Alexander Degen

Two diastereomers of 9-(2,6-di­chloro­phenyl)-4a-hydroxy-3,3,6,6-tetra­methyl-1,2,3,4,4a,5,6,7,8,9a-deca­hydro­xanthene-1,8-dione in the same crystal

The reaction of dimedone with 2,6-dichlorobenzaldehyde leads to the title compound, C(23)H(26)Cl(2)O(4). In principle, the reaction could yield eight different stereoisomers. We have found four of them in the same crystal as two enantiomeric pairs of diastereomers, which means that the asymmetric unit is built up of two different diastereomers. Two of the three chiral centres display the same configuration, while the third is different in the two molecules in the asymmetric unit. The packing of the molecules is stabilized by hydrogen bonds between the hydroxy group and the carbonyl group attached to the cyclohexene ring, forming chains in which the different diastereomers alternate.

Diels–Alder adducts of maleic ­anhydride and dienes: new compounds by crystallization

We have determined the crystal structures of bicyclo[2.2. 1]hept-2-ene-endo-cis-5,6-dicarboxylic anhydride, C(9)H(8)O(3), (I), 1,2,3,4,7,7-hexachlorobicyclo[2.2.1]hept-2-ene-endo-cis-5, 6-dicarboxylic anhydride diethyl ether solvate, C(9)H(2)Cl(6)O(3).0. 16C(4)H(10)O, (II), bicyclo[2.2.1]hept-2-ene-endo-cis-5, 6-dicarboxylic acid, C(9)H(10)O(4), (III), 1,2,3,4,7, 7-hexachlorobicyclo[2.2.1]hept-2-ene-endo-cis-5,6-dicarboxylic acid, C(9)H(4)Cl(6)O(4), (IVa) and (IVb), and ethyl 1,2,3,4,7, 7-hexachloro-6-carboxybicyclo[2.2. 1]hept-2-ene-endo-cis-5-carboxylate monohydrate, C(11)H(8)Cl(6)O(4). H(2)O, (V). Compounds (I) and (II) were prepared by a standard Diels-Alder reaction from maleic anhydride and cyclopentadiene or hexachlorocyclopentadiene, respectively. The crystal-growing processes of these compounds led to surprising results: rapid recrystallization of (I) from diethyl ether and (II) from petroleum ether gave crystals of these compounds, however, crystallization by slow evaporation techniques using common solvents yielded new compounds in which the five-membered heterocycle has been cleaved.

2,2′‐[(3‐Hydro­xy­phenyl)­methyl­ene]­bis(3‐hydroxy‐5,5‐di­methyl‐2‐cyclo­hexen‐1‐one) hydrate

The title compound is a bis-dimedone derivative which crystallizes as a monohydrate, C23H28O5·H2O. Its crystal packing is stabilized by hydrogen bonds involving water.

The twinned crystal structure of 3-indolyl­acetic acid

Crystals of the title compound, C10H9NO2, whose structure has already been determined four times by different research groups, were found to be twinned. Satisfactory refinement is only possible if the twinning is taken into account. This kind of twinning in the monoclinic crystal system can, in principle, occur if the following condition is met: a·|cosβ| = c/2, i.e. if the short diagonal of the ac plane is of the same length as a or c itself.

4-Phenyl­butan-2-one semi­carbazone

The title compound, C(11)H(15)N(3)O, crystallizes with two molecules in the asymmetric unit, which are held together by an extended network of hydrogen bonds. It is remarkable that only five of the six theoretically possible hydrogen bonds are formed.