Enantiomeric excess of 1,2-diols by formation of cyclic boronates: an improved method

A reliable method for determining the enantiomeric composition of 1,2-diols by the formation of diastereomeric cyclic esters with boronic acid is described. Starting from a previously reported structure of boronic chiral derivatizing agent (CDA), seven structurally related racemic CDAs were synthesized and their discriminating ability towards diols measured. The most promising amongst these was synthesized in its enantiomerically pure form according to Matteson’s protocol for the stereoselective homologation of pinanediol boronates; this CDA quantitatively and rapidly reacts with 1,2-diols in very mild conditions affording a couple of diastereoisomers, whose composition can be determined via 1H NMR analysis. In particular, an attractive feature is that the resonance used for the analysis originated from the CDA as a couple of baseline-separated singlets (Δδ up to 0.3 ppm) is useful for integration.

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(1S,2S,3R,5S)-Pinanediol(2-methylphenyl)methaneboronateC17H23BO2Ee >98%[α]D = −3.5 (c 2.0, CHCl3)

(1S,2S,3R,5S)-Pinanediol(1R)-1-benzamido-1-(2-methylphenyl)-methaneboronateC25H30BNO3Ee >98%[α]D = +6.1 (c 1.8, CHCl3)

(1R)-1-Benzamido-1-(2-methylphenyl)methaneboronic acidC15H16BNO3Ee >98%[α]D = +26.6 (c 0.7, CH3OH)