Isolation of Racemic 2,4-Pentanediol and 2,5-Hexanediol from Commercial Mixtures of Racemic and Meso Isomers by way of Cyclic Sulfites

Article abstract of DOI:10.1016/0957-4166(94)80028-6

Enantiomerically pure diols with C₂ symmetry such as 2,3-butanediol, 1, 2,4-pentanediol, 2, and 2,5-hexanediol, 3, are useful chiral auxiliaries, but they are expensive because chemists lack good synthetic routes that eliminate both the meso isomer and one enantiomer.Enzymic resolutions efficiently separate the enantiomer, but do not remove the meso isomer.To simplify enzymic resolutions of 2 and 3, we developed simple methods to isolate the racemic isomer from commercial mixtures of racemic and meso isomers 1 or 2, the meso isomer selectivity reacted with SOCl2 to give a cyclic sulfite that was removed by column chromatography to leave (+/-)-2, 92percent de, 1.4 g, 55percent yield.For 3, both meso and racemic isomers reacted with SOCl2 to give cyclic sulfites, but the sulfite derived from the meso isomer rearranged to trans-2,5-dimethyltetrahydrofuran under acidic conditions.Hydrolysis of the remaining sulfite gave (+/-)-3, 84percent de, 1.1 g, 37percent yield.Resolution of (+/-)-2 and (+/-)-3 using lipase from Pseudomonas cepacia yielded (2R,4R)-2-diacetate, 78 percent ee, >97percent de, 40percent of theory and (2R,5R)-3-diacetate, 94percent ee, >97percent de, 47percent of theory.Previously reported acetylations of 2 and 3 by lipase from Candida antarctica (CAL) or by lipase from Pseudomonas sp. (Amonolipase AK) are more enantioselective and thus, the best route to enantiomerically and diastereomerically pure 2 and 3 is removal of the meso isomer by way of cyclic sulfites followed by resolution with CAL or Amano lipase AK.