Dopachrome reduction

Subsequent investigations have shown that Raper s suggestion that dopachrome (4) and related aminochromes decompose by an internal oxidation-reduction process forming 5,6-dihydroxyindoles was essentially correct.73,118,120,184-137 The 5,6-dihydroxyindoles obtained from aminochromes such as dopachrome (4) and epino-chrome (27) (i.e. with no substitution in the 3-position) show only a relatively weak blue to blue-mauve fluorescence.118,120 The intense yellow-green fluorescence shown by the rearrangement products of aminochromes with a 3-hydroxyl group is due to the formation of... 

Cyclodopa (7) is an essential part of all betacyanins. Compound 7 is prepared by oxidative cyclization of (5)-dopa (93) or its methyl ester (94) with potassium ferricyanide followed by immediate reduction of the unstable dopachrome intermediate 74 with sodium dithionite. The first synthesis (Scheme 3) is suitable for seale-up and allows an easy aceess to the more stable 0,0-diacetyl methyl ester derivative 75. 

Addition of 5,6-dihydroxyindole 1 to dopachrome may occur during rearrangement and leads to a product that has been identified after reduction and acetylation as compond 148 (87G627). 

The internal oxidation-reduction reaction first described by Raper to explain the decolorisation of dopachrome (68) solutions [14], which were formed during the melanisation of DOPA, is undoubtedly the most characteristic reaction of the aminochromes [26-29]. 

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