Para-ascorbic acid

It is worthy of note that the substance maltol, 2-methyl-3-hydroxy-7-pyrone, which is related to XLII, occurs in pine needles along with L-ascorbic acid. This substance maltol, however, is not a growth promoter like L-ascorbic acid but an inhibitor, a point of interest inasmuch as it has been obtained in considerable quantity by the degradation of streptomycin. Since the naturally occurring para-ascorbic acid or hexenolactone, also a derivative of a-pyrone, is known to display anti-growth properties, it is not unlikely that other analogs of ascorbic acid may be growth inhibitors. 

Hydroxypyridine and pyridin-2-one are sufficiently reactive to undergo Elbs oxidation, and in both cases the substituent directs hydroxylation mainly para (Scheme 40) (58JA3717). Quinoline may be converted into 3-hydroxyquinoline (6% yield) by Udenfriend oxidation (ascorbic acid and oxygen in the presence of iron(II)) which is believed to involve attack by OH+ rather than radicals (54JBC(208)74i). 

The fat-soluble vitamins are A, D, E, and K. The water-soluble vitamins are thiamine (vitamin Bj), riboflavin, nicotinic acid (niacin) and nicotinamide, pyridoxine (vitamin B6), pantothenic acid, biotin, para-aminobenzoic acid, choline, inositol, and other lipotropic agents, ascorbic acid (vitamin C), the riboflavonoids, folate, and vitamin B12 (see Figure 66.1 and Figure 66.2, and Table 66.1). 

The organic compounds other than the essential amino acids which are required for health are called vitamins. Man is known to require at least thirteen vitamins Vitamin A, (thiamine), Bo (riboflavin), Bg (pyridoxin), B.o, C (ascorbic acid), D, K, niacin, pantothenic acid, inositol, para-aminobenzoic acid, and biotin. 

The main absorption band of benzoquinones appears around 260 nm in nonpolar solvents and at 280 nm iu water. Extinction coefficients are 1.3-1.5 x 10 M Upon reduction to hydroquinones, a four times smaller band at 290 nm is found. The most important property of quinones and related molecules is the relative stability of their one-electron reduction products, the semiquinone radicals. The parent compound 1,4-benzoquinone is reduced by FeCl, ascorbic acid, and many other reductants to the semiquinone anion radical which becomes protonated in aqueous media (pk = 5.1). Comparisons of the benzaldehyde reduction potential with some of the model quinones given below show that carbonyl anion radicals are much stronger reductants than semiquinone radicals and that ortho- and para-benzoquinones themselves are even relatively strong oxidants comparable to iron(III) ions in water (Table 7.2.1). This is presumably caused by the repulsive interactions between two electropositive keto oxygen atms, which are separated only by a carbon-carbon double bond. When this positive charge can be distributed into neighboring n systems, the oxidation potential drops significantly (Lenaz, 1985). 

Lindsey and Maxwell, 1949) phosphoric acid 0.005% (Eckey, 1934, 1935 Kraybill and Beadle, 1948) thiodipropionic acid 0.01% and its esters (O Leary, 1946) and lecithin in any amount (Evans, 1935 Olcott and Mattill, 1936). Many other compounds have been reported to have synergistic activity, including normal constituents of meat such as amino acids (Clausen et cd., 1947), ascorbic acid (Golumbic and Mattill, 1941 Calkins and Matthill, 1944), nicotinic acid (Taub and Simone, 1947), and para-aminobenzoic acid (Norris, 1949). 

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