Equatorial positions hydrogen

X-ray crystallographic analyses of various 3//-azepines indicate that, like the 1//-azepines, they have a boat conformation.28,36 38,289 In both isolable tautomers of the highly substituted 3//-azepine 11, the hydrogen at the sp3 carbon occupies an equatorial position with the carbon inclined at an angle of 57c to the planar hull. 28 3-[Hydroxy(diphenyl)methyl]-3//-azepine has a similar boat conformation, but with the 3-substituent in the equatorial position.39... 

Substituents may play a crucial role in the conformation of quinolizidine systems. Thus, compound 49 shows a trans-conformation 50 with all three hydroxyl groups in equatorial positions. For its diastereomer 51, a -conformation 52 was initially proposed, but the H NMR data point at the /raor-conformation 53, with axial orientation of the hydroxy substituents and presumably stabilized by an intramolecular hydrogen bond <2004T3009>. 

Phosphorus ligands are shown from NMR studies to take up equatorial positions, while pyridine and isonitrile bond to axial sites. For axial substitution, two isomers occur, depending on the orientation relative to the bridging hydrogen atom. The phosphine complexes lose CO on heating to yield the "unsaturated complexes H2Os3(CO)9L... 

Another possibility to increase the diastereoselectivity in an asymmetric synthesis can arise from different thermodynamic stabilities of the diasteieoisomeric products. If the thermodynamic stabilities of these are different enough, then, under conditions of equilibrium, a complete conversion of the less stable into the more stable can be achieved. For example, the diastereoselective hydrogenation of naphthalene derivates over Pd/C catalyst leads to a mixture of dihydronaphtalenes in which the cA-isomer predominates. The conversion of this isomer into the tram occurs by changing the properties of the reaction medium, namely by equilibration with a base. For such a purpose, NaOMe in IHF can be used [263], Generally, such an increase in stability in the six-membered rings can result from a rearrangement of the substituents from an axial to an equatorial position. 

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