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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

2000; American Chemical Society; Volume: 65; Issue: 10 Linguagem: Inglês

10.1021/jo0001582

1520-6904

Abimael D. Rodrı́guez, Jian‐Gong Shi, Yan‐Ping Shi,

Synthetic Organic Chemistry Methods

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.