Impact of CO 2 -solvent separators on the degradation of benzyl-2,3-dihydroxypiperidine-1-carboxylate during preparative supercritical fluid chromatographic (SFC) purification.

During a preparative separation of the cis enantiomeric pair of benzyl-2,3-dihydroxypiperidine-1-carboxylate using supercritical-fluid chromatography (SFC) with methanol modifier, significant degradation of the products in the collected fractions was observed when a Waters SFC-350 ^® (Milford, MA, USA) was used, but same was not observed when a Waters SFC-80q ^® (Milford, MA, USA) was used. Through a systematic investigation, we discovered that the compound degraded over time under an acidic condition created by the formation of methyl carbonic acid from methanol and CO _2. The extent of the product degradation was dependent on the time and the concentration of CO ₂ remained in the product fraction, which was governed by the efficiency of CO ₂ -methanol separation during the fraction collection. Hence, we demonstrated that the different designs of CO ₂ -solvent separator (high pressurized cyclone in Waters SFC-350 ^® and low-pressurized vortexing separator in Waters SFC-80q ^®® ) had a significant impact on the degradation of an acid-sensitive compound. The acidity caused by CO ₂ in methanol was supported by diminished degradation after a nitrogen purging or after neutralizing the collected fractions with a base. Three different solutions to overcome the degradation problem of the acid sensitive compounds using SFC-350 ^® with the high pressurized separator were investigated and demonstrated. The degraded products were isolated as four enantiomers and their relative stereochemistry were established based on 2D NMR data along with the plausible mechanism of degradation.
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