α-Stereoselective 3-Deoxy-d- manno -oct-2-ulosonoic Acid (Kdo) O -Glycosylation with a p -Toluenethioglycoside Donor by the ( p -Tol) 2 SO/Tf 2 O Preactivation Strategy.

A convenient and efficient approach was developed to synthesize α-Kdo O -glycosides based on the Tf 2 O/( p -Tol) 2 SO preactivation strategy using peracetylated Kdo thioglycoside as a donor. Under the optimized reaction conditions, several O -glycoside products, including α-(2 → 1)-, α-(2 → 2)-, α-(2 → 3)-, and α-(2 → 6)-Kdo products, were stereoselectively synthesized in high yields. Remarkably, a series of aromatic α-Kdo O -glycosides were first and successfully constructed in high yields. An S N 2-like mechanism was revealed by DFT calculations and experimental results.