Bicyclo- -nonane derivatives conformers

For bicyclo[3,3,l]nonane derivatives which are unsubstituted at positions 3 and 7, it has been shown in several cases that chair-chair conformations are preferred, although the cyclohexane rings are distinctly flattened with valence-angle increases (cc. 4°) beyond ideal tetrahedral values. However, derivatives with bulky endo substituents at positions 3 or 7 may adopt boat-chair conformations. Thus for 9-benzoyl-3a-bromo-9-azabicyclo[3,3,l]-nonan-2-one (74) and for the 2-hydroxy-analogue, it has been argued... 

Hyperflorin, an antibiotic isolated from St. John s wort Hypericum perforatum L.), has been found on the basis of chemical and spectroscopic evidence to possess structure (639). ° A review on the conformational analysis of bicyclo[3,3,l]nonanes has appeared,and the conformations of l,5-disubstituted-3,7-dimethyIenebicyclo-[3,3,l]nonane derivatives have been shown, on the basis of lanthanide-induced n.m.r. shifts, to be predominantly in the double-chair form. In a notable paper that contains a wealth of information pertinent to adamantane synthesis, it has been reported that bicyclo[3,3,l]nonane-3,7-dicarboxylic acid, its dimethyl ester, and various derivatives exist mainly in the chair-boat conformation. The synthetic entry to these systems is illustrated for the above diester (643) in which the starting point is adamantanone (640) thus, ring expansion with diazomethane followed by SeOj oxidation gave the a-diketone (641) which was cleaved with periodate to give the diacid (642). Esterification using diazomethane gave (643) which exists, as does the diacid, as... 

In bis compounds 256, 256+, and 257 (9-borabicyclononane, BBN), the bicyclo[3.3.1]nonane system adopts a chair-chair conformation according to X-ray studies (195,196). The CC conformation was also found for some other BBN derivatives (197-199). 

The conformation of derivatives of bicyclo[3.3.1]nonane has been the subject of many studies, based on proton nuclear magnetic resonance ( H-NMR), C-NMR, infrared (IR) and Raman spectroscopy, dipole measurements, X-ray crystallography, complexation experiments and various types of computational studies. Most of this work has been reviewed in detail (26,118, 119), and here we only report a summary of the general aspects. 

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