Non-holostane aglycones of sea cucumber triterpene glycosides. Structure, biosynthesis, evolution

Triterpene glycosides are known as characteristic metabolites of sea cucumbers (class Holothuroidea, phylum Echinodermata). The most of them have lanostane aglycones containing 18(20)-lactone, i.e. belongs to so-called holostane series. However non-holostane glycosides having no lactone (with the both normal and shortened side chains) or containing 18(16)-lactone function were also found in these invertebrates last a few decades. In addition, some very rare findings of the glycosides containing rearranged lanostane skeleton systems were reported. Probably, these natural products are formed in result of intramolecular aldol condensation of 1,6-diketo precursors as well as by Meinwald rearrangement of 1,2-epoxy precursor or pinacol-pinacolone rearrangement of fully substituted 1,2-diol precursor. Structures, biosynthesis and evolution of non-holostane aglycones of sea cucumber triterpene glycosides are discussed. Biological properties of the glycosides with hon-holostane aglycones are also discussed.