DFT Study on the Structure and Racemization Mechanism of 1,1 ＇- Bina phthalene-8,8 ＇-diol

Density functional theory （DFT） was applied to study the ground state geometries and isomerization processes of 1,1＇-binaphthalene-8,8＇-diol. Three isomers, denoted as ISO1, ISO2, and ISO3, were found, distinguished by different orientations of the OH groups, and each OH-orientational isomer has R- and S-enantiomer. The conformational stabilities of these isomers were investigated by tracking the energy change with respect to the ring-to-ring torsion. The inter-conversions between the three OH-orientational S-isomers were found to have quite low barriers owing to the nearly free rotation of OH groups around the O-C single bonds. The S-R enantiomerization of ISO1 and ISO2 can take place through the ring-ring torsion around the C1-C1/ single bond, either in the anti-rotation manner or in the syn-rotation manner. The barriers of the anti routes are lower than those of the corresponding syn routes by 87.95 and 75.04 kJ/mol. For the S-R enantiomerization of ISO3, only the anti route was found. The barriers for the anti route enantiomerizations of ISO1, ISO2, and ISO3 are 119.61, 120.43, and 121.59 kJ/mol, respectively. A parallel reaction mechanism via three anti enantiomerization routes was proposed for the racemization of 1,1＇-binaphthalene-8,8＇-diol.

Synthesis of (S)-N-(2-Ethyl-6-methylphenyl) alanine by Enzyme-catalyzed Resolution and Chemical Racemization

A practical synthesis of （S）-N-（2-ethyl-6-methylphenyl）alanine, a key intermediate for （S）-metolachlor, was completed by means of lipase-catalyzed hydrolytic kinetic resolution and chemical racemization of the remaining ester. The effects of operating temperature and enzyme concentration on the activity and enantioselectivity of enzyme were initially studied, and it was found that the enantioselectivity of CAL-B towards the resolution was not high enough to obtain enantiomerically pure compound（E=12.1）. When diethyl ether（15%, volume fraction） was added in the reaction medium, the lipase gave an excellent enantioselectivity（E=117.8）, which is about 9.7-fold that in pure buffered aqueous solution. For overcoming the limitation of a maximum theoretical yield of 50%, the acid product was separated from the remaining ester by a simple extraction procedure and the remaining ester was racemized with aldehyde and acetic acid under microwave irradiation or conventional heating condition, The results show the microwave irradiation was more effective than the conventional heating method and gave the desired （R,S）-N-（2- ethyl-6-methylphenyl）alanine methyl ester a high yield（92%） with R/S=50/50 in 1 h.

Two-water-assisted racemization of the succinimide intermediate formed in proteins. A computational model study

Racemization of aspartic acid (Asp) residues in proteins plays an important role in the molecular biology of aging. In the widely accepted mechanism of the Asp racemization, a succinimide (SI) intermediate is the species which actually undergo the direct racemization. In the present study, a two-water-assisted mechanism of the SI racemization was computationally investigated using a model compound in which an aminosuccinyl (Asu) residue is capped with acetyl and NMe groups on the N-and C-termini, respectively. The two water molecules catalyze the enolization of the Hα-Cα-C=O portion in the Asu residue by mediating proton relay from the α-carbon atom to the carboxyl oxygen atom. After the enolization, migration of the water molecules and conformational change lead to the mirror image of the initially formed enol two-water complex, and the racemization is completed by the following ketonization. The overall activation barrier (28.2 kcal·mol-1) corresponds to the enolization and ketonization steps, and falls within the available experimental activation energies (21.4-29.0 kcal·mol-1). Therefore, the two-water-assisted mechanism investigated here is plausible for the in vivo and in vitro racemization reactions of the SI intermediates formed in peptides and proteins.

Stereochemical editing:Catalytic racemization of secondary alcohols and amines

Chiral alcohols and amines are important structural units widely existing in pharmaceuticals,agrochemicals,and food additives.Dynamic kinetic resolution(DKR)is an efficient strategy to deliver optically active alcohols and amines from their racemates.For the development of DKR method,racemization catalyst plays as a crucial element with the requirement of compatibility with the kinetic resolution(KR)system.In this paper,recent advance in the catalytic racemization of secondary alcohols and amines is summarized based on different types of racemizing intermediates,which are redox racemization via ketone/imine intermediates,racemization via radical intermediates,and racemization via carbocation intermediates.Enzymatic racemization of secondary alcohols and amines is also enclosed.

Role of D-aspartate on biosynthesis,racemization,and potential functions:A mini-review

D-aspartate, a natural and endogenous amino acid, widely exists in animal tissues and can be synthesized through aspartate racemase and transformed by D-aspartate oxidase(DDO). D-aspartate mainly serves as a neurotransmitter and has been demonstrated to exhibit various physiological functions,including nutritional potential, regulation on reproduction and hormone biology, and neuron protection.This article mainly reviews the synthesis, racemization, and physiological functions of D-aspartate with emphasis on the potential in diseases.

Efficient and Practical Syntheses of Enantiomerically Pure (S)-(−)-Norcryptostyline Ⅰ, (S)-(−)-Norcryptostyline Ⅱ, (R)-(+)-Salsolidine and (S)-(−)-Norlaudanosine via a Resolution-Racemization Method

Four racemic tetrahydroisoquinolines(RS)-(±)-1-4 were prepared from homoveratrylamine via amidation,Bischler-Napieralski reaction and the subsequent reduction.The enantiomerically pure tetrahydroisoquinolines(S)-(−)-norcryptostyline Ⅰ[(S)-(−)-1],(S)-(−)-norcryptostyline Ⅱ[(S)-(−)-2],(R)-(+)-salsolidine[(R)-(+)-3]and(S)-(−)-norlaudanosine[(S)-(−)-4]were then obtained in 45%,40%,41%and 38%yields,respectively,via resolution of the racemic compounds(RS)-(±)-1-4 with half equivalent of chiral acids.In addition,the enantiomerically enriched compounds(R)-(+)-1,(R)-(+)-2,(S)-(−)-3 and(R)-(+)-4 from the mother liquors were efficiently racemized via a one-pot redox method in almost quantitative yields.

旋光法研究Co(phen)_3^(3+)同Co(Ⅱ)络合物的电子转移反应

应用Mureinik和Spiro方程到通过外球电子转移的(+)D-Co(phen)_3^(3+)外消旋反应,证明了电子转移反应对Co(Ⅲ)和Co(Ⅱ)络离子的浓度是一级的,V=k_2[Co(Ⅲ)][Co(Ⅱ)].测得在水溶液中其活化能Ea为24.5kJ/mol,活化熵ΔS^+为-151J/mol·K,20℃下的速度常数k_2为3.60 l/mol·S.作者还研究了水溶液中配体为H_2O、py、on、NH_3和bipy的六配位CO(Ⅱ)络离子对(+)D-Co(phen)_3^(3+)外消旋速度的影响,得出Co(Ⅱ)络离子同Co(phen)_3^(3+)的电子转移反应速度的顺序为: Co(H_2O)_6^(2+)<Co(py)_6^(2+)<Co(en)_3^(2+)<Co(NH_3)_6^(2+)<Co(phen)_3^(2+)<Co(bipy)_3^(2+)

1-Ethyl-2-Fluoro Pyridinium Salt: A Highly Efficient Coupling Reagents for Sterically Hindered Peptide Synthesis

1-Ethyl-2-fluoropyridinium tetrafluoroborate (FEP) was shown to be a very efficient coupling reagent for the synthesis of the hindered peptide with fast reaction speed, low racemization and good yields.

Is the required therapeutic effect always achieved by racemic switch of proton-pump inhibitors?

Many of the drugs currently used in medical practice are racemates. The enantiomers of a racemic drug differ in pharmacodynamics and/or pharmacokinetics, thus in some cases it is preferable to develop pure enantiomers by racemic switch. In a recent study by Pai et al, dexrabeprazole [R(+)-rabeprazole] (10 mg) was found to be more effective than rabeprazole (20 mg) in the treatment of gastroesophageal reflux disease. We read with great interest in this study and discussed whether such racemic switch would be applicable to other proton-pump inhibitors (PPIs). A literature review indicates that stereoselective pharmacokinetics, rather than stereoselective pharmacological activity, is the main cause of differences in clinical efficacy between pure enantiomer and racemic PPI. Racemic switches of PPI provide the therapeutic advantages such as reducing metabolic load on the body, simplifying pharmacokinetics, providing benefit to the non-responders to standard dose of racemate, more homogenous response to treatment and better efficacy with equal safety. Further studies in quantitative structure-activity relationships (QSARs) are needed to address the fact that the preferred enantiomer of PPI is not always in the same absolute configuration, i.e., S-form is for omeprazole, pantoprazole and tenatoprazole whereas R-form is for lansoprazole and rabeprazole.

Molecular Chirality and Chiral Superlattice in Crystal of Tetrakis[(pyrrol-1-yl)methyl]methane

The crystal structure of tetrakis[(pyrrol-1-yl)methyl]methane was determined by X-ray diffraction measurement, and the result shows that it belongs to monoclinic crystal system, space group is P2 1/n, with a=0.9284(3) nm, b=1.0950(6) nm, c=1.8749(8) nm; α=γ= 90.00(4)°, β=103.63(3)°, V=1.8523(14) nm 3, Z=4, ρ calcd. =1.192 kg/m 3, μ=0.072 nm -1 , F(000)=712, R 1=0.0854, wR 2=0.1884. It has been found that the molecules exist in two enantiomeric states. Enantioselective self-assemblies such as one-dimensional molecular stacks in a single handedness, homochiral monolayers and a chiral superlattice are specified in this racemic crystal. In addition, a simple technique is advocated to distinguish chiral states from tetrahedral molecules in the solid state. The present R/S nomenclature of the tetracooradinated carbon centers is used solely for its convenience to distinguish the two enantiomeric states, but not used to determine the absolute configurations.

Activity and Enantioselectivity of Lipase Immobilized in CTAB Microemulsion-based Gels

Introduction The formation of gelatin-containing mieroemulsionbased gels（MBGs） was first described in 1986 and the physical/structural characterization was carried out by a number of groups with a variety of techniques including tracer diffusion, electrical conductivity, NMR, X-ray and small angle neutron scattering. The MBGs were proposed to comprise an extensive, rigid, interconnected network of gelatin/water rods stabilized by a monolayer of surfactant, in coexistence with a po- pulation of conventional W/O microemulsion droplets.

Enantiomeric Resolution on L-Carnitine Selective Polymers Prepared by Molecular Imprinting

L-carnitine selective polymers were prepared by molecular imprinting using methacrylic acid as the functional monomer. The acid function of the monomer is expected to form hydrogen bond and ionic interactions with the amine function of the target molecule L-carnitine. The imprinted polymers were used as stationary phases in high-performance liquid chromatography (HPLC). It was shown that L-carnitine imprinted polymer exhibited a higher affinity to its template molecule, while the non-imprinted polymer had no affinity to the compounds tested. Racemic carnitine hydrochloride was efficiently resolved on the L-carnitine imprinted polymer, and the separation factor is 1.9.

Lipase-catalyzed Kinetic Resolution of Racemic 1-Trimethylsilylethanol in Organic Solvent

The enantioselective esterification of racemic 1-trimethylsilylethanol with acids catalyzed by lipase in organic solvent was successfully performed. The influence of some factors on the reaction was investigated. Among the four lipases explored, Candida rugosa lipase (CRL) showed the highest activity and enantioselectivity. Octanoic acid was the best acyl donor among the eleven acids studied and n-hexane was the most suitable medium for the reaction. The optimum shaking rate and temperature were found to be 150r·min-1 and 20℃ to 30℃, respectively. The enantiomeric excess of the remaining (5)-(-)-1-trimethylsilylethanol was 93% when substrate conversion was 53% upon incubation of the reaction mixture at 30℃, 150r·min-1 for 12 h.

ENZYME REACTION IN AQUEGUB-ORGANIC BYSTEM —KINETIC RESOLUTION OP RACEMIC OXIRANECARBOXYLIC ESTERS WITH LIPASE

The chiral oxiranecarboxylic acids or esters 2a-e were obtained with lipase (CCL) in aqueous-organic system.The ee and the absolute configurations of the products were determined.

A new shortcut synthesis route for (±) raphidecursinol

The new shortcut synthesis route of(±)raphidecursinol 1,a racemic 8,4′-oxyneolignan compound,can be more easily achieved by the synthesis route,starting from readily available inexpensive 3,4,5-trimethoxybenzaldehyde and 1,2,3-trihydroxybenzene.All structures were confirmed by ~1H NMR,IR and MS.

Proposed Extension to the Natta Projection Notation System for Enabling an Indication of Relative Stereochemistry and the Stereochemical State

A system of structure depiction, as an extension of the wedge and hashed wedge bonds (Natta projection), and text notation is herein suggested that embodies more explicit information—or reduced over-statement as circumstances warrant—on the stereochemical nature of the system at hand, in particular, for those cases where only the relative stereochemistry of a compound is known.

A New Organo-templated Layered Zincophosphate Synthesized from a Solvothermal System Containing Racemic 1,2-Diaminopropane

A new layered zincophosphate was synthesized under solvothermal conditions by employing racemic 1,2-diaminopropane as the structure-directing agent. The structure of the compound was solved by means of single-crystal X-ray diffraction analysis. It crystallized in the monoclinic space group P2_1/c (No. 14) with a= 1.047 0(3) nm, b=0.787 31(18) nm, c=0.662 68(16) nm, β=103.120(5)°, V=0.532 0(2) nm 3 and Z=4. The structure is made up of anionic zincophosphate sheets stacked in an AAAA sequence. The individual sheet contains three- and four-membered rings and the infinite Zn-O-Zn chains can also be envisaged in the sheet. The charge-balancing diprotonated racemic 1,2-diaminopropane cations are sandwiched between the layers, whereas the inorganic layers are stabilized by strong H-bonds formed between the N atoms of the amine and the O atoms in the inorganic sheets.

Synthesis of a Chain Aluminophosphate Containing Racemic 1, 2-Diaminopropane

A one-dimensional chain aluminophosphate C1.5H7.5N1.5Al0.5PO4 containing diprotonated racemic 1, 2-diaminopropane cations was synthesized from an alcoholic system. The compound consists of infinite inorganic [AlP2O8] chains of alternately connected AlO4 and PO2(=O)2 tetrahedral units and the racemic template molecules. The macroanionic chains contain corner-shared double four-membered rings.

Enantioseparation of some Racemic Compounds on Cellulose Tris (3,5-dimethylphenylcarbamate) Chiral Stationary Phase

Seven chiral compounds were resolved on cellulose tris (3,5-dimethylphenylcarbamate) chiral stationary phase (CDMPC-CSP) using n-hexane/alcohol as mobile phase. Solvent strength and structural characteristics of the compounds effecting on the retention and resolution were discussed. Satisfactory separation was obtained.

Effect of Crystallization Conditions on Polymorphic Diversity of Malic Acid RS-Racemate

Enantiomers （R （＋） and S（-））, RS-racemate mixture of enantiomers） of malic acid C4H605 have been （double compound） and （R ＋ S） -conglomerate （mechanical investigated by means of X-ray diffractometry and high tern- perature X-ray diffraction method. The RS-racemate was found to be able to form three polymorphic modifications, which we denominated as M1 （monoclinic, space group P21/c）, M2 （monoclinic, space group Cc）, and Tc （triclinic, space group P-l ）, the latter modification having been unknown before. Modification Tc was also described, and its X- ray diffraction characteristics, including interplanar spacings d, hkl indices, unit cell parameters, were defined. In addi- tion, X-ray diffraction characteristics for both reported earlier M1 and M2 monoclinic polymorphic modifications were measured with higher accuracy. The ability of RS-racemate to form one of the above three modifications （MI, M2, and Tc） or their mixtures containing various proportions and combinations of the components （M1 ＋ M2, M1 ＋ Tc, or M2 ＋ Tc） was found to depend on the type of crystallization medium （a melt, aqueous medium, ethanol or acetone solu- tion）, crystallization rate （from 2--3 minutes to 4 months）, and crystallization temperature. Heating S-enantiomer and M1 RS-racemate up to their respective melting points （100 ℃ and 124 ℃, correspondingly） only made them undergo thermal deformations, while heating （R ＋ S） -conglomerate in the temperature range of 96--110 ℃ resulted in its homogenization to form M2 RS-racemate, which, near the melting point （118 ℃）, namely, in the range of 112-116 ℃, was transformed into MI RS-racemate. Keywords： polvmorDhism; racemic, chiral