Formal [3 + 4] Annulation of α,β-Unsaturated Acyl Azoliums: Access to Enantioenriched N-H-Free 1,5-Benzothiazepines

NHC-Catalyzed Enantioselective Transformations Involving α-Bromoenals

α-Haloenals, especially, α-bromoenals considered as one of the important building blocks in organic synthesis. They can participate in various (3+2)-, (3+3)-, (3+4)-, and (2+4)-annulation reactions with other organic molecules in the presence of an NHC catalyst to produce enantioenriched carbo-, and heterocyclic compounds. Herein, we have described NHC-catalyzed enantioselective transformations of α-bromoenals in the synthesis of various heterocycles, and carbocycles, as well as acyclic organic compounds.

NBS-Promoted Synthesis of Thiocyanated Aminomaleimides and Site-Selective Intramolecular Cyclization Access to 1,4-Benzothiazepines via S-CN Bond Cleavage

A transition metal-free concise and efficient protocol for the synthesis of thiocyanated aminomaleimides and benzo[e][1,4]thiazepine derivatives has been developed. The method involves an initial α-C-H thiocyanation of aminomaleimides with KSCN and TEMPO-mediated tandem S-CN bond cleavage/intramolecular cyclization substitution processes, which enables the formation of seven-membered S/N-heterocycles. This synthetic strategy provides a reliable method for the synthesis of biologically interesting benzo[e][1,4]thiazepine derivatives by using KSCN as sulfur sources as well as expands the application of enaminones thiocyanation reactions in heterocycles synthesis.

Carbene-Catalyzed Access to Thiochromene Derivatives: Control of Reaction Pathways via Slow Release of Thiols from Disulfides

Substrates containing disulfide bonds, which are more stable and less smelling, could be used as thiophenol precursors in organic synthesis. Herein, an N-heterocyclic carbene (NHC)-catalyzed reaction between α-bromoenals and 2,2'-dithiodibenzaldehydes was developed. Through the sustained release strategy, the side reaction can be effectively inhibited, and the chiral thiochromene derivatives can be obtained with good yields and high optical purities. Application studies showed encouraging results when the desired products were explored for antimicrobial utilities in pesticide development.

Chemistry and Pharmacological diversity of Benzothiazepine - Excellent pathway to drug discovery

In this era of sporadic advancement in science and technology, a substantial amount of intervention is being set in motion to reduce health-related diseases. Discoveries from researchers have pinpointed the usefulness of heterocyclic compounds, amongst which benzothiazepine (BTZ) derivatives have been synthesized for their various pharmacological activities. This also contributes to their undeniable application in therapeutic medicine for the development of efficacious drugs. BTZs are compounds with a benzene ring fused with a thiazepine ring. This work contains several methods that have been used to synthesize 1,3-, 1,4-, 1,5-, and 4-1-benzothiazepine derivatives. In addition, up-to-date information about the crucial pharmacological activities of BTZ derivatives has been reviewed in this present study to appreciate their druggable potential in therapeutic medicine for drug development. Drug design and development have further been simplified with the implementation of computer aided approaches to predict biological interactions which can help in the design of several derivatives. Hence, the structural activity relationship (SAR), ADMET and the molecular docking studies of BTZ derivatives were discussed to further establish their interactions and safety in biological systems. This present work aims to expound on the reported chemistry and pharmacological propensity of BTZ moiety in relation to other relevant moieties to validate their potential as excellent pharmacophores in drug design and development.

Synthesis of the 1,5-Benzothiazepane Scaffold - Established Methods and New Developments

The 1,5-benzothiazepane structure is an important heterocyclic moiety present in a variety of commercial drugs and pharmaceuticals. This privileged scaffold exhibits a diversity of biological activities, including antimicrobial, antibacterial, anti-epileptic, anti-HIV, antidepressant, antithrombotic and anticancer properties. Its important pharmacological potential renders research into the development of new and efficient synthetic methods of high relevance. In the first part of this review, an overview of different synthetic approaches toward 1,5-benzothiazepane and its derivatives is provided, ranging from established protocols to recent (enantioselective) methods that promote sustainability. In the second part, several structural characteristics influencing biological activity are briefly explored, providing a few insights into the structure-activity relationships of these compounds.

Enantioselective Synthesis of δ-Carbolinones via N-Heterocyclic Carbene Catalysis

An enantioselective synthesis of δ-carbolinones was accomplished through N-heterocyclic carbene-catalyzed formal [3+3] annulation of 3-aminoindoles with 2-bromoenals. This protocol offers a rapid and efficient synthetic approach for accessing a wide range of enantioenriched structurally interesting δ-carbolinones under mild reaction conditions.

Total Synthesis of Mutanobactins A, B from the Human Microbiome: Macrocyclization and Thiazepanone Assembly in a Single Step

We report the first total syntheses of tricyclic mutanobactins A and B, lipopeptides incorporating a thiazepanone, isolated from Streptococcus mutans, a member of the human oral microbiome. A rapid, solid‐phase peptide synthesis (SPPS) based route delivers these natural products from a cascade of cyclization reactions. This versatile process was also employed in a streamlined synthesis of mutanobactin D. Additionally, we provide an independent synthesis of a truncated mutanobactin A analog, utilizing a novel thiazepanone amino acid building block.

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.

Revealing the Similarities of α,β-Unsaturated Iminiums and Acylazoliums in Organocatalysis

The secondary amine-catalyzed reactions proceeding via α,β-unsaturated iminiums and the N-heterocyclic carbene (NHC)-catalyzed transformations taking place via α,β-unsaturated acylazoliums are the two widely used electrophilic intermediates in organocatalysis. Over the last two decades, these two intermediates are extensively utilized for the enantioselective construction of valuable molecules. Both intermediates are generated by the covalent binding of catalysts to the substrates leading to LUMO activation of α,β-unsaturated carbonyls. A variety of soft nucleophiles are known to add to the α,β-unsaturated iminiums and acylazoliums in a conjugate fashion, and in many cases, striking similarity in reactivity has been observed. Having said this, there are few cases where these intermediates exhibit difference in reactivity. This Minireview is aimed at highlighting the resemblances in reactivity between α,β-unsaturated iminiums and acylazoliums thereby shedding light on the unnoticed parallels of the two intermediates in organocatalysis.

NHC-catalyzed covalent activation of heteroatoms for enantioselective reactions

Covalent activation of heteroatoms enabled by N-heterocyclic carbene (NHC) organic catalysts for enantioselective reactions is evaluated and summarized in this review. To date, sulfur, oxygen, and nitrogen atoms can be activated in this manner to react with another substrate to construct chiral carbon-heteroatom bonds with high optical enantioselectivities. The activation starts with addition of an NHC catalyst to the carbonyl moiety (aldehyde or imine) of substrates that contain heteroatoms. The key in this approach is the formation of intermediates covalently bound to the NHC catalyst, in which the heteroatom of the substrate is activated as a nucleophilic reactive site.

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

Theoretical study of the NHC-catalyzed C–S bond cleavage and reconstruction reaction: mechanism, stereoselectivity, and role of catalysts

The mechanism of the NHC-catalyzed C–S bond activation reaction has been theoretically studied and ELF analysis along with IRC results shows that the NHC promotes the cleavage of the C–S bond via an SN2 process.

Efficient Synthesis of 1,4-Thiazepanones and 1,4-Thiazepanes as 3D Fragments for Screening Libraries

1,4-Thiazepanes and 1,4-thiazepanones represent seven-membered ring systems with highly 3D character and are currently underrepresented in fragment screening libraries. A nuclear magnetic resonance (NMR) fragment screen identified 1,4-acylthiazepanes as new BET (bromodomain and extraterminal domain) bromodomain ligands; however, an efficient and readily diversified synthesis for library development has not been reported. Here we report a one-pot synthesis using α,β-unsaturated esters and 1,2-amino thiols to form 1,4-thiazepanones as precursors to 1,4-thiazepanes with high 3D character. This reaction proceeds in reasonable time (0.5-3 h) and in good yield and tolerates a broad scope of α,β-unsaturated esters. Several 1,4-thiazepanes were synthesized by a two-step transformation and were characterized as new BET bromodomain ligands using protein-observed 19F NMR. This synthesis should provide ready access to diverse 3D fragments for screening libraries.

Rh-Catalyzed Asymmetric Hydrogenation of Unsaturated Medium-Ring NH Lactams: Highly Enantioselective Synthesis of N-Unprotected 2,3-Dihydro-1,5-benzothiazepinones

A straightforward method to prepare 1,5-benzothiazepines was reported. Catalyzed by a Rh/Zhaophos complex, unsaturated cyclic NH lactams with a medium-size ring were hydrogenated smoothly, giving remarkably high enantioselectivities. The sulfur atom in the substrates did not bring an inhibition which was observed with commercially available bisphosphine ligands. This method was successfully applied in the scale-up synthesis of (R)-(-)-thiazesim.

Catalytic Enantioselective Synthesis of 1,4-Benzodioxepines

An efficient organocatalytic enantioselective synthesis of chiral 1,4-benzodioxepines is described. By proper incorporation of an intramolecular oxetane desymmetrization process, a range of benzylic alcohols bearing an internal oxetane reacted in the presence of a suitable chiral phosphoric acid catalyst to form chiral 1,4-benzodioxepines with high enantioselectivity. This process provides a new catalytic asymmetric example of direct synthesis of seven-membered heterocycles with good stereocontrol.

Computational study on NHC catalyzed [4+2] annulation between γ-chloroenals and pyrazolinones: mechanism and stereoselectivity

Mechanisms and origins of stereoselectivity of NHC-catalyzed reaction between γ-chloroenal and α-arylidene pyrazolinone.

Carbene-Catalyzed Enantioselective Addition of Thioamides to Bromoenals for Access to Thiazinone Heterocycles

A carbene-catalyzed reaction between α-bromoenals and thioamides was developed. Key steps include enantioselective 1,4-addition of thioamide sulfur atoms to α,β-unsaturated acyl azolium intermediate. Subsequent intramolecular annulation eventually affords thiazinone heterocycles with high optical purities. The introduction of Lewis acid additives such as Cu(OTf)₂ to this NHC catalytic reaction could provide small but consistent improvements to reaction enantioselectivities.

NHC-Catalyzed [3 + 3] Annulation of Thioamides and Modified Enals for the Enantioselective Synthesis of Functionalized Thiazinones

N-Heterocyclic carbene (NHC)-catalyzed [3 + 3] annulation of thioamides with modified enals allowing the enantioselective synthesis of functionalized 1,3-thiazin-4-ones is reported. The NHC generated from the chiral triazolium salt was optimal and the reaction is initiated by the thia-Michael addition to catalytically generated α,β-unsaturated acylazolium intermediates derived from 2-bromoenals, followed by intramolecular cyclization. This operationally simple procedure offers a straightforward and rapid access to target compounds in moderate to good yields and enantiomeric ratio values.

Access to Enantioenriched Spiro-ϵ-Lactam Oxindoles by an N-Heterocyclic Carbene-Catalyzed [4+3] Annulation of Flexible Oxotryptamines with Enals

Oxotryptamines were firstly used as flexible four-atom synthons in an NHC-catalyzed formal [4+3] annulation, providing a novel enantioselective method to access structurally diverse spiro-ϵ-lactam oxindoles with excellent enantioselectivities. This metal-free reaction features a broad substrate scope, excellent functional-group tolerance and proceeds under mild reaction conditions. Importantly, enantiopure privileged hexahydropyrroloindoles could be easily constructed by a one-pot process from the resulting spiro-ϵ-lactam oxindoles.

β-Sulfonylation of α-bromoenals enabled by N-heterocyclic carbene catalysis

An N-heterocyclic carbene (NHC)-catalyzed three-component tandem β-sulfonylation/esterification of α-bromoenals was developed.

Highly enantioselective synthesis of functionalized azepino[1,2-a]indolesviaNHC-catalyzed [3+4] annulation

The enantioselective [3+4] annulation of 3-formylindol-2-methylmalonates with 2-bromoenals catalyzed by NHCs is described to efficiently synthesise functionalized azepino[1,2-a]indoles.

N-Heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynoic acid esters with indolin-3-ones: access to functionalized pyrano[3,2-b]indol-2-ones

The structurally interesting pyrano[3,2-b]indol-2-ones were synthesized by an N-heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynoic acid esters.

[3 + 4] Annulation of Bromoenals and 1,2-Benzenedithiol: Base-Promoted [2 + 4] Reaction and N-Heterocyclic Carbene-Catalyzed Ring-Expansion

The [3 + 4] and [2 + 4] annulations of bromoenals and 1,2-benzenedithiol were developed in the presence or absence of N-heterocyclic carbene, respectively. Control experiment and kinetic investigation revealed the [3 + 4] annulation involves the base-promoted [2 + 4] annulation and the following carbene-catalyzed ring-expansion.

Catalytic enantioselective one-pot approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines

The first enantioselective catalytic approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines has been conveniently developed in a one-pot fashion, starting from α,β-unsaturated acyl pyrazoles and 2-aminothiophenol. The organocatalytic two-step sulfa-Michael/lactamization sequence is promoted by a readily available bifunctional thiourea and p-toluenesulfonic acid, respectively. The protocol enables access to both N-unprotected cis- and trans-diastereoisomers in moderate to satisfactory overall yields (up to 84%) and good to excellent ee values (up to 99%). Mechanistic investigations helped to shed light on the regio- and stereoselective outcome of the process.

Direct and Enantioselective Synthesis of N-H-Free 1,5-Benzodiazepin-2-ones by an N-Heterocyclic Carbene Catalyzed [3+4] Annulation Reaction

An NHC-catalyzed formal [3+4] annulation of α,β-unsaturated acylazoliums with protecting-group-free aryl 1,2-diamines was developed for a direct and highly enantioselective synthesis of 4-aryl N-H-free 1,5-benzodiazepin-2-ones. This methodology offers an efficient and rapid access to a wide range of enantioenriched target compounds from easily accessible starting materials. The protocol is also scalable and the desired products can easily undergo subsequent N-functionalization to afford diverse N-substituted derivatives. Additionally, a mechanism was proposed to explain the high enantioselectivity in this process.

N-Heterocyclic carbene-catalyzed annulation of ynals with amidines: access to 1,2,6-trisubstituted pyrimidin-4-ones

A thiazolium-catalyzed annulation of ynals and amidines has been reported to construct pyrimidin-4-ones.

Insights into N-heterocyclic carbene-catalyzed [3 + 4] annulation reactions of 2-bromoenals with N-Ts hydrazones

A DFT study toward N-heterocyclic carbene-catalyzed [3 + 4] annulation reactions of 2-bromoenals has been performed for the first time.