A synthesis of ascorbic acid (vitamin C, 1) starting from D-(+)-galactose (2) is shown below (Haworth, W.N., et al., J. Chem. Soc., 1933, 14191423). Consider the following questions about the design and reactions used in this synthesis: (a) Why did Haworth and co-workers introduce the acetal functional groups in 3? (b) Write a mechanism for the formation of one of the acetals. (c) Write a mechanism for the hydrolysis of one of the acetals (4 to 5). Assume that water was present in the reaction mixture. (d) In the reaction from 5 to 6 you can assume that there was acid (e.g., HCl) present with the sodium amalgam. What reaction occurred here and from what functional group did that reaction actually proceed? (e) Write a mechanism for the formation of a phenylhydrazone from the aldehyde carbonyl of 7. [Do not be concerned about the phenylhydrazone group at C2. We shall study the formation of bishydrazones of this type (called an osazone) in Chapter 22.] (f) What reaction was used to add the carbon atom that ultimately became the lactone carbonyl carbon in ascorbic acid (1)?
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