Iterative Synthesis of Leishmania Phosphoglycans by Solution, Solid-Phase, and Polycondensation Approaches without Involving Any Glycosylation

Abstract

A general strategy (solution, solid-phase, and polycondensation) for the synthesis of antigenic phosphoglycans (PG) of the protozoan parasite Leishmania is presented. Phosphoglycans constitute the variable structural and functional domain of major cell-surface lipophosphoglycan (LPG) and secreted proteophosphoglycan (PPG), the molecules involved in infectivity and survival of the Leishmania parasite inside human macrophages. We have shown that the chemically labile, anomerically phosphodiester-linked phosphoglycan repeats can be assembled in an iterative and efficient manner from a single key intermediate, without involving any glycosylation steps. Furthermore, the phosphoglycan chain can be extended toward either the nonreducing (6'-OH) or the reducing (1-OH) end. We also describe a new and efficient solid-phase methodology to construct phosphoglycans based on design and application of a novel cis-allylphosphoryl solid-phase linker that enabled the selective cleavage of the first anomeric-phosphodiester linkage without affecting any of the other internal anomeric-phosphodiester groups of the growing PG chain on the solid support. The strategy to construct larger phosphoglycans in a one-pot synthesis by polycondensation of a single key intermediate is also described, enabling CD spectrometric measurements to show the helical nature of phosphoglycans. Our versatile synthetic approach provides easy access to Leishmania phosphoglycans and the opportunity to address key immunological, biochemical, and biophysical questions pertaining to the phosphoglycan family (LPG and PPG) unique to the parasite.

Natural phosphoglycans containing glycosyl phosphate units: structural diversity and chemical synthesis

“…236 The branched phosphoglycans 236-238 238 The preparation of the L. donovani hexaglycosyl triphosphate structure 249 (which is similar to phosphoglycan 226, but lacking the dec-9-enyl moiety) was reported by the New Delhi group (Ruhela and Vishwakarma). 243,244 They operated a different protecting-group pattern (Scheme 23): acetic esters for permanent O-protection and the 6 0 -TBS ether for temporary protection. The first chain elongation (244+245) afforded the protected tetrasaccharide phosphate derivative 246 (74%), which then was converted into both the monohydroxyl derivative 247 (85%, through desilylation) and the Hphosphonate derivative 250 (86%, through consecutive anomeric deprotection and H-phosphonylation).…”

“…The range of apparent chain length was estimated as n = 4- 25. The preparation of the L. donovani phosphoglycan 306, which differs from 303 in the chain length, was reported by the New Delhi group. 244 Polycondensation of the bifunctional monomer 304, containing O-acetyl (instead of O-benzoyl) protecting groups, and subsequent oxidation seems to favour the formation of longer polymer chains. After deacetylation of the reaction product (305), polymer 306 with a degree of polymerisation n = 19-22 was isolated in 58% yield.…”

Total synthesis of the fully lipidated glycosylphosphatidylinositol (GPI) anchor of malarial parasite Plasmodium falciparum

“…Keeping in view the remarkable biological role [4][5][6][7] of Pf-GPIs in the malarial biology and immunology, efficient methods for synthesis of Pf-GPIs and their structural and functional mimics are critical for the further advances in this field of glycobiology. We have had continuing interest in the chemical biology of GPI molecules and related phospholycans, including new synthesis, 13,[16][17][18] biosynthesis, [19][20][21][22] and cell biology. 23 …”

Synthetic Fragments of Antigenic Lipophosphoglycans from Leishmania major and Leishmania mexicana and Their Use for Characterisation of the Leishmania Elongating α‐D‐Mannopyranosylphosphate Transferase

“…The preparation of compounds 5-9 was based on the principles of glycosyl hydrogenphosphonate chemistry, which was developed in this laboratory [19,23,24] for the synthesis of the phosphosaccharides 1-4 and was recently applied by the New Delhi group [27][28][29][30] for the synthesis of more Leishmania LPG synthetic fragments. The first examples of the hydrogenphosphonate method used for the preparation of glycoside phosphodiesters of biological origin were described by van Boom and co-workers [31] and by one of us (A.V.N.)…”