Redetermination of ethyl (3a-cis)-3a,8b-dihydroxy-2-methyl-4-oxo-3a,8b-dihydro-4H-indeno[1,2-b]furan-3-carboxylate monohydrate

The crystal structure of the title compound, C15H14O6·H2O, has been redetermined from single-crystal X-ray data. The structure was originally determined by Peet et al. [J. Heterocycl. Chem. (1995), 32, 33–41] but the atomic coordinates were not reported or deposited in the Cambridge Structural Database. The ethyl substituent is disordered over two sites with refined occupancies of 0.815 (6) and 0.185 (6). The indeno group is almost planar [maximum deviation 0.0922 (14) Å] and makes an angle of 68.81 (4)° with the furan ring. The fused ring molecules are assembled in pairs by intermolecular O—H⋯O hydrogen bonds. The resulting dimers are also hydrogen bonded to the water molecules, forming double-stranded chains running along the a axis.

The crystal structure of the title compound, C 15 H 14 O 6 ÁH 2 O, has been redetermined from single-crystal X-ray data. The structure was originally determined by Peet et al. [J. Heterocycl. Chem. (1995), 32, 33-41] but the atomic coordinates were not reported or deposited in the Cambridge Structural Database. The ethyl substituent is disordered over two sites with refined occupancies of 0.815 (6) and 0.185 (6). The indeno group is almost planar [maximum deviation 0.0922 (14) Å ] and makes an angle of 68.81 (4) with the furan ring. The fused ring molecules are assembled in pairs by intermolecular O-HÁ Á ÁO hydrogen bonds. The resulting dimers are also hydrogen bonded to the water molecules, forming double-stranded chains running along the a axis.
The ethyl substituent of the tile compound is disordered over two sites with refined occupancies of 0.815 (6) and 0.185 (6). The indeno moiety is almost planar, with atoms C11 and C12 deviating by -0.0574 (13) and 0.0922 (14) Å, respectively, from the indeno plane. The angle between the indeno group and the furan ring is 68.81 (4) °.

Experimental
A mixture of ninhydrin (1.78 g) and ethyl acetoacetate (1.27 ml) in molar ratio 1:1 were refluxed in acetone for thirty minutes in presence of Mg/HCl. The reaction mixture was filtered and dried at low pressure. The dried mass was crystallized with solvent system diethyl ether and hexane to give transparent crystals (mp 373-376 K) of title compound (2.68 g).
The melting point was determined on a Kofler block melting point apparatus and is uncorrected.

Refinement
Hydrogen atoms not belonging to the water molecule were placed at calculated positions and refined as riding on their parent atoms, using SHELXL97 (Sheldrick, 2008) defaults [C-H = 0.93 Å, N-H = 0.86 Å and U iso (H) = 1.2U eq (C,N)]. The hydrogen atoms of the water molecule were included in the refinement riding on the O atom with U iso = 1.5U eq (O).
We have chosen to model the positional disorder of the ethyl substituint with two groups. The ethyl C atoms were refined anisotropically with the U ij values restrained to behave isotropically, with the ISOR instruction, and each C atom of one group was given the same displacement parameters as the corresponding atom of the other group with EADP instructions.
The geometries of the two groups were made equivalent with SADI intructions.

Special details
Geometry. All e.s. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.