Atropine

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Slide 1: 

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Atropine: : 

Atropine: It is the most important alkaloid of tropane group and occurs in the roots of ‘deadly nightshade’ (Atropa belladona), ‘thorn apple’ (Datura stramonium). Atropine is the racemic form of l-hyoscyamine which readily racemises to atropine when warmed in an ethanol alkaline solution, thus atropine is (±)- or dl-hyoscyamine.

Isolation: : 

Isolation: Atropine is extracted either from belladona roots or from the juice of datura plant. In practice, the juice which also contains hyoscyamine is heated with potassium carbonate solution when hyoscyamine is racemised to atropine. Belladona roots Datura plant

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Then, the atropine is extracted with chloroform. The chloroform is recovered by evaporation and the residue is then extracted from the residue with dilute sulphuric acid. The solution is made alkaline with potassium carbonate when atropine is precipitated out. The precipitated atropine is extracted with ether and purified by converting it into oxalate or a sulphate.

Properties: : 

Properties: Atropine is a crystalline compound (m.p. 118 °C ) with bitter taste. Atropine is a tertiary base of pKa 10.0, which is quite high for an alkaloid. A single drop of solution containing 1 part of atropine in 40,000 parts of water is sufficient to dilate the pupil of the eye. Atropine has also been used to relieve the night sweats which are a distressing feature of tuberclusis and to deminish the activity of the salivary and gastric glands.

Constitution: : 

Constitution: 1) Molecular Formula: From the analytical data and molecular weight determination, it follows that the molecular formula of atropine is C17H23NO3 2) Atropine as an ester: When atropine is warmed with barium hydroxide solution, it undergoes hydrolysis to yield a racemic acid, (±)-tropic acid (C9H1003) and an optically inactive alcohol, tropine (C8H15NO).

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This reaction reveals that atropine is the tropeine, an ester of tropic acid i.e., it can be named as tropine tropate. Lanenberg further confirmed, that atropine is a tropine tropate ester, by evaporating a mixture of tropine and tropic acid in presence of hydrochloric acid and obtained atropine. Further, atropine cannot be amide because tropine, the product of hydrolysis, is a tertiary base

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3) Structure of Tropic acid: The structure of this acid is based on the following evidences: Its molecular formula has been found to be C9H1003 As tropic acid consumes one equivalent of alkali and does not add on bromine, it is a saturated monobasic acid. Tropic acid, when acetylated, forms a monoacetate, indicating that it must contain one hydroxyl group.

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When tropic acid is heated strongly, it loses a molecule of water to yield an optically inactive unsaturated acid, called atropic acid C9H8O2. Therefore, the hydroxyl group must be an alcoholic group. Atropic acid when oxidised, yields benzoic acid. The formation of benzoic acid reveals that atropic acid and hence tropic acid both will contain at least one benzene nucleus with a side chain containing a carboxylic group in their structure.

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As atropic acid is an unsaturated acid, it means that atropic acid may be either (I) or (II). Since, however, structure (I) is known to be cinnamic acid, structure (II) must be atropic acid. This is confirmed by the fact that atropic acid on oxidation with permanganate yields phenylglyoxal.

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Tropeine is the generic name for the esters of tropine. As atropic acid is formed by the dehydration of tropic acid, it means that addition of a molecule of the water to the former would therefore give tropic acid, consequently, must be either (III) or (IV).

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Tropic acid has been shown to be (IV) by synthesis, {e.g., Mackenzie and Wood synthesised tropic acid (IV) from acetophenone.

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Structure (III) is called atrolactic acid which on dehydration is converted into structure (II), thus confirming the structure of atropic acid. 4) Structure of Tropine: Its structure is established as follows: Its molecular formula has been found to be C8H15NO When tropine is treated with methyl iodide, it yields a crystalline compound. This reaction reveals that the nitrogen atom in tropine is tertiary.

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When fused with alkali, tropine yields methyl amine, indicating that it must contain a N-methyl, i.e., >N-CH3 group. This is also confirmed by the fact that tropine when heated with hydrogen iodide at 150 °C (Herzig-Meyer method) yields one mole of methyl iodide. As tropine forms a monoacetate and a monobenzoate, this indicates that it must contain a hydroxyl group.

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When tropine is oxidised with chromic acid, it yields a tropinone, C8H13NO, which gives characteristic reactions of a ketone. Therefore, the hydroxyl group must be secondary alcoholic (-CHOH-) group.

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Finally the structure of tropine has been confirmed by its synthesis. Robinson assumed that the skeleton of tropinone, when subjected to hydrolysis, would be broken down into the three units, namely, succindialdehyde, methylamine and acetone. Tropinone

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Furthermore, Robinson was of the opinion that these three units must be joined by means of double Mannich reaction to form tropinone in a single step. Tropinone

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A much better yield (40 %) was obtained if calcium acetone dicarboxylate or ethyl acetone-dicarboxylate was used instead of acetone. The calcium salt or ester so formed was converted into tropinone b warming with HCl e.g, (ca=Ca/2) Tropine Tropinone An ester

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5) Structure of atropine: We already know that atropine is a tropine tropate ester. On this basis, it is assigned (V). This structure has been confirmed by synthesis from tropine and tropic acid; this has been done by heating these two together in presence of HCl.

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6) Stereochemistry Tropines: When tropinone is reduced, it yields two alcohols, tropine and Ψ- tropine(psedotropine). However, the relative amounts of two alcohols are dependent upon the nature of reducing agents employed. Tropine and Ψ- tropine are considered to be epimers. In one of these epimers, the nitrogen bridge and the hydrogen of the secondary alcoholic group are on the same side(A) while in the other epimer, these are on the opposite side(B). (A) (B)

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‘O.D’

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