Methyl 3,5,5,6,8,8-hexamethyl-5,6,7,8-tetrahydronaphthalene-2-carboxylate (AHTN–COOMe)

Crystals of the title compound, C18H26O2, were grown from ethyl acetate. Due to the racemic precursor, the title compound is also obtained as a racemate. Disorder was observed during structure refinement, originating from two possible half-chair conformations of the non-aromatic ring. The disorder was refined by introducing split positions in the cyclo-hexane ring regarding the two possible R and S-enantiomers at the chiral CH group [ratio 0.744 (3):0.256 (3)]. The crystal structure features pairs of inversion-related molecules connected by pairs of non-classical C—H⋯O hydrogen bonds.


D-HÁ
The title compound was found in human breast milk and piscine fatty tissue by Valdersnes et al. (2006) proofing its ubiquitary occurrence.
The molecule crystallizes in the monoclinic space group P2 1 /n. The molecular structure of the compound and the atomlabeling scheme are shown in Fig 1. The structure is disordered in the non aromatic ring. This disorder can be described as pseudo mirror-symmetric with respect to the aromatic ring's plane, resulting in two moieties (ratio 0.744 (3):0.256 (3)).
Each molecule is surrounded by three next neighbors, whereas the centroids of the molecules are arranged in sheets parallel to the (202) plane.
A detailed description of the disorder treatment can be found in the refinement section. The molecules form pairs via non classical hydrogen bonds (C18-H26···O2) (see dashed green bonds in Fig. 2).

Refinement
The structure exhibits disorder originating from two possible half chair conformations in the non aromatic ring. The significant disorder around the atoms of the non aromatic ring was taken into account and the refinement was improved by introducing split positions for the atoms C10, C11, C13, C14, C15, C16 and C17. Equivalent bond distances within the two moieties were restrained to be the same within a standard deviation of 0.02Å, and equivalent disordered atoms were constrained to have identical ADPs. Refinement of the occupancy ratio converged to a value of 74.4 (3)% for the major and 25.6 (3)% for the minor moiety, respectively.
Hydrogen atoms were placed in calculated positions with C-H distances of 0.98 (CH 3 ), 0.99 (CH 2 ), 1.00 (CH sat ), and 0.95 (CH arom ) with U iso (H) = 1.2 of the parent atom U eq or 1.5 U eq (C methyl ). Fig. 1. : Left: ORTEP representation of the title compound with atomic labeling shown with 30% probability displacement ellipsoids. Right: Atoms belonging to the minor disordered moiety and their bonds are presented in light gray and as thin black lines respectively. Hydrogen atoms are omitted for the sake of clarity.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds