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PACIFIC SCIENCE, Vol. XXII, July 1968 
Non-aromatic double bonds: 3 
Theoretically possible variants: 8 
not hydrogena ted : 1 
mono hydrogenated 
(A 3 or A5 or Ay) : 3 
dihydrogenated 
(A 3 5 or A 3 7 or A 5 7).' 3 
fully hydrogenated: 1 
Naturally occurring 
(not hydrogenated, A 5 , A 5 7 ) : 3 
Fig. 3. Hydrogenation stages of the kawa lactones. 
methysticum (Klaproth, 1966). It is interesting 
that in both series of the unsubstituted lactones 
and of the p-methyl substituted ones we find 
three degrees of hydrogenation. In the dioxy- 
methylene derivatives one of the stages of 
hydrogenation is missing. It is the same one 
which, as the only representative of the series, 
occurs as a dimethoxy derivative. We do not 
know whether this vicarious occurrence of 
dimethoxy- and dioxymethylene, or for that 
matter the entire a-pyrone spectrum of the 
plant, is the result of evolutionary coincidence, 
or whether some day we shall be able to under- 
stand this development on causal biosynthetic 
grounds. However, let us not linger over fruit- 
less speculations regarding the origin of the 
pyrone spectrum, let us rather ask in which way 
the plant constructs these substances. I shall not 
be able to offer direct proof for definite bio- 
synthetic pathways since tracer studies or dy- 
namic biochemical studies have so far not been 
carried out. The ideas of the biosynthesis of 
the kawa pyrones which I shall discuss are 
based on a comparative study of molecular 
structure and on reasoning by analogy which 
has derived justification from the fundamental 
agreement in the metabolism of all green plants. 
IDEAS ON THE BIOSYNTHESIS OF THE kdWd 
pyrones: Let us consider briefly the structure of 
the kawa pyrones from a biogenetic point of 
view. It is striking that the benzene ring is 
substituted in the manner in which we know it 
from the phenylpropyl compounds (cinnamic 
acids, lignanes, and coumarins). The synthesis 
of these C 6 -C 3 -compounds goes back to shiki- 
mic and prephenic acids. The remaining four 
carbon atoms in the molecule of the kawa 
pyrones show two alternating oxygen functions, 
a feature which is characteristic of substances 
whose biosynthesis indicates polyacetate chains, 
that is, acetate metabolism. We thus arrive at 
the picture that the kawa pyrones are an example 
of so-called mixed formation. They are formed 
from phenylpropanes and from acetyl coenzyme 
A building blocks and, if one considers num- 
bers, from one phenylpropane and two acetate 
units: 
C 6 — C 3 -f- 2 C 2 > C 13 ( kawa pyrones) 
Such a scheme is outlined in Figure 4. Natural 
products which demonstrate such a mixed con- 
struction from phenylpropanes and acetates are 
no rarity in the plant kingdom. The most im- 
portant representatives are the flavonoids, which 
are made up of one phenylpropane unit and 
three acetate units (Geissman and Hinreiner, 
1952). 
C 6 -C 3 — f- 3 C 2 > C 15 (flavonoids) 
The kawa pyrones therefore appear to be 
nothing but variants of flavonoids. Of course 
this holds only if one considers the metabolic 
physiological and not the analytical chemical 
point of view, since no flavonoid-like C 15 - 
compounds are known with a-pyrone structure. 
Flavonoids and kawa pyrones seem to have a 
common precursor. Kawa pyrones appear to be 
precursors of flavonoids with one less acetate 
unit. It is very remarkable that it was possible 
to discover in the kawa plant the pyrones which, 
so to speak, correspond to flavonoids, C 13 - 
compounds which are analogs of C 15 -compounds 
(Fig. 5) (Hansel et al., 1963). 
Up to this point our biosynthetic scheme 
assigns to the kawa lactones a given position in 
the flavonoid metabolism of plants. It does not 
explain why only certain degrees of hydrogena- 
tion occur in nature. The following biosynthetic 
scheme combines the observation on the occur- 
rence of certain hydrogenation types and places 
