Tocopherylquinones

WENG x c and Gordon m h (1993) Antioxidant synergy between phosphatidyl ethanolamine and oc-tocopherylquinone . Food Chem, 48 (2) 165-8. 

Chromanoxylium cation 4 preferably adds nucleophiles in 8a-position producing 8a-substituted tocopherones 6, similar in structure to those obtained by radical recombination between C-8a of chromanoxyl 2 and coreacting radicals (Fig. 6.4). Addition of a hydroxyl ion to 4, for instance, results in a 8a-hydroxy-tocopherone, which in a subsequent step gives the /zara-tocopherylquinone (7), the main (and in most cases, the only) product of two-electron oxidation of tocopherol in aqueous media. A second interesting reaction of chromanoxylium cation 4 is the loss of aproton at C-5a, producing the o-QM 3. This reaction is mostly carried out starting from tocopherones 6 or /zora-tocopherylquinone (7) under acidic catalysis, so that chromanoxylium 4 is produced in the first step, followed by proton elimination from C-5a. In the overall reaction of a tocopherone 6, a [ 1,4] -elimination has occurred. The central species in the oxidation chemistry of a-tocopherol is the o-QM 3, which is discussed in detail subsequently. 

Antioxidants are compounds that inhibit autoxidation reactions by rapidly reacting with radical intermediates to form less-reactive radicals that are unable to continue the chain reaction. The chain reaction is effectively stopped, since the damaging radical becomes bound to the antioxidant. Thus, vitamin E (a-tocopherol) is used commercially to retard rancidity in fatty materials in food manufacturing. Its antioxidant effect is likely to arise by reaction with peroxyl radicals. These remove a hydrogen atom from the phenol group, generating a resonance-stabilized radical that does not propagate the radical reaction. Instead, it mops up further peroxyl radicals. In due course, the tocopheryl peroxide is hydrolysed to a-tocopherylquinone. 

When 6-hydroxychromans act as antioxidants, they are themselves oxidized to one or more of several compounds depending on the conditions. Amongst the commonest products from a-tocopherol are a-tocopherylquinone (702), a-tocored (703), the 5-benzoyloxy-methyl derivative (704) and the spiro dimer (705). 

The oxidation of a-hydroxy acids by benzyltrimethylammonium tribromide (BTMAB) to the corresponding carbonyl compounds shows a substantial solvent isotope effect, A (H20)/A (D20) = 3.57, but no KIE for a-deuteromandelic acid.133 The oxidation of glucose by hypobromous acid is first order in glucose and the acid.134 [l,l-2H2]Ethanol shows a substantial kinetic isotope effect when oxidized by hexamethylenetetramine-bromine (HABR) in acetic acid to aldehyde.135 Kinetics of the oxidation of aliphatic aldehydes by hexamethylenetetramine-bromine have been studied by the same group.136 Dioxoane dibromide oxidizes y-tocopherol to 5-bromomethyl-y-tocopherylquinone, which spontaneously cyclizes to 5-formyl-y-tocopherol.137... 

The rate constant for the bimolecular decay of the a-tocopheroxyl radical is only 3.5x 102M-1 s . Therefore, its half-life is several hours in chloroform at ambient temperature. This implies that vitamin E free radical can react with a second peroxyl radical. In biological membranes, a-tocopherylquinone is generally believed to be the major end-product, but the mechanism of its production remains controversial. It may arise either from the decomposition of a-tocopherone, or from dismutation of a-tocopheroxyl radicals. However, the steady-state concentration of a-tocopherylquinone is usually too low to be measurable ex vivo when tissue homogenization and extraction are performed in the presence of pyrogallol and butylated hydroxytoluene, respectively,... 

Vitamin E has also been noted for the function of tocopherylquinones as highly cytotoxic agents. Tocopherylquinones escape multidrug resistance in acute lymphoblastic leukemia cell lines (131). Ascorbate and a-tocopherol also have several poten-... 

Tocopherols are oxidized slowly by atmospheric oxygen and rapidly by ferric and silver salts. Oxidation products include tocopheroxide, tocopherylquinone, and tocopherylhydroqui-none, as well as dimers and trimers. Tocopherol esters are more stable to oxidation than the free tocopherols but are in consequence less effective antioxidants. See also Section 17. 

This is a specific reagent for the oxidation of o-tocopherol to a-tocopherylquinone. Addition of an aqueous solution of gold ehloride to a solution of a-tocopherol in... 

In the tocopherol series, 2,3,6-trimethyl-5-(3 -hydroxy-3 ,7M1, 15 -tetramethyl -hexadecany)phenol in benzene was added to a slurry of Fremy s salt and aqueous sodium carbonate containing tri-caprylmethylammonium chloride in benzene to give, after 2.5 hours reaction tocopherylquinone in 93% yield (ref. 50). 

Barr, R. and C.B. Arntzen Occurrence of 5-tocopherylquinone in higher plants and its relation to senescence Plant Physiol. 44 (1969) 593-598. 

Interestingly, there was only limited observable damage to the thylakoids of tissue exposed to light for short periods of time. This is reasonable because tocopherols and tocopherylquinones are located in the thylakoids (15). 

This allylation-oxidation reaction has been extended to hydroquinone derivatives to give compounds related to a- and -tocopherylquinones and to ubiquinone. Again /3-cyclodextrin mimics allylquinone synthetase. ... 

Titanium(IV) chloride-Zinc, 487-488 Titanocene dichloride, 507 Tocopherylquinones, 134 Toluene, 250, 253 p-Toluenesulfmic acid, 488 p-Toluenesulfonic acid, 139, 303, 488-489... 

Stocker, A. and Baumann, U., Supernatant protein factor in complex with RRR-alpha-tocopherylquinone a link between oxidized Vitamin E and cholesterol biosynthesis, J. Mol. Biol. 332 (4), 759-765, 2003. 

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