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 structural assignment of 2 was mainly based on 1D and 2D NMR and MS spectral data and was further confirmed by synthesis. The 2-C-alkoxylation of furanopseudopteranes 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 configuration. The facile solvolytic 2-C-acyloxylation of kallolide A was achieved readily under similar circumstances to afford kallolide A acetate (4) as the sole product. Mechanistic details in conversion of alcohols 1 and 8 into various solvolysis products, including dimeric ethers 2 and 9, were investigated in this study. Solvolysis of kallolide A and isokallolide A in [(18)O]-labeled 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. The chemical structures of kallolide A derivatives 3-7and those of isokallolide A congeners 9-11 were established by detailed analysis of the spectral data.
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Department of Chemistry, University of Puerto Rico, P.O. Box 23346, U.P.R. Station, San Juan, Puerto Rico 00931-3346, USA. arodrig@goliath.cnnet.clu.edu
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D000818:Animals; D003063:Cnidaria / Q000737:chemistry*; D005663:Furans / Q000138:chemical synthesis* / Q000494:pharmacology; D006575:Heterocyclic Compounds, 3-Ring / Q000138:chemical synthesis* / Q000494:pharmacology; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D008826:Microbial Sensitivity Tests; D009169:Mycobacterium tuberculosis / Q000187:drug effects; D012997:Solvents; D016339:Spectrometry, Mass, Fast Atom Bombardment; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet; D014407:Tumor Cells, Cultured
D005663:Furans; D006575:Heterocyclic Compounds, 3-Ring; D012997:Solvents; C412023:biskallolide A; C412024:isokallolide A