Large-scale supercritical fluid chromatography purification of unstable STING agonist intermediates

A Stereocontrolled Synthesis of a Phosphorothioate Cyclic Dinucleotide-Based STING Agonist

“…Methanol alone was a suitable solvent to affect the migration, although extended times (48 h) were needed to achieve 50% migration (Table ). Following the Usam protocol, the addition of 1% Et 3 N further accelerated the migration, with 50% Et 3 N in isopropanol (IPA) at 50 °C ultimately selected as the optimal combination to effect clean migration within 4 h. In the event, we developed a practical supercritical flow chromatography (SFC) method for this separation, which allowed for rapid purification on a multigram scale and was combined with continuous and immediate evaporation postpurification, resulting in <1% observed migration on a scale greater than 100 g. The undesired regioisomer was subjected to the IPA-Et 3 N conditions to effectively “recycle” into a 1:1 mixture within 5 h on a 100 g scale. Evaporation and resubjecting the material to SFC allowed the effective yield for this transformation to be increased from the aforementioned 31% on a gram scale to 65–75% on a scale greater than100 g, dependent on the number of times the SFC purification–recycle protocol was repeated.…”