Source, Etc

The cherry-laurel, Prunus Laurocerasus, Linne (N.O. Rosaceœ), is an evergreen shrub indigenous to Persia and Asia Minor but cultivated in most temperate regions. It was introduced into this country towards the end of the sixteenth century, but the poisonous property of the water distilled from the leaves was not known till 1731; the poisonous principle contained in it was identified as hydrocyanic acid soon after the discovery of the latter by- Scheele in 1782.

Description

Cherry-laurel leaves which are official in the fresh state only, average about 15 cm. in length by 5 cm. in breadth. The upper surface is dark green and glossy, the under surface paler. In outline they vary from oblong-lanceolate to nearly obovate, tapering towards both base and apex, the latter being shortly acute and recurved. The leaves are thick and coriaceous in texture, and are supported on short stout petioles. The margin is slightly recurved and is provided at intervals with short strong serrations. On the under surface of the leaf, near the base, and on either side of the midrib are from one to four yellowish depressed spots; these spots are the remains of glands in which a sugary substance is produced whilst the leaf is young. Both surfaces of the. leaf are quite glabrous.

When fresh and entire, cherry-laurel leaves are almost inodorous; but when crushed, or even scratched with a needle, the young leaves evolve an odour recalling that of oil of bitter almond and hydrocyanic acid. Old leaves, or dried leaves crushed and moistened with water, yield a much less perceptible odour.

Constituents

The principal constituent of cherry-laurel leaves is a glucoside. laurocerasin, first isolated by Lehmann (1885), and subsequently by Bourquelot and Herissey, who further investigated it and called it prulaurasin, by which name it is now generally known. Prulaurasin, C14H17N06, has been obtained in colourless, odourless, bitter prisms (m.pt. 120°; O.R. = - 53°.6). In contact with prunase (an enzyme also contained in the leaves) and water, prulaurasin is decomposed, yielding benzaldehyde (oil of bitter almonds), hydrocyanic acid, and dextrose.

Fig. 25.   Cherry laurel leaf. Under surface. Slightly reduced.

Fig. 25. - Cherry-laurel leaf. Under surface. Slightly reduced.

C14H17NO6

+

H1O

C6H12O6

+

HCN

+

C6H5CHO.

Prulaurasin

Water

Dextrose

Hydrocyanic acid

Benzaldehyde

As long as the leaves remain intact no decomposition takes place, for the emulsin is stored in the endodermis of the veins, whilst the prulaurasin is distributed through the parenchyma of the leaf. The two bodies cannot therefore come into contact with one another until the cells containing them are broken; directly this is effected the reaction takes place.

Fresh leaves from healthy plants yield on an average about 0.1 per cent, of hydrocyanic acid. In the autumn the yield is less than in the spring or early summer, the young leaves being especially rich in prulaurasin; newly unfolded leaves have yielded as much as 249 per cent, of hydrocyanic acid (Ven, 1898).

Prulaurasin resembles but is not identical with amygdalin. By acting upon amygdalin with yeast Fischer obtained an amygdonitrile glucoside, acid, laevo-phenylglycollic acid (mandelic acid),

This body is isomeric with prulaurasin, but differs in its optical rotation ( - 26°.9), and yields, when acted upon by hydrochloric

Constituents 47Constituents 48

Under similar conditions prulaurasin yields inactive (racemic) phenylglycollic acid Hence prulaurasin is the glucoside of racemic phenylglycollic acid, and the amygdonitrile glucoside that of laevo-phenylglycollic acid. Sambunigrin, an allied glucoside contained in the leaves of the elder, is the glucoside of dextro-phenylglycollic acid. Both the amygdonitrile glucoside and sambunigrin are readily converted into prulaurasin by dilute alkalies.

Several other glucosides have been discovered (e.g. manihotoxin, lotusin, dhurrin, phaseolunatin, gynocardin, linamarin, corynocarpin) which under the influence of enzymes yield hydrocyanic acid; they are collectively termed 'cyanogenetic' glucosides.

Uses

Cherry-laurel water is employed as a sedative, its action being due to the hydrocyanic acid it contains.