Preparation and cytotoxic evaluation of vouacapane oxidation products

Treatment of (−)-(5S,6R,8S,9S,10R,14R)-6-acetoxyvouacapane (1) with DDQ, mCPBA, or Cr(VI) reagents afforded diterpenoids with various degrees of oxidation at the furan and C rings. Oxidation of 1 using DDQ provided the known benzofuran 2, together with the new derivatives dimer 3, lactone 4, and aldehyde 5, while mCPBA oxidation gave 2, spirocassenolide 6, and cassenolides 7 and 8a. Oxidation of 1 with CrO3 gave 4, 6, 8a, and spirocassenolide 9, while the use of K2Cr2O7 yielded 4, 9, and spirocassenolide 10. The structures of the new compounds followed from HRMS, NMR measurements, and by single-crystal X-ray diffraction of 3, 4, 8b, and 10. Compounds 1, 2, and 6 were evaluated for their cytotoxic activity against the MCF-7 and HL-60 cancer cells, showing moderate cytotoxic activity.