Guidongnins I–J: Two New 6,7-<i>seco</i>-7,20-Olide-<i>ent</i>-kaurene Diterpenes with Unusual Structures from <i>Isodon rubescens</i>

Abstract

Two undescribed ent-kaurene diterpenes, named guidongnins I (1) and J (2), were isolated from the medicinal plant Isodon rubescens. Compound 1 was determined to contain an unprecedented 23 carbons in the skeleton by bearing an extra isopropyl group at C-17 out of the diterpenoid parent structure, and compound 2 was the first example of 6,7-seco-7,20-olide-ent-kaurenes with two fused-tetrahydrofuran rings formed between C-6 and C-19/C-20 through oxygen bridges. Their structures, including their absolute configurations, were determined using the analyses of the spectroscopic and X-ray diffraction data. Guidongnins I (1) and J (2) were assessed for their anti-cancer activities against the growth of various cancer cell lines, and 2 displayed cytotoxic potency against HepG2 at IC50 27.14 ± 3.43 μM.

Keywords

• <i>Isodon rubescens</i>
• <i>ent</i>-kaurene diterpenoid
• isopropyl group
• anti-cancer
• plausible biosynthetic pathway