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Directing a SmI2 radical cyclisation using a C−Si bond:A second generation approach to pestalotiopsin A, 6-epitaedolidol and punctaporonin C

Harb, Hassan Youssef

[Thesis]. Manchester, UK: The University of Manchester; 2011.

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Abstract

An improved second generation approach to pestalotiopsin A and 6-epitaedolidol has been developed.A silicon group bonded directly to carbon has been used as a stereocontrol element in cyclisation substrates for a SmI2 – mediated 4-exo-trig radical cyclisation. The silicon groups acts as a steric block to set up three new stereocentres in high to complete diastereocontrol. Several approaches to the cyclisation substrates are outlined, including a silyl transfer/aldol cascade and a novel C2-symmetric N-heterocyclic carbene – copper catalyst to asymmetrically install a C-Si bond. The silicon group is a versatile handle for further manipulation and has been oxidised using Fleming oxidation conditions to install the hydroxyl functionality required in the target compounds. The silicon group also simplifies the alcohol protecting group strategy in the approach to pestalotiopsin A and 6‐epitaedolidol.The first application of N-heterocyclic carbene – copper catalysed silyl transfer in natural product synthesis and the first use of a SmI2 – mediated cyclisation directed by a C-Si stereocontrol element are described.