Due to the depletion of fossil fuels, biomass-derived sugars have attracted increasing attention in recent years as an alternative carbon source. Although significant advances have been reported in the development of catalysts for the conversion of carbohydrates into key chemicals (e.g., degradation approaches based on the dehydration of hydroxyl groups or cleavage of C-C bonds via retro-aldol reactions), only a limited range of products can be obtained through such processes. Thus, the

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... ent of a novel and efficient strategy targeted towards the preparation of a range of compounds from biomass-derived sugars is required. We herein describe the highly-selective cascade syntheses of a range of useful compounds using biomass-derived sugars as carbon nucleophiles. We focus on the upgrade of C2 and C3 oxygenates generated from glucose to yield useful compounds via C-C bond formation. The establishment of this novel synthetic methodology to generate valuable chemical products from monosaccharides and their decomposed oxygenated materials renders carbohydrates a potential alternative carbon resource to fossil fuels. Molecules 2016, 21, 937 2 of 19 [4 + 2] retro-aldol reaction. In contrast, 1,3-dihydroxyacetone (DHA, C3) and glyceraldehyde (GLA, C3) were obtained via an isomerization of glucose to fructose followed by a [3 + 3] retro-aldol reaction. Molecules 2016, 21, 937 2 of 18 a [4 + 2] retro-aldol reaction. In contrast, 1,3-dihydroxyacetone (DHA, C3) and glyceraldehyde (GLA, C3) were obtained via an isomerization of glucose to fructose followed by a [3 + 3] retro-aldol reaction. Scheme 1. Transformation of lignocellulose-derived glucose into C2 (glycolaldehyde), C3 (1,3-dihydroxyacetone and glyceraldehyde), and C4 (erythrose) units via an isomerization and a retro-aldol reaction. As shown above, glucose can be theoretically converted into GA and erythrose via a [4 + 2] retroaldol reaction; however, isomerization to fructose followed by a [3 + 3] retro-aldol reaction is thermodynamically favored over the glucose reaction. In 2010, Holm et al. reported a one-pot transformation of mono-and disaccharides into alkyl lactate via carbon-carbon bond cleavage, dehydration, and a 1,2-hydride shift (Scheme 2) [9]. These alkyl lactates are useful chemicals, as they can be employed as renewable solvents or as building blocks in polyester synthesis. For example, Lewis acidic homogeneous and heterogeneous Sn catalysts catalyze the conversion of mono-and disaccharides to methyl lactate in methanol solution. In particular, Sn-Beta zeolite yields the optimal performances in the conversion of glucose into methyl lactate. Beta type zeolites are composed of a three-dimensional (3D) 12-membered ring pore structures (6.6 Å × 7.6 Å), which allow the larger carbohydrates to spread through the zeolite pore. The reaction pathway in the acid-catalyzed conversion is highly sensitive to the type of acid employed. Brønsted acids catalyze monosaccharide dehydration, leading primarily to the formation of HMF and its decomposition products, while Lewis acidic catalysts lead to retro-aldol reactions of the monosaccharides, and subsequent transformation to their lactic acid derivatives. Therefore, to achieve high selectivity in the Lewis acid-catalyzed pathway, it is important to reduce the catalytic effect of Brønsted acids.