Modular Synthesis of Ar‐BINMOL‐Phos for Catalytic Asymmetric Alkynylation of Aromatic Aldehydes with Unexpected Reversal of Enantioselectivity

An interesting group of multifunctional phosphines (Ar‐BINMOL‐Phos; Ar‐BINMOL=1,1′‐binaphthalene‐2‐α‐arylmethanol‐2′‐ol) with multi‐stereogenic centers of axial and sp³‐central chirality has been prepared successfully from a single chiral source through a concise synthetic route, in which the neighbouring lithium‐promoted [1,2]‐Wittig rearrangement proceeding with excellent diastereoselectivity and enantioselectivity is the key process in this approach. Also, in the catalytic alkynylation of aromatic aldehydes with terminal alkynes, the combination of these Ar‐BINMOL‐Phos ligands with dimethylzinc was found to be an effective catalyst system to afford predominantly the S‐configured propargylic alcohols, whereas the additional use of calcium hydride and n‐butyllithium along with the same Ar‐BINMOL‐Phos ligands gave the R‐configured products in high yields and excellent enantioselectivities (up to >99% ee).

     

The Discovery of Potent Nonstructural Protein 5A (NS5A) Inhibitors with a Unique Resistance Profile—Part 1

Nonstructural protein 5A (NS5A) represents a novel target for the treatment of hepatitis C virus (HCV). Daclatasvir, recently reported by Bristol–Myers–Squibb, is a potent NS5A inhibitor currently under investigation in phase 3 clinical trials. While the performance of daclatasvir has been impressive, the emergence of resistance could prove problematic and as such, improved analogues are being sought. By varying the biphenyl‐imidazole unit of daclatasvir, novel inhibitors of HCV NS5A were identified with an improved resistance profile against mutant strains of the virus while retaining the picomolar potency of daclatasvir. One compound in particular, methyl ((S)‐1‐((S)‐2‐(4‐(4‐(6‐(2‐((S)‐1‐((methoxycarbonyl)‐L ‐valyl)pyrrolidin‐2‐yl)‐1H‐imidazol‐5‐yl)quinoxalin‐2‐yl)phenyl)‐1H‐imidazol‐2‐yl)pyrrolidin‐1‐yl)‐3‐methyl‐1‐oxobutan‐2‐yl)carbamate ( 17 ), exhibited very promising activity and showed good absorption and a long predicted human pharmacokinetic half‐life. This compound represents a promising lead that warrants further evaluation.     

Reversal of Stereoselectivity in Lithiation of Ferrocenyl‐imidazolones: Epimeric Substrates lead to Planar Chiral Enantiomers

Diastereoselective induction of planar chirality in ferrocenes often employs chiral sulfur‐, carbon‐ or phosphorus‐based directing groups. The origin of stereoselectivity in these reactions may be classified as (a) cyclopentadiene (Cp) ring‐controlled or (b) base‐controlled. These two categories are represented by auxiliaries that typically have stereogenic centers α or γ to the ferrocene core, respectively. In this study, it is shown that (−)‐2‐ferrocenyl‐1S‐triethylsilyloxy‐7aS‐hexahydropyrrolo[1,2‐c]imidazol‐3‐one, the anti‐epimer of the previously reported syn‐(1R,7aS) substrate, induces lithiation of the pro‐R_p rather than the pro‐S_p Cp hydrogen in >95:5 dr, leading to enantiomers of the syn‐derived planar chiral imidazolones upon electrophile quench and elimination. This outcome provides a practical way to prepare planar chiral enantiomers in this series without having to synthesize a more expensive D ‐proline‐derived starting material, since both the syn and anti starting materials are available from a common L ‐proline‐derived intermediate. The origin of stereoselectivity in lithiation of the syn and anti epimers, which have β,γ‐stereogenic centers, appears to be driven primarily by the conformational bias exerted by the β‐silyloxy moiety in each chiral auxiliary, which positions the urea carbonyl within the proximity of one of the two prochiral ortho Cp hydrogens. As such, stereoselectivity is likely Cp ring‐controlled for both compounds despite their lack of α‐ferrocenyl stereogenic centers. This conclusion is supported by the insensitivity of lithiation selectivity to the bulkiness of the base, comparisons of enantiomers, deuteration experiments, nOe difference studies, and computational modeling of the ground states and lithiation transition states for both substrates.     

Synthesis and Pharmacological Evaluation of Enantiomerically Pure GluN2B Selective NMDA Receptor Antagonists

To determine the eutomers of potent GluN2B‐selective N‐methyl‐d ‐aspartate (NMDA) receptor antagonists with a 3‐benzazepine scaffold, 7‐benzyloxy‐3‐(4‐phenylbutyl)‐2,3,4,5‐tetrahydro‐1H‐3‐benzazepin‐1‐ols (S)‐ 2 and (R)‐ 2 were separated by chiral HPLC. Hydrogenolysis and subsequent methylation of the enantiomerically pure benzyl ethers of (S)‐ 2 and (R)‐ 2 provided the enantiomeric phenols (S)‐ 3 and (R)‐ 3 [3‐(4‐phenylbutyl)‐2,3,4,5‐tetrahydro‐1H‐3‐benzazepine‐1,7‐diol] and methyl ethers (S)‐ 4 and (R)‐ 4 . All enantiomers were obtained with high enantiomeric purity (≥99.7 % ee). The absolute configurations were determined by CD spectroscopy. R‐configured enantiomers turned out to be the eutomers in receptor binding studies and two‐electrode voltage clamp experiments. The most promising ligand of this compound series is the R‐configured phenol (R)‐ 3 , displaying high GluN2B affinity (K_i=30 nm ), high inhibition of ion flux (IC₅₀=61 nm ), and high cytoprotective activity (IC₅₀=93 nm ). Whereas the eudismic ratio in the receptor binding assay is 25, the eudismic ratio in the electrophysiological experiment is 3.     

5‐(Piperidin‐4‐yl)‐3‐Hydroxypyrazole: A Novel Scaffold for Probing the Orthosteric γ‐Aminobutyric Acid Type A Receptor Binding Site

A series of bioisosteric N₁‐ and N₂‐substituted 5‐(piperidin‐4‐yl)‐3‐hydroxypyrazole analogues of the partial GABA_AR agonists 4‐PIOL and 4‐PHP have been designed, synthesized, and characterized pharmacologically. The unsubstituted 3‐hydroxypyrazole analogue of 4‐PIOL ( 2 a ; IC₅₀～300 μM ) is a weak antagonist at the α₁β₂γ₂ GABA_AR, whereas substituting the N₁‐ or N₂‐position with alkyl or aryl substituents resulted in antagonists with binding affinities in the high nanomolar to low micromolar range at native rat GABA_ARs. Docking studies using a α₁β₂γ₂ GABA_AR homology model along with the obtained SAR indicate that the N₁‐substituted analogues of 4‐PIOL and 4‐PHP, 2 a – k , and previously reported 3‐substituted 4‐PHP analogues share a common binding mode to the orthosteric binding site in the receptor. Interestingly, the core scaffold of the N₂‐substituted analogues of 4‐PIOL and 4‐PHP, 3 b – k , are suggested to flip 180° thereby adapting to the binding pocket and addressing a cavity situated above the core scaffold.     

Design,Synthesis and Structure–Activity Relationships Studies on the D Ring of the Natural Product Triptolide

Triptolide is a diterpene triepoxide natural product isolated from Tripterygium wilfordii Hook F, a traditional Chinese medicinal herb. Triptolide has previously been shown to possess antitumor, anti‐inflammatory, immunosuppressive, and antifertility activities. Earlier reports suggested that the five‐membered unsaturated lactone ring (D ring) is essential for potent cytotoxicity, however, to the best of our knowledge, systematic structure–activity relationship studies have not yet been reported. Here, four types of D ring‐modified triptolide analogues were designed, synthesized and evaluated against human ovarian (SKOV‐3) and prostate (PC‐3) carcinoma cell lines. The results suggest that the D ring is essential to potency, however it can be modified, for example to C18 hydrogen bond acceptor and/or donor furan ring analogues, without complete loss of cytotoxic activity. Interestingly, evaluation of the key series of C19 analogues showed that this site is exquisitely sensitive to polarity. Together, these results will guide further optimization of this natural product lead compound for the development of potent and potentially clinically useful triptolide analogues.     

Bis‐(1,2,3,4‐tetrahydroisoquinolinium): A Chiral Scaffold for Developing High‐Affinity Ligands for SK Channels

N‐Methyl‐bis‐(1,2,3,4‐tetrahydroisoquinolinium) analogues derived from AG525 (1,1′‐(propane‐1,3‐diyl)‐bis‐(6,7‐dimethoxy‐2‐methyl‐1,2,3,4‐tetrahydroisoquinoline)) stereoisomers and tetrandrine, a rigid bis‐(1,2,3,4‐tetrahydroisoquinoline) analogue with an S,S configuration, were synthesized and tested for their affinity for small‐conductance calcium‐activated potassium channel (SK/K_Ca2) subtypes using radioligand binding assays. A significant increase in affinity was observed for the quaternized analogues over the parent 1,2,3,4‐tetrahydroisoquinoline compounds. Interestingly, the impact of stereochemistry was not the same in the two groups of compounds. For quaternized analogues, affinities of S,S and R,R isomers for SK2 and SK3 channels were similar and in both cases higher than that of the meso derivative. Among the bis‐tetrahydroisoquinoline compounds, the S,S isomers exhibited high affinity, while the R,R and meso isomers had similarly lower affinities. Furthermore, the SK2/SK3 selectivity ratio was slightly increased for quaternized analogues. Bis‐(1,2,3,4‐tetrahydroisoquinolinium) represents a new scaffold for the development of high‐affinity ligands for SK channel subtypes.     

Bivalent Argininamide‐Type Neuropeptide Y Y1 Antagonists Do Not Support the Hypothesis of Receptor Dimerisation

Bivalent ligands are potential tools to investigate the dimerisation of G‐protein‐coupled receptors. Based on the (R)‐argininamide BIBP 3226, a potent and selective neuropeptide Y Y₁ receptor (Y₁R) antagonist, we prepared a series of bivalent Y₁R ligands with a wide range of linker lengths (8–36 atoms). Exploiting the high eudismic ratio (>1000) of the parent compound, we synthesised sets of R,R‐, R,S‐ and S,S‐configured bivalent ligands to gain insight into the “bridging” of two Y₁Rs by simultaneous interaction with both binding sites of a putative receptor dimer. Except for the S,S isomers, the bivalent ligands are high‐affinity Y₁R antagonists, as determined by Ca²⁺ assays on HEL cells and radioligand competition assays on human Y₁R‐expressing SK‐N‐MC and MCF‐7 cells. Whereas the R,R enantiomers are most potent, no marked differences were observed relative to the corresponding meso forms. The difference between R,R and R,S diastereomers was most pronounced (about sixfold) in the case of the Y₁R antagonist containing a spacer of 20 atoms in length. Among the R,R enantiomers, linker length and structural diversity had little effect on Y₁R affinity. Although the bivalent ligands preferentially bind to the Y₁R, the selectivity toward human Y₂, Y₄, and Y₅ receptors was markedly lower than that of the monovalent argininamides. The results of this study neither support the presence of Y₁R dimers nor the simultaneous occupation of both binding pockets by the twin compounds. However, as the interaction with Y₁R dimers cannot be unequivocally ruled out, the preparation of a bivalent radioligand is suggested to determine the ligand–receptor stoichiometry. Aiming at such radiolabelled pharmacological tools, prototype twin compounds were synthesised, containing an N‐propionylated amino‐functionalised branched linker (K_i≥18 nM ), a tritiated form of which can be easily prepared.     

Binding Mode and Structure–Activity Relationships around Direct Inhibitors of the Nrf2–Keap1 Complex

An X‐ray crystal structure of Kelch‐like ECH‐associated protein (Keap1) co‐crystallised with (1S,2R)‐2‐[(1S)‐1‐[(1,3‐dioxo‐2,3‐dihydro‐1H‐isoindol‐2‐yl)methyl]‐1,2,3,4‐tetrahydroisoquinolin‐2‐carbonyl]cyclohexane‐1‐carboxylic acid (compound (S,R,S)‐ 1 a ) was obtained. This X‐ray crystal structure provides breakthrough experimental evidence for the true binding mode of the hit compound (S,R,S)‐ 1 a , as the ligand orientation was found to differ from that of the initial docking model, which was available at the start of the project. Crystallographic elucidation of this binding mode helped to focus and drive the drug design process more effectively and efficiently.     

Cerebrosides from a Far‐Eastern Glass Sponge Aulosaccus sp.

Nine new cerebrosides 1a–d , 2a , 2b , 3a–c were found in the extract of a Far‐Eastern glass sponge Aulosaccus sp. (class Hexactinellida). These β‐d ‐glucopyranosyl‐(1 → 1)‐ceramides contain sphingoid bases (2S,3S,4R,11Z)‐2‐aminoeicos‐11‐ene‐1,3,4‐triol (in 1a – d ), (2S,3S,4R,13Z)‐2‐aminoeicos‐13‐ene‐1,3,4‐triol (in 2a , b ) and (2S,3S,4R,13S*,14R*)‐2‐amino‐13,14‐methylene‐eicosane‐1,3,4‐triol (in 3a – c ), which are N‐acylated by (2R,15Z)‐2‐hydroxydocos‐15‐enoic (in 1a , 2a , 3a ), (2R,16Z)‐2‐hydroxytricos‐16‐enoic (in 1b , 2b , 3b ), (2R,17Z)‐2‐hydroxytetracos‐17‐enoic (in 1d ) and (2R)‐2‐hydroxydocosanoic (in 1c , 3c ) acids. The monoenoic and cyclopropane‐containing sphingoid bases of compounds 1a–d , 2a , 2b , 3a–c have not been found previously in any sphingolipids. The structures of the cerebrosides were elucidated on the basis of ¹H‐, ¹³C‐NMR spectroscopy, mass spectrometry, optical rotation data and chemical transformations. A simplified method for the assignment of the absolute configuration of 2‐hydroxy fatty acids by GC analysis of their (2R)‐ and (2S)‐oct‐2‐yl esters was proposed.     

New Insight into the Structure–Activity Relationships of the Selective Excitatory Amino Acid Transporter Subtype 1 (EAAT1) Inhibitors UCPH‐101 and UCPH‐102

In the present study, we made further investigations on the structure–activity requirements of the selective excitatory amino acid transporter 1 (EAAT1) inhibitor, 2‐amino‐4‐(4‐methoxyphenyl)‐7‐(naphthalen‐1‐yl)‐5‐oxo‐5,6,7,8‐tetrahydro‐4H‐chromene‐3‐carbonitrile (UCPH‐101), by exploring 15 different substituents (R¹) at the 7‐position in combination with eight different substituents (R²) at the 4‐position. Among the 63 new analogues synthesized, we identified a number of compounds that unexpectedly displayed inhibitory activities at EAAT1 in light of understanding the structure–activity relationship (SAR) of this inhibitor class extracted from previous studies. Moreover, the nature of the R¹ and R² substituents were observed to contribute to the functional properties of the various analogues in additive and non‐additive ways. Finally, separation of the four stereoisomers of analogue 14 g (2‐amino‐4‐([1,1′‐biphenyl]‐4‐yl)‐3‐cyano‐7‐isopropyl‐5‐oxo‐5,6,7,8‐tetrahydro‐4H‐chromene) was carried out, and in agreement with a study of a related scaffold, the R configuration at C4 was found to be mandatory for inhibitory activity, while both the C7 diastereomers were found to be active as EAAT1 inhibitors. A study of the stereochemical stability of the four pure stereoisomers 14 g ‐ A – D showed that epimerization takes places at C7 via a ring‐opening, C?C bond rotation, ring‐closing mechanism.     

MS Binding Assays for the Three Monoamine Transporters Using the Triple Reuptake Inhibitor (1R,3S)‐Indatraline as Native Marker

We herein present label‐free, mass‐spectrometry‐based binding assays (MS Binding Assays) for the human dopamine, norepinephrine, and serotonin transporters (hDAT, hNET, and hSERT). Using this approach both enantiomers of the triple reuptake inhibitor indatraline as well as its cis‐configured diastereomer were investigated toward hDAT, hNET, and hSERT in saturation experiments. The dissociation rate constants for (1R,3S)‐indatraline binding at hDAT, hNET, and hSERT were determined in kinetic studies. These experiments revealed an allosteric effect of clomipramine on the dissociation of (1R,3S)‐indatraline from hSERT. Finally, a comprehensive set of known monoamine transport inhibitors and substrates was studied in competition experiments at hDAT, hNET, and hSERT, using (1R,3S)‐indatraline as nonlabeled marker. The results are in excellent agreement with those reported for radioligand binding assays. Therefore, the established MS Binding Assays are a promising alternative to the latter for the characterization of new monoamine reuptake inhibitors at DAT, NET, and SERT.     

Discovery of α‐Substituted Imidazole‐4‐acetic Acid Analogues as a Novel Class of ρ1 γ‐Aminobutyric Acid Type A Receptor Antagonists with Effect on Retinal Vascular Tone

The ρ‐containing γ‐aminobutyric acid type A receptors (GABA_ARs) play an important role in controlling visual signaling. Therefore, ligands that selectively target these GABA_ARs are of interest. In this study, we demonstrate that the partial GABA_AR agonist imidazole‐4‐acetic acid (IAA) is able to penetrate the blood–brain barrier in vivo; we prepared a series of α‐ and N‐alkylated, as well as bicyclic analogues of IAA to explore the structure–activity relationship of this scaffold focusing on the acetic acid side chain of IAA. The compounds were prepared via IAA from l ‐histidine by an efficient minimal‐step synthesis, and their pharmacological properties were characterized at native rat GABA_ARs in a [³H]muscimol binding assay and at recombinant human α₁β₂γ_2S and ρ₁ GABA_ARs using the FLIPR? membrane potential assay. The (+)‐α‐methyl‐ and α‐cyclopropyl‐substituted IAA analogues ((+)‐ 6 a and 6 c , respectively) were identified as fairly potent antagonists of the ρ₁ GABA_AR that also displayed significant selectivity for this receptor over the α₁β₂γ_2S GABA_AR. Both 6 a and 6 c were shown to inhibit GABA‐induced relaxation of retinal arterioles from porcine eyes.     

Structural Exploration of (3S,6S)‐6‐Benzhydryl‐N‐benzyltetrahydro‐2H‐pyran‐3‐amine Analogues: Identification of Potent Triple Monoamine Reuptake Inhibitors as Potential Antidepressants

To further explore the basic structural motifs (3S,6S)‐6‐benzhydryl‐N‐benzyltetrahydro‐2H‐pyran‐3‐amine and (2S,4R,5R)‐2‐benzhydryl‐5‐(benzylamino)tetrahydro‐2H‐pyran‐4‐ol, developed by our research group, for monoamine transport inhibition, we designed and synthesized various structurally altered analogues. The new compounds were tested for their affinities for the dopamine transporter (DAT), the serotonin transporter (SERT), and the norepinephrine transporter (NET) in rat brain by measuring their capacity to inhibit the uptake of [³H]DA, [³H]5‐HT, and [³H]NE, respectively. Our results point to novel compounds with a TUI, DNRI, SNRI, or SSRI profile. Among the TUIs, compound 2 g exhibited a balanced potency for all three monoamine transporters (K_i: 60, 79, and 70.3 nM for DAT, SERT, and NET, respectively). In the rat forced swim test, compound 2 g produced a significant decrease in immobility in drug‐treated rats relative to vehicle, indicating a potential antidepressant property.     

Synthesis of 6‐amino acid substituted 4,6,7,12‐tetrahydro‐4‐oxoindolo[2,3‐a]quinolizines

In the presence of Na₂CO₃ (1S,3S)‐ and (1R,3S)‐1‐(2,2‐dimethoxyethyl)‐2‐(1,3‐dioxobutyl)‐3‐(1,3‐dioxo‐butyl)oxymethyl‐1,2,3,4‐tetrahydrocarboline ( 1 ) were transformed into (1S,3S)‐ and (1R,3S)‐1‐(2,2‐dimethoxyethyl)‐2‐(1,3‐dioxobutyl)‐3‐hydroxymethyl‐1,2,3,4‐tetrahydrocarboline ( 2 ), which were cyclized to (6S)‐3‐acetyl‐6‐hydroxymethyl‐4,6,7,12‐tetrahydro‐4‐oxoindolo[2,3‐a]quinolizine ( 4 ), via(6S,12bS)‐ and (6S,12bR)‐3‐acetyl‐2‐hydroxyl‐6‐hydroxymethyl‐1,2,3,4,6,7,12,12b‐octahydro‐4‐oxoindolo[2,3‐a]quinoline ( 3 ). (6S)‐ 4 was coupled with Boc‐Gly, Boc‐L‐Asp(β‐benzyl ester), or Boc‐L‐Gln to give 6‐amino acid substituted (6S)‐3‐acetyl‐4,6,7,12‐tetrahydro‐4‐oxoindolo[2,3‐a]quinolizines 5a , 5b , or 5c , respectively. After the removal of Boc from (6S)‐ 5a (6S)‐3‐acetyl‐6‐glycyl‐4,6,7,12‐tetrahydro‐4‐oxoindolo[2,3‐a]quinolizine ( 6 ) was obtained. The anticancer activities of (6S)‐ 5 and (6S)‐ 6 in vitro were tested.     

Characterization of the Rv3377c Gene Product,a Type‐B Diterpene Cyclase,from the Mycobacterium tuberculosis H37 Genome

The Rv3377c gene from the Mycobacterium tuberculosis H37 genome is specifically limited to those Mycobacterium species that cause tuberculosis. We have demonstrated that the gene product of Rv3377c is a diterpene cyclase that catalyzes the formation of tuberculosinol from geranylgeranyl diphosphate (GGPP). However, the characteristics of this enzyme had not previously been studied in detail with homogeneously purified enzyme. The purified enzyme catalyzed the synthesis of tuberculosinyl diphosphate from GGPP, but it did not bring about the synthesis of tuberculosinol. Optimal conditions for the highest activity were found to be as follows: pH 7.5, 30 °C, Mg^II (0.1 mM ), and Triton X‐100 (0.1 %). Under these conditions, the kinetic values of K_M and k_cat were determined to be 11.7±1.9 μM for GGPP and 12.7±0.7 min^?1, respectively, whereas the specific activity was 186 nmol min^?1 mg^?1. The enzyme activity was inhibited at substrate concentrations higher than 50 μM . The catalytic activity was strongly inhibited by 15‐aza‐dihydrogeranylgeraniol and 5‐isopropyl‐N,N,N,2‐tetramethyl‐4‐(piperidine‐1‐carbonyloxy)benzenaminium chloride (Amo‐1618). The DXDTT_293–297 motif, corresponding to the DXDDTA motif conserved among terpene cyclases, was mutated in order to investigate its function. The middle D295 was found to be the most crucial entity for the catalysis. D293 and two threonine residues function synergistically to enhance the acidity of D295, possibly through hydrogen‐bonding networks. The Rv3377c enzyme could also react with (14R/S)‐14,15‐oxidoGGPP to generate 3α‐ and 3β‐hydroxytuberculosinyl diphosphate. Conformational analyses were carried out with deuterium‐labeled GGPP and oxidoGGPP. We found that GGPP and (14R)‐oxidoGGPP adopted a chair/chair conformation, but (14S)‐oxidoGGPP adopted a boat/chair conformation. Interestingly, the conformations of oxidoGGPP for the A‐ring formation are the opposite of those of oxidosqualene when it is used as a substrate by squalene cyclases for the biosynthesis of hopene and tetrahymanol. (3R)‐Oxidosqualene is folded in a boat conformation, whereas (3S)‐2,3‐oxidosqualene folds into a chair conformation, for the formation of the A‐rings of the hopene and tetrahymanol skeletons, respectively.     

Synthesis and Highly Stereoselective Hydrogenation of the Statin Precursor Ethyl (5S)‐5,6‐Isopropylidenedioxy‐3‐oxohexanoate

A search for the large‐scale preparation of (5S)‐5,6‐(isopropylidenedioxy)‐3‐oxohexanoates ( 2 ) – a key intermediate in the synthesis of pharmacologially important statins – starting from (S)‐malic acid is described. The synthesis of the required initial compound methyl (3S)‐3,4‐(isopropylidenedioxy)butanoate ( 1 ) by Moriwake’s reduction of dimethyl (S)‐malate ( 3 ) has been improved. Direct 2‐C chain elongation of ester 1 using the lithium enolate of tert‐butyl acetate has been shown to be successful at a 3‐ to 5‐fold excess of the enolate. Unfortunately, the product, tert‐butyl (5S)‐5,6‐(isopropylidenedioxy)‐3‐oxohexanoate ( 2a ) is unstable during distillation. Ethyl (5S)‐5,6‐(isopropylidenedioxy)‐3‐oxohexanoate ( 2b ) was prepared alternatively on a multigram scale from (3S)‐3,4‐(isopropylidenedioxy)butanoic acid ( 7 ) by activation with N,N′‐carbonyldiimidazole and subsequent reaction with Mg(OOCCH₂COOEt)₂. A convenient pathway for the in situ preparation of the latter is also described. Ethyl ester ( 2b ) can be advantageously purified by distillation. The stereochemistry of the catalytic hydrogenation of β‐keto ester ( 2b ) to ethyl (5S)‐5,6‐(isopropylidenedioxy)‐3‐hydrohyhexanoate (syn‐ 6 and anti‐ 6 ) has been studied using a number of homogeneous achiral and chiral Rh(I) and Ru(II) complexes with phosphine ligands. A comparison of Rh(I) and Ru(II) catalysts with (S)‐ and (R)‐BINAP as chiral ligands revealed opposite activity in dependence on the polarity of the solvent. No influence of the chiral backbone of substrate 2b on the enantioselectivity was noted. A ratio of syn‐ 6 /anti‐ 6 =2.3 was observed with an achiral (Ph₃P)₃RuCl₂ catalyst. Ru[(R)‐Tol‐BINAP]Cl₂ neutralized with one equivalent of AcONa afforded the most efficient catalytic system for the production of optically pure syn‐(5S)‐5,6‐isopropylidenedioxy‐3‐hydroxyhexanoate (syn‐ 6 ) at a preparative substrate/catalyst ratio of 1000:1.     

E‐64c‐Hydrazide: A Lead Structure for the Development of Irreversible Cathepsin C Inhibitors

Cathepsin C is a papain‐like cysteine protease with dipeptidyl aminopeptidase activity that is thought to activate various granule‐associated serine proteases. Its exopeptidase activity is structurally explained by the so‐called exclusion domain, which blocks the active‐site cleft beyond the S2 site and, with its Asp 1 residue, provides an anchoring point for the N terminus of peptide and protein substrates. Here, the hydrazide of (2S,3S)‐trans‐epoxysuccinyl‐L ‐leucylamido‐3‐methylbutane (E‐64c) (k₂/K_i=140±5 M ^?1 s^?1) is demonstrated to be a lead structure for the development of irreversible cathepsin C inhibitors. The distal amino group of the hydrazide moiety addresses the acidic Asp 1 residue at the entrance of the S2 pocket by hydrogen bonding while also occupying the flat hydrophobic S1′–S2′ area with its leucine‐isoamylamide moiety. Furthermore, structure–activity relationship studies revealed that functionalization of this distal amino group with alkyl residues can be used to occupy the conserved hydrophobic S2 pocket. In particular, the n‐butyl derivative was identified as the most potent inhibitor of the series (k₂/K_i=56 000±1700 M ^?1 s^?1).     

Substrate‐Determined Diastereoselectivity in an Enzymatic Carboligation

Thiamine diphosphate‐dependent enzymes catalyze the formation of C?C bonds, thereby generating chiral secondary or tertiary alcohols. By the use of vibrational circular dichroism (VCD) spectroscopy we studied the stereoselectivity of carboligations catalyzed by YerE, a carbohydrate‐modifying enzyme from Yersinia pseudotuberculosis. Conversion of the non‐physiological substrate (R)‐3‐methylcyclohexanone led to an R,R‐configured tertiary alcohol (diastereomeric ratio (dr) >99:1), whereas the corresponding reaction with the S enantiomer gave the S,S‐configured product (dr>99:1). This suggests that YerE‐catalyzed carboligations can undergo either an R‐ or an S‐specific pathway. We show that, in this case, the high stereoselectivity of the YerE‐catalyzed reaction depends on the substrate's preference to acquire a low‐energy conformation.     

Zinc‐Catalyzed Stereo‐ and Regioselective 1,4‐Hydrative Fluorination of 3‐En‐1‐ynamides with Selectfluor

Zinc‐catalyzed 1,4‐oxofluorinations of 3‐en‐1‐ynamides with Selectfluor in acetonitrile/water proceeded with high regio‐ and stereoselectivity, giving E‐configured γ‐fluoro‐α,β‐unsaturated amides efficiently. Our control experiments indicate that kinetically unstable C‐bound zinc dienolates are chemically reactive to undergo S_E2′‐electrophilic fluorinations whereas the detectable O‐bound dienolates preferably undergo protodemetalation reactions instead.