Stereoselective separation of β-adrenergic blocking agents containing two chiral centers by countercurrent chromatography

- β-Adrenergic blocking agents containing two chiral centers were synthesized.
- A chiral stationary phase was selected for analytical separation by liquid chromatography.
- Stereoselective separation of β-blockers with two chiral centers by countercurrent chromatography was attempted.
- We report an alternative method for stereoselective separation of racemates with two chiral centers.

Four β-adrenergic blocking agents, including 1-[(1-methylethyl)amino]-3-phenoxy-2-propanol (1), 1-[(1-methylethyl)amino]-3-(3-methylphenoxy)-2-propanol (2), 1,1′-[1,4-phenylenebis(oxy)]bis[3-[(1-methylethyl)amino]-2-propanol (3) and 1,1′-[(4-methyl-1,2-phenylene)bis(oxy)]bis[3-[(1-methylethyl)amino]-2-propanol (4), were stereoselectively separated by countercurrent chromatography using di-n-hexyl l-tartrate and boric acid as chiral selector. The compounds (3) and (4) have four optical isomers since they contained two chiral centers. A two-phase solvent system composed of chloroform-0.05 mol L−1 of acetate buffer containing 0.10 mol L−1 of boric acid (1:1, v/v) was selected, in which 0.10 mol L−1 of di-n-hexyl l-tartrate was added in the organic phase as chiral selector. 20-42 mg of each racemate was stereoselectively separated by countercurrent chromatography in a single run with high purity of 96-98%, and the recovery of each separated compound reached around 87-93%. This is the first time report on successful stereoselective separation of optical isomeric compounds containing two chiral centers by countercurrent chromatography. At the same time, a chiral stationary phase was screened for analytical stereoselective separation of compounds (3) and (4) by high performance liquid chromatography.