New results in the isopropylidenation of galactopyranosides. Useful intermediates for the synthesis of galactose derivatives

An Efficient and Concise Synthesis of α-Galactosylceramide

A concise and stereoselective synthesis of α-galactosylceramide (α-GalCer) is described. The key features of the synthetic strategy are the use of a phytosphingosine in which the amine is masked as a tetrachlorophthalimide and the diol as an isopropylidene acetal, and the galactosyl donor is protected as a 4,6-benzylidene to improve the α selectivity of the glycosylation reaction. The pattern of protecting groups on the donor and the acceptor have proven to give an excellent match of reactivity, allowing the glycosylation reaction to take place stereoselectively. The overall synthesis gave α-GalCer in good yields and in few steps.

The use of the novel glycosyl acceptor and supramer analysis in the synthesis of sialyl-α(2-3)-galactose building block

A new glycosyl acceptor to be used in sialylation was designed as a 3-hydroxy derivative of 4-methoxyphenyl β-d-galactopyranoside with 2-O-acetyl group and O-4 and O-6 protected as benzylidene acetal. Two alternative syntheses of this compound were compared. Sialylation of 3-OH group of the glycosyl acceptor with O-chloroacetylated N-trifluoroacetylneuraminic acid phenyl thioglycoside (NIS, TfOH, MeCN, MS 3 Å, -40 °C) was studied in a wide concentration range (5-150 mmol L^-1). The outcome of sialylation generally followed the predictions of supramer analysis of solutions of sialyl donor in MeCN, which was performed by polarimetry and static light scattering and revealed two concentration ranges differing in solution structure and the structures of supramers of glycosyl donor. The optimized conditions of sialylation (C = 50 mmol L^-1) were used to synthesize protected Neu-α(2-3)-Gal disaccharide (78%, α:β = 13:1), which was then converted to sialyl-α(2-3)-galactose imidate building block useful for the synthesis of complex sialo-oligosaccharides.

Organoiridium complexes: efficient catalysts for the formation of sugar acetals and ketals

[Cp(∗)IrCl(2)](2) is used as an efficient promoter for the synthesis of sugar acetals and ketals with good to excellent yields. The catalyst is found to be general for a wide range of sugars.

Galactose-derived phosphonate analogues as potential inhibitors of phosphatidylinositol biosynthesis in mycobacteria

Galactose-based phosphonate analogues of myo-inositol-1-phosphate and phosphatidylinositol have been synthesized from methyl beta-d-galactopyranoside. Michaelis-Arbuzov reaction of isopropyl diphenyl phosphite or triisopropyl phosphite with a 6-iodo-3,4-isopropylidene galactoside afforded the corresponding phosphonates. Deprotection of the diphenyl phosphonate afforded methyl beta-d-galactoside 6-phosphonate, an analogue of myo-inositol-1-phosphate. The diisopropyl esters of the diisopropyl phosphonate were selectively deprotected and the corresponding anion was coupled with 1,2-dipalmitoyl-sn-glycerol using dicyclohexylcarbodiimide. Deprotection afforded a methyl beta-d-galactoside-derived analogue of phosphatidylinositol. The galactose-derived analogues of phosphatidylinositol and myo-inositol-1-phosphate were not substrates for mycobacterial mannosyltransferases (at concentrations up to 1 mM) involved in phosphatidylinositol mannoside biosynthesis in a cell-free extract of Mycobacterium smegmatis. The galactose-derived phosphonate analogue of phosphatidylinositol was shown to be an inhibitor at 0.01 mM of PimA mannosyltransferase involved in the synthesis of phosphatidylinositol mannoside from phosphatidylinositol, and a weaker inhibitor of the next mannosyltransferase(s), which catalyzes the mannosylation of phosphatidylinositol mannoside.

One-pot acetalation–acetylation of sugar derivatives employing perchloric acid immobilised on silica

Perchloric acid immobilised on silica gel has been used as an efficient promoter for per-O-acetylation, and acetalation and subsequent O-acetylation of glycosides and thioglycosides in one-pot using stoichiometric reagents.

Mixed acetals of cyclodextrins. Preparation of hexakis-, heptakis- and octakis[2,6-di-O-(methoxydimethyl)methyl]-alpha-, beta- and gamma-cyclodextrins

The proton-catalyzed addition of 2-methoxypropene to alpha-, beta- and gamma-cyclodextrins resulted in hexakis-, heptakis-, and octakis[2,6-di-O-(methoxydimethyl)methyl]-alpha-, beta- and gamma-cyclodextrins, but no reaction was observed of CD-s with 2,2-dimethoxypropane. The mixed acetal-type compounds can be alkylated under basic conditions. The preparation of hexakis(3-O-benzyl)-alpha-cyclodextrin demonstrates the synthetic value of this methodology.

Preparation and evaluation of the in vitro erythroid differentiation induction properties of some esters of methyl 3,4-O-isopropylidene-beta-D-galactopyranoside and 2,3-O-isopropylidene-D-mannofuranose

Two series of glycide esters of short fatty acids, designed for avoiding intramolecular transesterification, were prepared and tested for in vitro erythroid differentiation induction activities using the K562 cell line as experimental system. The 6-O-isobutiryl and pivaloyl derivatives of methyl 3,4-O-isopropylidene-beta-D-galactopyranosides as well the same 1-O-esters of 2,3-O-isopropylidene-alpha- and beta-D-mannofuranose exhibit biological activities much higher that the corresponding acids and could be proposed as possible agents to modulate production of embryo-fetal hemoglobins by human erythroid cells.

Scope and limitation of the application of the (methoxydimethyl)methyl group in the synthesis of 2'-O-, 6'-O- and 2',6'-di-O-(alpha-L-arabinofuranosyl)-beta-D-galactopyranosyl- (1-->6)-D-galactoses

For the characterisation of the anticipated epitope of an arabinogalactan, isolated from the extract of Echinacea purpurea, the trisaccharide alpha-L-Araf-(1-->2)-beta-D-Galp- (1-->6)-D-Gal was synthesized. The easily available 3,4-O-isopropylidene-6-O-(methoxydimethyl)methyl-beta-D-galactopyr anosyl- (1-->6)-1,2:3,4-di-O-isopropylidene-alpha-D-galactopyranose having the OH-2' free served as aglycone upon direct arabinosylation at the 2' position under Helferich conditions. The formed HgBr2 cleaved the acid sensitive protecting group at position 6', but under basic conditions the desired, fully protected trisaccharide, or its symmetrical dimerization derivative linked 6'- to 6'-position via an isopropylidene bridge, could be obtained. An alternative route involved arabinosylation of a hepta-O-acetylated digalactose with free OH-2'. Two other oligosaccharides, namely, alpha-L-Araf-(1-->6)-beta-D-Galp-(1-->6)-D-Gal and (alpha-L-Araf)2-(1-->2,6)-beta-D-Galp-(1-->6)-D-Gal were also synthesized and characterised.

New syntheses of D-tagatose and of 1,5-anhydro-D-tagatose from D-galactose derivatives

3,4-O-Isopropylidene-D-lyxo-hexopyranosid-2-ulose derivatives, obtained by oxidation of 3,4,6-protected D-galactopyranosides, can be alkylated in their anionic 2,6-pyranose forms to produce bis-glycosides containing the 2,5-dioxabicyclo[2.2.2]octane ring system. The 1-benzyl-2-methyl bis-glycoside 4b, when subjected to catalytic hydrogenolysis, produces the methyl D-lyxo-hexopyranos-2-uloside 10, existing as an 8:2 mixture of 1,5-pyranose anomers 9. Computational and NMR evidence is presented in favour of the hypothesis that the major anomer has the alpha configuration. Reduction of 9/10 with NaBH4 gives methyl 3,4-O-isopropylidene-beta-D-tagatopyranoside, that can be hydrolyzed to D-tagatose. A simple synthesis of 1,5-anhydro-D-tagatose, starting from 1,5-anhydro-D-galactitol, is also presented. All new compounds were fully characterized by elemental analysis and by 1H and 13C NMR spectroscopy.

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 a di-, tri-, and tetra-saccharide unit of the group B streptococcal common antigen

Condensation of methyl 2,3-O-isopropylidene-alpha-L-rhamnopyranoside with methyl 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-1-thio-beta-D-glucopyranoside activated by nitrosyl tetrafluoroborate gave an excellent yield of the protected disaccharide 9, which was transformed into glycosyl acceptor 11. Methyl 2,3,4,6-tetra-O-acetyl-1-thio-beta-D-galactopyranoside, obtained from D-galactose penta-acetate and methyl trimethylsilyl sulfide, under catalysis by boron trifluoride etherate, was converted into glycosyl donor 25, which was condensed with 11 under halide-ion catalysis to give the trisaccharide derivative 26. Rhamnosylation with 28 of 27, obtained by selective deprotection of 26, gave the protected tetrasaccharide 29. Deprotection of 10, 26, and 29 gave di- (2), tri- (3) and tetra-saccharide (4) methyl glycosides which form part of the group-specific polysaccharide antigen of Group B Streptococci.

Total synthesis of sialosylcerebroside, GM4

Described are total syntheses of O-[sodium (5-acetamido-3,5-dideoxy-D -glycero-alpha-D-galacto-2-nonulopyranosyl)onate]-(2----3)-O -beta-D -galactopyranosyl-(1----1)-(2R,3S,4E)-2-N-tetracosanoylsphingen ine,O-[sodium (5-acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosyl+ ++)onate] -(2----3)-O-alpha-D-galactopyranosyl-(1----1)-(2R,3S,4E)-2-N -tetracosanoylsphingenine, O-[sodium (5-acetamido-3,5-dideoxy-D-glycero-beta -D-galacto-2-nonulopyranosyl)onate]-(2----3)-O-beta-D-gal act opyranosyl -(1----1)-(2R,3S,4E)-2-N-tetracosanoylsphingenine, and O-[sodium (5-acetamido-3,5-dideoxy-D-glycero-beta-D-galacto-2-nonulopyranosyl++ +)onate] -(2----3)-O-alpha-D-galactopyranosyl-(1----1)-(2R,3S,4E)-2-N -tetracosanoylsphingenine by using O-[methyl (5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha-D -galacto-2-nonulopyranosyl)onate]-(2----3)-2,3,4,6-tetra-O-a cetyl-D -galactopyrano-syl trichloroacetimidate and O-[methyl (5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-beta -D-galacto-2-nonulopyranosyl)onate]-(2----3)-2,4,6-tri-O-ace tyl-D-galactopyranosyl trichloroacetimidate as key glycosyl donors, and (2S,3R,4E)-3 -O-benzoyl-2-N-tetracosanoylsphingenine as a key glycosyl acceptor.