As part of a project studying the secondary metabolites extracted from the Chilean flora, we report herein three new β-agarofuran sesquiterpenes, namely (1S,4S,5S,6R,7R,8R,9R,10S)-6-acetoxy-4,9-dihydroxy-2,2,5a,9-tetramethyloctahydro-2H-3,9a-methanobenzo[b]oxepine-5,10-diyl bis(furan-3-carboxylate), C27H32O11, (II), (1S,4S,5S,6R,7R,9S,10S)-6-acetoxy-9-hydroxy-2,2,5a,9-tetramethyloctahydro-2H-3,9a-methanobenzo[b]oxepine-5,10-diyl bis(furan-3-carboxylate), C27H32O10, (III), and (1S,4S,5S,6R,7R,9S,10S)-6-acetoxy-10-(benzoyloxy)-9-hydroxy-2,2,5a,9-tetramethyloctahydro-2H-3,9a-methanobenzo[b]oxepin-5-yl furan-3-carboxylate, C29H34O9, (IV), obtained from the seeds of Maytenus boaria and closely associated with a recently published relative [Paz et al. (2017). Acta Cryst. C73, 451-457]. In the (isomorphic) structures of (II) and (III), the central decalin system is esterified with an acetate group at site 1 and furoate groups at sites 6 and 9, and differ at site 8, with an OH group in (II) and no substituent in (III). This position is also unsubstituted in (IV), with site 6 being occupied by a benzoate group. The chirality of the skeletons is described as 1S,4S,5S,6R,7R,8R,9R,10S in (II) and 1S,4S,5S,6R,7R,9S,10S in (III) and (IV), matching the chirality suggested by NMR studies. This difference in the chirality sequence among the title structures (in spite of the fact that the three skeletons are absolutely isostructural) is due to the differences in the environment of site 8, i.e. OH in (II) and H in (III) and (IV). This diversity in substitution, in turn, is responsible for the differences in the hydrogen-bonding schemes, which is discussed.
Keywords: Celastraceae; DSC; Maytenus boaria; NMR; crystal structure; dihydro-β-agarofuran; sesquiterpene.