Synthesis of Phenol and Quinone Metabolites of Benzo[a]pyrene, a Carcinogenic Component of Tobacco Smoke Implicated in Lung Cancer

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental pollutants produced in the combustion of organic matter. PAHs are present in automobile exhaust and tobacco smoke, and they have recently been designated as human carcinogens. Current evidence indicates that PAHs are activated enzymatically to mutagenic metabolites that interact with DNA. There is evidence for three pathways of activation: the dial epoxide path, the radical-canon path, and the quinone path. The relative importance of these paths for human lung cancer has not been established. We now report syntheses of the principal phenol and quinone isomers of the prototype PAH carcinogen benzo[a]pyrene (BP) that are known or are suspected to be formed as metabolites of BP in human bronchoalveolar cells. The methods of synthesis were designed to be adaptable to the preparation of the C-13-labeled analogues of the BP metabolites. These compounds are needed as standards for sensitive LC-MS/MS methods for analysis of BP metabolites formed in lung cells. Efficient novel syntheses of the 1-, 3-, 6-, 9-, and 12-BP phenols and the BP 1,6-, 3,6-, 6,12-, and 9,10-quinones are now reported. The syntheses of the BP phenols (except 6-HO-BP) involve the key steps of Pd-catalyzed Suzuki-Miyaura cross-coupling of a naphthalene boronate ester with a substituted aryl bromide or triflate ester. The BP quinones were synthesized from the corresponding BP phenols by direct oxidation with the hypervalent iodine reagents IBX or TBI. These reagents exhibited different regiospecificities. IBX oxidation of the 7- and 9-BP phenols provided the ortho-quinone isomers (BP 7,8- and 9,10-diones, respectively), whereas TBI oxidation of the 1-, 3-, and 12-BP phenols furnished BP quinone isomers with carbonyl functions in separate rings (BP 1,6-, 3,6-, and 6,12-diones, respectively).