Galactose ascorbic acid synthesis

Synthesis of Ascorbic acid and of Disaccharides (i) The synthesis of Vitamin C (75), ascorbic acid from D-galactose (ref.72) and that from D-glucose (ref.73) were described in the same year which also saw the development of the the latter as an industrial route. Sorbitol obtained by the reduction of D-glucose was oxidised with the bacterium Acetobacter suboxvdans to L-sorbose which as the bis-acetonylidene derivative was next oxidised to the corresponding acid, L-2-oxogulonic acid, the methyl ester of which was base-catalysed to afford the final product as illustrated. 

A synthesis of ascorbic acid (vitamin C, 1) starting from D-(-l-)-galactose (2) is shown below (Haworth, W. N., et al.,/ Chem. Soc., 1933, 1419—1423). Consider the following questions about the design and reactions used in this synthesis ... 

The main biosynthetic pathway for ascorbic acid was worked out about 20 years later (also in the United Kingdom) by Mapson, Isherwood, and Chen at the University of Cambridge, but many others have contributed subsequently) and, just as Reichstein synthesis (Scheme 11.18) began from the readily available D-glucose, so it is that the biosynthesis is found to proceed from D-glucose (as well as from D-galactose) as shown in Scheme 11.19. 

We see that in this series it is essential that the configuration of the hydroxyls on carbon atoms 2 and 3 of the aldohexose should be n and L, respectively this is true of n-galactose but not of D-mannose. Consequently, we might expect to find the synthesis of ascorbic acid being promoted through an analogous series of intermediates from D-galactose as follows. 

E. herbicola. The resultant organism can carry out both steps a and b of Eq. 20-3 leaving only step c, a nonenzymatic acid-catalyzed reaction, to complete an efficient synthesis of vitamin C from glucose. Higher plants make large amounts of L-ascorbate, which in leaves may account for 10% of the soluble carbohydrate content. However, the pathway of synthesis differs from that in Fig. 20-2. Both D-mannose and L-galactose are efficient precursors. The pathway in Eq. 20-4, which starts with GDP-d-mannose and utilizes known enzymatic processes, has been suggested. The GDP-D-mannose-3,... 

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