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PubMed1993

All-alpha-D-linked tetra- and penta-saccharide substructures of Trestatin A by block syntheses with triflic anhydride as promoter.

The perbenzylated maltosyl and maltotriosyl fluorides 6 and 16 were treated with 2,3,2',3',6'-penta-O-benzyl-4,6-O-benzylidene-alpha,alpha-trehalose (7) using triflic anhydride as a promoter to give all-alpha-D-linked tetra- and penta-saccharides which were finally deblocked to the free oligosaccharides 4-O-alpha-maltosyl-9 and 4-O-alpha-maltotriosyl-alpha,alpha-trehaloses 18. The 1H NMR spectra of some of the compounds were fully analyzed by 1D TOCSY and ROESY experiments.

Anhydrides↗
PubMed1993

Chemical synthesis of a comb-shaped, branched stereoregular polysaccharide, 4-O-alpha-D-mannopyranosyl-(1-->6)-alpha-D-mannopyranan.

4-O-alpha-D-Mannopyranosyl-(1-->6)-alpha-D-mannopyranan (7) was prepared via ring-opening polymerization of 1,6-anhydro-2,3-di-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl-alpha-D- mannopyranosyl)-beta-D-mannopyranose (5) using phosphorus pentafluoride as initiator in dichloromethane at -60 degrees C, followed by debenzylation. Compound 5 was obtained via glycosidation of 1,6-anhydro-2,3-O- isopropylidene-beta-D-mannopyranose (1) with 2,3,4,6-tetra-O-benzyl-1-O-trichloroacetimidoyl-alpha-D-mannopyran ose (2) using p-toluenesulfonic acid as catalyst and subsequent transformation of the protecting groups of the resulting 1,6-anhydro-4-O-(2,3,4,6-tetra-O-benzyl-alpha-D-mannopyranosyl)-2,3- O-isopropylidene-beta-D-mannopyranose (3).

Carbohydrate Conformation↗
PubMed1993

Synthesis of p-trifluoroacetamidophenyl 6-deoxy-2-O-(3-O-[2-O-methyl-3-O- (2-O-methyl-alpha-D-rhamnopyranosyl)-alpha-L-fucopyranosyl]-alpha-L- rhamnopyranosyl)-alpha-L-talopyranoside: a spacer armed tetrasaccharide glycopeptidolipid antigen of Mycobacterium avium serovar 20.

The synthesis of the title tetrasaccharide glycoside 38 is reported. p-Nitrophenyl endo-3,4-O-benzylidene-6-deoxy-alpha-L-talopyranoside (4), 3-O-acetyl-2,4-di-O-benzyl-alpha-L-rhamnopyranosyl trichloroacetimidate (7), methyl 3-O-acetyl-4-O-benzyl-2-O-methyl-1-thio-beta-L-fucopyranoside (15), 3-O-acetyl-4-O-benzyl-2-O-methyl-alpha-L-fucopyranosyl bromide (16), and ethyl 3-O-acetyl-4-O-benzyl-2-O-methyl-1-thio-alpha-D-rhamnopyranoside (33) were prepared as intermediates. Compound 4 was glycosylated with imidate 7 as well as with methyl 3-O-acetyl-2,4-di-O-benzyl-1-thio-alpha-L-rhamnopyranoside (9), affording the same disaccharide derivative 8. Deacetylation of 8 gave crystalline 17. Condensation of 17 with both fucosyl donors 15 and 16 yielded the same trisaccharide derivative 18 stereoselectively. Compound 18 was also prepared by the coupling of 4 with disaccharide glycosyl donor 20. After deacetylation of 18 (-->34), methyl triflate-promoted glycosylation with compound 33 resulted in tetrasaccharide 35. Conversion of the p-nitrophenyl group of 35 into the p-trifluoroacetamidophenyl group (-->36) and removal of the protecting groups gave the title tetrasaccharide glycoside 38.

Carbohydrate Conformation↗
PubMed1993

Synthesis of the laminara-oligosaccharide methyl beta-glycosides of dp 3-8.

Ethyl 2,3,4,6-tetra-O-acetyl-1-thio-alpha-D-glucopyranoside has been prepared in a good yield by anomerization of the corresponding beta-thioglucoside with tin(IV) chloride and transformed, in three steps, into ethyl 2-O-benzoyl-4,6-O-benzylidene-1-thio-alpha-D-glucopyranoside (18). Chloroacetylation of 18, followed by treatment of the product with chlorine gave crystalline 2-O-benzoyl-4,6-O-benzylidene-3-O-chloroacetyl-beta-D-glucopyranosyl chloride (20). This was coupled with methanol in the presence of silver carbonate-silver perchlorate and the product was O-dechloroacetylated to afford methyl 2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranoside (22). Silver triflate-promoted glucosylation of 18 with 20 gave a beta-(1-->3)-linked disaccharide derivative, reaction of which with chlorine yielded crystalline O-(2-O-benzoyl-4,6-O-benzylidene-3-O-chloroacetyl-beta-D-glucopyranosyl) - (1-->3)-2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranosyl chloride (24). Likewise, condensation of 22 with 20 gave a beta-(1-->3)-linked disaccharide glycoside, which was partially deprotected to give methyl O-(2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranosyl)-(1-->3)-2-O-benz oyl- 4,6-O-benzylidene-beta-D-glucopyranoside (26). The methyl beta-glycosides of a homologous series of (1-->3)-linked beta-D-gluco-oligosaccharides from the tri- to the octa-saccharide have been synthesized in a blockwise manner by using 22 and 26 as the glycosyl acceptors, 24 as the disaccharide donor, and silver triflate as the promoter.

Carbohydrate Conformation↗
PubMed1993

Preparation of 4,6-cyclo-hexopyranoses by palladium-mediated intramolecular cyclodehalogenation.

4,6-Cyclo-4,6-dideoxy-hexapyronoses were obtained by palladium-mediated intramolecular cyclodehalogenation. Thus, methyl 2,3-di-O-acetyl-4,6-dideoxy-4,6-diiodo-beta-D-galactopyranoside (3) afforded methyl 2,3-di-O-acetyl-4,5-cyclo-4,6-dideoxy-beta-D-galactopyranoside (5) in 56% yield upon treatment with hydrogen in the presence of palladium-on-charcoal and diethylamine. The structure of 5 was proven by MS, NMR including NOE measurements, and by independent conversion of 4 to 5 by zinc-mediated Wurtz synthesis. Similarly, methyl 2,3-di-O-acetyl-4,6-cyclo-4,6-dideoxy-alpha-D-galactopyranoside (6) and O-(2,3,-di-O-acetyl-4,6-cyclo-4,6-dideoxy-alpha-D-galactopyranosyl)-(1-- >4)-1,2,3,6-tetra-O-acetyl-beta-D-glucopyranose (17) were obtained along with the respective 4,6-dideoxy analogues. Also methyl 2,3-di-O-acetyl-4,6-dideoxy-4,6-diiodo-beta-D-glucopyranoside (19) gave galacto-configured 5 stereoselectively.

Carbohydrate Conformation↗
PubMed1993

Convergent synthesis of an elusive hexasaccharide corresponding to the cell-wall polysaccharide of the beta-hemolytic Streptococcus group A.

A convergent synthesis of a hexasaccharide corresponding to the cell-wall polysaccharide of the beta-hemolytic Streptococcus Group A is described. The strategy relies on the preparation of a key linear trisaccharide unit beta-D-GlcpNAc-(1-->3)-alpha-L-Rhap-(1-->2)-alpha-L-Rhap which has previously resisted our efforts. The trisaccharide functions both as a glycosyl acceptor and donor to give an elusive hexasaccharide. This fully functionalized unit can serve, in turn, as a glycosyl acceptor or donor for the synthesis of higher-order structures. Deprotection gives a hitherto unknown hexasaccharide for use as a hapten in immunochemical studies. The characterization of all compounds by high-resolution 1H and 13C NMR spectroscopy is also described.

Carbohydrate Conformation↗
PubMed1993

Wittig reaction of unprotected D-aldopentoses with stabilized ylides: metal-ion effects.

Each of the four D-aldopentoses (1-4) reacts in the unprotected form with Ph3PCHCO2Me (5) in THF to give stereoselectively the corresponding trans-alpha,beta-unsaturated C7 Wittig adducts, isolated as the corresponding 4,5,6,7-tetraacetates (7-10) or 4,5:6,7-di-O-isopropylidene derivatives (11-14). Concurrently formed are also bicyclic, 1,4-lactone derivatives, isolated as their 5,7-diacetates (15-18) or 5,7-O-isopropylidene derivatives (19), arising through intramolecular Michael addition from the initial acyclic adducts. Formation of the cyclized products is suppressed by incorporation of Cu(OAc)2 in the reaction mixture, permitting preparative isolation of the hept-3-enonate derivatives 7-10, useful as dienophiles in Diels-Alder carbocyclizations with chirality transfer. Optimized yields were 25% (D-ribo), 50% (D-arabino), 49% (D-xylo), and 61% (D-lyxo). Under Cu(OAc)2-catalyzed conditions, the formation of small proportions of 3,7-anhydroheptonic acid esters (21 and 23) was observed in the D-arabino and D-xylo series, but not with the other two pentoses.

Carbohydrate Conformation↗
PubMed1993

Stereocontrol in Diels-Alder cycloaddition to unsaturated sugars: reactivities of acyclic seven-carbon trans dienophiles derived from aldopentoses.

Acyclic trans-2,3-unsaturated aldoheptonate derivatives (1-9) obtained from D-aldopentoses by Wittig chain-extension served as dienophiles for a detailed comparative study of their asymmetric Diels-Alder cycloaddition with cyclopentadiene. Cycloaddition under uncatalyzed thermal conditions gave mixtures of the four possible stereoisomeric norbornene adducts. The endo:exo ratios and the diastereofacial selectivities in the formation of the adducts were determined by NMR spectroscopy and by chemical transformations. The quantitative distribution of adducts as a function of stereochemistry of the dienophile is discussed.

Carbohydrate Conformation↗
PubMed1993

Stereocontrol in Diels-Alder cycloaddition to unsaturated sugars: reactivities of cis-dienophiles with cyclopentadiene.

Cycloaddition of cyclopentadiene with a D-arabinose-derived cis-dienophile, methyl (Z)-4,5,6,7-tetra-O-acetyl-2,3-dideoxy-D-arabino-hept-2-enonate (2), under thermal conditions gave essentially a single norbornene adduct, isolated crystalline in 81% yield and identified by NMR spectroscopy and X-ray crystallography as methyl (5R,6S)-6-endo-(1,2,3,4-tetra-O-acetyl-D-arabino-tetritol-1-yl)bic yclo [2.2.1]-hept-2-ene-5-endo-carboxylate (3). The diene adds exclusively from the si-face of the dienophile and give only the endo product. The same sequence starting from L-arabinose gave the enantiomer (7) of 3. In contrast, a related cis-dienophile (9) having a butenolide ring reacts with cyclopentadiene from the opposite (re) face giving mainly the endo adduct (5S,6R)-6-endo-(2,3,4-tri-O-acetyl)-D-arabino-tetritol-1-yl)bicycl o [2.2.1]hept-2-ene-5-endo-carboxylic acid 1,4-lactone (10), isolated crystalline in 70% yield, whose structure was again established by NMR spectroscopy, and firmly consolidated by X-ray crystallography. The minor (11%) product was the exo(5S,6R) isomer 11. A cis-enonate 14, analogous to 2 but deoxygenated at the allylic position, showed negligible diastereofacial selectivity and reacted with cyclopentadiene to give a mixture of all four possible adducts. A 6-membered ring dienophile 16 was also subjected to the same cycloaddition for comparison with the butenolide 9; it gave principally the two endo products 17 and 19 in 31 and 38% yields, respectively, accompanied by 12% of a mixture of the two exo products (18 and 20). The quantitative distribution of cycloaddition products as a function of dienophile stereochemistry is discussed. The high degree of asymmetric induction observed, especially with the readily accessible dienophiles 2 and 7, provides a valuable route of access to enantiomerically pure tetra-C-substituted cycloalkanes.

Carbohydrate Conformation↗
PubMed1993

Synthesis of the monodeoxy derivatives of 2-(trimethylsilyl)ethyl beta-lactoside.

Monodeoxy derivatives of 2-(trimethylsilyl)ethyl (Me3SiEt) beta-lactoside were synthesized, by deoxygenation at the disaccharide level, for the 2'-, 3'-, 4'-, and 6'-monodeoxylactosides. The 2-, 3-, and 6-deoxy derivatives were synthesized by beta-D-galactosylation of suitably protected monodeoxygenated Me3SiEt glucosides. Silver silicate was shown to be an efficient glycosylation promoter in the preparation of the 2- and 3-deoxylactosides.

Carbohydrate Sequence↗
PubMed1993

Synthesis of 3'- and 4'-deoxyfluorolactose and its Me3SiCH2CH2 and ceramide derivatives.

(2-Trimethylsilyl)ethyl (Me3SiCH2CH2)3'- and 4'-deoxyfluorolactosides (1 and 3) were synthesized by glycosylation of Me3SiCH2CH2 2,3,6-tri-O-benzyl-beta-D-glucopyranoside with 2,4,6-tri-O-acetyl-3-deoxy-3-fluoro-beta-D-galactopyranosyl bromide and 2,3,6-tri-O-benzyl-4-deoxy-4-fluoro-beta-D-galactopyranosyl bromide. Anomeric deblocking of the fully acetyled Me3SiCH2CH2 glycosides (12 and 13) gave the corresponding hemiacetals 14 and 15. Removal of the acetyl groups gave 3'- and 4'-deoxyfluorolactose (2 and 4). The deoxyfluorolactosylceramides 5 and 6 were synthesized via boron trifluoride etherate- or silver triflate-activation of the trichloroacetimidates prepared from 14 and 15. Silver triflate-mediated glycosylations showed lower reaction rates, and fewer byproducts were formed.

Carbohydrate Conformation↗