173
Spectral Assignments and Reference Data
Table 5. The 17O chemical shifts (ppm, relative to H2O) and line widths
Indeed, analysis of the respective coupling constants in com-
pounds 1, 3, 5, 7 and 8 (those having identical R0 and R00)
showed clearly separated ranges for a cis (3.2–4.0 Hz) and a
trans (7.7–10.1 Hz) coupling path. Thus, the mixed-substituted com-
pounds 2, 4 and 6 could be identified unambiguously as E isomers
by coupling constants of <3 Hz for the cis and ¾10 Hz for the trans
relation.
Because the measurement of this nucleus is very time consuming,
17O NMR spectra were recorded for only six of the title compounds
(Table 5). The shift of the enol oxygen atom is mainly determined
by the alkoxy residue (¾30 ppm difference between comparable
methoxy and ethoxy derivatives), whereas the influence of the nature
of R0 and R00 is less marked.
(Hz)
Compound
RO—CH CR2
R0, R00
1a
1b
3a
3b
4a
6b
105.8 (280)
140.4 (450)
84.2 (490)
116.9 (630)
101.1 (540)
130.4 (560)
—
—
566.9 (510), 518.6 (530)
564.6 (560), 516.2 (570)
321.4 (560), 130.5 (680)
318.9 (640), 157.9 (570)
REFERENCES
RESULTS AND DISCUSSION
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A full characterization of 1H–13C shifts as well as 1H–13C cou-
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obvious, the assignments are based primarily on the heteronu-
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and R00 residues additional two-dimensional experiments were
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of the heteronuclear coupling constants within the double bond 0(1J
[H-1, C-1], 2J [H-1, C-2] and to a smaller extent 3J [H-1, C(R )])
is significant. The effect of the various —CO—groups is much
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H-1 and the CO or CN carbon atoms of R0 and R00 as a means
of determining the stereochemistry of the double bond. Because
the dihedral angle is the most important factor influencing the
magnitude of a 3JC,H (although there is some effect of the nature
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between geometric isomers.
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Copyright 2004 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2005; 43: 171–173