Acid catalysis in aldol condensation of α-amino silyl ketene acetals. Diastereoselective synthesis of α-amino-β-hydroxyacids.

The aldol-type condensation between α-dibenzylamino trimethylsilyl ketene acetals and various achiral and chiral aldehydes in the presence of a Lewis acid furnishes α-amino-β -hydroxyacids in moderate yields with preferential C₂C₃$\text{threo}$ configuration.     

Development,validation and application of a 96-well enzymatic assay based on LC-ESI-MS/MS quantification for the screening of selective inhibitors against Mycobacterium tuberculosis purine nucleoside phosphorylase

Mycobacterium tuberculosis (Mtb) purine nucleoside phosphorylase (PNP, EC 2.4.2.1) has been identified as a target for the development of specific inhibitors with potential antimycobacterial activity. We hereby described the development and validation of a new 96-well LC-ESI-MS/MS method to assess the inhibition activity of nucleoside analogues towards MtbPNP and the human PNP (HsPNP). Enzyme activity was determined by monitoring the phosphorolysis of inosine (Ino) to hypoxanthine (Hpx). The enzymatic assay (v = 0.5 mL, enzyme<0.2 μg/well, T = 37 °C) was performed with an overall time of about 15 min/plate for sample processing and 2 min/sample for LC-MS analysis. Validation of the quantification method met the criteria of the CDER guidance of FDA. Kinetic parameters were in agreement with those reported in literature (HsPNP K_M = 0.150 ± 0.020 mM vs 0.133 ± 0.015 mM; MtbPNP K_M = 0.060 ± 0.009 mM vs 0.040 ± 0.003 mM for Ino), thus demonstrating the reliability of the newly developed enzymatic assay. Preliminary inhibition assays confirmed the effects reported for Acyclovir (Acv) and Formycin A (FA) against HsPNP and MtbPNP. The validated enzymatic assay was applied to the evaluation of a set of 8-halo-, 8-amino-, 8-O-alkyl-substituted purine ribonucleosides synthesized on purpose as potential inhibitors against MtbPNP. The assayed 8-substituted ribonucleosides did not exert a significant inhibitory effect against the tested enzymes up to 1 mM.     

Batch and Continuous Flow Hydrogenation of Liquid and Gaseous Alkenes Catalyzed by a Silica‐grafted Iridium(III) Hydride

The silica‐grafted hydride complex [IrH(SBA‐15)(PCP)] ( 2a ; PCP = 1,3‐bis((di‐tert‐butylphosphino)methyl)benzene) hydrogenates alkenes under ambient condition without prior activation. Compared to its POCOP analogue (POCOP = 1,3‐bis((di‐tert‐butylphosphino)oxy)benzene), the activity of catalyst 2a is significantly improved with liquid substrates and exceptionally boosted with ethene and propene. Under gas flow conditions, catalyst 2a hydrogenates ethene with a remarkable turnover frequency of 0.95 s^?1. A stability test indicates that the conversion is constant for at least 1 week.     

Highly enantioselective kinetic resolution of primary alcohols of the type Ph-X-CH(CH3)-CH2OH by Pseudomonas cepacia lipase: effect of acyl chain length and solvent

Although lipase from Pseudomonas cepacia (PCL) shows high enantioselectivity towards many secondary alcohols, it usually exhibits only low to moderate enantioselectivity towards primary alcohols. To increase this enantioselectivity, we optimised the reaction conditions for the PCL-catalysed hydrolysis of esters of three chiral primary alcohols: 2-methyl-3-phenyl-1-propanol 1, 2-phenoxy-1-propanol 2 and solketal 3. The enantioselectivity towards 1-acetate increased from E=16 to 38 upon changing the solvent from ethyl ether/phosphate buffer to 30% n-propanol in phosphate buffer and increased again to E ≥190 upon changing the substrate from 1-acetate to 1-heptanoate. The same changes increased the enantioselectivity towards alcohol 2 from E=17 to 70, but did not significantly increase the enantioselectivity towards alcohol 3. The best solvent was similar to the solvent used to crystallise the open form of PCL and likely stabilises the open form of PCL. This stabilisation may increase the enantioselectivity by removing kinetic contributions from a non-enantioselective lid-opening step. We determined the kinetic contribution of the lid-opening step by measuring the interfacial activation of PCL. The activation energy for the PCL-catalysed hydrolysis of ethyl acetate was at least 2.6 kcal/mol lower in the presence of a water–organic solvent interface.     

Carbocyclization reactions of 1-nitrocycloalkenes and cross-conjugated dienamines

Dodecahydro-8a-nitro-$\text{as}$-indacen-4 ( 2H )-one, dodecahy-dro-9a-nitro-4H-benz ¦e¦inden-4-one, dodecahydro-9b-nitro-5H-benz¦e¦inden-5-one and dodecahydro-4a-nitro-9 ( 1H )-phenanthrenone of $\text{cis}$-$\text{anti}$-$\text{cis}$ backbone are synthesized in quantitative yield by cycloaddition reaction of 1-nitro-cyclopent-1-ene and 1-nitro-cyclohex-1-ene with 1-( cyclopenten-1-yl )- and 1-( cyclohexen-1-yl )-1-( piperidin-1-yl )-ethenes respectively. The enamine intermediates are also isolated and characterized.     

Mirror-image packing in enantiomer discrimination molecular basis for the enantioselectivity of B.cepacia lipase toward 2-methyl-3-phenyl-1-propanol

Synthetic chemists often exploit the high enantioselectivity of lipases to prepare pure enantiomers of primary alcohols, but the molecular basis for this enantioselectivity is unknown. The crystal structures of two phosphonate transition-state analogs bound to Burkholderia cepacia lipase reveal this molecular basis for a typical primary alcohol: 2-methyl-3-phenyl-1-propanol. The enantiomeric alcohol moieties adopt surprisingly similar orientations, with only subtle differences that make it difficult to predict how to alter enantioselectivity. These structures, along with a survey of previous structures of enzyme bound enantiomers, reveal that binding of enantiomers does not involve an exchange of two substituent positions as most researchers assumed. Instead, the enantiomers adopt mirror-image packing, where three of the four substituents at the stereocenter lie in similar positions. The fourth substituent, hydrogen, points in opposite directions.     

Immobilized trypsin systems coupled on-line to separation methods: recent developments and analytical applications

The ability to rapidly and efficiently digest and identify an unknown protein is of great utility for proteome studies. Identification of proteins via peptide mapping is generally accomplished through proteolytic digestion with enzymes such as trypsin. Limitations of this approach consist in manual sample manipulation steps and extended reaction times for proteolytic digestion. The use of immobilized trypsin for cleavage of proteins is advantageous in comparison with application of its soluble form. Enzymes can be immobilized on different supports and used in flow systems such as immobilized enzyme reactors (IMERs). This review reports applications of immobilized trypsin reactors in which the IMER has been integrated into separation systems such as reversed-phase liquid chromatography or capillary electrophoresis, prior to MS analysis. Immobilization procedures including supports, mode of integration into separation systems, and methods are described.