Macrocyclic biphenyl tetraoxazoles: synthesis, evaluation as G-quadruplex stabilizers and cytotoxic activity.

A series of macrocyclic biphenyl tetraoxazoles was synthesized. The latter stages of the synthetic approach allowed for the addition of varied N-protected α-amino acids, which were subsequently deprotected and condensed to provide the desired macrocycles. Improved yields could be realized in the macrocyclization step of their synthesis relative to other macrocyclic G-quadruplex stabilizers. These 24-membered macrocycles were evaluated for their ability to stabilize G-quadruplex DNA and for their relative cytotoxicity against human tumor cells. These biphenyl tetraoxazoles were not strong ligands for G-quadruplex DNA relative to other macrocyclic polyoxazoles. This reduced stabilizing potential did correlate with their comparatively lower cytotoxic activity as observed in the human tumor cell lines, RPMI 8402 and KB3-1. These studies provide useful insights into the conformational requirements for the development of selective and more potent G-quadruplex ligands.