Hybrid Metal/Organo Relay Catalysis Enables Enynes To Be Latent Dienes for Asymmetric Diels–Alder Reaction

Enantioconvergent and diastereoselective synthesis of atropisomeric hydrazides bearing a cyclic quaternary stereocenter through ternary catalysis

An efficient and highly enantioconvergent and diastereoselective ternary catalysis in a one-pot process is reported, which represents an integrated strategy for the synthesis of atropisomeric hydrazides with defined vicinal central and axial chirality from readily available racemic α-amino-ynones, azodicarboxylates, and Morita-Baylis-Hillman (MBH) carbonates. This method utilizes in situ-generated racemic pyrrolin-4-ones via hydroamination of racemic α-amino-ynones by AuCl catalysis as a novel and versatile C1 synthon, which engage commercially available azodicarboxylates to generate amination products in high yields and uniformly excellent enantioselectivities under the catalysis of a chiral phosphoric acid. Following amination, N-alkylation catalyzed by diastereoselective organocatalyst afforded axially chiral hydrazides with excellent diastereoselectivities (>98 : 2 dr). The synthetic utility of the amination products and axially chiral hydrazides was also demonstrated by their facile conversion to diverse molecules in high yields with excellent stereopurity. Density functional theory calculations were performed to understand the origin of diastereoselectivity.

Phosphine-Promoted Tandem Intermolecular Diels-Alder Reactions with Pentadienyl 4-Nitrobenzoate as a Diene Precursor

A phosphine-promoted tandem Diels-Alder reaction using pentadienyl 4-nitrobenzoate (α-vinyl MBH adduct) as a diene precursor with 3-olefinic oxindoles or CF3-activated ketones as dienophiles has been developed. The reaction proceeds through the formation of a pentadienyl phosphonium intermediate via SN2'' addition, which acts as both a D-A diene and a precursor for the exomethylene moiety. This method offers a metal-free and step-efficient approach for synthesizing exomethylene-bearing spirooxindoles and dihydropyrans, which are privileged structures found in natural products.

One-Pot Direct Synthesis of Siloles from 1-Bromo-2-silylethynylbenzenes or 1-Bromo-4-silyl-1,3-enynes

We propose one-pot synthesis of siloles from readily available starting materials. This methodology is practical for the preparation of multisubstituted siloles in good to excellent yields with complete regioselectivity. Sequential reactions, such as lithiation, silylation, and diisobutylaluminum-hydride-promoted cyclization, enable the preparation of the siloles. This transformation provides siloles through two efficient C-Si bond formations in one vessel.

Copper-Catalyzed Regiospecific Amination of Heteroarenes with Quinoneimides

In this work, an unprecedented and widely applicable strategy for the regioselective C-3 amination of indoles and C-2 amination of heteroarenes (pyrrole and benzofuran) is presented in a simple, high-efficiency way. This protocol is also one of the few methods for the efficient construction of C-N bonds of quinoneimides by the 1,6-addition reaction.

A Powerful Chiral Super Brønsted C-H Acid for Asymmetric Catalysis

A new type of chiral super Brønsted C-H acids, BINOL-derived phosphoryl bis((trifluoromethyl)sulfonyl) methanes (BPTMs), were developed. As compared to widely utilized BINOL-derived chiral phosphoric acids (BPAs) and N-triflyl phosphoramides (NTPAs), BPTMs displayed much higher Brønsted acidity, resulting in dramatically improved activity and excellent enantioselectivity as demonstrated in catalytic asymmetric Mukaiyama-Mannich reaction, allylic amination, three-component coupling of allyltrimethylsilane with 9-fluorenylmethyl carbamate and aldehydes, and protonation of silyl enol ether. These new strong Brønsted C-H acids have provided a platform for expanding the chemistry of asymmetric Brønsted acid catalysis.

Asymmetric difluorocarbonylation reactions of non-active imines catalyzed by Bi(OAc)3/chiral phosphoric acid

Difluorinated carbonyl has been identified as the basic skeleton of multitudinous biologically active molecules.

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.

Transition Metal-Catalyzed Hydroalkoxylation of Alkynes: An Overview

Oxygen-bearing motifs, mainly the congener heterocycles are ubiquitous due to their presence in various natural products and bioactive scaffolds. Although in literature, several strategies have been developed for their synthesis, hydroalkoxylation of alkynes has come forward as a method of choice and has been used extensively. In particular, hydroalkoxylation of alkynes has gained enormous attention from the synthetic community due to the rapid access to a very useful and reactive synthetic intermediate like 'enol ether'. Furthermore, to manifold the utility of these methods, reports have been developed elaborating the generation of 'enol ether' using hydroalkoxylation and their usage in various reactions in cascade or tandem manner. This review focuses on recent development on the hydroalkoxylation of alkynes for the synthesis of oxygen-containing entities.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Gold(I)-Catalyzed Domino Reaction for Furopyrans Synthesis

We report herein an efficient synthesis of furopyran derivatives through a gold(I)-catalyzed domino reaction. The cascade reaction starts with two regioselective cyclizations, a 5-endo-dig and a 8-endo-dig, followed with a Grob-type fragmentation and a hetero Diels–Alder. The obtained furopyran derivatives contain fused and spiro-heterocycles. During this one-pot process, four bonds and four controlled stereogenic centers including a quaternary center are formed.

Enantioselective Redox-Divergent Chiral Phosphoric Acid Catalyzed Quinone Diels-Alder Reactions

An efficient enantioselective construction of tetrahydronaphthalene-1,4-diones as well as dihydronaphthalene-1,4-diols by a chiral phosphoric acid catalyzed quinone Diels-Alder reaction with dienecarbamates is reported. The nature of the protecting group on the diene is key to the success of achieving high enantioselectivity. The divergent "redox" selectivity is controlled by using an adequate amount of quinones. Reversible redox switching without erosion of enantioselectivity was possible from individual redox isomers.

Development of bifunctional organocatalysts and application to asymmetric total synthesis of naucleofficine I and II

The proline-type organocatalysts has been efficiently employed to catalyze a wide range of asymmetric transformations; however, there are still many synthetically useful and challenging transformations that remain unachievable in an asymmetric fashion. Herein, a chiral bifunctional organocatalyst with a spirocyclic pyrrolidine backbone-derived containing fluoro-alkyl and aryl sulfonamide functionalities, are designed, prepared, and examined in the asymmetric Mannich/acylation/Wittig reaction sequence of 3,4-dihydro-β-carboline with acetaldehyde, acyl halides, and Wittig reagents. As a result, the spirocyclic pyrrolidine trifluoromethanesulfonamide catalyst can facilitate this versatile sequence as demonstrated by 18 examples displaying excellent enantioselectivity (up to 94% ee), as well as moderate to good yields (up to 54% over 3 steps). As a practical application, the asymmetric total synthesis of naucleofficine I (1a) and II (1b) in ten steps have been accomplished.

Natural products often contain complex N-fused polycyclic structures with multiple substituents and stereocentres. Here, the authors developed a bifunctional organocatalyst that is instrumental in obtaining such structures and applied it to the total synthesis of naucleofficine I and II in 6 steps.

The Divergent Cascade Reactions of Arylalkynols with Homopropargylic Amines or Electron-Deficient Olefins: Access to the Spiro-Isobenzofuran- b-pyrroloquinolines or Bridged-Isobenzofuran Polycycles

Two divergent cascade reactions of arylalkynols with homopropargylic amines or electron-deficient olefins were developed to synthesize the spiro-isobenzofuran- b-pyrroloquinolines or bridged-isobenzofuran heterocycles in good yields, respectively. One reaction actually involved intramolecular 5- endo-dig hydroamination cyclization-protonation of homopropargylic amines to give cycloiminium ions and intramolecular 5- exo-dig hydroalkoxylation cyclization of arylalkynols to generate isobenzofuran with exocyclic double bond, followed by the nontypical Povarov-type reaction in the presence of PtCl₂/FeCl₃ cocatalysts. The other underwent intramolecular hydroalkoxylation cycloisomerization of alkynols with the subsequent normal [4 + 2] cycoaddition with dienophiles. Herein, the arylalkynols acted as both "masked" electron-rich olefins and "masked" electron-rich dienes.

Ir/PTC cooperatively catalyzed asymmetric umpolung allylation of α-imino ester enabled synthesis of α-quaternary amino acid derivatives bearing two vicinal stereocenters

A novel Ir/PTC (phase-transfer catalyst) cooperatively catalyzed asymmetric umpolung allylation of simple α-imino esters is developed and it provides facile access to α-quaternary amino acid derivatives bearing two vicinal stereocenters. Both the metal catalyst and the phase-transfer catalyst are crucial for this methodology. The reaction features good yields, high diastereo- and enantio-selectivities, ease of scale-up to gram scale and further transformation of the products (e.g. to quaternary proline analogues with multi stereocenters).

Manganese-Catalyzed Ring Opening of Benzofurans and Its Application to Insertion of Heteroatoms into the C2-O Bond

A new class of aromatic metamorphosis in which benzofurans are converted into diverse six-membered oxaheterocycles has been developed. This transformation is composed of two reactions in one pot: manganese-catalyzed arylative or alkylative ring-opening of benzofurans affording dianionic intermediates and subsequent trapping with multivalent heteroatom electrophiles. Various electrophiles containing silicon, boron, phosphorus, germanium, and titanium could be applied to this heteroatom insertion.

Cu-Catalyzed Cascade Annulation of Alkynols with 2-Azidobenzaldehydes: Access to 6H-Isochromeno[4,3-c]quinoline

A copper-catalyzed cascade reaction of alkynols and 2-azidobenzaldehydes has been achieved, giving 6H-isochromeno[4,3-c]quinoline in yields of 40-81%. This reaction provides a novel, concise strategy for rapidly constructing compounds with fused N- and O-containing heterocycles. In contrast to previously reported reactions of alkynols in which the first step is intramolecular cycloisomerization, the first step in this novel reaction of alkynols is entropically unfavorable intermolecular addition. The resulting hemiacetal intermediate then undergoes intramolecular cyclization and aromatization to afford the product.