Triose sugars

Fischer projections are a method of showing stereochemistry and particularly the relationship between chiral centers, with uncomplicated drawings. Thus, 16.46a is represented as 16.46b. For students with good spatial perception, some of the relationships described will be obvious, but this will not be true for everyone. If you can simply see what is happening, that s great—but for many people, a slow, mechanical, pedestrian approach, which will always work, is helpful. How can these projections be manipulated There are many rules in other textbooks, but we can make do with just one. Any pairwise swap reverses the absolute configuration at an asymmetric carbon atom. For those of you able to translate immediately between projection and three-dimensional structure, it may be obvious that 16.47a and 16.47c are identical, but the swapping system will let the rest of us catch up. 

What is the stereochemical relationship between each of the following pairs of compounds (enantiomers, diastereoisomers or identical)  

Under the Cahn-Ingold-Prelog system, OH COOH CHjOH H. In the three-dimensional drawing, H is pointing toward us, so we need to swap it with the COOH to get the opposite enantiomer. Our new structure is clearly S, so the starting material must have been R. 

With the Fischer projection, to assign the absolute stereochemistry, we need to put the lowest priority group at the top, by means of a swap  

our new molecule is clearly 5, so the original is R. Thus, we conclude that these two molecules are identical. 

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Glycerol (= 1,2,3-Propanetriol) (triose sugar alcohol) Universal in glycerol-based fats phospholipids Croton tiglium (Euphorbiaceae), Urtica dioica (Urticaceae) Sweet... 

Step 4 in the route to LA fi om cellulose deals with the dehydration of the triose sugars, according to a retro-Michael mechanism, forming the imstable intermediate pyruvic aldehyde (or methyl glyoxal). This reaction is typically performed by Brpnsted acid catalysis, although Lewis acids are also capable of this water... 

As triose sugars are not abtmdant in nature, they are not the preferred feedstock for LA production. Oxidation of glycerol might result in a mixture of trioses, but the selectivity at full conversion is currently not high enough [40, 115]. Yet reactions with trioses are crucial for mechanistic purposes. 

Taaming E, Saravanamurugan S, Spangsberg HM, Xiong J, West RM, Christensen CH (2009) Zeolite-catalyzed isomerization of triose sugars. ChemSusChem 2 625-627... 

The NADPH and ATP generated are used within the chloroplast to fix carbon dioxide (CO2) and reduce it to sugar. This sequence of reactions is called the Calvin-Benson-Bassham cycle and this cycle results in the production of the phosphorylated 3-carbon sugar glyceraldehyde-3-phosphate, also called triose. This triose sugar is exported out of the chloroplast and in this course we will refer to it as CH2O which is the chemical composition of sugar when normalized to 1 carbon atom. 

Unfortunately, literature shows only scarce examples of heterogeneous catalysts performing in the synthesis of lactic acid. The most efficient seems to be the Sn-BEA catalyst, reported by Taarning and coworkers [160] for the isomerization/esterification of triose sugars to ethyl lactate. 

Reaction network of the formation of lactic acid and alkyl lactate starting from triose sugars in aqueous conditions (R= H) or in alcoholic media (R = alkyl). Reproduced with permission of the Royal Society of Chemistry (Modified from Dusselier M, Van Wouwe P, Dewaele A Makshina E, Sels BE. Lactic acid as a platform chemical in the biobased economy the role ofchemocatalysis. Ener Environ Sci 2013 6 1415—42). 

Wang, J., Masui, Y, Onaka, M., 2011. Conversion of triose sugars with alcohols to alkyl lactates catalyzed by Bronsted acid tin ion-exchanged montmoriUonite. Applied Catalysis 107,135-139. 

The next reaction, catalysed by aldolase, splits the hexose sugar to give dihydroxyacetone phosphate and glyceraldehyde phosphate (both triose sugars) so that eventually two molecules of pyruvate (or lactate) are produced from each glucose molecule. 

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