Multistep synthesis of cationic lipid-like compounds based on a difluorotetrahydropyridine scaffold as amphiphilic nanocarriers

Abstract

A series of new 3,5-difluoro-2-methylene-4-aryl-2,3,4,5-tetrahydropyridines were synthesised via the reaction of 1,4-dihydropyridines (1,4-DHPs) with in situ generated N-fluoro quinine through one-step transfer-fluorination using Selectfluor®. Halogenation of these tetrahydropyridines with N-chloro-(or bromo)succinimide yielded 2-(chloromethylene)-2,3,4,5-tetrahydropyridine-3,5-dicarboxylates 7a or 2-(bromomethylene) derivatives 7b,c. Palladium-catalysed oxidative Heck reaction of 2-methylenetetrahydropyridine 2d with phenylboronic acid (11a) afforded new fluorinated phenyl-substituted alkene 13a, while pyridyl-substituted alkenes 13b,c were synthesised via Suzuki cross-coupling of brominated derivatives 7b,c with 4-pyridinylboronic acid (11c) or 4-pyridineboronic acid pinacol ester (11d). Quaternisation of the pyridine moiety of dialkyl 3,5-difluoro-6-methyl-4-phenyl-2-(pyridin-4-ylmethylene)-2,3,4,5-tetrahydropyridine-3,5-dicarboxylates 13b,c produced pyridium derivatives 14a–c, 15a–c with pyridinium moieties bearing alkyl chains of varying lengths. These quaternised pyridine derivatives demonstrated the ability to form liposomes. The resulting liposomes prepared using the injection method had average particle sizes of 61–199 nm with polydispersity index (PDI) values of 0.17–0.56 one day after preparation, 65–306 nm with PDI values of 0.16–0.54 after four days, and 62–253 nm with PDI values of 0.17–0.47 after seven days. The zeta potential (ζ-potential) of the liposomes ranged from 5.5 to 51.6 mV. Stable nanoparticles were consistently formed by compounds containing at least one dodecyl alkyl chain. This study demonstrates that quaternised pyridinium derivatives can form stable nano-liposomal carriers with tunable sizes and ζ-potentials exceeding +30 mV, ensuring colloidal stability.