Cyclic Terpene Aldehydes (Perillaldehyde, Safranal, Phellandral

2.6.5.3.2 Cyclic Terpene Aldehydes

The cyclic terpene aldehyde are of two types viz.,

monocyclic and bicyclic. A few typical examples from each category shall be discussed in the sections to follow:

A. Monocyclic Terpene Aldehydes

Examples: Perillaldehyde, Safranal and Phellandral

1. Perillaldehyde

Chemical Structure 4-(1-Methylethenyl)-1-cyclohexene-1- carboxaldehyde; (C₁₀H₁₄O).

Occurrence It is found in essential oils of Perilla arguta Benth; Labiatae; Sium latifolium L., Umbelliferae; Mandarin peel oil Citrus reticulata Blanco; Rutaceae etc.

Isolation It may be isolated from the respective essential oil, first by forming its crystalline sodium bisulphite adduct; and secondly, regenerating the desired product with alkali very carefully.

Characteristic Features The physical characteristic of the d- and l-isomers are as follows:

d-form : Liquid, bp₇₄₅ 273°C; bp₇ 98-100°C; d²⁰₄ 0.953; n²⁰_D 1.5058; [α]²⁰_D + 127° (C = 13.1 in CCl₄).

l-form : Liquid, bp₁₀ 104-105°C; d²⁰₄ 0.9645; n²⁰_D 1.5069; [α]²⁰_D – 146°.

Identification It forms oxime (C₁₀H₁₅NO) known as: 1-perillaldehyde-α-syn-oxime; perillartine; “perilla sugar”. Previously, it was commonly referred to as 1-perillaldehyde-α-anti-oxime. The needles have mp 102°C, UV max (alcohol). 232 nm (ε 21800). It is about 2000 times as sweet as sucrose.

Uses

1. The oxime is used as a sweetening agent in Japan.

2. Safranal

Chemical Structure 2,6,6-Trimethyl-1,3-cyclohexadiene-1-carboxaldehyde; (C₁₀H₁₄O).

Occurrence It is the chemical constituent obtained from the dried stigmas and tops of the styles of Crocus sativus (Fam; Iridaceae); The fresh drug comprises of protocrocin which upon drying undergoes decomposition to yield one mole of crocin (a coloured glycoside) and two moles of picrocrocin (a colourless bitter glycoside). It is the latter that on hydrolysis gives rise to safranal which is solely responsible for attributing the characteristic odour of the drug (saffron). These transformations are as given below:

Isolation The stigmas and tops of the styles of C. sativus are first dried under shade and then

subjected to hydrolysis in controlled conditions to yield safranal. The resulting product is treated with pure sodium bisulphite to obtain the safranal sodium bisulphite adduct from which the desired product is regenerated by treatment with dilute alkaline solution.

Characteristic Features It is a liquid having a pleasant characteristic odour. It has the following physical properties namely:

bp_1.0 70°C (bath temparature); d¹⁹₄ 0.9734; n¹⁹_D 1.5281. It is freely soluble in organic solvents like: methanol, ethanol, petroleum ether and glacial acetic acid.

Uses It is employed as a flavouring agent in confectionery products.

3. Phellandral

Chemical Structure 4-Isopropyl-1-cyclohexen aldehyde; (C₁₀H₁₅O).

Occurrence It was first and foremost found in the essential oil of Phellandrium aquaticum. It also occurs in the essential oil obtained from the flowers of lavender (Santolina chamaecyparissus L., family: Asteraceae) and several Eucalyptus species.

Isolation It is usually isolated through its sparingly soluble crystalline sodium bisulphite compound, from which phellandral is generated by treatment with dilute alkaline solution carefully.

Characteristic Features It is an oil having an odour very much reminiscent of cuminaldehyde. It undergoes rapid oxidation either on exposure to air or with silver oxide to give rise to phellandric acid (mp 144-145°C)

Identification It may be identified by preparing its corresponding derivative, such as: oxime, semicarbazone and phenyl hydrazine etc.

Uses Due to its resemblance of odour of cuminaldehyde it finds application in perfumery.

B. Bicyclic Terpene Aldehydes

Example: Myrtenal.

Chemical Structure

It is a naturally occurring oxygenated pinane derivative.

Occurrence The leaves of boldo contains essential oil to the extent of 2%* (Peumus boldus Molina, family: Monimiaceae).

Isolation The aldehyde is successfully isolated by preparing its sodium bisulphite adduct first and then regenerating it by treatment with mild alkaline solution cautiously.

Uses

1. The aromatic leaves are frequently used as mild diuretic, especially in liver ailments like jaundice.

2. It is also recommended for urogenital inflammations e.g.; gonorrhea in Latin America

-----------------------------------------------------------------------------

* Bruns, K., and Kohler, M, Uber die Zusammensetzung des boldoblatterols, parf kosm, 55, 225, 1975