Preparation of the four stereoisomers of 3-bromo-2-butanol or their acetates via lipase-catalysed resolutions of the racemates derived from dl- or meso-2,3-butanediol

The four stereoisomeric 3-bromo-2-butanols and/or their acetates were prepared via lipase-catalysed kinetic resolution by hydrolyses of the acetates of the (±)-syn- and (±)-anti-3-bromo-2-butanols, or via esterifications of the alcohols. The diastereomeric bromoacetates were obtained by syntheses from the dl- and meso-2,3-butanediols, respectively. On a preparative scale, the four stereoisomers, either as the free alcohols or as their acetates, were obtained in >95% ee, and in 35–40% yield (based on the starting racemates).

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(2R,3R)-3-Bromo-2-butanolC4H9OBrEe = 97% (by GC analysis on a Cyclosilb chiral column)dr = 100/0[α]D25=-20.2 (c 2, CHCl3)Source of chirality: enzymatic resolutionAbsolute configuration: (2R,3R)

(2R,3S)-3-Bromo-2-butanolC4H9OBrEe = 95% (by GC analysis on a Cyclosilb chiral column)dr = > 99.5/0.5[α]D20=+13.2 (neat)Source of chirality: enzymatic resolutionAbsolute configuration: (2R,3S)

(2S,3S)-2-Acetoxy-3-bromobutaneC6H11O2BrEe = 99.5% (by GC analysis on a Cyclosilb chiral column)dr = 94/6[α]D25=-2.2 (neat)Source of chirality: enzymatic resolutionAbsolute configuration: (2S,3S)

(2S,3R)-2-Acetoxy-3-bromobutaneC6H11O2BrEe = 99% (by GC analysis on a Cyclosilb chiral column)dr = > 99.5/0.5[α]D20=-16.6 (neat)Source of chirality: enzymatic resolutionAbsolute configuration: (2S,3R)