Autoxidation of hydrocarbons and other organic materials is one of the most important chemical processes known. Since the 1960's, the mechanism of inhibition of this process by antioxidants has been extensively studied, and antioxidants are now a key additive to many hydrocarbon products, including fuels, lubricant oils, rubber, polymers, chemicals, solvents and foodstuffs. However, it has only been in the last two decades that the importance of lipid peroxidation to human health has begun to become unearthed. Since then, substantial evidence has accumulated that implicate free radicals in aging, carcinogenesis and the pathogenesis of many conditions including heart disease (e.g. atherosclerosis), lung disease (e.g. emphysema) and several neurodegenerative disorders including Alzheimer's and Parkinson's diseases.
Consequently, the role of both enzyme and small-molecule antioxidants and the mechanisms of their protective function have been extensively studied. For example, it is now well accepted that the key initial event in the development of atherosclerosis involves free-radical mediated oxidative modification of low-density lipoprotein (LDL).
In support of this, it has been shown that a high intake of Vitamin E (-tocopherol, Formula 1 below), a potent lipid-soluble radical-trapping antioxidant, reduces the risk of coronary heart disease and that low levels of Vitamin E in serum correlate with an increased incidence of myocardial infarction. For example, see Gey, K. F. Nutr. Biochem., 1995, 6, 206-236, incorporated herein by reference.

Furthermore, probucol (Formula 2, below), a radical-scavenging antioxidant, is widely used to treat hypercholesterolemia and atherosclerosis. For example, see Barkley et al. Drugs 1986, 37, 761-800.

Given that antioxidants are of such tremendous industrial importance and have also been shown to possess preventive properties on the incidence of heart disease and many other degenerative diseases, various kinds of natural and synthetic antioxidants have been synthesized and studied both in vitro and in vivo. Unfortunately, few of them have demonstrated better radical-trapping activity than α-tocopherol, the major lipid-soluble radical-trapping antioxidant in plasma and LDL.
Phenols (of which α-tocopherol is an example) are the most abundant and widely used natural and synthetic antioxidants. Their mechanism of action as antioxidants, relies on their ability to transfer their phenolic H-atom to a chain-carrying peroxyl radical (LOO., Reaction 1) at a rate much faster than that at which the chain-propagating step of lipid peroxidation proceeds (Reaction 2).LOO.+ArOH→LOOH+ArO.  Reaction 1LOO.+LH→LOOH+L.(+O2→LOO.)  Reaction 2
A higher rate for Reaction 1 is expected with an increasingly weak ArO—H bond, and thus as the exothermicity of Reaction 1 increases relative to Reaction 2, one would expect that ArOH becomes a better chain-breaking antioxidant. Indeed, when the logarithm of the rate constant for Reaction 1 (logk1) is plotted against the phenolic O—H bond dissociation enthalpy (BDE) for several ArOH, a linear correlation of BDE (kcal/mol)=97.44−2.93 logk1(M−1s−1) is obtained. This correlation can be used to predict the rate constants for the reaction of peroxyl radicals with novel phenolic compounds whose O—H BDEs are known.
It is well-known that electron-donating (ED) groups substituted para and ortho to the phenolic hydroxyl lower the O—H bond dissociation enthalphy (BDE) and increase the rate of H-atom transfer to peroxyl radicals. However, efforts to design new phenolic antioxidants with increased rates of H-atom transfer to peroxyl radicals have remained unsuccessful. This is because, while the substitution of phenols with increasingly ED groups (e.g., —NH2 and —NH2) decreases their O—H BDEs, it also decreases their ionization potentials (IPs) such that they react directly with oxygen.
In 1985, Ingold and Burton investigated aminophenols as potential antioxidants. More specifically, they looked at Formula 3a and Formula 3b, below, as potential chain-breaking antioxidants, but found Formula 3a to be unstable in air and Formula 3b to react slowly with peroxyl radicals compared to -tocopherol. See Burton et al. J. Am. Chem. Soc. 1985, 107, 7053-7065, incorporated herein by reference.

These results may be explained in that the steric interaction between the meta-methyl and N-ethyl groups drives the N-ethyl group down out of the plane of the ring and removes the nitrogen lone pair from conjugation with the aromatic ring. This abolishes its stabilizing effect on the aryloxyl radical.
A known problem with aminophenols as antioxidants lies not only in the fact that they are very difficult to prepare and store, but also in their toxicity. In the case of para-aminophenol, this is related to its metabolic activation (oxidation, by cytochrome P450 among other possibilities) to a reactive intermediate that reacts with nucleophilic residues on proteins or DNA to form covalent intermediate that reacts with nucleophilic residues on proteins or DNA to form covalent intermediates or that can result in the depletion of glutathione stores.
Further substituted aminophenols (such as 4-N,N-dialkylaminophenol or 4b, above) are toxic because their oxidation no longer requires metabolism, but only a direct reaction with molecular oxygen to yield superoxide and the electrophilic species. This makes the compound a pro-oxidant and possible mutagen/carcinogen/teratogen rather than an antioxidant.
Based upon the above observations, the present inventors decided that a reasonable set of design criteria for new aminophenolic antioxidants are compounds with: (1) low phenolic O—H BDEs such that they have large logk1 value, but (2) high ionization potentials (IPs) such that they are not reactive to molecular oxygen.
U.S. Pat. No. 4,554,276 to LaMattina discloses 2-amino-5-hydroxy-4-methyl pyrimidines that are disclosed as being useful as inhibitors of leukotriene synthesis and for the treatment of pulmonary, inflammatory and cardiovascular diseases, cancer and psoriasis, and peptide ulcers.
U.S. Pat. No. 4,711,888 to Walker discloses hydroxy or alkoxy pyrimidines that are disclosed as being inhibitors of leukotriene synthesis and, as a result, are useful in the treatment of pulmonary, inflammatory, allergic, cardiovascular diseases, and peptide ulcers.
U.S. Pat. No. 5,187,175 to Belliotti et al., discloses 2-carbonyl subtituted-5-hydroxy-1,3-pyrimidines that are disclosed as being useful as inhibitors of 5-lipoxygenase, and thereby providing a treatment for inflammation, arthritis, pain, fever, and the like.
U.S. Pat. No. 5,196,431 to Belliotti discloses 2-substituted amino-4,6-di-tertiarybutyl-5-hydroxy-1,3-pyrimidines described as having activity as inhibitors of 5-lipoxygenase and/or cyclooxygenase providing treatment for inflammation, arthritis, pain, and fever.
U.S. Pat. No. 5,220,025 to Belliotti et al. is a divisional of the '431 patent discussed above.
U.S. Pat. No. 5,001,136 to Walker discloses 2-substituted methylamino-amino 5-(hydroxy or alkoxy) pyridines useful in the treatment of pulmonary, inflammatory, dermatological, allergic and cardiovascular diseases.
U.S. Pat. No. 5,284,949 to Belliotti et al. discloses 2-substituted amino-4,6-di-tertiarybutyl-5-hydroxy-1,3-pyrimidines described as having activity as inhibitors of 5-lipoxygenase and/or cyclooxygenase providing treatment for inflammation, arthritis, pain, and fever.
U.S. Pat. No. 5,177,079, to Connor et al. describe 2-substituted-4,6-di-tertiary-butyl-5-hydroxyl-1,3-pyrimidines disclosed as being useful as inhibitors of 5-lipoxygenase and/or cyclooxygenase providing treatment of conditions advantageously affected by such inhibition including inflammation, arthritis, pain, fever.