Dissecting the influence of oxazolidinones and cyclic carbonates in sialic acid chemistry

Probing the influence of a 4,6-O-acetal on the reactivity of galactopyranosyl donors: verification of the disarming influence of the trans-gauche conformation of C5-C6 bonds

The effect of a 4,6-O-alkylidene acetal on the rate of acid-catalyzed hydrolysis of methyl galactopyranosides and of spontaneous hydrolysis of 2,4-dinitrophenyl galactopyranosides has been studied through the synthesis and hydrolysis of analogs in which O6 is replaced by a methoxymethylene unit in which the methoxy group adopts either an equatorial or an axial position according to the configuration. Consistent with earlier studies under both acid-catalyzed and spontaneous hydrolysis conditions, the alkylidene acetal, or its 7-carba analog, retards hydrolysis with respect to comparable systems lacking the cyclic protecting group. The configuration at C6 in the 7-carba analogs does not influence the rate of acid-catalyzed hydrolysis but has a minor influence on the rate of spontaneous hydrolysis of the 2,4-dinitrophenyl galactosides, confirming earlier studies on the role played by the hydroxymethyl group conformation on glycoside reactivity. The benzylidene acetal is found to stabilize the α-anomer of galactopyranose derivatives relative to monocyclic analogs. Reasons for the α-selectivity of 4,6-O-benzylidene-protected galactopyranosyl donors bearing neighboring group-active protecting groups at O2 are discussed.

Exploration of the Oxazolidinthione Protecting System for the Synthesis of Sialic Acid Glycosides

An N-acetyl oxazolidinthione protected sialyl thioglycoside was synthesized and its use as a sialyl donor studied. The strongly electron-withdrawing nature of the oxazolidinthione moiety is such that activation could not be achieved at -78 °C. Couplings were therefore conducted at the lowest convenient temperature (-50 °C). Glycosides were formed in good yield but in two out three cases studied selectivities were lower than those seen with the corresponding N-acetyl oxazoldinone protected donor. The resulting N-acetyl oxazolidinthione protectd disaccharides were converted to the corresponding N-acetyl oxazolidinones by treatment with N-iodosuccinimide and triflic acid in the presence of water at 0 °C.