Presence in various plants: Stransky, Arch. Pharm. 258, 56 (1920); Ramstad, Pharm. Acta Helv. 28, 45 (1953). Structure: Verkade, Recl. Trav. Chim. Pays-Bas 43, 879 (1924). Synthesis: Riegel, Reinhard, J. Am. Chem. Soc. 48, 1334 (1926); Riegel, Zwilgmeyer, Org. Synth. coll. vol. II, 126 (1943); Toomey, Riegel, J. Org. Chem. 17, 1492 (1952).