Recent applications of chiral allenes in axial-to-central chirality transfer reactions

Challenging Task of Ni-Catalyzed Highly Regio-/Enantioselective Semihydrogenation of Racemic Tetrasubstituted Allenes via a Kinetic Resolution Process

The first earth-abundant transition metal Ni-catalyzed highly regio- and enantioselective semihydrogenation of racemic tetrasubstituted allenes via a kinetic resolution process as a challenging task was well established. This protocol furnishes expedient access to a diversity of structurally important enantioenriched tetrasubstituted allenes and chiral allylic molecules with high regio-, enantio-, and Z/E-selectivity. Remarkably, this semihydrogenation proceeded with one carbon-carbon double bond of allenes, which was regioselective complementary to the Rh-catalyzed asymmetric version. Deuterium labeling experiments and density functional theory (DFT) calculations were carried out to reveal the reasonable reaction mechanism and explain the regio-/stereoselectivity.

Organocatalytic regio- and enantioselective C1-arylation of β,γ-alkynyl-α-imino esters with pyrrolo[2,1-a]isoquinolines

We herein report regio- and enantioselective arylation of β,γ-alkynyl-α-imino esters with pyrroloisoquinolines. Using chiral phosphoric acid catalysts, a wide range of novel axially chiral tetrasubstituted α-amino allenoates were accessed in good yields with excellent enantioselectivities. Notably, this transformation occurred preferentially at the sterically more hindered C1-position of pyrroloisoquinolines. The potential scalability and late-stage functionalization demonstrated the utility of the current protocol.

Development of robust chiroptical systems through spirobifluorenes

Chiroptical responses are valuable for the structural determination of dissymmetric molecules. However, the development of everyday applications based on chiroptical systems is yet to come. We have been earlier using axially chiral allenes for the construction of linear, cyclic, and cage-shaped molecules that present remarkable chiroptical responses. Additionally, we have developed chiral surfaces through upstanding chiral architectures. Since the goal is to obtain robust chiroptical materials, more recently we have been studying spirobifluorenes (SBFs), a well-established building block in optoelectronic applications. After theoretical and experimental demonstration, the suitability of chiral SBFs for the development of robust chiroptical systems was certified by the construction all-carbon double helices, flexible shape-persistent macrocycles, chiral frameworks for surface functionalization, and structures featuring helical or spiroconjugated molecular orbitals. Here, we give an overview of our contribution to these matters.

Lewis Acid-Promoted [2 + 2] Cycloadditions of Allenes and Ketenes: Versatile Methods for Natural Product Synthesis

ConspectusCycloaddition reactions are an effective method to quickly build molecular complexity. As predicted by the Woodward-Hoffmann rules, concerted cycloadditions with alkenes allow for the constructions of all possible stereoisomers of product by use of either the Z or E geometry. While this feature of cycloadditions is widely used in, for example, [4 + 2] cycloadditions, translation to [2 + 2] cycloadditions is challenging because of the often stepwise and therefore stereoconvergent nature of these processes. Over the past decade, our lab has explored Lewis acid-promoted [2 + 2] cycloadditions of electron-deficient allenes or ketenes with alkenes. The concerted, asynchronous cycloadditions allow for the synthesis of various cyclobutanes with control of stereochemistry.Our lab developed the first examples of Lewis acid-promoted ketene-alkene [2 + 2] cycloadditions. Compared with traditional thermal conditions, Lewis acid-promoted conditions have several advantages, such as increased reactivity, increased yield, improved diastereoselectivity, and, for certain cases, inverse diastereoselectivity. Detailed mechanistic studies revealed that the diastereoselectivity was controlled by the size of the substituent and the barrier of a deconjugation event. However, these reactions required the use of stoichiometric amounts of EtAlCl2 because of the product inhibition, which led us to investigate catalytic enantioselective [2 + 2] cycloadditions of allenoates with alkenes. Through the use of chiral oxazaborolidines, a broad range of cyclobutanes can be prepared with the control of enantioselectivity. Mechanistic experiments, including 2D-labled alkenes and Hammett analysis, illuminate likely transition state models for the cycloadditions. Additional studies led to the development of Lewis acid-catalyzed intramolecular stereoselective [2 + 2] cycloadditions of chiral allenic ketones/esters with alkenes.The methods we developed have been instrumental in the synthesis of several families of natural products. Specifically, one key lactone motif in (±)-gracilioether F was constructed by a ketene-alkene [2 + 2] cycloaddition and subsequent regioselective Baeyer-Villiger oxidation sequence. Enantioselective allenoate-alkene [2 + 2] cycloadditions allowed for the synthesis of (-)-hebelophyllene E. Another attempt of applying this method in the synthesis of (+)-[5]-ladderanoic acid failed to deliver the desired cyclobutane because of an unexpected rearrangement. The key cyclobutane was later assembled by a stepwise carboboration/Zweifel olefination process. Finally, the stereoselective [2 + 2] cycloadditions of allenic ketones and alkenes was applied in the syntheses of (-)-[3]-ladderanol, (+)-hippolide J, and (-)-cajanusine.

Asymmetric α-allylic allenylation of β-ketocarbonyls and aldehydes by synergistic Pd/chiral primary amine catalysis

We herein describe an asymmetric α-allylic allenylation of β-ketocarbonyls and aldehydes with 1,3-enynes. A synergistic chiral primary amine/Pd catalyst was identified to facilitate the utilization of 1,3-enynes as atom-economic and achiral allene precursors. The synergistic catalysis enables the construction of all-carbon quaternary centers-tethered allenes bearing non-adjacent 1,3-axial central stereogenic centers in high level of diastereo- and enantio-selectivity. By switching the configurations of ligands and aminocatalysts, diastereodivergence can be achieved and any of the four diastereoisomers can be accessed in high diastereo- and enantio- selectivity.

Stereospecific [3+2] Cycloaddition of Chiral Arylallenes with C,N-Cyclic Azomethine Imines

A novel α,β-regioselective [3+2] cycloaddition reaction of arylallene with C,N-cyclic azomethine imine is reported. The axial-to-central chirality transfer phenomenon has been disclosed with chiral allenes in the reaction. The wide substrate scope, including different functional groups and natural products, reveals the generality of the methodology. Both experiments and density functional theory calculations have been used to elucidate a plausible mechanism.

Stereodivergent Synthesis of Allenes with α,β-Adjacent Central Chiralities Empowered by Synergistic Pd/Cu Catalysis

The stereodivergent synthesis of allene compounds bearing α,β-adjacent central chiralities has been realized via the Pd/Cu-catalyzed dynamic kinetic asymmetric alkylation of racemic allenylic esters. The matched reactivity of bimetallic catalytic system enables the challenging reaction of racemic aryl-substituted allenylic acetates with sterically crowded aldimine esters smoothly under mild reaction conditions. Various chiral non-natural amino acids bearing a terminal allenyl group are easily synthesized in high yields and with excellent diastereo- and enantioselectivities (up to >20 : 1 dr, >99 % ee). Importantly, all four stereoisomers of the product can be readily accessed by switching the configurations of the two chiral metal catalysts. Furthermore, the easy interconversion between the uncommon η³ -butadienyl palladium intermediate featuring a weak C=C/Pd coordination bond and a stable Csp² -Pd bond is beneficial for the dynamic kinetic asymmetric transformation process (DyKAT).

Ultrafast Imaging of Molecular Chirality with Photoelectron Vortices

Ultrafast imaging of molecular chirality is a key step toward the dream of imaging and interpreting electronic dynamics in complex and biologically relevant molecules. Here, we propose a new ultrafast chiral phenomenon exploiting recent advances in electron optics allowing access to the orbital angular momentum of free electrons. We show that strong-field ionization of a chiral target with a few-cycle linearly polarized 800 nm laser pulse yields photoelectron vortices, whose chirality reveals that of the target, and we discuss the mechanism underlying this phenomenon. Our Letter opens new perspectives in recollision-based chiral imaging.

Gold-catalyzed cycloadditions of allenes via metal carbenes

In recent years, gold-catalyzed cycloadditions of allenes, especially those involving a gold carbene intermediate, have received significant interest, as they avoid the utilization of potentially hazardous and inaccessible diazo compounds as starting materials for carbene generation. Cycloaddition reactions consisting of the uncomplicated addition of two or more unsaturated functional groups are one of the most efficient synthetic methodologies for the rapid assembly of carbo- and heterocyclic structures from simple acyclic precursors. In this review, we introduce an overview of the advances in the gold-catalyzed cycloaddition of allenes via a metal carbene intermediate and categorize these reactions according to the reaction types of the cycloadditions.

Allenes in Diels–Alder Cycloadditions

For a long time, allenes—and cumulenic systems in general—played a relatively minor role in Diels–Alder cycloadditions. This situation has changed, since allenes are more readily available and as their unique stereochemical features in [4+2]cycloadditions are more widely recognized. This review presents a comprehensive overview of allenes in Diels–Alder processes using selected examples. Allenes in dienes, dienophiles and cycloadducts are covered, inter- and intramolecular Diels–Alder cycloadditions are discussed, and stereochemical features of the addition process are described. Areas of emerging importance are also covered, including allenic components in dehydro-Diels–Alder processes, and dendralenic allenes in Diels–Alder sequences for the rapid generation of target-relevant molecular complexity. Preparatively useful methods for allenic precursor synthesis are also discussed.

1 Introduction

2 Allenic Dienes

2.1 Vinylallenes

2.2 Bisallenes

2.3 Cross-conjugated Allenes

3 Allenic Dienophiles

4 Intramolecular Diels–Alder Cycloadditions

5 Allenic Cycloadducts

6 Conclusions and Outlook

EATA Reaction Catalyzed by Copper Halides of Mixed Oxidation States and Its Application to Total Synthesis of Scorodonin

Naturally occurring conjugated allenynes are of general interest to the scientific community for their potent and various biological activities. The 1,5-H transfer of alka-1,4-diyn-3-yl amines would be one of the most straightforward yet challenging approach to this class of compounds since it may, in principle, form two regioisomeric products involving two different C-C triple bonds. Herein, a catalytic recipe of copper halides with mixed oxidation states, i.e., CuCl/CuBr 2 , has been identified to address the issues of the side reaction of conjugate addition and the selectivity of 1,5-H transfer of the key intermediate, alka-1,4-diyn-3-yl amines, in EATA (Enantioselective Allenation of Terminal Alkynes) reaction involving the conjugated 2-alkynals. This protocol could accommodate a wide range of functional groups providing a series of allenynes with a very high enantioselectivity (up to >99% ee). In addition, the enantioenriched allenynes can be readily transformed into various building blocks and applied to the highly enantioselective total synthesis of linear allenic natural product scorodonin for the first time. Mechanistic studies and DFT calculations elucidated the high regioselectivity for observed 1,5-H transfer within the intermediate of 1,4-diyn-3-yl amines. The calculated energy difference between two of the most stable transition states of 3.4 kcal/mol accounts for a selectivity of over 99:1, which is in perfect agreement with the experimental results.

Understanding Halide Counterion Effects in Enantioselective Ruthenium-Catalyzed Carbonyl (α-Aryl)allylation: Alkynes as Latent Allenes and Trifluoroethanol-Enhanced Turnover in The Conversion of Ethanol to Higher Alcohols via Hydrogen Auto-transfer

Crystallographic characterization of RuX(CO)(η3-C3H5)(JOSIPHOS), where X = Cl, Br, or I, reveals a halide-dependent diastereomeric preference that defines metal-centered stereogenicity and, therefrom, the enantioselectivity of C-C coupling in ruthenium-catalyzed anti-diastereo- and enantioselective C-C couplings of primary alcohols with 1-aryl-1-propynes to form products of carbonyl anti-(α-aryl)allylation. Computational studies reveal that a non-classical hydrogen bond between iodide and the aldehyde formyl CH bond stabilizes the favored transition state for carbonyl addition. An improved catalytic system enabling previously unattainable transformations was developed that employs an iodide-containing precatalyst, RuI(CO)3(η3-C3H5), in combination with trifluoroethanol, as illustrated by the first enantioselective ruthenium-catalyzed C-C couplings of ethanol to form higher alcohols.

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.

Enantioselective Ruthenium-BINAP-Catalyzed Carbonyl Reductive Coupling of Alkoxyallenes: Convergent Construction of syn-sec,tert-Diols via (Z)-σ-Allylmetal Intermediates

The first catalytic enantioselective ruthenium-catalyzed carbonyl reductive couplings of allene pronucleophiles is described. Using an iodide-modified ruthenium-BINAP-catalyst and O-benzhydryl alkoxyallene 1a, carbonyl (α-alkoxy)allylation occurs from the alcohol or aldehyde oxidation level to form enantiomerically enriched syn-sec,tert-diols. Internal chelation directs intervention of (Z)-σ-alkoxyallylruthenium isomers, which engage in stereospecific carbonyl addition.

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.

Synthesis of Aminoallenes via Selenium-π-Acid-Catalyzed Cross-Coupling of N-Fluorinated Sulfonimides with Simple Alkynes

The facile synthesis of aminoallenes, accomplished by a selenium-π-acid-catalyzed cross-coupling of an N-fluorinated sulfonimide with simple, non-activated alkynes, is reported. Until now, aminoallenes were difficult to be accessed by customary means, inasmuch as pre-activated and, in part, intricate starting materials were necessary for their synthesis. In sharp contrast, the current study shows that ordinary internal alkynes can serve as simple and readily available precursors for the construction of the aminoallene motif. The operating reaction conditions tolerate numerous functional groups such as esters, nitriles, (silyl)ethers, acetals, and halogen substituents, furnishing the target compounds in up to 86 % yield.

Selectivity for Alkynyl or Allenyl Imidamides and Imidates in Copper-Catalyzed Reactions of Terminal 1,3-Diynes and Azides

Copper-catalyzed reactions of terminal 1,3-diynes with electron-deficient azides to generate either 3-alkynyl or 2,3-dienyl imidamides and imidates are described. The selectivity depends on the diyne substituents and the nucleophile that reacts with the ketenimide intermediate generated from the corresponding triazole precursor. Reactions of 1,3-diynes containing a propargylic acetate afford [3]cumulenyl imidamides, while reactions using methanol as the trapping agent selectively generate 2,3-dienyl imidates. Five-membered heterocycles were obtained from 1,3-diynes containing a homopropargylic hydroxyl or amine substituent.

Acid-catalyzed allenylation of pyrazolones with propargyl alcohols

A TsOH-catalyzed allenylation of pyrazolones with propargylic alcohols has been developed. The established reaction system is well tolerated by a wide scope of pyrazolones and propargylic alcohols. The process has the salient features of operational simplicity, facile scale-up and high yield. In particular, the integration of the pharmaceutical-related pyrazolone skeleton and the allenyl group into a single molecule not only enriches the structural diversity of the pyrazolone scaffold, but potentially also contributes to a broader spectrum of biological activity. Furthermore, it is easy to synthesize 3aa in gram-scale with the yield and efficiency basically maintained, making the practical application of this process more prominent.

Point-to-Axial Chirality Transmission: A Highly Sensitive Triaryl Chirality Probe for Stereochemical Assignments of Amines

A readily available stereodynamic and the electronic circular dichroism (ECD)-silent 2,5-di(1-naphthyl)-terephthalaldehyde-based probe has been applied for chirality sensing of primary amines. The chiral amine (the inductor) forces a change in the structure of the chromophore system through the point-to-axial chirality transmission mechanism. As a result, efficient induction of optical activity in the chromophoric system is observed. The butterflylike structure of the probe, with the terminal aryl groups acting as changeable "wings", allowed for the generation of exciton Cotton effects in the region of 1Bb electronic transition in the naphthalene chromophores. The sign of the exciton couplets observed for inductor-reporter systems might be correlated with an absolute configuration of the inductor, whereas the linear relationship between amplitudes of the specific Cotton effect and enantiomeric excess of the parent amine gives potentiality for quantitative chirality sensing. Despite the structural simplicity, the probe turned out to be unprecedentedly highly sensitive to even subtle differences in the inductor structure (i.e., O vs CH2).

Iridium-Catalyzed Intramolecular Cycloisomerization between Functionalized Alkyne with Aryl Vinyl Ether: Synthesis of 2-Vinyl-3-functionalized Methylbenzofurans

We have developed cycloisomerization between an aryl vinyl ether and a functionalized alkyne, such as silylalkyne, to give 2,3-disubstituted benzofuran derivatives using [IrCl(cod)]₂, PCy₃, and NaBAr^F₄. This catalyst system not only catalyzes the above cycloisomerization but also isomerize a terminal olefin to give an aryl vinyl ether.

Stereochemical Relay through a Cationic Intermediate: Helical Preorganization Dictates Direction of Conrotation in the halo-Nazarov Cyclization

A stereocontrolled halo-Prins/halo-Nazarov cyclization protocol is reported, where chiral information from a secondary alcohol is relayed through several intermediates yielding halocyclopentene products diastereoselectively. An enantiopure product is obtained when a nonracemic secondary alcohol is used. Experimental and computational studies are described, enabling the design and synthesis of systems that ionize and cyclize with >95% chirality transfer through a mechanism involving the creation and preservation of transient helical chirality in a pentadienyl cation intermediate. First, a diastereoselective alkynyl Prins cyclization is executed to synthesize a conformationally distorted dihydropyran intermediate with a curved backbone and high reactivity. This chiral precursor adopts a specific helical alignment early in the subsequent cationic ionization/halo-Nazarov cyclization process, dictating the direction of conrotation in the electrocyclization. Notably, despite the ablation of an sp³ stereogenic center during ionization, the low halo-Nazarov barrier enables efficient capture of a cationic intermediate with dynamic conformational chirality. The ionization and electrocyclization thus occur with "memory of chirality".

Diastereo- and Enantioselective 1,4-Difunctionalization of Borylenynes by Catalytic Conjunctive Cross-Coupling

Enantioselective conjunctive cross-coupling of enyne-derived boronate complexes occurs with 1,4 addition of the electrophile and migrating group across the π system. This reaction pathway furnishes α-boryl allenes as the reaction product. In the presence of a chiral catalyst, both the central and axial chirality of the product can be controlled during product formation.

The Mechanism of Copper-Catalyzed Trifunctionalization of Terminal Allenes

A highly selective copper-catalyzed trifunctionalization of allenes has been established based on diborylation/cyanation with bis(pinacolato)diboron (B₂ pin₂ ) and N-cyano-N-phenyl-p-toluenesulfonamide (NCTS). The Cu-catalyzed trifunctionalization of terminal allenes is composed of three catalytic reactions (first borocupration, electrophilic cyanation, and second borocupration) that provide a densely functionalized product with regio-, chemo- and diastereoselectivity. Allene substrates have multiple reaction-sites, and the selectivities are determined by the suitable interactions (e.g., electronic and steric demands) between the catalyst and substrates. We employed DFT calculations to understand the cascade copper-catalyzed trifunctionalization of terminal allenes, providing densely-functionalized organic molecules with outstanding regio-, chemo- and diastereoselectivity in high yields. The selectivity challenges presented by cumulated π-systems are addressed by systematic computational studies; these give insight to the catalytic multiple-functionalization strategies and explain the high selectivities that we see for these reactions.

Nickel-Catalyzed Hydrocyanation of Allenes and Its Application

Cyano (CN) groups are equivalent to carbonyl as well as amino- and hydroxymethyl groups. Therefore, their catalytic introduction under metal catalysis is an important issue in synthetic organic chemistry. Ni-catalyzed hydrocyanation is one of the most well-investigated, powerful tools for installing a CN group. However, it is still difficult to control chemo- and regioselectivity. In this review, the author uses allenes to enable regio-, stereo-, and face-selective transformations to natural product synthesis and axial chirality transfer.

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.

Design and Synthesis of Flavonoidal Ethers and Their Anti-Cancer Activity In Vitro

Flavonoids are well-characterized polyphenolic compounds with pharmacological and therapeutic activities. However, most flavonoids have not been developed into clinical drugs, due to poor bioavailability. Herein, we report a strategy to increase the drugability of flavonoids by constructing C(sp²)-O bonds and stereo- as well as regioselective alkenylation of hydroxyl groups of flavonoids with ethyl-2,3-butadienoate allenes. Twenty-three modified flavonoid derivatives were designed, synthesized, and evaluated for their anti-cancer activities. The results showed that compounds 4b, 4c, 4e, 5e, and 6b exhibited better in vitro inhibitory activity against several cancer cell lines than their precursors. Preliminary structure–activity relationship studies indicated that, in most of the cancer cell lines evaluated, the substitution on position 7 was essential for increasing cytotoxicity. The results of this study might facilitate the preparation or late-stage modification of complex flavonoids as anti-cancer drug candidates.

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.

Pd-Catalyzed coupling reaction of cyclobutanols with propargylic carbonates

Pd-Catalyzed ring opening coupling reaction of cyclobutanols with propargylic carbonates afforded δ-allenyl ketones efficiently.

Preparation of Carbodiimides with One-Handed Axial Chirality

The axial chirality of carbodiimide was proposed in 1932, but the synthesis of carbodiimide with one-handed axial chirality has not been achieved because of the low barrier of racemization. This work presents a strategy to use a conformationally restrained cyclic structure for creating carbodiimides whose biases of the axial chirality (labeled as S_NCN/ R_NCN) are higher than 100:1, as determined by vibrational circular dichroism spectroscopy and density functional theory calculations.

Gold-catalyzed stereoselective cycloisomerization of allenoic acids for two types of common natural γ-butyrolactones

γ-(E)-Vinylic and γ-alkylic γ-butyrolactones are two different types of lactones existing extensively in animals and plants and many of them show interesting biological activities. Nature makes alkylic γ-butyrolactones by many different enzymatic lactonization processes. Scientists have been mimicking the natural strategy by developing new catalysts. However, direct and efficient access to γ-(E)-vinylic γ-butyrolactones is still extremely limited. Here, we wish to present our modular allene approach, which provides an efficient asymmetric approach to (E)-vinylic γ-butyrolactones from allenoic acids by identifying a new gold complex as the catalyst. Based on this cycloisomerization strategy, the first syntheses of racemic xestospongiene and xestospongienes E, F, G, and H have been realized and the absolute configurations of the chiral centers in xestospongienes E and F have been revised. In addition, by applying a C–O bond cleavage-free hydrogenation, the syntheses of naturally occurring γ-alkylic γ-lactones, (R)-4-tetradecalactone, (S)-4-tetradecalactone, (R)-γ-palmitolactone, and (R)-4-decalactone, have also been achieved.

Gamma-butyrolactones are widespread in Nature, however direct catalytic methods to access them are limited. Here, the authors report a gold-catalyzed cycloisomerization of allenoic acids to acces γ-butyrolactones and apply it to the asymmetric synthesis of xestospongienes E, F, G, and H and other naturally occurring lactones.