Extending the Range of Laboratory-Scale Purification by Utilising Supercritical Fluid Chromatography for Unattended Overnight Operation

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Supercritical Fluid Chromatography (SFC) remains the preferred choice for preparative enantioseparation within Drug Discovery Research, where semi-preparative SFC is routinely employed to purify intermediates and final products in quantities up to 50 g. As compounds progress through the Medicinal Chemistry pathway, greater quantities of material are required; this can be in the region of 200 g of pure enantiomer for those compounds requiring toxicological evaluations. To meet the increasing demand for larger scale purification within Discovery, and to deliver in a timely manner, SFC can be applied in an uninterrupted batch process where purification campaigns are run unattended overnight across multiple days. In this article the challenges in extending the upper limit of laboratory-scale purification from 50 g to 500 g (racemate) are highlighted and the controls required to satisfy Health and Safety when handling large volumes of SFC co-solvents. Examples of chiral SFC purifications are presented to illustrate the benefit of utilising night-time capacity to process up to 1 kg of material.

Introduction

The application and benefits of SFC are well documented and recently published [1-8].  Within Lilly SFC remains the technique of choice for the chiral purification of Discovery samples less than 50 g [9]. The low viscosity of supercritical fluids enables 3-5 times higher flow rates compared to HPLC, with increased efficiencies due to the higher diffusivity of analytes in a CO2 based mobile phase. Compared to the organic solvents employed in HPLC, CO2 is inexpensive, non-toxic and non-flammable. Most importantly, SFC significantly reduces organic solvent consumption, is easier to handle and permits rapid fraction drying. Handling large volumes of organic solvent, within a Discovery laboratory environment, is highly restrictive and in our opinion the solvent handling benefits of SFC over HPLC are understated. 

Miller [10] and Zhang et.al [11] discuss the application of single-column batch preparative SFC for material up to 1 kg in quantity. At the Lilly Research site in the UK, we recently reviewed our purification capabilities to meet the increasing demand for larger scale samples. The result of the review was the development of a SFC workflow to accommodate the following requirements:

i.unattended overnight operation at a flow rate up to 400 g/min.
ii.the capacity to deliver and collect up to 100 litres of co-solvent across a 12 hour  purification period.
iii.semi-automated solvent top up and fraction solvent manipulation without interrupting the purification campaign.
iv. parallel fraction solvent evaporation.
v. simple and quick sample to sample changeover.
vi. compliance to local solvent handling and waste stream protocols.

Experimental

Carbon dioxide was supplied by BOC Gases (Worsley, Manchester, UK). CO2 gas was purified and pressurised to 1500 psi using a booster system supplied from Va-Tran Systems, Inc (Chula Vista, CA, USA). Methanol, ethanol and isopropanol were HPLC grade and pre-blended with diethylmethylamine (DEMA) additive (0.2%), supplied by Romil (Cambridge, UK). Coated Polysaccharide chiral SFC Columns packed with Chiralpak AD and Chiralpak AS (amylose derivatives) and Chiralcel OD and OJ (cellulose derivatives) were used. Immobilised Polysaccharide chiral SFC columns packed with Chiralpak IA (amylose derivative) and Chiralpak IC (cellulose derivative) were also used. All columns were supplied by Chiral Technologies (Chiral Technologies Europe, Illkirch, France). Preparative column dimensions were 25 cm x 3 cm i.d. or 25 cm x 5 cm i.d. 5 µm particle size.