^<13>C-核磁気共鳴スペクトルにおけるオルト置換フェノールのO-メチル化効果(第2報)

概要

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O-Methylation effects on carbon-13 nuclear magnetic resonance chemical shifts (in DMSO-d_6) of o-, p-disubstituted phenols were investigated. In phenols (7-24), o-substituted phenols with another substituent conjugated to the benzene ring at para position, O-methylation caused downfield shifts by averages 1.1,1.0 and 0.8 ppm for ipso-C (C-1), substituted ortho-C (C-2) and para-C (C-4) respectively, and upfield shifts by averages 1.1,0.6 and 4.5 ppm for meta-C's (C-3 and C-5) and ortho-methine C (C-6) respectively. The trend of O-methylation effect on ipso-C (a smaller downfield shift) and meta-C's (larger upfield shifts) was different from that reported on o-substituted phenols with a proton or a nonconjugated substituent at para position ; however this trend was observed also on O-methylation of p-monosubstituted phenols (1-6). The regularity of a downfield shift by about 1.0 ppm for substituted ortho-C and an upfield shift by about 4.1-4.5 ppm for ortho-methine C was found to be held not only on O-methylation of o-monosubstituted phenols but also on that of o-substituted phenols with other substituents at meta and/or para positions. The above results were successfully applied to the ^<13>C-NMR spectral interpretation of some natural products including coumarins (25-35) and flavonoids (36-42).
1985-03-25