Au(I)/Sc(III)-co-catalyzed tandem spiroannulation/cycloisomerization of 3-(2-ethynylaryl)-N-tosylaziridines with indoles to access 5H-benzo[b]carbazoles

An unusual spiroannulation/cycloisomerization cascade of 3-(2-ethynylaryl)-N-tosylaziridines with indoles enabled by cooperative gold/scandium catalysis is presented, which facilitates the synthesis of 5H-benzo[b]carbazoles in moderate to excellent yields. This protocol features a broad substrate scope and good functional-group compatibility. Additionally, the resulting 5H-benzo[b]carbazoles exhibit good fluorescence properties, demonstrating the synthetic practicality of this method. Moreover, control experiments were performed to illustrate the reaction mechanism.

Metal-mediated synthesis of pyrrolines

The five-membered, nitrogen-containing pyrroline ring is a privileged structure. This ring is present in many bioactive compounds from natural sources. Pyrrolines-the dihydro derivatives of pyrroles-have three structural isomer classes, depending on the location of the double bond: 1-pyrrolines (3,4-dihydro-2H-pyrroles), 2-pyrrolines (2,3-dihydro-1H-pyrroles) and 3-pyrrolines (2,5-dihydro-1H-pyrroles). This review aims to describe the latest advances for the synthesis of pyrrolines by transition metal-catalyzed cyclizations. Only reactions in which the pyrroline ring is formed by metal promotion are described. Transformations of the pyrroline ring in other heterocycles, and the structural manipulations of the pyrroline itself are not discussed. The review is organized into three parts, each covering the metal-mediated synthesis of the three pyrroline isomers. Each part is subdivided according to the metal involved, and concludes with a brief description of notable biological activities within the class.

Au-Catalyzed Hexannulation and Pt-Catalyzed Pentannulation of Propargylic Ester Bearing a 2-Alkynyl-phenyl Substituent: A Comparative DFT Study

The mechanistic pathways of metal-catalyzed pentannulation and hexannulation of aromatic enediyne were studied quantum mechanically with Pt and Au salts. In agreement with the experimental facts, our result shows that the pentannulation favors over the hexannulation under Pt-catalyzed conditions and the reverse possibility favors when the Pt salt is replaced with an Au one. The Pt-catalyzed reaction involves a long-range acyl migration that follows the cyclization step. Our study reveals that such migration takes place under the assistance of a ligand of the metal atom. Moreover, the variation of aromaticity (probed by the change of the nucleus-independent chemical shift (0) value) in the cyclization steps shows that both processes maintain the development of the aromatic character of the generated intermediate during the progress of the reaction.

Breaking aziridines to construct morpholines with a gold(i)-catalyzed tandem ring-opening and cycloisomerization reaction

A convenient synthetic method for the construction of morpholine derivatives from easily available aziridines and propargyl alcohols has been successfully developed. A tandem nucleophilic ring-opening of aziridine/6-exo-dig cyclization/double bond isomerization sequence was achieved by using a single gold(i) catalyst under mild conditions. The gold(i) catalyst served as a π acid and also a σ acid to realize the dual activation of both reactants in this reaction. The obtained unsaturated morpholine products could be easily hydrogenated to achieve target morpholine derivatives with good diastereoselectivities in high yields.

Exploiting the σ-phylic properties of cationic gold(i) catalysts in the ring opening reactions of aziridines with indoles

Gold(i) species catalyze the ring opening reactions of aziridines with indoles as nucleophiles for the synthesis of tryptamines and pyrroloindolines.

Au(I)-catalyzed annulation of propargyl amine with aldehydes: one-pot cascade synthesis of 2,5-dimethylpyrazines

3-Substituted 2,5-dimethylpyrazines were synthesized in high yields via a one-pot cascade annulation of easily available propargyl amine with aldehydes catalyzed by Au(PPh2Cy)Cl.

Efficient synthesis of tetrahydronaphthalene- or isochroman-fused spirooxindoles using tandem reactions

The cascade reaction involving a Michael–aldol or vinylogous Henry-acetalization relay is described. We have used the cascade reaction to assemble tetrahydronaphthalene- or isochroman-fused spirooxindoles and other drug-like spirocyclic scaffolds.

Gold-catalyzed synthesis of functionalized pyridines by using 2H-azirines as synthetic equivalents of alkenyl nitrenes

2H-Azirines are easily synthesized from the corresponding ketones and, despite possessing a CN bond embedded in a strained three-membered cycle, they are sufficiently stable to be isolated, stored, and manipulated. 2H-Azirines can be regarded as valuable synthetic equivalents of alkenyl nitrenes, however, reactions capitalizing on the cyclic strain of the heterocyclic motif and involving the cleavage of the CN single bond remain surprisingly limited. A gold-catalyzed reaction that allows the formation of polysubstituted functionalized pyridines from easily accessible 2-propargyl 2H-azirine derivatives was developed. This transformation, which corresponds to an unprecedented intramolecular transfer of an alkenyl nitrene to an alkyne, proceeds with low catalyst loading, is efficient, and exhibits a high functional-group tolerance and a wide substrate scope.

Gold(I)-catalyzed rearrangement of N-aryl 2-alkynylazetidines to pyrrolo[1,2-a]indoles

Various N-aryl-2-alkynylazetidines were very efficiently converted to pyrrolo[1,2-a]indoles with gold catalysts, especially the 2-biphenyl-dicyclohexylphosphino-gold(I) hexafluoroantimonate, in dichloromethane at room temperature. Additionally, two formal syntheses of bioactive non-natural compounds, i.e. 7-methoxymitosene and an 5-HT(2C) receptor agonist, have been achieved.

Gold versus silver catalyzed intramolecular hydroarylation reactions of [(3-arylprop-2-ynyl)oxy]benzene derivatives

The scope and the generality of gold versus silver catalyzed intramolecular hydroarylation reactions of 3-[(3-arylprop-2-ynyl)oxy]benzene derivatives in terms of rings substitution were investigated. Only products deriving from 6-endo cyclization were exclusively formed. The features of substituents had a considerable effect on the reaction outcome in the presence of silver catalysis, whereas gold catalysis revealed a unique blend of reactivity and selectivity and represented the only choice for the intramolecular hydroarylation reaction of the starting substrates bearing electron deficient arenes.

Mechanistic and sterochemical insights on the Pt-catalyzed rearrangement of oxiranylpropargylic esters to cyclopentenones

A mechanism for the rearrangement of oxiranylpropargylic esters to cyclopentenones catalyzed by PtCl(2) is proposed based on DFT calculations (M06/6-31++G(d,p)). Although the basic steps are coincidental with those proposed by Sarpong et al., who characterized a 2H-pyran as intermediate, calculations have revealed other intricate details of this complex rearrangement. The 2H-pyran is proposed to result from the ring-opening of a bicyclic oxonium ion that follows the nucleophilic capture by the epoxide of a platinum carbene generated by an initial Pt-mediated 1,2-propargylic rearrangement. The key steps in the evolution of this system are the electrocyclic ring-opening of the 2H-pyran to a α-methoxycarbonyl dienone and an iso-Nazarov ring closure. Prior to those, changes in hapticity and in the conformation of the dienone are required in order to produce the helical conformation needed to generate a single diastereomer of the cyclopentenone product obtained experimentally. The metal is needed well beyond the first step of the mechanism, and both electrocyclic reactions are favored by coordination to the metal when compared to their uncomplexed counterparts. Moreover, we have experimentally demonstrated that the rearrangement is stereoconvergent, a feature that is traced back to the initial configuration of the epoxide, which determines the somewhat counterthermodynamic placement of the metal syn to the methyl group of the stereogenic center in the 2H-pyran intermediate. Finally, starting from enantiopure oxiranylpropargylic ester 13, a racemate of cyclopentenone (R*,S*)-16 was obtained. Thus, the sequence does not proceed with memory of chirality, and the absolute stereochemical information is already lost at the stage of the 2H-pyran 14.