Carbohydrate-based building blocks

Wright, L.W. and Brandner, J.D. (1964) Catalytic isomerization of polyhydric alcohols. II. The isomerization of isosorbide to isomannide and isoidide. Journal of Organic Chemistry, 29 (10), 2979-2982. Wu, J. (2012) Carbohydrate-Based Building Blocks and Step-growth Polymers Synthesis, Characterization and Structure-Property Relations. PhD thesis, Eindhoven Technical University. 

Table 5.2 The 13 key plant carbohydrate-derived" building blocks identified in the EU BREW project (based on Patel et al. [2]).

Ring B and the central three-carbon bridge forming the C ring (see Fig. 5.1) originate from the amino acid phenylalanine, itself a product of the shikimate pathway, a plastid-based process which generates aromatic amino acids from simple carbohydrate building blocks. Phenylalanine, and to a lesser extent tyrosine, are then fed into flavonoid biosynthesis via phenylpropanoid (C6-C3) metabolism (see Fig. 5.1). 

A nice and convergent approach to both compounds makes use of RCM to form the 5-membered building block 71, which mimics the carbohydrate part of the nucleosides. The necessary diene precursor 69 is readily assembled via Evans aldol chemistry. RCM then affords the ring in almost quantitative yield (69->70), leaving the chiral centers and the free hydroxyl group intact. Removal of the chiral auxiliary by reductive cleavage, attachment of the base by means of jt-allylpalladium chemistry, and a final deprotection step complete these highly efficient syntheses [46]. 

It is worth pointing out that, besides o-sorbitol 19 and D-mannitol 36, other low-molecular weight building blocks have been already obtained on the ton-scale from low cost or waste polymeric carbohydrates (starch, cellulose, hemicellulose, chitin) [80, 81]. Most of these compounds are densely functionalized enantiopure molecules that can be easily converted into high-value added products, including chiral ionic liquids. Therefore, further studies are required to develop other synthetic approaches to environmentally sustainable ionic liquids based on renewable raw materials. 

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