Regioselective Benzoylation Research Articles

Benzoylation of Tetrols: A Comparison of Regioselectivity Patterns for O- and S-Glycosides of d-Galactose.

Efficient protecting group strategies are important for glycan synthesis and represent a unique synthetic challenge in differentiating sugar ring hydroxyl groups. Direct methods to enable regioselective protecting group installation are thus desirable. Herein, we explore a one-step regioselective benzoylation to deliver 2,3,6-protected d-galactose building blocks from tetrols across a variety of α- and β-, O- and S-glycoside substrates. We focus on benzoyl chloride as the esterifying reagent and a reaction temperature of -40 °C to screen the regioselectivity outcome for twenty-two different glycosides, based on isolated yields. Using this methodology, we demonstrate the capability for α-linked aryl and alkyl glycosides (O- and S- d-galactosides, d-galactosamines, and l-fucose), delivering consistent isolated yields (>65%) for 2,3,6-benzoylated products. We extend to explore β-linked systems, where the observed regioselectivity is not paralleled. We posit that both steric and electronic factors from the anomeric substituent contribute to modulating the reactivity competition between 2-OH and 4-OH, enabling the formation of regioisomeric mixtures. However, a certain balance of these factors within the aglycon can deliver 2,3,6-regioselectivity, notably for β-O-Et and β-O-CH2CF3 glycosides. The methodology contributes toward understanding the peculiarities of regioselective carbohydrate-protecting group installation, exploring the importance of the anomeric substituent upon ring hydroxyl group reactivity.

Convergent synthesis of glycoalkaloids solasonine and its saponin derivative.

We present a practical and convergent synthesis of glycoalkaloids solasonine 1 and its saponin derivative 2, incorporating a {3-O-α-L-rhamnopyranosyl-(1→2)-O-[β-D-glucopyranosyl-(1→3)]-β-D-galactopyranoside} moiety. The key features of this strategy include the following: (1) AuCl3-tBuCN cooperative catalysis enabling 1,2-trans stereoselective glycosidation of 2-branched trisaccharide trichloroacetimidate donors with steroidal aglycons, in the absence of neighboring group participation; (2) "cyanide effect" mediated regioselective benzoylation of the 4- and 6-hydroxyl groups of galactopyranosyl disaccharide; and (3) an effective approach to prevent orthoester byproduct formation.

Substrate specific closed-loop optimization of carbohydrate protective group chemistry using Bayesian optimization and transfer learning.

A new way of performing reaction optimization within carbohydrate chemistry is presented. This is done by performing closed-loop optimization of regioselective benzoylation of unprotected glycosides using Bayesian optimization. Both 6-O-monobenzoylations and 3,6-O-dibenzoylations of three different monosaccharides are optimized. A novel transfer learning approach, where data from previous optimizations of different substrates is used to speed up the optimizations, has also been developed. The optimal conditions found by the Bayesian optimization algorithm provide new insight into substrate specificity, as the conditions found are significantly different. In most cases, the optimal conditions include Et3N and benzoic anhydride, a new reagent combination for these reactions, discovered by the algorithm, demonstrating the power of this concept to widen the chemical space. Further, the developed procedures include ambient conditions and short reaction times.

Regioselective benzoylation of unprotected β-glycopyranosides with benzoyl cyanide and an amine catalyst – application to saponin synthesis

Regioselective protection of trans-trans triol and tetrol moieties in carbohydrates was achieved with BzCN as the benzoylating agent and amine catalysts. The protocols are useful for the chemical synthesis of oligosaccharides and saponins.

Catalytic Regioselective Benzoylation of 1,2-trans-Diols in Carbohydrates with Benzoyl Cyanide: The Axial Oxy Group Effect and the Action of Achiral and Chiral Amine Catalysts

Regioselective protection of the polyfunctional carbohydrates with acyl groups under catalytic conditions is a prerequisite for efficient structural modification and chain extension via glycosidati...

Synthesis and Structural Analysis of Aspergillus fumigatus Galactosaminogalactans Featuring α‐Galactose, α‐Galactosamine and α‐N‐Acetyl Galactosamine Linkages

AbstractGalactosaminogalactan (GAG) is a prominent cell wall component of the opportunistic fungal pathogen Aspergillus fumigatus. GAG is a heteropolysaccharide composed of α‐1,4‐linked galactose, galactosamine and N‐acetylgalactosamine residues. To enable biochemical studies, a library of GAG‐fragments was constructed featuring specimens containing α‐galactose‐, α‐galactosamine and α‐N‐acetyl galactosamine linkages. Key features of the synthetic strategy include the use of di‐tert‐butylsilylidene directed α‐galactosylation methodology and regioselective benzoylation reactions using benzoyl‐hydroxybenzotriazole (Bz‐OBt). Structural analysis of the Gal, GalN and GalNAc oligomers by a combination of NMR and MD approaches revealed that the oligomers adopt an elongated, almost straight, structure, stabilized by inter‐residue H‐bonds, one of which is a non‐conventional C−H⋅⋅⋅O hydrogen bond between H5 of the residue (i+1) and O3 of the residue (i). The structures position the C‐2 substituents almost perpendicular to the oligosaccharide main chain axis, pointing to the bulk solvent and available for interactions with antibodies or other binding partners.

Synthesis and Structural Analysis of Aspergillus fumigatus Galactosaminogalactans Featuring α-Galactose, α-Galactosamine and α-N-Acetyl Galactosamine Linkages.

Galactosaminogalactan (GAG) is a prominent cell wall component of the opportunistic fungal pathogen Aspergillus fumigatus. GAG is a heteropolysaccharide composed of α-1,4-linked galactose, galactosamine and N-acetylgalactosamine residues. To enable biochemical studies, a library of GAG-fragments was constructed featuring specimens containing α-galactose-, α-galactosamine and α-N-acetyl galactosamine linkages. Key features of the synthetic strategy include the use of di-tert-butylsilylidene directed α-galactosylation methodology and regioselective benzoylation reactions using benzoyl-hydroxybenzotriazole (Bz-OBt). Structural analysis of the Gal, GalN and GalNAc oligomers by a combination of NMR and MD approaches revealed that the oligomers adopt an elongated, almost straight, structure, stabilized by inter-residue H-bonds, one of which is a non-conventional C-H⋅⋅⋅O hydrogen bond between H5 of the residue (i+1) and O3 of the residue (i). The structures position the C-2 substituents almost perpendicular to the oligosaccharide main chain axis, pointing to the bulk solvent and available for interactions with antibodies or other binding partners.

Regioselectively substituted cellulose mixed esters synthesized by two-steps route to understand chiral recognition mechanism and fabricate high-performance chiral stationary phases

It is challenging to design and fabricate new and high-performance cellulose-based chiral stationary phases (CSPs), due to the indistinct chiral recognition mechanism and the inherent difficulty to control the structure of cellulose derivatives. Herein, taking advantage of the high regioselective benzoylation of cellulose in 1-allyl-3-methylimidazolium chloride, a series of regioselectively substituted cellulose mixed esters, cellulose 6-benzoate-2,3-phenylcarbamate, are directly obtained by a facile two-steps route without protecting and deprotecting process. The resultant cellulose mixed esters exhibit high chiral recognition capability. In particular, when the benzoate group has an electron-donating substituent on phenyl ring, such as 4-tert-butyl group, the corresponding regioselectively substituted cellulose mixed esters have much better enantioseparation capability than cellulose tri(3,5-dimethylphenylcarbamate), which is commercially available as Chiralcel OD column, one of the most powerful CSPs. More importantly, via adjusting the chemical structure of cellulose derivatives and adding a post-treatment process to optimize their chiral recognition properties, the chiral recognition mechanism is clearly revealed. The synergy of the hydrophobic helical conformation, weak hydrogen-bond donating ability and appropriate distribution of substituents of cellulose derivatives is essential to fabricate high-performance CSPs.

Selective Protection/Deprotection in 1-Deoxynojirimycin Scaffold: Regioselective Mono-Benzoylation and Alkylation using TBAB-NaOH Catalytic System

Protecting groups play an important role in the carbohydrate chemistry and considerably influence the reactivity of substrate. A study of the substitution of various protecting groups in 1-deoxynojirimycin was carried out. Substrate N-benzyloxycarbonyl- 1,5-dideoxy-1,5-imino-4,6-O-isopropylidene-D-glucitol was subjected to alkylation at positions 2- and 3- to obtain di-substituted products and N-Cbz group was selectively removed by using NaOH in EtOH/H2O. Regioselective benzoylation and alkylation of N-benzyloxycarbonyl-1,5-dideoxy-1,5-imino-4,6-O-isopropylidene-D-glucitol were conducted under the action of TBAB-NaOH catalytic system. It was found that all protected and mono-protected analogs form simultaneously and their structures were confirmed by spectroscopic means. The results showed that electrophiles play an important role in determining the product distribution.

Stereoselective total synthesis of (−)-zeylenol, a key intermediate for the synthesis of (+)-pipoxide, (−)-uvarigranol G and (−)-tonkinenin A

Total synthesis of (−)-zeylenol, a key intermediate for the synthesis of (+)-pipoxide, (−)-uvarigranol G and (−)-tonkinenin A was achieved from commercially available starting material d-mannose. The key steps are mixed aldol condensation, Grignard reaction, ring closing metathesis and regioselective benzoylation.

ChemInform Abstract: Synthetic Studies on Dermacozines: First Synthesis of Dermacozines A, B and C.

We herein, report the first total synthesis of Dermacozine A and B in moderate to high yields from a commercially available methyl 2-bromo (or chloro)-3-nitrobenzoate in an atom efficient route. The key carbon–nitrogen and carbon-carbon bond forming reactions involve inter and intra-molecular Pd (0) catalyzed N-arylations and regioselective Friedel-Crafts benzoylation respectively. Nitrile group was used as a masked carboxylic group to differentially functionalize two carboxylic acids to achieve Dermacozine C.

Regioselective Benzoylation of Diols and Carbohydrates by Catalytic Amounts of Organobase.

A novel metal-free organobase-catalyzed regioselective benzoylation of diols and carbohydrates has been developed. Treatment of diol and carbohydrate substrates with 1.1 equiv. of 1-benzoylimidazole and 0.2 equiv. of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in MeCN under mild conditions resulted in highly regioselective benzoylation for the primary hydroxyl group. Importantly, compared to most commonly used protecting bulky groups for primary hydroxyl groups, the benzoyl protective group offers a new protection strategy.

A green and convenient method for regioselective mono and multiple benzoylation of diols and polyols

An efficient method for regioselective benzoylation of diols and polyols was developed. The benzoylation is catalyzed by only 0.2 equiv of benzoate anion in acetonitrile with the addition of a stoichiometric amount of benzoic anhydride under very mild condition, leading to high yields. Compared with all other methods, this method shows particular advantage in regioselective multiple benzoylation of polyols, and in avoiding the use of any metal-based catalysts and any amine bases, which is more environment-friendly.

ChemInform Abstract: Regioselective Benzoylation of 4,6‐O‐Benzylidene Acetals of Glycopyranosides in the Presence of Transition Metals.

AbstractThe benzoylation of 4,6‐O‐benzylidene acetals of α‐D‐glucopyranosides and α‐D‐mannopyranosides with benzoic anhydride in the presence of (F3C‐COO)2Cu gives 2‐benzoates in good yields with high regioselectivity.

Regioselective Benzoylation of 4,6-O-Benzylidene Acetals of Glycopyranosides in the Presence of Transition Metals

GRAPHICAL ABSTRACT

ChemInform Abstract: Directly Regioselective Protection of Secondary Hydroxyl Group on Ribosides in Aqueous Solution.

AbstractThe treatment with benzoyl chloride and Na2CO3 in aqueous acetonitrile allows regioselective benzoylation of secondary hydroxyl moieties in ribosides.

Synthesis And Characterization Of Methyl 4, 6-O-Enzylidene- ?-D-Glucopyranoside Derivatives

Regioselective benzoylation of methyl 4,6-O-benzylidene-?-D-glucopyranoside (1) using direct acylation method furnished the methyl 4,6-O-benzylidene-2-O-(4-t-butylbenzoyl)- ?-D-glucopyranoside (2) in an excellent yield. In order to obtain newer products for antimicrobial screening studies, the 2-O-4-t-butylbenzoyl derivative was further transformed to a series of 3-O-acyl derivatives (2-12) containing a wide variety of functionalities in a single molecular framework. Journal of Bangladesh Chemical Society, Vol. 25(2), 101-109, 2012 DOI: http://dx.doi.org/10.3329/jbcs.v25i2.15045

Regioselective benzoylation of glycopyranosides by benzoic anhydride in the presence of Cu(CF3COO)2

Benzoylation of methyl and benzyl glycopyranosides by benzoic anhydride in acetonitrile in the presence of copper(II) trifluoroacetate as a promoter has given monobenzoates with a good yield and high regioselectivity. The composition of monobenzoates depended both on a configuration of hydroxyl groups and on a configuration of aglycone. The simple syntheses of the monobenzoates of some glycosides are offered.

Direct aroylation of β-cyclodextrin on the secondary hydroxyl face: a convenient equilibrium-dependent preparation of mono-3-aroyl-β-cyclodextrin via C2-O-aroyl to C3-O-aroyl group migration

A new and convenient method is presented for the preparation of mono-3-O-benzoyl-β-cyclodextrin by direct regioselective benzoylation of β-cyclodextrin using N-benzoylimidazole in carbonate buffer solution. This process involved equilibrium-dependent C2-O-aroyl to C3-O-aroyl group migration to improve the yield.

ChemInform Abstract: Regioselective Benzoylation of Glycopyranosides by Benzoyl Chloride in the Presence of MoO2(acac)2.

AbstractThe MoO2(acac)2‐catalyzed regioselective benzoylation of glycopyranosides leads to a variety of 3‐benzoylpyranosides, e.g. compounds (III) and (V).