Constituents of genus petasites—IV

A group of a new type of sesquiterpene, named bakkenolide-A, B, C, D, and E was isolated from the buds of Petasites japonicus (Sieb. et Zucc.) Maxim. subsp. giganteus (Fr. Schm.) Kitam. (Japanese name, Akita-buki). They have a novel spiro-γ-lactone grouping. The structure and stereochemistry including absolute configuration was established for bakkenolide-A (B-A) (1) on the basis of chemical degradations (Chart 1, 2) and physical measurements....