Dimethylformamide-Modulated Kdo Glycosylation for Stereoselective Synthesis of α-Kdo Glycosides

A simple and direct DMF-modulated alpha-selective Kdo glycosylation approach for the stereoselective synthesis of the alpha-linked Kdo glycosides is developed. Glycosylation of the readily available peracetylated Kdo ortho-hexynylbenzoate with common acceptor alcohols using SPhosAuNTf(2) as a promoter and DMF as a modulating molecule afforded a range of Kdo glycosides with good alpha-selectivities. Furthermore, the present method is effectively applied in the latent-active synthesis of the alpha-linked di-Kdo glycoside bearing a linker at the reducing end. Finally, the first observation of a Kdo imidinium ion in the low-temperature NMR provides evidence for the plausible mechanism of the DMF-modulated a-selective Kdo glycosylation.