Researchers from the Zelinsky Institute synthesized a fragment of the repeating unit of bacteria Azospirillum brasilense sp7 polysaccharide and proposed a new stereocontrol mechanism for glycosylation
Gram-negative bacteria of Azospirillum genus promote plant growth and form symbiotic association with various plants. These microorganisms have a number of promising applications in agricultural biotechnology due to their ability to fix atmospheric nitrogen, produce phytohormones, increase the solubility of phosphates, and act as antagonists against many phytopathogens. At the present time, synthesis of the various fragments of chemical repeating units of Azospirillum polysaccharides and their subsequent use for the inhibition of the biological activity of Azospirillum polysaccharides is the only approach for the determination of the monosaccharide sequence (the structure of biological repeating unit) in O-specific polysaccharides, which is important for understanding the processes of plant– microbe symbiosis
During the synthesis of protected tetrasaccharide fragment of the repeating unit of polysaccharide of Azospirillum brasilense sp7 researchers from the Zelinsky Institute found that glycosylation with a mixture of protected N-trifluoroacetyl-protected glucosamine N-phenyltrifluoroacetimidate and the corresponding 2-trifluoromethyl-oxazoline led to 1,2-cis-linked glycoside along with the expected 1,2-trans-linked isomer. A new mechanism for glycosylation stereocontrol was proposed for the explanation of this quite unusual fact. It involves hydrogen-bond mediated complexation of hydrogen atom of NHTFA at C-2 of oxacarbenium ion derived from glycosyl donor and oxygen atom of hydroxy group of the incoming nucleophile.
The obtained protected tetrasaccharide and intermediate oligosaccharides with cleavable pre-spacer 4-(2-chloroethoxy)phenyl aglycon will be further used in the synthesis of more complex oligosaccharide fragments of rhizobacterium A. brasilense sp7 and neoglycoconjugates thereof.
Source:
P. Abronina, A. Zinin, A. Chizhov, L. Kononov, Unusual outcome of glycosylation: hydrogen-bond mediated control of stereoselectivity by N-trifluoroacetyl group? Eur. J. Org. Chem., accepted manuscript, DOI: 10.1002/ejoc.202000520