3,5,5,6,8,8-Hexamethyl-5,6,7,8-tetrahydro-2-naphthoic acid (AHTN–COOH)

The title compound, C17H24O2, is the product of a haloform reaction of 6-acetyl-1,1,2,4,4,7-hexamethyltetraline (AHTN). The compound is a racemic mixture with a disorder in its aliphatic ring [occupany ratio 0.683 (4):0.317 (4)] due to two possible half-chair forms. The carboxylic acid unit is slightly twisted out of coplanarity with the aromatic system [dihedral angle = 29.26 (6)°]. In the crystal, pairs of short classical intermolecular O—H⋯O hydrogen bonds link pairs of molecules around a center of symmetry.

AHTN itself is a widely used fragrance in cosmetics and cleaning products. Its commonest trade name is Tonalide. The crystal structure of AHTN was shown by De Ridder (De Ridder et al., 1990). AHTN is produced in the ton-scale, in 2000, approx. 343,000 kg (Kupper et al., 2004) and introduced into the environment mainly by sewage treatment plants supplied by municipal wastewater. It can be found in surface water at low µg/L concentration (Heberer, 2003). Due to the low estrogenic potential of AHTN (Bitsch et al., 2002) this might induce a health concern.
The industrial synthesis of AHTN 1 is shown in Figure 2. Both starting chemicals are inexpensive and readily available: para-cymene and neo-hexene, the later results from an olefin metathesis of di-iso-butylene with ethylene (Sell, 2006). AHTN 1 is obtained as racemic mixture in industrial-scale. Hence, the title compound AHTN-COOH 2 is obtained as racemic mixture, too.
The compound (Fig. 3) is crystallizing in the monoclinic space group P21/c. It shows disorder (68:32) within the cyclohexane moiety: the two possible half-chair forms are present.
The compound exhibits a short classical intermolecular H bond which links the molecules into pairs around a center of symmetry ( Fig. 4). A summary of these interactions is compiled in Table 1.

Experimental
NaOCl was added to a solution of racemic AHTN in acetonitrile. The mixture was stirred for 72 h at room temperature.
Afterwards water was added to dissolve precipitated salt, sodium sulfite to quench free chlorine and 6 M hydrochloric acid to adjust the pH to one. The organic layer was extracted with diethyl ether. The extracts were combined, dried over anhydrous  5, 151.6, 143.4, 137.8, 131.3, 130.5, 128.5, 44.7, 38.9, 35.8, 35.0, 32.8, 32.4, 28.9, 25.1, 21.9, 17 Non-H atoms were refined anisotropically and all H atoms were placed in calculated positions and refined using a riding model with C-H distances of 0.93 Å for the CH groups, 0.97 Å for the CH 2 groups and 0.96 Å for the CH 3 groups, U iso (H) = 1.5U eq (non-H), except for the H2 atom which was found in the electron density map and fixed in its position. Methyl groups were allowed to rotate as rigid groups.