Dehydroascorbic monomers

Derivatives of Dehydroascorbic Monomers. Monomeric dehydro-ascorbic acid presumably is formed only in solvents that prevent the formation of dimers, for example, water and alcohols. The unstable evasive nature of the compound impedes its crystallization, and intractable syrups are often the result. However, derivatives of the monomer could be obtained with satisfactory crystalline quality for a regular x-ray study. 

The first step in the mechanism involves the reduction of Cu(II) to Cu(I) by ascorbyl-6-hexadeeanoate giving dehydroascorbic acid and a weak acid HY benzoic acid). In fact this stage of the process has no importance since Cu(I) benzoate may directly be used to initiate the polymerization by reducing the pyridinium salt. The strong Bronsted acid formed attacks the monomer and initiates the polymerization. Notably, lower polymer yields were obtained by using pyridium salt rather than iodonium salt. 

The Dehydroascorbic Acid Dimer. Of the possible versions of piure dehydroascorbic acid, only the symmetric dimer has been obtained in crystalline state and investigated by diffraction methods. However, derivatives of monomers are known and will be discussed below. 

An improved synthesis of dehydroascorbic acid has been reported (42). The oxidation of ascorbic acid in absolute methanol with oxygen over activated charcoal catalyst is reported to aflFord 28 in 95% yield. Dehydroascorbic acid has been characterized in solution as the monomer, 28 (43), and as the dimer (44,45) and its tetra acetyl derivative 29 (46). Several studies of mono- and di-hydrazone (48-53) and osazone (54) derivatives of dehydroascorbic acid have been reported. Hydrazone derivatives of dehydroascorbic acid have been used in the reductive synthesis of 2,3-diaza-2,3-dideoxy- and 2-aza-2-deoxyascorbic acid derivatives 30, 31, and 32 (55,56). Recently the reaction product of dehydro-L-ascorbic acid and L-phenylalanine in aqueous solution has been isolated and identified as tris(2-deoxy-2-L-ascorbyl)amine, 33, based on spectral and chemical data and its symmetry properties (57). 

Dehydroascorbic acid is now readily prepared in pure form by the Ohmori-Takagi method in which ascorbic acid is dissolved in ethanol and oxygenated in the presence of charcoal. The structure of dehydro-L-ascorbic acid is solvent dependent. In water, dehydro-L-ascorbic acid exists almost exclusively in the bicyclic hydrated monomer in which 6-OH... 

Figure 4.20 Dehydroascorbic acid monomer (DHA) and dimer (BDHA)...

Ascorbyl radical is acid pK = —0.96), which exists in solution as a resonance stabilised anion (as a bicyclic compound, apparently with a double bond between carbons C-2 and C-3) and with the unpaired electron located in the C-4 region. Dehydroascorbic acid occurs as the bicycHc hydrated monomer (3,6-anhydro-L-xylo-hexulono-l,4-lactone hydrate, Figure 5.27) in aqueous solutions. 

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