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

esonance showed HMBC correlations to the other two olefinic resonances at δC 150.90
and δC 124.67 as well as the carbonyl resonance at δC 185.76. These resonances were
therefore assigned to C-12 (δC 150.9), C-13 (δC 124.67) and C-14 (δC 185.76). The C-12
and C-13 resonances were distinguished based on chemical shift as the more deshielded
resonance of C-12 at δC 150.90 was assigned to the olefinic carbon where the hydroxyl
group is situated. The hydroxyl group proton resonance at δH 7.21 showed HMBC
correlations to both C-12 and C-13 and to the other carbonyl resonance at δC 183.28,
which was then assigned to C-11.
One of the 2H-1 resonances was present at δH 2.62 and the other overlapped with the 3H-
16 and 3H-17 methyl resonances at δH 1.18, determined by the HSQC correlation with C-
1. The other two non equivalent methylene resonances at C-2 and C-3 were assigned due
to the C-3 carbon resonance at δC 42.27 showing a HMBC correlation with the two
methyl resonances at δH 1.23 (3H-19ax) and δH 0.92 (3H-18eq). The C-2 carbon
resonance was assigned to the remaining methylene resonance. The resonance at δH 1.62
was assigned to 3H-20 since C-5 showed correlations to all three methyl resonances, 3H-
18eq, 3H-19ax and 3H-20ax.
Of the remaining two quaternary resonances at δC 33.54 and δC 38.63, the resonance at δC
38.63 was assigned to C-4 based on a HMBC correlation with H-6.
The stereochemistry of H-5, H-6 and H-7 were ascertained from coupling constants and
using a molecular model to determine their dihedral angles and assuming the H-5 proton
to be axial. Since the H-5, H-6 and H-7 proton resonances were all slightly broadened
singlets, we were looking for a model where the dihedral angles between H-5 and H-6,
and H-6 and H-7 were close to 90°. This was possible if the H-5 proton was axial
(alpha), H-6 was equatorial (alpha) and H-7 was axial (alpha). This indicates that the
hydroxyl group at C-6 is axial (beta) and that the acetoxy group at C-7 is equatorial (beta)
(Figure 16). This also supports the downfield chemical shift of δH 1.62 of 3H-20 as the
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