Synthetic approaches toward napthazarin derivatives

Abstract

The sea urchin pigments, spinochromes A, C, D, and E (I, II, III, and IV) are derivatives of naphthazarin (V). They constitute a group of compounds which have been studied for- a quarter of a century, but whose structures were determined only in recent years. In order to confirm the assigned structures and to make these compounds and their derivatives available for physical and chemical studies these four compounds were synthesized from readily available starting materials. All four synthetic pigments proved to be identical with the natural compounds. The electronic spectra of some fifty derivatives and synthetic intermediates were measured. The observed shifts in wavelength were correlated with structural parameters. Attempts to explore new synthetic pathways toward naphthazarins and to introduce the acetyl function into the polyhydroxy naphthoquinone nucleus met with little success. An x-ray crystallographic analysis was undertaken of a dimethyl ether of spinochrome A in order to solve a structural ambiguity which seemed insoluble by other physical and chemical methods. In spite of probable twinning of the crystal the x-ray diffraction studies resulted in the assignment of VI as the most probable structure of spinochrome A dimethyl ether, m.p. 181-182°.