Kawa Constituents — Hansel 
303 
TABLE 4 
Some Diagnostic Tests to Distinguish Between 
Tautomeric a- and y-Hydroxypyrone 
Methyl Ethers 1 
TEST 
4-methoxy- 
Y-pyrone 
2-methoxy- 
y-PYRONE 
Basicity 
(a) ether-insoluble 
oxonium salt 
formation 
+ 
(b) picrate formation 
— 
+ 
IR carbonyl frequency 
(a) 1724 cm -1 
( a-pyrone ) 
+ 
(b) 1667 cm -1 
(y-pyrone) 
— 
+ 
Diels-Alder reaction 2 
+ 
— 
1 Chmielewska et al., 1958; Bullock and Smith, I960. 
2 Alder and Rickert, 1937. 
chose a y-pyrone formulation for the following 
reasons. 
By the action of alkali under mild conditions 
yangonin can be saponified to an acid and 
methanol, analogous to a methyl ester of a 
carboxylic acid. Kawain, on the other hand, the 
constitution of which was secured as an a- 
pyrone, behaved differently. Under analogous 
yangonin 
OH 
Ar-CH=CH 
OH ^OH 
ester of yangona acid.” Borsche’s ideas are 
summarized in Figure 8. If we consider Bor- 
sche’s yangonin formula in some detail, we are 
rather surprised that the following points did 
not concern him. 
(a) Ring opening with alkali is generally 
effected more easily with a-pyrones than with 
y-pyrones. 
(b) Borsche’s yangonin formula has the 
structural characteristic of an enol acetal. Acetals 
are generally more resistant to alkali than are 
esters and vinylogous esters (enol ethers). It 
appears to me that it would have been possible 
to arrive at the opposite conclusion that the 
alkali liability of the ring demands formulation 
of an a-pyrone. 
In 1954 a Polish team reopened the question 
of the structure of yangonin. They systematic- 
ally synthesized a-pyrones and the analogous 
y-pyrones and investigated both types of com- 
pounds with IR and UV spectroscopy. IR 
spectroscopy in particular proved to be an ex- 
cellent aid for distinguishing between the two 
isomeric pyrones. All y-pyrones showed a 
carbonyl band at 6.0p (1667 cm -1 ), while the 
a-pyrone band appears at 5.8p (1724 cm -1 ), 
which is the characteristic band of unsaturated 
lactones. 
These investigations made it apparent that 
the pyrones which occur in the kawa rhizome 
differ from one another by their substituents 
and by their degree of hydrogenation, and not, 
Ar-CH=CH 
kawain 
* C " 3 °Vh- 
•OH- 
^ Ar-CH=CH 
0CH 3 
,0 
OH 
conditions kawain furnished no methanol, nor 
did kawaic acid which was produced by action 
of alkali. Borsche attempted to explain this 
striking difference in the behavior of kawain 
and yangonin in the following manner. Since 
the enol-ether linkage of kawain is not saponifi- 
able, yangonin, which splits off methanol readily, 
cannot contain an enol-ether linkage. He there- 
fore searched for a formula of yangonin which 
would combine an ester-like bound methanol 
with a cyclic structure (differing by one mole of 
water from an open chain ester). Borsche for- 
mulated yangonin as a y-pyrone, which he also 
considered to be the "anhydride of the methanol 
OH 
Ar-CH'=CH-i[ Cf° 
OH ^0H 
yangona acid 
ch 2 n 2 
OH 
Ar -CH=CH — k Cy 
OH 0CH 3 
methyl yangonate 
+ CH3OH 
0H~ 
Ar-CH=CH— IL c^' 0CH 3 
OH ^OH 
-H 2 0 
Ar CH-CH -v. 0 ^oCH 3 
Borsche's yangonin 
Fig. 8. Borsche’s argument for the structure of 
yangonin. 
