Synthesis of Glycosylphosphatidylinositol Analogues with an Unnatural -D-Glucosamine-(1→6)--Inositol Motif.

Glycosylphosphatidylinositol (GPI) anchors contain a unique α-D-glucosamine-(1→6)--inositol [αGlcN(1,6)Ins] motif in their conserved core structure. To facilitate investigations of the functional roles of this structural motif, two GPI analogues containing unnatural βGlcN(1,6)Ins, instead of αGlcN(1,6)Ins, and an alkyne group at different positions of the GPI core were designed and synthesized. To this end, an orthogonally protected pseudopentasaccharide derivative of GPIs with the βGlcN(1,6)Ins motif was convergently constructed via [3+2] glycosylation and used as the common intermediate to prepare both GPI analogues by streamlined synthetic protocols. The pseudopentasaccharide intermediate and developed protocols can be widely applicable to access various GPI analogues with the βGlcN(1,6)Ins motif. The target GPI analogues contain an alkyne, which allows their further modification to introduce various molecular labels via click chemistry, making them useful probes for the study of GPI anchorage. The differences in reactivity and NMR behavior of the two GPI analogues, as well as the differences of these analogues from previously reported GPI derivatives of similar structure containing an αGlcN(1,6)Ins motif, suggest that the 2-O-phosphoethanolamine moiety on mannose-I and the linkage form of GlcN in GPIs can have a decisive impact on the structure, which is likely relevant to biology.