From here, we followed a procedure utilizing excess p-toluenesulfonic acid (pTsOH, Scheme 1) in toluene established by Agbaje et al. to dehydrate the hexahydropyrimidine (Compound 4). Notably, a dehydrated trifluoromonastrol compound has not been previously reported in the literature . Other dehydration conditions screened (Table ) did not give the desired product. Through a series of experiments where two reagents were added and the conversion of both the imine and enamine intermediates were forced to equilibrium, and third reagent was subsequently added dropwise, it was found that the initial step of the reaction progresses faster upon initial equilibrating formation of enamine E-3 (Figure ). This differs from the proposed mechanism of the synthesis of the non-fluorinated monastrol, which putatively proceeds through the imine pathways . We postulate that this difference might be caused by the highly electron-withdrawing effects of the trifluoromethyl functionality on the ketone carbonyl, such that the α,α,α-trifluoroketone of the ketoester outcompetes the aryl aldehyde as the preferred electrophile for initial Schiff base condensation with thiourea. We further probed the thermodynamics of the three possible mechanistic pathways by performing density functional theory (DFT) free energy calculations on various reactive intermediates en route to the hexahydropyrimidine in each of the mechanistic possibilities (Figure ). We observed that, regardless of the identity of the aryl aldehyde modeled, the initial thiourea addition to the α,α,α-trifluoroketone to form hemiaminal E-1 was the most thermodynamically favorable (ΔG = -1.4 kcal/mol) pathway, which is consistent with our mechanistic hypothesis that the trifluorinated keto ester outcompetes the aryl aldehyde as the best electrophile, thereby committing the Biginelli cyclocondensation to the enamine route. This result, as shown by computer modeling, is consistent with our observation of initial accumulation of the intermediate identified as the enamine (δ = -82.88 ppm in CH3CN), regardless of the identity of the aryl aldehyde. Moreover, we observed that the hydroxyl elimination prior to the final cyclization was highly thermodynamically unfavorable, which corresponds to the observed Hammett LFER which pointed to this elimination as the rate determining step. The rate of the final cyclization is dependent on the concentration of the dehydrated iminoester E-6, whose rate is in turn dependent on the equilibrium of the preceding β-hydroxy elimination step.