Hexafluoroantimonic Acid Catalysis: Formal [3+2+2] Cycloaddition of Aziridines with Two Alkynes

Synthesis of Tetrahydroazepines through Silyl Aza-Prins Cyclization Mediated by Iron(III) Salts

A new methodology for the synthesis of seven-membered unsaturated azacycles (tetrahydroazepines) was developed. It is based on the powerful aza-Prins cyclization in combination with the Peterson-type elimination reaction. In a single reaction step, a C–N, C–C bond and an endocyclic double bond are formed. Under mild reaction conditions and using iron(III) salts as sustainable catalysts, tetrahydroazepines with different degrees of substitution are obtained directly and efficiently. DFT calculations supported the proposed mechanism.

Iodine-Mediated Pyridine Ring Expansion for the Construction of Azepines

A synthesis of azepines by the expansion of pyridine rings promoted by iodine in air is reported. The scope of the method is demonstrated with 27 examples. Two iodinations are involved in the reaction, in which the second iodination leads to the opening of the six-membered ring.

Catalyst-controlled cycloisomerization/[4+3] cycloaddition sequence to construct 2,3-furan-fused dihydroazepines and 2,3-pyrrole-fused dihydrooxepines

A novel catalyst-controlled cycloisomerization/[4+3]cycloaddition sequence of readily available acyclic enyne-amides and crotonate-derived sulfur ylides is reported. This strategy enables the rapid and efficient construction of a series of bicyclic 2,3-furan-fused...

Copper and Rhodium Relay Catalysis for Selective Access to cis-2,3-Dihydroazepines

A new catalytic protocol to access synthetically challenging cis-2,3-dihydroazepines is reported. The reaction starts with readily available dienals, alkynes, and sulfonyl azides as the substrates and employs copper and rhodium as relay catalysts. Key steps include a copper-catalyzed reaction between an alkyne and a sulfonyl azide to form a triazole intermediate. The subsequent activation of this triazole intermediate by a rhodium catalyst, followed by a reaction with the dienal substrate, eventually leads to the dihydroazepine product. The regio- and stereochemistries of the products are believed to be controlled through a stereospecific conrotatory 8π-electrocyclization process against a possible competing 6π-electrocyclization process.

Copper-Catalyzed Cycloisomerization of Unactivated Allene-Tethered O-Propargyl Oximes: A Domino Reaction Sequence toward the Synthesis of Hexahydropyrrolo[3,4-b]azepin-5(4H)-ones

A novel copper-catalyzed cycloisomerization of unactivated allene-tethered O-propargyl oximes has been developed for the synthesis of hexahydropyrrolo[3,4-b]azepin-5(4H)-ones. This one-pot domino reaction proceeds via a [2,3]-sigmatropic rearrangement, a [3 + 2] cycloaddition, and another [3,3]-sigmatropic rearrangement. The methodology offers a practical and straightforward route for the rapid assembly of both ring components of the fused bicyclic motifs from acyclic precursors by simultaneously forming four new bonds (a C═O, a C═N, and two C-C bonds) in a single step.

Catalytic Asymmetric Construction of a 1,2,4-Benzotriazepine Skeleton via Diastereo- and Enantioselective Decarboxylative [4 + 3] Cyclization

We have developed a protocol for palladium-catalyzed decarboxylative [4 + 3] cycloaddition reactions between 4-vinyl benzoxazinanones and azomethine imines to generate moderate to good yields of structurally diverse 1,2,4-benzotriazepines bearing two stereogenic centers with good to excellent stereoselectivities. This protocol not only addresses the challenge of asymmetrically constructing compounds with a 1,2,4-benzotriazepine skeleton but also demonstrates the utility of decarboxylative cycloadditions for the synthesis of enantioenriched polycyclic compounds.

Aziridine used as a vinylidene unit in palladium-catalyzed [2 + 2 + 1] domino annulation

A series of chromone fused methylenecyclopentanes are efficiently constructed in moderate to good yields by Pd-catalyzed [2 + 2 + 1] annulation, in which aziridine is used as a vinylidene unit by cleavage of two C–N bonds for the first time.

Asymmetric β,γ′-regioselective [4 + 3] and [4 + 2] annulations of α-vinylenals via cascade iminium ion-dienamine catalysis

α-Vinylenal substrates have been designed and applied in β,γ′-regioselective asymmetric [4 + 3] and [4 + 2] annulation reactions via cascade aminocatalysis.

Recent Advances in Cycloaddition Reactions with Alkynes to Construct Heterocycles

Heterocyclic compounds, especially N-heterocycles and O-heterocycles, are prominent structural motifs present in numerous natural products and medically and/or economically important compounds. This review aims to describe the development of transition-metal-catalyzed cycloaddition reactions of functionalized m-atom partners with alkynes to access a wide range of five-, six-, and seven-membered heterocycles, that is functionalized N-heterocycles and O-heterocycles such as azepines, isoquinolines, isocoumarins, spiroheterocycles, indoles, furans, and pyrroles, in a selectively controlled manner with an emphasis on scope and limitations and with a discussion of the mechanisms.

1 Introduction

2 Intermolecular Cycloaddition To Construct Azepine Derivatives

2.1 [5+2] Cycloaddition

2.2 [3+2+2] Cycloaddition

2.3 [3+2]/[5+2] Cycloaddition

3 Intermolecular [4+2] Cycloaddition To Construct Isoquinolines or Isocoumarins

4 Intermolecular [3+2] Cycloaddition To Construct Spirohetero­cyclic Compounds, Indoles, Furans, and Pyrroles

5 Summary and Outlook

Metal-free aza-Claisen type ring expansion of vinyl aziridines: an expeditious synthesis of seven membered N-heterocycles

A metal-free approach for the synthesis of 7-membered aza-heterocycles has been developed by the intermolecular [5 + 2] cycloaddition of non-activated vinylaziridines and alkynes. This method has a broad substrate scope under mild reaction conditions to afford structurally diverse 7-membered N-heterocycles in high yield up to 92%.

Ring-opening formal hetero-[5+2] cycloaddition of 1-tosyl-2,3-dihydro-1H-pyrroles with terminal alkynes: entry to 1-tosyl-2,3-dihydro 2,3-dihydro-1H-azepines

Lewis acid-catalyzed formal hetero-[5+2] cycloaddition of 2,3-dihydro-1H-pyrroles with alkynes through C(sp²)–N bond cleavage toward 2,3-dihydro-1H-azepines is described.

Oxidative Cascade Reaction of N-Aryl-3-alkylideneazetidines and Carboxylic Acids: Access to Fused Pyridines

A versatile silver-promoted oxidative cascade reaction of N-aryl-3-alkylideneazetidines with carboxylic acids is reported, providing a very efficient pathway to functionalized fused pyridines. This method allows introduction of fused pyridine ring systems to heterocycles, drugs, and natural products. A mechanistic study revealed that silver salt is essential for the chemo- and regioselective ring expansion, sequential oxidative nucleophilic additions, and oxidative aromatization. This approach represents the first example of the strained N-heterocycles undergoing a cascade reaction with a π bond and a nucleophile together.

Nucleophilic ring opening reactions of aziridines

Aziridine ring opening reactions have gained tremendous importance in the synthesis of nitrogen containing biologically active molecules. During recent years, a great effort has been put forward by scientists toward unique bond construction methodologies via ring opening of aziridines. In this regard, a wide range of chiral metal- and organo-catalyzed desymmetrization reactions of aziridines have been reported with carbon, sulfur, oxygen, nitrogen, halogen, and other nucleophiles. In this review, an outline of methodologies adopted by a number of scientists during 2013-2017 for aziridine ring opening reactions as well as their synthetic applications is described.

Ring-opening and cyclization of aziridines with aryl azides: metal-free synthesis of 6-(triflyloxy)quinolines

A metal-free synthesis of 6-(triflyloxy)quinolines has been developed via the ring-opening and cyclization of 2-aryl-1-tosylaziridines with 2-azidobenzaldehydes in the presence of TfOH.

Alkene functionalization for the stereospecific synthesis of substituted aziridines by visible-light photoredox catalysis

A novel strategy involving visible-light-induced functionalization of alkenes for the stereospecific synthesis of substituted aziridines was developed.

Accessing benzooxadiazepinesviaformal [4 + 3] cycloadditions of aza-o-quinone methides with nitrones

Seven membered N-heterocycles were constructed through a formal [4 + 3] cycloaddition of nitrones within situgenerated aza-o-quinone methides.

AgOTf-catalyzed dehydrative [3+2] annulation of aziridines with 2-naphthols

The reaction of 2-naphthols with aziridines in the presence of AgOTf resulting in a dehydrative, formal [3+2] annulation is reported. The reaction allows the synthesis of functionalized benzoindolines, and tolerates a broad range of functional groups. A preliminary study on the mechanism of this reaction indicates an S_N1-type ring-opening of aziridines. This method is demonstrated for the one-pot synthesis of benzoindoles.

[4 + 3] Cycloaddition of Phthalazinium Dicyanomethanides with Azoalkenes Formed in Situ: Synthesis of Triazepine Derivatives

[4 + 3] cycloaddition of phthalazinium dicyanomethanides with in situ formed azoalkenes was achieved, providing an access to various 1,2,4-triazepine derivatives in moderate to excellent yields.