On the synthesis and conformational analysis of specifically deuterated 8-tert.-Butyldecahydroquinolines and their N-Nitrosoderivatives

The tert.-butyl-functionality is an important 'holding group' in conformational analysis. Because of its space demand the position on a sixmembered (carbocyclic or heterocyclic) ring is generally completely equatorial. In 8β-tert.-butyl-cis-decahydroquinoline (as in other 1-tert.-butyl-2-trans-disubstituted cyclohexanes) the conformational equilibrium is in favor of a tert.-butyl-axial conformer A; a 1,2-diequatorial trans-interaction is thus avoided. In order to establish the shape of the cyclohexane ring in conformer A (either chair or twist-boat) coupling constants of protons acting as probes had to be measured in a specimen (6) which was multideuterated in other positions to simplify the spectrum. The synthesis of 6 was carried out in the following way: two synthons, N-(2-tert.-butyl-4,4,5,5-tetradeuterocyclohex-1-yliden)cyclohexylamine and 1-bromo-1,1,2,2,3,3-hexadeutero-3-(2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopent-1-yl)-propane were prepared in a manner described and reacted to give 8-tert.-butyl-2,2,3,3,4,4,5,5,6,6-decadeutero-trans-decahydro-quinolines. Via the corresponding N-nitrosoderivatives and base-catalysed equilibration in DMSO or DMSO-d6 the N-nitroso-8β-tert.-butyl-cis-decahydroquinoline-d8 or -d11 were prepared and their conformational condition investigated by 1H-NMR. The compound was found to exist exclusively in conformation with the tert.-butyl group in an equatorial position at the cyclohexane ring (B). The nitrosamines were converted in the free secondary amines, and the conformational situation was again looked into by 1H-NMR. Low temperature 13C-NMR had determined the equilibrium A↔B to favor A by 80% at 173K. NOE-experiments and determination of the coupling constants of the remaining protons indicate the cyclohexane part of A to be largely if not exclusively twist-shaped. 93 refs., 15 figs., 1 tab. (Author)