Sialic acids are nine carbon keto sugar acids that are essential in human pathology. The most common sialic acid, N-acetylneuraminic acid (NeuAc), is the parent sialic acid in the family. Legionaminic acid (Leg) and pseudaminic acid (Pse) are 9-deoxy analogues of NeuAc that are rare but essential. Gram-negative pathogenic bacteria contain legionaminic acid and pseudaminic acid glycosides, which are important in their pathogenic processes. Chapter one describes the biology and chemistry of legionaminic acid and pseudaminic acid including their previously reported syntheses.The development of improved syntheses of legionaminic acid (Leg) and pseudaminic acid (Pse) donors is described in chapter two. Cleavage of the N-acetylneuraminic acid side chain at C7 gives the corresponding aldehyde, which, after conversion to the Ellman S or R sulfinimines, was coupled with acetaldehyde in a samarium iodide-mediated process to install the new side chains stereoselectively. This side chain exchange strategy afforded legionaminic acid, 8-epi-legionaminic acid, acetaminic acid, and 8-epi-acetaminic acid donors in 12 steps, along with a pseudaminic acid donor in 15 steps from N-acetylneuraminic acid. These conformationally constrained donors showed different reactivities and selectivities in glycosylation reactions compared to their mono cyclic donors. Therefore, side chain conformation was identified as an important factor, which influences reactivity and selectivity in the glycosylation reactions. Chapter three reviews, influence of side chain conformations in various glycosyl donors on glycosylation reactivity and selectivity. Chapter four describes the syntheses of four hexofuranosyl thiglycoside donors, analysis of their side chain conformations, and their glycosylation reactions. A L-idofuranosyl donor and a D-altrofuranosyl donor with predominant gt conformation and a L-galactofuranosyl donor with the gg conformation showed 1,2-cis-selectivity in their glycosylations. In contrast, a D-glucofuranosyl donor with the most electron withdrawing tg conformation was 1,2-trans-selective. Low temperature NMR studies revealed the formation of configuration dependent complex mixtures from the activated D-glucofuranosyl and L-idofuranosyl donors, which were interpreted as indicative of the formation of bridged bicyclic oxonium ions by participation of various benzyl ethers.