Dehydro vitamin

Condensation of Dehydrophytol with TMFIQ to yield Dehydro-Vitamin E. 

The dehydro-Vitamin E obtained is hydrogenated with a platinum catalyst in methanol. Good yields of Vitamin E (8) (a-tocopherol) are obtained. 

Heilbron hat das Carbinol LXI nach Oppenauer zum Keton LXII oxydiert. Dieses Keton wurde nach Reformatzki mit Bromessigester umgesetzt, und nach Wasserabspaltung und Verseifung wurde die Desmethyl-dehydro-Vitamin A-Saure (LXIII) erhalten. Diese Saure hat sich als schwach biologisch wirksam erwiesen. 

A steroid very closely related structurally to cholesterol is its 7 dehydro derivative 7 Dehydrocholesterol is formed by enzymatic oxidation of cholesterol and has a conju gated diene unit m its B ring 7 Dehydrocholesterol is present m the tissues of the skin where it is transformed to vitamin D3 by a sunlight induced photochemical reaction... 

The most significant chemical characteristic of L-ascorbic acid (1) is its oxidation to dehydro-L-ascorbic acid (L-// fi (9-2,3-hexodiulosonic acid y-lactone) (3) (Fig. 1). Vitamin C is a redox system containing at least three substances L-ascorbic acid, monodehydro-L-ascorbic acid, and dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid and the intermediate product of the oxidation, the monodehydro-L-ascorbic acid free radical (2), have antiscorbutic activity equal to L-ascorbic acid. 

The reversible oxidation of L-ascorbic acid to dehydro-L-ascorbic acid is the basis for its known physiological activities, stabiUties, and technical apphcations (2). The importance of vitamin C in nutrition and the maintenance of good health is well documented. Over 22,000 references relating only to L-ascorbic acid have appeared since 1966. 

This synthesis was the first step toward industrial vitamin production, which began in 1936. The synthetic product was shown to have the same biological activity as the natural substance. It is reversibly oxidized in the body to dehydro-L-ascorbic acid (3) (L-// fi (9-2,3-hexodiulosonic acid y-lactone), a potent antiscorbutic agent with hiU vitamin activity. In 1937, Haworth and Szent-Gyn rgyi received the Nobel Prize for their work on vitamin C. 

Properties 7-Dehydro-cholesterol Ergosterol Vitamin D2 Vitamin... 

Ascorbic acid oxidase (MW = 1,40,000 8 Cu). It is widely distributed in plants and micro-organisms. It catalyses oxidation of ascorbic acid (vitamin C) to dehydro ascorbic acid. 

L-dehydroascorbic acid, are active forms of the vitamin. However, once the dehydro form is further oxidized to diketogiuconic acid, no vitamin C activity is retained. The thermal half life of dehydroascorbic acid is less than 1 minute at lOO C (pH 6), and 2 minutes at 7O C Vitamin C is also... 

Fruits and vegetables are the most common food sources for vitamin C (15-18). In food, vitamin C exists as two vitamers L-ascorbic acid (AA) and its oxidation product, dehydro-L-ascor-bic acid (DHAA) (Fig. 1) (15,17-19). Total Vitamin C is the sum of the AA and the DHAA contents. A stereoisomer, isoascorbic acid (IAA) is often added to food as an antioxidant. Both LAA and its oxidation product, dehydroisoascorbic acid (DHIAA), can interfere in Total Vitamin C determinations. 

The ring fission of 1,3-cyclohexadienes observed in the vitamin D series (Sec. IVA) is a general reaction for 1,3-cyclohexadienes and 2,4-cyclohexadienones (Sec. IIB). Irradiation of 1,3-cyclohexadiene (Formula 330) gives 1,3,5-hexatriene (Formula 331) (142,143), and a-phellendrene (Formula 332) gives a triene which is probably Formula 333 (143). The evidence for Structure 333 is not compelling. Methyl dehydro-... 

The ease of oxidation of reduced ascorbic acid is the basis for a simple method of analysis by dye titration (58j. Ascorbic acid as it occurs in citrus juice is in the reduced form. When subjected to oxidation, ascorbic acid changes to the dehydro form. Dehydroascorbic acid has nearly the same physiological activity as the reduced form and is easily converted to the latter. Further oxidation of the dehydroascorbic acid converts it to 2,3-diketo-gulonic acid. This reaction is irreversible, and the oxidized product is devoid of biological activity. These reactions are shown in Figure 2. Nearly 90 percent or more of the vitamin C found in citrus juice and citrus products is in the reduced form (Table X) (59). 

Vitamin D-active substances are required in the diets of growing children and pregnant women, but normal adults receiving sufficient doses of sunshine can manufacture sufficient amounts of these compounds to meet their needs. Active vitamin D compounds can by synthesized in such individuals from 7-dehydro-cholesterol (see Table 6.2), an intermediate in cholesterol biosynthesis. Dietary sources also include cholecalciferol, which is produced from 7-dehydrocholesterol and ergosterol (Table 6.2). 7-Dehydrocholesterol and ergosterol are often referred to as provitamins. 

L-ascorbic acid (Figure 9-9) is a lactone (internal ester of a hydroxycarboxylic acid) and is characterized by the enediol group, which makes it a strongly reducing compound. The D form has no biological activity. One of the isomers, D-isoascorbic acid, or erythorbic acid, is produced commercially for use as a food additive. L-ascorbic acid is readily and reversibly oxidized to dehydro-L-ascorbic acid (Figure 9-10), which retains vitamin C activity. This compound can be further oxidized to diketo-L-gulonic acid, in a... 

By the use of well authenticated reactions, cholest-5-en-3/3,4a-dioI diacetate has been converted into its 7-dehydro-analogue and thence into 4a-hydroxy-vitamin D3... 

Q-11 What is the structural difference between pro-vitamin D (7-dehydro cholesterol) in the skin and ergosterol ... 

The results in Figure 9.51 demonstrate that the major vitamin D-related compound in skin not exposed (0 minutes) to ultraviolet (UV) light was 7-dehydro-... 

There are various physiological forms known as vitamins D, namely vitamin D2 (calciferol, ergocaldferol), vitamin D3 (cholecalciferol), phosphate esters of D2, D3, 25-hydroxycholecalciferol, 1,25-dihydroxychole-caldferol, and 5,25-dihydroxycholecalciferol. There are active anologs and related compounds known as vitamins D, namely 22-dihydroergosterol (vitamin D4), 2-dehydrostigmasterol (vitamin Dg), and 7-dehydro-sitosterol (vitamin D5) [3]. 

Nitrogen derivatives of L-ascorbic acid are important because they have been used extensively for the vitamin s determination (J) in the form of the bis(2,4-dinitrophenylhydrazone) of dehydro-L-ascorbic acid (l). In addition, because of the commercial availability of L-ascorbic acid with a relatively low price as well as the widespread use of hetero-... 

The oxidative product of ascorbic acid, dehydroascorbic acid, is the preferred form of the vitamin for uptake by neutrophils, erythrocytes, and lymphocytes (27). Once within the erythrocyte, dehydroascorbic acid is reduced to ascorbic acid by a glutathione-dependent, dehydro-ascorbic-acid-reducing enzyme (20,28). However, the reduced form of ascorbic acid is found in most other tissues, that is, liver, lungs, kidneys, skin, and pituitary and adrenal glands (20,29). From these studies, ascorbic acid is taken up by several tissues by an energy-dependent and Na -sensitive process, but the transport of the oxidized vitamin form follows the principles of diflFusion. 

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