Synthesis of carboxyl-reduced analogues related to the Chlamydia-specific Kdo trisaccharide epitope

Synthesis of deoxy analogues of (2→8)-linked 3-deoxy-α-D-manno-oct-2-ulopyranosylonic acid (Kdo) disaccharides for binding studies with Chlamydia specific monoclonal antibodies

Deoxy analogues of the Chlamydia-specific, α-(2→8)-linked Kdo disaccharide epitope modified at the pyranose ring of the terminal Kdo unit have been prepared. Utilizing the 3,5-dideoxy-D-arabino-oct-2-ulosonate bromide donor [1] and the acceptor [2] under Helferich conditions, the 5-deoxy-α-Kdo-(2→8)-α-Kdo disaccharide [3] was obtained as the minor product together with unsaturated, α-(2→8)-glycosidically and (4→8)-ether-linked derivatives [5] and [7] as the major components. Deprotection afforded the disaccharide allyl glycosides [4], [6], and [8]. Further transformation of protected intermediates by hydrogenation followed by deblocking gave the propyl glycosides [12], [14] and [17]. The compounds may be used for binding studies with Chlamydia-specific and cross-reactive, Kdospecific monoclonal antibodies.

Synthesis of Kdo-trisaccharide derivatives of chlamydial and enterobacterial LPS containing carboxyl-reduced or β-configurated Kdo-residues

Five allyl glycosides corresponding to the 3-deoxy-D- manno-2-octulosonic acid (Kdo) containing genus-specific LPS epitope of Chlamydia were synthesized. Compounds 5 and 22 contain one carboxyl-reduced Kdo moiety linked to O-4 of the proximal Kdo unit, whereas the analogues 28, 34 and 36 each contain one β-linked Kdo-residue within the trisaccharide sequence Kdo p-(2→8)-Kdo p-(2→4)-Kdo p. Elaboration of the carboxyl-reduced derivatives was achieved by BF3•Et2O-catalyzed coupling of Kdo-fluoride derivatives 1 or 14 with the 7,8- O-carbonyl-derivative 2. The β-linked oligosaccharides were obtained by Helferich-glycosidation of the respective Kdo-disaccharide bromide derivatives 26 and 31. The deprotected compounds - characterized by H and 13C NMR spectroscopy - are suitable haptens for the immunochemical study of monoclonal antibodies directed against the Kdo-region of chlamydial and enterobacterial LPS.

Synthesis of chlamydia lipopolysaccharide haptens through the use of α-specific 3-iodo-Kdo fluoride glycosyl donors

A scalable approach towards high-yielding and (stereo)selective glycosyl donors of the 2-ulosonic acid Kdo (3-deoxy-D-manno-oct-2-ulosonic acid) is a fundamental requirement for the development of vaccines against Gram-negative bacteria. Herein, we disclose a short synthetic route to 3-iodo Kdo fluoride donors from Kdo glycal esters that enable efficient α-specific glycosylations and significantly suppress the elimination side reaction. The potency of these donors is demonstrated in a straightforward, six-step synthesis of a branched Chlamydia-related Kdo-trisaccharide ligand without the need for protecting groups at the Kdo glycosyl acceptor. The approach was further extended to include sequential iteration of the basic concept to produce the linear Chlamydia-specific α-Kdo-(2→8)-α-Kdo-(2→4)-α-Kdo trisaccharide in a good overall yield.

Synthesis of Neoglycoconjugates Containing 4-Amino-4-deoxy-l-arabinose Epitopes Corresponding to the Inner Core of Burkholderia and Proteus Lipopolysaccharides

Disaccharides that contain 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) and d-glycero-d-talo-oct-2-ulosonic acid (Ko) substituted at the 8-position by 4-amino-4-deoxy-β-l-arabinopyranosyl (Ara4N) residues have been prepared. Coupling an N-phenyltrifluoroacetimidate-4-azido-4-deoxy-l-arabinosylglycosyl donor to acetyl-protected allyl glycosides of Kdo and Ko afforded anomeric mixtures of disaccharide products in 74 and 90 % yield, respectively, which were separated by chromatography. Further extension of an intermediate Ara4N-(1→8)-Kdo disaccharide acceptor, which capitalized on a regioselective glycosylation with a Kdo bromide donor under Helferich conditions, afforded the branched trisaccharide α-Kdo-(2→4)[β-l-Ara4N-(1→8)]-α-Kdo derivative. Deprotection of the protected di- and trisaccharide allyl glycosides was accomplished by TiCl₄-promoted benzyl ether cleavage followed by the removal of ester groups and reduction of the azido group with thiol or Staudinger reagents, respectively. The reaction of the anomeric allyl group with 1,3-propanedithiol under radical conditions afforded the thioether-bridged spacer glycosides, which were efficiently coupled to maleimide-activated bovine serum albumin. The neoglycoconjugates serve as immunoreagents with specificity for inner core epitopes of Burkholderia and Proteus lipopolysaccharides.

Synthesis of spacer-containing chlamydial disaccharides as analogues of the alpha-Kdop-(2-->8)-alpha-Kdop-(2-->4)-alpha-Kdop trisaccharide epitope

On the basis of high-resolution crystal structures of the antigen binding fragment of the chlamydia-specific monoclonal antibody S25-2 in complex with the trisaccharide alpha-Kdop-(2-->8)-alpha-Kdop-(2-->4)-alpha-Kdop and part structures thereof, seven modified alpha-Kdop-(2-->8)-alpha-Kdop disaccharide derivatives were synthesized starting from the protected disaccharide allyl ketoside 1. Hydroboration and subsequent oxidation as well as ozonolysis, respectively, followed by Wittig-reaction for chain elongation were used to install a terminal carboxylic group on spacer entities of various chain lengths. Furthermore, addition of methyl 2-thioacetate to the allyl group furnished the corresponding thioether derivative. Standard deprotection gave the target disaccharides as simplified trisaccharide analogues, which will be used to probe the contribution of the proximal carboxylic group in the binding of chlamydia-specific di- and trisaccharide-reactive monoclonal antibodies.