Solvent scales comparison by using α-nitrocyclohexanone as probe in ionic liquids,organic solvents and CH3CN/CHCl3 mixtures

The UV–vis spectra of α-nitrocyclohexanone were determined at 25 °C in a series of solvents, including room-temperature ionic liquids, water and CH₃CN/CHCl₃ binary mixtures. A multiple linear regression analysis was performed by applying the Catalán empirical multiparameter solvent approach. Polarizability plays the major role in the solute-solvent interactions, as demonstrated by the quantification of the single solvent contributions, while an excellent linear correlation was found between the experimental and calculated solvent-dependent maxima adsorption bands. The solvatochromic trend of α-nitrocyclohexanone in the investigated media was confirmed by applying the Spange empirical solvent parameters for the ionic liquids in the Kamlet–Taft scale.     

Photochemical Reactions. 153th communication. Photochemistry of acylsilanes: Photolyses and thermolyses of α,β-epoxy silyl ketones

The photolyses and thermolyses of the α,β-epoxy silyl ketones 5 and 6 are described. On n,π*-excitation, the silyl ketones 5 and 6 were transformed to the ketone 7 and the ketene 8 in quantitative yield. The formation of 8 may be explained by initial cleavage of the C(α)? O bond and subsequent C(1)→C(2) migration of the (t-Bu)Me₂Si group. In contrast to the acylsilanes 5 and 6 , the photolyses of the analogous methyl ketones 11 and 12 gave a very complex mixture of products. On thermolysis, 5 and 6 yielded the ketone 7 and the acetylenic compound 9 , which were probably formed via a siloxycarbene intermediate. In addition, the 1,3-dioxle 10 was formed via an initial C(α)? C(β) bond cleavage leading to the ylide g and subsequent intramolecular addition of the carbonyl group. The analogous 1,3-dioxole 13 was obtained on pyrolysis of the methyl ketones 11 and 12 .     

Synthetic Studies on Sialoglycoconjugates 23: Total Synthesis of Sialyl-α(2↠6)-Lactotetraosylceramide and Sialyl-α(2↠6)-Neolactotetraosylceramide

ABSTRACT

The first total syntheses of sialyl-α(2→6)-lactotetraosylceramide (29, IV⁶NeuAcLc₄Cer) and sialyl-α(2→6)-neolatotetraosylceramide (33, IV⁶NeuAcnLc₄Cer) are described. Methyl O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-α-D-galacto-2-nonulopyranosylonate)-(2→6)-2,4-di-O-benzoyl-3-O-benzyl-1-thio-ß-D-galactopyranoside (11), the key glycosyl donor was prepared, via glycosylation of 2-(trimethylsilyl)ethyl 3-O-benzyl-ß-D-galactopyranoside (2) with the methyl α-thioglycoside 3 of N-acetylneuraminic acid, benzoylation, replacement of the 2-(trimethylsilyl)ethyl group by acetyl, and introduction of the methylthio group with (methylthio)trimethylsilane. Each coupling of 2-(trimethylsilyl)ethyl O-(2-acetamido-4,6-O-benzylidene-2-deoxy-ß-D-glucopyranosyl)-(1→3′)-per-O-benzyl-ß-lactoside (12) or 2-(trimethylsilyl)ethyl O-(2-acetamido-3-O-acetyl-6-O-benzyl-2-deozy-ß-D-glucopyranosyl)-(1→3′)-per-O-benzyl-ß-D-lactoside (14) prepared from 12 by O-acetylation and reductive opening of the benzylidene acetal, with 11 gave the pentasaccharides 16 and 20 in good yields. Compounds 16 and 20 were converted into the corresponding α-trichloroacetimidates 19 and 24 which, on coupling with (2S, 3R, 4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol (25), gave the ß-glycosides 26 and 30, respectively. Finally, 26 and 30 were transformed, via selective reduction of the azide group, condensation with octadecanoic acid, O-deacylation, and hydrolysis of the methyl ester group, into 29 and 33, respectively.     

A New Synthesis of 2-(6-Methoxycarbonylhexyl)-Cyclopent-2-EN-1-One Using α-Silylallyl Sulphide

Acylation of the α-trimethylsilylallyl phenyl sulphide (2). by reaction with the acid chloride (3), catalyzed by aluminium chloride in CH₂Cl₂ at ?78°C, gave methyl 9-oxo-12-phenylthio-11-dodecene (4). Hydrolysis of (4) followed by aldol condensation gave 2-(6-methoxycarbonylhexyl)-cyclopent-2-en-1-one (6).     

Synthesis and Biological Properties of N-Acetyl-4-deoxy-D-neuraminic Acid

N-Acetylneuraminic acid ( 1 ) can be transformed into the methyl α-D -ketoside 2 which, by reaction with methanesulfonyl chloride, yields the corresponding 4-O-mesylate 3 and the 4,7-di-O-mesylate 4 as a by-product. Compound 3 reacts with Nal giving the 4-deoxy-4-iodo compound 5 with equatorial orientation of the I-atom. As second product, the dihydrooxazole 6 is produced. Catalytic hydrogenation of 5 is followed by ester cleavage and removal of the isopropylidene group yielding the methyl α-D -ketoside 8 which affords the title compound, N-acetyl-4-deoxyneuraminic acid ( 9 ), by reaction with fowl plague virus sialidase. Further biochemical activities of 8 and 9 are reported.     

Synthetic Studies on Sialoglycoconjugates 27: Synthesis of Sialyl-α(2→6)-Lewis X

ABSTRACT

Synthesis of a positional isomer of sialyl Lewis X with regard to the substitution of the terminal galactose residue of the pentasaccharide by N-acetylneuraminic acid is described. Dimethyl(methylthio)sulfonium triflate-promoted coupling of 2-(trimethylsilyl)ethyl O-(2,3,4-tri-O-benzyl-α-L-fucopyranosyl)-(1→3)-O-(2-acetamido-6-O-benzyl-2-deoxy-ß-D-glucopyranosyl)-(1→3)-O-(2,4,6-tri-O-benzyl-ß-D-galactopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-ß-D-glucopyranoside (1) with methyl O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-α-D-galacto-2-nonulopyranosylonate)-(2→6)-2,4-di-O-benzoyl-3-O-benzyl-1-thio-ß-D-galactopyranoside (2) gave the desired hexasaccharide 3. Compound 3 was converted into the α-trichloro-acetimidate 6, via reductive removal of the benzyl groups, O-acetylation, removal of the 2-(trimethylsilyl)ethyl group, and treatment with trichloro-acetonitrile, which, on coupling with (2S, 3R, 4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol (7), gave the ß-glycoside 8. Finally, 8 was transformed, via selective reduction of the azide group, coupling with octadecanoic acid, O-deacylation, and hydrolysis of the methyl ester group, into the title ganglioside 11 in good yield.     

Synthesis of an arabinogalactan-type octa- and two isomeric nonasaccharides. Suitable tuning of protecting groups

An arabinogalactan-type double-branched octa- and two isomeric nonasaccharides were synthesized using the (2-naphthyl)methyl (NAP) and the acid sensitive but base stable (methoxydimethyl)methyl (MIP) protecting groups. The β-(1→6)-linked hexagalactan skeleton was synthesized having a benzyl and a (2-naphthyl)methyl (NAP) group at positions 2 of the second and the penultimate galactopyranosyl units, and this made possible sequential introduction of α-l-arabinofuranosyl or α-l-arabinofuranosyl-(1→5)-α-l-arabinofuranosyl side chains.     

Syntheses of (-)-Isocitric Acid Lactone and (-)-Homoisocitric Acid. A New Method of Conversion of Alkynylsilanes into the Alkynyl Thioether and Corresponding Carboxylic Acids

A simple, stereoselective synthesis of natural isocitric and homoisocitric acids from a common alkynylsilane correlates the stereochemistry of these acids. Starting with dimethyl D-malate dianion, methyl 2-hydroxy-3-carbomethoxy-6-(trimethylsilyl)-5-hexynoate (6a) was prepared with a good stereoselectivity (threo/erythro 90/10). Oxidative cleavage of the triple bond provided isocitric acid lactone (8') in 15% overall yield starting from D-malic acid diester 1. The synthesis of homoisocitric acid relied on a new method of conversion of alkynylsilane to alkynyl thioether, which is converted to the carboxylic acid of the same chain length. Addition of benzenesulfenyl chloride to (trimethylsilyl)alkyne 6b and elimination of trimethylsilyl chloride gave the corresponding thioether 10, which by acid hydrolysis gave homoisocitric acid (11) in a 24% yield from D-malic acid ester. This novel method of conversion of alkynylsilane to the corresponding acid was illustrated with several other alkynyltrimethylsilanes.     

Alternative Syntheses of (l→3)-β-D-Galacto-Oligosaccharides

ABSTRACT

Galactosyl halides bearing different substituents at O-3 [i.e. acetyl (15), benzoyl (14), benzyl (3), bromoacetyl (12), and the 2, 3, 4, 6-tetra-O-benzoyl-β-D-galactopyranosyl group (17)] have been prepared, and used to study the stereoselectivity of the coupling reaction to position O-3 of different galactose derivatives [i.e. methyl 2, 4, 6-tri-O-acetyl-(9) and 2, 4, 6-tri-O-benzoyl-β-D-galactopyranoside (7), l, 2, 4, 6-tetra-O-benzoyl-β-D-galactose (6) and O-(2, 4, 6-tri-O-benzoyl-β-D-galactopyranosyl)-(1→3)-β-D-galactose (33)], as well as to benzoic acid. In more polar solvents, using silver trifluoro-methanesulfonate as the promoter, a higher proportion of β-linked products was formed, whereas with silver perchlorate as the promoter the α-linked product predominated. Under basic conditions, applied to prevent anomerisation of 1-O-benzoylated nucleophiles 6 and 33, no orthoesters were found as end products. Under those conditions, a better overall yield of the β-(l→3)-linked galactotriose 31 was obtained by condensation of die disaccharide glycosyl donor 17 and the monosaccharide glycosyl acceptor 6 than by condensation of 14 and 33. The disaccharide glycosyl chloride 17 was obtained in 75% yield by the cleavage of the corresponding methyl glycoside with dichloromethyl methyl ether.     

LIQUID CRYSTALLINE BEHAVIOR OF AROMATIC COPOLYAMIDE IN CONCENTRATED SULPHURIC ACID

The liquid crystalline behavior of anisotropic solutions in 100% sulphuric acid ofaromatic copolyamide obtained by low-temperature solution copolycondensation of tere-phthalic acid chloride (TPC), p-phenylene diamine (PPD) and 4, 4'-diamino-diphenylether(DAPE) has been studied by optical microscopy and X-ray diffeaction. The effects ofinherent viscosity, concentration of copolyamide in sulphuric acid, the content of the thirdmonomer (DAPE) and sequence distribution of copolyamide on the critical concentration,isotropic temperature, phase diagram and texture of liquid crystal were investigated. Theschlieren texture was observed and the results of X-ray diffraction indicate that the con-centrated solutions of copolyamide exhibit nematic liquid crystalline behavior.     

Mannosazide methyl uronate donors. Glycosylating properties and use in the construction of β-ManNAcA-containing oligosaccharides

Mannosazide methyl uronate donors equipped with a variety of anomeric leaving groups (β- and α-S-phenyl, β- and α-N-phenyltrifluoroacetimidates, hydroxyl, β-sulfoxide, and (R(s))- and (S(s))-α-sulfoxides) were subjected to activating conditions, and the results were monitored by (1)H NMR. While the S-phenyl and imidate donors all gave a conformational mixture of anomeric α-triflates, the hemiacetal and β- and α-sulfoxides produced an oxosulfonium triflate and β- and α-sulfonium bistriflates, respectively. The β-S-phenyl mannosazide methyl uronate performed best in both activation experiments and glycosylation studies and provided the 1,2-cis mannosidic linkage with excellent selectivity. Consequently, an α-Glc-(1→4)-β-ManN(3)A-SPh disaccharide, constructed by the stereoselective glycosylation of a 6-O-Fmoc-protected glucoside and β-S-phenyl mannosazide methyl uronate, was used as the repetitive donor building block in the synthesis of tri-, penta-, and heptasaccharide fragments corresponding to the Micrococcus luteus teichuronic acid.     

Oxidative degradation of eicosapentaenoic acid into polyunsaturated aldehydes

Eicosapentaenoic acid is converted in good overall yield to an α,β-ethylenic epoxide derivative. The oxidative cleavage of the epoxide ring with periodic acid in ether proceeded in part with acid-catalyzed rearrangement of the vinyl epoxide moiety prior to cleavage. Among the products were 2E- and 2Z, 5Z, 8Z, 11Z-tetradecatetraenal, 4-hydroxy-2E,6Z,9Z,12Z-pentadecatetraenal and (all-Z)-2-methoxy-3,6,9,12-pentadecatetraenal.     

Isomorphism in copolyamides of long repeating chain units containing oxa- and thia-alkylene linkages

Various copolyamides of long repeating chain units were prepared from hexamethylenediamine (HMDA) and p-xylylenediamine (PXDA) with aliphatic dicarboxylic acids of three structural types: α,ω-alkanedioic, α,ω-oxaalkanedioic, and α,ω-thiaalkanedioic acids. Both binary and ternary combinations of these dicarboxylic acids having the same number of chain atoms with the diamine afforded highly crystalline copolyamides. In all cases of these copolymers, the plots of the melting points versus the compositions are expressed by linear relations, even in the ternary systems. For example, the melting points of the copolyamides of HMDA with 6-oxaundecanedioic and 6-thiaundecanedioic acids are practically unchanged in all ranges of composition. The same relation is also observed in the corresponding copolyamides of PXDA. The relation between the densities and the composition is plotted with good linearity in every case. From x-ray examination, the lattice spacings of each copolyamide are ascertained to be unchanged by the composition. These results reveal that methylene, ether, and thioether linkages are in the relation of isomorphous replacements for each other in these copolyamide systems. Moreover, the linear relationship between the melting point and the composition is explained by assuming that the entropy of fusion in these copolyamides changes linearly according to the change of the composition.     

Stereoselective synthesis of highly enantioenriched 3-methyl-2-cyclohexen-1-ones possessing an asymmetric quaternary carbon as C-4 or C-6: a sugar template approach

The 1,4-addition of the enolate generated from α-methylated acetoacetate incorporated at C-4 of methyl 6-deoxy-2,3-di-O-(tert-butyldimethylsilyl)-α-d-glucopyranoside to methyl vinyl ketone, followed by aldol condensation of the resulting 1,4-addition product under two base-mediated conditions, provided 4-O-functionalized d-glucose derivatives with high diastereoselectivity. These products install a 3-methyl-2-cyclohexen-1-one-4- (or -6-) carboxylic acid as the O-4 ester, in which C-4 or C-6 is an asymmetric quaternary carbon. Removal of the sugar template from those aldol condensation products provided synthetically useful 3,6-dimethyl-2-cyclohexen-1-one-6-carboxylic acid and 3,4-dimethyl-2-cyclohexen-1-one-4-carboxylic acid derivatives both in high enantioenriched forms.     

Synthesis of permethylated α-d-mannosylacetic acid, a new type of bioconjugate

A concise stereoselective 3-step conversion of methyl α-d-mannopyranoside to α-d-2,3,4,6-tetra-O-methyl-mannosylacetic acid is described. After methylation of the alcohol functions, an allylation is performed. The resulting alkene undergoes oxidative cleavage to the acid, an alkylated C-sugar, appropriate for attachment to peptides or other drug candidates for solubility enhancement.     

Asymmetric Strecker synthesis of α-arylglycines

A practically simple three-component Strecker reaction for the asymmetric synthesis of enantiopure α-arylglycines has been developed. Addition of a range of aryl-aldehydes to a solution of sodium cyanide and (S)-1-(4-methoxyphenyl)ethylamine affords highly crystalline (S,S)-α-aminonitriles that are easily obtained in diastereomerically pure form. Heating the resultant (S,S)-α-aminonitriles in 6 M aqueous HCl at reflux resulted in cleavage of their chiral auxiliary fragments and concomitant hydrolysis of their nitrile groups to afford enantiopure (S)-α-arylglycines. The enantiopurities of these (S)-α-arylglycines were determined via derivatization of their corresponding methyl esters with 2-formylphenylboronic acid and (S)-BINOL, followed by (1)H NMR spectroscopic analysis of the resultant mixtures of diastereomeric iminoboronate esters.     

Total synthesis of (+)-kalafungin using a tandem Michael-Dieckmann approach

A stereoselective synthesis of the antibiotic kalafungin 1 is reported. A key step involved the tandem Michael-Dieckmann reaction between methyl 2-methoxy-6-methylbenzoate 11 and the α,β-unsaturated lactone (R)-6-(2-(tert-butyldimethylsilyloxy)ethyl)-4-methoxy-5,6-dihydropyran-2-one 10, which was prepared from (S)-aspartic acid. The C5 alkyl substituent was introduced by the use of methylmagnesium bromide and subsequent stereoselective reduction. A sequence of oxidations followed by acid-catalyzed epimerization delivered (+)-kalafungin 1.     

4-Methylumbelliferyl 5-Acetamido-3,4,5-trideoxy-α-D-manno-2-nonulopyranosidonic Acid: Synthesis and Resistance to Bacterial Sialidases

The synthesis of 4-methylumbelliferyl α-D -glycoside 13 of N-acetyl-4-deoxyneuraminic acid and its behaviour towards bacterial sialidases is described. N-Acetyl-4-deoxyneuraminic acid ( 1 ) was transformed into its methyl ester 2 and then acetylated to give the anomeric pentaacetates 3 and 4 of methyl 4-deoxyneuraminate and the enolacetate 5 (Scheme). A mixture 3/4 was treated with HCl/AcCl to give the glycosyl chloride, which was directly converted into the 4-methylumbelliferyl α-D -glycoside 9 of methyl 7-O,8-O,9-O,N-tetraacetylneuraminate and into the 2,3-dehydrosialic acid 11 . The ketoside 9 was de-O-acetylated to 12 with NaOMe in MeOH. Saponification (NaOH) of the methyl ester 12 followed by acidification gave the free 13 , which was also converted into the sodium salt 14 by passage through Dowex 50 (Na⁺). The 4-deoxy α-D -glycoside 13 is not hydrolyzed at significant rates by Vibrio cholerae and Arthrobacter ureafaciens sialidase. Neither the free N-acetyl-4-deoxyneuraminic acid ( 1 ), nor the α-D -glycoside 13 inhibit the activity of these sialidases.     

Stereocontrolled Synthesis of 6′-Deoxy-6′-Fluoro Derivatives of Methyl α-Sophoroside, α-Laminaribioside, α-Kojibioside and α-Nigeroside

Abstract

Sequential tritylation, benzoylation and detritylation of D-glucose, followed by resolution of the crude product by chromatograpEy gave crystalline 1,2,3,4-tetra-O-benzoyl-α- (1) and β-D-glucopyranose (2). Compound 1, 2, and the corresponding methyl α-glycoside 5 were treated with dimethylaminosulfur trifluoride (methyl DAST) to give, respectively, the 6-deoxy-6-fluoro derivatives 3, 4, and 6. Crystalline 2,3,4-tri-O-benzoyl-6-deoxy-6-fluoro-α-D-glucopyranosyl chloride (10) could be obtained from either 3, 4, or 5 by reaction with dichloromethyl methyl ether in the presence of anhydrous zinc chloride. Silver trifluoromethanesulfonate-promoted reaction of 10 with methyl 2-O-(9) and 3-O-benzyl-4,6-O-benzylidene-α-D-glucopyranoside (8) gave the corresponding, (β-linked disaccharidës in high yield. Subsequent deprotection afforded the 6′-deoxy-6′-fluoro derivatives of methyl α-sophoroside (13) and methyl 6′ -deoxy-o′-fluoro-α-laminaribioside (16). Condensation of 8 and 9 with 6-O-acetyl-2,3,4-tri-O-benzyl-α-D-glucopyranosyl chloride in the presence of silver perchlorate was highly stereoselective and produced the α-linked disaccharidës 17 and 21, respectively, in excellent yield. Deacetylation of 17 and 21, followed by fluorination of the resulting alcohols 18 and 22 with methyl DAST and subsequent hydrogenolysis, gave 6′-deoxy-6′-fluoro derivatives of methyl α-kojibioside and methyl α-nigeroside 20 and 24, respectively.