Practical Asymmetric Synthesis of a Potent PDE4 Inhibitor via Stereoselective Enolate Alkylation of a Chiral Aryl−Heteroaryl Secondary Tosylate

Generation of difluorocarbenes and introduction of fluorinated one carbon units into carbonyl and related compounds

Difluorocarbene is a simple and versatile one-carbon unit for synthesizing acyclic and cyclic organofluorine compounds. However, the use of difluorocarbene in organic synthesis has been relatively limited because of the harsh conditions required for its generation, the toxicity of the precursors, and undesired dimerization. This feature article provides an account of (i) the generation of free and metal difluorocarbenes from trimethylsilyl 2,2-difluoro-2-(fluorosulfonyl)acetate (TFDA) or BrCF₂CO₂Li/Na and (ii) their application to the facile synthesis of valuable organofluorine compounds. The difluorocarbenes thus generated react with (thio)carbonyl compounds and silyl dienol ethers to provide a wide variety of products such as (a) difluoromethyl (thio)ethers, (b) fluorinated thiophenes, (c) fluorinated thia/oxazoles, (d) fluorinated cyclopentanones and (e) difluoroalkenes.

In vivo effective dibenzo[b,d]furan-1-yl-thiazoles as novel PDE-4 inhibitors

Herein we report the synthesis, PDE-4B and TNF-α inhibitory activities of a few dibenzo[b,d]furan-1-yl-thiazole derivatives. The hydroxycyclohexanol amide derivatives 14, 18, 24, 29, 31 and 33 exhibited promising in vitro PDE-4B and TNF-α inhibitory activities. Compound 24 showed good systemic availability in preclinical animal models and was also found to be non-toxic (exploratory mutagenicity test). Further it exhibited promising results in in vivo asthma/COPD and Uveitis models.

Stereospecific Coupling of Boronic Esters with N-Heteroaromatic Compounds

A protocol for the stereospecific coupling of chiral secondary and tertiary boronic esters with lithiated N-heteroaromatics is described. The process involves initial boronate complex formation followed by addition of Troc-Cl, which activates the nitrogen and induces 1,2-migration. Oxidative workup furnishes the coupled product with >98% es.

Structure-activity relationship studies and sleep-promoting activity of novel 1-chloro-5,6,7,8-tetrahydroimidazo[1,5-a]pyrazine derivatives as dual orexin receptor antagonists. Part 2

Replacement of the dimethoxyphenyl moiety in the core skeleton of almorexant by appropriately substituted imidazoles afforded novel 1-chloro-5,6,7,8-tetrahydroimidazo[1,5-a]pyrazine derivatives as potent dual orexin receptor antagonists. We describe in this Letter our efforts to further optimize the potency and brain penetration of these derivatives by fine-tuning of the pivotal phenethyl motif, and we comment on the sleep-promoting activity of selected compounds in a rat electroencephalographic (EEG) model.

A novel class of tunable cyclopropanation reagents (RXZnCH2Y) and their synthetic applications

The Simmons-Smith cyclopropanation is a widely used method to synthesize cyclopropanes from alkenes using methylene iodide and a zinc reagent. A novel class of organozinc species, RXZnCH(2)Y, has been found to efficiently cyclopropanate alkenes, including traditionally unreactive unfunctionalized alkenes. The reactivity and selectivity of this class of organozinc reagents can be regulated by tuning the electronic and/or steric nature of the RX group attached to Zn. During recent years, this class of organozinc reagent has been widely used in organic synthesis as a reagent for cyclopropanation and other useful synthetic transformations. Catalytic, asymmetric versions of this reaction have been developed providing high enantiomeric excess for unfunctionalized olefins.

Synthesis and structure-activity relationships of aza- and diazabiphenyl analogues of the antitubercular drug (6S)-2-nitro-6-{[4-(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine (PA-824)

New heterocyclic analogues of the potent biphenyl class derived from antitubercular drug PA-824 were prepared, aiming to improve aqueous solubility but maintain high metabolic stability and efficacy. The strategy involved replacement of one or both phenyl groups by pyridine, pyridazine, pyrazine, or pyrimidine, in order to reduce lipophilicity. For para-linked biaryls, hydrophilicities (ClogP) correlated with measured solubilities, but highly soluble bipyridine analogues displayed weak antitubercular activities. A terminal pyridine or proximal heterocycle allowed retention of potency and provided solubility improvements, particularly at low pH, with examples from the latter classes displaying the better in vivo efficacies, high metabolic stabilities, and excellent pharmacokinetics. Five such compounds were >100-fold better than the parent drug in a mouse model of acute Mycobacterium tuberculosis infection, and two orally bioavailable pyridine analogues (3-4-fold more soluble than the parent at low pH) were superior to antitubercular drug OPC-67683 in a chronic infection model.

Synthesis and structure-activity studies of biphenyl analogues of the tuberculosis drug (6S)-2-nitro-6-{[4-(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine (PA-824)

A series of biphenyl analogues of the new tuberculosis drug PA-824 was prepared, primarily by coupling the known (6S)-2-nitro-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazin-6-ol with iodobenzyl halides, followed by Suzuki coupling of these iodides with appropriate arylboronic acids or by assembly of the complete biaryl side chain prior to coupling with the above alcohol. Antitubercular activity was determined under both replicating (MABA) and nonreplicating (LORA) conditions. para-Linked biaryls were the most active, followed by meta-linked and then ortho-linked analogues. A more detailed study of a larger group of para-linked analogues showed a significant correlation between potency (MABA) and both lipophilicity (CLOGP) and the electron-withdrawing properties of terminal ring substituents ( summation operatorsigma). Selected compounds were evaluated for their efficacy in a mouse model of acute Mycobacterium tuberculosis infection. In vivo activity correlated well with the stability of compounds to microsomal metabolism. Three compounds bearing combinations of lipophilic, electron-withdrawing groups achieved >200-fold higher efficacies than the parent drug.

Enzyme-Catalyzed Enantioselective Diaryl Ketone Reductions

The synthesis of diarylmethanols via the reduction of a range of substituted benzophenone and benzoylpyridine derivatives with ketoreductase enzymes (KREDs) has afforded chiral products with high yield (>90%) and ee (up to >99%). Ortho, meta, and para substitutions with a variety of electron-donating, electron-withdrawing, and halogen groups were examined. Substitution at the ortho position and/or highly electronically dissymmetric molecules were not required for good selectivity, as is the case with conventional chemical catalyst reductions. [reaction: see text].

2-Chloro-2,2-difluoroacetophenone: A Non-ODS-Based Difluorocarbene Precursor and Its Use in the Difluoromethylation of Phenol Derivatives

A novel and non-ODS-based (ODS = ozone-depleting substance) preparation of 2-chloro-2,2-difluoroacetophenone (1) was achieved in high yield by using 2,2,2-trifluoroacetophenone as the starting material. Compound 1 was found to act as a good difluorocarbene reagent, which readily reacts with a variety of structurally diverse phenol derivatives 4 in the presence of potassium hydroxide or potassium carbonate to produce aryl difluoromethyl ethers 5 in good yields. This new and easy-to-handle synthetic methodology offers an environmentally friendly alternative to other Freon- or Halon-based difluoromethylating approaches.

Facile One-Pot Synthesis of N-Difluoromethyl-2-pyridone Derivatives

[reaction: see text] A novel one-pot synthesis of N-difluoromethyl-2-pyridones is described. N-(Pyridin-2-yl)acetamide derivatives were excellent precursors for the preparation of N-difluoromethyl-2-pyridone derivatives. Difluoromethylation of 2-acetaminopyridine derivatives was achieved with sodium chlorodifluoroacetate as a difluorocarbene source in the presence of a catalytic amount of 18-crown-6. Subsequent in situ hydrolysis of resultant 1,2-dihydro-2-acetimino-1-difluoromethylpyridines proceeded under mild acidic conditions to afford the corresponding N-difluoromethyl-2-pyridones in moderate to good yields.

Stereoselective formation of carbon-carbon bonds via SN2-displacement: synthesis of substituted cycloalkyl[b]indoles

A general asymmetric synthesis of substituted cycloalkyl[b]indoles has been accomplished. The key features of this approach are (1) the utilization of a Japp-Klingemann condensation/Fischer cyclization to prepare cycloalkyl[b]indolones, (2) the asymmetric borane reduction of these heterocyclic ketones with (S)-OAB to obtain enantiomerically pure alcohols, and (3) the stereoselective S(N)2-displacement of these indole alcohol substrates with a carbon nucleophile under Mitsunobu conditions to set the C1 or C3 tertiary carbon stereocenter. The use of trimethylphosphine (PMe3) and bis(2,2,2-trichloroethyl) azodicarboxylate (TCEAD) was found to have an effect on the Mitsunobu dehydrative alkylation.

Asymmetric Synthesis of Functionalized Diarylmethanols Catalyzed by a New γ-Amino Thiol

[reaction: see text] A mild asymmetric arylation of aromatic aldehydes catalyzed by gamma-amino thiol 5 gave the corresponding diarylmethanols with 95 to >99.5% ee.