Reversible Photohydration of Trenbolone Acetate Metabolites: Mechanistic Understanding of Product-To-Parent Reversion Through Complementary Experimental and Theoretical Approaches

J. Baltrusaitis
E.V. Patterson
M. O'Connor
S. Qu
E.P. Kolodziej, University of Washington Tacoma
D.M. Cwiertny


Photolysis experiments (in H2O and D2O) and quantum chemical calculations were performed to explore the pH-dependent, reversible photohydration of trenbolone acetate (TBA) metabolites. Photohydration of 17α-trenbolone (17α-TBOH) and 17β-trenbolone (17β-TBOH) occurred readily in simulated sunlight to yield hydrated products with incorporated H+ at C4 and OH- at either C5 (5-OH-TBOH) or C12 (12-OH-TBOH) in the tetracyclic steroid backbone. Although unable to be elucidated analytically, theory suggests preferred orientations of cis-12-OH-TBOH (relative to C13 methyl) and trans-5-OH-TBOH, with the former most thermodynamically stable overall. Both experiment and theory indicate limited stability of trans-5-OH-TBOH at acidic pH where it undergoes concurrent, carbocation-mediated thermal rearrangement to cis-12-OH-TBOH and dehydration to regenerate its parent structure. Experiments revealed cis-12-OH-TBOH to be more stable at acidic pH, which is the only condition where its reversion to parent TBA metabolite occurred. At basic pH cis-12-OH-TBOH decayed quickly via hydroxide/water addition, behavior that theory attributes to the formation of a stable enolate resistant to dehydration but prone to thermal hydration. In a noteworthy deviation from predicted theoretical stability, 17α-TBOH photohydration yields major trans-5-OH-TBOH and minor cis-12-OH-TBOH, a distribution also opposite that observed for 17β-TBOH. Because H+ and OH- loss from adjacent carbon centers allows trans-5-OH-TBOH to dehydrate at all pH values, the presumed kinetically controlled yield of 17α-TBOH photohydrates results in a greater propensity for 17α-TBOH reversion than 17β-TBOH. Additional calculations explored minor, but potentially bioactive, trenbolone analogs that could be generated via alternative rearrangement of the acidic carbocation intermediate. © 2016 American Chemical Society.