Why Are the Hydroxy Groups of Partially Protected N -Acetylglucosamine Derivatives Such Poor Glycosyl Acceptors, and What Can Be Done about It? A Comparative Study of the Reactivity of N -Acetyl-, N -Phthalimido-, and 2-Azido-2-deoxy-glucosamine Derivatives in Glycosylation. 2-Picolinyl Ethers as Reactivity-Enhancing Replacements for Benzyl Ethers