Abstract

A series of five compounds containing the bicyclo-[3.3.0]octa-2,6-diene skeleton are described, namely tetra-methyl cis,cis-3,7-dihydroxy-bicyclo-[3.3. 0]octa-2,6-diene-2,4-exo,6,8-exo-tetracarboxyl-ate, C16H18O10, (I), tetra-methyl cis,cis-3,7-di-hydr-oxy-1,5-dimethyl-bicyclo-[3.3.0]octa-2,6-diene-2,4-exo,6,8- exo-tetra-carboxyl-ate, C18H22O10, (II), tetra-methyl cis,cis-3,7- dimethoxybicyclo-[3.3.0]octa-2,6-diene-2,4-exo,6,8-exo-tetra-carboxyl-ate, C18H22O10, (III), tetra-methyl cis,cis-3,7-dimeth-oxy-1,5-dimethyl-bicyclo-[3.3. 0]octa-2,6-diene-2,4-exo,6,8-exo-tetra-carboxylate, C20H26O10, (IV), and tetra-methyl cis,cis-3,7-diacetoxy-bicyclo-[3.3.0]octa-2,6-diene-2,4-exo,6,8- exo-tetra-carboxyl-ate, C20H22O12, (V). The bicyclic core is substituted in all cases at positions 2, 4, 6 and 8 with methoxy-carbonyl groups and additionally at positions 3 and 7 with hydroxy [in (I) and (II)], methoxy [in (III) and (IV)] or acet-oxy [in (V)] groups. The conformations of the methoxy-carbonyl groups at positions 2 and 4 are exo for all five compounds. Each C5 ring of the bicyclic skeleton is almost planar, but the rings are not coplanar, with dihedral angles of 54.93 (7), 69.85 (5), 64.07 (4), 80.74 (5) and 66.91 (7)° for (I)-(V), respectively, and the bicyclooctadiene system adopts a butterfly-like conformation. Strong intra-molecular hydrogen bonds exist between the -OH and C=O groups in (I) and (II), with O⋯O distances of 2.660 (2) and 2.672 (2) Å in (I), and 2.653 (2) and 2.635 (2) Å in (II). The mol-ecular packing is stabilized by weaker C - H⋯O(=C) inter-actions, leading to dimers in (I)-(III) and to a chain structure in (V). The structure series presented in this article shows how the geometry of the cycloocta-2,6-diene skeleton changes upon substitution in different positions and, consequently, how the packing is modified, although the intermolecular interactions are basically the same across the series. © 2008 International Union of Crystallography.