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)A, 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.