18. 



Heterocycles 



a. FURANS AND RELATED SUBSTANCES 



Apparently there has been no investigation of the biosynthetic 

 origin of the furans listed here, but it is known that furans can 

 be formed in several different ways. 



The relationship of furans to sugars is recognized in the desig- 

 nation of the five-membered ring hemi-acetal form of sugars as 

 the furanose form. Dilute acid converts glucose to 5-hydroxy- 

 methylfurfural. The latter compound may be a precursor of 

 Sumiki's acid, although the transformations are probably enzy- 

 matic. The four carbon atom sugar erythrose also is a likely 

 furan precursor as pointed out by Wenkert.^ 



The furans with carbon chains at the 2-position are obviously 

 terpenoid. Since they were isolated from a sweet potato me- 

 dium, their direct derivation from glucose cannot be assumed. 

 The simpler substances may arise from oxidation of the more 

 complex. 



It is interesting to note that the lactone side-chain of digitoxi- 

 genin is derived from acetate rather than from mevalonic acid.^ 

 Such lactones as well as the related tetronic acids, would seem 



Digitoxigenin 



HO 



to be potential furan precursors. 



1 Ernest Wenkert, Experientia 15 165 (1959). 



^ E. Leete, Seventh Medicinal Chemistry Symposium of the Amer- 

 ican Chemical Society, Kingston, Rhode Island, 1960. 



