Active Allenes Research Articles

Pd-Catalyzed Enantioselective Creation of All-Carbon Quaternary Center with 2,3-Allenylic Carbonates.

Enantioselective construction of all-carbon quaternary centers has been achieved via the palladium-catalyzed highly enantioselective allenylation of oxindoles with 2,3-allenylic carbonates to afford a variety of optically active allene products, which contain oxindole units with different functional groups, in high ee. The corresponding synthetic applications have also been demonstrated.

Iron‐Catalyzed Cross‐Coupling of Propargyl Ethers with Grignard Reagents for the Synthesis of Functionalized Allenes and Allenols

AbstractHerein we disclose an iron‐catalyzed cross‐coupling reaction of propargyl ethers with Grignard reagents. The reaction was demonstrated to be stereospecific and allows for a facile preparation of optically active allenes via efficient chirality transfer. Various tri‐ and tetrasubstituted fluoroalkyl allenes can be obtained in good to excellent yields. In addition, an iron‐catalyzed cross‐coupling of Grignard reagents with α‐alkynyl oxetanes and tetrahydrofurans is disclosed herein, which constitutes a straightforward approach towards fully substituted β‐ or γ‐allenols, respectively.

Iron-Catalyzed Cross-Coupling of Propargyl Ethers with Grignard Reagents for the Synthesis of Functionalized Allenes and Allenols.

Herein we disclose an iron‐catalyzed cross‐coupling reaction of propargyl ethers with Grignard reagents. The reaction was demonstrated to be stereospecific and allows for a facile preparation of optically active allenes via efficient chirality transfer. Various tri‐ and tetrasubstituted fluoroalkyl allenes can be obtained in good to excellent yields. In addition, an iron‐catalyzed cross‐coupling of Grignard reagents with α‐alkynyl oxetanes and tetrahydrofurans is disclosed herein, which constitutes a straightforward approach towards fully substituted β‐ or γ‐allenols, respectively.

Catalytic Enantioselective Simultaneous Control of Axial Chirality and Central Chirality in Allenes

AbstractChiral molecules, which may contain one or more different type(s) of stereocentres, such as central, axial, planar, and helical chiralities, etc., are indispensable in chemistry, pharmaceutical industry, and life science. Despite many advances for the preparation of chiral molecules usually with a single type of chirality have been realized, simultaneous construction of different types of chiralities is still a significant challenge. Here, we wish to report a protocol for preparation of chiral allenes with both central and axial chiralities via a catalytic asymmetric allenylation of different biologically or synthetically useful fluorinated or non‐fluorinated nucleophiles with readily available racemic allenes by using a single chiral ligand. An echoing between the central chirality and axial chirality for the enantioselectivity was observed. This strategy provides a general and practical approach to functionalized optically active allenes bearing both central and axial chiralities with an excellent enantioselectivity under mild conditions.

Rhodium(I)‐Catalyzed Arylation/Dehydroxylation of tert‐Propargylic Alcohols Leading to Tetrasubstituted Allenes

AbstractDiverse tetrasubstituted allenes are obtained selectively by the reaction of tert‐propargylic alcohols and arylboroxines under rhodium catalysis. The reaction is assumed to proceed through an arylation/dehydroxylation process, which involves β‐hydroxide elimination of a β‐hydroxy alkenyl‐rhodium intermediate that is generated by regioselective arylrhodation of the tert‐propargylic alcohol. In addition, when enantioenriched propargylic alcohol was used to prepare optically active allene, high efficiency of central‐to‐axial chirality transfer was observed. The application of current method to structural modification of pharmaceutical drugs was also showcased by a highly diastereoselective transformation of mifepristone.magnified image

Identification of Jasmonic Acid and Jasmonoyl-Isoleucine, and Characterization of AOS, AOC, OPR and JAR1 in the Model Lycophyte Selaginella moellendorffii.

Jasmonic acid (JA) is involved in a variety of physiological responses in seed plants. However, the detection and role of JA in lycophytes, a group of seedless vascular plants, have remained elusive until recently. This study provides the first evidence of 12-oxo-phytodienoic acid (OPDA), JA and jasmonoyl-isoleucine (JA-Ile) in the model lycophyte Selaginella moellendorffii. Mechanical wounding stimulated the accumulation of OPDA, JA and JA-Ile. These data were corroborated by the detection of enzymatically active allene oxide synthase (AOS), allene oxide cyclase (AOC), 12-oxo-phytodienoic acid reductase 3 (OPR3) and JA-Ile synthase (JAR1) in S. moellendorffii. SmAOS2 is involved in the first committed step of JA biosynthesis. SmAOC1 is a crucial enzyme for generating the basic structure of jasmonates and is actively involved in the formation of OPDA. SmOPR5, a functionally active OPR3-like enzyme, is also vital for the reduction of (+)-cis-OPDA, the only isomer of the JA precursor. The conjugation of JA to Ile by SmJAR1 demonstrates that S. moellendorffii produces JA-Ile. Thus, the four active enzymes have characteristics similar to those in seed plants. Wounding and JA treatment induced the expression of SmAOC1 and SmOPR5. Furthermore, JA inhibited the growth of shoots in S. moellendorffii, which suggests that JA functions as a signaling molecule in S. moellendorffii. This study proposes that JA evolved as a plant hormone for stress adaptation, beginning with the emergence of vascular plants.

Chiral Induction in Intramolecular Rhodium‐Catalyzed [2+2+2] Cycloadditions of Optically Active Allene–ene/yne–allene Substrates

AbstractAllene‐yne‐allene and allene‐ene‐allene N‐tosyl‐linked substrates with two chiral centres in the α‐position of the allene moiety were satisfactorily prepared starting both from racemic and chiral propargylic alcohols. The Wilkinson's complex‐catalyzed [2+2+2] cycloaddition reaction of these substrates was evaluated. In the case of enantiomerically pure bisallenes, high stereoselectivity was observed, giving a diastereomerically pure cycloadduct. The chirality of starting bisallene substrates can be completely transferred to the cycloadducts, representing an atom‐economical and enantiospecific process for the construction of fused polycycles. However, when reacting an oxygen‐linked allene‐ene‐allene substrate, the stereoselectivity decreased and two diastereoisomers were formed. A detailed characterization study of the resulting cycloadducts allowed us to identify the enantioisomer generated in the cycloaddition.magnified image

Methyltrioxorhenium-catalyzed highly selective dihydroxylation of 1,2-allenylic diphenyl phosphine oxides.

For the first time, methyltrioxorhenium (MTO) has been applied as a catalyst for the dihydroxylation of allenes in the presence of hydrogen peroxide as the oxidant. The regioselectivities turn out to be well controlled, affording β-carbonyl-γ-hydroxyl diphenyl phosphine oxides as the only product. The axial chirality of optically active allenes can also be nicely transferred to the chirality center of the products. Based on chirality transfer experiments and ESI-MS studies of (18)O-labeled products, a possible mechanism, proceeding via regioselective epoxidation of the electron-rich carbon-carbon double bond, a subsequent intermolecular nucleophilic attack of a water molecule on the in situ formed epoxide via neighboring group participation (NGP), followed by a rearrangement has been proposed as the major reaction pathway.

Morphological self-assembly of enantiopure allenes for upstanding chiral architectures at interfaces.

Chiroptically active allenes are employed for the construction of surface-confined nanostructures. Morphological complementarity between the homochiral units leads to self-assembly of two highly-ordered, upstanding, diastereomeric architectures. The novel, intertwined self-assembled layer structures feature reactive terminal alkynes for further functionalization and carry potential for widespread applications exploiting chiroptical amplification.

Optically Active Allenes from an Allylic Substitution/Isomerization Sequence

It takes two to trisubstitute: A two-step procedure involving enantioselective allylic alkylation followed by 1,3 hydrogen shift enables the catalytic asymmetric synthesis of trisubstituted allenes.

ChemInform Abstract: Optically Active 2,3‐Allenoates via Palladium‐Catalyzed Carbonylation of Enantioenriched Propargylic Mesylates.

AbstractThe reaction allows to introduce various alkyl‐, allyl‐ and benzyl substituents in the desired optically active allene derivatives.

ChemInform Abstract: Preparation of C2‐Symmetric Allenes by Palladium‐Catalyzed Double‐Nucleophilic Substitution on 3‐Bromopenta‐2,4‐dienyl Acetate.

AbstractIn the presence of chiral ligands optically active allenes are available.

ChemInform Abstract: Catalytic Asymmetric Synthesis of Optically Active Allenes from Terminal Alkynes.

AbstractChiral allenols are prepared in moderate to good yields and high enantioselectivities from terminal secondary or tertiary propargylic alcohols and aldehydes according to the three‐step sequence shown.

Catalytic Asymmetric Synthesis of Optically Active Allenes from Terminal Alkynes

CuBr and ZnI(2) have been developed as catalysts or subcatalysts for the efficient asymmetric synthesis of axially chiral allenols with up to 97% ee from readily available propargylic alcohols, aliphatic or aromatic aldehyde, pyrrolidine, and commerically available ligands. The alcohol unit in the terminal alkynes plays a very important role for ensuring high enantioselectivity via coordination.

ChemInform Abstract: Stereochemical Aspects on the Formation of Chiral Allenes from Propargylic Ethers and Epoxides

Chiral propargylic ethers react with organocopper reagents to afford optically active allenes by a u addition to the triple bond followed by an anti p- elimination of the resulting alkenyl copper species. However, the same reaction, run with a Grignard reagent and a catalytic amount of copper (I) salt affords allenes through an anti or svn process. The crucial step is the p-elimination of the intermediate alkenyl metal species, which is of & type with FWa and of type with RMgCJ. Propargylic acetates, which also afford allenes in this reaction, but through a Cu(II1) intermediate, are not sensitive to this halogen effect. By close analogy to ethers, propargylic epoxides react with Grignard reagents and catalytic amount of Cu(1) salt, leading to a-allenic alcohols. The reaction is highly diastereoselective and its stereochemical outcome can be fully controlled.The diastereomer, probably arising through an addition-elimination mechanism, is better obtained with RM&l and copper (I) bromide, whereas the anti diastereomer is best obtained with RM& and a complexed copper (I) salt.

ChemInform Abstract: Bicyclization of Dienes, Enynes, and Diynes with Ti(II) Reagent. New Developments Towards Asymmetric Synthesis

The title bis-unsaturated compounds undergo bicyclization with (h 2 -propene)Ti(O-i- Pr)2 to give dialkoxytitanacycles, the characteristic reactions of which are highlighted. Substrate- controlled asymmetric cyclizations starting from optically active allenes, enynes having a chiral acetal as a leaving group, and optically active enynoates were found promising to give cyclic compounds with high enantiopurity.

Chemoenzymatic Dynamic Kinetic Resolution of Axially Chiral Allenes

At the palladium: Dimeric palladium bromide complexes bearing monodentate N-heterocyclic carbene ligands have been identified as efficient catalysts for the chemoselective racemization of axially chiral allenyl alcohols. In combination with porcine pancreatic lipase as biocatalyst, a dynamic kinetic resolution has been developed, giving access to optically active allenes in good yield and high enantiomeric purity (see scheme).

Alkylidenesilacyclopropanes Derived from Allenes: Applications to the Selective Synthesis of Triols and Homoallylic Alcohols

Several alkylidenesilacyclopropanes were prepared by silver-mediated silylene transfer to allenes. Oxasilacyclopentanes derived from allenes were prepared with high regio- and diastereoselectivity by a two-step, one-flask silacyclopropanation/carbonyl insertion reaction. Triols and homoallylic alcohols were formed diastereoselectively by functionalizing the oxasilacyclopentanes. An optically active allene (>98% ee) was utilized to synthesize an enantiopure homoallylic alcohol in 96% ee.

Enzymatic kinetic resolution of primary allenic alcohols. Application to the total synthesis and stereochemical assignment of striatisporolide A

Crude Porcine pancreatic lipase was successfully used for the kinetic resolution of axially chiral primary allenic alcohols providing very high enantioselectivities with E values above 200. This simple access to optically active allenes was applied to the total synthesis of the fungal metabolite (-)-striatisporolide A, allowing its unambiguous stereochemical assignment.

Multiple chirality transfers in the enantioselective synthesis of 11- O-debenzoyltashironin. Chiroptical analysis of the key cascade

The mechanism of the cascade oxidative dearomatization–transannular Diels–Alder was investigated in the context of an asymmetric route to (−)-11- O-debenzoyltashironin. Although the oxidative dearomatization provides two acetal intermediates, the transannular Diels–Alder proceeds spontaneously from only one of the acetal isomers. Access to enantioenriched tetracyclic adduct was gained through the use of optically active allene.