Synthesis and Identification of Major Metabolites of Environmental Pollutant Dibenzo[c,mno]chrysene

Dibenzo[c,mno]chrysene commonly known as naphtho[1,2-a]pyrene (N[1,2-a]P) is an environmental pollutant, recently identified in coal tar extract, in air-borne particulate matter, in marine sediment, and in cigarette-smoke condensate. We recently reported an efficient synthesis of N[1,2-a]P and examined its in vitro metabolism by male Sprague Dawley rat liver S9 fraction, which resulted in a number of dihydrodiol and phenolic metabolites. The synthesis of 10-hydroxy-N[1,2-a]P and fjord region N[1,2-a]P trans-9,10-dihydrodiol, which were identified among the various metabolites, was assigned earlier by comparing with the synthetic standards. The other major metabolites were separated by HPLC and, based on the 1H NMR analysis, were tentatively suggested to be the two K-region dihydrodiols, that is, N[1,2-a]P trans-4,5-dihydrodiol (6) and N[1,2-a]P trans-7,8-dihydrodiol (7), and the hydroxy derivatives of N[1,2-a]P. To unequivocally assign the structure to each of the peaks and to have them in larger amounts for toxicological studies, we have now synthesized the two K-region dihydrodiols and the 1-/3-hydroxy-N[1,2-a]P, short-listed based on the proton NMR of the collected peaks. The K-region dihydrodiols 6 and 7 were prepared by the treatment of N[1,2-a]P with OsO(4) to give a mixture of cis dihydrodiols 2 and 3, followed by pyridinium chlorochromate-assisted oxidation to quinones 4 and 5, and finally reduction with NaBH(4) to afford the dihydrodiols 6 and 7 with the desired trans stereochemistry. The 1-hydroxy-N[1,2-a]P (22) and 3-hydroxy-N[1,2-a]P (23) were synthesized using a photochemical approach. As expected, all the synthesized dihydrodiol and phenolic derivatives of N[1,2-a]P identified with those obtained from in vitro metabolism enabling the assignment of all the major metabolites.