The constituents of phebalium nudum hook-I The bark

A large number of natural essentially non-terpenoid compounds contain isoprene units. The C₅ units may have their origin based on senecioic acid, or mevalonic acid, but the fundamental stage seems to be an -hydroxy-γ,γ-dimethylallyl system, which can undergo a number of modifications giving rise to types of compounds listed below under (A) to (F) in increasing order of complexity. The simpler furan derivatives are also now considered to be derived from C₅ units by the loss of three carbon atoms by oxidation. This is based not only on co-occurrence of types but also on the experimental feasibility of converting the dimethylallyl (C₅) and allyl groups into furans. In this connection, syntheses of angelicin and psoralen are described. Further the furan rings of furanoquinolines should be considered to have a similar origin.