A new effect of hydrogen bond formation

Summary

A marked difference in the behaviour of naringin and of butrin towards hydrolysing agents has been noticed previously. Thus naringin yields the flavanone, naringenin, while butrin yields a mixture of the flavanone, butin, and the chalkone, butein. Similar difference in behaviour is also exhibited by the flavanones, naringenin and butin, when treated with mineral acids or alkalis.

Methylation has now been conducted of the glycosides, naringin and butrin and their aglucones naringenin and butin using varying molar proportions of dimethyl sulphate in the presence of anhydrous potassium carbonate and in acetone solution. From the study of the products and also of a number of simpler flavanone derivatives it is clear that the presence of a free 5-hydroxyl gives stability to the pyranone ring. This stability is not found when the 5-hydroxyl is non-existent as in butrin and butin or gets methylated as in some of the methyl ethers of naringin and naringenin and other flavanones; the products are chalkones in these cases.

The special influence of the 5-hydroxyl group in stabilising flavanone structure is attributed to the existence of chelation between this hydroxyl and the carbonyl group of the pyranone ring. The mechanism of the chalkone-flavanone conversion is discussed.