2.2 Phenylpropenes
Phenylpropenes have gained their legitimate cognizance in phytochemistry by virtue of their vital contributions to the volatile oil flavours and aroma of medicinal plants. In general, the phenylpropenesare normally isolated in the volatile oil component of plant tissues, along with the volatile terpenes. It is pertinent to note here that these are evidently lipid-soluble, a distinct deviation from a majority of other phenolic compounds.
A few typical examples of important members of phenylpropenes are, namely:
(a) Eugenol : A major constituent of oil of cloves;
(Section 2.6.1.4.3 Chapter 5)
(b) Anethole: A principle of anise and fennel;
(Section 2.6.5.6A. Chapter 5)
(c) Myristicin: A component of nutmeg;
(Section 2.6.5.6C. Chapter 5)
Synonyms Cinnamal; Phenylacrolein; Cinnamic aldehyde;
Biological Sources It is obtained from ceylon cinnamon oil Cinnamomum verum J.S. Presler (Lauraceae)-Ceylon Cinnamon; Myroxylon balsamum var. Pereirae (Royle) Harms. (Fabaceae)-Balsam of Peru; and Syzygium oromaticum (L.) Merr. & Perry (Myrtaceae)-Clovers, Clavos.
Preparation Cassia oil (Cinnamomum cassia Blume., family: Lauraceae) contains volatile oil (1-2%). This volatile oil contains cinnamaldehyde (80-85%) which is isolated by subjecting it to fractional distillation under vacuo.
Characteristic Features Cinnamaldehyde is a yellowish oily liquid having a strong odour of cinnamon. Its physical parameters are: d2525 1.048-1.052; bp100177.7°C, bp200 199.3°C and bp760 246°C, n20D1.618-1.623. It dissolves in about 700 parts of water and in about 7 volumes of 60% ethanol. It is, however, miscible with ethanol, ether, chloroform and oils.
Chemical Test On addition of a drop of FeCl3 (1% w/v) solution to a few drops of cinnamaldehyde a distinct brown colour is produced.
Uses
1. It is used extensively in the perfume industry.
2. It is employed for flavouring foods and beverages.
Interestingly, it has been observed that the pairs of the allyl (CH2==CH—CH2—) and propenyl (CH3CH=CH–) isomers, such as: eugenol and isoeugenol invariably occur together in the same medicinal plant as stated below:
(i) Cananga odorata (Lam.) Hook. f. & Thoms. (Annonaceae)-Cananaga, Ylang-Ylang; and
(ii) Myristica fragrans Houtt. (Myristicaceae)-Mael, Nutmeg
Note Isomerization of the allyl to the propenyl form may also be accomplished in the laboratory, but only under very drastic and specific experimental parameters i.e., in the presence of strong alkali. However, such isomerization rarely takes place under normal conditions of isolation from natural products, such as: solvent extraction with ether etc.
