Synthesis of Core-class 2 O-Linked Glycopeptides: a Benzyl-protected Tetrasaccharyl Serine and its Derivative Carrying a Hydrophobic Cholestanyl Group

A core-class 2 tetrasaccharide-linked serine was synthesized in a convergent manner. The coupling reaction of disaccharide glycosyl donor 3 and acceptor 4 stereoselectively afforded tetrasaccharide 15, which was converted to glycosyl fluoride 20. Glycosylation of Fmoc serine allyl ester 5 with 20 produced α- and β-glycosides in 40% and 33% yields, respectively. α-Isomer 21 was converted into 1, a useful building block for the solid-phase synthesis of glycopeptides. On the other hand, 21 was N-deprotected and condensed with hydrophobic cholestanol through a succinyl spacer. The same compound was alternatively synthesized by coupling 20 and 28. Functional group manipulation and hydrogenation afforded core 2 tetrasaccharide-cholestanol conjugate 2.