Naphthoquinoid pigments of sea urchins. Trimethyl orthoacetate as an effective reagent for O-methylation of β-OH groups of spinazarin, spinochromes D and E, and echinochrome A

Trimethyl orthoacetate (MeC(OMe)₃, TMOA) is a convenient reagent for both selective and complete O-methylation of the β-OH groups of sea urchin pigments such as spinazarin, spinochromes D and E, and echinochrome A. The reactions of these compounds with TMOA make it possible to furnish preparative amounts of 6-monomethyl ethers of spinochrome D and echinochrome A, 7-monomethyl ether (namakochrome) and 6,7-dimethyl ether of spinochrome E, which are difficult to prepare by other methods, and to carry out complete O-methylation of all discussed pigments. Among the known O-methylating agents, TMOA is most suitable for the preparative syntheses of methyl ethers of polyhydroxynaphthazarins.