O-Methylation effect on the carbon-13 nuclear magnetic resonance signals of ortho-disubstituted phenols and its application to structure determination of new phthalides from Aspergillus silvaticus.

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Abstract
The O-methylation effect on the 13-CNMR chemical shifts of o-disubstituted phenols was investigated. In phenols with nonconjugated o-disubstituents (1-10), O-methylation caused a downfield shift by an average of 5.2 ppm for the o-carbons (C-2 and -6), and a downfield shift by an average of 4.6 ppm for the p-carbon (C-4). These shift values are significantly different from those observed in o-monosubstituted and o-nonsubstituted phenols. This regularity is very useful for the spectral interpretation of some natural products with an o-disubstituted phenol group and for the determination of the position of the methoxy group in such compounds. Two new phthalides, silvaticol (15) and O-methylsilvaticol (16), were isolated along with nidulol (17) from Aspergillus silvaticus [in the course of screening for antimicrobial metabolites]. In the course of the structure determination of these compounds, the O-methylation effects of o-mono- and o-disubstituted phenols were successfully applied to these phthalides. The structures of silvaticol and O-methylsilvaticol were determined as 6-hydroxy-4-methoxy-5-methylphthalide and 4,6-dimethoxy-5-methylphthalide, respectively.