Switchable Diastereoselectivity in the Fluoride-Promoted Vinylogous Mukaiyama-Michael Reaction of 2-[(Trimethylsilyl)oxy]furan Catalyzed by Crown Ethers.

The fluoride-promoted vinylogous Mukaiyama-Michael reaction of 2-[(trimethylsilyl)oxy]furan with diverse α,β-unsaturated ketones is described. The TBAF-catalyzed VMMR afforded high anti-diastereoselectivity irrespective of the solvents used. The KF/crown ethers catalytic systems proved to be highly efficient in terms of yields and resulted in a highly diastereoselective unprecedented solvent/catalyst switchable reaction. Anti-adducts were obtained as single diastereomers or with excellent diastereoselectivities when benzo-15-crown-5 in CH2Cl2 was employed. On the other hand, high syn-diastereoselectivities (from 73:27 to 96:4) were achieved by employing dicyclohexane-18-crown-6 in toluene. On the basis of DFT calculations, the catalysts/solvent-dependent switchable diastereoselectivities are proposed to be the result of loose or tight cation-dienolate ion pairs.