Crystal structures of the two epimers from the unusual thermal C6-epimerization of 5-oxo-1,2,3,5,5a,6,7,9b-octa-hydro-7,9a-ep-oxy-pyrrolo-[2,1-]iso-indole-6-carb-oxy-lic acid, 5a(),6(),7(),9a(),9b() and 5a(),6(),7(),9a(),9b().

The isomeric title compounds, CHNO (I) and CHNO (II), the products of an usual thermal C6-epimerization of 5-oxo-1,2,3,5,5a,6,7,9b-octa-hydro-7,9a-ep-oxy-pyrrolo-[2,1-]iso-indole-6-carb-oxy-lic acid, represent the two different diastereomers and have very similar mol-ecular geometries. The mol-ecules of both compounds comprise a fused tetra-cyclic system containing four five-membered rings (pyrrolidine, pyrrolidinone, di-hydro-furan and tetra-hydro-furan), all of which adopt the usual envelope conformations. The dihedral angle between the basal planes of the pyrrolidine and pyrrolidinone rings are 14.3 (2) and 16.50 (11)°, respectively, for (I) and (II). The nitro-gen atom has a slightly pyramidalized geometry [bond-angle sum = 355.9 and 355.3°, for (I) and (II)], respectively. In the crystal of (I), mol-ecules form zigzag-like hydrogen-bonded chains along [010] through strong O-H⋯O hydrogen bonds and are further linked by weak C-H⋯O hydrogen bonds into complex two-tier layers parallel to (100). Unlike (I), the crystal of (II) contains centrosymmetric cyclic hydrogen-bonded dimers [graph set (14)], formed through strong O-H⋯O hydrogen bonds and are further linked by weak C-H⋯O hydrogen bonds into ribbons extending across [101].