Isolation, structural characterization, and synthesis of a naturally occurring bisfuranopseudopterane ether: Biskallolide A. Evidence for a carbocation intermediate during the facile conversion of kallolide A and isokallolide A into various solvolysis products

The West Indian alcyonacean Pseudopterogorgia bipinnata (Verrill, 1864) is shown to contain a novel bisditerpenoid ether: biskallolide A (2). The stru ctural assignment of 2 was mainly based on 1D and 2D NMR and MS spectral da ta and was further confirmed by synthesis, The 2-C-alkoxylation of furanops eudopteranes kallolide A (1) and isokallolide A (8) occurs spontaneously in some solvents and involves replacement of the C2 hydroxyl with an alkoxyl group to yield solvolysis products that display net retention of configurat ion. The facile solvolytic 2-C-acyloxylation of kallolide A was achieved re adily under similar circumstances to afford kallolide A acetate (4) as the sole product. Mechanistic details in conversion of alcohols 1 and 8 into va rious solvolysis products, including dimeric ethers 2 and 9, were investiga ted in this study. Solvolysis of kallolide A and isokallolide A in [O-18]-l abeled solvent demonstrated that the C2 alkoxyl of the solvolysis products originated from the solvent, suggesting that these conversions may proceed through an S(N)1 mechanism with generation of a carbocation intermediate. T he chemical structures of kallolide A derivatives 3-7 and those of isokallo lide A congeners 9-11 were established by detailed analysis of the spectral data.