Fixation of natural furanoeremophilane by Diels-Alder reaction

Diels-Alder reaction of furanoeremophilan-6β-ol (petasalbin) and its synthetic analogue, 4-hydroxy-4,5,6,7-tetra-hydrobenzofuran, was studied using N-ethyl- and N-phenylmaleimides as the dienophiles, affording corresponding adducts as mixtures of stereoisomers. The adduct of petasalbin and maleimide was also obtained when the latter compound was added to a crude extracted solution of Petasites japonicus var. giganteus. By using this method, it was estimated that the contents of unstable furanoeremophilan-10β-ol in L. cymbulifera is more than 32% of the extract.