Chiral Brønsted Acid and Gold Catalyzed Enantioselective Synthesis of 1,8‐Dihydroindeno[2,1‐ b ]pyrroles

Enantioselective Assembly of Fully Substituted α-Amino Allenoates Through a Mannich Addition and Stepwise [3,3]-σ Rearrangement Sequence

Chiral fully-substituted allenes are synthetically significant and pivotal building blocks that can engage in diverse transformations toward a variety of bioactive molecules. The enantioselective assembly of these skeletons using readily available reactants offers significant advantages but remains challenging. Herein, an asymmetric formal Michael-type addition of alkynyl imines with the key alkylgold intermediates derived in situ from N-propargylamides is accomplished under gold-complex and chiral quinine-derived squaramide (QN-SQA) synergetic catalysis. Control experiments and the density functional theory (DFT) calculations indicated that this cascade reaction involves a Mannich-type addition and stepwise [3,3]-σ rearrangement sequence, leading to the fully substituted α-amino allenoates, which are elusive and take multi-step to prepare with other methods, in high yields and excellent enantioselectivity.

Synthesis of chiral systems featuring the pyrrole unit: a review

Synthetic strategies towards pyrroles within chiral frameworks are summarised, focussing on reports published 2010-2023. The synthesis of pyrroles featuring substituents bearing chiral centres are summarised, as are those whereby pyrroles are located within axially chiral systems courtesy of restricted bond rotation.

Catalytic (4+2) Annulation via Regio- and Enantioselective Interception of in-situ Generated Alkylgold Intermediate

A regio- and stereoselective stepwise (4+2) annulation of N-propargylamides and α,β-unsaturated imines/ketones has been accomplished with synergetic catalysis by a combination of a gold-complex and a chiral quinine-derived squaramide (QN-SQA), leading to highly functionalized chiral tetrahydropyridines/dihydropyrans in good to high yields with generally excellent enantioselectivity. Mechanistic studies and DFT calculations indicate that the in situ formed alkylgold species is the key intermediate in this transformation, and the amide group served as a traceless directing group in this highly selective transformation. This method complements the enantioselective (4+2) annulation of allene reagents, providing the formal internal C-C π-bond cycloaddition products, which is challenging and remains elusive.

Enantioselective merged gold/organocatalysis

Gold complexes, because of their unique carbophilic nature, have evolved as efficient catalysts for catalyzing various functionalization reactions of C-C multiple bonds. However, the realization of enantioselective transformations via gold catalysis remains challenging due to the geometrical constraints and coordination behaviors of gold complexes. In this context, merged gold/organocatalysis has emerged as one of the intriguing strategies to achieve enantioselective transformations which could not be possible by using a single catalytic system. Historically, in 2009, this field started with the merging of gold with axially chiral Brønsted acids and chiral amines to achieve enantioselective transformations. Since then, based on the unique reactivity profiles offered by each catalyst, several reports utilizing gold in conjunction with various chiral organocatalysts such as amines, Brønsted acids, N-heterocyclic carbenes, hydrogen-bonding and phosphine catalysts have been documented in the literature. This article demonstrates an up-to-date development in this field, especially focusing on the mechanistic interplay of gold catalysts with chiral organocatalysts.

Brønsted Acid-Catalyzed Dehydrative Nazarov-type Cyclization of CF3-Substituted 3-Indolylallyl Alcohols: Divergent Synthesis of 1-Trifluoromethylated Cyclopenta[b]indoles

An expedient and efficient synthetic method for the divergent synthesis of 1-trifluoromethylated cyclopenta[b]indoles that relies on Brønsted acid-catalyzed dehydrative Nazarov-type cyclization of CF3-substituted 3-indolylallyl alcohols is described. Two classes of 1-trifluoromethylated cyclopenta[b]indoles can be easily accessed simply by changing the NH-protecting group of indoles. With arylsulfonyl protected 3-indolylallyl alcohols as starting materials, the reaction provided the arylsulfonyl protected 1-trifluoromethylated cyclopenta[b]indoles in good to excellent yields, whereas pivaloyl (Piv) protected substrates led to the formation of NH-free 1-trifluoromethylated cyclolopenta[b]indoles with another alkenyl isomer. This protocol features tunable chemoselectivity, operational simplicity, excellent functional group compatibility, and mild metal-free conditions.

Brønsted Acid-Catalyzed Dehydrative Nazarov-Type Cyclization/C2-N1 Cleavage Cascade of Perfluoroalkylated 3-Indolyl(2-benzothienyl)methanols

A novel and unprecedented p-toluenesulfonic acid-catalyzed dehydrative Nazarov-type cyclization/C2-N1 bond cleavage cascade reaction of perfluoroalkylated 3-indolyl(2-benzothienyl)methanols has been developed. This reaction provides an efficient and practical protocol for the construction of highly functionalized benzothiophene-fused cyclopentenones with exclusive stereoselectivity. In addition, this cascade transformation also delineates a rare example of the involvement of the selective C2-N1 bond cleavage of indoles.

N-Triflylphosphoramides: highly acidic catalysts for asymmetric transformations

N-Triflylphosphoramides (NTPA), have become increasingly popular catalysts in the development of enantioselective transformations as they are stronger Brønsted acids than the corresponding phosphoric acids (PA). Their highly acidic, asymmetric active site can activate difficult, unreactive substrates. In this review, we present an account of asymmetric transformations using this type of catalyst that have been reported in the past ten years and we classify these reactions using the enantio-determining step as the key criterion. This compendium of NTPA-catalysed reactions is organised into the following categories: (1) cycloadditions, (2) electrocyclisations, polyene and related cyclisations, (3) addition reactions to imines, (4) electrophilic aromatic substitutions, (5) addition reactions to carbocations, (6) aldol and related reactions, (7) addition reactions to double bonds, and (8) rearrangements and desymmetrisations. We highlight the use of NTPA in total synthesis and suggest mnemonics which account for their enantioselectivity.

Direct synthesis of 2,3,5-trisubstituted pyrroles via copper-mediated one-pot multicomponent reaction

We have developed a copper-mediated one-pot synthesis of 2,3,5-trisubstituted pyrroles from 1,3-dicarbonyl compounds and acrylates using ammonium acetate as a nitrogen source. The reaction achieves C-C and C-N bond formation and provides an efficient approach to access highly functionalized pyrroles without further raw material preparation. This method is operationally simple, compatible with a wide range of functional groups, and provides the target products in moderate to good yields.

Gold Catalysed 1,4-Enyne Acetate Strategy for the Synthesis of 1H-Indenes and Partially Hydrogenated Methanonaphtho[1,2-c]furan-1,3(4H)-diones

A synthetic method to prepare 1H-indenes and partially hydrogenated methanonaphtho[1,2-c]furan-1,3(4H)-diones from gold(i)-catalysed 1,4-enyne acetate cycloisomerisation and oxidation or Diels–Alder reaction with maleic anhydride is described. The proposed mechanism involves Rautenstrauch rearrangement of the 1,4-enyne motif to give an insitu formed 1,3-cyclopentadiene intermediate. This is followed by 6-endo-dig cyclisation of the cyclic adduct and oxidation to give the aromatic carbocycle or Diels–Alder reaction with maleic anhydride to afford the bridged furan product.

Tandem Organocatalytic Cycloaromatization/Intramolecular Friedel-Crafts Alkylation Sequence for the Synthesis of Indolizinones and Pyrrolo-azepinone Derivatives

The organocatalytic synthesis of indolizinones and pyrrolo-azepinones has been accomplished in a tandem fashion through a sequence that comprises initial cycloaromatization followed by intramolecular Friedel-Crafts alkylation. The process takes place under Brønsted acid catalysis, giving rise to final products in moderate to good yields. Attempts to carry out the tandem protocol in an enantioselective fashion were performed with chiral (R)-BINOL-derived N-triflyl phosphoramides. After initial optimization, the tandem process took place with moderate levels of enantioselectivity.

Gold-Catalysed Oxidative Cycloisomerisation of 1,6-Diyne Acetates to 1-Naphthyl Ketones

A synthetic method to prepare 1-naphthyl ketones from gold(i)-catalysed oxidative cycloisomerisation of 1,6-diyne acetates is described. The proposed mechanism involves cyclopropenation–cycloreversion of the 1,6-diyne motif initiated by a 1,2-acyloxy migration. This is followed by nucleophilic attack of the ensuing gold carbenoid species by a molecule of water and autoxidation to give the aromatic product.