A General Strategy for the Synthesis of Rare Sugars via Ru(II)-catalyzed and Boron-mediated Selective Epimerization of 1,2-trans-diols to 1,2-cis-diols

Human glycans are primarily composed of nine common sugar building blocks. On the other hand, several hundred monosaccharides have been discovered in bacteria and most of them are not readily available. The ability to access these rare sugars and the corresponding glycocon-jugates can facilitate the studies of various fundamentally important biological processes in bacteria, including interactions between microbiota and the human host. Many rare sugars also exist in a variety of natural products and pharmaceutical reagents with significant biological activi-ties. Although methods have been developed for the synthesis of rare monosaccharides, most of them involve lengthy steps. Herein we report an efficient and general strategy that can provide access to rare sugars from commercially available common monosaccharides via a one-step Ru(II)-catalyzed and boron-mediated selective epimerization of 1,2-trans-diols to 1,2-cis-diols. The formation of boronate esters drives the equilibrium towards 1,2-cis-diol products, which can be immediately used for further selective functionalization and glycosylation. The utility of this strategy was demonstrated by the efficient construction of glycoside skeletons in natural products or bioactive compounds.

Rare sugars: Metabolic Impacts and Mechanisms of Action – a Scoping Review

Food manufacturers are under increasing pressure to limit the amount of free sugars in their products. Many have reformulated products to replace sucrose, glucose and fructose with alternative sweeteners, but some of these have been associated with additional health concerns. Rare sugars are “monosaccharides and their derivatives that hardly exist in nature”, and there is increasing evidence that they could have health benefits. This review aimed to scope the existing literature in order to identify the most commonly researched rare sugars, to ascertain their proposed health benefits, mechanisms of action and potential uses, and to highlight knowledge gaps. A process of iterative database searching identified 55 relevant articles. The reported effects of rare sugars were noted, along with details of the research methodologies conducted. Our results indicated that the most common rare sugars investigated are D-psicose and D-tagatose, with the potential health benefits divided into three topics: glycaemic control, body composition and cardiovascular disease. All the rare sugars investigated have the potential to suppress postprandial elevation of blood glucose and improve glycaemic control in both human and animal models. Some animal studies have suggested that certain rare sugars may also improve lipid profiles, alter the gut microbiome and reduce pro-inflammatory cytokine expression. The present review demonstrates that rare sugars could play a role in reducing the development of obesity, type 2 diabetes, and/or cardiovascular disease. However, understanding of the mechanisms by which rare sugars may exert their effects is limited, and their effectiveness when used in reformulated products is unknown.

Production of D-Allose From D-Allulose Using Commercial Immobilized Glucose Isomerase

Rare sugars are regarded as functional biological materials due to their potential applications as low-calorie sweeteners, antioxidants, nucleoside analogs, and immunosuppressants. D-Allose is a rare sugar that has attracted substantial attention in recent years, owing to its pharmaceutical activities, but it is still not widely available. To address this limitation, we continuously produced D-allose from D-allulose using a packed bed reactor with commercial glucose isomerase (Sweetzyme IT). The optimal conditions for D-allose production were determined to be pH 8.0 and 60°C, with 500 g/L D-allulose as a substrate at a dilution rate of 0.24/h. Using these optimum conditions, the commercial glucose isomerase produced an average of 150 g/L D-allose over 20 days, with a productivity of 36 g/L/h and a conversion yield of 30%. This is the first report of the successful continuous production of D-allose from D-allulose by commercial glucose isomerase using a packed bed reactor, which can potentially provide a continuous production system for industrial applications of D-allose.

Theoretical Study of Hydrogen Bond Formation in Four Rare Sugars: Gulose, Allose, Altrose, and Talose

Rare sugars are monosaccharides with tremendous potential for applications in pharmaceutical, cosmetics, nutraceutical, and flavors industries. The four rare sugars, including; gulose, allose, altrose and talose are stereoisomers that are different in the hydroxyl group orientation (axial or equatorial) on the C2-4 atoms. The DFT, AIM, and, NBO calculations were used to probe the probability of formation of internal H-bonds in four rare sugars. The AIM analysis identified that altrose and talose can form three predominantly intramolecular H-bonds, whereas gulose and allose revealed one and two H-bonds, respectively and these normal intramolecular H-bonds are mostly closed-shell interactions. The theoretical calculated O-H stretching FT-IR vibrational frequencies confirmed that the intramolecular H-bonds shifted toward low frequencies in comparison to the free hydroxyl group, which caused the red-shift. Also, the lowest IR frequency in each sugar was related to the structure with the highest stabilization energy and the most strongest intramolecular H-bonds.

Rare Sugars: Recent Advances and Their Potential Role in Sustainable Crop Protection

Rare sugars are monosaccharides with a limited availability in the nature and almost unknown biological functions. The use of industrial enzymatic and microbial processes greatly reduced their production costs, making research on these molecules more accessible. Since then, the number of studies on their medical/clinical applications grew and rare sugars emerged as potential candidates to replace conventional sugars in human nutrition thanks to their beneficial health effects. More recently, the potential use of rare sugars in agriculture was also highlighted. However, overviews and critical evaluations on this topic are missing. This review aims to provide the current knowledge about the effects of rare sugars on the organisms of the farming ecosystem, with an emphasis on their mode of action and practical use as an innovative tool for sustainable agriculture. Some rare sugars can impact the plant growth and immune responses by affecting metabolic homeostasis and the hormonal signaling pathways. These properties could be used for the development of new herbicides, plant growth regulators and resistance inducers. Other rare sugars also showed antinutritional properties on some phytopathogens and biocidal activity against some plant pests, highlighting their promising potential for the development of new sustainable pesticides. Their low risk for human health also makes them safe and ecofriendly alternatives to agrochemicals.