Synthesis of 2-Deoxyglycosides with Exclusive β-Configuration Using 2-SAc Glycosyl Bromide Donors

In this study, we developed an indirect method for the synthesis of 2-deoxyglycosides with an exclusive β-configuration using glucosyl and galactosyl bromide donors with 2-thioacetyl (SAc) groups. The 2-SAc glucosyl and galactosyl bromide donors were easily obtained through the treatment of 1-OAc, 2-SAc glucose and galactose with HBr-CH3COOH solution, respectively. The glycosylation of such donors with acceptors under an improved Koenigs-Knorr condition resulted in glycosylation products with an exclusive β-configuration in excellent yields. The synthetic approach of 2-SAc glycosyl donors using glycals as the starting materials was also investigated. Based on these studies, the synthetic method of using 2-deoxyglycosides with an exclusive β-configuration through desulfurization will have more practical applications.

The sugar cube: Network control and emergence in stereoediting reactions

Stereochemical editing strategies have recently enabled the transformation of readily accessible substrates into rare and valuable products. Typically, site selectivity is achieved by minimizing kinetic complexity by using protecting groups to suppress reactivity at undesired sites (substrate control) or by using catalysts with tailored shapes to drive reactivity at the desired site (catalyst control). We propose "network control," a contrasting paradigm that exploits hidden interactions between rate constants to greatly amplify modest intrinsic biases and enable precise multisite editing. When network control is applied to the photochemical isomerization of hexoses, six of the eight possible diastereomers can be selectively obtained. The amplification effect can be viewed as a mesoscale phenomenon between the limiting regimes of kinetic control in simple chemical systems and metabolic regulation in complex biological systems.

Improved Synthesis of Sulfur-Containing Glycosides by Suppressing Thioacetyl Migration

Complex mixtures were often observed when we attempted to synthesize 4-thio- and 2,4-dithio-glycoside derivatives by double parallel and double serial inversion, thus leading to no or low yields of target products. The reason was later found to be that many unexpected side products were produced when a nucleophile substituted the leaving group on the substrate containing the thioacetate group. We hypothesized that thioacetyl migration is prone to occur due to the labile thioacetate group even under weak basic conditions caused by the nucleophile, leading to this result. Therefore, we managed to inhibit the generation of thiol groups from thioacetate groups by the addition of an appropriate amount of conjugate acid/anhydride, successfully improving the synthesis of 4-thio- and 2,4-dithio-glycoside derivatives. The target products which were previously difficult to synthesize, were herein obtained in relatively high yields. Finally, 4-deoxy- and 2,4-dideoxy-glycoside derivatives were efficiently synthesized through the removal of thioacetate groups under UV light, starting from 4-thio- and 2,4-dithio-glycoside derivatives.

Regioselective Sulfonylation/Acylation of Carbohydrates Catalyzed by FeCl3 Combined with Benzoyltrifluoroacetone and Its Mechanism Study

A catalytic amount of FeCl₃ combined with benzoyl trifluoroacetone (Hbtfa) (FeCl₃/Hbtfa = 1/2) was used to catalyze sulfonylation/acylation of diols and polyols using diisopropylethylamine (DIPEA) or potassium carbonate (K₂CO₃) as a base. The catalytic system exhibited high catalytic activity, leading to excellent isolated yields of sulfonylation/acylation products with high regioselectivities. Mechanism studies indicated that FeCl₃ initially formed [Fe(btfa)₃] (btfa = benzoyl trifluoroacetonate) with twice the amount of Hbtfa under basic conditions in the solvent acetonitrile at room temperature. Then, Fe(btfa)₃ and two hydroxyl groups of the substrates formed a five- or six-membered ring intermediate in the presence of the base. The subsequent reaction between the cyclic intermediate and a sulfonylation reagent led to the selective sulfonylation of the substrate. All key intermediates were captured in the high-resolution mass spectrometry assay, therefore demonstrating this mechanism for the first time.

Regio/Site-Selective Benzoylation of Carbohydrates by Catalytic Amounts of FeCl3

This work uncovered the regio/site-selective benzoylation of 1,2- and 1,3-diols and glycosides containing a cis-vicinal diol using a catalytic amount of FeCl3 with the assistance of acetylacetone. FeCl3 may initially form [Fe(acac)3] (acac = acetylacetonate) with excess acetylacetone in the presence of diisopropylethylamine (DIPEA) in acetonitrile at room temperature. Then, benzoylation was catalyzed by Fe(acac)3 with added benzoyl chloride in the presence of DIPEA under mild conditions as reported. This reaction produced selectivities and isolated yields similar to or slightly lower than the reaction using Fe(acac)3 as a catalyst in most cases. The result provides not only the green and convenient selective benzoylation method associated with the most inexpensive catalysts but also the possibility that the effects of various metal salts and ligands on the regioselective protection can be extensively investigated in future study to obtain the optimized catalytic system.

Straightforward S-S Bond Formation via the Oxidation of S-Acetyl by Iodine in the Presence of N-Iodosuccinimide

Straightforward S-S bond formation via the oxidation of S-acetyl group by iodine was reported here. The reaction was further applied in the synthesis of per-O-acetylated glycosyl disulfides. These studies demonstrated great improvement in reaction rate, yield, and general convenience in the presence of N-iodosuccinimide. Furthermore, selectively deacetylated glycosyl thiols were obtained in high yields when these per-O-acetylated glycosyl disulfides were reduced with tris(2-carboxyethyl)-phosphine (TCEP). Our method supplied an efficient way to obtain both per-O-acetylated glycosyl disulfides and per-O-acetylated glycosyl thiols in which the sulfur group was located at any position.

A carbohydrate-anion recognition system in aprotic solvents

A carbohydrate–anion recognition system in nonpolar solvents is reported, in which complexes form at the B-faces of β-d-pyranosides with H1-, H3-, and H5-cis patterns similar to carbohydrate–π interactions. The complexation effect was evaluated for a range of carbohydrate structures; it resulted in either 1:1 carbohydrate–anion complexes, or 1:2 complex formation depending on the protection pattern of the carbohydrate. The interaction was also evaluated with different anions and solvents. In both cases it resulted in significant binding differences. The results indicate that complexation originates from van der Waals interactions or weak CH⋅⋅⋅A⁻ hydrogen bonds between the binding partners and is related to electron-withdrawing groups of the carbohydrates as well as increased hydrogen-bond-accepting capability of the anions.

H-bonding activation in highly regioselective acetylation of diols

H-bonding activation in the regioselective acetylation of vicinal and 1,3-diols is presented. Herein, the acetylation of the hydroxyl group with acetic anhydride can be activated by the formation of H-bonds between the hydroxyl group and anions. The reaction exhibits high regioselectivity when a catalytic amount of tetrabutylammonium acetate is employed. Mechanistic studies indicated that acetate anion forms dual H-bonding complexes with the diol, which facilitates the subsequent regioselective monoacetylation.