Stereoselective Anomeric Phosphorylation under Modified Mitsunobu Reaction Conditions

Glycosyl phosphates are vital components of various biomolecules and serve as intermediates in the biosynthesis of substrates for glycosyltransferases such as nucleotide-activated sugars and dolichol-phosphate sugars. Their limited availability underscores the importance of stereoselective chemical synthesis of glycosyl phosphates with a defined anomeric configuration. A particular challenge lies in the generation of glycosyl phosphates with an anomeric configuration opposite to that of the starting lactol, as exemplified by l/d-<i>glycero</i>-β-d-<i>manno</i>-heptosyl phosphate-precursors in the synthesis of the pathogen-associated molecular pattern ADP-heptose, which triggers innate immune signaling through interaction with α-kinase-1 (ALPK1). The modification of the Mitsunobu reaction enabled stereospecific anomeric phosphorylation and efficient synthesis of challenging β-heptosyl phosphates. Mechanistic studies using NMR spectroscopy guided the differentiation between two possible reaction pathways, proceeding either with the inversion or with retention of the anomeric configuration. The identification of (diglycosyloxy)phosphoranes as reactive species enabled the development of specialized reaction protocols that facilitate the stereoselective synthesis of glycosyl phosphates with inversion of the anomeric configuration via an SN2 mechanism. The use of auxiliary reagents altered the reaction pathway, leading instead to the formation of glycosyl phosphates with the retention of the anomeric configuration.