An efficient diastereo- and enantioselective Ag-catalyzed method for additions of a commercially available siloxyfuran to α-ketoimine esters is disclosed. Catalytic transformations require an inexpensive metal salt (AgOAc) and an air stable chiral ligand that is prepared in three steps from commercially available materials in 42% overall yield. Aryl- as well as heterocyclic substituted ketoimines can be used effectively in the Ag-catalyzed process. Additionally, two examples regarding reactions of alkyl-substituted ketoimines are presented. An electronically modified N-aryl group is introduced that is responsible for high reaction efficiency (>98% conversion, 72−95% yields after purification), diastereo- (up to >98:2 dr) and enantioselectivity (up to 97:3 er or 94% ee). The new N-aryl unit is crucial for conversion of the asymmetric vinylogous Mannich (AVM) products to the unprotected amines in high yields. Spectroscopic and X-ray data are among the physical evidence provided that shed light on the identity of the Ag-based chiral catalysts and some of the mechanistic subtleties of this class of enantioselective C—C bond forming processes.
An Al-catalyzed enantioselective method for additions of Me(2)Zn and Et(2)Zn to alpha-ketoesters bearing aromatic, alkenyl, and alkyl substituents is disclosed. Transformations are promoted in the presence of a readily available amino acid-based ligand and afford the desired products in excellent yields and in up to 95% ee. Investigations described illustrate that the presence of a Lewis basic additive can lead to significant enhancements in efficiency and enantioselectivity. A mechanistic model that provides a rationale for such effects is provided.
Bismuth triflate is a highly efficient catalyst (0.1-1 mol %) for the deprotection of acetals and ketals. The procedure is very facile and selective for acetals derived from ketones and conjugated aldehydes. tert-Butyldimethylsilyl ethers are stable to the reaction conditions. The highly catalytic nature of bismuth triflate and the use of a relatively nontoxic solvent system (THF/H(2)O) make this procedure particularly attractive for large-scale synthesis.
Abstract-The allylation of acetals using allyltrimethylsilane is efficiently catalyzed by bismuth triflate (1.0 mol%). The reaction proceeds smoothly at room temperature to afford the corresponding homoallyl ether in good yield. The mild reaction conditions, the low toxicity of bismuth salts, and the high catalytic efficiency of the system make this procedure particularly attractive for large-scale synthesis.
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