NHC-catalyzed covalent activation of heteroatoms for enantioselective reactions

Covalent activation of heteroatoms enabled by N-heterocyclic carbene (NHC) organic catalysts for enantioselective reactions is evaluated and summarized in this review. To date, sulfur, oxygen, and nitrogen atoms can be activated in this manner to react with another substrate to construct chiral carbon–heteroatom bonds with high optical enantioselectivities. The activation starts with addition of an NHC catalyst to the carbonyl moiety (aldehyde or imine) of substrates that contain heteroatoms. The key in this approach is the formation of intermediates covalently bound to the NHC catalyst, in which the heteroatom of the substrate is activated as a nucleophilic reactive site.

Covalent activation of heteroatoms enabled by N-heterocyclic carbene (NHC) organic catalysts for enantioselective reactions is evaluated and summarized in this review.     

Enantioselective organocatalytic Michael additions of aldehydes to enones with imidazolidinones: cocatalyst effects and evidence for an enamine intermediate

An enantioselective intermolecular Michael addition of aldehydes to enones catalyzed by imidazolidinones has been achieved. Chemoselectivity (Michael addition vs aldol) can be controlled through judicious choice of hydrogen-bond-donating cocatalysts. The optimal imidazolidinone/hydrogen-bond-donor pair affords Michael addition products in excess of 90% ee. Furthermore, we have isolated and characterized an enamine intermediate and demonstrated its efficacy as a nucleophile in the observed Michael addition reactions.     

Carbene-Catalyzed Intermolecular Dehydrogenative Coupling of Aldehydes with C(sp3)−H Bonds

The development of catalyst-controlled methods for direct functionalization of two distinct C−H bonds represents an appealing approach for C−C formations in synthetic chemistry. Herein, we describe an organocatalytic approach for straightforward acylation of C(sp³)−H bonds employing readily available aldehyde as “acyl source” involving dehydrogenative coupling of aldehydes with ether, amine, or benzylic C(sp³)−H bonds. The developed method affords a broad range of ketones under mild conditions. Mechanistically, simple ortho-cyanoiodobenzene is essential in the oxidative radical N-heterocyclic carbene catalysis to give a ketyl radical and C(sp³) radical through a rarely explored intermolecular hydrogen atom transfer pathway, rendering the acylative C−C formations in high efficiency under a metal- and light-free catalytic conditions. Moreover, the prepared products show promising anti-bacterial activities that shall encourage further investigations on novel agrochemical development.     

Comparative pharmacokinetic profiles of five poorly soluble pulchinenosides in different formulations from Pulsatilla chinensis saponins extracts for enhanced bioavailability

Pulsatilla chinensis, a traditional Chinese medicine (TCM), has been used for treating amoebic diseases, vaginal trichomoniasis and bacterial infections over a long history. Now growing attention has been attracted to its antitumor activities. The purpose of this work was to compare the pharmacokinetic profiles of pulchinenosides in different formulations and to improve their oral bioavailability. Extracts of P. chinensis saponins were prepared for PRS‐Na (salt forming), PRS‐HPβCD (hydroxypropyl‐β‐cyclodextrin inclusion complex), PRS‐O/W (oil‐in‐water emulsion) and PRS‐silica (micronization), respectively. A simpler and more durable LC‐MS/MS method was developed in this study for quantitative analysis of pulsatilla sapoin D, B7, B10, B11 and sapoin PD simultaneously. The four formulations enhanced saponins oral bioavailability to varying degrees, as PRS‐HPβCD > PRS‐silica > PRS‐O/W > PRS‐Na, which indicated that water‐soluble preparations can obviously improve the solubility of saponins, and are helpful to increase bioavailability. In particular, hydroxypropyl‐β‐cyclodextrin inclusion complex was the most effective way to promote absorption of saponins, raising the F values (bioavailability) >20 times. Therefore, P. chinensis saponin molecules can be slowly released by emulsion and micronization, which can avoid the enormous C_max appearing in HPβCD, considering the pharmacokinetics profiles. However, appropriate pharmacokinetic parameters were observed in PRS‐Na, although the F value was minimum among the four preparations. Copyright © 2015 John Wiley & Sons, Ltd.     

青海高原道地药材羌活和铁棒锤微量元素含量比较研究

为研究青海高原道地药材羌活和铁棒锤的特性,采用电热板消解法处理样品,原子吸收分光光度法测定了两种药材中Ca、Mg、Cu、Zn、Fe、Mn 6种金属元素的含量。结果表明,Ca、Mg、Cu、Zn、Fe、Mn 6种微量元素含量在两种道地药材中,含量最高的是Ca,平均值分别为(55.78±12.10)mg/g和(26.48±5.79)mg/g;含量最低的元素为Cu,其值分别为(0.28±0.28)mg/g和(0.09±0.06)mg/g;其它元素含量从高到低依次为Mg,Fe,Zn,Mn。可见不同地区的羌活其微量元素的含量有明显的差异,且羌活微量元素含量也存在种间差异,细叶羌活的Ca、Mg元素含量比宽叶羌活的要低,相反,Fe,Cu,Zn 3种元素含量细叶羌活的比宽叶的要高。     

Enantioselective Separation and Determination of Carnosine in Rat Plasma by Fluorescence LC for Stereoselective Pharmacokinetic Studies

A sensitive fluorescence liquid chromatographic analytical method was developed for the simultaneous determination of carnosine enantiomers in rat plasma. The method was applied to pharmacokinetic studies. Chiral separation of carnosine enantiomers was achieved by pre-column derivatization with o-phthaldialdehyde and the thiol N-acety-l-cysteine as derivating reagents. They were separated on an ODS column and detected by fluorescence detection (λ_ex = 350 nm, λ_em = 450 nm). γ-Aminobutyric acid was used as internal standard. The method was linear up to 6,000 ng mL^?1 for l-carnosine, 4,000 ng mL^?1 for d-carnosine. Low limit of quantitation (LLOQ) was 40 ng mL^?1 for each isomer. The relative standard deviations obtained for intra- and inter-day precision were lower than 12% and the recoveries were higher than 75% for both enantiomers. The method was applied to a stereoselective study on the pharmacokinetics of carnosine after oral administration with a single dose (carnosine, 75 mg kg^?1 for each isomer) to a rat. The initial data indicated that l-carnosine had a larger value of the highest plasma concentration than d-carnosine (C _max 5,344 vs. 1,914 ng mL^?1), and that of l-carnosine had a lower value of AUC_(0?∞) and t _1/2(h) (AUC_(0?∞) 5,306 vs. 6,321 ng h mL^?1, t _1/2 1.43 vs. 3.37 h). Our results indicated that the pharmacokinetic of l-carnosine and d-carnosine revealed enantioselective properties significantly.     

Lithium amidoborane, a highly chemoselective reagent for the reduction of α,β-unsaturated ketones to allylic alcohols

Lithium amidoborane (LiNH(2)BH(3), LiAB for short), is capable of chemoselectively reducing α,β-unsaturated ketones to the corresponding allylic alcohols at ambient temperature. A mechanistic study shows that the reduction is via a double hydrogen transfer process. The protic H(N) and hydridic H(B) in amidoborane add to the O and C sites of the carbonyl group, respectively.     

Addition of indoles to oxyallyl cations for facile access to α-indole carbonyl compounds

A direct coupling of unprotected indoles and α-halo ketones via in situ generated oxyallyl cation intermediates is described. The reactions efficiently afford α-indole carbonyl compounds with good to quantitative yields.