Diastereoseparation of C-7 Pentacyclic Oxindole Alkaloids and Their Vibrational Circular Dichroism Characteristics
Column chromatography of methanolic leaf extracts of Uncaria longiflora var. pteropoda has eluted fractions containing a mixture of diastereomeric C-7 pentacyclic oxindole alkaloids (POAs) known as the isopteropodine (1) and pteropodine (2). Previously, multiple developments of a preparative thin layer chromatography (PTLC) technique managed to separate the two diastereomers from the stem extract of the same plant. However, due to their similar retention times, the separation process was tedious and the purity of these compounds was unsatisfactory. The present work has successfully improved these unfavourable separation conditions within two complete cycles of a 74 minute recycling-High Performance Liquid Chromatography (r-HPLC) run. In addition, their respective C-7S and C-7R diastereomerism were unambiguously distinguished by their opposite vibrational circular dichroism (VCD) signals appearing at the regions ca. 1700 and 1100 cm-1. The present work describes for the first time the r-HPLC separation technique and VCD spectral characteristics of these molecules. The findings revealed that the combination of r-HPLC and VCD are able to separate, characterize and distinguish the diastereomers.