Conjugated nitrosoalkenes as Michael acceptors in carbon-carbon bond forming reactions: a review and perspective

Regioselective [3 + 2]/[4 + 2] Annulations of α-Nitrosostyrenes with Triazines

Reported herein are the regioselective [3 + 2]/[4 + 2] annulations of α-nitrosostyrenes with hexahydro-1,3,5-triazines, which provide five- and six-membered heterocycles in moderate yields under mild reaction conditions. Compared to previous reports, this unusual [3 + 2] annulation features the use of rhodium catalysts to control the regioselectivity of α-nitrosostyrenes, generated in situ from α-bromooximes 1. Meanwhile, the [4 + 2] annulation process was primarily regulated by 1,4-dioxane and temperature. In addition, DFT calculations reveal that the activation barrier of the [3 + 2] annulation is lower than the [4 + 2] process (1.7 kcal/mol) in the presence of a Rh catalyst. Finally, synthetic transformation of cyclic nitrones 3 and oxadiazines 4 was also demonstrated.

Building Up a Piperazine Ring from a Primary Amino Group via Catalytic Reductive Cyclization of Dioximes

Piperazine is one of the most frequently found scaffolds in small-molecule FDA-approved drugs. In this study, a general approach to the synthesis of piperazines bearing substituents at carbon and nitrogen atoms utilizing primary amines and nitrosoalkenes as synthons was developed. The method relies on sequential double Michael addition of nitrosoalkenes to amines to give bis(oximinoalkyl)amines, followed by stereoselective catalytic reductive cyclization of the oxime groups. The method that we developed allows a straightforward structural modification of bioactive molecules (e.g., α-amino acids) by the conversion of a primary amino group into a piperazine ring.

Iridium-Catalyzed and pH-Dependent Reductions of Nitroalkenes to Ketones

A highly chemoselective conversion of α,β-disubstituted nitroalkenes to ketones is developed. An acid-compatible iridium catalyst serves as the key to the conversion. At a 2500 S/C ratio, nitroalkenes were readily converted to ketones in up to 72% isolated yields. A new mechanistic mode involving the reduction of nitroalkene to nitrosoalkene and N-alkenyl hydroxylamine is proposed. This conversion is ready to amplify to a gram-scale synthesis. The pH value plays an indispensable role in controlling the chemoselectivity.

Unified Total Synthesis of the Limonoid Alkaloids: Strategies for the De Novo Synthesis of Highly Substituted Pyridine Scaffolds

Highly substituted pyridine scaffolds are found in many biologically active natural products and therapeutics. Accordingly, numerous complementary de novo approaches to obtain differentially substituted pyridines have been disclosed. This article delineates the evolution of the synthetic strategies designed to assemble the demanding tetrasubstituted pyridine core present in the limonoid alkaloids isolated from Xylocarpus granatum, including xylogranatopyridine B, granatumine A and related congeners. In addition, NMR calculations suggested structural misassignment of several limonoid alkaloids, and predicted their C3-epimers as the correct structures, which was further validated unequivocally through chemical synthesis. The materials produced in this study were evaluated for cytotoxicity, anti-oxidant effects, anti-inflammatory action, PTP1B and Nlrp3 inflammasome inhibition, which led to compelling anti-inflammatory activity and anti-oxidant effects being discovered.

α-Nitrosostyrenes as Three-Atom Units for the (3+1) Cyclization Reaction: Facile Access to 2,3-Dihydrodiazete N-Oxides and Their Diversified Synthetic Conversions

An unprecedented (3+1) cyclization of α-nitrosostyrenes, generated in situ from α-bromooximes, and N-tosyloxycarbamates was developed, which enables the synthesis of a range of structurally unique and hitherto unexplored 2,3-dihydrodiazete N-oxides in moderate to high yields. The products possess a highly strained four-membered ring structure containing two nitrogen atoms. The synthetic applicability of the products was also demonstrated by many important conversions to diverse nitrogen-containing compounds.

Mild synthesis of isoxazoline derivatives via an efficient [4 + 1] annulation reaction of transient nitrosoalkenes and sulfur ylides

An efficient [4 + 1] annulation between α-bromooximes and sulfur ylides via in situ generation of nitrosoalkenes under mild basic reaction conditions has been developed, providing an expeditious and scalable approach to synthesize biologically interesting isoxazoline derivatives with good to excellent yields.

Identification of rapid access to polycyclic systems via a base-catalyzed cascade cyclization reaction and their biological evaluation

A base-mediated cascade reaction between malonate esters and acrolein was developed to access complex polycyclic systems. This novel tandem reaction enables the simultaneous generation of up to seven new bonds and at least three new stereogenic centers. Mechanistic studies indicate a series of nucleophilic 1,4 and 1,6 Michael addition reactions occur, followed by an aldol condensation reaction, culminating in the formation of three fused rings. The compounds were characterized by NMR studies and the stereochemistry was confirmed by X-ray analysis. The ability to generate multigram quantities of such complex molecular scaffolds renders the method promising for medicinal chemistry campaigns. Herein, we also demonstrate that the lead compounds display promising anti-proliferative activities against human cancer cell models.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Michael Addition of P-Nucleophiles to Conjugated Nitrosoalkenes

A general approach to various α-phosphorus-substituted oximes (β-oximinoalkyl-substituted phosphonates, phosphine oxides, phosphine-borane complexes, and phosphonium salts) was developed. The strategy exploits hitherto unknown Michael addition of PH-containing compounds (diphenylphosphine oxide, diisopropyl phosphite, phosphine-borane complexes, and triphenylphosphonium bromide) to unstable conjugated nitrosoalkenes, which are generated in situ from corresponding nitrosoacetals. The resulting α-phosphorus-substituted oximes can be considered as useful P-, N-, and O-ligands for catalysis and precursors to valuable β-aminophosphonates.

Recent Advances in the Chemistry of Conjugated Nitrosoalkenes and Azoalkenes

This review aims to present the most recent contributions in the chemistry of nitrosoalkenes and azoalkenes, highlighting the chemical behavior that makes them important and versatile building blocks in organic synthesis. These are heterodienes used in the assembly of a variety of heterocyclic systems, spanning from five- to seven-membered heterocycles, as well as for the functionalization of heterocycles.