Showing papers on "Glucal published in 2005"

α-N-acetylmannosamine (ManNAc) synthesis via rhodium(II)-catalyzed oxidative cyclization of glucal 3-carbamates

Abstract: Glucal 3-carbamates 1 and 7 underwent oxidative cyclization with iodobenzene diacetate or iodosobenzene in the presence of Rh2(OAc)4, providing mannosamine 2-N,3-O-oxazolidinones. With iodosobenzene, incorporation of 4-penten-1-ol provided a readily separable anomeric mixture of n-pentenyl glycosides, with the anomers exhibiting pronounced differences in reactivity as glycosyl donors. N-acylation of the sugar oxazolidinones led to α-selective glycosyl donors for the elaboration of various 2-mannosamine frameworks. Alternatively, the anomeric n-pentenyl glycosides of N-Cbz 2-mannosamine oxazolidinones were converted separately to oxazolidinone-opened derivatives 28α and 28β. These served as stereoconvergent glycosyl donors, and the α-linked products were readily advanced to a variety of N-acetylmannosamine (ManNAc) frameworks, using an intramolecular O→N acetyl transfer as the final step.

Metal nitrates catalysed O-glucosylation using acetyl glucal in organic solvents and ionic liquids: A comparative investigation

Abstract: The Ferrier glucosylation of alcohols with 3,4,6-tri- O-acetyl-d-glucal has been investigated with several metal nitrates and the optimal catalyst is bismuth nitrate pentahydrate (BNP). Good yields of pseudoglycals were also obtained with ferric nitrate nonahydrate (FNN), heightening the catalyst dosage (50 mol%) being required however. The BNP-mediated reactions showed remarkable solvent dependency and in the optimal protocol, the amount of BNP as low as 10 mol% was effective, furnishing excellent yields of O-glucosides with good anomeric selectivity in acetonitrile. A comparison of BNP and FNN in terms of yields and selectivity of the product has been made. In comparison to the reactions in acetonitrile, the catalytic ability of BNP was found to enhance drastically in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF 6]. © 2005 Elsevier B.V. All rights reserved.

C(1 -> 4)-linked disaccharides through carbonylative Stille cross-coupling

Abstract: A tinglucal tributyl[4,6-O-bis(t-butyl)silylidene-3-O-tris(isopropyl)silyl]tin 7 and a triflate derived from isolevoglucosenone (1R,4R,5R)-4-benzyloxy-6,8-dioxabicyclo[3.2.1]oct-2-en-2-yl trifluoromethanesulfonate 10 undergo the carbonylative Stille condensation under special conditions requiring AsPh3, LiCl, and powdered charcoal as co-catalysts to give a cross-conjugated dienone 6 in which the bicyclic alkene moiety is more reactive than the glucal alkene moiety. This allows the regio- and stereoselective hydrogenation of the bicyclic alkene moiety giving an enone 21 that can be reduced stereoselectively to an allylic alcohol 22. Hydroboration of the glucal and bicyclic acetal opening generates a C(1-->4) linked disaccharide 25 in which a protected form of beta-D-glucopyranose is attached at position C(4) of a alpha-D-3-deoxy-ribo-hexopyranoside derivative via a (S)-hydroxymethano linker. (C) 2004 Elsevier Ltd. All rights reserved.

Enzymatic synthesis of alkyl α-2-deoxyglucosides by alkyl alcohol resistant α-glucosidase from Aspergillus niger

Abstract: Aspergillus niger α-glucosidase (ANGase) was used for an efficient syntheses of alkyl α- d -2-deoxyglucosides (A2DGs) and for regioselectivity studies of alkoxy-hydro additions of d -glucal in the presence of alkyl alcohols. ANGase showed a high stability with respect to the high concentration of alkyl alcohols. The reaction conditions were optimized for pH, temperature, alkyl alcohol concentration, and d -glucal concentration. On the basis of MS and NMR analyses, A2DGs were confirmed to have only an α-2-deoxyglucosidic bond and the two-dimensional NMR (HMBC) spectra showed to be made up of 2-deoxyglucosyl and alkyl moieties.

Unusual Dimethyl Ketal from Cycloadduct of 4,6-O-Benzylidene-D-allal and Dichloroketene

Abstract: Glycal derivatives are useful building blocks in organic synthesis as well as in carbohydrate chemistry. Cycloadditions of dichloroketene to tri-O-benzyl or tri-O-acetylD-glucal and D-galactal were reported to produce α-oriented cyclobutanone ring, and the resulting bicyclic cyclobutanones were converted into lactone compounds by oxidation. Although they showed high stereo and regioselectivity with moderate yield, this methodology has not been widely applied for synthetic purpose. For the last few years, we have studied the cycloaddition of ketene to glucal, galactal, and allal derivatives and their application for the synthesis of C-glycoside derivatives and modified nucleosides. Even though, in case of allal, β-oriented cyclobutane ring formation is reported, no direct evidence supporting the face selectivity of dichloroketene cycloaddition to allal derivatives has been reported yet. Thus herein we report the X-ray structure of an unusual ketal from allal-derivative as the evidence that the facial selectivity of cycloaddition to glycal is controlled by the stereochemistry of C-3 constituent and discuss the mechanism for the formation of the unsual dimethyl ketal. To substantiate the evidence for facial selectivity, glucal and allal derivatives (1 and 3, respectively) were subject to dichloroketene cycloaddition reaction and treated with sodium methoxide in methanol. As expected, 3-O-benzyl4,6-O-benzilidene-D-glucal (1) was converted to C-1 and C-2 dialkylated C-glycoside, 3-O-benzyl-4,6-O-benzylidene1-dichloromethyl-2-methoxycarbonyl-1,2-dideoxy-α-D-glucopyranoside (2) in 73% yield. To our surprise, 3-O-benzyl4,6-O-benzylidene-D-allal (3) provided two products, O-benzyl-4,6-O-benzylidene-1-dichloromethyl-2-methoxycarbonyl-1,2-dideoxy-β-D-altropyranoside (4a) and dimethyl ketal (5a), in 36% and 29% isolated yield from allal (3), respectively (Scheme 1). The stereochemistry of C2 of 2 was determined by coupling constant between H2 and H3 (9.3 Hz, 1,2-diaxial orientation) and the previous results from glucal and galactal. Although C-glycoside from allalderivative has not been reported in the literature, methoxy carbonyl group on C2 of 4a was determined to orient axially based on coupling constant between H2 and H3 (3 Hz, 1,2diequatorial). This assignment of stereochemistry of C2 was confirmed by X-ray structure of 5a. In solid state the ketal (5a) has chair conformation of benzlylidene unit, twisted chair conformation of six membered sugar ring, and axial orientation of C3 benzyloxy substituent. Besides, more importantly, cyclobutanone is located on β-face of sugar ring, showed in Figure 1. This stereochemistry is attributed to the steric hindrance of bulky group on C3, which prevented dichloroketene from reacting on the α-face of allal effectively. To the best of our knowledge, the x-ray structure of allal-derived cycloaddition products has not been disclosed yet. Accordingly, we can

InCl 3 -catalyzed stereoselective synthesis of 1,5-benzodiazepines

Abstract: o-Phenylenediamines (OPDA) undergo smooth condensation with 4,6-di-O-benzyl- or 4,6-di-O- acetyl- or 4,6-di-O-ethyl-2,3-dideoxy-aldehydo-D-erythro-trans-hex-2-enose derived from D- glucal in the presence of 2 mol% of InCl3 under mild conditions to afford a new class of 1,5- benzodiazepines in good yields.

A new entry to 4,6-O-benzylidene glucal from phenyl 1-seleno-α-D-mannopyranoside

Abstract: The two-step synthesis of 4,6-O-benzylidene glucal, in 59% overall yield, from phenyl 1-seleno-α- d -mannopyranoside is described.

C(1→4)‐Linked Disaccharides Through Carbonylative Stille Cross‐Coupling.

Abstract: A tinglucal tributyl[4,6-O-bis(t-butyl)silylidene-3-O-tris(isopropyl)silyl]tin 7 and a triflate derived from isolevoglucosenone (1R,4R,5R)-4-benzyloxy-6,8-dioxabicyclo[3.2.1]oct-2-en-2-yl trifluoromethanesulfonate 10 undergo the carbonylative Stille condensation under special conditions requiring AsPh3, LiCl, and powdered charcoal as co-catalysts to give a cross-conjugated dienone 6 in which the bicyclic alkene moiety is more reactive than the glucal alkene moiety. This allows the regio- and stereoselective hydrogenation of the bicyclic alkene moiety giving an enone 21 that can be reduced stereoselectively to an allylic alcohol 22. Hydroboration of the glucal and bicyclic acetal opening generates a C(1-->4) linked disaccharide 25 in which a protected form of beta-D-glucopyranose is attached at position C(4) of a alpha-D-3-deoxy-ribo-hexopyranoside derivative via a (S)-hydroxymethano linker. (C) 2004 Elsevier Ltd. All rights reserved.

Crystal Structure of (3R,4R,5R)-3,4,6-Tri-O-benzyl-D-glucal

Abstract: The title compound, C27H28O4, consisting of three O-benzyl modieties and one glucal modiety, belongs to the space group P21 with cell parameters a = 12.0729(5), b = 4.7002(2), c = 19.6720(8)A and β = 90.989(4)°. The glucal moiety has a twist-boat conformation.

Protic Acid (HClO4 Supported on Silica Gel)‐Mediated Synthesis of 2,3‐Unsaturated‐O‐glucosides and a Chiral Furan Diol from 2,3‐Glycals.

Abstract: Perchloric acid supported on silica gel acts as an excellent reagent system in converting glucals into 2,3-unsaturated-O-glucosides in good to excellent yields in short reaction time with good a selectivity. Primary, secondary, and allylic alcohols, phenols, and thiols react with 3,4,6-tri-O-acetyl glucal with equal ease. In addition to this, a chiral furan diol is obtained from unprotected D-glucal or D-galactal in good yields.

Synthesis of Silylallene Glycosides and Diene Diglycosides by C‐Glycosidation of D‐Glucal with 1,4‐Bis(trimethylsilyl)‐2‐butyne.

Abstract: Silylmethylallenyl glycosides, symmetrical and unsymmetrical diene glycosides, were synthesized by C-glycosidation with 1,4-bis(trimethylsilyl)-2-butyne in good yield. The nature of the product is controlled by the choice of Lewis acid, BF3·OEt2, or SnCl4. The efficient construction of unsymmetrical diene glycosides was achieved in one pot on the basis of the order of addition of sugar starting materials.

Synthesis of α-2-Deoxyglycosides by Acid-Mediated Conjugate Addition.

Abstract: A new method for the synthesis of α‐2‐deoxyglycosides from hex‐1‐en‐3‐uloses is reported. Several acids were surveyed for their ability to mediate the conjugate addition of cyclohexanol to a glucal derived α,β‐unsaturated ketone. Although several acids successfully afforded the α‐2‐deoxy‐3‐uloside as a single anomer, Ph3P · HBr in benzene afforded the highest yield of product. These conditions were then applied to other alcohol nucleophiles, and it was found that nonsterically hindered alcohols readily undergo conjugate addition. Finally, the stereoselectivity of reduction of the resulting α‐2‐deoxy‐3‐uloside was determined for several hydride‐reducing agents.

Unusual Dimethyl Ketal from Cycloadduct of 4,6‐O‐Benzylidene‐D‐allal and Dichloroketene.

Abstract: Glycal derivatives are useful building blocks in organic synthesis as well as in carbohydrate chemistry. Cycloadditions of dichloroketene to tri-O-benzyl or tri-O-acetylD-glucal and D-galactal were reported to produce α-oriented cyclobutanone ring, and the resulting bicyclic cyclobutanones were converted into lactone compounds by oxidation. Although they showed high stereo and regioselectivity with moderate yield, this methodology has not been widely applied for synthetic purpose. For the last few years, we have studied the cycloaddition of ketene to glucal, galactal, and allal derivatives and their application for the synthesis of C-glycoside derivatives and modified nucleosides. Even though, in case of allal, β-oriented cyclobutane ring formation is reported, no direct evidence supporting the face selectivity of dichloroketene cycloaddition to allal derivatives has been reported yet. Thus herein we report the X-ray structure of an unusual ketal from allal-derivative as the evidence that the facial selectivity of cycloaddition to glycal is controlled by the stereochemistry of C-3 constituent and discuss the mechanism for the formation of the unsual dimethyl ketal. To substantiate the evidence for facial selectivity, glucal and allal derivatives (1 and 3, respectively) were subject to dichloroketene cycloaddition reaction and treated with sodium methoxide in methanol. As expected, 3-O-benzyl4,6-O-benzilidene-D-glucal (1) was converted to C-1 and C-2 dialkylated C-glycoside, 3-O-benzyl-4,6-O-benzylidene1-dichloromethyl-2-methoxycarbonyl-1,2-dideoxy-α-D-glucopyranoside (2) in 73% yield. To our surprise, 3-O-benzyl4,6-O-benzylidene-D-allal (3) provided two products, O-benzyl-4,6-O-benzylidene-1-dichloromethyl-2-methoxycarbonyl-1,2-dideoxy-β-D-altropyranoside (4a) and dimethyl ketal (5a), in 36% and 29% isolated yield from allal (3), respectively (Scheme 1). The stereochemistry of C2 of 2 was determined by coupling constant between H2 and H3 (9.3 Hz, 1,2-diaxial orientation) and the previous results from glucal and galactal. Although C-glycoside from allalderivative has not been reported in the literature, methoxy carbonyl group on C2 of 4a was determined to orient axially based on coupling constant between H2 and H3 (3 Hz, 1,2diequatorial). This assignment of stereochemistry of C2 was confirmed by X-ray structure of 5a. In solid state the ketal (5a) has chair conformation of benzlylidene unit, twisted chair conformation of six membered sugar ring, and axial orientation of C3 benzyloxy substituent. Besides, more importantly, cyclobutanone is located on β-face of sugar ring, showed in Figure 1. This stereochemistry is attributed to the steric hindrance of bulky group on C3, which prevented dichloroketene from reacting on the α-face of allal effectively. To the best of our knowledge, the x-ray structure of allal-derived cycloaddition products has not been disclosed yet. Accordingly, we can