The genus Cinnamomum

Chemistry of Cinnamon and Cassia 91
synthesised elsewhere in the plant and transported to the sites of lignification.
Experimental evidence has shown that the stem (heartwood) has a low content of
cinnamic aldehyde as well as eugenol. This is as expected since free eugenol and cinnamic
acid would have been converted to non-volatile lignin.
Both cinnamic aldehyde and eugenol are relatively insoluble in water, hence they
have to be converted to water-soluble glucosides for transportation within the plant
(Senanayake, 1977; Senanayake et al., 1977). There was a high content of these glucosides
in the leaves in comparison to other parts of the plant. Glucoside formation was
related to photosynthesis. This fact indicated that cinnamic aldehyde and eugenol
should be in a site more accessible to the photosynthetic glucose present in the leaves,
so that they could be converted to glucosides. The petiole-dip method clearly demonstrated
that detached leaves were capable of forming these compounds; the bark had
a low incorporation of precursors into these compounds when cuttings were used. This
indicated that bark contributed very little to the biosynthesis of these two compounds.
No attempts were made by Senanayake and collaborators (1977) to isolate lignin
and determine the labelling pattern (this would indicate the precursor of the lignin),
however, the data in this study pointed to the possibility that both cinnamic aldehyde
and eugenol were formed in the leaves. Weissenbôck et al. (1971) and Loffelhardt and
Kindle (1975), in similar biosynthetic studies with PA, have shown that chloroplasts
and thylakoid membranes are capable of converting PA into cinnamic acid and related
products. The latter authors have also indicated that the chloroplast could be the
predominant, if not exclusive, site of benzoic acid formation in Astilbe chinensis.
Biosynthesis of phenylpropanoids
Both eugenol and cinnamic aldehyde belong to the group of compounds consisting of a benzene
ring with a propane side branch, viz. C6-C3. The side branch may have some degree
of unsaturation and the benzene ring may or may not contain some functional groups.
The origin of the aromatic ring of many natural phenylpropanoid compounds is now
regarded to be the cyclohexane derivatives that arise by the cyclisation of sedoheptulose,
a C7 sugar molecule (Neish, 1960; Geissman, 1963). The key compound in this biosynthetic
scheme is the now well recognised shikimic acid.
The shikimic acid pathway was established largely from the experimental work of
Davis and Sprinson, which has been exhaustively reviewed by Davis (1955, 1958),
Sprinson (1960), Neish (1960, 1968), and Bohm (1965). The key to this scheme was
the discovery by Davis (1951) that a requirement of a mutant strain of Escherichia coli
for five aromatic compounds – phenylalanine, tyrosine, tryptophan, p-aminobenzoic
acid and p-hydroxybenzoic acid – could be completely satisfied by the single compound,
shikimic acid. Thus shikimic acid was established as an obligate intermediate for the
biosynthesis of aromatic rings in E. coli.
Both cinnamic aldehyde and eugenol are formed through the Shikimic acid pathway
leading to lignin. The exact point of cinnamic aldehyde and eugenol formation,
however, is unclear. It would appear that cinnamic aldehyde should be formed by a single
step reduction of cinnamic acid. A further step reduction would yield cinnamic
alcohol, which could contribute towards the formation of lignin.
In species such as Cinnamomum, it may well be that a genetic block occurs at the
conversion of cinnamic aldehyde to cinnamyl alcohol, and as a result there can arise
an accumulation of cinnamic aldehyde. Birch (1963) suggested the loss of ability to