2-(1-Amino-4-tert-butylcyclohexyl)acetic acid (tBu-β3,3-Ac6c) hemihydrate1

The title compound, C12H23NO2·0.5H2O, crystallized with two 2-(1-amino-4-tert-butylcyclohexyl)acetic acid molecules, which are present as zwitterions, and one water molecule in the asymmetric unit. The molecular structure of each zwitterion is stabilized by an intramolecular six-membered (C 6 ) N—H⋯O hydrogen bond. In the crystal, the two independent zwitterions are linked head-to-head by N—H⋯O hydrogen bonds. Further O—H⋯O and N—H⋯O hydrogen bonds link the zwitterions and the water molecules, forming sandwich-like layers, with a hydrophilic filling and a hydrophobic exterior, lying parallel to the ab plane.

The title compound, C 12 H 23 NO 2 Á0.5H 2 O, crystallized with two 2-(1-amino-4-tert-butylcyclohexyl)acetic acid molecules, which are present as zwitterions, and one water molecule in the asymmetric unit. The molecular structure of each zwitterion is stabilized by an intramolecular six-membered (C 6 ) N-HÁ Á ÁO hydrogen bond. In the crystal, the two independent zwitterions are linked head-to-head by N-HÁ Á ÁO hydrogen bonds. Further O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds link the zwitterions and the water molecules, forming sandwich-like layers, with a hydrophilic filling and a hydrophobic exterior, lying parallel to the ab plane.
The crystal structure shows a six membered (C 6 ) NH···O intramolecular hydrogen bond between NH and the carbonyl group of tBu-β 3,3 -Ac 6 c. In the structure the cyclohexane ring adopts a chair conformation with equatorial tert-butyl and amino groups. The carboxymethyl group occupies the axial position. Fig. 3 shows the packing of molecules in the crystal.
Intermolecular O···HO and NH···O hydrogen bonds stabilize the structure leading to the formation of hydrophobic and hydrophilic layers as shown in Fig. 3.

Experimental
To a solution of 4-tert-cyclohexanone (7.70 g, 50 mmol), malonic acid (5.20 g, 50 mmol) in 100 ml of ethanol was added 11.55 g (150 mmol) of ammonium acetate. The reaction mixture was refluxed for 24 h. After completion of the reaction, the reaction mixture was allowed to cool to room temperature and ethyl alcohol was evaporated under vacuum. The residue was triturated with acetone (3 x 50 ml) and dried to yield a white solid (Yield 6.5 g, 61%). M.P. 265-267°C.
Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a methanol/water (8:2) mixture.

Refinement
H atoms were located in a difference Fourier map and both their coordinrates and U iso were refined.  The molecular structure of the compound. The thermal ellipsoids are drawn at the 40% probability level. H atoms are shown as small spheres of arbitrary radii.

Figure 2
The packing arrangement of molecules viewed down the a-axis. Hydrogen bonds are shown in dotted lines.  The space filling model showing the alternative hydrophobic and hydrophilic layers in crystal lattice. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.21 e Å −3 Δρ min = −0.17 e Å −3 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 R-factors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq