Chemical investigation of a Korean marine sponge, Monanchora sp., yielded nine new sesterterpenoids (1-9) along with phorbaketals A C (10-12). The planar structures were established on the basis of NMR and MS analysis, and the absolute configurations of 1-9 were defined using the modified Mosher's method and CD spectroscopic data analysis. Compounds 1-8, designated as phorbaketals D K, possess a spiroketal-modified benzopyran moiety such as phorbaketal A, and their structural variations are due to oxidation and/or reduction of the tricyclic core or the side chain. Compound 9, designated as phorbin A, has a monocyclic structure and is proposed to be a possible biogenetic precursor of the phorbaketals. Compounds 1-9 were evaluated for cytotoxicity against four human cancer cell lines (A498, ACHN, MIA-paca, and PANC-1), and a few of them were found to exhibit cytotoxic activity.
Unified Asymmetric Total Syntheses of (−)-Alotaketals A-D and (−)-Phorbaketal A
alotane-type sesterterpenoids showed strikingly different biological activities and potency with subtle structural alterations. Asymmetric totalsyntheses of the tricyclic sesterterpenoids (−)-alotaketals A–D and (−)-phorbaketal A were accomplished [29–31 steps from (−)-malic acid] in a collective way for the first time. The key features of the strategy included 1) a new cascade cyclization of vinyl epoxy