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

1,3-diaxial interaction between 3H-20 and the 6β-hydroxy group results in a
paramagnetic shift of the methyl resonance, supporting this assignment.
3
2
4
1
19
18
5
HO
20
H ax
O
10
OH
6
12
16 17
11
H
9
15
13
7
H
14
8
O
O
OCCH 3
Figure 16: Chair conformation of compound I showing the relationship between H-
5, H-6 and H-7
There have been two reports of compound I being isolated previously (Mehrotra et al.,
1989; Rasikari, 2007). The NMR data compares well with literature (Rasikari, 2007),
however the C-4 and C-10 resonances are switched around, but we have assigned C-4
based on an HMBC correlation with H-6 and are confident in our assignment. With
regard to the stereochemistry of the molecule, in Rasikari (2007) it is important to point
out that the stereochemistry in the name of the compound does not correspond with the
structure. We think that the compound was incorrectly named as 7α-acetoxy instead of
7β-acetoxy as a coupling constant of 3.5 Hz is used as a basis for this assignment.
Mehrotra et al. (1989) is also used as a comparison in the identification in Rasikari et al.
(2007). However, Mehrotra et al. (1989) observed a JH-6, H-7 of 3.5 Hz, close enough to
that observed by Rasikari (2007) of JH-6, H-7 of 2.0 Hz, and concluded a 6β-axial, 7β-
equatorial configuration. The structure of the molecule in Rasikari (2007) is however
correctly depicted as the 6β-ax, 7β-eq form.
While the 1 H NMR data also compares well to that in Mehrotra et al. (1989), the carbon
resonances of C-6, 13, 15-17, 19 and 20 all differ by 1-3 ppm and the carbon resonance
of C-18 is very different (33.68 as opposed to 21.00 in the literature). The carbonyl
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