Regioselective activation of glycosyl acceptors by a diarylborinic acid-derived catalyst

A derivative of diphenylborinic acid promotes catalytic, regioselective Koenigs-Knorr glycosylations of carbohydrate derivatives bearing multiple secondary hydroxyl groups. Robust levels of selectivity for the equatorial OH group of cis-1,2-diol motifs are demonstrated in reactions of seven acceptors derived from galactose, mannose, fucose, and arabinose using a variety of glycosyl halide donors. Catalyst control presents a new means of generating defined glycosidic linkages from unprotected or minimally protected carbohydrate feedstocks.     

Borinic acid-catalyzed regioselective acylation of carbohydrate derivatives

Reversible covalent interactions of organoboron compounds are exploited as the basis for regioselective borinic acid-catalyzed acylations of polyols. This catalytic protocol enables differentiation of the secondary OH groups of a wide range of carbohydrate derivatives with diverse acid chloride and chloroformate reagents, using a structurally simple diarylborinic acid-derived catalyst.     

Novel Selectivity in Carbohydrate Reactions,IV: DABCO-Mediated Regioselective Primary Hydroxyl Protection of Carbohydrates

An efficient procedure for the regioselective tritylation of primary hydroxyl group of aldohexopyranosides and nucleosides using trityl chloride in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) in dichloromethane has been developed. Subsequent acetylation of the tritylated products in the same pot has been made possible, thereby providing an efficient route to the fully protected carbohydrate derivatives that can be discriminated chemoselectively.     

Directing-protecting groups for carbohydrates. Design, conformational study, synthesis and application to regioselective functionalization

A novel concept of regioselective transformation of secondary hydroxyl groups in carbohydrates is presented. First, the relative reactivity of the free hydroxyl groups of onoprotected d-glucose derivatives was assessed using acetylation as a model reaction. As a result, acylation of these polyols gave a mixture of monosubstituted products in which the 3-O functionalized derivatives predominated. Novel hydrogen bond acceptor protecting groups were next designed to modulate the 4-OH and 3-OH reactivity in the hope to mediate higher regioselective transformations. A molecular modeling study later validated by spectroscopic analysis predicted additional intramolecular hydrogen bonds between the hydroxyl groups and pyridyl-containing protecting groups. Taking advantage of this induced hydrogen bond network, we achieved regioselective acetylation of the hydroxyl group at position 3 without protecting any secondary hydroxyl groups of the carbohydrate moiety. This designed protecting/directing group increased the nucleophilicity and the steric hindrance of position 3. As a result, optimization of the reaction conditions enabled the monoacetylation (not affected by steric hindrance) of 6-O-protected glucopyranosides at position 3 and selective silylation (affected by steric hindrance) of position 2 in high isolated yields and regioselectivities. This result certainly opens doors to the regioselective open glycosylation of carbohydrates.     

Synthesis of a carbon analogue of N-acetylmannosamine via acetolysis on a relatively stable ozonide

After a 2-methylpropenyl group was added to a carbohydrate framework through regioselective opening of a glucose-derived epoxide, hydrolysis or acetolysis of the methyl glycoside in various derivatives was problematic. Ozonolysis of the olefin followed by brief treatment with dimethyl sulfide gave a mixture of diastereomeric ozonides that proved to be stable for weeks at room temperature. Acetolysis of these ozonides at low temperature allowed selective cleavage of the methyl glycoside in the presence of the 1,2,4-trioxolane as well as selective formation of the alpha-acetate.     

Silicon-Modified Carbohydrate Surfactants II: Siloxanyl Moieties Containing Branched Structures

Branched siloxanyl-modified carbohydrate surfactants have been synthesized by coupling mono-, di- and poly-functional siloxanes to carbohydrate units either via a branched spacer or by attaching a separate modifying element to a straight-chained structure. Hydrophilic as well as extremely hydrophobic elements have been incorporated successfully. Siloxanyl-modified carbohydrates bearing a secondary amino function were alkylated in regioselective reactions by different epoxides ranging from glycidol- to siloxanyl-modified allyl glycidyl ether derivatives. Alternatively, carbohydrate-modified piperazinyl structures yielded cyclic subunits after alkylation. Structures bearing two identical hydrophilic groups are accessible by alkylation of carbohydrate-modified bisamides. The derivatives synthesized were characterized by means of GC, NMR and elemental analysis.     

Hydrogen bonds in regioselectively substituted cellulose derivatives

Formation of hydrogen bonds in various cellulose derivatives, 2,3-di-O- and 6-O-substituted cellulose ethers, were characterized by FTIR and solid-state CP/MAS¹³C-NMR. The polymers were synthesized by regioselective substitution of hydroxyl groups and had a uniform structure. Since their three hydroxyl groups (OH) are selectively blocked, the cellulose derivatives appeared to form specific inter- and intramolecular hydrogen bonds. The characteristic OH stretching frequencies in IR spectra and the C-1 chemical shift in CP/MAS spectra of 6-O-substituted cellulose derivatives indicated existence of two equivalent intramolecular hydrogen bonds between ether oxygen and OH groups at 3-OH-O5′ and O6-HO-2′ [Figure 3(C)]. Influence of the substituents at the C-6 position on the formation was not significant except trityl group. Behavior of the hydrogen bonds in 6-O-tritylcellulose were also discussed. © 1994 John Wiley & Sons, Inc.     

Synthesis of chondramide A analogues with modified β-tyrosine and their biological evaluation

Starting from cinnamates 9 , obtained by Wittig reaction or Heck coupling, the diols 17 were prepared by asymmetric dihydroxylation. This was followed by a regioselective substitution of the 3‐OH group with hydrazoic acid under Mitsunobu conditions. Methylation of the 2‐OH group and reduction of the azide group led to the β‐tyrosine derivatives 8 . Condensation with the dipeptide acid 6 furnished the tripeptide part of the chondramides. The derived acids 21 were combined with the hydroxy ester 7 to the esters 22 . Cleavage of the tert‐butyl groups and intramolecular lactam formation gave rise to the chondramide A analogues 2 b – k . Growth inhibition assays showed most of the analogues to be biologically active. Some of them even reach the activity of jasplakinolide. It can be concluded that the 4‐position of the aryl ring in the β‐tyrosine of chondramide A tolerates structural modifications quite well.     

Selective cleavage of sugar anomeric O-acyl groups using FeCl3·6H2O

A convenient method has been developed for regioselective anomeric deacylation of carbohydrate derivatives using FeCl₃·6H₂O in CH₃CN. Operational simplicity, economic consideration, high yield, and low toxicity are key features associated with this protocol.     

Preparation and Regioselective Metalation of Bis(trimethylsilyl)methyl‐Substituted Aryl Derivatives

A range of bis(trimethylsilyl)methyl‐substituted aryl derivatives was prepared by using a Kumada–Corriu cross‐coupling reaction. The regioselective metalation of the resulting bis(trimethylsilyl)methyl‐substituted aryl derivatives bearing this bulky silyl group allowed the generation of functionalized aromatics. A regioselective switch in the presence or in the absence of the bis(trimethylsilyl)methyl group has been demonstrated. Furthermore, this silyl group was converted into a formyl group or a styryl group, enhancing the scope of application of such bis(trimethylsilyl)methyl‐substituted arenes.     

Efficient synthesis of beta-D-mannosides and beta-D-talosides by double parallel or double serial inversion

A neighboring equatorial ester group plays a highly important role in the Lattrell-Dax (nitrite-mediated) carbohydrate epimerization reaction, inducing the formation of inversion compounds in good yields. On the basis of this effect, efficient synthetic routes to beta-D-mannosides and beta-D-talosides, from the corresponding beta-D-galactosides and beta-D-glucosides, have been designed. The present routes are based on multiple regioselective acylation via the respective stannylene intermediates, followed by inversions to the corresponding manno- and talopyranoside structures by nitrite or acetate substitution. It was found that the ester group was able to induce the inversion of its two neighboring groups in high yields following either a double parallel or a double serial inversion process. By combination of direct inversion, and neighboring- as well as remote-group participation, several beta-d-mannoside and beta-D-taloside derivatives were very conveniently obtained in good yields.     

6-endo Versus 5-exo radical cyclization: streamlined syntheses of carbahexopyranoses and derivatives by 6-endo-trig radical cyclization

Three factors that can direct 6-endo radical cyclization over 5-exo ring closure: substitution at C-5, vinyl radical cyclization and ring strain, have been considered in the context of the preparation of carbapyranoses from carbohydrate derivatives. As a result, alkyl radicals in substrates containing a strain inducing 2,3-O-isopropylidene ring, and vinyl radical in non-strained compounds undergo a completely regioselective 6-endo-trig ring closure leading to carbasugar derivatives.     

Iron(III) chloride-tandem catalysis for a one-pot regioselective protection of glycopyranosides

Tandem catalysis by using iron(III) chloride hexahydrate leads to carbohydrate building blocks displaying an orthogonal protecting group pattern as illustrated by the regioselective protection of trehalose and maltose disaccharides.     

Cobalt(II)–catalyzed C8H alkoxylation of 1-naphthylamine derivatives with alcohols

An efficient protocol for cobalt (II)-catalyzed regioselective aryl C8H bond alkoxylation of 1-naphthylamine derivatives with readily available alcohols has been developed, demonstrating an efficient approach to 8-alkoxy-1-naphthylamine derivatives with good functional group tolerance. Note that the picolinamide moiety as a bidentate directing group may play a key role in this regioselective transformation.     

A prototype calix[4]arene-based receptor for carbohydrate recognition containing peptide and phosphate binding groups

A novel class of macrobicyclic receptors for carbohydrate recognition based on upper rim, peptide-bridged calix[4]arenes has been designed and synthesized. Receptor 12, in which a charged phosphate group cooperates with peptide hydrogen-bonding donor and acceptor groups in the binding process, is the most efficient and selective in the complexation of simple carbohydrate derivatives. The selectivity observed is toward beta-glucoside 13a, which is better bound (DeltaG degrees = 19.6 kJ mol(-)(1)) compared to the corresponding alpha anomer 13b (DeltaG degrees = 17.0 kJ mol(-)(1)) and to the beta-galactoside 13c (DeltaG degrees = 17.7 kJ mol(-)(1)) in CDCl(3). A substantial drop in the stability constant is observed by esterification of the phosphate group in the host 12 or by alkylation of the OH groups in the 2 and 3 positions in the beta-glucoside and beta-galactoside derivatives. On the basis of a careful analysis of the (1)H NMR data available, a binding mode of the beta-octylglucoside 13a to receptor 12 is proposed.     

Regioselective intramolecular [3+2] annulation of allene-nitrones

The regioselective intramolecular 1,3-dipolar cycloaddition of the phenylsulfonylallene-nitrone derivatives has been developed. This reaction showed that the distal double bond of the allene exclusively reacted with the nitrone group to produce the bicyclic isoxazolidine derivatives regardless of the substitution pattern on the allenyl moiety.     

Basic Ionic Liquid as Catalyst and Reaction Media for the One‐pot Three‐component Regioselective Synthesis of Various Thiazol‐2‐imine Derivatives

Thiazol‐2‐imine derivatives were synthesized in an easy and efficient route from the regioselective one‐pot three‐component reaction of aryl amine, alkylisothiocyanate, and various α‐haloketones in [omim][OH] basic ionic liquid. Recyclable reaction media, catalyst free, and short reaction times are the main advantages of this protocol. This method produces the products at room temperature in excellent yields (68–96%). Furthermore, the target compound 6b was determined by X‐ray crystallographic analysis.     

Hydrogen-bonding cooperativity: using an intramolecular hydrogen bond to design a carbohydrate derivative with a cooperative hydrogen-bond donor centre

Neighbouring groups can be strategically located to polarise HO.OH intramolecular hydrogen bonds in an intended direction. A group with a unique hydrogen-bond donor or acceptor character, located at hydrogen-bonding distance to a particular OH group, has been used to initiate the hydrogen-bond network and to polarise a HO.OH hydrogen bond in a predicted direction. This enhanced the donor character of a particular OH group and made it a cooperative hydrogen-bond centre. We have proved that a five-membered-ring intramolecular hydrogen bond established between an amide NH group and a hydroxy group (1,2-e,a), which is additionally located in a 1,3-cis-diaxial relationship to a second hydroxy group, can be used to select a unique direction on the six-membered-ring intramolecular hydrogen bond between the two axial OH groups, so that one of them behaves as an efficient cooperative donor. Talose derivative 3 was designed and synthesised to prove this hydrogen-bonding network by NMR spectroscopy, and the mannopyranoside derivatives 1 and 2 were used as models to demonstrate the presence in solution of the 1,2-(e,a)/five-membered-ring intramolecular hydrogen bond. Once a well-defined hydrogen-bond is formed between the OH and the amido groups of a pyranose ring, these hydrogen-bonding groups no longer act as independent hydrogen-bonding centres, but as hydrogen-bonding arrays. This introduces a new perspective on the properties of carbohydrate OH groups and it is important for the de novo design of molecular recognition processes, at least in nonpolar media. Carbohydrates 1-3 have shown to be efficient phosphate binders in nonpolar solvents owing to the presence of cooperative hydroxy centres in the molecule.     

Regioselective reduction of 2-perfluoroalkanoylcyclohexane-1,3-diones and their enamino derivatives

Ionic hydrogenation of 2-perfluoroalkanoylcyclohexane-1,3-diones and their endocyclic enamino derivatives containing a secondary amino group by the action of triethylsilane in trifluoroacetic acid in the presence of a catalytic amount of lithium perchlorate involved regioselective reduction of the side-chain carbonyl group to hydroxy with formation of the corresponding hydroxy diketones and hydroxy amino ketones, respectively. Under analogous conditions endocyclic enamino derivatives possessing a tertiary amino group underwent deacylation to give enamino ketones.     

Regioselective synthesis of 2,8-disubstituted 4-aminopyrido[3,2-d]pyrimidine-6-carboxylic acid methyl ester compounds

We report herein the synthesis of 4-amino-2,8-dichloropyrido[3,2-d]pyrimidine derivatives 2 and their regioselective diversification through S(N)Ar and metal-catalyzed cross-coupling reactions. While amination of 2 took place selectively at C-2, the regioselectivity of thiol or thiolate addition depended on the reaction conditions. Selective C-8 addition was obtained in DMF with Hünig's base and C-2 addition in (i)PrOH. These C-2 or C-8 regioselective thiolations provided an opportunistic way to selectively activate either of the two positions toward the metal-catalyzed cross-coupling reaction. The chloride could be efficiently substituted by Suzuki-Miyaura reaction and the sulfanyl group by Liebeskind-Srogl cross-coupling reaction, demonstrating the orthogonality of both reactive centers. The development of regioselective conditions for these different transformations yielded the synthesis of 4-amino-2,6,8-trisubstituted pyrido[3,2-d]pyrimidine derivatives, with various substituents.