Guidongnins I-J: Two New 6,7- seco -7,20-Olide- ent -kaurene Diterpenes with Unusual Structures from Isodon rubescens .

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 IC 50 27.14 ± 3.43 μM.