The Metabolites of Secoiridoid Enantiomers and Diastereomers with 9-Carbon Skeleton in Gentiana macrophylla Pall.

INTRODUCTION

The secoiridoid metabolites with a 10-carbon skeleton, such as gentiopicroside, swertiamarin, and sweroside, were identified as the main components in Gentiana macrophylla. However, various unpredictable rearrangement metabolites might be generated due to the high reactivity of the hemiacetal in their structures. Thus, our ongoing research aimed to explore additional metabolites with new carbon skeleton features from G. macrophylla.

METHODS

Metabolites were separated and purified using multi-chromatographic techniques, such as silica gel, HP-20 resin gel, LH-20 dextran gel, C18 gel, and chiral CHIRALPAK AD-H column chromatography; and characterized by multi-spectroscopic methods, including NMR and X-ray spectroscopy, experimental CD measurement compared with ECD calculation, and carbon NMR calculation. Moreover, their biosynthetic pathways were deduced according to the KEGG pathway and their correlations with normal secoiridoid metabolites in G. macrophylla.

RESULTS

A pair of norsecoiridoid diastereomers, (3R, 6R, 8S)-3-methoxy-6-hydroxy-swerimuslactone A (1) and (3S, 6R, 8R)-3-methoxy-6-hydroxy-swerimuslactone A (2), were isolated and identified from G. macrophylla, along with two pairs of racemates (3) and (4) with [α]²⁰_D = 0. These racemates were further separated by chiral column chromatography into two pairs of norsecoiridoid enantiomers: (-)-(5S, 8S)- gentimaclactone A (3a) and (+)-(5R, 8R)-gentimaclactone A (3b), and (+)-(3S, 8R)-swerimilegenin E (4a) and (-)-(3R, 8S)-swerimilegenin E (4b), respectively.

DISCUSSION: All compounds were determined to have a 9-carbon skeleton, derived from the normal 10- carbon secoiridoids in G. macrophylla. Their biosynthetic pathways were deduced to occur via ring-opening of the hemiacetal at C-1, ring-closing with the hydroxyl group at C-8, followed by degradation of a CHO group.

CONCLUSION

Four norsecoiridoids with a 9-carbon skeleton, derived from the normal 10-carbon secoiridoids, were isolated from G. macrophylla. They were present as pairs of diastereomers or racemates, which were further separated into pairs of enantiomers. Among them, the diastereomer pair (3R, 6R, 8S)- 3-methoxy-6-hydroxy-swerimuslactone A (1) and (3S, 6R, 8R)-3-methoxy-6-hydroxy-swerimuslactone A (2), and the enantiomer pair (-)-(5S, 8S)-gentimaclactone A (3a) and (+)-(5R, 8R)-gentimaclactone A (3b) were identified as new compounds. Additionally, the plausible biosynthetic pathways of these compounds were proposed for future studies.