Oxidative coupling of dopa with resorcinol and phloroglucinol: Isolation of adducts with an unusual tetrahydromethanobenzofuro[2,3-d]azocine skeleton

Abstract

Under biomimetic conditions, both phloroglucinol and resorcinol cause significant inhibition of the oxidative conversion of dopa to dopachrome, a key step in the biosynthesis of melanin pigments. The major product (λmax = 377 nm) from oxidation of dopa in the presence of phloroglucinol was characterized as 2, containing the unusual tetrahydromethanobenzofuroazocine ring system. Another product, formed in somewhat smaller amount, was formulated as 3, arising from coupling of phloroglucinol with 2 dopaquinone derived units. Analogous cross coupling adducts 4 and 5 were obtained by reaction of dopa or dopa methylester with resorcinol, respectively. These results suggest the trapping of labile dopaquinone, generated in situ from oxidation of dopa, by the reactive phenolic compounds, leading to the novel adducts 2, 4 and 5.