university of kwazulu-natal faculty of science and agriculture school ...

molecular formula of C20H32O2 and fragment ion peaks at m/z 286 and 268. These
fragment ion peaks are brought about when compound III loses one and two water
molecules consecutively. The IR spectrum showed O-H stretching bands at 3396 cm -1
and C-H stretching bands at 2930 cm -1 . The UV spectrum showed a maximum
absorbance at a wavelength of 228 nm (log ε = 1.70).
The 1 H NMR spectrum indicated the presence of four tertiary methyl groups with
resonances at δΗ 1.01, 0.99, 0.78 and 0.75, two double bonds with four olefinic proton
resonances at δH 5.72, δH 5.20, δH 4.90 and δH 4.88 and two oxygenated methine groups
with resonances at δΗ 3.26 and δΗ 3.85. The resonances at δH 5.72 (dd, J = 17.21, 10.57
Hz), δH 4.90 (dd, J = 10.57, 1.16 Hz) and δH 4.88 (dd, J = 17.21, 1.16 Hz) was typical of
a vinyl group (H-15, H-16a and H-16b, respectively) (Lyder et al., 1998). A deshielded
methine resonance at δΗ 3.25 is typical for compounds with a hydroxyl group at the 3-
position (H-3, dd, J = 3.84, 10.84 Hz) (Kalauni et al., 2005).
The 13 C NMR spectrum showed the presence of twenty carbon resonances typical of a
diterpenoid of which four were olefinic 146.18 (CH=), δC 137.46 (C), 127.64 (CH=) and
112.69 (CH2) indicating two double bonds, three were quaternary (δC 39.11, 39.05 and
39.57), four were aliphatic methine carbon groups, five were methylene and four were
methyl.
The interesting point about this molecule is that the methyl resonance at δH 1.01 showed
HMBC correlations to the vinylic carbon resonance (C-15), the olefinic resonance at δC
127.64 (C-14) and a methylene resonance at δC 45.85 (C-12). Since this methyl group
was aliphatic, indicated by its chemical shift, the two double bonds could not be
conjugated and had to be separated by an aliphatic carbon. Ring C and its constituents
could then be constructed as in figure 17 and the methyl proton resonance was assigned
to 3H-17.
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