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Lipophilicity and ionization constant of sixteen fentanyl analogues were investigated by reversed-phase thin- layer chromatography (RP-TLC). Fourteen compounds have nitrogen containing groups at C-3 position of the piperidine ring and two compounds are 3-carbomethoxy derivatives of fentanyl. Among them, five are diaste- reomeric cis/trans couples and six novel trans diastereomers. In addition, the lipophilicity and ionization constant of fentanyl were also determined by the same method, as a reference. The physicochemical property, lip- ophilicity, expressed as retention indices RM 0 , b, and C0, as well as PC1, was determined and correlated with in silico values. Ionization constants were determined on the basis of the relationships between analyte’s retention expressed as RF and mobile phase pH. Calculated structural descriptors together with the retention indices, were subjected to the principal component analysis – PCA and hierarchical cluster analysis – HCA, in order to provide basic insights into the similarities among the studied compounds. The blood – brain barrier (BBB) permeation was investigated in the function of experimentally obtained values of lipophilicity, polar surface area and mo- lecular weight. In general, results of the present research corroborate well with previously determined anti- nociceptive activities of the investigated compounds, pointing out that besides the cis/trans isomerism, another set of important parameters should be taken into account when designing new derivatives of C-3 substituted fentanyl, especially lipophilicity, voluminosity and HBD/A character of the substituents. Accordingly, RP-TLC can be considered as a valuable asset in the ligand-based drug design

Lipophilicity and ionization constant of sixteen fentanyl analogues were investigated by reversed-phase thin-layer chromatography (RP-TLC). Fourteen compounds have nitrogen containing groups at C-3 position of thepiperidine ring and two compounds are 3-carbomethoxy derivatives of fentanyl. Among them, five are diaste-reomeric cis/trans couples and six novel trans diastereomers. In addition, the lipophilicity and ionization constantof fentanyl were also determined by the same method, as a reference. The physicochemical property, lip-ophilicity, expressed as retention indices RM0 , b, and C0, as well as PC1, was determined and correlated with insilico values. Ionization constants were determined on the basis of the relationships between analyte’s retentionexpressed as RF and mobile phase pH. Calculated structural descriptors together with the retention indices, weresubjected to the principal component analysis – PCA and hierarchical cluster analysis – HCA, in order to providebasic insights into the similarities among the studied compounds. The blood – brain barrier (BBB) permeationwas investigated in the function of experimentally obtained values of lipophilicity, polar surface area and mo-lecular weight. In general, results of the present research corroborate well with previously determined anti-nociceptive activities of the investigated compounds, pointing out that besides the cis/trans isomerism, anotherset of important parameters should be taken into account when designing new derivatives of C-3 substitutedfentanyl, especially lipophilicity, voluminosity and HBD/A character of the substituents. Accordingly, RP-TLCcan be considered as a valuable asset in the ligand-based drug design