Aromatase is a unique cytochrome P-450 enzyme that catalyzed the synthesis of estrone and estradiol from the 4-en-3-one androgens

catalyzed the synthesis of estrone and estradiol from the 4-en-3-one androgens androstenedione (AD) and testosterone. The aromatization process appears to proceed with three oxygenations at the C-19 position of the androgens, followed by the eventual loss of the C-19 angular methyl group and the elimination of the 1b ,2b-hydrogens, resulting in aromatization of the A-ring of the androgen molecule to the estrogen. Inhibitors of aromatase are useful in treating estrogen-dependent diseases such as a breast cancer. For this reason, a number of aromatase inhibitors, analogs of the substrate AD, have been described. Among the inhibitors, suicide substrates are of interest due to their high selectivity. 19-Ethynyl, 4-hydroxy-, 6-oxo, and 6b-bromo-ADs are the earliest discovered suicide substrates. The three former steroids act by oxygenation of the 19-angular methyl moiety in the inactivation of human placental aromatase in a suicide manner, whereas the inactivation mechanism for the 6b-bromo inhibitor 1 (Fig. 1), which has a very high affinity to aromatase, is currently unknown. On the other hand, Furth et al. previously reported that the 2,2-dimethyl substrate analog 2,2-dimethylAD (3) is an excellent competitive inhibitor of aromatase. In addition, it was also reported that the 2,2-dimethyl analogs of the suicide substrates 4-hydroxyAD and 6-oxoAD lost their enzyme-inactivating ability via a suicide mechanism without diminution of their competitive inhibitory potency. This, along with the previous findings regarding the aromatase reaction of a series of 2-methylAD derivatives, indicates that the 2,2-dimethyl group prevents the aromatase-catalyzed 19-oxygenation of the two suicide substrates, producing a reactive electrophile that binds irreversibly to a nucleophilic residue of the active site of the enzyme. Taken together, to determine whether or not 19-oxygenation is involved in the suicidal inactivation of aromatase by 6b-bromoAD (1), we focused on the 2,2-dimethyl derivative. This paper describes the synthesis of 2,2-dimethyl-6aand 6b-bromoADs (4, 5) and their aromatase inactivation ability. Steroids 4 and 5 did not deactivate aromatase in a suicide manner.