Page 94 - phytochemistry II -pharmD general

Purine alkaloids

Purines are derivatives of a heterocyclic nucleus consists of six membered ring
(pyrimidine) fused to a five membered ring (imidazole).

The most important purine alkaloids are methylated derivatives of xanthine
nucleus. Caffeine is 1, 3, 7 trimethyl xanthine, theobromine is 3, 7 dimethyl
xanthine, theophylline is 1, 3 dimethyl xanthine.

O R3 Caffeine R1 R2 R3
N CH3 CH3 CH3
1 Theobromine
7 Theophylline H CH3 CH3
R1 N CH3 CH3 H
N
O 3

N

R2

Xanthine alkaloids are very weak bases, forming salts with only very strong
acids which can be easily hydrolyzed in water. They do not give precipitate with
Mayer’s reagent. They all give Murexide positive test.

a. Caffeine

The alkaloid Caffeine occurs in seeds of Coffee ( Coffea arabica, family
Sterculiaceae) (1-2%), in the seeds of different species of Cola family
Sterculiaceae, (2-3%); in the leaves of Tea (Camelia sinensis, family Theaceae)
(2-5%); and in Guarana seeds Paulinia cupana (Sapindaceae) (2.5-5%).

Properties

Caffeine occurs as a white powder or white glistening needles. It is odorless
sublimable (on heating) without decomposition. Caffeine is soluble in water (1:50),
more soluble in hot water (1:6), in acetone (1:50), in benzene (1:100) and sparingly
soluble in ether. The stability of caffeine in water in increased by addition of organic
acids or their salts e.g. citric acid, sodium benzoate and salicylate.

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