5-N,4-O-Carbonyl-7,8,9-tri-O-chloroacetyl-Protected Sialyl Donor for the Stereoselective Synthesis of α-(2→9)-Tetrasialic Acid

An efficient stereoselective synthesis of α-(2→9)-tetrasialic acid was achieved using tri-O-chloroacetyl-derivatized sialyl donor and a triol sialyl acceptor. Both the acceptor and the donor were also protected with a cyclic 5-N-4-O-carbonyl protecting group. The donor is highly reactive and enabled α-selective sialylation with various primary, secondary, and tertiary acceptors under in situ activation conditions (NIS/TfOH, −78 °C, acetonitrile/dichloromethane). The trans-fused oxazolidinone ring and O-chloroacetyl protecting groups were easily removed under mild reaction conditions to provide the fully deprotected α(2→9)-tetrasialic acid.