Nitrones as Trapping Reagents of α,β-Unsaturated Carbene Intermediates - [1,2]Oxazino[5,4-b]indoles by a Platinum- Catalyzed Intermolecular [3+3] Cycloaddition

Ascending of Cycloaddition Strategy for N–O Heterocycles

The N–O heterocycles are biologically relevant scaffolds and versatile building blocks in contemporary organic synthesis. In this short review, we showcase the involvement and elevation of various cycloaddition strategies towards the production of the N–O heterocycles; 1,2-oxazines and 1,2-oxazinanes, 1,2-oxazepanes, and 1,2-oxazetidines. An overview of the advantages and challenges associated with these synthetic endeavors is provided.

1 Introduction

2 Six-Membered N–O Heterocycles (1,2-Oxazines and 1,2-Oxazinanes)

3 Seven-Membered N–O Heterocycles (1,2-Oxazepanes)

4 Four-Membered N–O Heterocycles (1,2-Oxazetidines)

5 Summary and Outlook

Regioselectivity Influences in Platinum-Catalyzed Intramolecular Alkyne O-H and N-H Additions

The steric and electronic drivers of regioselectivity in platinum-catalyzed intramolecular hydroalkoxylation are elucidated. A branch point is found that divides the process between 5-exo and 6-endo selective processes, and enol ethers can be accessed in good yields for both oxygen heterocycles. The main influence arises from an electronic effect, where the alkyne substituent induces a polarization of the alkyne that leads to preferential heteroatom attack at the more electron-deficient carbon. The electronic effects are studied in other contexts, including hydroacyloxylation and hydroamination, and similar trends in directionality are predominant although not uniformly observed.

A novel Pd-catalysed sequential carbonylation/cyclization approach toward bis-N-heterocycles: rationalization by electronic structure calculations

An unprecedented palladium-catalysed sequential aminocarbonylation/cyclization synthetic strategy, using carbon monoxide and structurally different aliphatic diamines as N-nucleophiles, gives access, in one pot, to a new family of indole-based N-heterocyclic derivatives (hydropyrazinones, benzodiazepinones and hydroquinoxalines). Optimization of the reaction conditions towards double carbonylation (P _CO = 30 bar, T = 80°C, iodoindole/diamine ratio = 1 : 1.5, toluene as solvent) allowed the target cyclic products, which are formed in situ via intramolecular cyclization of the ketocarboxamide intermediates, to be obtained through a nucleophilic addition/elimination reaction with the pendant terminal amine groups. The structure of the diamine nucleophile was revealed to affect the reaction's selectivity, with the best yields for the cyclic products being obtained in the presence of (1S,2S)-(+)-cyclohexane-1,2-diamine (a) as the nucleophile, using either 5- or 7-iodoindole as the substrate. The reaction's selectivity was rationalized based on electronic structure calculations, which explain the effect of the diamine structure on the predominant formation of the cyclic products.

α-Imino Gold Carbenes from 1,2,4-Oxadiazoles: Atom-Economical Access to Fully Substituted 4-Aminoimidazoles

A novel and atom-economical synthesis of fully substituted 4-aminoimidazoles via gold-catalyzed selective [3 + 2] annulation of 1,2,4-oxadiazoles with ynamides is reported. This protocol represents a new strategy to access α-imino gold carbenes, which corresponds to an unprecedented intermolecular transfer of N-acylimino nitrenes to ynamides. Moreover, the reaction proceeds with 100% atom economy, exhibits good functional group tolerance, and can be conducted in gram scale.

An efficient approach to generate aryl carbenes: gold-catalyzed sequential activation of 1,6-diynes

An efficient approach to generate aryl gold carbenes has been developed.

A formal intermolecular [4 + 2] cycloaddition reaction of 1,3-disubstituted indoles and alkylquinones

A formal [4 + 2] cycloaddition reaction of 1,3-disubstituted indoles and alkylquinones was realized to afford polycyclic indolines in good yields.

Platinum-Catalyzed α,β-Unsaturated Carbene Formation in the Formal Syntheses of Frondosin B and Liphagal

Formal syntheses of tetracyclic terpenoids frondosin B and liphagal are described. Both synthetic routes rely on the use of platinum-catalyzed α,β-unsaturated carbene formation for the key C-C bond forming transformations. The successful route toward frondosin B utilizes a formal (4 + 3) cycloaddition, while the liphagal synthesis features the vinylogous addition of an enol nucleophile as a key step. Both synthetic routes are discussed, revealing insights into structural requirements in the catalytic α,β-unsaturated carbene reaction manifold.

Lewis Acid Mediated Vinylogous Additions of Enol Nucleophiles into an α,β-Unsaturated Platinum Carbene

A variety of substituted indoles and benzofurans are accessed via a platinum catalyzed annulation and vinylogous addition of enol nucleophiles. Several β-dicarbonyl compounds participate in the reaction, as do α-nitro and α-cyano carbonyl species. Subjecting the indole products to acidic conditions results in the formation of fused heterocycles.

Highly diastereoselective and regioselective copper-catalyzed nitrosoformate dearomatization reaction under aerobic-oxidation conditions

An unprecedented copper-catalyzed acylnitroso dearomatization reaction, which expands the traditional acylnitroso ene reaction and acylnitroso Diels-Alder reaction to a new type of transformation, has been developed under aerobic oxidation. Intermolecular and intra-/intermolecular reaction modes demonstrate an entirely different N- or O-acylnitroso selectivity. Hence, we can utilize this reaction as a highly diastereoselective access to a series of new pyrroloindoline derivatives, which are important structural motifs for natural-product synthesis.

Flexible and practical synthesis of 3-oxyindoles through gold-catalyzed intermolecular oxidation of o-ethynylanilines

A novel gold-catalyzed intermolecular oxidation of o-ethynylanilines has been developed, which provides a reliable access to synthetically useful 3-oxyindoles. Importantly, this gold-catalyzed oxidative process outcompetes the typical indole formation.