Copper-Catalyzed Asymmetric Hydroboration of α-Dehydroamino Acid Derivatives: Facile Synthesis of Chiral β-Hydroxy-α-amino Acids

Enantioselective γ-borylation of unsaturated amides and stereoretentive Suzuki-Miyaura cross-coupling

The rhodium-catalyzed, directed catalytic asymmetric hydroboration of γ,δ-unsaturated amides affords a direct route to chiral acyclic secondary γ-borylated carbonyl derivatives in high enantiomeric purity. In contrast to a similar β-borylated amide derivative, the γ-borylated amide undergoes Suzuki-Miyaura cross-coupling with stereoretention. The utility of the boronic ester products is further illustrated by other stereospecific C-B bond transformations leading to γ-amino acid derivatives, 1,4-amino alcohols, and 5-substituted-γ-lactone and γ-lactam ring systems.

Synthesis of Chiral Tertiary Boronic Esters: Phosphonate-Directed Catalytic Asymmetric Hydroboration of Trisubstituted Alkenes

Highly enantioselective rhodium-catalyzed hydroboration of allylic phosphonates by pinacolborane affords chiral tertiary boronic esters. The β-borylated phosphonates are readily converted to chiral β- and γ-hydroxyphosphonates and aminophosphonates and to phosphonates bearing a quaternary carbon stereocenter. The utility of the latter is illustrated by the synthesis of (S)-(+)-bakuchiol methyl ether.

Conformational preferences and synthesis of isomers Z and E of oxazole-dehydrophenylalanine

Dehydrophenylalanine, ΔPhe, is the most commonly studied α,β-dehydroamino acid. In nature, further modifications of the α,β-dehydroamino acids were found, for example, replacement of the C-terminal amide group by oxazole ring. The conformational properties of oxazole-dehydrophenylalanine residue (ΔPhe-Ozl), both isomers Z and E, were investigated. To determine all possible conformations, theoretical calculations were performed using Ac-(Z/E)-ΔPhe-Ozl(4-Me) model compounds at M06-2X/6-31++G(d,p) level of theory. Ac-(Z/E)-ΔPhe-Ozl-4-COOEt compounds were synthesized and the conformational preferences of each isomer, Z and E, were investigated using FTIR and NMR-NOE in solutions of increasing polarity (CHCl3 , DMSO-d6). The solid-state low-temperature structures of Ac-(Z)-ΔPhe-Ozl-4-COOEt and its intermediate analog Ac-(Z)-ΔPhe-Ozn(4-OH)-4-COOEt were also determined. In a weakly polar environment, the ΔPhe-Ozl residue has a tendency to adopt the conformation β2 with the calculated φ and ψ angles of -127° and 0° for the isomer Z and -170° and 26° for the isomer E. The increase of environment polarity favors the helical conformation α and the beta-turn like conformation β, but the conformation β2 seems to be still accessible. The (E)-ΔPhe-Ozl residue can be obtained from the isomer Z in photoisomerization reaction. However, hydroxyl-oxazoline-dehydrophenylalanine ΔPhe-Ozn(4-OH) decomposes in such conditions. Alternatively, (E)-ΔPhe-NH2 can be applied as a substrate in the Hantzsch reaction. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 283-294, 2016.

Copper-Catalyzed Asymmetric Borylation: Construction of a Stereogenic Carbon Center Bearing Both CF3 and Organoboron Functional Groups

Copper-catalyzed borylation of β-trifluoromethyl-α,β-unsaturated ketones was efficiently achieved by means of bis(pinacolato)diboron (B₂pin₂), affording the enantioenriched products in good yields with high enantioselectivities. CuI and (R,S)-Josiphos consist of the most efficient catalyst system under mild conditions. In the absence of the chiral ligand, the reactions could be performed more efficiently to form β-ketone derivatives which were directly borylated and indirectly trifluoromethylated at the β-carbon atom of the α,β-unsaturated ketone substrates. The present protocol provides a promising method to access a stereogenic carbon center bearing both CF₃ and organoboron functional groups.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.

Palladium(II)-Catalyzed Cross-Dehydrogenative Coupling (CDC) of N-Phthaloyl Dehydroalanine Esters with Simple Arenes: Stereoselective Synthesis of Z-Dehydrophenylalanine Derivatives

Pd(II)-catalyzed cross-dehydrogenative coupling (CDC) of methyl N-phthaloyl dehydroalanine esters with simple aromatic hydrocarbons is reported. The reaction, which involves the cleavage of two sp(2) C-H bonds followed by C-C bond formation, stereoselectively generates highly valuable Z-dehydrophenylalanine skeletons in a practical, versatile, and atom economical manner. In addition, a perfluorinated product was expediently converted into important nonproteinogenic amino acid building blocks through copper-catalyzed conjugate additions of boron, silicon, and hydride moieties.

Enantioselective Synthesis of Chiral Piperidines via the Stepwise Dearomatization/Borylation of Pyridines

We have developed a novel approach for the synthesis of enantioenriched 3-boryl-tetrahydropyridines via the Cu(I)-catalyzed regio-, diastereo-, and enantioselective protoborylation of 1,2-dihydropyridines, which were obtained by the partial reduction of the pyridine derivatives. This dearomatization/enantioselective borylation stepwise strategy provides facile access to chiral piperidines together with the stereospecific transformation of a stereogenic C-B bond from readily available starting materials. Furthermore, the utility of this method is demonstrated for the concise synthesis of the antidepressant drug (-)-paroxetine. A theoretical study of the reaction mechanism is also described.

Synthesis of Chiral Tertiary Boronic Esters by Oxime-Directed Catalytic Asymmetric Hydroboration

Chiral boronic esters are useful intermediates in asymmetric synthesis. We have previously shown that carbonyl-directed catalytic asymmetric hydroboration (CAHB) is an efficient approach to the synthesis of functionalized primary and secondary chiral boronic esters. We now report that the oxime-directed CAHB of alkyl-substituted methylidene and trisubstituted alkene substrates by pinacolborane (pinBH) affords oxime-containing chiral tertiary boronic esters with yields up to 87% and enantiomeric ratios up to 96:4 e.r. The utility of the method is demonstrated by the formation of chiral diols and O-substituted hydroxylamines, the generation of quaternary carbon stereocenters through carbon-carbon coupling reactions, and the preparation of chiral 3,4,4-trisubstituted isoxazolines.

E,Z-Stereodivergent synthesis of N-tosyl α,β-dehydroamino esters via a Mukaiyama–Michael addition

E or Z dehydroamino esters are stereoselectively obtained from a same set of reactants depending on the reaction conditions.

Bimetallic Cu(i) complex with a pyridine-bridged bis(1,2,3-triazole-5-ylidene) ligand

Expanded π-conjugated system catalyst for hydroboration of alkenes.

Surprisingly Facile C-H Activation in the Course of Oxime-Directed Catalytic Asymmetric Hydroboration

Deuterium-labeling studies carried out in conjunction with investigations into the directed catalytic asymmetric hydroboration of unsaturated oxime ethers reveal a surprisingly facile ortho-metallation or σ-bond metathesis pathway that that diverts the expected course of CAHB to a tandem C-H activation/hydroboration reaction pathway.

Enantioselective desymmetrization via carbonyl-directed catalytic asymmetric hydroboration and Suzuki-Miyaura cross-coupling

The rhodium-catalyzed enantioselective desymmetrization of symmetric γ,δ-unsaturated amides via carbonyl-directed catalytic asymmetric hydroboration (directed CAHB) affords chiral secondary organoboronates with up to 98% ee. The chiral γ-borylated products undergo palladium-catalyzed Suzuki-Miyaura cross-coupling via the trifluoroborate salt with stereoretention.

Asymmetric boron conjugate addition to α,β-unsaturated carbonyl compounds catalyzed by CuOTf/Josiphos under non-alkaline conditions

The asymmetric boron conjugate addition onto α,β-unsaturated ketones and esters has been developed by using the CuOTf/Josiphos complex as the catalyst under non-alkaline conditions.

Crystal structure of N-(tert-but-oxy-carbon-yl)phenyl-alanylde-hydro-alanine isopropyl ester (Boc-Phe-ΔAla-OiPr)

In the crystal structure of the de­hydro­dipeptide (Boc-Phe-ΔAla-OiPr), the mol­ecule has a trans configuration of the N-methyl­amide group. Its geometry is different from saturated peptides but is in excellent agreement with other de­hydro­alanine compounds. In the crystal, an N—H⋯O hydrogen bond links the mol­ecules in a herringbone packing arrangement.

In the title compound, the de­hydro­dipeptide (Boc–Phe–ΔAla–OiPr, C₂₀H₂₈N₂O₅), the mol­ecule has a trans conformation of the N-methyl­amide group. The geometry of the de­hydro­alanine moiety is to some extent different from those usually found in simple peptides, indicating conjugation between the H₂C=C group and the peptide bond. The bond angles around de­hydro­alanine have unusually high values due to the steric hindrance, the same inter­action influencing the slight distortion from planarity of the de­hydro­alanine. The mol­ecule is stabilized by intra­molecular inter­actions between the isopropyl group and the N atoms of the peptide main chain. In the crystal, an N—H⋯O hydrogen bond links the mol­ecules into ribbons, giving a herringbone head-to-head packing arrangement extending along the [100] direction. In the stacks, the mol­ecules are linked by weak C—H⋯O hydrogen-bonding associations.

Copper(i)-catalyzed enantioselective hydroboration of cyclopropenes: facile synthesis of optically active cyclopropylboronates

Copper(i)-catalyzed enantioselective hydroboration of 3-aryl substituted cyclopropene-3-carboxylate is described, providing chiral cyclopropylboronates with excellent enantioselectivities (89–95% ee) in moderate to high yields (55–86%).