Synthesis of beta-D-GlcpNAc-(1-->2)-5a-carba-alpha-D-Man p-(1-->6)-beta-D- Glcp-O(CH2)7CH3: a reactive acceptor analog for N-acetylglucosaminyltransferase-V

Synthesis of 5a,5a'-dicarba-d-glucobioses from conformationally restricted carbaglucosyl triflates using SN2-type inversion with carbaglucosyl nucleophiles

Novel carbohydrate mimics were designed which contain two 5a-carba-d-glucose residues, one each at reducing and nonreducing end, and thus these mimics are 5a,5a'-dicarba-d-glucobioses. Dicarbadisaccharides have attractive features such as stability against endogenous degradative enzymes and being resistant to glycation reactions such as the Maillard reaction. For the synthesis of dicarba-β-d-isomaltose derivatives, the carbaglucosyl triflate locked in ⁴C₁ conformation was synthesized by protecting with butane-2,3-diacetal group or benzylidene group. Then, 5a,5a'-dicarba-β-d-maltose and 5a,5a'-dicarba-α,β-d-trehalose were synthesized by the S_N2-type inversion reaction using 4,6-O-benzylidene carbaglucosyl triflate with 4-OH and 1-OH carba-β-d-glucose derivatives, respectively, and similarly 5a,5a'-dicarba-α-d-isomaltose with 6-OH carba-α-d-glucose derivative.

From 1,4-Disaccharide to 1,3-Glycosyl Carbasugar: Synthesis of a Bespoke Inhibitor of Family GH99 Endo-α-mannosidase

Understanding the enzyme reaction mechanism can lead to the design of enzyme inhibitors. A Claisen rearrangement was used to allow conversion of an α-1,4-disaccharide into an α-1,3-linked glycosyl carbasugar to target the endo-α-mannosidase from the GH99 glycosidase family, which, unusually, is believed to act through a 1,2-anhydrosugar "epoxide" intermediate. Using NMR and X-ray crystallography, it is shown that glucosyl carbasugar α-aziridines can act as reasonably potent endo-α-mannosidase inhibitors, likely by virtue of their shape mimicry and the interactions of the aziridine nitrogen with the conserved catalytic acid/base of the enzyme active site.

Biosynthesis and Biological Activity of Carbasugars

The first synthesis of carbasugars, compounds in which the ring oxygen of a monosaccharide had been replaced by a methylene moiety, was described in 1966 by Professor G. E. McCasland’s group. Seven years later, the first true natural carbasugar (5a-carba-R-D-galactopyranose) was isolated from a fermentation broth of Streptomyces sp. MA-4145. In the following decades, the chemistry and biology of carbasugars have been extensively studied. Most of these compounds show interesting biological properties, especially enzymatic inhibitory activities, and, in consequence, an important number of analogues have also been prepared in the search for improved biological activities. The aim of this review is to give coverage on the progress made in two important aspects of these compounds: the elucidation of their biosynthesis and the consideration of their biological properties, including the extensively studied carbapyranoses as well as the much less studied carbafuranoses.

Three-dimensional homology model of GlcNAc-TV glycosyltransferase

The enzyme UDP-N-acetylglucosamine: α-d-mannoside β-1-6 N-acetylglucosaminyltransferase V (GnT-V) catalyzes the transfer of GlcNAc from the UDP-GlcNAc donor to the α-1-6-linked mannose of the trimannosyl core structure of glycoproteins to produce the β-1-6-linked branching of N-linked oligosaccharides. β-1-6-GlcNAc-branched N-glycans are associated with cancer growth and metastasis. Therefore, the inhibition of GnT-V represents a key target for anti-cancer drug development. However, the development of potent and specific inhibitors of GnT-V is hampered by the lack of information on the three-dimensional structure of the enzyme and on the binding characteristics of its substrates. Here we present the first 3D structure of GnT-V as a result of homology modeling. Various alignment methods, docking the donor and acceptor substrates, and molecular dynamics simulation were used to construct seven homology models of GnT-V and characterize the binding of its substrates. The best homology model is consistent with available experimental data. The three-dimensional model, the structure of the enzyme catalytic site and binding information obtained for the donor and acceptor can be useful in studies of the catalytic mechanism and design of inhibitors of GnT-V.

A Convenient Approach for Access to Both Carbapentofuranoses and Carbahexopyranoses. Stereocontrolled Synthesis of Enantiopure Carba-d-ribofuranoses, Carba-d-arabinofuranoses and Carba-l-gulopyranose

A new approach to carbasugars in enantiomerically pure form is reported. The key step involves ring-closing metathesis of dienols 6 derived from a (R)-(+)-glyceraldehyde derivative 4 to form the substituted cyclopentenol 9 and cyclohexenol 34a. Stereocontrolled addition of hydroxyl groups followed by conversion of the ketal unit to hydroxymethyl group in these intermediates led to carbapentoses and -hexoses. Stereoselectivity during introduction of hydroxyl groups arises through the steric hindrance posed by the allylic substituents. A remarkable feature of the present approach is the accessibility of both d- and l-series of carbapentoses as illustrated by the synthesis of beta-D- and beta-L-carbaribofuranoses 17 and 20, respectively. Carba-alpha-D-ribofuranose 25, the biosynthetic intermediate to the antibiotic aristeromycin, has also been synthesized from the same cyclopentenol 9. Functional group manipulation in the cyclopentenol 9a also enabled access to carbaarabinofuranose 32. The present synthetic strategy can be extended for the synthesis of carbahexopyranose, as illustrated by the synthesis of carba-alpha-L-gulopyranose 40b.

Synthesis and conformational investigation of methyl 4a-carba-D-arabinofuranosides

The synthesis of carbasugar analogues of methyl alpha-D-arabinofuranoside and methyl beta-D-arabinofuranoside (3 and 4) is reported. The route developed involves the conversion of D-mannose into a suitably protected diene (13), which is then cyclized via olefin metathesis. The resulting cyclopentene (14) is stereoselectively hydrogenated to provide an intermediate that can be used for the synthesis of both targets. Through the use of NMR spectroscopy, we have probed the ring conformation of 3 and 4, as well as the rotamer populations about the C(4)-C(5) and C(1)-O(1) bonds. These studies have demonstrated that there are differences in ring conformation between these carbasugars and their glycoside parents (1 and 2). However, only minor differences are seen in the rotameric equilibrium about the C(4)-C(5) bond in 3 and 4 relative to 1 and 2. In regard to the C(1)-O(1) bond, NOE data from 3 and 4 suggest that the favored position about this bond is similar to that in the glycosides; that is, the methyl group is anti to C(2). However, confirmation of this preference through measurement of (3)J(C,C) between the methyl group and C(2) or C(4a) was not successful.

Total synthesis of both methyl 4a-carba-D-arabinofuranosides

[reaction: see text] The total synthesis of methyl 4a-carba-alpha-D-arabinofuranoside (1) and methyl 4a-carba-beta-D-arabinofuranoside (2) has been achieved starting from D-mannose (5). Key transformations included a ring-closing metathesis of diene 11 employing Schrock's catalyst followed by a stereoselective hydrogenation with Wilkinson's catalyst.

Galactosyl transfer ability of beta-(1-->4)-galactosyltransferase toward 5a-carba-sugars

Bovine beta-(1-->4)-galactosyltransferase was assayed with a series of 5a-carba-sugars, i.e., sugar analogues in which the ring oxygen of pyranose is replaced by a methylene group. The analogues are 5a-carba-sugar of 2-acetamido-2-deoxy-alpha-DL-galactopyranose, both alpha and beta anomers of 2-acetamido-2-deoxy-DL-glucopyranose (5a-carba-DL-GlcNAc), and 2-acetamido-2-deoxy-DL-mannopyranose. Of these analogues, both alpha and beta anomers of 5a-carba-DL-GlcNAc act as an acceptor. Enzymatic synthesis using the alpha and beta anomers of 5a-carba-DL-GlcNAc afforded the corresponding D-Gal-beta-(1-->4)-5a-carba-alpha-D-GlcNAc and D-Gal-beta-(1-->4)-5a-carba-beta-D-GlcNAc on a practical scale, and these structures were confirmed by NMR spectroscopy. These results indicate that the ring oxygen atom in the 5a-carba-D-GlcNAc is not used for specific recognition by bovine beta-(1-->4)-galactosyltransferase.

Utilization of glycosyltransferases to change oligosaccharide structures

Carbohydrates on cell surfaces are important biomolecules in various biological recognition processes. Elucidation of the biological roles of complex oligosaccharides necessitates an efficient methodology to synthesize these compounds and their analogs. Enzymatic synthesis renders itself to be useful in the construction of an oligosaccharide structure owing to its mild reaction condition, high regio- and stereoselectivity. This review article focuses on the recent progress in oligosaccharide syntheses catalyzed by glycosyltransferases, namely sialyltransferase, galactosyltransferase, fucosyltransferase, and N-acetylglucosaminyltransferase. A survey of the latest patent and literature related to this field is also included.

Unusual beta-D-xylosides that prime glycosaminoglycans in animal cells

The biosynthesis of glycosaminoglycans (GAG) takes place while the polysaccharide chains are usually attached to a proteoglycan core protein. Cells also will assemble GAG chains on beta-D-xylosides containing hydrophobic aglycones. In order to evaluate the relationship of the structure of the sugar to priming activity of the glycoside, we synthesized beta-D-xyloside analogs in which the hydroxyls were substituted with hydrogen, fluorine, -O-methyl, amino, -O-isopropyl, and -O-benzyl groups. Epimers at the 2-, 3-, and 4-position of xylose also were made. Their ability to prime GAGs was tested in Chinese hamster ovary cells by measuring 35SO4 incorporation into polysaccharide chains and by assaying the transfer of galactose to the xylosides by galactosyltransferase I (UDP-D-galactose:xylose beta1-4-galactosyltransferase) in vitro. All of the analogs failed to act as primers of GAGs in vivo and as substrates in vitro with the following exceptions. Substitution of 2-OH and 3-OH with -OCH3 were active at high concentration (1 mM), but the deoxygenated derivatives were inactive. Efficient priming also occurred on a derivative with fluorine instead of the 3-OH group, suggesting that the oxygen atoms at C-2 and C-3 were involved as hydrogen bond acceptors. Methylated and deoxy analogs at C-4 were inactive, due to the loss of the acceptor hydroxyl group. Interestingly, benzyl-beta-D-threo-pentopyranos-4-uloside (4-keto derivative) and benzyl-4-methyl-beta-D-xyloside, with a methyl group in place of an axial hydrogen at C-4, primed GAG chains. Priming by these unusual xylosides suggests the possibility of designing inhibitors of GAG synthesis based on xyloside analogs with reactive groups in key positions.

Synthesis of carbocyclic analogues of the mannosyl trisaccharide: ether- and imino-linked methyl 3,6-bis(5a-carba-alpha-D-mannopyranosyl)-3,6-dideoxy-alpha-D-mannopyran osides

Carbocyclic analogues 2 and 3 of the "trimannosyl structure 1", methyl 3,6-bis(5a-carba-alpha-D-mannopyranosyl)-3,6-dideoxy-alpha-D-ma nnopyranosides bonded by way of respective ether and imino linkages, were synthesized. The ether 2 had no inhibitory activity against alpha-D-mannosidase; in contrast, the imino compound 3 was a mild inhibitor. Furthermore, the inhibitory activity of 4, related to 3 by introduction of unsaturation between C-5 and C-5a of the carba-sugar moieties, was shown to be somewhat greater.