Asymmetric Organocatalytic Synthesis of 2,3-Allenamides from Hydrogen-Bond-Stabilized Enynamides

Novel Stereo-Induction Pattern in Pudovik Addition/Phospha-Brook Rearrangement Towards Chiral Trisubstituted Allenes

Despite the significance of chiral allene skeletons in catalysis, organic synthesis and medicinal chemistry et al., there is a scarcity of reports on axially chiral allenyl phosphorus compounds. Here, we disclosed an efficient and straightforward cascade reaction between ethynyl ketones and phosphine oxides, resulting in a broad array of trisubstituted allenes incorporating a phosphorus moiety in high yields with excellent stereoselectivities facilitated by peptide-mimic phosphonium salt (PPS) catalysis, Additionally, comprehensive series of mechanistic experiments have been conducted to elucidate that this cascade reaction proceeds via an asymmetric Pudovik addition reaction followed by a subsequent phospha-Brook rearrangement that occurs concomitantly with kinetic resolution, representing a stereospecific rearrangement and protonation process facilitating central-to-axial chirality transfer in a cascade manner. We anticipate that our research will pave the way for a promising exploration of novel stereo-induction pattern in the Pudovik addition/phospha-Brook rearrangement cascade reaction.

Diastereoselective Synthesis of Allenes through Phosphine-Catalyzed Cascade Isomerization/Annulation

Phosphine-catalyzed cascade isomerization/annulation has been developed to realize a diastereoselective synthesis of allenes installed on the hexahydropentalene skeleton, which contains five chiral centers (and one axial chirality). This reaction tolerated a broad range of allenoates and enynes. The allene products were transformed to various halogen-substituted fused-ring compounds.

Stereoselective Sulfa-Michael/Aldol Reaction Promoted by an Axially Chiral Styrene-Based Organocatalyst

Herein, we describe a stereoselective sulfa-Michael/aldol cyclization reaction promoted by a rationally designed novel axially chiral styrene-based organocatalyst. A variety of highly substituted tetrahydrothiophenes featuring an alkyne-substituted quaternary stereogenic center are obtained in good yields, excellent stereoselectivities, and exclusive trans selectivities. This process tolerates a broad range of alkynyl-substituted acrylamides under mind conditions. The utility of this approach is highlighted in its excellent asymmetric introduction, scalability, and attractive product diversification.

Axially chiral styrene-based organocatalysts and their application in asymmetric cascade Michael/cyclization reaction

An axially chiral styrene-based organocatalyst, featuring a combination of axially chiral styrene-based structure and a pyrrole ring, has been designed and synthesized. This catalyst demonstrates remarkable capabilities in producing a wide range of densely substituted spirooxindoles that feature an alkyne-substituted quaternary stereogenic center. These spirooxindoles are generated through mild cascade Michael/cyclization reactions, resulting in high conversion rates and exceptional enantioselectivity. Our catalytic model, based on experiments, X-ray structure analysis and DFT calculations suggests that chiral matched π-π interactions and multiple H-bonds between the organocatalyst and substrates play significant roles in controlling the stereoselectivity of the reaction.

Intramolecular Hydrogen-Bond Activation: Strategies, Benefits, and Influence in Catalysis

The activation of molecules through intramolecular hydrogen-bond formation to promote chemical reactions appears as a suitable strategy in organic synthesis, especially for the preparation of chiral compounds under metal and organocatalytic conditions. The use of this interaction has enabled reactivity enhancement of reagents, as well as stabilization of the chemical species and enantiocontrol of the processes.

Copper-Catalyzed Enantioselective 1,4-Protosilylation of Alkynyl-substituted Enones to Synthesize the Highly Diastereomeric Chiral Homoallenylsilanes

A copper-catalyzed 1,4-protosilylation of α-alkynyl-enones to form the functionalized chiral homoallenylsilanes was developed. In the presence of a chiral monopyridine oxazoline ligand, a variety of trisubstituted allene derivatives bearing a contiguous stereogenic center and axis were prepared in good yields with excellent enantioselectivities and diastereoselectivities.

Access to Allene-Containing Molecules via Enantioselective Reactions of Azolium Cumulenolate Intermediates

Azolium cumulenolates are a special type of intermediates in N-heterocyclic carbene catalysis. They contain elongated linear structures with three contiguous C=C bonds and sterically unhindered α-carbon atoms. These structural features make it difficult to develop enantioselective reactions for these intermediates. Here we disclose the first carbene-catalyzed highly enantioselective addition reactions of azolium cumulenolates. The reaction starts with alkynals as the precursors for azolium cumulenolate intermediates that undergo enantioselective addition to activated ketones. From the same set of substrates, both allene and spirooxindole products can be obtained with high yields and excellent enantioselectivities. The allene moieties in our optically enriched products carry rich reactivities and can be transformed to diverse molecules. The spirooxindole scaffolds in our products are important structural motifs in natural products and medicines.

Highly enantioselective one-pot sequential synthesis of valerolactones and pyrazolones bearing all-carbon quaternary stereocentres

Highly enantiopure and bioactive δ-valerolactones and pyrazolones, bearing α-all-carbon quaternary stereocentres, were successfully and sequentially prepared via a one-pot procedure starting from readily available, inexpensive materials, catalysed by a new chiral squaramide under mild reaction conditions. An organocatalytic Michael reaction afforded the valerolactones, while a one-pot Michael-hydrazinolysis-imidization cascade yielded the pyrazolones. This procedure is economically efficient and environmentally benign.

A General and Highly Selective Palladium-Catalyzed Hydroamidation of 1,3-Diynes

A chemo-, regio-, and stereoselective mono-hydroamidation of (un)symmetrical 1,3-diynes is described. Key for the success of this novel transformation is the utilization of an advanced palladium catalyst system with the specific ligand Neolephos. The synthetic value of this general approach to synthetically useful α-alkynyl-α, β-unsaturated amides is showcased by diversification of several structurally complex molecules and marketed drugs. Control experiments and density-functional theory (M06L-SMD) computations also suggest the crucial role of the substrate in controlling the regioselectivity of unsymmetrical 1,3-diynes.

Chiral phosphoric acid-catalyzed stereodivergent synthesis of trisubstituted allenes and computational mechanistic studies

Chiral molecules with multiple stereocenters are widely present in natural products and pharmaceuticals, whose absolute and relative configurations are both critically important for their physiological activities. In spite of the fact that a series of ingenious strategies have been developed for asymmetric diastereodivergent catalysis, most of these methods are limited to the divergent construction of point chirality. Here we report an enantioselective and diastereodivergent synthesis of trisubstituted allenes by asymmetric additions of oxazolones to activated 1,3-enynes enabled by chiral phosphoric acid (CPA) catalysis, where the divergence of the allenic axial stereogenicity is realized by modifications of CPA catalysts. Density functional theory (DFT) calculations are performed to elucidate the origin of diastereodivergence by the stacking- and stagger-form in the transition state (TS) of allene formation step, as well as to disclose a Münchnone-type activation mode of oxazolones under Brønsted acid catalysis.

Copper(I)-catalyzed diastereo- and enantio-selective construction of optically pure exocyclic allenes

Among about 150 identified allenic natural products, the exocyclic allenes constitute a major subclass. Substantial efforts are devoted to the construction of axially chiral allenes, however, the strategies to prepare chiral exocyclic allenes are still rare. Herein, we show an efficient strategy for the asymmetric synthesis of chiral exocyclic allenes with the simultaneous control of axial and central chirality through copper(I)-catalyzed asymmetric intramolecular reductive coupling of 1,3-enynes to cyclohexadienones. This tandem reaction exhibits good functional group compatibility and the corresponding optically pure exocyclic allenes bearing cis-hydrobenzofuran, cis-hydroindole, and cis-hydroindene frameworks, are obtained with high yields (up to 99% yield), excellent diastereoselectivities (generally >20:1 dr) and enantioselectivities (mostly >99% ee). Furthermore, a gram-scale experiment and several synthetic transformations of the chiral exocyclic allenes are also presented.

Photochemical Deracemization of Allenes and Subsequent Chirality Transfer

Trisubstituted allenes with a 3-(1'-alkenylidene)-pyrrolidin-2-one motif were successfully deracemized (13 examples, 86-98 % ee) employing visible light (λ=420 nm) and a chiral triplet sensitizer as the catalyst (2.5 mol %). The photocatalyst likely operates by selective recognition of one allene enantiomer via hydrogen bonds and by a triplet-sensitized racemization process. Even a tetrasubstituted allene (45 % ee) and a seven-membered 3-(1'-alkenylidene)-azepan-2-one (62 % ee) could be enantiomerically enriched under the chosen conditions. It was shown that the axial chirality of the allenes can be converted into point chirality by a Diels-Alder (94-97 % ee) or a bromination reaction (91 % ee). Ring opening of the five-membered pyrrolidin-2-one was achieved without significantly compromising the integrity of the chirality axis (92 % ee).

2-Activated 1,3-enynes in enantioselective synthesis

The rapid enantioselective synthesis of valuable building blocks and pharmaceutically important compounds from easily accessible precursors is one of the major areas of focus in organic chemistry. In this context, 2-activated 1,3-enyne has emerged as a powerful synthon in recent years for the efficient synthesis of enantioenriched furans, allenes, 4-H-pyrans, and 4-isoxazolines, which are privileged scaffolds in bioactive compounds and natural products. This review will cover the history of the development of 2-activated 1,3-enyne in enantioselective synthesis along with the corresponding mechanisms, which may motivate further development in this area to forge more complex and valuable molecules.

Rhodium(iii)-catalysed cascade [3 + 2] annulation of N-aryloxyacetamides with 3-(hetero)arylpropiolic acids: synthesis of benzofuran-2(3H)-ones

Herein, a cascade [3 + 2] annulation of N-aryloxyacetamides with 3-(hetero)arylpropiolic acids affording benzofuran-2(3H)-ones via rhodium(iii)-catalyzed redox-neutral C-H functionalization/isomerization/lactonization using an internal oxidative directing group O-NHAc was achieved. This catalytic system provides a regio- and stereoselective approach to synthesize (Z)-3-(amino(aryl)methylene)benzofuran-2(3H)-ones with exclusive Z configuration selectivity, acceptable yields and good functional group tolerance. Preliminary investigations on ultraviolet-visible and fluorescence behaviors reveal that the annulation products may be applied as a promising fluorescent probe for sensing metal cations, especially for cerium (Ce3+).