Absolute configuration determination of donor-acceptor [2.2]paracyclophanes by comparison of theoretical and experimental vibrational circular dichroism spectra

Vibrational circular dichroism (VCD) spectra were obtained for the assignments of the absolute configurations of diastereomeric 4,7-bis(methoxycarbonyl)-12,15-dimethoxy-[2.2]paracyclophanes (1 and 2), and density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d) level for all possible conformations of both 1 and 2 to give the theoretical VCD spectra. Comparisons of the experimental and theoretical VCD spectra obtained unambiguously established the absolute configurations of the dextrorotatory (+)-enantiomers as (4S(p);12S(p))-1 and (4S(p);12R(p))-2, respectively.     

Binding and photochemistry of enantiomeric 2-(3-benzoylphenyl)propionic acid (ketoprofen) in the human serum albumin environment.

Global analysis of circular dichroism multiwavelength data and time resolved fluorescence was applied to investigate the interaction of R(-)- and S(+)-ketoprofen (KP) with human serum albumin (HSA) in buffer solution at neutral pH. The most stable drug:protein adducts of 1 : 1 and 2 : 1 stoichiometry were characterized as regards the stability constants and the absolute circular dichroism spectra. The spectra of the diastereomeric 1 : 1 conjugates are negative with minima at ca. 350 nm for R(-)-KP and 330 nm for S(+)-KP, those of the 2 : 1 complexes are both negative with minimum at 340 nm and quite similar in shape to each other, thereby showing that the protein loses chiral recognition capability upon multiple binding. HSA intrinsic time resolved fluorescence data obtained exciting at 295 nm point to Trp 214 being located in the secondary binding site for both KP enantiomers. The photodegradation of the S(+)- and R(-)-KP:HSA complexes was studied by steady state photolysis using lambda(irr) > 320 nm. No decrease of the photodegradation quantum yields was observed in 1 : 1 complexes. An induction time for the photodegradation course in 2 : 1 complexes was observed. Transient absorption spectroscopy at lambda(exc) = 355 nm showed that triplet KP species were formed with stereo-differentiated lifetimes and high quantum yields (0.7-0.9). Secondary transients were consistent with the occurrence of photodecarboxylation and/or photoreduction within the protein matrix.     

Stereoselective synthesis of 2,3,7-trimethylcyclooctanone and related compounds and determination of their relative and absolute configurations by the MalphaNP acid method

The copper/chiral phosphoramidite (L(1))-catalyzed conjugate addition of dimethylzinc to cycloocta-2,7-dienone 4, followed by the methylation of the intermediate enolate, yielded a single isomer of 7,8-dimethylcyclooct-2-enone (+)-5. Compound (+)-5 was subjected to the second conjugate addition with ent-L(1) giving only one stereoisomer of 2,3,7-trimethylcyclooctanone (+)-6, which was converted to 2,3,7-trimethylcyclooctanol 7. To determine the relative and absolute configurations of these compounds, the (1)H NMR anisotropy method using (S)-(+)-2-methoxy-2-(1-naphthyl)propionic acid {(S)-(+)-MalphaNP acid} 1 was applied. Racemic alcohol (+/-)-7 was esterified with (S)-(+)-MalphaNP acid 1 yielding diastereomeric esters, which were efficiently separated by HPLC on silica gel affording the first-eluted MalphaNP ester (-)-10a and the second-eluted one (-)-10b. The relative and absolute configurations of ester (-)-10a were determined to be (S;1R,2S,3R,7S) by analyzing the (1)H and (13)C NMR spectra of (-)-10a and (-)-10b, especially their HSQC-TOCSY and NOESY spectra, and by applying the MalphaNP anisotropy method. The alcohol 7 formed from (+)-6 was similarly esterified with (S)-(+)-MalphaNP acid 1 yielding an MalphaNP ester, which was identical with (-)-10a, and the relative and absolute configurations of 2,3,7-trimethylcyclooctanone (+)-6 were determined to be (2S,3R,7S).     

Pairwise interaction enthalpies of enantiomers of β-amino alcohols in DMSO + H2O mixtures at 298.15 K

The dilution enthalpies of enantiomers of six β-amino alcohols, namely (R)-(-)-2-amino-1-propanol versus (S)-(+)-2-amino-1-propanol, (R)-(-)-2-amino-1-butanol versus (S)-(+)-2-amino-1-butanol, and (R)-(-)-2-amino-1-pentanol versus (S)-(+)-2-amino-1-pentanol in dimethylsulfoxide (DMSO) + H(2)O mixtures (mass fractions of DMSO w = 0 to 0.3) have been determined respectively using an isothermal titration calorimeter (MicroCal ITC200, Northampton, MA, USA) at 298.15 K. According to the McMillan-Mayer theory, the corresponding homochiral enthalpic pairwise interaction coefficients (h(XX)) of the six amino alcohols have been calculated. It is found that across the whole studied composition range of mixed solvent, values of h(XX) for S-enantiomer are almost universally higher than those of R-enantiomer for each amino alcohol and that the variations of h(XX) depend largely on the composition of mixed solvent. The results were interpreted from the point of view of solute-solute interaction mediated by cosolvent DMSO, as well as competition equilibrium between hydrophobic-hydrophobic, hydrophilic-hydrophilic, and hydrophobic-hydrophilic interactions.     

Stereo-selective interaction of enantiomers of diniconazole, a fungicide, with purified P-450/14DM from yeast

R(-) isomer of diniconazole [S-3308L, (E)-1-(2,4-dichlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-l-yl)-1-+ ++penten-3-ol], a newly developed fungicide strongly inhibited lanosterol 14 alpha-demethylation catalyzed by a yeast cytochrome P-450 (P-450/14DM). On the other hand, S(+) isomer of diniconazole was a weaker inhibitor for P-450/14DM. The R(-) isomer combined with both ferric and ferrous P-450/14DM and interfered binding of CO to the cytochrome. The S(+) isomer also interacted with both forms of P-450/14DM but the absorption spectra of the S(+)-diniconazole complexes were different from those of the R(-)-diniconazole complexes. Furthermore, S(+) isomer did not significantly interfere the binding of CO to P-450/14DM. These observations suggest that P-450/14DM discriminates enantiomers of diniconazole and the R(-) isomer is more favorably fit for the active site of the cytochrome.     

Chiral discrimination phenomena in mixed-ligand copper(II) complexes with amino acids and 1,3-dicarbonyl compounds

Circular dichroism (CD) spectra of individual mixed-ligand copper(II) complexes of 1,3-dicarbonyl compounds, (1S)- or (1R)-3-hydroxymethylene camphor, (1S)-3-trifluoroacetyl camphor, or (1R)-2-hydroxymethylene menthone, and α-amino acids, alanine, valine, proline, or their N-alkyl derivatives, were calculated from CD spectra of equilibrium solutions containing the above constituents in methanol or ethylene dichloride. Diastereomeric mixed-ligand complexes incorporating identical dicarbonyl but enantiomeric N-alkyl-α-amino acid ligands exhibit quasi-enantiomeric CD spectra. Unsubstituted amino acids, on the contrary, will make no decisive contributions to the net optical activity spectrum of the mixed-ligand complexes. Formation constants of diastereomeric mixed-ligand complexes have been calculated from data on disproportionation of the latter into corresponding equally paired complexes. Enantioselectivity was demonstrated to amount to up to 700 cal/mol. Possible steric structures of mixed-ligand complexes are discussed. © 1993 Wiley-Liss, Inc.     

Stereochemical studies of chiral resolving agents, M9PP and H9PP acids

The stereoselective Grignard reaction of (1R,2S,5R)-(-)-2-isopropyl-5-methylcyclohexyl pyruvate (menthyl pyruvate) with 9-phenanthrylmagnesium bromide yielded diastereomeric hydroxy-esters, where intramolecular OH em leader O=C hydrogen bond was observed in IR and (1)H NMR spectra. The alkaline hydrolysis of the major product gave (+)-2-hydroxy-2-(9-phenanthryl)propionic acid (H9PP acid (3)), whose absolute configuration was assigned as S based on the chemical correlation with (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl ester of (S)-2-methoxy-2-(9-phenanthryl)propionic acid (M9PP acid (2)); the absolute configuration of 2 had been previously established by X-ray crystallography. The enantioresolution of (+/-)-6-methyl-5-hepten-2-ol, sulcatol, an insect pheromone, was carried out using (S)-(+)-M9PP acid 2.     

Substrate specificity and stereoselectivity of horse liver alcohol dehydrogenase. Kinetic evaluation of binding and activation parameters controlling the catalytic cycles of unbranched, acyclic secondary alcohols and ketones as substrates of the native and active-site-specific Co(II)-substituted enzyme

1. The steady-state parameters kcat and Km and the rate constants of hydride transfer for the substrates isopropanol/acetone; (S)-2-butanol, (R)-2-butanol/2-butanone; (S)-2-pentanol, (R)-2-pentanol/2-pentanone; 3-pentanol/3-pentanone; (S)-2-octanol and (R)-2-octanol have been determined for the native Zn(II)-containing horse-liver alcohol dehydrogenase (LADH) and the specific active-site-substituted Co(II)LADH. 2. A combined evaluation of steady-state kinetic data and rate constants obtained from stopped-flow measurements, allowed the determination of all rate constants of the following ordered bi-bi mechanism: E in equilibrium E.NAD in equilibrium E.NAD.R1R2 CHOH in equilibrium E.NADH.R1R2CO in equilibrium E.NADH in equilibrium E. 3. On the basis of the different substrate specificities of LADH and yeast alcohol dehydrogenase (YADH), a procedure has been developed to evaluate the enantiomeric product composition of ketone reductions. 2-Butanone and 2-pentanone reductions revealed (S)-2-butanol (86%) and (S)-2-pentanol (95%) as the major products. 4. The observed enantioselectivity implies the existence of two productive ternary complexes; E.NADH.(pro-S) 2-butanone and E.NADH.(pro-R) 2-butanone. All rate constants describing the kinetic pathways of the system (S)-2-butanol, (R)-2-butanol/2-butanone have been determined. These data have been used to estimate the expected enantiomer product composition of 2-butanone reductions using apparent kcat/Km values for the two different ternary-complex configurations of 2-butanone. Additionally, these data have been used for computer simulations of the corresponding reaction cycles. Calculated, simulated and experimental data were found to be in good agreement. Thus, the system (S)-2-butanol, (R)-2-butanol/2-butanone is the first example of a LADH-catalyzed reaction for which the stereochemical course could be described in terms of rate constants of the underlying mechanism. 5. The effects of Co(II) substitution on the different steps of the kinetic pathway have been investigated. The free energy of activation is higher for alcohol oxidation and lower for ketone reduction when catalyzed by Co(II)LADH in comparison to Zn(II)LADH. However, the free energies of binding are affected by metal substitution in such a way that the enantioselectivity of ketone reduction is not significantly changed by the substitution of Co(II) for Zn(II). 6. Evaluation of the data shows that substrate specificity and stereoselectivity result from combination of the free energies of binding and activation, with differences in binding energies as the dominating factors. In this regard, the interactions of substrate molecules with the protein moiety are dominant over the interactions with the catalytic metal ion.     

Mosher ester analysis for the determination of absolute configuration of stereogenic (chiral) carbinol carbons

This protocol details the most commonly used nuclear magnetic resonance (NMR)-based method for deducing the configuration of otherwise unknown stereogenic, secondary carbinol (alcohol) centers (R1R2CHOH (or the analogous amines where OH is replaced by NH2)). This 'Mosher ester analysis' relies on the fact that the protons in diastereomeric alpha-methoxy-alpha-trifluoromethylphenylacetic acid (MTPA) esters (i.e., those derived from conjugation of the carbinol under interrogation with MTPA) display different arrays of chemical shifts (deltas) in their 1H NMR spectra. The protocol consists of the following: (i) preparation of each of the diastereomeric S- and R-MTPA esters and (ii) comparative (Delta delta(SR)) analysis of the 1H NMR spectral data of these two esters. By analyzing the sign of the difference in chemical shifts for a number of analogous pairs of protons (the set of Delta delta(SR) values) in the diastereomeric esters (or amides), the absolute configuration of the original carbinol (or amino) stereocenter can be reliably deduced. A typical Mosher ester analysis requires approximately 4-6 h of active effort over a 1- to 2-d period.     

gamma-Glutamyl dipeptides in Petiveria alliacea

Three gamma-glutamyl dipeptides have been isolated from Petiveria alliacea L. roots. These dipeptides include (S(C2)R(C7))-gamma-glutamyl-S-benzylcysteine together with two diastereomeric sulfoxides, namely (S(C2)R(C7)R(S))- and (S(C2)R(C7)R(S))-gamma-glutamyl-S-benzylcysteine S-oxides (gamma-glutamyl-petiveriins A and B, respectively). Their structures and absolute configurations have been determined by NMR, MALDI-HRMS, IR and CD spectroscopy, and confirmed by comparison with authentic compounds obtained by synthesis.     

Facile synthetic route to enantiopure unsymmetric cis-2,5-disubstituted pyrrolidines

The (+/-)-cis-5-arylcarbamoyl-2-ethoxycarbonylpyrrolidines 6a-g were firstly synthesized in 53-64% yields by using meso-diethyl-2,5-dibromoadipate 3 and (S)-(-)-1-phenylethylamine in three steps. The diastereomeric mixture (S;2S,5R)-(-)-7 and (S;2R,5S)-(+)-8 were prepared by the Grignard reaction and separated by a flash column chromatography in 29 and 52% yields. The absolute configurations of (+)-8 was confirmed by X-ray crystallographic analysis and the enantiopure pyrrolidines (2S,5R)-(-)-9/(2R,5S)-(+)-9 and (2S,5R)-(-)-10/(2R,5S)-(+)-10 were obtained in good yields.     

Enantioselectivity in glutathione conjugation of 1,2-epoxy-1,2,3,4-tetrahydronaphthalene by hepatic glutathione S-transferase

Enantiomers of 1,2-epoxy-1,2,3,4-tetrahydronaphthalene (ETN) were conjugated with glutathione (GSH) specifically at their benzylic oxiran carbons, with a marked difference in rate [(1R,2S)-(+)- less than (1S,2R)-(-)-ETNs] as well as in affinity for GSH S-transferase [Km: (1S,2R)-(-)- less than (1R, 2S)-(+)-ETNs], in rat liver cytosol to yield two diastereomeric S-(2-hydroxy-1,2,3,4-tetrahydronaphth-1-yl)glutathiones which were separable by reverse partition hplc. Enzymatic GSH conjugation of racemic ETN occurred preferentially with the (1S,2R)-(-)-component as a result of its retarding effect on the conjugation of the (1R,2S)-(+)-counterpart, one half of which remained in enantiomerically pure form in the incubation medium when the (1S,2R)-(-)-component had been completely conjugated.     

Stereospecific oxidation of secondary alcohols by human alcohol dehydrogenases

The human liver alpha alpha alcohol dehydrogenase exhibits a different substrate specificity and stereospecificity for secondary alcohols than the human beta 1 beta 1, and gamma 1 gamma 1 or horse liver alcohol dehydrogenases. All of the enzymes efficiently oxidize primary alcohols, but alpha alpha oxidizes secondary alcohols far more efficiently than human beta 1 beta 1 and gamma 1 gamma 1 or horse liver alcohol dehydrogenase. Specifically, alpha alpha oxidizes four- and five-carbon secondary alcohols with efficiencies 0.06-2.2 times that of primary homologs and oxidizes these secondary alcohols with efficiencies up to 3 orders of magnitude greater than those of the three other isoenzymes. Whereas the human beta 1 beta 1, gamma 1 gamma 1 and horse isoenzymes show a distinct preference toward (S)-(+)-3-methyl-2-butanol, the alpha alpha isoenzyme prefers (R)-(-)-3-methyl-2-butanol. Computer-simulated graphics demonstrate that the horse subunit accommodates (S)-(+)-3-methyl-2-butanol within the active site much better than the opposite stereoisomer, primarily due to steric hindrance caused by Phe-93. Human alpha may accommodate (R)-(-)-3-methyl-2-butanol better than (S)-(+)-3-methyl-2-butanol because of close contacts between the latter and Thr-48. These observations suggest that substitutions at positions 93 and 48 in the active site of human liver alcohol dehydrogenase isoenzymes may determine their substrate specificity for secondary alcohols.     

Synthesis, absolute configuration and biological activity of both enantiomers of 2-(5,6-dichloro-3-indolyl)propionic acid: new dichloroindole auxins

Racemic 2-(5,6-dichloro-3-indolyl)propionic acid (5,6-Cl2-2-IPA) was synthesized from 5,6-dichloroindole-3-acetic acid (5,6-Cl2-IAA) by successive esterification, methoxycarbonylation, methylation, and double hydrolysis. The racemate was converted to the diastereomeric esters of (S)-(-)-1-phenylethyl alcohol. These were separated by HPLC into two optically active diastereomers and then hydrolyzed with p-TsOH to the optically active enantiomers of 5,6-Cl2-2-IPA. The absolute configurations of both the 5,6-Cl2-2-IPA enantiomers were determined by comparing the 1H-NMR spectra of their diastereomeric (S)-(-)-1-phenylethyl esters with those of the diastereomeric (S)-(-)-1-phenylethyl esters of 2-(3-indolyl)propionic acid (2-IPA) whose absolute configurations are already known. There was no essential difference between (S)-(+)- and (R)-(-)-5,6-Cl2-2-IPA in hypocotyl growth-inhibiting activity toward Chinese cabbage, but their inhibitory activities were stronger than that of the potent mother auxin, 5,6-Cl2-IAA. No essential difference in the coleoptile elongating activity of Avena sativa was apparent for the enantiomers, this activity being about one-third that of 5,6-Cl2-IAA.     

Enantioconvergent synthesis of (-)-(S)- and (+)-(R)-2-acetyl-3,6-dihydroxycyclohex-2-enone starting from rac-6-hydroxy-3-methoxycyclohex-2-enone

The synthesis of enantiomerically pure (-)-(S)- and (+)-(R)-2-acyl-3,6-dihydroxycyclohex-2-enone starting from diastereomerically pure N-tosyl-(S)-proline esters 3-methoxy-6-hydroxycyclohex-2-enone 1 is presented. An enantioconvergent synthesis of either (-)-(S)- and (+)-(R)-2-acyl-3,6-dihydroxycyclohex-2-enone starting with the racemic alpha-ketol 1 through a conversion of ( approximately 1:1) mixture of diastereomeric esters into one diastereomer by a repeated crystallization, followed by dimethylaminopyridine-catalyzed equilibration as key steps is described.     

Chiral recognition based on enantioselective interactions of propranolol enantiomers with cyclosophoraoses isolated from Rhizobium meliloti

Cyclosophoraoses isolated from Rhizobium meliloti, as an NMR chiral shift agent, were used to discriminate propranolol enantiomers. Continuous variation plot made from the complex of cyclosophoraoses with propranolol showed that the diastereomeric complex had predominantly 1:1 stoichiometry through UV spectroscopic analysis. The chiral recognition of propranolol enantiomers by cyclosophoraoses was investigated through the determination of binding constant based on the (13)C NMR chemical shift changes. The averaged K(obs) values from the plots were 55.7 M(-1) for (R)-(+)-propranolol and 36.6 M(-1) for (S)-(-)-propranolol, respectively. Enantioselectivity (alpha = K(R+)/K(S(-)) of 1.52 was then obtained. Computational calculation also revealed that (R)-(+) propranolol was more tightly bound with cyclosophoraose than (S)-(-)-propranolol due to the enhanced van der Waals interaction.     

Complex of Burkholderia cepacia lipase with transition state analogue of 1-phenoxy-2-acetoxybutane: biocatalytic, structural and modelling study.

In a series of four racemic phenoxyalkyl-alkyl carbinols, 1-phenoxy-2-hydroxybutane (1) is enantioselectively acetylated by Burkholderia cepacia (formerly Pseudomonas cepacia) lipase with an E value > or = 200, whereas for the other three racemates E was found to be < or = 4. To explain the high preference of B. cepacia lipase for (R)-(+)-1, a precursor of its transition state analogue with a tetrahedral P-atom, (R(P),S(P))-O-(2R)-(1-phenoxybut-2-yl)methylphosphonic acid chloride was prepared and crystallized in complex with B. cepacia lipase. The X-ray structure of the complex was determined, allowing to compare the conformation of the inhibitor with results of molecular modelling.     

First direct discrimination of chiral phosphorus thionate (P=S) derivatives by multinuclear magnetic resonance spectroscopy in the presence of a chiral dirhodium complex

Enantiomeric ratios of compounds with P=S functionalities can be determined by 1H, 13C, and 31P NMR spectroscopic inspection of their diastereomeric complexes with (R)-Rh2(MTPA)4 (MTPA-H identical with methoxytrifluoromethylphenylacetic acid; Mosher's acid). This is the first facile and rapid spectroscopic method for chiral recognition in this class of compounds. Whereas complexation shifts Deltadelta are moderate or even negligible, significant signal dispersions Delta(nu) can be observed. Some rationalization about the complexation mode is presented. The NMR spectral characteristics of the free P=S compounds 1-9 are described in detail.     

Transfer of chirality by dithiophosphate ligands and chiral discrimination in the stereoselective formation of square-planar Ni(II) complexes

In the formation reaction of Ni(2+) with the chiral racemic ligand, (R)(R)bdtp(-)/(S)(S)bdtp(-), bdtp(-) = [SSPOCH)CH(3))CH(CH(3))O](-), cyclo- O,O'-[1,2-dimethylethylene] dithiophosphato ion, the meso-complex Ni[(R)(R)(lambda)bdtp][(S)(S)(delta)-bdtp] is stereoselectively produced. The meso-complex was compared with the enantiopure crystals of (+)(589)Ni[(R)(R)(lambda)bdtp](2) or (-)(589)Ni[(S)(S)(delta)bdtp](2), as well as racemic crystals, rac-(+/-)Ni[bdtp](2), which were prepared from the solution containing the two enantiomers in a 1:1 ratio. Dissociation constants in solutions indicate different stability of the meso and enantiopure complexes depending on the solvent, whereas a more efficient crystal packing, weak H-bonding, and nonbonding interactions contribute to stabilization of the meso-species over the racemic one. Molecular structures show that the outer five-membered ligand ring adopts the half-chair conformation C(2) with either the lambda or the delta chirality and the methyl groups are in equatorial (e) positions. Enantiopure ligands of (+)(589)Ni[(R)(R)(lambda)bdtp](2) and (-)(589)Ni[(S)(S)(delta)bdtp](2) induce chirality into the symmetric SSNiSS chromophore with slightly helical distortion. Thus, their CD spectra exhibit weak negative or positive Cotton effects at 662 nm. CD spectra in L(+)- and D(-)diethyltartrate of the meso-complex and racemic crystal, rac-(+/-)Ni[bdtp](2), exhibit different weak Cotton effects of opposite sign. Complexes dissociate in methanol; rac-(+/-)Ni[bdtp](2) in methanol undergoes a crystallization-induced second-order asymmetric transformation which finally yields crystals of the meso-Ni[(R)(R)(lambda)bdtp][(S)(S)(delta)bdtp] complex.     

New route to enantiopure MalphaNP acid, a powerful resolution and chiral 1H NMR anisotropy reagent

MalphaNP acid (+/-)-1, 2-methoxy-2-(1-naphthyl)propionic acid, was enantioresolved by the use of phenylalaninol (S)-(-)-4; a diastereomeric mixture of amides formed from acid (+/-)-1 and amine (S)-(-)-4 was easily separated by fractional recrystallization and/or HPLC on silica gel, yielding amides (R;S)-(-)-5a and (S;S)-(+)-5b. Their absolute configurations were determined by X-ray crystallography by reference to the S configuration of the phenylalaninol moiety. Amide (R;S)-(-)-5a was converted to oxazoline (R;S)-(+)-8a, from which enantiopure MalphaNP acid (R)-(-)-1 was recovered. In a similar way, enantiopure MalphaNP acid (S)-(+)-1 was obtained from amide (S;S)-(+)-5b. These reactions provide a new route for the large-scale preparation of enantiopure MalphaNP acid, a powerful chiral reagent for the enantioresolution of alcohols and simultaneous determination of their absolute configurations by (1)H NMR anisotropy.