Direct enantiomeric resolution of cyclic alcohol derivatives of polycyclic aromatic hydrocarbons by chiral stationary phase high-performance liquid chromatography

Regio- and stereospecific oxidation of 9,10-dihydroanthracene and 9,10-dihydrophenanthrene by naphthalene dioxygenase: structure and absolute stereochemistry of metabolites

The oxidation of 9,10-dihydroanthracene and 9,10-dihydrophenanthrene was examined with mutant and recombinant strains expressing naphthalene dioxygenase from Pseudomonas putida (NCIB 9816.4. Salicylate-induced cells of P. putida strain 9816/11 and isopropylthiogalactopyranoside-induced cells of Escherichia coli JM109(DE3)(pDTG141) oxidized 9,10-dihydroanthracene to (+)-cis-1R,2S)-1,2-dihydroxy-1,2,9,10-tetrahydroanthracene (> 95% relative yield; > 95% enantiomeric excess) as the major product. 9-Hydroxy-9,10-dihydroanthracene (< 5% relative yield) was a minor product formed by both organisms. The same cells oxidized 9,10-dihydrophenanthrene to (+)-cis-(3S,4R)-3,4-dihydroxy-3,4,9,10-tetrahydrophenanthrene (70% relative yield; > 95% enantiomeric excess) and (+)-(S)-9-hydroxy-9,10-dihydrophenanthrene (30% relative yield). The major reaction catalyzed by naphthalene dioxygenase with 9,10-dihydroanthracene and 9,10-dihydrophenanthrene was stereospecific dihydroxylation in which both of the previously undescribed cis-diene diols were of R configuration at the benzylic center adjacent to the bridgehead carbon atom. The results suggest that for benzocylic substrates, the location of benzylic carbons influences the type of reaction(s) catalyzed by naphthalene dioxygenase.

endo-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2-carboxylic acid, a superior resolving agent for the high-performance liquid chromatographic separation of enantiomers of hydroxylated derivatives of two azaaromatic hydrocarbons

The high-performance liquid chromatographic (HPLC) separation of enantiomers of oxide and hydroxy derivatives of dibenz[a,j]acridine and 7-methylbenz[c]acridine was investigated on a chiral stationary phase chromatography column using commercially available columns. In most cases either poor or no separation of enantiomers was achieved. Normal-phase separation of diastereoisomeric ester derivatives of the hydroxy compounds, prepared from commercially available (-)-menthoxyacetic acid or (+)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetic acid, was investigated. No separation of the diastereoisomeric esters of trans-3,4-dihydroxy-3,4-dihydrodibenz[a,j]acridine was observed. However, diastereoisomeric esters prepared from (+)-endo-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2-carboxyl ic acid [(+)-HCA] were easily separated. Using the three chiral acids, diastereoisomers were prepared from sixteen hydroxy derivatives of dibenz[a,j]acridine and 7-methylbenz[c]acridine. (+)-HCA esters gave good to excellent HPLC separations which were superior to those achieved using other chiral acids in most cases. The enantiomeric composition of trans-3,4-dihydroxy-3,4-dihydrodibenz[a,j]acridine formed as a major rodent liver microsomal metabolite of dibenz[a,j]acridine was determined using (+)-HCA.

Stereoselectivity of cytochrome P-450 isozymes and epoxide hydrolase in the metabolism of polycyclic aromatic hydrocarbons

Enantiomeric compositions of epoxides formed in the metabolism of planar benz[a]anthracene (BA), benzo[a]pyrene (BaP), and chrysene (CR), and nonplanar benzo[c]phenanthrene (BcPh), 12-methylbenz[a]anthracene (12-MBA) and 7,12-dimethylbenz[a]anthracene (7,12-DMBA) by liver microsomes from untreated, phenobarbital-treated, and 3-methylcholanthrene-treated rats are determined either by direct chiral stationary phase HPLC analysis or by the enantiomeric compositions of metabolically formed trans-dihydrodiols. Cytochrome P-450 isozymes contained in various liver microsomal preparations have varying degrees of stereoselectivity in catalyzing the epoxidation reactions at various formal double bonds of the polycyclic aromatic hydrocarbons studied. In general, cytochrome P-450c, the major cytochrome P-450 isozyme contained in liver microsomes from 3-methylcholanthrene-treated rats, has the highest degree of stereoselectivity. Regardless of absolute configuration, non-K-region epoxides are converted to trans-dihydrodiols by epoxide hydrolase-catalyzed water attack at the allylic carbon. The S-center of K-region S,R-epoxide enantiomers derived from planar BA, BaP and CR is the major site of epoxide hydrolase-catalyzed water attack. In contrast, the R-center of K-region S,R-epoxide enantiomers derived from nonplanar BcPh, 12-MBA and 7,12-DMBA is the major site of epoxide hydrolase-catalyzed water attack. However, the K-region R,S-epoxide enantiomers of the six polycyclic aromatic hydrocarbons studied are hydrated by microsomal epoxide hydrolase with varying degrees of regioselectivity. Thus the enantiomeric composition of a metabolically formed dihydrodiol is determined by (i) the stereoselective epoxidation at a formal double bond of a parent hydrocarbon by microsomal cytochrome P-450 isozymes and (ii) the enantioselective and regioselective hydration of the metabolically formed epoxide by microsomal epoxide hydrolase.

Stereoselective formation and hydration of 12-methylbenz[a]anthracene 5,6-epoxide enantiomers by rat liver microsomal enzymes

The K-region trans-5,6-dihydrodiols formed in the metabolism of 12-methylbenz[a]anthracene (12-MBA) by liver microsomal preparations from untreated, phenobarbital-treated and 3-methylcholanthrene-treated male Sprague-Dawley rats were found by chiral stationary-phase h.p.l.c. (c.s.p.-h.p.l.c.) analyses to contain (5S,6S)/(5R,6R) enantiomer ratios of 93:7, 88:12 and 97:3 respectively. The absolute stereochemistry of a 12-MBA trans-5,6-dihydrodiol enantiomer was elucidated by the exciton-chirality c.d. method. The 5,6-epoxides formed in the metabolism of 12-MBA by liver microsomal preparations from untreated, phenobarbital-treated and 3-methylcholanthrene-treated male Sprague-Dawley rats in the presence of the epoxide hydrolase inhibitor 3,3,3-trichloropropylene 1,2-oxide were isolated from a mixture of metabolites by normal-phase h.p.l.c., and their (5S,6R)/(5R,6S) enantiomer ratios were found by c.s.p.-h.p.l.c. analyses to be 73:27, 78:22 and 99:1 respectively. The absolute configurations of 12-MBA 5,6-epoxide enantiomers, resolved by c.s.p.-h.p.l.c., were determined via high-resolution (500 MHz) proton-n.m.r. and c.d. spectral analyses of the two isomeric methoxylation products derived from each of the 12-MBA 5,6-epoxide enantiomers. Enantiomeric pairs of the two methoxylation products were resolved by c.s.p.-h.p.l.c. The results indicate that enantiomeric 5S,6R-epoxide and 5S,6S-dihydrodiol were the major enantiomers preferentially formed in the metabolism at the K-region 5,6-double bond of 12-MBA by all three rat liver microsomal preparations. Optically pure 12-MBA 5S,6R-epoxide was hydrated predominantly at the C(6) position (R centre) to form 12-MBA trans-5,6-dihydrodiol with a (5S,6S)/(5R,6R) enantiomer ratio of 97:3. However, optically pure 12-MBA 5R,6S-epoxide was hydrated nearly equally at both C(5) and C(6) positions to form 12-MBA trans-5,6-dihydrodiol with a (5S,6S)/(5R,6R) enantiomer ratio of 57:43.

Direct separation of non-K-region mono-ol and diol enantiomers of phenanthrene, benz[a]anthracene, and chrysene by high-performance liquid chromatography with chiral stationary phases

The direct separation of 26 bay region and non-bay region mono-ol and diol enantiomers of phenanthrene, benz[a]anthracene, and chrysene was compared by high-performance liquid chromatography on commercially available columns, packed with gamma-aminopropylsilanized silica to which either (R)-N-(3,5-dinitrobenzoyl)phenylglycine(R-DNBPG) or (S)-N-(3,5-dinitrobenzoyl)leucine(S-DNBL) was either ionically or covalently bonded. In general, enantiomers of bay region mono-ols and diols are more efficiently resolved than those of non-bay region derivatives. Elution orders of enantiomers on either chiral stationary phase are the same, regardless of whether the chiral stationary phase is ionically or covalently bonded. Except for the enantiomers of 4-hydroxy-4-methyl-1,2,3,4-tetrahydrobenz[a]anthracene, 1,2,3,4-tetrahydrobenz[a]anthracene trans-1,2-diol, and benz[a]anthracene trans-1,2-dihydrodiol, elution orders of resolved enantiomers on R-DNBPG are reversed on S-DNBL. The enantiomers are generally more efficiently resolved on R-DNBPG than on S-DNBL. With the exception of the elution order of the enantiomeric 4-hydroxy-1,2,3,4-tetrahydrochrysene, the results of this study are consistent with the chiral recognition mechanisms proposed by Pirkle and co-workers, who developed the chiral stationary phases used in this study.

Elution order-absolute configuration relationship of K-region dihydrodiol enantiomers of benz[a]anthracene derivatives in chiral stationary phase high-performance liquid chromatography

The direct resolution of K-region cis- and trans-dihydrodiol enantiomers of 14 unsubstituted and methyl- and bromo-substituted benz[a]anthracene (BA) derivatives was investigated by high-performance liquid chromatography with commercially available columns, packed with gamma-aminopropylsilanized silica to which either (R)-N-(3,5-dinitrobenzoyl)phenylglycine (R-DNBPG) or (S)-N-(3,5-dinitrobenzoyl)leucine (S-DNBL) is either ionically or covalently bonded. BA derivatives used in this study include: BA, 1-methyl-BA, 4-methyl-BA, 7-methyl-BA, 8-methyl-BA, 10-methyl-BA, 11-methyl-BA, 12-methyl-BA, 7,12-dimethyl-BA, 7-bromo-BA, 7-bromo-1-methyl-BA, 7-bromo-11-methyl-BA, 7-bromo-12-methyl-BA, and 3-methylcholanthrene. The enantiomers of BA trans-5,6-dihydrodiol were the only compounds not resolved by any of the four chiral stationary phases (CSPs) tested. The results indicate that conformational preference of the hydroxyl group is one of the most important factor in determining the elution order of dihydrodiol enantiomers. The presence and the location of a substituent and the molecular size and shape of the dihydrodiols can significantly affect the efficiency of enantiomeric resolution. In general, the ionically bonded R-DNBPG provides the best resolution of enantiomeric quasidiequatorial trans-dihydrodiols and the R,R enantiomers are consistently more strongly retained. In contrast, the enantiomeric pairs of quasidiaxial trans-dihydrodiols are generally better resolved by the covalently bonded R-DNBPG, and the S,S enantiomers are more strongly retained. The enantiomers of cis-dihydrodiols having hydroxyl groups that adopt quasiequatorial-quasiaxial and/or quasiaxial-quasiequatorial conformations are more consistently resolved by the ionically bonded S-DNBL and in all cases the S,R enantiomers are more strongly retained. Thus, it is possible to choose a CSP which resolves the K-region dihydrodiol enantiomers with a predictable elution order.

Cytochrome P-450-catalyzed stereoselective epoxidation at the K region of benz[a]anthracene and benzo[a]pyrene

The enantiomers of K-region benz[a]anthracene (BA) 5,6-epoxide and benzo[a]pyrene (BP) 4,5-epoxide were resolved by chiral stationary-phase high-performance liquid chromatography (CSP-HPLC). The K-region epoxides formed in the metabolism of BA by liver microsomes from untreated (control), phenobarbital (PB)-treated, and 3-methylcholanthrene (MC)-treated male Sprague-Dawley rats were determined by CSP-HPLC to have a 5R,6S/5S,6R enantiomer ratio of 25:75, 21:79, and 4:96, respectively. The K-region 4,5-epoxide formed in the metabolism of BP by the same rat liver microsomal preparations contained a 4R,5S/4S,5R enantiomer ratio of 48:52 (control), 40:60 (PB), and 5:95 (MC), respectively. The results indicate that various cytochrome P-450 isozymes of rat liver exhibit different stereoselective properties in catalyzing the epoxidation reactions at the K region of BA and of BP.

Resolution of epoxide enantiomers of polycyclic aromatic hydrocarbons by chiral stationary-phase high-performance liquid chromatography

Enantiomers of nine K-region and one non-K-region epoxides of polycyclic aromatic hydrocarbons have been resolved by high-performance liquid chromatography with chiral stationary phases either ionically or covalently bonded to gamma-aminopropylsilanized silica. Resolution of enantiomers was confirmed by ultraviolet-visible absorption, circular dichroism, and mass spectral analyses. This method has been applied to the determination of optical purity and absolute configuration of the K-region epoxides formed in the metabolism of 1-methylbenz[a]anthracene, 7-methylbenz[a]anthracene, and 12-methylbenz[a]anthracene by rat liver microsomes.

Stereoselective hydroxylation at the aliphatic carbons of 7,8- and 9,10-dihydrobenzo[a]pyrenes by rat liver microsomes

Optically active 7-hydroxy-7,8-dihydrobenzo[a]pyrene and 8-hydroxy-7,8-dihydrobenzo[a]pyrene were identified as two of the major metabolites formed by incubation of 7,8-dihydrobenzo[a]pyrene with rat liver microsomes. Optically active 9-hydroxy-9,10-dihydrobenzo[a]pyrene and 10-hydroxy-9,10-dihydrobenzo[a]pyrene were similarly identified as two of the minor metabolites of 9,10-dihydrobenzo[a]pyrene. The formation of these metabolites was abolished either by prior treatment of liver microsomes with carbon monoxide or the absence of NADPH, but was not inhibited by an epoxide hydrolase inhibitor. The results indicate that the aliphatic carbons of dihydro polycyclic aromatic hydrocarbons may undergo stereoselective hydroxylation reactions catalyzed by the cytochrome P-450 system of rat liver microsomes.

Direct resolution of mono- and diol enantiomers of unsubstituted and methyl-substituted benz[a]anthracene and benzo[a]pyrene by high-performance liquid chromatography with a chiral stationary phase

The direct resolution of 86 structurally related monomethyl, mono-ol, and trans- and cis-diol enantiomers of unsubstituted and methyl-substituted benz[a]anthracene and benzo[a]pyrene was investigated by high-performance liquid chromatography with a commercially available column, packed with an (R)-N-(3,5-dinitrobenzoyl)phenylglycine, ionically bonded to gamma-aminopropylsilanized silica. The results indicate that structural factors, such as conformation, presence of a methyl substituent, molecular size and shape, and ring saturation all contribute to chiral interactions between the chiral stationary phase and the solutes. Detailed chiral recognition mechanisms can not yet be established, due to complex structural factors that influence enantiomeric resolutions and the lack of data on the absolute configurations of the resolved enantiomers. Nevertheless, the chromatographic method can be applied to the determination of enantiomeric purity of mono- and diol metabolites of polycyclic aromatic hydrocarbons. The absolute configurations of a limited number of resolved enantiomers have been established.