Catalytic Asymmetric 1,4-Hydrocarbonation of 1,3-Enynes via Photoredox/Cobalt/Chromium Triple Catalysis

A synergistic photoredox/cobalt/chromium triple catalysis system for regioselective, enantioselective, and diastereoselective 1,4-hydrocarbonation of readily available 1,3-enyne precursors was explored, providing a modular synthetic platform for various trisubstituted axially chiral allenes bearing an extra central chirality. The protocol features a broad substrate scope, good functional group tolerance, excellent selectivity, and mild reaction conditions. Furthermore, a possible reaction mechanism is proposed based on numerous control experiments and density functional theory calculations.

Palladium-Catalyzed, Regio-/Stereo- and Enantiospecific Anti-Carboxylation of Unactivated Internal Allenes

We report herein a directing group-controlled, palladium-catalyzed, regio-, stereo-, and enantiospecific anti-carboxylation of unactivated, internal allenes enabled via the synergistic interplay of a rationally designed bidentate directing group, palladium catalyst, and a multifunctional acetate ligand. The corresponding trans allyl ester was obtained in excellent yields with exclusive δ-regioselectivity and anti-carboxypalladation stereocontrol. The acetate ligand of the palladium catalyst controls the regio-, stereo- and enantioselectivity in the desired transformation. The potential of this concept has been demonstrated by the development of the chiral version of this transformation by using axial-to-central chirality transfer with good yields and enantioselectivities. Detailed investigations, including kinetic studies, order studies, and DFT studies, were performed to validate the ligand-assisted nucleopalladation process and the rationale behind the observed racemization of chiral allenes. The studies also indicated that the anti-carboxypalladation step was the rate-limiting as well as the stereo- and enantiodetermining step.

Multivalent pyrrolidines acting as pharmacological chaperones against Gaucher disease

A concise asymmetric synthesis of clickable enantiomeric pyrrolidines was achieved using Crabbé-Ma allenation. The synthesized iminosugars were grafted by copper-free strain-promoted alkyne-azide cycloaddition onto phosphorus dendrimers. The hexavalent and dodecavalent pyrrolidines were evaluated as β-glucocerebrosidase inhibitors. The level of inhibition suggests that monofluorocyclooctatriazole group may contribute to the affinity for the protein leading to potent multivalent inhibitors. Docking studies were carried out to rationalize these results. Then, the iminosugars clusters were evaluated as pharmacological chaperones in Gaucher patients' fibroblasts. An increase in β-glucocerebrosidase activity was observed with hexavalent and dodecavalent pyrrolidines at concentrations as low as 1 µM and 0.1 µM, respectively. These iminosugar clusters constitute the first example of multivalent pyrrolidines acting as pharmacological chaperones against Gaucher disease.

Relay Catalysis for Selective Aerobic Oxidative Esterification of Primary Alcohols with Methanol

Esters are bulk and fine chemicals and ubiquitous in polymers, bioactive compounds, and natural products. Their traditional synthetic approach is the esterification of carboxylic acids or their activated derivatives with alcohols. Herein, a bimetallic relay catalytic protocol was developed for the aerobic esterification of one alcohol in the presence of a slowly oxidizing alcohol, which has been identified as methanol. A concise synthesis of phlomic acid was executed to demonstrate the practicality and potential of this reaction.

A Pd-catalyzed highly selective three-component protocol for trisubstituted allenes

Herein we report the first example of a Pd-catalyzed highly selective three-component reaction of alkynyl-1,4-diol dicarbonates, organoboronic acids, and malonate anions for the efficient synthesis of trisubstituted 2,3-allenyl malonates not readily available by the known protocols. The reaction demonstrates an excellent regio- and chemo-selectivity for both the oxidative addition referring to the two C-O bonds and the subsequent coupling with the nucleophile with a remarkable functional group compatibility. A series of control experiments confirm a unique mechanism involving β-O elimination forming alka-1,2,3-triene and the subsequent insertion of its terminal C[double bond, length as m-dash]C bond into the Ar-Pd bond.

Alkyl amines and ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions

The use of alkyl amines and ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions represents a promising strategy that greatly enriches redox-neutral hydride transfer chemistry. This review summarizes the remarkable progress made in this field, and focuses on (1) alkyl amines as traceless hydride donors in cascade [1,5]-hydride transfer/elimination reactions and (2) alkyl ethers as traceless hydride donors in [1,5]-hydride transfer cascade reactions. The reaction mechanisms, features, scope, limitations, and synthetic applications are included, where appropriate. Importantly, its powerful ability in allene synthesis and the combination with [Re]-vinylidene and carbocation chemistries render this strategy attractive enough to inspire chemists to develop colorful reactions for building molecular complexity.

Scope and limitations of absolute configuration determination of allenic natural products using the CCC stretching VCD signal

The allene functional group in natural products isolated so far exists in a non-racemic form, but its axial chirality is difficult to elucidate. Allenes exhibit a characteristic antisymmetric CCC stretching mode at around 1950 cm^-1, and their VCD properties have not been studied in detail. This work, for the first time, applied VCD spectroscopy to allenic natural products and allenic molecules with other asymmetric centers focusing on the antisymmetric CCC stretching mode. This vibrational mode yielded a negligibly weak VCD signal for several molecules, but in the presence of electron-withdrawing and/or conjugating substituents, it generated a stronger one. Its sign was found to be influenced by the nature of substituents. These findings should deepen the understanding of the VCD properties of the allene functional group and should be useful for future studies of chiral allenes.

Zinc Iodide Catalyzed Synthesis of Trisubstituted Allenes from Terminal Alkynes and Ketones

A straightforward, user-friendly, efficient protocol for the one pot, ZnI₂-catalyzed allenylation of terminal alkynes with pyrrolidine and ketones, toward trisubstituted allenes, is described. Trisubstituted allenes can be obtained under either conventional heating or microwave irradiation conditions, which significantly reduces the reaction time. A sustainable, widely available, and low-cost metal salt catalyst is employed, and the reactions are carried out under solvent-free conditions. Among others, synthetically valuable allenes bearing functionalities such as amide, hydroxyl, or phthalimide can be efficiently prepared. Mechanistic experiments, including kinetic isotope effect measurements and density functional theory (DFT) calculations, suggest a rate-determining [1,5]-hydride transfer during the transformation of the intermediate propargylamine to the final allene.

Concise asymmetric synthesis of new enantiomeric C-alkyl pyrrolidines acting as pharmacological chaperones against Gaucher disease

A concise and asymmetric synthesis of the enantiomeric pyrrolidines 2 and ent-2 are herein reported. Both enantiomers were assessed as β-GCase inhibitors. While compound ent-2 acted as a poor competitive inhibitor, its enantiomer 2 proved to be a potent non-competitive inhibitor. Docking studies were carried out to substantiate their respective protein binding mode. Both pyrrolidines were also able to enhance lysosomal β-GCase residual activity in N370S homozygous Gaucher fibroblasts. Notably, the non-competitive inhibitor 2 displayed an enzyme activity enhancement comparable to that of reference compounds IFG and NN-DNJ. This work highlights the impact of inhibitors chirality on their protein binding mode and shows that, beyond competitive inhibitors, the study of non-competitive ones can lead to the identification of new relevant parmacological chaperones.

Azaheterocyclic diphenylmethanol chiral solvating agents for the NMR chiral discrimination of alpha-substituted carboxylic acids

A series of small-membered heterocycle probes, so-called azaheterocycle-containing diphenylmethanol chiral solvating agents (CSAs), have been developed for NMR enantiodiscrimination. These chiral sensors were readily synthesized were inexpensive and efficiently used for the chiral analysis of alpha-substituted carboxylic acids. The sensing method was operationally simple and the processing was straightforward. Notably, we propose (S)-aziridinyl diphenylmethanol as a promising CSA, which has excellent chiral discriminating properties and offers multiple detectable possibilities pertaining to the 1H NMR signals of diagnostic split protons (including 25 examples, up to 0.194 ppm, 77.6 Hz). Its ability to detect the molecular recognition of fluorinated carboxylic acids were further investigated, with a good level of discrimination via the 19F NMR spectroscopic analysis. In addition, an accurate enantiomeric excess (ee) analysis of the p-methoxyl-mandelic acid with different optical compositions have been calculated based on the integration of well-separated proton signals.

Copper Hydride Catalyzed Enantioselective Synthesis of Axially Chiral 1,3-Disubstituted Allenes

The general enantioselective synthesis of axially chiral disubstituted allenes from prochiral starting materials remains a long-standing challenge in organic synthesis. Here, we report an efficient enantio- and chemoselective copper hydride catalyzed semireduction of conjugated enynes to furnish 1,3-disubstituted allenes using water as the proton source. This protocol is sufficiently mild to accommodate an assortment of functional groups including keto, ester, amino, halo, and hydroxyl groups. Additionally, applications of this method for the selective synthesis of monodeuterated allenes and chiral 2,5-dihydropyrroles are described.

Hydroxy group-enabled highly regio- and stereo-selective hydrocarboxylation of alkynes

Hydrogen bonding-enabled highly regio- and stereo-selective hydrocarboxylation of alkynes has been successfully developed to afford 3-hydroxy-2(E)-alkenoates with up to 97% yield.

Here we present an example of utilizing hydroxy groups for regioselectivity control in the addition reaction of alkynes—a highly efficient Pd-catalyzed syn-hydrocarboxylation of readily available 2-alkynylic alcohols with CO in the presence of alcohols with an unprecedented regioselectivity affording 3-hydroxy-2(E)-alkenoates. The role of the hydroxy group has been carefully studied. The synthetic potential of the products has also been demonstrated.

Allenation of Terminal Alkynes with Aldehydes and Ketones

So far, over 150 natural products and pharmaceuticals containing an allene moiety have been identified. During the last two decades, allenes have also been demonstrated as synthetically versatile starting materials in organic synthesis. In comparison to alkenes and alkynes, allenes are unique unsaturated hydrocarbons due to their axial chirality, which could be transformed to central chirality via chirality transfer to provide an irreplaceable entry to chiral molecules. Thus, methods for allene synthesis from readily available chemicals are of great interest. In 1979, Crabbé et al. reported the first CuBr-mediated allenation of terminal alkynes (ATA) reaction to form monosubstituted allenes from 1-alkynes and paraformaldehyde in the presence of diisopropylamine. During the following 30 years, the ATA reactions were limited to paraformaldehyde. This Account describes our efforts toward the development of ATA reactions in the last ten years. First, we improved the yields and scope greatly for the synthesis of monosubstituted allenes by modifying the original Crabbé recipe. Next we developed the ZnI₂-promoted or CuI-catalyzed ATA reactions for the synthesis of 1,3-disubstituted allenes from terminal alkyne and normal aldehydes. Furthermore, we first realized the CdI₂-promoted ATA reaction of ketones with pyrrolidine as the matched amine for the preparation of trisubstituted allenes. Due to the toxicity of CdI₂, we also developed two alternative approaches utilizing CuBr/ZnI₂ or CuI/ZnBr₂/Ti(OEt)₄. The asymmetric version of ATA reactions for the synthesis of optically active 1,3-disubstituted allenes has also been achieved in this group with two strategies. One is called "chiral ligand" strategy, using terminal alkynes, aldehydes, and nonchiral amine with the assistance of a proper chiral ligand. The other is the "chiral amine" strategy, applying terminal alkynes, aldehydes, and chiral amines such as ( S)- or ( R)-α,α-diphenylprolinol or ( S)- or ( R)-α,α-dimethylprolinol. Optically active 1,3-disubstituted allenes containing different synthetically useful functionalities such as alcohol, amide, sulfamide, malonate, carboxylate, and carbohydrate units could be prepared without protection with the newly developed CuBr₂-catalyzed chiral amine strategy. Recently, we have applied these enantioselective allenation of terminal alkyne (EATA) reactions to the syntheses of some natural allenes such as laballenic acid, insect pheromone, methyl ( R)-8-hydroxyocta-5,6-dienoate, phlomic acid, and lamenallenic acid, as well as some non-allene natural γ-butyrolactones such as xestospongienes (E, F, G, and H), ( R)-4-tetradecalactone, ( S)-4-tetradecalactone, ( R)-γ-palmitolactone, and ( R)-4-decalactone.

Catalytic enantioselective construction of axial chirality in 1,3-disubstituted allenes

Metal-catalyzed enantioselective construction of the loosening axial allene chirality spreading over three carbon atoms using a chiral ligand is still a significant challenge. In the literature, steric effect of the substrates is the major strategy applied for such a purpose. Herein, we present a general palladium-catalyzed asymmetrization of readily available racemic 2,3-allenylic carbonates with different types of non-substituted and 2-substituted malonates using (R)-(−)-DTBM-SEGPHOS as the preferred ligand to afford 1,3-disubstituted chiral allenes with 90~96% ee. This protocol has been applied to the first enantioselective synthesis of natural product, (R)-traumatic lactone. Control experiments showed that in addition to the chiral ligand, conducting this transformation via Procedure C, which excludes the extensive prior coordination of the allene unit in the starting allene with Pd forming a species without the influence of the chiral ligand, is crucial for the observed high enantioselectivity.

Highly enantioselective synthesis of allenes has been relying, so far, on the steric hindrance of substrates. Here the authors achieve excellent stereocontrol in the synthesis of chiral allenes with a palladium-DTBM-SEGPHOS catalytic system in a non-substrate-dependent manner.

A metal-catalyzed new approach for α-alkynylation of cyclic amines

The first catalytic α-alkynylation of cyclic amines with the help of the N-propargylic group with an exclusive high E-stereoselectivity has been realized.

The first catalytic α-alkynylation of cyclic amines with the help of the N-propargylic group to afford 2-(1-alkynyl) N-allylic cyclic amines with an exclusive E-stereoselectivity for the in situ formed C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond has been realized. Based on mechanistic studies, it is proven that the reaction proceeds through metal-mediated anti-1,5-hydride transfer forming an iminonium intermediate, which accepts the addition of the in situ generated 1-alkynyl metal species. The synthetic application has also been demonstrated.

Catalytic transient leaving group for atom-economic synthesis of allenes from 2-alkynols

An atom economic approach from readily available propargylic alcohols to allenes, the first carboxylation of propargylic alcohols, has been established. Through the cooperative binary catalysis of Pd and a phosphoric acid, the reaction afforded multi-substituted allenoates with a broad scope tolerating useful functional groups. The synthetic potential of the obtained products has been demonstrated.

Enantioselective Synthesis of Allenes by Catalytic Traceless Petasis Reactions

Allenes are useful functional groups in synthesis as a result of their inherent chemical properties and established reactivity patterns. One property of chemical bonding renders 1,3-substituted allenes chiral, making them attractive targets for asymmetric synthesis. While there are many enantioselective methods to synthesize chiral allenes from chiral starting materials, fewer methods exist to directly synthesize enantioenriched chiral allenes from achiral precursors. We report here an asymmetric boronate addition to sulfonyl hydrazones catalyzed by chiral biphenols to access enantioenriched allenes in a traceless Petasis reaction. The resulting Mannich product from nucleophilic addition eliminates sulfinic acid, yielding a propargylic diazene that performs an alkyne walk to afford the allene. Two enantioselective approaches have been developed; alkynyl boronates add to glycolaldehyde imine to afford allylic hydroxyl allenes, and allyl boronates add to alkynyl imines to form 1,3-alkenyl allenes. In both cases, the products are obtained in high yields and enantioselectivities.

Aerobic oxidation and EATA-based highly enantioselective synthesis of lamenallenic acid

A highly enantioselective synthesis of (−)-lamenallenic acid utilizing the recently developed EATA reaction and aerobic oxidation reaction has been accomplished.

Recent advances in catalytic asymmetric synthesis of allenes

This minireview summarizes the recent advances in the synthesis of allenes with catalytic asymmetric methods.

Enantiodiscrimination of carboxylic acids using the diphenylprolinol NMR chiral solvating agents

Enantiopure diphenylprolinols have been developed as effective and practical NMR chiral solvating agents for the enantiodiscrimination of diverse α-carboxylic acids.

A concise construction of carbohydrate-tethered axially chiral allenes via copper catalysis

Here we observed that CuBr₂ may smoothly catalyze the highly diastereoselective three-component reaction of carbohydrates bearing a terminal alkyne unit, aliphatic or aromatic aldehydes, and (R) or (S)-α,α-diphenylprolinol, affording carbohydrates bearing the chiral 1,3-substituted allene entity in 94–99% de.