The genus Cinnamomum

92 U.M. Senanayake and R.O.B. Wijesekera
introduce para-oxygen to the ring as a possible explanation for the cinnamic aldehyde
remaining as such. Similarly a two-step reduction of the side-chain of ferulic acid will
yield coniferyl-alcohol. Elimination of the terminal hydroxy group in the side-chain
and re-arrangement of the double bond will yield eugenol. In Cinnamomum species there
may be some enzyme responsible for such a transformation of coniferyl alcohol. Some
further speculations have been made on the basis of mechanistic consideration
(Wijesekera, 1978; Wijesekera et al., 1975).
In view of the structural relationship with lignin, it is logical that cinnamic aldehyde
should be widespread in the plant kingdom. It is found in Ascomycetes,
Basidiomycetes and in many higher plants (Guenther, 1949; Birch, 1963; Zenk and
Gross, 1972; Gottlieb, 1972). Among the commercially important essential oils,
cinnamic aldehyde is found in cinnamon bark and leaf oils, cassia bark and leaf
oils, patchouli leaf oil and myrrh oil (Guenther, 1949). Due, as postulated here, to
a possible block in the pathway (as in the case of a mutation), both cinnamic aldehyde
and eugenol appear to accumulate, especially in cinnamon and in cassia. Gottlieb
(1972) pointed out that the family Lauraceae is at the bottom of the evolutionary ladder
concerning phenylpropanoid biosynthesis and it is therefore reasonable to expect some
anomaly in the shikimic acid pathway leading to lignin formation.
So far no reports have appeared on tracer studies of cinnamic aldehyde biosynthesis
in situ in plants. However, the tracer studies of Kaneko (1960, 1961 and 1962),
Canonica et al. (1971), Manitto et al. (1974, 1975a,b) and Senanayake (1977) have
shown interesting developments in cinnamic aldehyde and eugenol biosynthesis. It is
generally assumed that the allyl and propenyl groups attached to phenolic nuclei in
many plant constituents, such as in anethole, chavicol, estrogole and eugenol, originate
from the cinnamic acid side-chain through the reductive steps as shown in Fig. 3.3
(Birch, 1963; Bullock, 1965).
It now appears that formation of the side-chain may occur through a different mechanism
(Canonica et al., 1971). The difference seems to lie in the position of the double
bond in the side-chain. In cinnamic aldehyde the double bond is between carbons
1 and 2 (Fig. 3.3). In such a situation the phenyl-propane skeleton of L-phenylalanine
is incorporated into the molecule with the retention of all carbon atoms (Kaneko,
1962; Manito et al., 1974). When DL-phenylalanine, labelled with (14C) at the sidechain
in all three possible positions, was fed for example to Pimpinella anisum and
Foeniculum vulgare, the anethole recovered was labelled in the same position as that of
DL-phenylalanine originally fed to the plant.
In a separate study, Canonica et al. (1971) showed that when L-phenylalanine,
labelled with (14C) in all three possible positions in the side-chain, was fed to Ocimum
basilicum L., the eugenol recovered was not labelled when L-phenyl-(1–14 C) alanine
was used. This indicated that the carboxyl group was lost during complete conversion
of the ferulic acid to the allyl group. In their subsequent studies, Manitto et al. (1974,
1975a) showed that the decarboxylation of the side-chain took place at the ferulic acid
stage and an extra carbon atom was introduced to the side-chain, probably donated by
S-adenosyl-methionine or an equivalent compound. Decarboxylation at the ferulic acid
stage was established as labelled ferulic acid and incorporated into eugenol in an appreciable
quantity (Manitto et al., 1975b). Thus it appears that the prophenyl and allyl
side-chains have independent origins.
Furthermore, as in the case of eugenol, the allyl group only occurs when there is
a p-oxygen attached to the ring. Thus Birch’s hypothesis (Birch, 1963) has been rejected