Stannylene Acetals Research Articles

Halide-mediated regioselective 6-O-glycosylation of unprotected hexopyranosides with perbenzylated glycosyl bromide donors

The regio- and stereoselective glycosylation at the 6-position in 2,3,4,6-unprotected hexopyranosides has been investigated with dibutyltin oxide as the directing agent. Perbenzylated hexopyranosyl bromides were employed as the donors and the glycosylations were promoted by tetrabutylammonium bromide. The couplings were completely selective for both glucose and galactose donors and acceptors as long as the stannylene acetal of the acceptor was soluble in dichloromethane. This gave rise to a number of 1,2-cis-linked disaccharides in reasonable yields. Mannose donors and acceptors, on the other hand, did not react in the glycosylation under these conditions.

Stannylene‐Mediated Regioselective 6‐O‐Glycosylation of Unprotected Phenyl 1‐Thioglycopyranosides

AbstractA straightforward procedure is described for the synthesis of (1→6)‐linked saccharides by regioselective glycosylation of unprotected glycosyl acceptors. Phenyl 1‐thioglycopyranosides derived from D‐glucose, D‐galactose and D‐mannose were treated with dibutyltin oxide to introduce a stannylene acetal, and then subjected to selective glycosylation at the 6‐position with the Koenigs–Knorr protocol. Peracylated glycosyl bromides of D‐glucose, D‐galactose, D‐mannose and D‐glucosamine were employed as the donors to give the corresponding (1→6)‐linked disaccharides in moderate to good yields. The best results were obtained with glycosyl donors and acceptors derived from D‐glucose and D‐galactose. Fully acylated disaccharide thioglycosides could also serve as glycosyl donors for the regioselective coupling. Brominolysis and subsequent Koenigs–Knorr coupling with the stannylene acetal of phenyl 1‐thio‐β‐D‐glucopyranoside gave rise to the corresponding (1→6)‐linked trisaccharides in moderate yields.

An efficient method for selective oxidation of 1,2-diols in water catalyzed by Me2SnCl2

Dimethyltin(IV)dichloride-catalyzed selective oxidation of 1,2-diols in water was achieved using dibromoisocyanuric acid (DBI) or Br2 as oxidants. The catalyst activates the 1,2-diol moiety through the formation of stannylene acetal in addition to enhancing selectivity. Various cyclic and acyclic 1,2-diol substrates have been selectively oxidized affording α-hydroxyketones in good to excellent yields. This method is safe and simple in operation.

ChemInform Abstract: Direct Selective and Controlled Protection of Multiple Hydroxyl Groups in Polyols via Iterative Regeneration of Stannylene Acetals.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Direct selective and controlled protection of multiple hydroxyl groups in polyols via iterative regeneration of stannylene acetals

A direct selective protection (O-benzylation) of two or more hydroxyl groups in polyols displaying diverse structural patterns was made possible by the establishment of conditions that enable an efficient turnover for the Bu 2Sn group, initially at the corresponding stannylene acetals (only ∼1.0 mol equiv of Bu 2SnO was employed). It was also demonstrated that one might exert control over the number of protected groups, by means of appropriate tuning of reaction conditions. The feasibility of a substoichiometric (tin source) catalytic protocol was demonstrated as well.

Short synthetic sequence for 2-sulfation of α- l-iduronate glycosides

Hunter syndrome (mucopolysaccharidosis-II) is caused by deficiency of the lysosomal enzyme iduronate-2-sulfatase. The assay of this sulfatase requires the use of α- l-iduronate glycosides containing a sulfate at the 2-position. We report a simple, three-step procedure for the introduction of sulfate at the 2-position starting with the methyl ester of α- l-iduronate glycosides. The procedure involves protection of the 2- and 4-hydroxyl groups of the iduronate moiety as the dibutyl stannylene acetal, selective sulfation with sulfur trioxide–trimethylamine, and deprotection of the methyl ester to afford the desired 2-sulfate in 61% overall yield.

Synthesis of regioselectively sulfated xylodextrins and crystal structure of sodium methyl β- d-xylopyranoside 4- O-sulfate hemihydrate

Methyl xylobioside and methyl xylotrioside were prepared from the peracetylated anomeric xylosyl trichloroacetimidates by reaction with methanol followed by Zemplén deacetylation. Methyl β- d-xylopyranoside, methyl β- d-xylobioside and methyl β- d-xylotrioside were subjected to treatment with dibutyltin oxide followed by reaction with the trimethylamine/sulfur trioxide complex in tetrahydrofuran. This way, preferential sulfation of the terminal 4-hydroxy group at the nonreducing xylopyranosyl unit was achieved. In addition, partial sulfation at position 2 of the distal xylose unit was observed. The substitution pattern was derived from NMR spectroscopic data and was confirmed by the X-ray structure determination of sodium methyl β- d-xylopyranoside 4- O-sulfate. The compound crystallized as a hemihydrate in a triclinic lattice of space group P1 and possesses a pseudomonoclinic 2D supramolecular structure. The sulfation of free pentose oligomers via their intermediate stannylene acetals may thus be exploited to generate biologically active oligosaccharides for biomedical applications.

Regioselective formation of 6- O-acylsucroses and 6,3′-di- O-acylsucroses via the stannylene acetal method

Regioselective formation of 6- O-acylsucroses and 6,3′-di- O-acylsucroses in one pot with good yields was achieved for the first time by a typical acylation method of sucrose via its dibutylstannylene acetal. Pure monoesters at OH-6 and diesters at OH-6,3′ obtained by these procedures were readily isolated by simple column chromatography, thus overcoming the main difficulties associated with regioselectivity, efficiency, and isolation techniques for the practical preparation. Explanations for the regioselectivities observed during this stannylene acetal-mediated reaction were also proposed based on the structures of the stannylene acetal in solution and the intramolecular migration of stannylenes.

A novel application of tin-based alkoxide anion for deacetylation

A stannylene acetal, in the presence of CsF, is able to cleave both aromatic and aliphatic acetates rapidly and efficiently under room temperature. Control experiments indicate that both the stannylene acetal and CsF are required and play significant roles in the deacetylation process. Thus, it is likely that the tin-based alkoxide anion generated by a stannylene acetal and CsF is responsible for deacetylation.

Regioselective Synthesis of Vinylic Derivatives of Common Monosccarides Through Their Activated Stannylene Acetal Intermediates

The regioselective C-2-O-acrylation and metacrylation of methyl 4,6-O-benzylidene-α-D-glucopyranoside and methyl 4,6-O-benzylidene-α-D-galactopyranoside through their corresponding organotin intermediates have been studied. Regioselectivity was achived through the formation of a tin chelate of the 2,3-diols. Thus, methyl 4,6-O-benzylidene-α-D-glucopyranoside and methyl 4,6-O-benzylidene-α-D-galactopyranoside were reacted with dibutylstannylene to give the corresponding dibutylstannylene acetal intermediates that were then reacted in a regioselective manner with acryloyl chloride or metacryloyl chloride in the presence of triethylamine (TEA) or pyridine to give the vinylic type monomeric compounds. The monomeric products containing glucose and galactose units from each reaction were separated by column chromatography using a gradient of n-hexane and ethyl acetate as eluant. The structure of the obtained compounds were confirmed using 1H-, 13C- and 2D NMR spectroscopy.

Regioselectivity in Alkylation Reactions of 1,2‐O‐Stannylene Acetals of D‐Arabinofuranose.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Di-n-butyltin Oxide (DBTO)

Di-n-butyltin oxide (DBTO), with the composition Bu2SnO, is an amorphous, polymeric powder insoluble in nonreacting solvents. It has been widely used for the ­selective manipulation of hydroxyl groups and polyols. The largest application of DBTO in organic chemistry is in the generation and reactions of stannylenes from polyhydroxy compounds. The stannylene acetals may be prepared by refluxing equimolar mixtures of substrate and DBTO in benzene or toluene, with a Dean-Stark apparatus. [1] Recently, Simas has reported that the stannylene intermediate formation does not require removal of water. [2] The synthesis of di-n-butylstannylene is faster in methanol, but this solvent must be removed prior to addition of the electrophile. [3] Formation of these tin derivatives can be greatly accelerated by microwave irradiation. [4]

Regioselectivity in Alkylation Reactions of 1,2‐O‐Stannylene Acetals of d‐Arabinofuranose

The synthesis of β‐arabinofuranosides via alkylation of 1,2‐O‐stannylene acetal intermediates has been studied. With reactive alkyl halides (benzyl bromide, allyl bromide, and p‐methoxybenzyl chloride), the method provides a mixture of β‐arabinofuranosides and 2‐O‐alkylated lactols in ratios of 4:1 to 1:1.5. However, with carbohydrate‐derived electrophiles, no alkylated products are produced. It appears, therefore, that the method is limited to the preparation of β‐arabinofuranosides of simple alcohols. Through the use of computational chemistry, we have explored the conformational properties of one of these stannylene acetals and propose that these species exist in more than one conformation in solution and that this contributes to the relatively poor regioselectivity in these reactions.

Synthesis and Taste Properties of 4,1′,4′,6′‐Tetrahalodeoxysucrose Analogues

The synthesis of a series of 1,4,6‐trideoxy‐1,4,6‐trihalo‐β‐d‐hexulofuranosyl 4‐deoxy‐4‐halo‐α‐d‐hexopyranosides is described. The 4‐chloro‐, 4‐bromo‐ and 4‐iodo‐4‐deoxy‐β‐d‐fructofuranosyl analogues were synthesized from a 3′,4′‐lyxo‐epoxide using the respective alkali metal halides. The corresponding 4‐halodeoxytagatofuranosyl analogues, on the other hand, were obtained by direct halide displacement of the 4′‐O‐trifluoromethanesulfonyl derivative, which was derived by regioselective sulfonylation of 1,6‐di‐O‐trityl‐β‐d‐fructofuranosyl 6‐O‐trityl‐α‐d‐glucopyranoside via its stannylene acetal. The sweetness intensities of these tetrahalodeoxy compounds strongly suggest that both size and configuration of the halogen substituents at C‐4 and C‐4′ are critical for sweetness enhancement.

ChemInform Abstract: Substituted Diether Diols by Ring‐Opening of Carbocyclic and Stannylene Acetals.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Substituted diether diols by ring-opening of carbocyclic and stannylene acetals

Reduction of malonaldehyde bis(ethylene and propylene acetals) with borane or monochloroborane produces diether diols 1 and 2 in high yield. Similar reduction of glyoxal bis(ethylene acetals) has only limited utility for the preparation of tetrasubstituted triethylene glycols 3. Organotin chemistry is complementary: stannylene acetals prepared from disubstituted vicinal diols can be alkylated with half an equivalent of 1,2-dibromoethane to produce tetrasubstituted triethylene glycols 3, or with two equivalents of 2-chloroethanol to produce disubstituted triethylene glycols 4.

Glycosylation via locked anomeric configuration: stereospecific synthesis of oligosaccharides containing the β- d-mannopyranosyl and β- l-rhamnopyranosyl linkage

cis-1,2-Stannylene acetals of d-mannose and l-rhamnose, formed preferentially from the free sugars treated with dibutyltin oxide, are capable of displacing the trifluoromethanesulfonyl (triflyl) leaving groups in carbohydrates to give, with retention of configuration at the anomeric center in the nucleophile, cis-1,2-linked oligosaccharides. In the case of secondary triflates, the new glycosidic linkage is formed with complete inversion of configuration in the electrophile. Both the reactivity of the electrophile and nucleophilicity of oxygens in the stannylene complex affect the overall outcome of the reaction. From the comparison of results of a number of glycosylations via stannylene acetals, it appears that nucleophilicity of oxygens involved in the cis-1,2-acetals decreases in the order: equatorial anomeric > equatorial non-anomeric > axial anomeric. Consequently, treatment of the stannylene acetal prepared from d-mannose (mainly the cis-1,2-stannylene compound in admixture with a small proportion of the cis-2,3-stannylene acetal) with methyl 2,3,4-tri- O-benzoyl-6- O-trifluoromethanesulfonyl- α- d-glucopyranoside yielded, in addition to the expected β- d-mannopyranoside (major), a product of non-anomeric alkylation at O-3. On the other hand, glycosylation of the stannylene acetal derived from maltose with methyl 2,3,6-tri- O-benzoyl-4- O-trifluoromethanesulfonyl- α- d-galactopyranoside gave almost exclusively a non-glycosidically, (2→4)-linked pseudo-trisaccharide. Combination of the glycosylation via locked anomeric configuration with conventional glycosylations, to yield higher oligosaccharides, is also demonstrated.

ChemInform Abstract: Manipulation of Free Carbohydrates via Stannylene Acetals. Preparation of β‐Per‐O‐acyl Derivatives of D‐Mannose, L‐Rhamnose, 6‐O‐Trityl‐D‐talose, and D‐Lyxose.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Manipulation of free carbohydrates via stannylene acetals. Preparation of β-per- O-acyl derivatives of d-mannose, l-rhamnose, [formula omitted], and d-lyxose

A simple and high-yielding method for the preparation of 1- O- β-acyl derivatives of carbohydrates with an axial OH group at C-2 is described. It utilizes the property of unprotected carbohydrates to preferentially form 1,2- O- cis stannylene acetals, when treated with dibutyltin oxide. These acetals can be acylated with retention of configuration at the anomeric position.

Improvements in the Regioselectivity of Alkylation Reactions of Stannylene Acetals

Abstract The regioselectivity of benzylation of stannylene acetals of trans-diols on pyranose rings is improved by performing the reaction in benzyl bromide, for instance, for methyl 4,6-O-benzylidene-2,3-O-dibutylstannylene-α-D-glucopyranoside, the ratio of 2-O-benzyl ether to 3-O-benzyl ether changed from 70:20 in DMF to 74:8 in benzyl bromide, then improved further to 84:2 if the dihexylstannylene acetal was used. Standard conditions for methylation of stannylene acetals are methyl iodide in DMF. Non-polar conditions give much improved regioselectivity, the O-2 to O-3 product ratio for the above dibutylstannylene acetal changed from 57:30 in DMF to 84:13 with methyl iodide in 1,1,2,2-tetrachloroethane or 90:8 with methyl triflate in chloroform. Examples of benzylation and methylation reactions on different dialkylstannylene acetals on three different trans-diols are included.