C-glycosidic analogues of lipid A and lipid X: synthesis and biological activities.

The synthesis of a series of novel analogues of lipid A, the lipophilic terminal of lipopolysaccharides (LPS), and lipid X, the reducing monosaccharide unit in lipid A, is reported. In these compounds, the native 1-O-phosphate group has been replaced by a "bioisosteric" CH2COOH substituent. The new N,O-acylated monosaccharide C-glycosides were obtained by Witting reaction of suitably protected glucosamine derivatives. These lipid X analogues were recognized as substrates by the enzyme lipid A synthase and could be coupled with UDP-lipid X to afford the corresponding disaccharide analogues of the lipid A precursor on preparative scale. All compounds were characterized by NMR, MS, and elemental analysis, and were tested for their ability to enhance nonspecific resistance to infection in mice and also for endotoxicity. The results clearly show that the new compounds express biological activities similar to those of their O-phosphorylated natural counterparts. Furthermore, these compounds exhibit a better therapeutic index in mouse models than the standard LPS obtained from Salmonella abortus equi.