Trimethyl orthoacetate as a convenient reagent for selective methylation of β-OH groups of (poly)hydroxynaphthazarins

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Trimethyl orthoacetate was found to be a convenient reagent for methylation of the β-OH groups of (poly)hydroxynaphthazarins. The substrates bearing one β-OH group react with MeC(OMe)3 to give the corresponding methoxy derivatives in 79–89% yields. Depending on the reaction conditions, methylation of substrates with two β-OH groups on the different and the same rings affords either the corresponding mono-O-methylated (43–70%) or di-Omethylated (71–78%) derivatives. Trimethyl orthoacetate offers a good alternative to CH2N2 in the preparative synthesis of O-methylated (poly)hydroxynaphthazarin derivatives.