Betulin (lup-20(29)-ene-3β,28-diol) 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.
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^Tilford, Gregory L. (1997), Edible and Medicinal Plants of the West, Missoula, MO: Mountain Press, ISBN0-87842-359-1.