Stereochemistry of the major rat liver microsomal metabolites of the carcinogen 7-methylbenz[c]acridine

The major metabolites of the carcinogen 7-methylbenz[c]acridine (7MBAC), trans-5,6-dihydro-5,6-dihydroxy-7-methylbenz[c]acridine (7MBAC-5,6-DHD), and trans-8,9-dihydro-8,9-dihydroxy-7-methylbenz[c]acridine (7MBAC-8,9-DHD) were characterized as their enantiomers after separation of their bis-(+)-(1R,2S,4S)-endo-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5 -ene-2-carboxylic acid [(+)-HCA] esters and hydrolysis. The synthetic precursor, trans-3,4-dihydroxy-7-methyl-1,2,3,4-tetrahydrobenz[c]acridine (7MBAC-3,4-THD), was similarly separated into enantiomers, and the dihydrodiol trans-3(S),4(S)-dihydro-3,4-dihydroxy-7-methylbenz[c]acridine (7MBAC-3,4-DHD) was prepared from 7MBAC-3(S),4(S)-THD. Absolute configurations were assigned by the chiral exciton coupling of the bis-p-(dimethylamino)benzoate of 7MBAC-3(R),4(R)-THD, and by the semiempirical methods based on the biaryl chromophores of the enantiomers of 7MBAC-5,6-DHD and of the methanolysis products of the 5,6-oxide of 7MBAC which were resolved as their (+)-HCA esters. X-ray crystallography was used for 7MBAC-8(S),9(S)-DHD bis-(+)-HCA ester, and assignments were correlated with chiral exciton coupling of the bis-4-(dimethylamino)cinnamates of 7MBAC-5(R),6(R)-DHD and 7MBAC-8(S),9(S)-DHD. The stereochemical compositions of four metabolites (three dihydrodiols and 7MBAC-5,6-oxide) formed in incubations with rat liver microsomes from control and induced liver were determined by normal-phase separations of bis-(+)-HCA esters, and by chiral stationary-phase separation of the 5,6-oxide methanolysis products. The 3(R),4(R)-enantiomer of 7MBAC-3,4-dihydrodiol predominated, 74-98% enantiomeric purity, and purity for the oxide varied from about 71% 5(R),6(S)-oxide for control microsomes to about 28% 5(R),6(S)-oxide for liver microsomes obtained from 3-methylcholanthrene-pretreated rats.