Structural Elucidation and Chemistry of Betanidin

Betanidin (1) constitutes the basic structural unit of all betacyanin pigments. The compound occurs as its glucoside betanin (5) in red beets and is therefore easily accessible. The chemistry of 1 has been intensively studied by Dreiding s 

On treatment of 11 with palladium, an interesting disproportionation reaction takes place which yields the colorless indole derivative 14 (27,28). Reaction of betanidin with acetyl chloride in trifluoroacetic acid afforded di-C -acetyl-betanidin (13), which could be hydrolyzed back to 1 with concentrated hydrochloric acid (5). This demonstrated the presence of two phenolic hydroxyl groups in betanidin, and from consideration of the H-NMR data formula 1 was assigned to betanidin. 

The (25) configuration of betanidin has been determined by the identification of degradation product 7 as (5)-cyclodopa (5). For the determination of the absolute configuration at C-15, studies on the stereochemical relationship of betanidin (1) and isobetanidin (6) were of importance. It was found that on hydrolysis of betanin (5) under drastic conditions (18% HCl, 87°C) the ratio of 6 to 1 formed increased with the reaction time, whereas with concentrated HCl at room temperature the aglycone contained only 5% of 6 (29). On treatment of either pure 1 or 6 with 0.4 N KOH at 25°C with exclusion of oxygen, a 1 2 equilibrium mixture of the two isomers was formed. That the epimerization takes place at C-15 is indicated by the fact that both isomers gave the same optically 

The absolute configuration at C-15 was finally determined by peracid degradation of indicaxanthin (2), which had been previously correlated with betanidin (1) 

Betanidin hydrochloride is smoothly decarboxylated in boiling ethanol (33). In order to investigate the mechanism of this reaction, the decarboxylation of 1 was performed in monodeuterioethanol (34). The resulting decarboxybetanidin (19), 

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