Tetrahydro-1,8-naphthyridinol Analogues of α-Tocopherol as Antioxidants in Lipid Membranes and Low-Density Lipoproteins

Recently we demonstrated that the C(7)-unsubstituted tetrahydro-1,8-naphthyridin-3-ol has more than an order of magnitude better peroxyl radical trapping activity than α-tocopherol (α-TOH) in inhibited autoxidations in benzene. In order to prepare analogues more structurally related to α-TOH for further studies in vitro and in vivo, we developed synthetic approaches to C(7)-monoalkyl and C(7)-dialkyl analogues using a sequence involving (1) AgNO₃-mediated hydroxymethyl radical addition to 1,8-naphthyridine, (2) regioselective alkyllithium addition by cyclic chelation in a nonpolar solvent, (3) iodination of the naphthyridine at C(3), and (4) CuI-medidated benzyloxylation of the aryl iodide followed by catalytic hydrogenolysis. An α-TOH isostere was prepared by a Wittig coupling of a C₁₆ side chain identical to that of α-TOH to the naphthyridinols. The C(7)-mono- and dialkyl analogues exhibited more than an order of magnitude higher antioxidant activity (k_inh = (5.3−6.1) × 10⁷ M^-1 s^-1) than α-TOH (k_inh = 0.35 × 10⁷ M^-1 s^-1) in benzene, as determined by a newly developed peroxyl radical clock. In addition to the strong antioxidant activity in benzene, the closest α-TOH analogue (naphthyridinol-based tocopherol, N-TOH) showed excellent inhibition of the oxidation of cholesteryl esters in human low-density lipoprotein and spared endogenous α-TOH in these experiments. Lateral diffusion of N-TOH in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine liposomes was comparable to that of α-TOH, suggesting that it will have good antioxidant characteristics in both membranes and lipoproteins. Furthermore, a binding assay using a fluorescent tocopherol analogue showed that N-TOH binds to recombinant human tocopherol transfer protein better than α-TOH itself, suggesting that distribution of unnatural antioxidants such as N-TOH in vivo is possible.