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.
^Green, Brian; Bentley, Michael D.; Chung, Bong Y.; Lynch, Nicholas G.; Jensen, Bruce L. (2007), Isolation of Betulin and Rearrangement to Allobetulin A Biomimetic Natural Product Synthesis, J. Chem. Educ.84: 1985, doi:10.1021/ed084p1985.
^Gao, Y; Xu, H; Lu, Z; Xu, Z (2009). "Quantitative determination of steroids in the fruiting bodies and submerged-cultured mycelia of Inonotus obliquus". Se pu27 (6): 745–9. PMID20352924.
^Tilford, Gregory L. (1997), Edible and Medicinal Plants of the West, Missoula, MO: Mountain Press, ISBN0-87842-359-1.