13C NMR spectroscopy of [4.n.0] bicyclic compounds: Assignment of the ring junction configuration from ring junction configuration from ring junction 13C chemical shifts—application and limits
✍ Scribed by P. Metzger; E. Casadevall; M. J. Pouet
- Publisher
- John Wiley and Sons
- Year
- 1982
- Tongue
- English
- Weight
- 505 KB
- Volume
- 19
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
REFERENCE DATA
of the alcohol into the acetate (7) and ketone (ll), respectively. The C-29 signal in 2, however, is virtually unaffected by acetylation (6), while it is appreciably shielded (3.9 ppm) on conversion to ketone 10. Similar shift behaviour was also observed for the C-29 and C-30 signals of 4, 8 and 12. The absorption position of the C-18 methyl signal was virtually constant. Although the 'HNMR spectra of isomeric 4cu-and 40methyl sterols are similar to each other,14 the 13C NMR spectra can, thus, be used not only to differentiate between the isomers, but also to characterize the axial or equatorial configuration of a C-4 methyl group in an unknown sterol.
EXPERIMENTAL
The ',CNMR spectra were recorded at 22.53 MHz using a JEOL FX-90Q Fourier transform spectrometer combined with a JEOL JEC-980B computer system. Conditions were as follows: lock, internal; temperature, 25 "C; pulse width, 9 F S ; flip angle 45"; spectral width, 4000, 4504 or 6024 Hz; pulse repetition h i e , 1.0 s; number of data points, 4096. The samples (c. 4mg) were each dissolved in CDCI, (40 &I) containing 5 O h of tetramethylsilane (TMS) as an internal reference and analyzed in 1.7 mm (0.d.) micro sa'mple tubes. Chemical shifts were expressed in ppm relative to internal TMS. The 13C NMR spectra were first recorded in the proton noise-decoupled mode in order to measure the exact chemical shifts. The degree of substitution of each carbon atom was then determined from spectra recorded in the single-frequency off-resonance decoupled mode. All the compounds examined were prepared in our laboratory, and their preparation and physical data have been reported previously.