Quinone diacetals synthesis

Indeed, the biosynthesis of the biopolymer melanin involves the oxidative cyclization of dihydroxyphenylala-nine (DOPA) to phenylalanine-3,4-quinone (dopaquinone), which eventnally forms 5,6-dihydroxyindole (DHI). Polymerization of DHI affords melanin [1], Lim and Patil have exploited this biochemical transformation using commercial mushroom tyrosinase in a synthesis of 5,6-dihydroxyindoles protected as the diacetates (2) (Scheme 1) [2], The parent indole (R = R =H) is obtained in less than 10% yield. Carpender has reported a similar oxidative cyclization using manganese dioxide to give 2 (R =Me) in 80% overall yield from epinine (1, R =Me, R =H) [3]. Other oxidants (H, Hp /FeSO, O, NaOCl, NaClOj/ VjOj) gave little or no product. Choi, Nam, and colleagues have effected an electrochemical oxidation of dopamine (1, R = R =H) to 5,6-dihydroxyindole that polymerizes to form films of polydopamine suitable for neural attachment and function [4]. 

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