Chin-Sheng Chao

Low‐Concentration 1,2‐trans β‐Selective Glycosylation Strategy and Its Applications in Oligosaccharide Synthesis

This study develops an operationally easy, efficient, and general 1,2-trans beta-selective glycosylation reaction that proceeds in the absence of a C2 acyl function. This process employs chemically stable thioglycosyl donors and low substrate concentrations to achieve excellent beta-selectivities in glycosylation reactions. This method is widely applicable to a range of glycosyl substrates irrespective of their structures and hydroxyl-protecting functions. This low-concentration 1,2-trans beta-selective glycosylation in carbohydrate chemistry removes the restriction of using highly reactive thioglycosides to construct 1,2-trans beta-glycosidic bonds. This is beneficial to the design of new strategies for oligosaccharide synthesis, as illustrated in the preparation of the biologically relevant beta-(1-->6)-glucan trisaccharide, beta-linked Gb(3) and isoGb(3) derivatives.

Neighboring-Group Participation by C-2 Ether Functions in Glycosylations Directed by Nitrile Solvents

Ether-protecting functions at C-2 hydroxy groups have been found to play participating roles in glycosylations when the reactions are conducted in nitrile solvent mixtures. The participation mechanism is based on intramolecular interaction between the lone electron pair of the oxygen atom of the C-2 ether function and the nitrile molecule when they are positioned in a cis configuration. A 1,2-cis glycosyl oxazolinium intermediate is formed. This participation, in conjunction with the anomeric effect of the glycosyl donor, confers high 1,2-trans selectivities on glycosylations. Further application of this concept has led to efficient preparations of α-(1→5)-arabinan oligomers.

Ultrasonication-assisted spray ionization mass spectrometry for on-line monitoring of organic reactions

A straightforward on-line monitoring of organic reactions by ultrasonication-assisted spray ionization mass spectrometry (UASI MS) is demonstrated in this work.

Tuning Reactivity of Glycosyl Imidinium Intermediate for 2-Azido-2-deoxyglycosyl Donors in α-Glycosidic Bond Formation

The chemical properties of nucleophile additives were investigated in a modulated glycosylation context. N-Formylmorpholine (NFM) was found to be an effective modulator for glycosylation with less reactive 2-azido-2-deoxythioglucosyl and thiogalactosyl donors.

Online monitoring of chemical reactions by contactless atmospheric pressure ionization mass spectrometry

Online monitoring of the appearance of the intermediates and products during chemical reactions is helpful to understand reaction mechanisms. Various analytical tools including nuclear magnetic resonance spectroscopy, Raman spectroscopy and mass spectrometry (MS) have been employed for the study. MS is one of the most promising means among analytical tools that can provide information of mass-to-charge ratios for intermediates and products generated from chemical reactions. The main challenge for online monitoring of species generated from chemical reactions by MS is the interface design. The success of online monitoring of chemical reactions using MS relies on real-time recording of the generated species from reactions without sacrificing kinetic information during coupling. In addition, MS can only detect gas-phase ions. Nevertheless, most chemical reactions occur in liquid phase. Thus, generating gas-phase ions from liquid samples is vital for the success of this study. Electrospray ionization (ESI) is one of the most common atmospheric pressure ionization (API) methods that generate gas-phase ions from liquid-reacting species/products in the proximity of a mass spectrometer for online monitoring of chemical reactions. In addition, some recently developed ambient ionization methods such as desorption ESI, extractive ESI, electrospray desorption ionization, fused droplet ESI, low-temperature plasma probe for ambient desorption ionization and ultrasonication-assisted spray ionization [27] also have shown their usefulness in monitoring reactions in real time. High-voltage electric power supply, gas supply, or laser is generally required for these approaches. High-voltage free approaches such as Venturi easy ambient sonic-spray ionization with gas supply only have been used to online monitoring of organic reactions. A comprehensive description about the progress using MS to monitor chemical reactions can refer to the recently published book written by Santos. Some of these techniques may be suitable for monitoring fast reactions that take place during desorption/ionization process. However, our current study is focused on using a recently developed API technique for monitoring the changes of reacting species from an organic reaction requiring a long reaction time to reach the end of the reaction. A continuous-flow API method was recently explored, called contactless API (C-API), which uses a short tapered-capillary tip (~ 1 cm) as the spray emitter for the generation of gas-phase ions from a liquid solution. High-voltage electric power supply, gas supply and laser are not required for the C-API approach. The C-API approach only requires a tapered capillary for the generation of gas-phase ions in the proximity of MS (Fig. 1). The sample solution can flow continuously from the sample inlet

Versatile acetylation of carbohydrate substrates with bench-top sulfonic acids and application to one-pot syntheses of peracetylated thioglycosides

Inexpensive and readily available sulfonic acids, p-toluenesulfonic acid, and sulfuric acid are versatile and efficient catalysts for the peracetylation of a broad spectrum of carbohydrate substrates in good yield and in a practical time frame. Three appealing features in sulfonic acid-catalyzed acetylation of free sugars were explored including (1) suppression of furanosyl acetate formation for D-galactose and L-fucose; (2) high yielding chemoselective acetylation of sialic acid under appropriate conditions; and (3) peracetylation of amino sugars with different amino protecting functions. Simple one-pot two step acetylation-thioglycosidation methods for the expeditious synthesis of p-tolyl per-O-acetyl thioglycosides were also delineated.