Having synthesized the needed products, we next studied the influence of the trifluoromethyl-diazirine fragment on the basicity of the nitrogen atom. For that, we values of hydrochlorides 1g-11g by standard titration method (Figure ). Incorporation of the diazirine unit at the β-position of the azetidine ring decreased its basicity by more than two orders of magnitude: pK a HCl) = 8.8. A similar effect was observed during the incorporation of the trifluoromethyl-diazirine position of the pyrrolidine ring -basicity of the nitrogen atom decreased by more than two orders of magnitude: HCl) = 9.1. Introduction of the diazirine fragment at the α-position of the pyrrolidine ring led to an even more dramatic increase of basicityby more than four orders of magnitude: pK a (17*HCl) = 11.3 vs r trend was observed with piperidine ): incorporation of the trifluoromethyl-diazirine unit at the most ) led to a decrease in basicity by ca. one position (8g) -by two magnitudes, at by more than four magnitudes of order: pK a HCl) = 9.7 vs pK a (8g‧HCl) = 9.1 vs pK a In a brief summary, two conclusions can be made: 1. Incorporation of the trifluoromethyl-diazirine fragment at the structure of azetidine, pyrrolidine, and piperidine leads to a significant decrease in the basicity of the nitrogen atom with ΔpK a = -(1.5-4.3). These experiments indicate that trifluoromethyldiazirine substituent is strongly electron-withdrawing, similar to other fluoroalkyl groups. 2. Even though the basicity of the nitrogen atom in 1g-11g is reduced compared to the original secondary amines, it is still sufficient for the standard amide coupling -the most popular reaction in medicinal chemistry (see Table ). 5) Physicochemical properties. We also studied the effect of the trifluoromethyl-diazirine fragment on solubility and lipophilicity of azetidine, pyrrolidine, and piperidine. Therefore, we first synthesized model compounds 19-27 (Table ) by standard acylation with para-phenyl benzoic acid. Indeed, despite reduced basicity, all amines 1g-11g were compatible with the standard protocol of amide coupling under standard conditions (please, see SI). Incorporation of the trifluoromethyl-diazirine unit into model compounds 19, 21 and 24 significantly decreased their water solubility: 78-331 µM (19, 21, 24) vs <10 µM (20, 22, 23, 25, 26, 27) (Table ). Analogously, lipophilicity (logD) of the synthesized trifluoromethyl-diazirine containing derivatives was higher: 3.0-3.3 (19, 21, 24) vs 3.9-5.0 µM (20, 22, 23, 25, 26, 27).