Tyrosinase-catalyzed oxidation of 3,4-dihydroxyphenylglycine

The oxidation chemistry of dopa in relation to melanin biosynthesis has been extensively studied. However, the oxidation of its lower homolog viz., 3,4-dihydroxyphenylglycine has not been described. Using 3,4-dimethoxybenzaldehyde as the starting material, the chemical synthesis of 3,4-dihydroxyphenylglycine was accomplished by Strecker's synthesis and demethylation reactions. Tyrosinase readily oxidized this unusual amino acid to the expected quinone. The glycyl-o-benzoquinone thus formed was highly unstable like its higher analog, dopaquinone. However, unlike its counterpart, it failed to exhibit intramolecular cyclization reaction. Rather, glycyl-o-benzoquinone exhibited facile transformation(s) to ultimately generate 3,4-dihydroxybenzaldehyde as an isolatable product. A probable mechanism involving intermediary formation of unstable quinone methide and carbinolamine intermediates is proposed to account for the novel transformation of glycyl-o-benzoquinone to 3,4-dihydroxybenzaldehyde.