Sesquiterpenoids as chemosystematic markers in the subtribe Hypochaeridinae (Lactuceae, Asteraceae)

Sesquiterpene lactones are well established as chemosystematic markers in the Asteraceae family. From the Lactuceae tribe of the Asteraceae family a large number of sesquiterpene lactones - mainly of the guaiane type - have been isolated. One of the 11 subtribes of the Lactuceae recognized by Bremer is the subtribe Hypochaeridinae, which encompasses 10 genera with approximately 170 species. The present communication summarizes the sesquiterpene derivatives reported from these 10 genera, points out, which constituents are characteristic for particular groups, and discusses the occurrence of these secondary metabolites in a chemosystematic context. To this end, each of the reported sesquiterpenoids reported for the Hypochaeridinae is classified into one of three main compound classes (MCCs; eudesmane-, germacrane-, guaiane-derivatives) and into one of a number of compound classes (CCs) within these principal groups. The distribution of sesquiterpenoids belonging to these particular classes of sesquiterpenoids generally follows the currently accepted generic limits. However, the genus Helminthoteca, which is included into Picris by many authors, possesses an array of compounds implying a closer relationship to the genus Hypochaeris than to the remainder of the genus Picris. Furthermore, Leontodon subgenus Oporinia shows closer similarity in secondary metabolite patterns to the genus Picris than to Leontodon subgenus Leontodon. On the other hand Leontodon subgenus Leontodon has more chemical characters in common with the genus Hedypnois than with Leontodon subgenus Oporinia. These findings are in-line with recent results of molecular analyses, which imply that current generic limits within the Hypochaeridinae might not reflect the phylogeny of the subtribe. Furthermore, a simple hierarchical clustering approach for the reproducible exploration of phytochemical literature data is proposed.