Bestimmung des Chiralitätssinns der enantiomeren 2,6‐Adamantandiole

Determination of the Chirality Sense of the Enantiomeric 2,6‐Adamantanediols The enantiomers of 2,6‐adamantanediol ( 1 ) are resolved via the diastereoisomeric camphanoates. The (2 R ,6 R )‐chirality sense for (−)‐ 1 and (2 S ,6 S ) for (+)‐ 1 was determined by chemical correlation with (−)‐(1 R ,5 R )‐bicyclo[3.3.1]nonan‐2,6‐dion ((1 R ,5 R )‐ 3 ) of known absolute configuration in the following way: alkylation of the bis(pyrrolidine enamine) of (−)‐(1 R ,5 R )‐ 3 with CD 2 I 2 and hydrolysis of the product gives the enantiomer 4 of (4,4‐D 2 )‐2,6‐adamantanedione. Reduction of 4 with LiAlH 4 leads to one enantiomer ( Scheme 2 ) of each of the three diols 5 – 7 of known absolute configuration. The three diols are themselves configurational isomers due to the presence of the CD 2 group, but correspond otherwise entirely to the enantiomeric diols 1 . Accordingly, they can also be separated by means of their diastereoisomeric camphanoates to give the diols 5 / 6 and 7 . These samples are easily distinguished and identified by their characteristic 1 H‐NMR spectra ( cf. Fig. 2 ). This allows to identify the (2 R ,6 R )‐ and (2 S ,6 S )‐enantiomer of 1 on the basis of their behavior in the resolution experiment analogous to that of the diols 5 / 6 and 7 , respectively. The diol (−)‐ 1 must have the (2 R ,6 R )‐configuration because it forms, like the diols 5 / 6 , with (−)‐camphanic acid the diastereoisomeric ester less soluble in benzene. The diol (+)‐ 1 has (2 S ,6 S )‐configuration, because it forms, like 7 , with (+)‐camphanic acid the diastereoisomeric ester less soluble in benzene. The bis(4‐methoxybenzoate) of (−)‐(2 R ,6 R )‐ 1 shows chiroptical properties which are in accordance with Nakanishi 's rule for two chromophores having coupled electric dipol transition moments arranged with a left‐handed torsion angle.