Collective Synthesis of Highly Oxygenated (Furano)germacranolides Derived from Elephantopus mollis and Elephantopus tomentosus

Abstract

Germacranolides, secondary metabolites produced by plants, have garnered academic and industrial interest due to their diverse and complex topology as well as a wide array of pharmacological activities. Molephantin, a highly oxygenated germacranolide isolated from medicinal plants, Elephantopus mollis and Elephantopus tomentosus, has exhibited anti‐tumor, inflammatory, and leishmanicidal activities. Its chemical structure is based on a highly strained ten‐membered macrocyclic backbone with an (E,Z)‐dienone moiety, which is fused with an α‐methylene‐γ‐butyrolactone and adorned with four successive stereogenic centers. Herein, we report the first synthesis of molephantin via 12 steps starting from readily available building blocks. The synthesis features the highly diastereoselective intermolecular Barbier allylation of the β,γ‐unsaturated aldehyde with optically active 3‐bromomethyl‐5H‐furan‐2‐one intermediate and ensuing Nozaki‐Hiyama‐Kishi (NHK) macrocyclization for the construction of the highly oxygenated ten‐membered macrocyclic framework. This synthetic route enabled access to another germacranolide congener, tomenphantopin F. Furthermore, cycloisomerization of molephantin into 2‐deethoxy‐2β‐hydroxyphantomolin could be facilitated by irradiation with ultraviolet A light (λmax = 370 nm), which opened a versatile and concise access to the related furanogermacranolides such as EM‐2, phantomolin, 2‐O‐demethyltomephantopin C, and tomenphantopin C.