Spinochrome synthesis

Abstract

Of the five widely distributed structural pigments of the sea urchins (echinoids) one (spinochrome B) is a derivative of juglone, while four (spinochromes A, C, D, and E) are derivatives of naphthazarin. In addition to the two peri OH groups of naphthazarin these compounds bear one or more β-hydroxyls and/or an acetyl side chain. The synthesis of these four naphthazarin derivatives is described. The naphthalene skeleton was constructed by condensing 1,2-dihydroxy-3,4-dimethoxybenzene with either chloro- or dichloromaleic anhydride in an aluminum-sodium chloride melt. Further oxygen functions were introduced by nucleophilic substitution of chlorine by methoxide ion or by the Thiele method. An acetyl side chain was attached to the polyhydroxynaphthoquinone by preparing the leucoacetate and treating it with acetic anhydride-boron trifluoride. The naphthazarin system was regenerated after hydrolysis and oxidation during work-up.