Stereoselective Isomerization Research Articles

Regio- and Stereo-Selective Isomerization of Borylated 1,3-Dienes Enabled by Selective Energy Transfer Catalysis.

Configurationally-defined dienes are pervasive across the bioactive natural product spectrum, where they typically manifest themselves as sorbic acid-based fragments. These C5 motifs reflect the biosynthesis algorithms that facilitate their construction. To complement established biosynthetic paradigms, a chemical platform to facilitate the construction of stereochemically defined, functionalizable dienes by light-enabled isomerization has been devised. Enabled by selective energy transfer catalysis, a variety of substituted β-boryl sorbic acid derivatives can be isomerized in a regio- and stereo-selective manner (up to 97 : 3). Directionality is guided by a stabilizing nO→pB interaction in the product: this constitutes a formal anti-hydroboration of the starting alkyne. This operationally simple reaction employs low catalyst loadings (1 mol %) and is complete in 1 h. X-ray analysis supports the hypothesis that the nO→pB interaction leads to chromophore bifurcation: this provides a structural foundation for selective energy transfer.

Stereodivergent Biocatalytic Formal Reduction of α-Angelica Lactone to (R)- and (S)-γ-Valerolactone in a One-Pot Cascade.

The formal asymmetric and stereodivergent enzymatic reduction of α-angelica lactone to both enantiomers of γ-valerolactone was achieved in a one-pot cascade by uniting the promiscuous stereoselective isomerization activity of Old Yellow Enzymes with their native reductase activity. In addition to running the cascade with one enzyme for each catalytic step, a bifunctional isomerase-reductase biocatalyst was designed by fusing two Old Yellow Enzymes, thereby generating an unprecedented case of an artificial enzyme catalyzing the reduction of nonactivated C=C bonds to access (R)-valerolactone in overall 41 % conversion and up to 91 % ee. The enzyme BfOYE4 could be used as single biocatalyst for both steps and delivered (S)-valerolactone in up to 84 % ee and 41 % overall conversion. The reducing equivalents were provided by a nicotinamide recycling system based on formate and formate dehydrogenase, added in a second step. This enzymatic system provides an asymmetric route to valuable chiral building blocks from an abundant bio-based chemical.

Nitrile modulated-Ni(0) phosphines in trans-selective phenylpropenoids isomerization: An allylic route by a regular η1-N(end-on) or an alkyl route via a flipped-nitrile?

Acetonitrile has been widely viewed as a weakly coordinating solvent in metal-catalyzed reactions. Evidence is presented herein that it worked as an actor ligand in nickel-catalyzed stereoselective isomerizations of eugenol, methyl eugenol, estragole, and allylphenol, with trans-isomerized products of ≥95%. Nickel(0)(nitrile)phosphines were generated in situ from Ni(II) salt in the presence of phosphines, Zn, and nitrile with Ni/phosphines/Zn/nitrile mole ratio as low as 1:2:1:1. The isomerization of eugenol gave trans-isoeugenol of >97% in 30 min at r.t. with Ni turnover frequency (TOF) up to 1,116 h−1 and the activation energy of 61 kJ·mol−1. High selectivity of trans-isoeugenol was retained in continuous and multi-gram trials. Density functional theory (DFT)/PBE0-D3 calculations rationalized the possible formation of nickel hydride (i.e., Ni(H)(η1-CH2CN)(phosphine) complex) from η1-N(end-on)-Ni complex. This nitrile modulated-Ni(0) complex indicated the complex followed an alkyl-type isomerization, instead of a conventional allylic-type one.

T-BuOK promoted stereoselective isomerization of allyl aryl ethers

The t-BuOK promoted stereoselective isomerization of allyl aryl ethers has been developed. The reactions proceeded well in methyl tert-butyl ether (MTBE), providing the corresponding products in good to excellent yields (83–96%). Most of the substrates were isomerized predominantly to the Z isomers with excellent stereoselectivities. Gram-scale synthesis was also achieved on a 10 mmol scale. Moreover, the cascade Cannizzaro and isomerization reactions, and the one-pot isomerization and styryl sulfide formation reactions, were also realized, affording a potential synthetic route to access interesting compounds.

Stereoselective Rhodium-Catalyzed Isomerization of Stereoisomeric Mixtures of Arylalkenes

A new efficient method for the synthesis of a high ratio of E-alkenes from E/Z mixtures of alkenes with B2pin2 in the presence of a rhodium catalyst is described. This reaction features mild reaction conditions, broad functional group tolerance, and highly great application potential.

Stereoselective Thermal Isomerization of Triphosphasumanenes, Utilized for Stereocontrolled Synthesis

We found stereoselective thermal isomerizations at the phosphorous centers of triphosphasumanenes in the solid state. Theoretical calculations and thermal analysis indicate that the isomerization i...

Multicatalytic Stereoselective Synthesis of Highly Substituted Alkenes by Sequential Isomerization/Cross-Coupling Reactions.

Starting from readily available alkenyl methyl ethers, the stereoselective preparation of highly substituted alkenes by two complementary multicatalytic sequential isomerization/cross-coupling sequences is described. Both elementary steps of these sequences are challenging processes when considered independently. A cationic iridium catalyst was identified for the stereoselective isomerization of allyl methyl ethers and was found to be compatible with a nickel catalyst for the subsequent cross-coupling of the in situ generated methyl vinyl ethers with various Grignard reagents. The method is compatible with sensitive functional groups and a multitude of olefinic substitution patterns to deliver products with high control of the newly generated C═C bond. A highly enantioselective variant of this [Ir/Ni] sequence has been established using a chiral iridium precatalyst. A complementary [Pd/Ni] catalytic sequence has been optimized for alkenyl methyl ethers with a remote C═C bond. The final alkenes were isolated with a lower level of stereocontrol. Upon proper choice of the Grignard reagent, we demonstrated that C( sp2)-C( sp2) and C( sp2)-C( sp3) bonds can be constructed with both systems delivering products that would be difficult to access by conventional methods.

Biocatalytic Formal Anti-Markovnikov Hydroamination and Hydration of Aryl Alkenes

Biocatalytic anti-Markovnikov alkene hydroamination and hydration were achieved based on two concepts involving enzyme cascades: epoxidation–isomerization–amination for hydroamination and epoxidation–isomerization–reduction for hydration. An Escherichia coli strain coexpressing styrene monooxygenase (SMO), styrene oxide isomerase (SOI), ω-transaminase (CvTA), and alanine dehydrogenase (AlaDH) catalyzed the hydroamination of 12 aryl alkenes to give the corresponding valuable terminal amines in high conversion (many ≥86%) and exclusive anti-Markovnikov selectivity (>99:1). Another E. coli strain coexpressing SMO, SOI, and phenylacetaldehyde reductase (PAR) catalyzed the hydration of 12 aryl alkenes to the corresponding useful terminal alcohols in high conversion (many ≥80%) and very high anti-Markovnikov selectivity (>99:1). Importantly, SOI was discovered for stereoselective isomerization of a chiral epoxide to a chiral aldehyde, providing some insights on enzymatic epoxide rearrangement. Harnessing this s...

Tertiary Amino Group in Cationic Gold Catalyst: Tethered Frustrated Lewis Pairs That Enable Ligand-Controlled Regiodivergent and Stereoselective Isomerizations of Propargylic Esters

The development of novel ligands specifically tailored for homogeneous gold catalysis is essential for a new generation of gold catalysis. In this work, we report the development of remotely functionalized biphenyl-2-ylphosphine ligands based on strategic positioning in cationic gold catalysts of frustrated Lewis pairs (FLP), that is, basic tertiary amine moieties and the acidic gold center. By rationally tuning the location and size of the basic group, these Lewis pairs exhibit little or modest quenching and importantly enable regiodivergent and stereoselective isomerizations of propargylic esters into synthetically versatile dienyl esters under exceptionally mild conditions. Notably, the implementation of the concept of FLP in gold catalysis is rare.

Palladium−Hydride‐Catalyzed Regiodivergent Isomerization of 1‐Alkenes

AbstractNew stable palladium−hydride complexes catalyze the highly regio‐ and stereoselective isomerization of 1‐alkenes at multiple positions. Tuning the ligand and the reaction conditions allowed the position and geometry of the double bonds to be controlled with excellent conversions and selectivities. The catalytic species, a Pd−H complex, was prepared by reacting [LPdCl2] with a silver salt, followed by conversion into Pd−H by using a silyl hydride. The structures of the Pd complexes were determined by using X‐ray crystallography and NMR spectroscopy.

Catalytic asymmetric [2+2] cycloaddition between quinones and fulvenes and a subsequent stereoselective isomerization to 2,3-dihydrobenzofurans.

The catalytic enantioselective [2+2] cycloaddition between quinones and fulvenes was achieved, for the first time, by the use of a chiral copper(ii) complex catalyst. The transformation afforded a series of enantiomerically enriched [6,4,5]-tricyclic cyclobutane derivatives in good yields with excellent regio- and stereoselectivities. Furthermore, the [2+2] adducts could be easily converted into formal [3+2] adducts efficiently and stereoselectively.

ChemInform Abstract: Tandem Allylic Alcohol Isomerization/Oxo‐Michael Addition Reaction Promoted by Re2O7.

AbstractA regioselective and stereoselective isomerization of cyclohexadienone‐tethered allylic alcohols followed by diastereoselective ring‐closure via oxa‐Michael addition leads to bicyclic enones.

MoV Reagents in Organic Synthesis

AbstractThe use of MoV reagents, and in particular MoCl5, in organic synthesis is surveyed. The oxidative treatment of aromatic substrates is the most common application. The unique properties of these reagents are due to their high oxidative power combined with exquisite Lewis acid properties. In several examples MoV reagents outperform other common oxidative coupling reagents. C–C bond formation through inter‐ and intramolecular oxidative coupling can lead to selective formation of five‐ to eight‐membered ring systems. Mechanistic investigations of the courses of reactions involving MoV reagents and aromatic substrates indicate that radical cations are initially formed, entering the oxidative coupling scenery. Mechanistic studies reveal that Scholl‐type conversion is unlikely. However, over‐oxidation of the product seems to be the rationale for the selective conversion. Reduction occurs during workup, through the action of the lower‐valency molybdenum salts present. This opens up the pathway to oxidative domino sequences. MoCl5 has already been used in the key steps of the syntheses of several natural products or their analogues. In addition, MoCl5 can serve in chlorination reactions and in stereoselective isomerization by template formation, and also as a precursor for alkyne trimerization.

The Sialic Acids Waltz: Novel Stereoselective Isomerization of the 1,7‐Lactones of N‐Acetylneuraminic Acids into the Corresponding γ‐Lactones and Back to the Free Sialic Acids

AbstractAn internal rearrangement of the 1,7‐lactone of N‐acetylneuraminic acid into its γ‐lactone in high yield is reported, together with the successive conversion into the free parent sialic acid. Overall, these reactions further characterize the chemical behavior of sialic acids and their derivatives, which are increasingly considered to be key players in many biological processes, including ischemic heart disease.

Palladium Hydride Promoted Stereoselective Isomerization of Unactivated Di(exo)methylenes to Endocyclic Dienes

The exomethylenes of 2,6-disubstituted bicyclo[3.3.1]nonan-9-ones 2 are readily isomerized over a palladium catalyst under an atmosphere of hydrogen to predominantly form the isomer 3 with C2 symmetry with very little formation of the analogous product with Cs symmetry. A hydrogen source is essential to effect the rearrangement.

ChemInform Abstract: TMSOTf-Catalyzed Stereoselective Isomerization of Acetates of the Baylis-Hillman Adducts.

TMSOTf-catalyzed isomerization of acetates of the Baylis - Hillman adducts, i.e. methyl 3-acetoxy-3-aryl-2-methylenepropanoates and 3-acetoxy-3-aryl-2-methylenepropanenitriles providing methyl (2E)-2-(acetoxymethyl)-3-arylprop-2-enoates and (2E)-2-(acetoxymethyl)-3-arylprop-2-enenitriles, respectively, is described.

Efficient Tandem Biocatalytic Process for the Kinetic Resolution of Aromatic β‐Amino Acids

AbstractWe describe a simple and efficient enzymatic tandem reaction for the preparation of enantiomerically pure β‐phenylalanine and its analogues from the corresponding racemates. In this process, phenylalanine aminomutase (PAM) catalyzes the stereoselective isomerization of (R)‐β‐phenylalanines to (S)‐α‐phenylalanines, which are in situ transformed to cinnamic acids by phenylalanine ammonia lyase (PAL). Preparative scale conversions are done with a mutated PAM with enhanced catalytic activity.

ChemInform Abstract: Regio‐ and Stereoselective Isomerizations of Allenamides: Synthesis of 2‐Amido‐dienes and Their Tandem Isomerization‐Electrocyclic Ring‐Closure.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Regio- and Stereoselective Isomerizations of Allenamides: Synthesis of 2-Amido-Dienes and Their Tandem Isomerization-Electrocyclic Ring-Closure

A regio- and stereoselective isomerization of allenamides is described, leading to preparations of de novo 2-amido-dienes and a tandem isomerization-6pi-electron electrocyclic ring-closure.

A first one-pot synthesis, isomerization and synthetic utility of mono- and bis Morita–Baylis–Hillman adducts of 1,1′-ferrocenedialdehyde

A one-pot synthesis of mono- and bis-Morita–Baylis–Hillman adducts of 1,1′-ferrocenedialdehyde has been achieved. These adducts undergo a facile and efficient stereoselective isomerization with a number of saturated, unsaturated, aromatic alcohols, phenols and thiophenol with a montmorillonite K10 clay catalyst to afford highly functionalized trisubstituted alkene derivatives of ferrocene. The synthetic utility of isomerized derivatives has been demonstrated by a ferrocene appended novel macrocycle synthesis, a ferrocenyl bis-triazole synthesis and an evaluation of the liquid crystalline property of a cholesterol derivative of ferrocene.