An efficient stereoselective synthesis of alpha-(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 alpha-selective sialylation with various primary, secondary, and tertiary acceptors under in situ activation conditions (NIS/TfOH, -78 degrees 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 alpha(2-->9)-tetrasialic acid.
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