Molecular structure of two derivatives of the germacranolide sesquiterpene lactone melnerin

Abstract

Two biomodified esters (1) and (2) of a new germacranolide sesquiterpene lactone, melnerin, have been isolated from the leaves of Melapodium cinereum. Their structures have been characterized by ¹H and ¹³C n.m.r. spectroscopy, and gas chromatography–mass spectrometry of trimethylsilyl-derivatives, and single-crystal X-ray diffraction. Melnerin A, (1), C₂₀H₂₈O₇, with an isobutyrate side-chain, and melnerin B, (2), C₂₁H₃₀O₇ with a 2-methylbutyrate side-chain, co-crystallize in ca. 4 : 1 ratio, in the orthorhombic space group P2₁2₁2₁, with a= 9.322(2), b= 11.655(2), c= 19.032(4)Å, Z= 4. The crystal structure, which is disordered in the ester side-chain, was solved by direct phasing procedures and refined by weighted full-matrix least-squares to R 8.2% 1 711 reflections. The Δ¹⁽¹⁰⁾-cis-germacrene ring has the same conformation as cis-cyclodecene. Substituents at C(4)(hydroxymethylene) and C(10)(methoxycarbonyl) are syn and α-oriented, while the ester group attached to C(8) is β-oriented. The α-methylene-γ-lactone, trans-fused at C(6) and C(7), is non-planar with right-handed chirality. The absolute configuration is assigned from biogenetic considerations and correlation with known structures.