Specific features of phase transfer catalytic glycosylation of aromatic hydroxy acids

Reactions of peracetylated α-D-glucosaminyl chloride with isomeric hydroxybenzoic and 1-hydroxy-2-naphthoic acids in a solid phase transfer system of potassium carbonate-acetonitrile were studied. It was found that the nature of carboxylic acids, lipophilicity of the phase transfer catalyst, and reaction temperatures affected the reaction composition and product yields. The O-β-glycosyl esters of ortho-hydroxyaromatic acids were first found to form anomeric 1,2-cis derivatives in the presence of potassium carbonate. The structures of the synthesized compounds were confirmed by ¹H NMR spectroscopy. As was shown in vivo experiments, the analgesic activities of glycosyl esters of salicylic acid and peracetylated 2-carboxyphenylglucosaminide were comparable with that of aspirin.     

Improved preparations of some per-O-acetylated aldohexopyranosyl cyanides

3,4,6-Tri-O-acetyl-1,2-O-[1-(exo-, endo-cyano)ethylidene]-α-d-galacto- (1a/b), -α-d-gluco- (2a/b), and -β-d-manno-pyranose (3a/b) were stereoselectively isomerized to the corresponding per-O-acetylated 1,2-trans-aldohexopyranosyl cyanides in 75, 16, and 62% yield, respectively, by treatment with boron trifluoride etherate in dry nitromethane. The corresponding per-O-acetylated 1,2-cis-aldohexopyranosyl cyanides were obtained concurrently in respective yields of 1.9, 0.9, and 4.8%. The per-O-acetylaldohexopyranosyl cyanide products were found stable to the reaction conditions and were readily isolated following completion of the rearrangement. It had previously been proved that reaction of 2,3,4,6-tetra-O-acetyl-α-d-manno- and -gluco-pyranosyl bromide with mercuric cyanide in nitromethane generates, in the ratio of ∼1:1, the desired 1,2-trans-glycosyl cyanides and the corresponding 1,2-O-(1-cyanoethylidene) isomers (3a/b and 2a/b, respectively). Treatment of these reaction-mixtures with boron trifluoride etherate in nitromethane effected the rearrangement of 3a/b and 2a/b, thereby facilitating the isolation, and increasing the overall yields, of the per-O-acetylated 1,2-trans-d-manno and -gluco-pyranosyl cyanides (58 and 30% total yield, respectively) relative to the earlier procedures. The boron trifluoride etherate-mediated reaction of per-O-acetyl-α- and -β-d-galacto, -α- and -β-d-gluco-, -α-d-manno-, and -2-deoxy-2-phthalimido-β-d-gluco-pyranoses with trimethylsilyl cyanide in nitromethane was also investigated. This reaction provides a “one-flask” synthesis of the corresponding per-O-acetylated 1,2-trans-aldohexopyranosyl cyanides in which 1,2-O-(1-cyanoethylidene) derivatives are isomerized in situ. Finally, improved preparations of the (not readily accessible) per-O-acetylated 1,2-cis-d-manno- and -gluco-pyranosyl cyanides are described. Thus, 2,3,4,6-tetra-O-acetyl-α- and -β-d-mannopyranosyl cyanide (48 and 16% total yield, respectively) and -α- and -β-d-glucopyranosyl cyanide (12 and 39% total yield, respectively) were synthesized by fusion of the corresponding -α-d-glycosyl bromides with mercuric cyanide.     

Reaction of organocuprate reagents with protected 1,2-anhydro sugars. Stereocontrolled synthesis of 2-deoxy-C-glycosyl compounds

The reaction of protected 1,2-anhydro-α-d-gluco- and β-d-manno-pyranoses with alkyl and phenyl organocuprates afforded the corresponding C-glycosyl compounds in acceptable to high yield. Complete stereocontrol was obtained, leading respectively to the β-d or the α-d anomer. With the perbenzylated manno derivative, deoxygenation at C-2 was achieved in high yield, affording 2-deoxy-α-d-C-glycosyl compounds.     

Über die orthoesterbildung als konkurrenzreaktion zur glykosylierung

1,2-(Alkyl orthoacetates) (orthoesters) were obtained in good yields from acetylated cis-glycosyl halides and alcohols in tetrahydrofuran in the presence of silver salicylate. The scope and the possible mechanism of the reaction were investigated. The presence of an intermediate oxonium ion (13) derived from the sugar derivative and tetrahydrofuran was deduced, from the occurrence of 4-bromobutyl 2,3,4,6-tetra-O-acetyl-β-d-glucopyranoside as a by-product of the reaction, and from semi-quantitative kinetic investigations. After an intramolecular reaction of this ion to form an acetoxonium ion (14), the reaction with an alcohol yields the orthoester (e.g.1-7). In a similar manner, and in concurrence with direct glycosylation, ortho esters and products arising from orthoesters may be formed in the usual glycoside synthesis, using the reaction just described with the solvent or other nucleophiles.     

Photo-oxygenation of glycosylfurans. Rearrangement of C-glycosyl into O-glycosyl derivatives

Photo-oxygenation of 3-ethoxycarbonyl-5-(2,3-O-isopropylidene-β-d-erythrofuranosyl)-2-methylfuran and 3-hydroxymethyl-5-(2,3-O-isopropylidene-β-d-erythrofuranosyl)-2-methylfuran yields the corresponding endo-peroxides which rearrange at room temperature into the O-glycosyl derivatives ethyl 2,3-O-isopropylidene-β-d-erythrofuranosyl 2-acetylfumarate and 2,3-O-isopropylidene-β-d-erythrofuranosyl 3-acetyl-3-hydroxymethylacrylate, respectively. The endo-peroxides can be reduced without rearrangement, yielding C-glycosyl derivatives. Alcoholysis of the O-glycosyl derivatives yields 2,3-O-isopropylidene-d-erythrose, dialkyl 2-acetyl-3-alkoxysuccinates, 4-ethoxycarbonyl-5-methoxy-5-methyl-2-oxo-2,5-dihydrofuran and 4-hydroxymethyl-5-methoxy-5-methyl-2-oxo-2,5-dihydrofuran.     

Thioglycosylation of 1,2-cis-glycosyl acetates: a long-standing overlooked issue in preparative carbohydrate chemistry

1,2-cis-Glycosyl acetates with the α-configuration were revealed to be very unreactive towards Lewis acid catalyzed thioglycosylations. Optimal and cost-effective conditions for enabling this direct conversion is absent in the literature. Our studies have shown that elevating the reaction temperature with a catalytic amount of BF₃·OEt₂ was more effective than changing Lewis acids with higher acidities to accommodate the low reactivity of α-glycosyl acetates. The effect of impurities in stored BF₃·OEt₂ on the reaction is also discussed.     

Oxidation products of acylated anthocyanins under acidic and neutral conditions

Acylated anthocyanidin-3,5-diglucosides are oxidized with H₂O₂ under acidic conditions to acylated ortho-benzoyloxyphenylacetic acid esters. When the same reaction is carried out under neutral conditions, the reaction product is the 3-O-acyl-glucosyl-5-O-glucosyl-7-hydroxy coumarin.     

Preparation and conformational analysis of C-glycosyl β- and β/β-peptides

Ten C-glycosyl β²- and β/β²-peptides with three to eight amino acid residues have been prepared. Solution and solid-phase peptide syntheses were employed to assemble β²-amino acids in which C-glycosylic substituents are attached to the C-2 position of β-amino acids. Conformational analysis of the C-glycosyl β²-peptides using NMR and CD spectra indicates that the tripeptide can form a helical secondary structure. Besides, helix directions of the C-glycosyl β/β²-peptides are governed by the configuration at the α-carbon of the peptide backbone that originates from the stereocenter of the C-glycosyl β²-amino acids.     

Facile synthesis of 1,2-trans-O-acetyl glycosyl chloride derivatives of cellobiose,lactose, and D-glucose

Solutions of O-acetyl-α-glycosyl bromide derivatives of d-glucose, cellobiose, and lactose in hexamethylphosphoramide were converted into corresponding β-chlorides at room temperature by the action of lithium chloride. At 3:1 mM ratios of chloride ion to glycose, 5–10% w/v solutions of glycosyl bromide formed α- and β-chlorides in ratios of (or greater than) 1:19 within 2–13 min and produced crystalline β-chlorides in 70–80% yields. Anomeric compositions were determined by n.m.r. spectroscopy in hexamethylphosphoramide. Older methods of preparing 1,2-trans-O-acetylgIycosyl chlorides, with aluminum chloride or titanium tetrachloride, gave the α- and β-cellobiosyl and -Iactosyl chlorides in ratios that varied from 2:3 to 1:4 and reached 85–95% levels of β-chloride only with β-d-glucose pentaacetate. When hydrolyzed under conditions that controlled solution acidity, the β-cellobiosyl and -Iactosyl chlorides each gave 2-hydroxy derivatives in yields that could be varied from 16 to 60%. Hepta-O-acetyl-2-O-methyl-α-cellobiose was prepared to demonstrate how these hydrolysis mixtures can be used to synthesize a 2-O-substituted derivative.     

New approaches to synthetic receptors. Studies on the synthesis and properties of macrocyclic C-glycosyl compounds as chiral,water-soluble cyclophanes

In an approach for the preparation of macrocyclic C-glycosyl compounds, the C-glycosyl residue is synthesized by acid-assisted reduction of a cyclic hemiacetal with sodium cyanoborohydride. Macrocycle formation is effected by the reaction of a symmetrical bis(C-glycosyl)derived diamine with a dicarboxylic acid dichloride. The product macrocycles are presented as the first examples of a new type of chiral, water-soluble cyclophane. Molecules of this type are of interest as synthetic receptors for lipophilic substrates.     

A new method for the detritylation of 1,2-diradyl-3-O-tritylglycerols

The 3-O-trityl derivatives of 1,2-diacylglycerols, 1-O-alkyl-2-acylglycerols and 1,2-di-O-alkylglycerols alkyglycerols are detritylated with boron trifluoride—methanol in methylene chloride. The reaction is complete within 30 min at 0°C. Acyl migration does not occur.     

Preparation of 1-aryl-2-(benzylthio)-(1,2-dideoxy-d-glycero-β-l-gluco-heptofurano)[1,2-d]-2-imidazolines,and new,acyclic C-nucleosides of the imidazole

The reaction of 1-aryl-(1,2-dideoxy-d-glycero-β-l-gluco-heptofurano)[1,2-d]imidazolidine-2-thiones with benzyl chloride and an equivalent amount of sodium hydrogencarbonate yields 1-aryl-2-(benzylthio)-(1,2-dideoxy-d-glycero-β-l-gluco-heptofurano)[1,2-d]-2-imidazolines (2). If the reaction is carried out in the absence of sodium hydrogencarbonate, the 1-aryl-2-(benzylthio)-4-(d-galacto-pentitol-1-yl)imidazoles are obtained. These compounds are also obtained by acid-catalyzed isomerization of compounds 2.     

Regioselective O-deacylation of fully acylated glycosides and 1,2-O-isopropylidenealdofuranose derivatives with hydrazine hydrate

On hydrazinolysis in 1:4 acetic acid-pyridine, and in pyridine, partial O-deacylation of fully acylated methyl glycosides and some other glycosyl compounds (23 compounds) was found to be induced, to give, in good yields, products bearing one free hydroxyl group; the results obtained indicated that, among the primary and secondary O-acyl groups, the 2-O-acyl groups were, in general, the most labile toward the nucleophile (hydrazine). Hydrazinolysis of 1,2-O-isopropylidenealdofuranose acylates (3 compounds), on the other hand, gave, in high yield, the corresponding monoacyl derivatives having the protecting group on their primary hydroxyl group. The factors possibly involved in the regioselectivity of the hydrazinolysis were discussed.     

Spiro(imidazo[1,2-a]pyrano[2,3-c]pyridine-9-indenes) as inhibitors of gastric acid secretion

Asymmetric and symmetric spiro(imidazo[1,2-a]pyrano[2,3-c]pyridine-9-indenes) were prepared using a synthetic approach that comprised a cross-metathesis reaction and an acid-catalyzed cycloisomerisation as key steps. The target compounds constitute potent inhibitors of the gastric proton pump enzyme with inhibitory activity comparable to potassium-competitive acid blockers (P-CABs) belonging to the known 9-aryl-7H-8,9-dihydropyrano[2,3-c]imidazo[1,2-a]pyridine series. Spiro(imidazo[1,2-a]pyrano[2,3-c]pyridine-9,2′-indenes) represent the first example for P-CABs, in which the distance between the heterocyclic scaffold and the aryl residue has been modified, and are promising candidates for further development as anti-ulcer drugs.     

Synthesis of higher-carbon sugars via vinyltin derivatives of simple monosaccharides

6-O-Benzyl-7,8-dideoxy-1,2:3,4-di-O-isopropylidene-l-glycero-α-d-galacto-oct-7-ynopyranose reacted with tributyltin hydride to afford (Z-6-O-benzyl-7,8-dideoxy-1,2:3,4-di-O-isopropylidene-8-(tributylstannyl)-l-glycero-α-d-galacto-oct-7-enopyranose, which was subsequently isomerized to the E-olefin 4. Replacement of the tributyltin moietey with lithium in 4 afforded the vinyl anion which reacted with 3-O-benzyl-1,2-O-isopropylidene-α-d-xylo-pentodialdo-1,4-furanose, furnishing 3-O-benzyl-6-C-[(E)-6-O-benzyl-7-deoxy-1,2:3,4-di-O-isopropylidene-l-glycero-α-d-galacto-heptopyranos-7-ylidene] -60-deoxy-1,2-O-isopropylidene-α-d-gluco- (6) and -β-l-ido-furanose (7) in yields of ∼70 or ∼87% (depending on the temperature of the reaction). The configurations of the new chiral centers in 6 and 7 were determined by their conversion into 3-O-benzyl-1,2-O-isopropylidene-α-d-gluco- and -β-l-ido-furanose, respectively. Oxidation of 6 and 7 gave the same enone, 3-O-benzyl-6-C-[(E)-6-O-benzyl-7-deoxy-1,2:3,4-di-O-isopropylidene-l-glycero-α-d-galacto- heoptopyranos-7-ylidene]-6-deoxy-1,2-O-isopropylidene-α-d-xylo-hexofuranos-5-ulose.     

Discriminative flavonoid patterns within a preliminary collection of the Setaria italica specific complex

One hundred and eighty individuals of Setaria italica ssp. italica, reflecting the worldwide variability of cultivated foxtail millet, and 40 individuals of wild S. i. ssp. viridis, were analysed for their flavonoid content. S. i. spp. italica accumulates native or acylated O-glycosyl derivatives of 8-Cglycosylflavones and O-glycosylfiavones, showing a large qualitative as well as quantitative inter-varietal diversity. The subspecies is strongly organized into seven chemotypes clearly defined by the presence or absence of O-pentosylation and acylation, and by the balance between apigenin and luteolin derivatives. This organization only partly corroborates a previous classification based on morphological and electrophoretical data, chemotypes affording an additional discriminative criterion in most cases. Flavonoid patterns of S. i. ssp. viridis fall in the chemotypes defined from the cereal, and underlines the conspecific nature of the two taxa. Similarities between wild and cultivated foxtail millet from same geographical origin suggest independent domestication from already differentiated gene pools.     

Synthesis of some C-glycosyl- and polyhydroxyalkyl-pyridazines

Photo-oxygenation of 3-hydroxymethyl-5-(2,3-O-isopropylidene-β-d-erythrofuranosyl)-2-methylfuran, 5-(1,2:3,4-di-O-isopropylidene-d-arabino-tetritol-1-yl)-3-(1-hydroxyethyl)-2-methylfuran (8a), and 2-methyl-5-(1,2,3,4-tetra-O-acetyl-d-arabino-tetritol-1-yl)-3-furoic acid (8b) yielded the corresponding endo-peroxides, which were transformed into 4-hydroxymethyl-6-(2,3-O-isopropylidene-β-d-erythrofuranosyl)-3-methylpyridazine, 6-(1,2:3,4-di-O-isopropylidene-d-arabino-tetritol-1-yl)-4-(1-hydroxyethyl)-3-methylpyridazine, and 6-(d-arabino-tetritol-1-yl)-3-methylpyridazine by treatment with hydrazine. The γ-di-ketones (Z)-1-(1,2:3,4-di-O-isopropylidene-d-arabino-tetritol-1-yl)-3-(1-hydroxyethyl)pent-2-ene-1,4-dione and d-arabino-6,7,8,9-tetraacetoxy-4-methoxynonane-2,5-dione can be obtained by reduction of the endo-peroxides 9a and 9b (derived from 8a and 8b, respectively) with dimethyl sulphide. The C → O rearrangement reported for C-glycosyl endo-peroxides was also observed for 9a.     

Studies of carbohydrate derivatives having the -CH2F function

The following primary sulphonates have been converted into the corresponding deoxyfluoro derivatives by reaction with potassium fluoride in ethylene glycol:1,2:3,4-di-O-isopropylidene-6-O-tosyl α-D-galactopyranose (1), methyl 2,3-O2-isopropyliden-5-O-tosyl-α,β-D-ribofuranoside (2), 1,2:3,4-di-O-methylene-6-O-tosyl-α-D-glucofuranose (3), 3,5-di-O-benzylidene-1,2-O-isopropylidene-6-O-tosyl-α-D-glucofuranose (4), and 1,2:3,5-di-O-isopropylidene-6-O-tosyl-α-D-glucofuranose (5). The yields were generally poor; in the reaction of 1, a major by-product was 6-O-(2-hydroxyethyl)-1,2:3,4-di-O-isopropylidene-α-D-galactopyranose (11). The reaction of the primary hydroxyl precursor of each of the above tosylates with N2-(2-chloro- 1,1,2-trifluoroethyl)-N,N-diethylamine generally yielded the O-chlorofluoroacetyl derivative; however, 1,2:3,5-di-O-methylene-α-D-glucofuranose (12) was converted into the 6-deoxy-6-fluoro derivative (8). The ¹⁹F resonances of compounds containing the CH₂F moiety fall between φ_C +213 and φ_C +235 p.p.m. The differences between the vicinal¹⁹F-¹H couplings of compounds having the D-gluco and D-galacto configurations clearly reflect the influence of the C-4O-4 substitutents on the populations of the C-5C-6 rotamers. A novel type of noise-modulated, heteronuclear decoupling experiment is described.     

Structural determination of 6-C-diglycosyl-8-C-glycosyl-flavones and 6-C-glycosyl-8-C-diglycosylflavones by mass spectrometry of their permethyl ethers

Permethylated 6-C-diglycosyl-8-C-glycosylflavones and 6-C-glycosyl-8-C-diglycosylflavones gave well defined EIMS including the molecular peak and a fragmentation pattern characteristic of the 6-C-glycosyl residue. X″′-O-glycosides (8-C-disaccharides) are thus easily distinguished from X″-O-glycosides (6-C-disaccharides) and, in the latter, the position of the O-glycosidic bond should be deduced from the MS, after acid hydrolysis. Three new C-glycosylflavones have been characterized in this way from Spergularia rubra and Stellaria holostea.     

An approach to the synthesis of branched-chain amino sugars from c-methylene sugars

The reaction of 1,2:5,6-di-O-isopropylidene-3-C-methylene-α-D-ribo-hexofuranose (4) with mercuric azide in hot 50% aqueous tetrahydrofuran yielded, after reductive demercuration, 3-azido-3-deoxy-1,2:5,6-di-O-isopropylidene-3-C-methyl-α-D-glucofuranose (5). Partial, acid hydrolysis of5 afforded the diol7, which gave 3-azido-3-deoxy-1,2-O-isopropylidene-5,6-di-O-methanesulphonyl-3-C-methyl-α-D-glucofuranose (8) on sulphonylation. On hydrogenation over a platinum catalyst and N-acetylation, the dimethanesulphonate 8 furnished 3,6-acetylepimino-3,6-dideoxy-1,2-O-isopropylidene-5-O-methanesulphonyl-3-C-methyl-α-D-glucofuranose (9), which was also prepared by an analogous sequence of reactions on 3-azido-3-deoxy-1,2-O-isopropylidene-5-O-methanesulphonyl-3-C-methyl-6-O-toluene-p-sulphonyl-α-D-glucofuranose (13). The formation of the N-acetylepimine 9 establishes the D-gluco configuration for 5.1,2-O-Isopropylidene-3-C-methylene-α-D-ribo-hexofuranose (20) reacted with mercuric azide in aqueous tetrahydrofuran at ≈85° to give 3,6-anhydro-1,2-O-isopropylidene-3-C-methyl-α-D-glucofuranose (22) as a result of intramolecular participation by the C-6 hydroxyl group in the initial intermediate.