The cycloaddition of azomethine imines catalyzed by organic bases such as prolinol, proline, alkaloids, 1,4-diazabicyclo[2.2.2]octane, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), primary amines, tertiary amines, N-heterocyclic carbene, and phosphine are discussed here. The mechanistic and electronic aspects, including broad functional group tolerance, catalyst loading, and reaction conditions, are evaluated. This review also demonstrates the scope and potential reactivity of azomethine imines in organocatalytic cycloadditions. We are positive that the reactions of azomethine imines discussed here will aid in discovering new and efficient reaction pathways in synthesizing biologically active and industrially relevant molecules.     

An interrupted Ugi reaction enables the preparation of substituted indoxyls and aminoindoles

In a variation of the classical Ugi reaction, an acid-promoted reaction between imines and isocyanides forms both 3-aminoindoles and substituted indoxyls. A recently reported triflyl phosphoramide is shown to be critical to obtain high yields under mild conditions.     

Switch in stereoselectivity caused by the isocyanide structure in the rhodium-catalyzed silylimination of alkynes

The reaction of terminal alkynes with hydrosilanes and tert-alkyl isocyanides in the presence of Rh(4)(CO)(12) gives (Z)-β-silyl-α,β-unsaturated imines in good yields. On the other hand, the use of aryl isocyanides in place of tert-alkyl isocyanides leads to the formation of E isomers.     

Phosphine-catalyzed annulations of azomethine imines: allene-dependent [3 + 2], [3 + 3], [4 + 3], and [3 + 2 + 3] pathways

In this paper we describe the phosphine-catalyzed [3 + 2], [3 + 3], [4 + 3], and [3 + 2 + 3] annulations of azomethine imines and allenoates. These processes mark the first use of azomethine imines in nucleophilic phosphine catalysis, producing dinitrogen-fused heterocycles, including tetrahydropyrazolo-pyrazolones, -pyridazinones, -diazepinones, and -diazocinones. Counting the two different reaction modes in the [3 + 3] cyclizations, there are five distinct reaction pathways-the choice of which depends on the structure and chemical properties of the allenoate. All reactions are operationally simple and proceed smoothly under mild reaction conditions, affording a broad range of 1,2-dinitrogen-containing heterocycles in moderate to excellent yields. A zwitterionic intermediate formed from a phosphine and two molecules of ethyl 2,3-butadienoate acted as a 1,5-dipole in the annulations of azomethine imines, leading to the [3 + 2 + 3] tetrahydropyrazolo-diazocinone products. The incorporation of two molecules of an allenoate into an eight-membered-ring product represents a new application of this versatile class of molecules in nucleophilic phosphine catalysis. The salient features of this protocol--the facile access to a diverse range of nitrogen-containing heterocycles and the simple preparation of azomethine imine substrates--suggest that it might find extensive applications in heterocycle synthesis.     

1,3-Dipolar cycloaddition of arynes with azomethine imines: synthesis of 1,2-dihydropyrazolo[1,2-a]indazol-3(9H)-ones

A [3+2] 1,3-dipolar cycloaddition reaction of arynes with stable azomethine imines has been developed. The reaction rapidly assembles tricyclic pyrazoloindazolone derivatives in moderate yields under mild reaction conditions.     

Synthesis of novel spiro-[3H-indole-3,3′-[1,2,4]triazolidine]-2-ones via azomethine imines

Novel spiroindoles 5 are prepared easily via a one-pot, 1,3-dipolar cycloaddition reaction of azomethine imines with isatin imines under thermal conditions.     

Amine‐Catalyzed Enantioselective 1,3‐Dipolar Cycloadditions of Aldehydes to C,N‐Cyclic Azomethine Imines

Amine‐catalyzed enantioselective 1,3‐dipolar cycloadditions of aldehydes to C,N‐cyclic azomethine imines were developed. The reactions between diversely substituted C,N‐cyclic azomethine imines and aldehydes proceeded smoothly in the presence of chiral prolinol silyl ether catalyst and gave the C‐1‐substituted tetrahydroisoquinolines in a highly stereoselective manner. These tetrahydroisoquinolines could be efficiently transformed to several other useful polycyclic frameworks.     

Kinetic resolutions of azomethine imines via copper-catalyzed [3 + 2] cycloadditions

An effective method has been developed for the kinetic resolution of racemic azomethine imines via [3 + 2] cycloadditions with alkynes catalyzed by a chiral copper complex. Efficient kinetic resolution is observed for a variety of N1 and C5 substituents on the dipole, thereby furnishing a wide array of useful enantioenriched azomethine imines, which can readily be transformed into monocyclic and bicyclic pyrazolidinones.     

Silver acetate-catalysed asymmetric 1,3-dipolar cycloadditions of imines and chiral acrylamides

N-Metallated azomethine ylides were generated by the reaction of arylidene glycine imines with AgOAc and triethylamine. These azomethine ylides undergo cycloaddition to chiral acrylamides with excellent diastereoselectivity. The configuration of two of the cycloadducts was confirmed by X-ray crystallography.     

Asymmetric synthesis of multifunctionalized pyrrolines by a ruthenium porphyrin-catalyzed three-component coupling reaction

[reaction: see text] Chiral multifunctionalized pyrrolines have been synthesized by a ruthenium porphyrin catalyzed three-component coupling reaction. In a one-pot reaction, ruthenium porphyrins catalyzed in situ generation of chiral azomethine ylides from chiral diazo esters and imines. Asymmetric 1,3-dipolar cycloaddition reactions of the chiral azomethine ylides with dipolarophiles afforded the corresponding pyrrolines in good yields and high diastereoselectivity (up to 92% de).     

Catalytic Asymmetric Inverse‐Electron‐Demand 1,3‐Dipolar Cycloaddition of C,N‐Cyclic Azomethine Imines with Azlactones: Access to Chiral Tricyclic Tetrahydroisoquinolines

Reported herein is a bifunctional‐organocatalyst‐mediated enantioselective inverse‐electron‐demand 1,3‐dipolar cycloaddition of C,N‐cyclic azomethine imines with azlactones. The strategy provides concise access to enantioenriched C1‐substituted tetrahydroisoquinolines featuring a pyrazolidinone scaffold. Moreover, the scalability and practical utility of this protocol was well demonstrated by employing a gram‐scale reaction and some representative transformations.     

Direct synthesis of pyrroles from imines, alkynes, and acid chlorides: an isocyanide-mediated reaction

[reaction: see text] A direct synthesis of pyrroles from imines, acid chlorides, and alkynes mediated by isocyanides is reported. This reaction proceeds with a range of each of these three substrates, providing a method to generate families of pyrroles in high yield. Mechanistic studies suggest this process proceeds via the generation of imino analogues of munchnones, which can undergo in situ coupling with alkynes to liberate isocyanate and form the pyrrole product.     

An asymmetric Ugi three-component reaction induced by chiral cyclic imines: synthesis of morpholin- or piperazine-keto-carboxamide derivatives

A series of chiral 5,6-dihydro-1,4-oxazin-2-one substrates, as preformed cyclic aldimines and ketoimines, were employed to develop a new asymmetric Ugi three-component reaction for the first time. The Ugi reaction of the imines, isocyanides, and carboxylic acids opens an efficient access to novel morpholin-2-one-3-carboxamide compounds. The chiral imines showed promising stereoinduction for the new chiral center of the Ugi products, and predominant trans-isomers were obtained in the most cases. Addition of some Lewis acids or proton acids could improve the diastereoselectivity further but usually led to a drop in total yield. The Ugi-3CR could be extended to the stereoselective synthesis of ketopiperazine-2-carboxamide derivatives.     

Solvent-free 1,3-Dipolar Cycloaddition of Azomethine Imines with Terminal Alkynes Promoted by Calcium Fluoride Under the Ball Milling Condition

A convenient and efficient procedure for the synthesis of N,N-bicyclic pyrazolidinone derivatives has been developed via the reaction of azomethine imines with terminal alkynes under the ball milling condition without solvent, which was promoted by calcium fluoride with a catalyst of copper(I) salt. The cyclization reactions exhibited moderate to high yields.     

Kinetic Resolution of Azomethine Imines by Brønsted Acid Catalyzed Enantioselective Reduction

Azomethine imines are valuable substrates in asymmetric catalysis, and can be precursors to β‐amino carbonyl compounds and complex hydrazines. However, their utility is limited because complex and enantioenriched azomethine imines are often unavailable. Reported herein is a kinetic resolution of N,N′‐cyclic azomethine imines by enantioselective reduction (s=13–43). This resolution was accomplished using a Brønsted acid catalyst, and represents the first example of the asymmetric reduction of azomethine imines. The pyrazolidinone product (up to 86 % ee) and the recovered azomethine imine (up to 99 % ee) can both be used to access the opposite enantiomers of valuable products.     

Cycloadditions of aromatic azomethine imines with 1,1-cyclopropane diesters

The cycloaddition of aromatic azomethine imines to 1,1-cyclopropane diesters was achieved using Ni(ClO4)2 as catalyst. The methodology gives access to unique tricyclic dihydroquinoline derivatives with dr up to 6.6:1. A nonconcerted mechanism is proposed on the basis of stereochemical analysis of the reaction.