Selected aldohexoses (D-glucose, D-mannose, and D-galactose) and aldopentoses (D-xylose, L-arabinose, and D-ribose) are readily available components of biopolymers. Isomerization reactions of these substances are very attractive as carbon-efficient processes to broaden the portfolio of abundant monosaccharides. This review focuses on the chemocatalytic isomerization of aldoses into the corresponding ketoses as well as epimerization of aldoses at C2. Recent advances in the fields of catalysis by bases and Lewis acids are considered. The emphasis is laid on newly uncovered catalytic systems and mechanisms of carbohydrate transformations.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.201501577

Catalytic Isomerization of Biomass-Derived Aldoses: A Review.

Recent examples of the use of a very old synthetic method are discussed. Discovery of the cation-catalyzed epimerization explains some of the previously controversial results. The mechanism of the reaction is discussed. Suggestions are made for improvements in yield and in the method of work-up.

The Lobry de Bruyn-Alberda van Ekenstein Transformation and Related Reactions

Kinetic study of detoxification of dilute-acid hydrolyzates by Ca(OH) 2

While the mechanisms of glucose transformation via heterogeneous metal-substituted BEA zeolites have recently been elucidated, understanding of the same chemistry in homogeneous metal salt catalysts in water is still limited. Here, we investigate the mechanisms of various Lewis acid metal(III) chlorides in glucose isomerization, epimerization, and other interconversions using nuclear magnetic resonance spectroscopy (13C NMR and 1H NMR). We show that the metal chlorides isomerize glucose to fructose via a C2–C1 intramolecular hydride transfer, despite their wide range of catalytic activity. Glucose epimerization to mannose proceeds via two parallel mechanisms, a reverse C2–C1 hydride transfer, and a C1–C2 intramolecular carbon shift (the Bilik reaction), with the hydride transfer being predominant. We hypothesize that the relative activity for the Bilik reaction correlates with the ionic radius of the catalyzing metal aqua cation, with an optimum at ∼1 A for lanthanide cations. In addition, we show that th...

Mechanistic Insights into Lewis Acid Metal Salt-Catalyzed Glucose Chemistry in Aqueous Solution

We report a synthetic glycosylation reaction between sucrosyl acceptors and glycosyl fluoride donors to yield the derived trisaccharides. This reaction proceeds at room temperature in an aqueous solvent mixture. Calcium salts and a tertiary amine base promote the reaction with high site-selectivity for either the 3′-position or 1′-position of the fructofuranoside unit. Because nonenzymatic aqueous oligosaccharide syntheses are underdeveloped, mechanistic studies were carried out in order to identify the origin of the selectivity, which we hypothesized was related to the structure of the hydroxyl group array in sucrose. The solution conformation of various monodeoxysucrose analogs revealed the co-operative nature of the hydroxyl groups in mediating both this aqueous glycosyl bond-forming reaction and the site-selectivity at the same time.

/pdf/aqueous-glycosylation-of-unprotected-sucrose-employing-25s6vkdinv.pdf

Aqueous Glycosylation of Unprotected Sucrose Employing Glycosyl Fluorides in the Presence of Calcium Ion and Trimethylamine

Aldoses were epimerized at C-2 by Ca2+ in aqueous or alcoholic basic solutions through a stereospecific [1-2] rearrangement of the carbon skeletons of the aldose. The skeletal rearrangement was confirmed by 13C NMR analysis of the reaction products. The reaction of [1-13C]- and [2-13C]-D-glucose afforded [2-13C]- and [1-13C]-D-mannose, respectively, as major products. The effects of metal ions, bases, and solvents were examined, and it was found that a high concentration of Ca2+ and a base (>pH 12.3) were especially effective for the present rearrangement. Separation of the reaction products was also effected by using Ca2+ cation-exchange chromatography. Thus, under the optimized conditions, D-mannose and D-lyxose, which are rare in nature and expensive, were easily obtained in high isolated yields from D-glucose and D-xylose, respectively.

Kaori Soeda

Papers

C-2 Epimerization of Aldoses by Calcium Ion in Basic Solutions. A Simple System to Transform D-Glucose and D-Xylose into D-Mannose and D-Lyxose.

C‐2 Epimerization of Aldoses by Calcium Ion in Basic Solutions. A Simple System to Transform D‐Glucose and D‐Xylose into D‐Mannose and D‐ Lyxose.