ChemSpider none Molar mass 538.89 g·mol | Formula C24H23ClO12 | |
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Sporolides A and B are novel polycyclic macrolides from the obligate marine bacterium Salinispora tropica (which is found in ocean sediment) that are composed of a chlorinated cyclopenta[a]indene ring and a cyclohexenone moiety. They are only the second class of compounds isolated from Salinispora, and their unique carbon skeleton provides a clear indication of the tremendous potential of marine actinomycetes as a source of novel secondary metabolites. The structures and absolute stereochemistries of both metabolites were elucidated using a combination of NMR spectroscopy and X-ray crystallography, and are considered unique for two reasons:
- both compounds appear to be polyketides and therefore derived from acetate units,
- the number of oxidized carbons is amazing, with 21 of 24 carbons either oxygenated or sp2 - hybridized.
The complex aromatic structure of the sporolides is hypothesized to be derived from an unstable nine-membered ring enediyne precursor, which undergoes Bergman cyclization to generate a p-benzyne intermediate. A nucleophilic attack by chloride accounts for the 1:1 mixtures and for the single chlorine in enediyne-derived natural products. This mechanism has been recently demonstrated in laboratory experiments, and can account for the biosynthesis of sporolides.
Total Synthesis
First stereoselective synthesis was reported by K. C. Nicolaou, more specifically, Sporolide B, through a highly stereoselective and convergent strategy that involves two important cycloaddition reactions: a thermally induced intramolecular [4+2] cycloaddition reaction involving an o-quinone and a tetrasubstituted olefin to form the macrocyclic structure of the molecule, and a ruthenium-catalyzed, intermolecular [2+2+2] cycloaddition reaction between two acetylenic units.