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Total Synthesis of Berkelic Acid

MPS-Authors
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Snaddon,  Thomas N.
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Buchgraber,  Philipp
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Schulthoff,  Saskia
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Wirtz,  Cornelia
Service Department Mynott (NMR), Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Mynott,  Richard
Service Department Mynott (NMR), Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Fürstner,  Alois
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Citation

Snaddon, T. N., Buchgraber, P., Schulthoff, S., Wirtz, C., Mynott, R., & Fürstner, A. (2010). Total Synthesis of Berkelic Acid. Chemistry – A European Journal, 16(40), 12133-12140. doi:10.1002/chem.201001133.


Cite as: http://hdl.handle.net/11858/00-001M-0000-000F-8DA7-A
Abstract
A productive total synthesis of both enantiomers of berkelic acid (1) is outlined that takes the structure revision of this bioactive fungal metabolite previously proposed by our group into account. The successful route relies on a fully optimized triple-deprotection/1,4-addition/spiroacetalization cascade reaction sequence, which delivers the tetracyclic core 32 of the target as a single isomer in excellent yield. The required cyclization precursor 31 is assembled from the polysubstituted benzaldehyde derivative 20 and methyl ketone 25 by an aldol condensation, in which the acetyl residue in 20 transforms from a passive protecting group into an active participant. Access to fragment 25 takes advantage of the Collum–Godenschwager variant of the ester enolate Claisen rearrangement, which clearly surpasses the classical Ireland–Claisen procedure in terms of diastereoselectivity. Although it is possible to elaborate 32 into the target without any additional manipulations of protecting groups, a short detour consisting in the conversion of the phenolic –OH into the corresponding TBS-ether is beneficial. It tempers the sensitivity of the compound toward oxidation and hence improves the efficiency and reliability of the final stages. Orthogonal ester groups for the benzoate and the aliphatic carboxylate terminus of the side chain secure an efficient liberation of free berkelic acid in the final step of the route.