Dimethylcyclohexa-1,3-dien-5-ol-6-one, dimer

The C NMR spectrum of the metabolite shows 16 signals instead of 8 as expected from the elemental composition determined by high-resolution mass spectrometry. Moreover, aromaticity of the 2,6-xylenol is obviously lost after metabolism because two ketonic carbonyl carbon atoms (5c = 203.1 and 214.4) and four instead of twelve carbon signals are observed in the shift range of trigonal carbon nuclei (5c = 133.1, 135.4, 135.6 and 139.4) in the C NMR spectra. To conclude, metabolism involves oxidation of the benzenoid ring. 

The connectivities found in the HH COSY diagram reveal two partial stuctures A and B according to Table 44.1. These are supported by the identical proton-proton coupling constants of the coupling protons. 

Carbon-proton bonds are then assigned by means of the one-bond CH connectivities observed in the CH COSY diagram (Table 44.2). This completes partial structures A and B to the CH skeletons C and D. 

Two- and three-bond CH connectivities of the methyl protons detected in the CH COLOC diagram identify the partial structures E and F. Only one bond from 5c = 72.6 to 5c = 44.5 could not be directly identified because of the absence of the corresponding cross signal (Sh =1.17 with 6c = 44.5). Nevertheless, partial structure E is reasonable, assuming the six-membered ring to be retained during metabolism. 

Fragments E and F which include all 16 carbon atoms detected by C NMR can be attached to each other in two ways G or H the structure G is realised, as follows from the NOE difference spectra, which show a significant NOE between the methyl protons at Sh = 1.28 and the alkene proton at Sh - 6.28 and vice versa (Table 44.3). 

---