Constit. of Corylus avellana (filbert) and Vicia faba Betulin is an abundant naturally occurring triterpene. It is commonly isolated from the bark of birch trees and forms up to 30% of the dry weight of the extractive. The purpose of the compound in the bark is not known. It can be converted to betulinic acid (the alcohol group replaced by a carboxylic acid group), which is biologically more active than betulin itself.; Chemically, betulin is a triterpenoid of lupane structure. It has a pentacyclic ring structure, and hydroxyl groups in positions C3 and C28.