Structure of Enhydrin, a Germacranolide from Enhydra fluctuans Lour.

Abstract

Based on spectral and chemical evidence, enhydrin (C₂₃H₈₈O₁₀, m.p. 185-6° M⁺ 464), a germacranolide isolated from the stem and leaves of Enhydra fluctuans Lour., has been shown to have the structure (IV). The lactone grouping together with three ester functions account for nine oxygen atoms. The tenth oxygen atom is involved in epoxide formation. The placement of the γ-lactone ring with respect to the epoxide and the two acyl groups has been decided on the spin-decoupling experiments. Selective hydrolysis of enhydrin and dihydroenhydrin confirms the presence of the acetoxyl at G-5 and α-methyl-α,β-expoxybutyryl residue at G-6. Enhydrin resembles in many respects (TLC, IR and NMR) with uvedalin epoxide isolated earlier from Poly-mnia uvedalia Linn., but differs in melting point. Dihydroenhydrin is however, different from di-hydrouvedalin epoxide. This is expected as these are epimeric. (24S)-Ethyl-cholesta-5,22,25-triene-3-β-ol (I) has also been isolated from this plant.