Inter- and Intramolecular [4 + 3] Cycloadditions Using Epoxy Enol Silanes As Functionalized Oxyallyl Cation Precursors

Wing Ki Chung; Sze Kui Lam; Brian Lo; Lok Lok Liu; Wing-Tak Wong; Pauline Chiu

doi:10.1021/ja807566t

journal article Mar 12, 2009

Inter- and Intramolecular [4 + 3] Cycloadditions Using Epoxy Enol Silanes As Functionalized Oxyallyl Cation Precursors

Wing Ki Chung Sze Kui Lam Brian Lo Lok Lok Liu Wing-Tak Wong Pauline Chiu

Journal of the American Chemical Society Vol. 131 No. 13 pp. 4556-4557 · American Chemical Society (ACS)

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The TES enol ether2appears to be optimized for reactivity, as the TBS-analogue reacts sluggishly resulting in lower yields, while the TMS-derivative1is too reactive.

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TES-3ais obtained by quenching with Et3N, and3ais obtained by quenching with aqueous NaHCO3and fluoride (TBAF or HF−pyridine).

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Other acids [Yb(OTf)3, Sc(OTf)3, BF3·Et2O, Bu2BOTf, BBr3, SnCl4, TiCl4] induced the cycloaddition stoichiometrically or in poor yields.

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An extended Table1showing results of experiments for optimization and scope appears in theSI.

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The relative stereochemistries of8a−gwere determined by NOE correlations, and the structures of8a, desilylated-8c,8g, and9ghave been additionally confirmed by X-ray crystallography.

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For simplicity, Scheme1shows full C−O bond cleavage at the epoxide to afford the siloxyallyl cation. However, the actual extent of bond dissociation may or may not be complete prior to C−C bond formation.

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Transition stateBaccounts for the formation of9fand9g. Apparently the selectivity for cycloaddition via transition stateAis disfavored only when the substrate is hindered, as in7gin which both the enol ether and the furan are substituted.

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Details

Published: Mar 12, 2009
Vol/Issue: 131(13)
Pages: 4556-4557

Authors

W

Wing Ki Chung