In this study, we designed novel double carbonyl-protected sialyl donors that are able to control the conformation of glycerol side chains to enable highly α-selective α(2,8)-sialylation. BothN-acetyl and N-glycolyl derivatives were prepared and applied to the synthesis of structurally defined oligosialic acids. Using iterative cycles of 8-O-deprotection and glycosylation, we achieved efficient chain elongation in both disialic and octasialic acids in high yields, with complete α-selectivity at each step. Notably, the strategy was effective for both Neu5Ac- and Neu5Gc-containing sequences with loss of neither selectivity nor reactivity. Optimized deprotection conditions involving S-alkylthiocarbamate formation, selective oxidation, and mild basic hydrolysis allowed complete removal of protecting groups without N-acyl cleavage, which afforded α(2,8)-linked oligosialic acids in good yields. This method provides a robust platform for the synthesis of long-chain sialic acid oligomers, which includes the Neu5Gc-containing analogues that are valuable for studying biological functions and developing glycan-based probes for Siglec recognition.