Structure and synthesis of Vitamines

Inhaltsverzeichnis

1. Vitamin A

2. Vitamin B6

3. Vitamin H

4. Vitamin B1

5. Vitamin C

Vitamin A

- Another Name: vitamin Al/ Axerophthol/ Retinol

- Molecule formula:C20H30O

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- Vitamin Al influences growth in animals, and also apparently increases resistance to disease.
- Night blindness is due to vitamin Al deficiency in the human diet, and a prolonged deficiency leads to hardening of the cornea, etc.
- Vitamin Al occurs free and as esters in fats, in fish livers and in blood.
- It was originally isolated as viscous yellow oil, but later it was obtained as a crystalline solid, m.p. 63-64°C.
- Vitamin Al is estimated by the blue colour reaction it gives with a solution of antimony trichloride in chloroform. It is also estimated by light.
- On catalytic hydrogenation, vitamin Al is converted into perhydrovitamin A1, C20H400; thus vitamin Al contains five double bonds.
- vitamin Al forms an ester with p-nitrobenzoic acid, it follows that vitamin Al contains a hydroxyl group. Thus the parent hydrocarbon of vitamin Al is C20H40, and consequently the molecule contains one ring.
- Ozonolysis of vitamin Al produces one molecule of geronic acid per molecule of vitamin Al, and so there must be one β- ionone nucleus present.
- Oxidation of vitamin Al with permanganate produces acetic acid; this suggests that there are some —C(CH3)= groups in the chain.
- All of the foregoing facts are in keeping with the suggestion that vitamin Al is half the β-carotene structure.
- When heated with an ethanolic solution of hydrogen chloride, vitamin Al is converted into some compound (II) which, on dehydrogenation with selenium forms 1: 6- dimethyl naphthalene, III. Heilbron assumed I as the structure of vitamin A1, and explained the course of the reaction as follows:

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Perhydrovitamin Al has been synthesised from β-ionone and was shown to be identical with the compound obtained by reducing vitamin A1.

Synthesis: Isler et al. (1947).

1. Methyl vinyl ketone to produce compound IV.
2. Compound V is prepared from β-ionone by means of the Darzens reaction.
3. Combination of IV and V, etc.

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Vitamin B6

Another Nmae: Pyridoxin/Adermin Molecule Formula:[illustration not visible in this excerpt]

- obtained from rice bran and yeast.
- it cures dermatitis in rats.
- Pyridoxin behaves as a weak base.
- Application of the Zerewitinoff method showed the presence of three active hydrogen atoms.
- When treated with diazomethane, pyridoxin formed a monomethyl ether which, on acetylation, gave a diacetyl derivative. It therefore appears that the three oxygen atoms in pyridoxin are present as hydroxyl groups, and since one is readily methylated, this one is probably phenolic. This conclusion is supported by the fact that pyridoxin gives the ferric chloride colour reaction of phenols. Thus the other two hydroxyl groups are alcoholic.
- Examination of the ultraviolet absorption spectrum of pyridoxin showed that it is similar to that of 3-hydroxypyridine. It was therefore inferred that pyridoxin is a pyridine derivative with the phenolic group in position 3.
- Lead tetra-acetate has no action on the monomethyl ether of pyridoxin, this leads to the conclusion that the two alcoholic groups are not on adjacent carbon atoms in a side-chain.
- When this methyl ether is very carefully oxidised with alkaline potassium permanganate, the product is a methoxypyridinetricarboxylic acid, C9H707N. This acid gave a blood-red colour with ferrous sulphate, a reaction which is characteristic of pyridine-2-carboxylic acid; thus one of the three carboxyl groups is in the 2- position.
- When the methyl ether of pyridoxin was oxidised with alkaline permanganate under the usual conditions, the products were carbon dioxide and the anhydride of a dicarboxylic acid, C8H504N ; thus these two carboxyl groups are in the ortho-position. - Furthermore, since this anhydride, on hydrolysis to its corresponding acid, did not give a red colour with ferrous sulphate, there is no carboxyl group in the 2-position. It therefore follows that, on decarboxylation, the tricarboxylic acid eliminates the 2- carboxyl group to form the anhydride; thus the tricarboxylic acid could have either of the following structures.

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- Now pyridoxin methyl ether contains three oxygen atoms (one as methoxyl and the other two alcoholic) ; it is therefore possible that two carboxyl groups in the tricarboxylic acid could arise from two CH2OH groups, and the third from a methyl group, i.e., pyridoxin could be either of the following :

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A decision between the two structures was made on the following evidence.

- When pyridoxin methyl ether was oxidised with barium permanganate, the product was a dicarboxylic acid, C3H905N, which did not give a red colour with ferrous sulphate; thus there is no carboxyl group in the 2-position.

- Also, since the dicarboxylic acid formed an anhydride and gave a phthalein on fusion with resorcinol, the two carboxyl groups must be in the ortho position.

- Furthermore, analysis of both the dicarboxylic acid and its anhydride showed the presence of a methyl group. Thus the structure of this dicarboxylic acid is either I or II.

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Kuhn et al. (1939) showed that the anhydride was that of I from its formation by the oxidation of 4-methoxy-3-methyl-isoquinoline (a synthetic compound of known structure).

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Hence, on the foregoing evidence, pyridoxin is

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Synthesis: Harris and Folkers (1939)

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