A study of transesterification of chiral (-)-pinanediol methylboronic ester with various structurally modified diols

The transesterification of chiral (-)-pinanediol methylboronic ester was studied with various structurally modified diols by H-1 NMR to understand the factors influencing the unusual stability of this boronic ester as well as to find ways of recovering pinanediol from its methylboronic ester. In all the cases, reactions were allowed to proceed to equilibrium. The preliminary experiments indeed have shown some encouraging results (displacement of pinanediol up to 40-53%). Amongst cyclopentane-based cis-1,2-diols, endo-2-phenyl-exo,exo-2,3-norbornane-diol appeared to be the most effective diol in displacing pinanediol (38%). In the cases of pinane-based diols, the best result was obtained with 2-ethyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol (53%). It was interesting to observe that the transesterification with 2-phenyl-6,6-dimethylbicyclo[3.1.1]heptane-cis-2,3-diol resulted in a 50% conversion after 4 days only, whereas the former diol took 24 days to reach equilibrium.