Synthesis of 3-Azabicyclo[3.1.0]hexane Derivates

3-Azabicyclo[3.1.0]hexanes are an important class of nitrogen-containing heterocycles that have been found to be key structural features in a wide range of biologically active natural products, drugs, and agrochemicals. As a cutting-edge area, the synthesis of these derivatives has made spectacular progress in recent decades, with various transition-metal-catalyzed and transition-metal-free catalytic systems being developed. In this review, we provide an overview of recent advances in the efficient methods for the synthesis of 3-azabicyclo[3.1.0]hexane derivatives since 2010, emphasizing the scope of substrates and synthesis' applications, as well as the mechanisms of these reactions.

Palladium-Catalyzed Borylative Cyclization and Cyclopropanation of Terminal Alkyne-Derived Enynes

Described herein is a palladium-catalyzed borylative cyclization and cyclopropanation of terminal alkyne-derived enynes, affording borylated bicycles, fused cycles, and bridged cycles in good isolated yields. The synthetic utility of this protocol was fully demonstrated by large scale reaction and synthetic derivatization of the borate group.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Synthesis of γ-substituted carbonyl compounds from DMSO-mediated oxidation of enynamides: mechanistic insights and carbon- and hetero-functionalizations

1,3-Enynamides underwent oxygenative coupling with carbon- and heteroatom nucleophiles with high remote selectivity. Kinetic analysis revealed a continuum mechanism between concerted S_N2′′ and via a carbocation, depending on the nucleophiles used.

Oxidative coupling of 1,3-enynamides using DMSO as a terminal oxidant has been developed. Carbon as well as unmodified heteroatom nucleophiles, including aliphatic alcohols, thiols, and hydrazides, could be efficiently alkylated at the γ-position in a highly regioselective fashion. The kinetic analysis suggested a nucleophile-dependent mechanism ranging from a concerted S_N2′′ to a carbocationic mechanism. Thus, the remote site-selectivity was ascribed to the partial positive charge developing at the terminal carbocationic center.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes, one of the most important strained rings, have gained much attention for more than a century because of their interesting and unique reactivity. They not only exist in many natural products, but have also been widely used in the fields of organic synthesis, medicinal chemistry and materials science as versatile building blocks. Based on the sustainable development in this area, this review mainly focuses on the recent advances in the synthesis of cyclopropanes classified by the type of catalytic system, including regio-, diastereo-, and enantio-selective reactions.

Diastereospecific Gold(I)-Catalyzed Cyclization Cascade for the Controlled Preparation of N- and N,O-Heterocycles

The reaction of oxime-tethered 1,6-enynes with a cationic gold(I) catalyst demonstrates a great potential for the synthesis of a range of heterocycles in a diastereospecific fashion. The control of the configuration of the oxime and the alkene of the enyne moiety is the key to selectively obtain dihydro-1,2-oxazines, isoxazolines or dihydropyrrole-N-oxides as single diastereoisomers. As supported by DFT calculations, these cascade reactions proceed stepwise, by the intramolecular addition of the O or N atom of the oxime onto cyclopropyl gold(I) carbene intermediates. In this study, a rare [3,3]-sigmatropic rearrangement of nitrones is also observed.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes have gained much attention by virtue of their interesting structure and unique reactivity. This review discusses the recent advances in the synthesis of cyclopropanes, and some of the related applications will be discussed.

Brønsted Acid Catalyzed Oxygenative Bimolecular Friedel-Crafts-type Coupling of Ynamides

A non-metal approach for accessing α-oxo carbene surrogates for a C-C bond-forming bimolecular coupling between ynamides and nucleophilic arenes was developed. This acid-catalyzed coupling features mild temperature, which is critical for the required temporal chemoselectivity among nucleophiles. The scope of nucleophiles includes indoles, pyrroles, anilines, phenols and silyl enolethers. Furthermore, a direct test of S_N 2' mechanism has been provided by employing chiral N,N'-dioxides which also enlightens the nature of the intermediates in related metal-catalyzed processes.

Intramolecular cyclization of N-allyl propiolamides: a facile synthetic route to highly substituted γ-lactams (a review)

The development of simple and efficient methods for construction of substituted γ-lactams is an important synthetic goal because such ring skeletons are present in numerous natural compounds that display diverse biological activities.

Alkynyl sulfoxides as α-sulfinyl carbene equivalents: gold-catalysed oxidative cyclopropanation

Gold catalysed oxidation of alkynyl sulfoxides provides access to a desirable α-sulfinyl metal carbene reactivity profile that is inaccessible using diazo chemistry.

Construction of Polycyclic γ-Lactams and Related Heterocycles via Electron Catalysis

Cascade radical cyclization of 1,6-enynes for the construction of biologically important polycyclic γ-lactams and related heterocycles is reported. In these radical cascade processes, three new C–C bonds are formed and transition metals are not required to run these sequences. The mild reaction conditions, broad substrate scope, and the importance of the heterocyclic products render the approach valuable.

Gold carbenes, gold-stabilized carbocations, and cationic intermediates relevant to gold-catalysed enyne cycloaddition

Cationic gold complexes in which gold is bound to a formally divalent carbon atom, typically formulated as gold carbenes or α-metallocarbenium ions, have been widely invoked in a range of gold-catalyzed transformations, most notably in the gold-catalyzed cycloisomerization of 1,n-enynes. Although the existence of gold carbene complexes as intermediates in gold-catalyzed transformations is supported by a wealth of indirect experimental data and by computation, until recently no examples of cationic gold carbenes/α-metallocarbenium ions had been synthesized nor had any cationic intermediates generated via gold-catalyzed enyne cycloaddition been directly observed. Largely for this reason, there has been considerable debate regarding the electronic structure of these cationic complexes, in particular the relative contributions of the carbene (LAu(+)[double bond, length as m-dash]CR2) and α-metallocarbenium (LAu-CR2(+)) forms, which is intimately related to the extent of d → p backbonding from gold to the C1 carbon atom. However, over the past ∼ seven years, a number of cationic gold carbene complexes have been synthesized in solution and generated in the gas phase and cationic intermediates have been directly observed in the gold-catalyzed cycloaddition of enynes. Together, these advances provide insight into the nature and electronic structure of gold carbene/α-metallocarbenium complexes and the cationic intermediates generated via gold-catalyzed enyne cycloaddition. Herein we review recent advances in this area.

Regioselective functionalisation of dibenzothiophenes through gold-catalysed intermolecular alkyne oxyarylation

A protocol has been developed for direct Csp(3)-Csp(2) bond formation at the 4- and 6-positions of dibenzothiophenes using a gold(I) catalyst with terminal alkynes and dibenzothiophene-S-oxides. The sulfoxide acts as a traceless directing group to avoid the need to prefunctionalise at carbon. The iterative use of this protocol is possible and has been employed in the preparation of novel macrocyclic structures. In addition, a cascade process shows how oxyarylations can be combined with other processes resulting in complex, highly efficient transformations.

Au-catalyzed ring-opening reactions of 2-(1-alkynyl-cyclopropyl)pyridines with nucleophiles

A novel method for the C-C bond cleavage of cyclopropanes was developed by gold-catalyzed cycloisomerization of 2-(1-alkynyl-cyclopropyl)pyridine with nucleophiles, which provides efficient access to structurally diverse indolizines under mild conditions. A series of N-, C- and O-based nucleophiles were involved in this reaction to afford the corresponding indolizines in modest to excellent yields.

Gold-catalyzed cyclopropanation reactions using a carbenoid precursor toolbox

This review highlights recent advances in gold-catalyzed selective cyclopropanation divided by the type of carbenoid precursors.

Copper-mediated oxidative transformation of N-allyl enamine carboxylates toward synthesis of azaheterocycles

A method for synthesis of 3-azabicyclo[3.1.0]hex-2-enes has been developed by intramolecular cyclopropanation of readily available N-allyl enamine carboxylates. Two complementary reaction conditions, CuBr-mediated aerobic and CuBr2-catalyzed-PhIO2-mediated systems effectively induced stepwise cyclopropanation via carbocupration of alkenes. Oxidative cyclopropane ring-opening of 5-substituted 3-azabicyclo[3.1.0]hex-2-enes was also developed for synthesis of highly substituted pyridines. In addition, diastereoselective reduction of 3-azabicyclo[3.1.0]hex-2-enes to 3-azabicyclo[3.1.0]hexanes was achieved using NaBH3CN in the presence of acetic acid.

Mechanistic intricacies of gold-catalyzed intermolecular cycloadditions between allenamides and dienes

The mechanism of the gold-catalyzed intermolecular cycloaddition between allenamides and 1,3-dienes has been explored by means of a combined experimental and computational approach. The formation of the major [4+2] cycloaddition products can be explained by invoking different pathways, the preferred ones being determined by the nature of the diene (electron neutral vs. electron rich) and the type of the gold catalyst (AuCl vs. [IPrAu](+), IPr=1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). Therefore, in reactions catalyzed by AuCl, electron-neutral dienes favor a concerted [4+3] cycloaddition followed by a ring contraction event, whereas electron-rich dienes prefer a stepwise cationic pathway to give the same type of formal [4+2] products. On the other hand, the theoretical data suggest that by using a cationic gold catalyst, such as [IPrAuCl]/AgSbF6, the mechanism involves a direct [4+2] cycloaddition between the diene and the gold-activated allenamide. The theoretical data are also consistent with the observed regioselectivity as well as with the high selectivity towards the formation of the enamide products with a Z configuration. Finally, our data also explain the formation of the minor [2+2] products that are obtained in certain cases.