A new efficient route to the phenolic derivatives of chrysene and 5-methylchrysene, precursors to dihydrodiol and diol epoxide metabolites of chrysene and 5-methylchrysene, through Suzuki cross-coupling reaction

A new, abbreviated synthesis of 5-methylchrysene (17), 2-hydroxychrysene (16), 8-hydroxy-5-methylchrysene (23), and 2-hydroxy-5-methylchrysene (24) is reported. The phenolic derivatives 16, 23, and 24 can easily be converted to carcinogenic dihydrodiol and diol epoxide metabolites of chrysene and 5-methylchrysene. The method entails the initial Suzuki cross-coupling reaction of naphthalene-2-boronic acid (1) and/or 6-methoxynaphthalene-2-boronic acid (2) with 2-bromo-5-methoxybenzaldehyde (3), methyl 2-bromophenylacetate (4), 2-bromophenylacetone (5), and/or 2-iodo-5-methoxyphenylacetone (6) to produce 2-(2-naphthyl)-5-methoxybenzaldehyde (7), methyl 2-(6-methoxy-2-naphthyl)phenylacetate (8), 2-(2-naphthyl)phenylacetone (9), 2-(2-naphthyl)-5-methoxyphenylacetone (10), and 2-(6-methoxy-2-naphthyl)phenylacetone (11) in 55-98% yields. 2-Methoxychrysene (15) was obtained with high regioselectivity by two different procedures. In the first procedure, the aldehyde function of 7 was elongated with trimethylsulfonium iodide under phase transfer conditions to generate the ethylene oxide 12 which after methanesulfonic acid treatment produced 15. The second procedure involved modification of ester 8 to its aldehyde analogue 14 which was subsequently treated with methanesulfonic acid to produce 15. Phenylacetone 10 was converted by methanesulfonic acid treatment into 8-methoxy-5-methylchrysene (18) with 90% regioselectivity. However, the similar cyclization of phenylacetones 9 and 11 to 5-methylchrysene (17) and 2-methoxy-5-methylchrysene (19) occurred with only 33-50% regioselectivity. The separation of 17 and 19 from their chromatographically similar 6-methylbenz[a]anthracene byproducts 20 and 22 was readily achieved by a chemical method. The methoxy derivatives of chrysene were finally demethylated with boron tribromide to the corresponding phenolic compounds in 90-98% yields.