Recent Advances in the Chemistry of the Terpenoids

Abstract

Terpenoids are ubiquitous natural products that are characterised by a carbon skeleton which is assembled from isoprene units. Despite this common building block there are a wide range of structural types, many of which are chiral. They include compounds with a variety of biological activities. Consequently the terpenoids are of considerable interest in phytochemical, synthetic, biological and environmental research. Current studies include the elucidation of the structures of novel terpenoids particularly from plants that have been used in folk medicine. Since some terpenoids are readily available, their structures and in particular their chirality, make them useful starting materials for the synthesis of rarer natural products not only to confirm the structure of the latter but also to explore structure-biological activity relationships.Articles published in the Journal of Chemical Research between January 2012 and June 2015, exemplify a wide variety of studies in terpenoid natural product chemistry. Reviews have been published on the aromatisation of terpenoids and steroids by dehydrogenation 1 and on the use of the readily available diterpenoid, manool in the synthesis of sesquiterpenoids and diterpenoids. 2,3 Research papers are grouped into those concerning monoterpenoids,sesquiterpenoids, diterpenoids and triterpenoids. The monoterpenoid camphor has been the source of many useful chiral auxiliaries and building blocks. A new camphor-derived chiral auxiliary and its application to the asymmetric Darzens reaction has been reported 4 whilst an environmentally friendly preparation of 3-bromocamphor and camphorquinone has been described. 5 The role of endophytic micro-organisms and their metabolites in the ecological chemistry of their host plants is a subject of considerable interest. Novel sesquiterpenes with a tremulane carbon skeleton including ceriponol P, have been isolated 6,7 from the fungus Ceriporia lacerate which is an endophyte in Huperzia serrata, the clubmoss that is the source of the alkaloids huperzine A and B which are used for the treatment of age-related memory loss. Sesquiterpenoid lactones are characteristic of the Compositae (Asteraceae). A new guaianolide glycoside has been isolated 8 from the roots of Pterocypsela elata.Over 500 tetracyclic diterpenoids, many of which have tumour-inhibitory properties, have been obtained from Isodon (Lamiaceae) species. These plants are widely used in traditional Chinese medicines. Further examples of these diterpenoids have been reported from I. lophanthoides, 9 I. japonicus 10 and I. serra. 11 Readily available diterpenoids have been used as carriers for pharmacophores in the treatment of malaria 12 and cytotoxic components such as chlorambucil. 13 The synthesis of rosmic acid derivatives from the more readily available rosmanol and carnosol has been described 14 in the context of their biological activity. Andrographolide is a labdane whose biological activity as an anti-cancer agent is attracting some interest. An unexpected elimination reaction has ...