Synthesis of Azepine- and Azocine-Annulated Heterocycles by Aromatic Aza-Claisen Rearrangement and Ring-Closing Metathesis

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.

Reactivity switch-over of 4-hydroxydithiocoumarins under various conditions and their application in organic synthesis

4-Hydroxydithiocoumarin is a valuable organic precursor to architect important heterocycles. The three different reactive nucleophilic sites in 4-hydroxydithiocoumarins display intriguing regioselectivity in their reaction towards various electrophiles. Previously, 4-hydroxydithiocoumarins have been used in the synthesis of heterocycles using Claisen and thio-Claisen reactions. Hetero-Diels-Alder reactions involving 4-hydroxydithiocoumarins have proven to be very convenient in assembling complex molecular organic frameworks from readily available feedstocks. Development of multicomponent reactions employing 4-hydroxydithiocoumarins has given rise to several important reaction protocols to access polycyclic compounds. Recently, this moiety has been used in forging some unusual S-S, S-N and S-O bonds under oxidative conditions which further explores its hidden reactivity in organic synthesis. Besides that, hydrothiolation of various alkynes and alkenes using 4-hydroxydithiocoumarins has led to the synthesis of some potential lead molecules. This mini-review provides an account of the reactivity pattern of 4-hydroxydithiocoumarins and their strategic applications in various reactions for the synthesis of several heterocycles and other important organic syntheses.

Convenient synthesis of quinoline-fused triazolo-azepine/oxepine derivatives through Pd-catalyzed C–H functionalisation of triazoles

The convenient synthesis of a novel polycyclic fused system comprising three different heterocycles, viz., quinolines, azepines/oxepines and triazoles is presented in high yields.

Ring-closing-metathesis-based synthesis of annellated coumarins from 8-allylcoumarins

8-Allylcoumarins are conveniently accessible through a microwave-promoted tandem Claisen rearrangement/Wittig olefination/cyclization sequence. They serve as a versatile platform for the annellation of five- to seven-membered rings using ring-closing olefin metathesis (RCM). Furano-, pyrano-, oxepino- and azepinocoumarins were synthesized from the same set of precursors using Ru-catalyzed double bond isomerizations and RCM in a defined order. One class of products, pyrano[2,3-f]chromene-2,8-diones, were inaccessible through direct RCM of an acrylate, but became available from the analogous allyl ether via an assisted tandem catalytic RCM/allylic oxidation sequence.

Synthesis of medium-sized aryl-fused nitrogenous heterocycles via sequential aryne aza-Claisen rearrangement/ring-closing metathesis

The reaction of arynes and secondary allylamines furnished ortho-allyl-substituted N-arylanilines via an aza-Claisen rearrangement. In this transformation, the sequential formation of C–C and C–N bonds occurred by involving two aryne molecules under metal-free reaction conditions to provide moderate to good yields of the products.

O,N,B-Containing eight-membered heterocycles by ring expansion of boroles with nitrones

A route to novel O,N,B-containing eight-membered rings, which represent a new structural motif in heterocyclic chemistry, was developed by ring expansion of boroles with nitrones.

Photoassisted diversity-oriented synthesis: accessing 2,6-epoxyazocane (oxamorphan) cores

The modular synthesis of photoprecursors and their photoinduced cyclization into substituted 1-benzazocanes of two distinct topologies is described. The key step producing an extended polyheterocyclic system involves the photogeneration of azaxylylenes and their subsequent intramolecular cycloaddition with furan-containing pendants tethered either via the aniline nitrogen or through the carbonyl group containing arm. The primary photoproducts-secondary or tertiary anilines which are not acylated at the nitrogen atom-undergo facile acid-catalyzed or spontaneous ring-opening-ring-closing rearrangement to yield fused polyheterocyclic structures possessing a 2,6-epoxyazocane (or oxamorphan) core.

Facile cyclization in the synthesis of highly fused diaza cyclooctanoid compounds using retrievable nano magnetite-supported sulfonic acid catalyst

Magnetically separable Fe₃O₄–SO₃H nano particles have been prepared and established as an efficient catalyst for the synthesis of dihydrofuran fused cyclooctanoids via the dehydration reaction of dihydroxy cyclooctanoid heterocylces.

N-allyl-N-sulfonyl ynamides as synthetic precursors to amidines and vinylogous amidines. An unexpected N-to-C 1,3-sulfonyl shift in nitrile synthesis

A detailed study of amidine synthesis from N-allyl-N-sulfonyl ynamides is described here. Mechanistically, this is a fascinating reaction consisting of diverging pathways that could lead to deallylation or allyl transfer depending upon the oxidation state of palladium catalysts, the nucleophilicity of amines, and the nature of the ligands. It essentially constitutes a Pd(0)-catalyzed aza-Claisen rearrangement of N-allyl ynamides, which can also be accomplished thermally. An observation of N-to-C 1,3-sulfonyl shift was made when examining these aza-Claisen rearrangements thermally. This represents a useful approach to nitrile synthesis. While attempts to render this 1,3-sulfonyl shift stereoselective failed, we uncovered another set of tandem sigmatropic rearrangements, leading to vinyl imidate formation. Collectively, this work showcases the rich array of chemistry one can discover using these ynamides.

A divergent mechanistic course of Pd(0)-catalyzed aza-Claisen rearrangement and aza-Rautenstrauch-type cyclization of N-allyl ynamides

A fascinating mechanistic study of ynamido-palladium-pi-allyl complexes is described that features isolation of a unique silyl ketenimine via aza-Claisen rearrangement, which can be accompanied by an unusual thermal N-to-C 1,3-Ts shift in the formation of tertiary nitriles and a novel cyclopentenimine formation via a palladium-catalyzed aza-Rautenstrauch-type cyclization pathway.