Stereoselective metabolism of 7-methylbenz[a]anthracene: absolute configuration of five dihydrodiol metabolites and the effect of dihydrodiol conformation on circular dichroism spectra

7-Methylbenz[a]anthracene (7-MBA) was metabolized stereoselectively by rat liver microsomes to form five optically active dihydrodiols as the predominant metabolites. The dihydrodiols were purified by a combination of reversed-phase and normal-phase high performance liquid chromatography (HPLC). By comparison of their circular dichroism (CD) spectra with the corresponding benz[a]anthracene (BA) dihydrodiols of known absolute stereochemistry, the major dihydrodiol enantiomers of 7-MBA have been determined to have 1R,2R-, 3R,4R- and 10R , 11R - absolute configurations, respectively. Due to their quasi- diaxial conformations, the absolute configuration of trans-5,6- and trans-8,9-dihydrodiols, the two most abundant metabolites of 7-MBA, could not be determined by simple comparisons of their circular dichroism spectra with those of the quasidi -equatorial BA 5R, 6R - and 8R , 9R -dihydrodiols. The major enantiomers of the quasi- diaxial trans-5,6- and trans-8,9-dihydrodiol metabolites of 7-MBA were determined by comparison to the CD spectrum of 7-bromo-BA 5R, 6R -dihydrodiol and by the exciton chirality method to have R,R absolute stereochemistry. This study also revealed that the circular dichroism Cotton effects of an enantiomeric dihydrodiol of polycyclic aromatic hydrocarbons can be drastically altered if the conformation (quasi- diaxial vs. quasi di-equatorial ) of the dihydrodiol is changed.     

Enantioselective aliphatic hydroxylations of racemic 1-hydroxy-3-methylcholanthrene by rat liver microsomes

Enantiomeric pairs of 1-hydroxy-3-hydroxymethylcholanthrene (1-OH-3-OHMC), 3-methylcholanthrene (3MC) trans- and cis-1,2-diols, and 1-hydroxy-3-methylcholanthrene (1-OH-3MC) were resolved by HPLC using a covalently bonded (R)-N-(3,5-dinitrobenzoyl)phenylglycine chiral stationary phase (Pirkle type 1A) column. The absolute configuration of an enantiomeric 3MC trans-1,2-diol was established by the exciton chirality CD method following conversion to a bis-p-N,N-dimethylaminobenzoate. Incubation of an enantiomeric 1-OH-3MC with rat liver microsomes resulted in the formation of enantiomeric 3MC trans- and cis-1,2-diols; the absolute configurations of the enantiomeric 1-OH-3MC and 3MC cis-1,2-diol were established on the basis of the absolute configuration of an enantiomeric 3MC trans-1,2-diol. Absolute configurations of enantiomeric 1-OH-3-OHMC were determined by comparing their CD spectra with those of enantiomeric 1-OH-3MC. The relative amount of three aliphatic hydroxylation products formed by rat liver microsomal metabolism of racemic 1-OH-3MC was 1-OH-3-OHMC greater than 3MC cis-1,2-diol greater than 3MC trans-1,2-diol. Enzymatic hydroxylation at C2 of racemic 1-OH-3MC was enantioselective toward the 1S-enantiomer over the 1R-enantiomer (approximately 3/1); hydroxylation at the C3-methyl group was enantioselective toward the 1R-enantiomer over the 1S-enantiomer (approximately 58/42). Rat liver microsomal C2-hydroxylation of racemic 1-OH-3MC resulted in a 3MC trans-1,2-diol with a (1S,2S)/(1R,2R) ratio of 63/37 and a 3MC cis-1,2-diol with a (1S,2R)/(1R,2S) ratio of 12/88, respectively.     

Isoflavonoids from Crotalaria albida Inhibit Adipocyte Differentiation and Lipid Accumulation in 3T3-L1 Cells via Suppression of PPAR-γ Pathway

Two 2″-isopropenyl dihydrofuran isoflavonoids (1 and 3), one 2″-isopropenyl dihydrofuran chromone (2), as well as 13 known compounds were isolated from the herbs of Crotalaria albida. Their structures and relative configurations were elucidated via NMR and HRESIMS analyses. The 2″ S absolute configuration of 1 and 2 were deduced by comparing their NOESY spectra with that of 3, which was determined via single crystal X-ray diffraction (CuKα). The 3R absolute configuration of 1 was determined by CD. Compounds 1, 2, and 3 inhibit the adipocyte differentiation and lipid accumulation of 3T3-L1 through down-regulation of PPAR-γ activity.     

X-ray diffraction studies on the absolute configuration of alpha- and beta-anordrins

The molecular structures and absolute configurations of alpha- and beta-anordrins are reported. Pure alpha- and beta-epimers were obtained with recrystallization and column chromatography combined with high-pressure liquid chromatography methods; they were identified by high-resolution infrared and mass spectra and 1H and 13C nuclear magnetic resonance. By single crystal x-ray diffraction analysis, the crystals of alpha- and beta-epimers were found to belong to the orthorhombic space groups P2(1)2(1)2(1) and P2(1)2(1)2, respectively. The molecular structures of these two epimers were determined. The absolute configurations were deduced by conformation analysis, 1H nuclear magnetic resonance, and comparison with the absolute configuration of the starting material. The absolute configurations of asymmetric centers of alpha- and beta-epimers were observed to be 2R, 5S, 8R, 9S, 10S, 13S, 14S, 17R, and 2S, 5S, 8R, 9S, 10S, 13S, 14S, 17R, respectively. These results were confirmed by the x-ray diffraction determination of the absolute configuration of 2 alpha,17 alpha-diethynyl-A-nor-5 alpha- androstane-2 beta, 17 beta-diol dichloroacetate.     

Chiral cruciferous phytoalexins: preparation, absolute configuration, and biological activity

Synthesized by an efficient one-pot spirocyclization method, two chiral cruciferous phytoalexins, 1-methoxyspirobrassinin (2) and 1-methoxyspirobrassinol methyl ether (4a), were prepared through optical resolution using the chiral HPLC method of corresponding racemates. The absolute configuration of natural (+)-2 was elucidated as R by using the direct comparison of ECD and VCD spectra with those of known (S)-(-)-spirobrassinin (1). Another chiral phytoalexin, (-)-4a, had its absolute configuration 2R,3R elucidated through the comparison of observed and calculated VCD. Interestingly, the absolute configurations of natural (S)-(-)-spirobrassinin (1) and (R)-(+)-1-methoxyspirobrassinin (2) were opposite of each other, even though their structures are almost similar, with the exception of an N-methoxy group. A significant difference in the antiproliferative activity between (2R,3R)-(-) and (2S,3S)-(+)-4a was observed.     

Application of (S)‐TBMB carboxylic acid‐based on‐line HPLC‐exciton CD analysis to acyclic vicinal alcohols and amino alcohols

Applications of the on‐line HPLC‐exciton CD analysis using (S)‐2‐tert‐butyl‐2‐methyl‐1,3‐benzodioxole‐4‐carboxylic acid [(S)‐TBMBC‐OH] that can simultaneously determine the enantiomeric compositions and the absolute configuration of cyclohexane‐1,2‐diols and diamines as well as acyclic vicinal diols and amino alcohols were studied. Di‐O‐ or di‐N,O‐(S)‐TBMBC derivatives of acyclic terminal vicinal diols, 2‐hydroxy‐1‐amines, and nonterminal vicinal diols gave symmetrical exciton CD spectra between enantiomers, indicating their absolute configurations. However, Di‐N,O‐(S)‐TBMBC derivatives of 2‐amino‐1‐ols did not always give symmetrical exciton CD spectra between enantiomers, but their 2‐phthalimido‐1‐O‐(S)‐TBMBC derivatives gave symmetrical exciton CD spectra, indicating their absolute configurations. All these (S)‐TBMBC derivatives were separated by normal‐phase HPLC and unequivocally determined by the on‐line HPLC‐exciton CD analysis without recourse to reference samples. Chirality 11:149–159, 1999. © 1999 Wiley‐Liss, Inc.     

Synthesis and stereochemical characterization of optically active 1,2-diarylethane-1,2-diols: useful chiral controllers in the Ti-mediated enantioselective sulfoxidation

The series of phenylsubstituted 1,2-diphenylethane-1,2-diols 2a-h was prepared in high chemical (70--80%) and optical yields (approximately 90%) by Sharpless syn-dihydroxylation of the corresponding (E)-1,2-diarylethenes, in turn obtained by McMurry or Wittig reactions. The enantiomeric excesses of the samples were determined by HPLC analysis using Chiralcel OD chiral stationary phase (CSP). This CSP was able to resolve all the diols, except for 2g, with alpha values ranging between 1.10--1.64. In all cases the (R,R) antipode was eluted first. (R,R) absolute configuration was assigned to the dextrorotatory (CHCl(3)) diols 2a--h by analyzing the CD spectra of their 2,2-dimethyl-1,3-dioxolanes 3a--h. In fact, the CD spectra of all these dioxolanes present a positive couplet (210--180 nm range) which can be nonempirically related to an (R,R) absolute configuration of the two stereocenters.     

CD exciton chirality method for determination of the absolute configuration of beta-hydroxy-alpha-amino acid derivatives

The absolute configuration of beta-hydroxy-alpha-amino acids was studied by CD exciton chirality method using 7-diethylaminocoumarin-3-carboxylate as a red-shifted chromophore. The CD spectra of bischromophoric derivatives of (S)-serine and (2S,3R)-threonine methyl esters (2 and 7) were compared with those of acyclic vic-aminoalcohols and diols (3--6 and 8--9). This study indicates that the polar carboxylate group of beta-hydroxy-alpha-amino acids makes them a unique subclass of vic-aminoalcohols. By combining the data of CD and NMR coupling constants, we are able to correlate their preferred conformer B and positive CD to the corresponding absolute configuration.     

Absolute configuration of eremophilanoids by vibrational circular dichroism

The absolute configurations (AC) of natural occurring 6-hydroxyeuryopsin (1), of its acetyl derivative 2, and of eremophilanolide 8 were confirmed by comparison of the experimental vibrational circular dichroism (VCD) spectra with theoretical curves generated from density functional theory (DFT) calculations. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field. All MMFF94 conformers were further optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by calculations of their vibrational frequencies at the B3LYP/6-31G(d,p); the VCD spectra of 2 and 8 were also calculated at the B3PW91/DGDZVP level of theory. Good agreement between theoretical and experimental VCD curves unambiguously verified the 4S,5R,6S absolute configuration for 1 and 2, and the 1S,4S,5R,6S,8S,10S configuration for 8.     

Absolute stereochemistry of guaianolides, of matricin and its epimers, of yarrow proazulenes, and of chamazulene carboxylic acid

Known determinations of the absolute configuration of guaianolides were collected and found to be few. The absolute configurations of guaianolides rest on the assumption that 7-H always has alpha-orientation. For matricin, only the relative configuration was determined. On the basis of a detailed study of the NMR spectra of matricin and its epimers, and of synthetic, NMR, and CD studies with its decomposition product, chamazulene carboxylic acid, we were able to reconfirm the accepted 3S,3aR,4S,9R,9aS,9bS configuration of matricin.     

Absolute configuration of pentahydroxy bile alcohols excreted by patients with cerebrotendinous xanthomatosis: a circular dichroism study

The absolute configurations of the C27 pentahydroxy bile alcohols present in bile and feces of two patients with cerebrotendinous xanthomatosis (CTX) were determined by circular dichroism (CD) spectroscopy. The CD spectra of 5beta-cholestane-3alpha,7alpha,12alpha,24alpha,25-pentol in the presence of Eu(fod)3 [tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionato) europium (III)] exhibited a negative Cotton effect and was assigned to 24R absolute configuration. Conversely, 5beta-cholestane-3alpha,7alpha,12alpha,24beta,25-pentol showed a strong positive Cotton effect and was assigned the 24S configuration. These assignments were based upon comparison with a model compound, 5-cholestene-3beta,24(R),25-triol, whose single-crystal X-ray structure has been determined. The importance of these data is to establish a structural mechanism for the conversion of 5beta-cholestane-3alpha,7alpha,12alpha,24S,25-pentol rather than 5beta-cholestane-3alpha,7alpha,12alpha,24R,25-pentol into cholic acid in man as well as in animals.     

Absolute configuration of tropane alkaloids bearing two alpha,beta-unsaturated ester functions using electronic CD spectroscopy: application to (R,R)-trans-3-hydroxysenecioyloxy-6-senecioyloxytropane

The absolute configuration of heterocyclic natural products substituted with two mobile alpha,beta-unsaturated esters was studied using electronic circular dichroism (CD) spectroscopy. The conformational flexibility of the side chains imposed the use of density functional theory calculation to determine the set of the most probable conformations in solution. The electronic CD and UV spectra were calculated by Boltzmann-weighted average of the simulated spectra using the results of the excited states calculation of a set of simplified structures. Comparison with the experimental CD spectrum allowed to determine whether the calculations were made with the right enantiomer. The method was applied to the determination of the absolute configuration of (R,R)-trans-3-hydroxysenecioyloxy-6-senecioyloxytropane.     

Determination of the absolute configurations of the anteiso acid moieties of glycoglycerolipid S365A isolated from Corynebacterium aquaticum

The absolute configurations of the two acid moieties, 12-methyltetradecanoate and 14-methylhexadecanoate, of glycoglycerolipid S365A isolated from Corynebacterium aquaticum were determined by an HPLC analysis after their conversion with the chiral fluorescent labeling reagents, (1S,2S)- and (1R,2R)-2-(2,3-anthracenedicarboximido)cyclohexanol. Both anteiso acids had the S configuration.     

Trans-diaryl epoxides: asymmetric synthesis, ring-opening, and absolute configuration

Anthryl-phenyl, phenanthryl-phenyl, and naphthyl-phenyl trans-epoxides (1, 2, and 3, respectively) having enantiomeric purities of 95%, 99%, and 96% were synthesized from a diastereo and enantiopure sulfonium salt derived from Eliel's oxathiane. The determination of their (1R,2R) absolute configurations was achieved by application of the CD exciton chirality method using a Zn-porphyrin tweezer on the corresponding alcohols obtained after opening of these epoxides with LiAlH(4). The R-configuration at C2 of these epoxides, (-)-1, (+)-2, and (-)-3, is consistent with our previous results concerning asymmetric synthesis of monoaryl epoxides, cyclopropanes, and aziridines. The (1S,2R)-configuration of the cis isomer (when present) was also confirmed. Moreover, the agreement between the negative exciton chirality for conjugates of (S)-configuration predicted by molecular modeling and the observed CD spectra helps to clarify the relative steric size of phenyl and CH(2)-aryl (phenanthryl or anthryl), which is critical when the tweezer method is applied for absolute configurational assignment (phenyl = medium group; anthacenyl CH(2) and phenanthryl CH(2) = large group).     

Absolute stereochemistry of the strevertenes

The CD exciton chirality method was applied to determine the absolute stereochemistry of the strevertenes, antifungal pentaene macrolides produced by Streptoverticillium sp. LL-30F848. The CD difference spectrum of strevertene A methyl ester 15-dimethylaminobenzoate showed a positive couplet between the dimethylaminobenzoate and the pentaene chromophores, and therefore established the 15R configuration. Thus, by considering the relative configurations of the remaining stereogenic centers as derived from X-ray crystallography and ROESY experiments, the absolute stereochemistry of the strevertenes is established as 2R, 3S, 5S, 7S, 11R, 13R, 14R, 15R, 26S and 27R.     

Synthesis and glycosidase inhibitory activities of 2-(aminoalkyl)pyrrolidine-3,4-diol derivatives

Several 2-(aminomethyl)-and 2-(2-aminoethyl)-pyrrolidine-3,4-diol derivatives have been assayed for their inhibitory activities towards glycosidases. Good inhibitors of alpha-mannosidases must have the (2R,3R,4S) configuration and possess 2-(benzylamino)methyl substituents. Stereomers with the (2S,3R,4S) configuration are also competitive inhibitors of alpha-mannosidases, but less potent as they share the configuration of C(1), C(2), C(3) of beta-D-mannosides rather than that of alpha-D-mannosides. Interestingly, (2S,3R,4S)-2-[2-[(4-phenyl)phenylamino]ethyl]pyrrolidine-3,4-diol (12g) inhibits several enzymes, for instance alpha-L-fucosidase from bovine epididymis (K(i)=6.5microM, competitive), alpha-galactosidase from bovine liver (K(i)=5microM, mixed) and alpha-mannosidase from jack bean (K(i)=102microM, mixed). Diamines such as (2R,3S,4R)-2-[2-(phenylamino) or 2-(benzylamino)ethyl]pyrrolidine-3,4-diol (ent-12a, ent-12b) inhibit beta-glucosidase from almonds (K(i)=13-40microM, competitive).     

Elucidation of the absolute configuration of JNJ-27553292, a CCR2 receptor antagonist, by vibrational circular dichroism analysis of two precursors

The absolute configurations of two precursors, that is, 1-(3',4'-dichlorophenyl)-propanol and 1-(3',4'-dichlorophenyl)-propanamine, of a potent 2-mercapto-imidazole CCR-2 receptor antagonist, JNJ-27553292, were determined using vibrational circular dichroism. As a consequence, the absolute configuration of the antagonist itself was also determined. The two precursor compounds were subjected to a thorough conformational analysis and rotational strengths were calculated at the B3LYP/cc-pVTZ level of theory. Based on these data, vibrational circular dichroism spectra were simulated, which in turn were compared with experimental spectra. Agreement between the spectra allowed the assignment of the absolute configuration, which is in agreement with the proposed configuration based on stereospecific reactions on similar compounds.     

Guaiane- and aristolane-type sesquiterpenoids of Nardostachys chinensis roots

Two guaiane-type compounds, nardoguaianone J and K (1 and 2) and two aristolane-type compounds, kanshone F and G (3 and 4), were isolated from Nardostachys chinensis roots. The structures including the absolute configurations were elucidated by spectral means and by comparison of their CD spectra.     

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.