Hydrangdione C, a novel triterpenoid with an unprecedented skeleton from Salvia hydrangea DC. ex Benth

The genus Salvia is a rich source of terpenoids with structural diversity. Apart from sesterterpenoids as unusual constituents of Salvia species [1,2], it is a source of di- and triterpenoids with unprecedented carbon skeletons [3]. Aiming at identifying structurally interesting and bioactive metabolites from the Salvia species, we examined the extract of Salvia hydrangea DC. ex Benth. In our search for new bioactive natural products, a novel triterpenoid (hydrangdione C, 1) was isolated from the hexane extract of this plant. The skeletal type displayed by hydrangdione C was noticeable for its unusual carbon ring skeleton with a unique five membered ring D substituted by an acetyl group. This is the first report of a natural triterpenoid with a five-membered ring D. The structure of 1 was established by comprehensive 1D NMR, 2D NMR, and HRMS spectroscopic analysis and subsequently confirmed by a single-crystal X-ray diffraction study.

Figure 1: Hydrangdione C

A plausible biosynthetic pathway for 1 was proposed. The formation of 1 could be explained through a pathway including a [3 + 2] cycloaddition type reaction between a monoterpenic unit (trans-β-ocimene) and a diterpenic unit (an icetexone precursor) followed by an oxidation reaction.

References: (1) Moghaddam FM et al. (2010)J Nat Prod 71: 1601–1605.

(2) Rustaiyan A, Masoud S, Anaraki MT (2007) Nat Prod Comm 10: 1031–1042.

(3) Xu G et al. (2007) Org Lett 9: 291–293.