Improved chromatographic diastereoresolution of cyclopropyl dafachronic acid derivatives using chiral anion...

In the present paper we describe the optimization and the application of a chromatographic method suitable to get all four diastereoisomers of C24-C25 cyclopropyl dafachronic acid derivatives in sufficient amount for their biological appraisal towards the nuclear hormone receptor transcription factor DAF-12. A preliminary column screening of six anion exchange type Cinchona alkaloid-based chiral stationary phases (CSPs) allowed to identify the one with a quinine scaffold and carrying a O-9-(3,5-bis(trifluoromethyl)phenyl) moiety at C9 position as the best CSP. Few modifications of the experimental conditions revealed that a content of 18mM acetic acid used as counterion and displacer in acetonitrile and a column temperature fixed at 35°C were optimal for the simultaneous discrimination of all four diastereoisomers with a 1.0mL/min flow rate. With such conditions, transoid (S,S) and (R,R) diastereoisomers were resolved with R_S>1.4. With non chiral reversed-phase columns, neither the cisoid nor the transoid diastereoisomers could be resolved. This way, ca. 1.0mg of each stereoisomer was isolated with a diastereomeric purity >98%, suitable for the following biological tests. The indirect stereochemical assignments of the four diastereoisomers, and hence the corresponding chromatographic elution order (24R,25R)<(24S,25S)<(24R,25S)<(24S,25R) were made in an analogy manner on the basis of the resolution of fully assigned and structurally very similar ursodeoxycholic acid derivatives. As support of this indirect way of assigning the absolute configuration of the C24 and C25 chiral centre a molecular modeling procedure based on dynamic simulation was successfully applied.