Multiwavelength Optical Rotation Detection: An Effective Approach for the Recognition of Analytical and Semi-Preparative HPLC Enantioseparation of the Chiral Pheromone Olean and Its Stereochemical Characterization.

Olean is the chiral spiroacetal sex pheromone of the female olive fruit fly Bactrocera oleae. Laboratory tests have demonstrated that the (R)-(-)-olean enantiomer is active on males, whereas females respond to (S)-(+)-olean. Here we present the first HPLC enantioseparation of olean using polysaccharide derivatives as chiral stationary phases and a polarimetric detector equipped with a micro-flow cell capable of detecting optical rotation at six different wavelengths. Unlike blind UV detection, the polarimetric detector allows the chiral spiroacetal, which lacks a chromophore, to be detected as bisegnate peaks, indicating the opposite sign of the optical rotation. The HPLC enantioseparation was optimized on the coated type amylose-based Chiralpak AS-H CSP using a mobile phase consisting of n-hexane-2-propanol 99.99:0.01 (v/v). These conditions were scaled up to a semi-preparative level and allowed resolution of 2.5 mg of racemic sample in 5 min. Multiwavelength optical rotation detection during HPLC enantioseparation of racemic samples provides a direct readout of the stereochemistry of olean and allows tracking of virtual optical rotation dispersion curves without the need for preliminary collection of enantiomeric samples by semi-preparative HPLC.