Kawa Constituents — Hansel 
297 
TABLE 1 
Structural Variants of the Kawa Lactones and Their Occurrence in Nature 1 
it into a hypothetical biosynthetic scheme. In 
this scheme (Fig. 6) the paths which lead on 
one hand to kawain and on the other hand to 
yangonin branch already at the stage of the |3, 5- 
diketocarboxylic acid. Reduction of the 8-keto 
group to an alcohol leads after eye lization to the 
enolic kawain and dihydrokawain. Enolization 
of the 5-keto group, however, followed by 
shikimic or 
prephenic acid 
kawa lactones 
cyclization leads to the dienolic yangonin. In 
order to arrive at the hypothetical (synthetically 
easily accessible, but not occurring in nature) 
7,8-dihydroyangonin derivatives, the long con- 
jugated system of yangonin would have to be 
interrupted. One therefore suspects that hydro- 
genation of the double bond at this point is not 
possible for energetic reasons. In the case of 
/C 
CH 
h 2 
0 
I II 
CH 
C = 0 
OH 
OH 
2 acetyl - Co A 
methyl donator 
Fig. 4. A hypothetical scheme for the biosynthesis of the kawa lactones. 
