Improved synthesis of the 2-, 3-, and 4-deoxy derivatives from methyl beta-D-galactopyranoside

Improved Synthesis of Sulfur-Containing Glycosides by Suppressing Thioacetyl Migration

Complex mixtures were often observed when we attempted to synthesize 4-thio- and 2,4-dithio-glycoside derivatives by double parallel and double serial inversion, thus leading to no or low yields of target products. The reason was later found to be that many unexpected side products were produced when a nucleophile substituted the leaving group on the substrate containing the thioacetate group. We hypothesized that thioacetyl migration is prone to occur due to the labile thioacetate group even under weak basic conditions caused by the nucleophile, leading to this result. Therefore, we managed to inhibit the generation of thiol groups from thioacetate groups by the addition of an appropriate amount of conjugate acid/anhydride, successfully improving the synthesis of 4-thio- and 2,4-dithio-glycoside derivatives. The target products which were previously difficult to synthesize, were herein obtained in relatively high yields. Finally, 4-deoxy- and 2,4-dideoxy-glycoside derivatives were efficiently synthesized through the removal of thioacetate groups under UV light, starting from 4-thio- and 2,4-dithio-glycoside derivatives.

Regioselective Manipulation of GlcNAc Provides Allosamine, Lividosamine, and Related Compounds

Palladium-catalyzed oxidation of isopropyl N-acetyl-α-d-glucosamine (GlcNAc) is used to prepare the rare sugars allosamine, lividosamine, and related compounds with unprecedented selectivity. The Passerini reaction applied on 3-keto-GlcNAc provides an entry into branching of the carbon skeleton in this compound.

Stannane-Free Chemoselective Hydrodehalogenation of 4-Halotetrahydropyrans: Scope and Application to Natural Product Synthesis

A stannane-free hydrodehalogenation of 4-halotetrahydropyran under very mild conditions has been developed. This methodology allows selective one-pot dehalogenation and/or debenzylation depending on the type of halide-substrate used. The applicability of this methodology is well demonstrated in the synthesis of a key intermediate toward the enantioselective synthesis of (+)-SCH 351448.

Synthesis of 4-Deoxy-l-(and d-)hexoses from Chiral Noncarbohydrate Building Blocks

4-Deoxy-l-hexoses were synthesized starting from our previously reported reagent 1 and (R)-benzyl glycidyl ether, which led in few steps to a substituted dihydropyran 6. The stereocontrolled hydroxylation of the latter afforded the corresponding 4-deoxy-l-hexoses 7a, 9, and 11. The same procedure, starting from (S)-benzyl glycidyl ether, enabled the preparation of their d-series enantiomers.

Negative ion fast atom bombardment and collision-induced dissociation mass spectrometry of the 2-, 3-, 4- and 6-deoxy derivatives from methyl beta-D-galactopyranoside and related compounds

The fast atom bombardment collision-induced dissociation mass spectra of the [M-H]- ions of the 2-, 3-, 4- and 6-deoxy derivatives from methyl beta-D-galactopyranoside and some related compounds have been recorded. The fragmentation reactions of these quasimolecular ions and of OD-labelled analogues have been examined and related to the molecular structure. In some cases distinct and common mechanisms can be derived, but it is also clear from these experiments that not only the site of deprotonation of the molecules, but also the anticipated charge localization in the fragment ions strongly direct the fragmentation reactions.

Synthesis of methyl O-alpha-L-rhamnopyranosyl-(1-->2)-alpha-D-galactopyranosides specifically deoxygenated at position 3, 4, or 6 of the galactose residue

The title disaccharides were synthesized by condensation of 2,3,4-tri-O-benzoyl-alpha-L-rhamnopyranosyl bromide with suitably protected, deoxygenated derivatives of methyl alpha-D-galactopyranoside. Deoxygenation was achieved via activation of a protected methyl alpha-D-gluco- or galacto-pyranoside with N,N'-thiocarbonyldiimidazole followed by treatment with tributyltin hydride and azobisisobutyronitrile. At position 3, the deoxygenation was more successful when performed with the tri-O-benzoylated precursor, rather than the tri-O-benzylated one. The corresponding nucleophile was obtained by benzylidenation of the methyl 3-deoxy-alpha-D-xylo-hexopyranoside. The preparation of the glycosyl acceptor deoxygenated at position 4 could be pursued starting from derivatives having either the D-galacto or the D-gluco configuration. The pathway involving the former was found superior.

Synthesis of specifically deoxygenated ligands related to (1----6)-beta-D-galacto-oligosaccharides, and studies on their binding to monoclonal antigalactan antibodies

Synthetic deoxygenated derivatives of methyl beta-glycosides of (1----6)-beta-D-galacto-oligosaccharides were prepared, and their binding to antigalactan monoclonal antibodies X24 and J539 (Fab') was studied. The results suggest the involvement of an additional, critical hydrogen bond in the highest affinity subsite (A), which now appears to require two hydrogen bonds from the 2- and 3-hydroxyls of the galactosyl residue to the protein, and one from the protein to O-4 of that residue. The data obtained with a series of oligosaccharides deoxygenated at position 3(1), 3(2), 3(3), 4(1), 4(2), or 4(3) support the binding patterns and subsite-arrangement inferred previously from studies with large numbers of deoxyfluoro-substituted ligands and this family of antibodies.

Synthesis of a series of methyl beta-glycosides of (1----6)-beta-D-galacto-oligosaccharides having one residue deoxygenated at position 3

Methyl beta-glycosides of beta-(1----6)-linked D-galactobioses (13 and 16) and -galactotrioses (21, 24, and 26) containing a 3-deoxy-beta-D-xylo-hexopyranosyl moiety either as one of the end units or the internal unit have been synthesized. The extension of the oligosaccharide chain was achieved, inter alia, by the use of two newly synthesized glycosyl donors derived from 3-deoxy-D-xylo-hexopyranose, namely, 2,4,6-tri-O-benzoyl-3-deoxy-a-D-xylo-hexopyranosyl chloride (8) and 2,4-di-O-benzoyl-6-O-bromoacetyl-3-deoxy-a-D-xylo-hexopyranosyl chloride (10). Glycosylation reactions were mediated by silver triflate under base-deficient conditions.