13C NMR spectral studies of some methoxycoumarin derivatives. A re-assignment for citropten (limettin) and an examination of Peri-proximity effects for the methyl-methoxy and methoxy-methyl couples
✍ Scribed by Alan G. Osborne
- Publisher
- John Wiley and Sons
- Year
- 1989
- Tongue
- English
- Weight
- 513 KB
- Volume
- 27
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
The "C chemical shifts and "C-'H coupling constants of some methoxycoumarins are reported. Some earlier spectral assignments, including those for citropten (limettin), require revision. Methoxy substituent effects on longrange "C-'H couplings in coumarins are highlighted. Peri-proximity effects for the methyl-methoxy and methoxy-methyl couples are derived, and the value of these effects in the assignment of peri-substituted compounds is illustrated.
KEY WORDS 13C NMR Methoxycoumarins Citropten Limettin I3C-'H coupling constants Peri-proximity effects for 2 from previous data3 (see Table 1) were more inconsistent than could be attributed to instrumental error alone.
Introduction
Although I3C NMR spectral studies of methoxycoumarins have already attracted considerable intere ~t , ' -~ this study indicates that many of the original assignment^,^.^.^ some of which have been perpetuated in more recent are in need of correction. Amongst the assignments that require significant r e ~i s i o n ~. ~ are those of the natural product citropten (limettin) (5).