Chiral synthesis via organoboranes. 38. Selective reductions. 48. Asymmetric reduction of trifluoromethyl ketones by B-chlorodiisopinocampheylborane in high enantiomeric purity

TiCl4-Catalyzed Hydroboration of Ketones with Ammonia Borane

Investigation of a variety of Lewis acids for the hydroboration-hydrolysis (reduction) of ketones with amine-boranes has revealed that catalytic (10 mol %) titanium tetrachloride (TiCl₄) in diethyl ether at room temperature immensely accelerates the reaction of ammonia borane. The product alcohols are produced in good to excellent yields within 30 min, even with ketones which typically requires 24 h or longer to reduce under uncatalyzed conditions. Several potentially reactive functionalities are tolerated, and substituted cycloalkanones are reduced diastereoselectively to the thermodynamic product. A deuterium labeling study and ¹¹B NMR analysis of the reaction have been performed to verify the proposed hydroboration mechanism.

2,2'-Bipyridine-α,α'-trifluoromethyl-diol ligand: synthesis and application in the asymmetric Et2Zn alkylation of aldehydes

A chiral 2,2'-bipyridine ligand (1) bearing α,α'-trifluoromethyl-alcohols at 6,6'-positions was designed in five steps affording either the R,R or S,S enantiomer with excellent stereoselectivities, i.e. 97% de, >99% ee and >99.5% de, >99.5% ee, respectively. The key step for reaching high levels of stereoselectivity was demonstrated to be the resolution of the α-CF₃-alcohol using (S)-ibuprofen as the resolving agent. An initial application for the 2,2'-bipyridine-α,α'-CF₃-diol ligand was highlighted in the Zn^II-catalyzed asymmetric ethylation reaction of aromatic, heteroaromatic, and aliphatic aldehydes. Synergistic electron deficiency and steric hindrance properties of the newly developed ligand afforded the corresponding alcohols in good to excellent yields (up to 99%) and enantioselectivities (up to 95% ee). As observed from single crystal diffraction analysis, the complexation of the 2,2'-bipyridine-α,α'-CF₃-diol ligand generates an unusual hexacoordinated Zn^II.

Asymmetric hydrogenation of trifluoromethyl ketones: application in the synthesis of Odanacatib and LX-1031

The asymmetric hydrogenation of trifluoromethyl ketones via iridium catalysis to access chiral secondary 2,2,2-trifluoroethanols is presented. The key intermediates of Odanacatib and LX1301 were prepared with high yields and enantioselectivities.

Asymmetric Hydrogenation of Aryl Perfluoroalkyl Ketones Catalyzed by Rhodium(III) Monohydride Complexes Bearing Josiphos Ligands

The asymmetric hydrogenation of 2,2,2-trifluoroacetophenones and aryl perfluoroalkyl ketones was developed using a unique, well-defined chloride-bridged dinuclear rhodium(III) complex bearing Josiphos-type diphosphine ligands. These complexes were prepared from [RhCl(cod)]₂ , Josiphos ligands, and hydrochloric acid. As catalyst precursors, they allow for the efficient and enantioselective synthesis (up to 99 % ee) of chiral secondary alcohols with perfluoroalkyl groups. This system does not require an activating base for the hydrogenation of 2,2,2-trifluoroacetophenones. Additionally, the enantioselective C=O hydrogenations of 2-phenyl-3-(haloacetyl)-indoles, a class of privileged structures in medicinal chemistry, is reported for the first time.

Synthesis of chiral α-trifluoromethyl alcohols and ethers via enantioselective Hiyama cross-couplings of bisfunctionalized electrophiles

Methods for synthesis of chiral organic compounds bearing trifluoromethyl-substituted stereocenters are of great interest for agrochemical and pharmaceutical labs and industries in their search for new bioactive materials. We report on employment of bisfunctionalized electrophiles, bearing both a trifluoromethyl and a functional group as direct substituents of the reactive center, in cross-coupling reactions. We exemplify this concept in the asymmetric synthesis of enantioenriched α-trifluoromethyl- and perfluoroalkyl-containing benzylic and allylic ethers and alcohols by nickel-catalyzed stereoconvergent Hiyama cross-coupling reaction. Substrate electrophiles are conveniently prepared in few steps from trifluoroacetic acid. The method represents a conceptually different approach to chiral CF₃-substituted alcohols and ethers and allows for a rapid catalytic preparation of a wide range of these valuable compounds in high yields and enantioselectivity.

Compounds containing stereocenters bearing -CF₃ groups, especially in proximity of heteroatoms, are of great interest for drug and agrochemicals development. Here, the authors report a nickel-catalyzed Hiyama cross-coupling methodology for the asymmetric synthesis of valuable alpha-trifluoromethyl alcohols and ethers.

Chemo- and Enantioselective Addition and β-Hydrogen Transfer Reduction of Carbonyl Compounds with Diethylzinc Reagent in One Pot Catalyzed by a Single Chiral Organometallic Catalyst

Using a single chiral phosphoramide-Zn(II) complex as the catalyst, the asymmetric β-H transfer reduction of aromatic α-trifluoromethyl ketones and enantioselective addition of aromatic aldehydes with Et2Zn in one pot were successfully realized, affording the corresponding additive products of secondary alcohols in high yields (up to 99%) with excellent enantioselectivities (up to 98% ee) and the reduction products of α-trifluoromethyl alcohols in good to excellent yields with up to 77% ee.

Design, synthesis, and subtype selectivity of 3,6-disubstituted β-carbolines at Bz/GABA(A)ergic receptors. SAR and studies directed toward agents for treatment of alcohol abuse

A series of 3,6-disubstituted β-carbolines was synthesized and evaluated for their in vitro affinities at α(x)β(3)γ(2) GABA(A)/benzodiazepine receptor subtypes by radioligand binding assays in search of α(1) subtype selective ligands to treat alcohol abuse. Analogues of β-carboline-3-carboxylate-t-butyl ester (βCCt, 1) were synthesized via a CDI-mediated process and the related 6-substituted β-carboline-3-carboxylates 6 including WYS8 (7) were synthesized via a Sonogashira or Stille coupling processes from 6-iodo-βCCt (5). The bivalent ligands of βCCt (32 and 33) were also designed and prepared via a palladium-catalyzed homocoupling process to expand the structure-activity relationships (SAR) to larger ligands. Based on the pharmacophore/receptor model, a preliminary SAR study on 34 analogues illustrated that large substituents at position-6 of the β-carbolines were well tolerated. As expected, these groups are proposed to project into the extracellular domain (L(Di) region) of GABA(A)/Bz receptors (see 32 and 33). Moreover, substituents located at position-3 of the β-carboline nucleus exhibited a conserved stereo interaction in lipophilic pocket L(1), while N(2) presumably underwent a hydrogen bonding interaction with H(1). Three novel β-carboline ligands (βCCt, 3PBC and WYS8), which preferentially bound to α1 BzR subtypes permitted a comparison of the pharmacological efficacies with a range of classical BzR antagonists (flumazenil, ZK93426) from several different structural groups and indicated these β-carbolines were 'near GABA neutral antagonists'. Based on the SAR, the most potent (in vitro) α(1) selective ligand was the 6-substituted acetylenyl βCCt (WYS8, 7). Earlier both βCCt and 3PBC had been shown to reduce alcohol self-administration in alcohol preferring (P) and high alcohol drinking (HAD) rats but had little or no effect on sucrose self-administration.(1-3) Moreover, these two β-carbolines were orally active, and in addition, were anxiolytic in P rats but were only weakly anxiolytic in rodents. These data prompted the synthesis of the β-carbolines presented here.

Synthesis and characterization of novel 6-fluoro-4-piperidinyl-1,2-benzisoxazole amides and 6-fluoro-chroman-2-carboxamides: antimicrobial studies

Novel derivatives of 6-fluoro-4-piperidinyl-1,2-benzisoxazole amides 4(I-VI) were obtained by the condensation of different acid chlorides with 6-fluoro-3-piperidin-4yl-benzo[d]isoxazole. Also, 6-fluoro-chroman-2-carboxamides 6(I-III) were synthesized by using nebulic acid chloride with different amines in presence of triethylamine as acid scavenger and dichloroethane as solvent. The synthesized compounds were characterized by IR, 1H NMR, and CHN analysis. These molecules were evaluated for their efficacy as antimicrobials in vitro by disc diffusion and microdilution method against pathogenic strains such as Bacillus substilis, Escherichia coli, Pseudomonas fluorescens, Xanthomonas campestris pvs, X. oryzae, Aspergillus niger, A. flavus, Fusarium oxysporum, Trichoderma species, F. monaliforme, and Penicillum species. Compounds 4I, 4IV, 4V, 6I, 6II and 6III showed better inhibitory activity than compared to standard drugs. Among these compounds, 4IV and 6III showed potent inhibitory activity against all the strains and found to be nonstrain dependent. The title compounds represent a novel class of potent antimicrobial agents.

Highly regioselective hydrogenolysis of bis(alpha-methylbenzyl)amine derivatives affected by the trifluoromethyl substituent on the aromatic ring

[reaction: see text] The highly regioselective hydrogenolysis of bis(alpha-methylbenzyl)amine derivatives proceeded with influence not from the electronic effect but from the steric effect of the trifluoromethyl substituent on the aromatic ring to provide a practical asymmetric synthesis of trifluoromethyl-substituted alpha-phenylethylamines.

An exceptional hydroboration of substituted fluoroolefins providing tertiary alcohols

[reaction--see text] A rare hydroboration-oxidation providing tertiary-alcohols has been achieved in the case of 1,1,2-perfluoroalkyl(aryl)ethylenes. The hydroboration of substituted perfluoroalkyl(aryl)ethylenes with dichloroborane reveals that the regioselectivity does not entirely depend on the electronics of the fluoroolefins.

Metabolism of fluorine-containing drugs

This article reviews current knowledge of the metabolism of drugs that contain fluorine. The strategic value of fluorine substitution in drug design is discussed in terms of chemical structure and basic concepts in drug metabolism and drug toxicity.

Two classes of enzymes of opposite stereochemistry in an organism: one for fluorinated and another for nonfluorinated substrates

Reduction of methyl ketones by dried cells of Geotrichum candidum (APG4) afforded (S)-alcohols in excellent enantiomeric excess (ee), whereas the reduction of trifluoromethyl ketones gave the corresponding alcohols of the opposite configuration also in excellent ee. The replacement of the methyl moiety with a trifluoromethyl group alters both the bulkiness and the electronic properties, the effect of which on the stereoselectivity was examined. No inversion in stereochemistry was observed in the reduction of hindered ketones such as isopropyl ketone, while the stereoselectivity was inverted in the reduction of ketones with electron-withdrawing atoms such as chlorine. The mechanism for the inversion in stereochemistry was investigated by enzymatic studies. Several enzymes with different stereoselectivities were isolated; one of them catalyzed the reduction of methyl ketones, and another with the opposite stereoselectivity catalyzed the reduction of trifluoromethyl ketones. Furthermore, both APG4 and the isolated enzyme were applied to the reduction of fluorinated ketones on a preparative scale, which resulted in the synthesis of chiral fluorinated alcohols with excellent ee.

Synthesis of the nanomolar photoaffinity GABA(B) receptor ligand CGP 71872 reveals diversity in the tissue distribution of GABA(B) receptor forms

A radioiodinated probe, [125I]-CGP 71872, containing an azido group that can be photoactivated, was synthesized and used to characterize GABA(B) receptors. Photoaffinity labeling experiments using crude membranes prepared from rat brain revealed two predominant ligand binding species at approximately 130 and approximately 100 kDa believed to represent the long (GABA(B)R1a) and short (GABA(B)R1b) forms of the receptor. Indeed, these ligand binding proteins were immunoprecipitated using a GABA(B) receptor-specific antibody confirming the receptor specificity of the photoaffinity probe. Most convincingly, [125I]-CGP 71872 binding was competitively inhibited in a dose-dependent manner by cold CGP 71872, GABA, saclofen, (-)-baclofen, (+)-baclofen and (L)-glutamic acid with a rank order and stereospecificity characteristic of the GABA(B) receptor. Photoaffinity labeling experiments revealed that the recombinant GABA(B)R2 receptor does not bind [125I]-CGP 71872, providing surprising and direct evidence that CGP 71872 is a GABA(B)R1 selective antagonist. Photoaffinity labeling experiments using rat tissues showed that both GABA(B)R1a and GABA(B)R1b are co-expressed in the brain, spinal cord, stomach and testis, but only the short GABA(B)R1b receptor form was detected in kidney and liver whereas the long GABA(B)R1a form was selectively expressed in the adrenal gland, pituitary, spleen and prostate. We report herein the synthesis and biochemical characterization of the nanomolar affinity [125I]-CGP 71872 and CGP 71872 GABA(B)R1 ligands, and differential tissue expression of the long GABA(B)R1a and short GABA(B)R1b receptor forms in rat and dog.

The mechanism of inactivation of S-adenosylhomocysteine hydrolase by fluorinated analogs of 5'-methylthioadenosine

5'-Deoxy-5'-difluoromethylthioadenosine (DFMTA) 1a and 5'-deoxy-5'-trifluoromethyl-thioadenosine (TFMTA) 1b are inhibitors of beef liver S-adenosyl-L-homocysteine hydrolase. DFMTA and TFMTA are time-dependent and irreversible inhibitors of the enzyme. Both 1a and 1b are oxidized by E-NAD+ to produce E-NADH and fluoride anion is formed in the inactivation reaction (2.2 mol of fluoride/mole of enzyme subunit and 3.1 fluoride/mole of enzyme subunit from DFMTA and TFMTA respectively). Using [8-3H]-1a or [8-3H]-1b no trace of labelled adenosine was detected during the inactivation reaction but adenine was formed. The mechanism of inhibition of S-adenosyl-L-homocysteine hydrolase by these two fluorinated nucleosides is discussed.