Synthesis and application of sialic acid-containing building blocks for glycopeptide libraries. Establishing glycosylation conditions

Three different sialic acid-containing building blocks (6-8) were synthesiz ed for use in solid-phase glycopeptide libraries. Investigation of the cond itions for glycosylation of threonine (Thr) with various sialic acid donors revealed that the best results were obtained by coupling glycosyl xanthate 2 to the acceptors Fmoc-Thr-OH (5) or the alpha-azido acid analogue of Thr , 4. Among several catalysts employed, phenylsulfanyl triflate (PST) afford ed the best yields. Both the N-Fmoc and alpha-azido analogues of Thr allowe d glycosylation with good stereoselectivity in 80% (--> 8) and 84% (--> 6) yield, respectively. Introduction of a phenylthio group in the 3 position o f the sialic acid donor 3, to assist the stereoselective outcome of the gly cosylation reaction, gave good results; however difficulties in the removal of the phenylthio auxiliary group made this route less attractive. Both building blocks 6 and 8 were successfully introduced in solid-phase gl ycopeptide synthesis. Interestingly, alkaline deprotection of the Fmoc grou p of 8, necessary for subsequent introduction of amino acids, resulted in a n immediate attack of the alpha-amino group on the sialic acid methyl ester to form the lactam 14. This side reaction was also observed during reducti on of the azido acid building block 6 under alkaline conditions, but could be suppressed by performing the reduction under acidic conditions. Lactam f ormation was completely avoided by hydrolysis of the methyl ester prior to reduction of the azide.