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DOI: 10.1055/s-0040-1707127
Evolution of a Cycloaddition–Rearrangement Approach to the Squalestatins: A Quarter-Century Odyssey
For studentship funding we thank the Engineering and Physical Sciences Research Council (EPSRC), the University of Oxford, the Higher Education Commission, Pakistan, the Sultanate of Oman, and the Higher Committee for Education Development in Iraq.Publication History
Received: 03 April 2020
Accepted after revision: 02 May 2020
Publication Date:
04 June 2020 (online)
Abstract
The highs, lows, and diversions of a journey leading to two syntheses of 6,7-dideoxysqualestatin H5 is described. Both syntheses relied on highly diastereoselective n-alkylations of a tartrate acetonide enolate and subsequent oxidation–hydrolysis to provide an asymmetric entry to β-hydroxy-α-ketoester motifs. The latter were differentially elaborated to diazoketones which underwent stereo- and regioselective Rh(II)-catalysed cyclic carbonyl ylide formation–cycloaddition and then acid-catalysed transketalisation to generate the 2,8-dioxabicyclo[3.2.1]octane core of the squalestatins/zaragozic acids at the correct tricarboxylate oxidation level. The unsaturated side chain was either protected with a bromide substituent during the transketalisation or introduced afterwards by a stereoretentive Ni-catalyzed Csp3–Csp2 cross-electrophile coupling.
1 Introduction
2 Racemic Model Studies to the Squalestatin/Zaragozic Acid Core
3 Asymmetric Model Studies to a Keto α-Diazoester
3.1 Dialkyl Squarate Desymmetrisation
3.2 Tartrate Alkylation
3.2.1 Further Studies on Seebach’s Alkylation Chemistry
4 Failure at the Penultimate Step to DDSQ
5 Second-Generation Approach to DDSQ: A Bromide Substituent Strategy
5.1 Stereoselective Routes to E-Alkenyl Halides via β-Oxido Phosphonium Ylides
5.2 Back to DDSQ Synthesis
6 An Alternative Strategy to DDSQ: By Cross-Electrophile Coupling
7 Alkene Ozonolysis in the Presence of Diazo Functionality: Accessing α-Ketoester Intermediates
8 Summary
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References and Notes
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1 Permanent address: University of Kufa, Najaf Governorate, Iraq.
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