Synthesis of bromoundecyl resorc[4]arenes and applications of the cone stereoisomer as selector for liquid chromatography

As an extension of our studies on the multifaceted properties of C-alkylated resorc[4]arenes, we planned to immobilize on a solid support resorc[4]arenes with C11-long side chains in the lower rim. To this purpose, we synthesized two conformationally diverse resorc[4]arenes containing a bromoundecyl moiety in the four axial pendants. The cone stereoisomer 6a (30% yield) was selected for the reaction with an aminopropylated silica gel (APSG) obtained from spherical Kromasil Si 100, 5 μm particles, to give the corresponding immobilized SP-C11-resorc[4]arene system. The resulting polar-embedded stationary phase was fully characterized and investigated in the HPLC discrimination of the E/Z stereoisomers of naturally occurring and semi-synthetic combretastatins, a family of (Z)-stilbene anticancer drugs. The chair stereoisomer 6b (20% yield), when submitted to X-ray diffraction analysis, showed a noteworthy self-assembly in the crystal lattice, with intercalated hydrophobic and polar layers as a result of intermolecular Br• • •O halogen-bond interactions, according to a unique stacking motif. The potential and versatility of the SP-C11-resorc[4]arene stationary phase were shown as well in the separation of highly polar natural products (namely, flavonoids), under reversed-phase (RP) conditions, and of fullerenes C60 and C70, by using apolar solvents as mobile phases.