Synthesis of 6,8,9-Trihydroxy-2-Methyl-2H-Naphtho [2,3-b]Pyran-5,10-Dione, a Pigment of Echinothrix diadema, and Its Analogs

6,8,9-Trihydroxy-2-methyl-2H-naphtho[2,3-b]pyran-5,10-dione, a pigment of Echinothrix diadema, was synthesized. It existed as a mixture of two tautomers in CHCl3 solution according to IR spectroscopy. Its analogs 6,9-dihydroxy-2-methyl-2H-naphtho[2,3-b]pyran-5,10-dione and 6,9-dihydroxy-2-methyl-7,8-dichloro-2H-naphtho[2,3-b]pyran-5,10-dione were synthesized.