Synthesis of Functionalized Tetrahydroquinoline Containing Indole Scaffold via Chemoselective Annulation of Aza-ortho-quinone Methide Precursor

The chemoselective annulation of aza-ortho-quinone methide generated by in situ o-chloromethyl sulfonamide has been achieved with bifunctional acyclic olefin. This efficient approach provides access to the diastereoselective synthesis of functionalized tetrahydroquinoline derivatives containing indole scaffolds through the inverse-electron-demand aza-Diels-Alder reaction under mild reaction conditions with excellent results (up to 93% yield, > 20:1 dr). Moreover, this article realized the cyclization of α-halogeno hydrazone with electron-deficient alkene affording the tetrahydropyridazine derivatives, which had never been reported.

A Novel Method to Construct 2-Aminobenzofurans via [4 + 1] Cycloaddition Reaction of In Situ Generated Ortho-Quinone Methides with Isocyanides

A new approach for the synthesis of 2-aminobenzofurans has been described via Sc(OTf)₃ mediated formal cycloaddition of isocyanides with the in situ generated ortho-quinone methides (o-QMs) from o-hydroxybenzhydryl alcohol. Notably, as a class of readily available and highly active intermediates, o-QMs were first used in the construction of benzofurans. This [4 + 1] cycloaddition reaction provides a straightforward and efficient methodology for the construction of 2-aminobenzofurans scaffold in good yield (up to 93% yield) under mild conditions.

Chemodivergent Synthesis of Aza-Heterocycles with a Quarternary Carbon Center via [4 + 1] Annulation between Azoalkenes and α-Bromo Carbonyl Compounds

An efficient [4 + 1] annulation reaction between in situ generated azoalkene intermediates and α-bromocarbonyls has been established. A series of skeletally diverse aza-heterocycles with a functionalized quaternary center were obtained in up to 89% yield under mild conditions.

[4+n] Annulation Reactions Using ortho-Chloromethyl Anilines as Aza-ortho-Quinone Methide Precursors

Aza-ortho-quinone methides are important reactive intermediates that have found broad applications in synthetic chemistry. Recently, 1,4-elimination of ortho-chloromethyl aniline derivatives has emerged as a novel, powerful and convenient method for aza-ortho-quinone methide generation. This review will highlight the recent applications of aza-ortho-quinone methide precursors in annulation reactions to access various biologically important nitrogen-containing heterocycles. The general mechanisms are briefly discussed as well.

1 Introduction

2 [4+n] Annulation Reactions Using ortho-Chloromethyl Anilines as Aza-ortho-Quinone Methide Precursors

2.1 [4+2] Annulation Reactions

2.2 [4+1] Annulation Reactions

2.3 [4+3] Annulation Reactions

3 Conclusion and Perspective

Synthesis of Benzo[e][1,4]thiazepines by Base-Induced Formal [4+3] Annulation Reaction of Aza-o-quinone Methides and Pyridinium 1,4-Zwitterionic Thiolates

The base-induced formal [4+3] annulation reaction of in situ-formed aza-o-quinone methides and pyridinium 1,4-zwitterionic thiolates is reported. This protocol provides a novel and reliable method for the synthesis of biologically interesting benzo[e][1,4]thiazepine derivatives in synthetically useful yields. In addition, postsynthetic modification results in the formation of its sulfoxide and sulfone derivatives.

A [4+3] Cycloaddition Reaction of Aza-ortho-quinone Methides with C,N-Cyclic Azomethine Imines for Synthesis of 1,2,4-Triazepines

The base-induced formal [4+3] cycloaddition reaction of C,N-cyclic azomethine imines with aza-ortho-quinone methides, generated in situ, is reported. This protocol provided an efficient method for the synthesis of biologically important 1,2,4-triazepine derivatives, with a wide substrate scope and excellent functional-group tolerance, and it gives moderate to excellent yields under mild conditions. Several of the derivatives exhibited in vitro antitumor activities against the A2780 cell line in a screening of the cancer cell lines HCT-116, H2228, and A2780 by an MTT assay.

Recent Advances in Transition-Metal-Free (4+3)-Annulations

(4+3)-Annulations are incredibly versatile reactions which combine a 4-atom synthon and a 3-atom synthon to form both 7-membered carbocycles as well as heterocycles. We have previously reviewed transition-metal-catalyzed (4+3)-annulations. In this review, we will cover examples involving bases, NHCs, phosphines, Lewis and Brønsted acids as well as some rare examples of boronic acid catalysis and photocatalysis. In analogy to our previous review, we exclude annulations involving cyclic dienes like furan, pyrrole, cyclohexadiene or cyclopentadiene, as Chiu, Harmata, Fernándes and others have recently published reviews encompassing such substrates. We will however discuss the recent additions (2010–2020) to the literature on (4+3)-annulations involving other types of 4-atom-synthons.

1 Introduction

2 Bases

3 Annulations Using N-Heterocyclic Carbenes

3.1 N-Heterocyclic Carbenes (NHCs)

3.2 N-Heterocyclic Carbenes and Base Dual-Activation

4 Phosphines

5 Acids

5.1 Lewis Acids

5.2 Brønsted Acids

6 Boronic Acid Catalysis and Photocatalysis

7 Conclusion

(4 + 2) cyclization of aza-o-quinone methides with azlactones: construction of biologically important dihydroquinolinone frameworks

A base-promoted (4 + 2) cyclization of aza-o-quinone methides (aza-o-QMs) in situ generated from N-(o-chloromethyl)aryl amides was established. In this approach, azlactones were utilized as competent two-atom reaction partners to undergo (4 + 2) cyclization with aza-o-QMs, which afforded a series of dihydroquinolinone derivatives in overall good yields (up to 98%). This protocol has not only advanced the development of aza-o-QM-involved reactions, but also offered a useful method for constructing biologically important dihydroquinolinone frameworks.

Palladium-catalyzed [4 + 3] dearomatizing cycloaddition reaction of N-iminoquinolinium ylides

An efficient approach to obtain saturated seven-membered ring containing three heteroatoms has been developed by using a palladium-catalyzed [4 + 3] dearomatizing cycloaddition. This approach features mild conditions and good functional group tolerance.

An efficient and facile strategy for trifluoromethylation and perfluoroalkylation of isoquinolines and heteroarenes

An effective and regioselective strategy for trifluoromethylation and perfluoroalkylation of isoquinolines and heteroarenes was developed.

A concise construction of 4-alkynylquinazolines via [4 + 2] annulation of 4-alkynylbenzoxazinanones with acylhydroxamates under transition-metal-free conditions

A concise and highly efficient method for the construction of valuable 4-alkynylquinazolines under transition-metal-free conditions was developed via [4 + 2] annulation of 4-alkynylbenzoxazinanones with acylhydroxamates in good to excellent yields.