Grayanane Diterpenoid Glucosides from the Leaves of Rhododendron micranthum and Their Bioactivities Evaluation

Thirteen new grayanane diterpenoid glucosides, 3- epi-grayanoside B (1), micranthanosides A-E (2-6), 7α-hydroxygrayanoside C (7), micranthanoside F (8), 14β-acetyoxymicranthanoside F (9), micranthanoside G (10), 14- O-acetylmicranthanoside G (11), 14β-hydroxypieroside A (12), and micranthanoside H (13), and six known analogues (14-19) were isolated from the leaves of Rhododendron micranthum. The structures of 1-19 were elucidated based on spectroscopic analysis, comparison with literature, and chemical methods. The absolute configurations of 3- epi-grayanoside B (1) and micranthanosides A (2) and C (4) were defined by single-crystal X-ray diffraction analysis. This is the first report of the crystal structures of grayanane diterpenoid glucosides. 3- epi-Grayanoside B (1) represents the first example of a 3α-oxygrayanane diterpenoid glucoside, and micranthanosides A-D (2-5) are the first examples of 5α-hydroxy-1-β H-grayanane diterpenoids. In addition, micranthanosides C-F (4-6 and 8) and 14β-acetyoxymicranthanoside F (9) represent the first examples of grayanane glucosides with the glucosylation at C-16. All the grayanane diterpenoid glucosides 1-19 were assayed for their anti-inflammatory, antitumor, and PTP1B inhibitory activities, but did not show significant activities at 40 μM. Grayanane diterpenoid glucosides 1-18 were evaluated for their antinociceptive activity, and compounds 2, 3, 7-10, 12, 13, and 16 showed significant antinociceptive effects with percentage inhibitions in excess of 50%.