Streamlined Iterative Assembly of Thio-Oligosaccharides by Aqueous S-Glycosylation of Diverse Deoxythio Sugars

A streamlined iterative assembly of thio-oligosaccharides was developed by aqueous glycosylation. Facile syntheses of various deoxythio sugars with the sulfur on different positions from commercially available starting materials were described. These syntheses featured efficient chemical methods including our recently reported BTM-catalyzed site-selective acylation. The resulting deoxythio sugars could then be used for the Ca(OH)₂ -promoted protecting group-free S-glycosylation in water at room temperature. The aqueous glycosylation reaction proceeded smoothly to afford the corresponding 1,2-trans S-glycosides in good yields with high chemo- and stereoselectivity. An appropriate choice of protecting groups for the thiol in the glycosyl donor was necessary for the development of iterative synthesis of thio-oligosaccharides. The aqueous glycosylation was then applied to the synthesis of a trimannoside moiety of N-linked glycans core region.

Bronsted acidic surfactants: efficient organocatalysts for diverse organic transformations

Organic transformations using efficient, atom-economical, cost-effective and environmentally benign strategies for the construction of diversified molecules have attracted synthetic chemists worldwide in recent years. These processes often minimize the waste production and avoid the use of hazardous flammable organic solvents. Among various green protocols, the procedures using surfactant-based catalytic systems have received a considerable attention in organic synthesis. In this context, Bronsted acidic surfactants have emerged as efficient catalysts for various C–C, C–O, C–N and C–S bond forming reactions. Many of these reactions occur in water, as Bronsted acidic surfactants have a unique ability of creating hydrophobic pocket through micelle formation in aqueous medium and the substrate molecules react efficiently to afford the targeted products in good yields. In the past, Bronsted acidic surfactant combined catalysts successfully displayed their potential to accelerate the reaction rates of diverse organic transformations. This chapter presents a complete overview on Bronsted acidic surfactants catalyzed organic reactions to construct a variety of aromatic and heteroaromatic molecular frameworks.

Surfactant-mediated thioglycosylation of 1-hydroxy sugars in water

Thioglycosides are an important class of sugars, since they can be used as non-ionic biosurfactants, biomimetic glycosides, and building blocks for carbohydrate synthesis. Previously, Brønsted- or Lewis-acid-catalyzed dehydrative glycosylations between a 1-hydroxy sugar and a thiol have been reported to yield open-chain dithioacetal sugars as the major products instead of the desired thioglycosides. These dithioacetal sugars are by-products derived from the endocyclic bond cleavage of the thioglycosides. Herein, we report dehydrative glycosylation in water mediated by a Brønsted acid-surfactant combined catalyst (BASC). Glycosylations between 1-hydroxy furanosyl/pyranosyl sugars and primary, secondary, and tertiary aliphatic/aromatic thiols in the presence of dodecyl benzenesulfonic acid (DBSA) provided the thioglycoside products in moderate to good yields. Microwave irradiation led to improvements in the yields and a shortening of the reaction time. Remarkably, open-chain dithioacetal sugars were not detected in the DBSA-mediated glycosylations in water. This method is a simple, convenient, and rapid approach to produce a library of thioglycosides without the requirement of anhydrous conditions. Moreover, this work also provides an excellent example of complementary reactivity profiles of glycosylation in organic solvents and water.

Direct Dehydrative Glycosylation Catalyzed by Diphenylammonium Triflate

Methods for direct dehydrative glycosylations of carbohydrate hemiacetals catalyzed by diphenylammonium triflate under microwave irradiation are described. Both armed and disarmed glycosyl-C1-hemiacetal donors were efficiently glycosylated in moderate to excellent yields without the need for any drying agents and stoichiometric additives. This method has been successfully applied to a solid-phase glycosylation.

Direct Dehydrative Glycosylation of C1-Alcohols

Due to the central role played by carbohydrates in a multitude of biological processes, there has been a sustained interest in developing effective glycosylation methods to enable more thorough investigation of their essential functions. Among the myriad technologies available for stereoselective glycoside bond formation, dehydrative glycosylation possesses a distinct advantage given the unique properties of C1-alcohols such as straightforward preparation, stability, and a general reactivity compatible with a diverse set of reaction conditions. In this Focus Review, a survey of direct dehydrative glycosylations of C1-alcohols is provided with an emphasis on recent achievements, pervading limitations, mechanistic insights, and applications in total synthesis.

Chiral phosphoric acid-catalyzed desymmetrizative glycosylation of 2-deoxystreptamine and its application to aminoglycoside synthesis

This work describes chiral phosphoric acid (CPA)-catalyzed desymmetrizative glycosylation ofmeso-diol derived from 2-deoxystreptamine.

Recovery of acetic acid from dilute aqueous solutions using catalytic dehydrative esterification with ethanol

We have developed a direct esterification of aqueous acetic acid with ethanol (molar ratio=1:1) catalyzed by polystyrene-supported or homogeneous sulfonic acids toward the recovery of acetic acid from wastewater in chemical plants. The equilibrium yield was significantly increased by the addition of toluene, which had a high ability to extract ethyl acetate from the aqueous phase. It was shown that low-loading and alkylated polystyrene-supported sulfonic acid efficiently accelerated the reaction. These results suggest that the construction of hydrophobic reaction environments in water was critical in improving the chemical yield. Addition of inorganic salts was also effective for the reaction under not only biphasic conditions (toluene-water) but also toluene-free conditions, because the mutual solubility of ethyl acetate and water was suppressed by the salting-out effect. Among the tested salts, CaCl(2) was found to be the most suitable for this reaction system.

Ultrasound-enhanced one-pot synthesis of 3-(Het)arylmethyl-4-hydroxycoumarins in water

3-(Aryl)methyl-4-hydroxycoumarins were produced in good to excellent yields by reaction between 4-hydroxycoumarin and (hetero)aromatic aldehydes in the presence of Hantzsch 1,4-dihydropyridine (HEH) which works as an hydride donor (i.e., in a sequential Knoevenagel-reductive Michael addition). The sonochemical-assisted procedure (method B) provides an improved and accelerated conversion when compared to conventional silent reactions (method A). Experiments carried out according to method B showed that the reaction could be more efficiently run in the absence of organic solvents, at 30-40°C in open vessel, without the need of an excess HEH and with simplified work-up and separation procedures.

Unusual Rate Acceleration in Brønsted Acid Catalyzed Dehydration Reactions: Local Hydrophobic Environment in AggregatedN-(2,6-diphenylphenyl)-N-mesitylammonium Pentafluorobenzenesulfonates

Bulky diarylammonium pentafluorobenzenesulfonates effectively promote dehydration reactions, such as condensation reactions to give esters and the dehydrative cyclization of 1,3,5-triketones. In particular, N-(2,6-diphenylphenyl)-N-mesitylammonium pentafluorobenzenesulfonate shows much higher catalytic activity than C6F5SO3H under reaction conditions without the removal of generated water, even though the former is a weaker acid. Its crystallization gives an aggregated cyclic ion pair, which is composed of two diarylammonium cations, four pentafluorobenzenesulfonate anions, and two oxonium cations. This ion pair is strongly stabilized by four intermolecular and two intramolecular pi-pi attractive interactions and 10 hydrogen bonds. The extremely high catalytic activity of N-(2,6-diphenylphenyl)-N-mesitylammonium pentafluorobenzenesulfonate in the dehydration reactions may be ascribed to the local hydrophobic environment of the tightly aggregated ammonium salts.

[Development of efficient methods for synthesis of nitrogen-containing compounds using carbamates, acylhydrazines, and ammonia]

For the efficient synthesis of divergent nitrogen-containing compounds of pharmaceutical and agricultural importance, the development of efficient, complementary, and new synthetic methodologies is essential. One of the key subjects is how to introduce nitrogen atoms in to organic molecules. This review summaries our recent efforts on this issue, focusing on the use of carbamates, acylhydrazines, and ammonia as nitrogen sources.

Surfactant-Type Brønsted Acid Catalyzed Dehydrative Nucleophilic Substitutions of Alcohols in Water

A protocol for the dehydrative nucleophilic substitution of benzyl alcohols with a variety of carbon- and heteroatom-centered nucleophiles using dodecylbenzenesulfonic acid (DBSA) as a surfactant-type Brønsted acid catalyst in water has been developed. The reaction system can be applied to the stereoselective C-glycosylation of 1-hydroxy sugars in water. [reaction: see text].