Synthetic, Crystallographic, Computational, and Biological Studies of 1,4-Difluorobenzo[<i>c</i>]phenanthrene and Its Metabolites

1,4-Difluorobenzo[<i>c</i>]phenanthrene (1,4-DFB<i>c</i>Ph) and its putative metabolites, the dihydrodiol and diol epoxides, have been synthesized and structurally characterized, and the extent of DNA binding by the metabolites has been assessed. 1,4-DFB<i>c</i>Ph and 1,4-difluoro-10-methoxybenzo[<i>c</i>]phenanthrene were prepared by photochemical cyclization of appropriate naphthylphenylethylenes. The dihydrodiol was synthesized from 1,4-difluoro-10-methoxybenzo[<i>c</i>]phenanthrene, and the diol epoxides were diastereoselectively synthesized from the dihydrodiol. Interesting differences were noted in <sup>1</sup>H NMR spectra of the series 1 (<i>syn</i>) diol epoxides of benzo[<i>c</i>]phenanthrene (B<i>c</i>Ph) and 1,4-DFB<i>c</i>Ph; the B<i>c</i>Ph diol epoxide displays a quasi-diequatorial orientation of the hydroxyl groups, but in the 1,4-DFB<i>c</i>Ph case these are diaxially disposed. This difference probably stems from the presence of the fjord-region fluorine atom in 1,4-DFB<i>c</i>Ph. A through-space, fjord-region H−F coupling has also been observed for 1,4-DFB<i>c</i>Ph and its derivatives. Comparative X-ray crystallographic analyses of B<i>c</i>Ph and 1,4-DFB<i>c</i>Ph and their dihydrodiols show that introduction of fluorine increases the molecular distortion by about 6−7°. As a guide to estimating the molecular distortion and its effects, and for comparison with the X-ray structures in known cases, optimized structures of B<i>c</i>Ph, 1,4-DFB<i>c</i>Ph, and 1,4-DMB<i>c</i>Ph (the dimethyl analogue) as well as their dihydrodiols and diol epoxides were computed. Relative aromaticities of these compounds were assessed by nucleus-independent chemical shift calculations, and <sup>13</sup>C NMR chemical shifts were computed by gauge-inducing atomic orbital calculations. 1,4-DFB<i>c</i>Ph and its dihydrodiol were subjected to metabolism, and the amount of DNA binding in human breast cancer MCF-7 cells was assessed. The extent of DNA binding was then compared with that for B<i>c</i>Ph and its dihydrodiol and the potent carcinogen benzo[<i>a</i>]pyrene. The 1,4-DFB<i>c</i>Ph series 2 (<i>anti</i>) diol epoxide-derived DNA adducts were also compared with those arising from intracellular oxidation of the dihydrodiol with subsequent DNA binding. These experiments showed that increased molecular distortion decreased metabolic activation to the terminal metabolites but that diol epoxide metabolites that are formed are the DNA-damaging species.