Methyl group-induced helicity in 1,4-dimethylbenzo[c]phenanthrene and its metabolites: Synthesis, physical, and biological properties

1,4-Dimethylbenzo[c]phenanthrene (1,4-DMBcPh) is the dimethylated analogue of the benzo[c]phenanthrene (BcPh), where one of its two methyl groups resi des in the highly congested fjord-region. A comparative X-ray crystallograp hic analysis, described herein, shows that BcPh is distorted out-of-plane s o that there is an angle of 27 degrees between the outermost rings. The add itional methyl groups in I,4-DMBcPh increase this nonplanarity to an angle of 37 degrees. This methyl group-induced disruption of planarity results in P and M enantiomers of 1,4-DMBcPh, and this helicity is observed in a pron ounced manner in its putative metabolites, the dihydrodiol and diol epoxide . Synthetically, photochemical cyclization offers convenient access to 1,4- DMBcPh as well as its metabolites. For this, Wittig reaction of 2,5-dimethy lbenzyltriphenylphosphoninm chloride and 2-naphthaldehyde provided a cis/tr ans mixture of alkenes which, when subjected to photolysis, provided],4-DMB cPh. Despite the high steric congestion in the fjord-region, this reaction proceeds with respectable yields. Correspondingly, a Wittig reaction of the same phosphonium chloride derivative with 6-methoxy-2-naphthaldehyde affor ded an alkene mixture that, upon photochemical ring closure, provided 10-me thoxy-1,4-dimethylbenzo[c]phenanthrene. The methoxy group was cleaved and t he resulting phenol oxidized to the o-quinone. Reduction of the o-quinone w ith NaBH4 under an oxygen atmosphere provided the dihydrodiol, (+/-)-trans- 9, 10-dihydroxy-9, 10-dihydro-1,4-dimethylbenzo[c]phenanthrene. This dihydr odiol was found to be an approximately 3:1 mixture of diastereomers, which was produced as follows. Reduction of the quinone proceeds in a stereoselec tive manner producing both the (R,R)- and (S,S)-trans-diols. However, this factor, when coupled with the P, M atropisomerism of the hydrocarbon, resul ts in two diastereomeric pairs of enantiomeric dihydrodiols. Due to steric constraints imposed by the fjord-region methyl group, the P --> M interconv ersion is slow, making the proton resonances of the diastereomeric dihydrod iols distinctly observable by NMR. Assignment of major and minor dihydrodio l isomers has been achieved by NOESY experiments. Finally, epoxidation prov ides a mixture of diol epoxides that reflects the dihydrodiol ratio. The me tabolic activation of these compounds to reactive intermediates was studied through analysis of their binding to DNA. DNA binding data using human mam mary carcinoma MCF-7 cells reveal that the level of DNA binding of BcPh is not statistically different from that of 1,4-DMBcPh. However, there is an I i-fold increased activation of BcPh dihydrodiol as compared to the 1,4-DMBc Ph dihydrodiol. In contrast to the planar benzo[a]pyrene, BcPh is only poor ly adducted to DNA in culture cells. Thus, it appears that increasing the n onplanarity in this type of PAH lowers its ability to be metabolically acti vated to form DNA-damaging adducts, although in the case of 1,4-DMBcPh, the presence of the two methyl groups in one of the angular rings may also con tribute to the decrease.