Structure and properties of spinochrome H

Abstract

Spinochrome H, the major purple pigment in the spines of Echinometra oblonga(Blainville) and Colobocentrotus (Podophora) atratus (Linn.), was isolated in yields of 0.06 and 0.05% respectively by a previously unreported method utilizing acid-washed silica gel in column chromatography. Trace amounts of spinochrome H were also isolated from other Hawaiian sea urchins, Tripneustes gratilla (Linn.) and Echinothrix diadema (Linn.). Spinochrome H crystallizes with one mole of methanol, m.p. 183-184°. The solvent may be removed at 80° under high vacuum over phosphorus pentoxide yielding solvent-free pigment, m.p. 192-193°. The molecular formula of C12H80? is supported by combustion analyses and mass spectrum (M+, 264). The compound possesses an acetyl group, one nucleoid proton, and four hydroxy groups of two types, free and hydrogen-bonded. Spinochrome H is a polyhydroxy-1,4-naphthoquinone and, specifically, a naphthazarin analog as shown by its ultraviolet spectrum in methanolic aluminum chloride. Spectral and chemical behavior of spinochrome H, of its monomethoxy derivative, m.p. 239-242°, and of its dimethoxy derivative, m.p. 224-2270 , place one of the unassigned two hydroxy groups ortho to the acetyl group. No decision can be made on the basis of available evidence regarding the fourth hydroxy group. The expression IA⇌IB may therefore be written as a representation of spinochrome H. The remaining structural ambiguity will have to be solved by synthesis or X-ray diffraction. A similarity of spinochrome H with spinochrome A is suggested on the basis of ultraviolet and visible spectra, melting point, and reported behavior during calcium carbonate column chromatography. Spinochrome A, isolated from the Mediterranean sea urchins, Paracentrotus lividus (Lam.) and Echinus esculentus (Linn.), however, was assigned structure II. No conclusive evidence in support Of 11 has been published. A biogenetic scheme is presented which favors the 2,7-dihydroxy-6-acetylnaphthazarin structure over its 3,7-isomer on the basis of head-to-tail linkage of activated acetate units. Several synthetic reaction pathwavs, designed to lead to spinochrome H or its isomers, have resulted in structural elucidations of some naphthazarin intermediates.