MAGNETIC RESONANCE IN CHEMISTRY
Magn. Reson. Chem. 2005; 43: 861–863
Spectral Assignments and Reference Data
13C NMR spectral assignment of five
epimeric 3a- versus 3b-functionalized
cholestane pairs
RESULTS AND DISCUSSION
Cholestan-3ˇ-ol (2) was synthesized following previously described
procedures.6 The epimerization to cholestan-3˛-ol (1) with the
hydroxyl group in the axial position7 was accomplished8 starting
from tosylation of 2 followed by reaction with KNO2 in dry DMF.
All derivatives 3–10 were prepared following typical procedures
described in the literature (Scheme 1).
S. S. Ramos,1 L. Santos,1 P. Almeida,2 W. B. Motherwell3
and M. C. Costa1∗
The benzoylated derivatives 3 and 4 were prepared by reaction
of the cholestan-3-ols 1 and 2, respectively, with benzoyl chloride in
dry pyridine.9 The preparation of xanthates 5 and 6 was carried out
by reaction of the cholestan-3-ol 1 and 2, respectively, with sodium
hydride in dry THF in the presence of a catalytic amount of imidazole
followed by treatment with carbon disulfide and methyl iodide.5,10
Lawesson’s reagent11 was used to prepare the thiobenzoates 7 and
8 from benzoylated 3 and 4, respectively. Thiocarbamates 9 and 10
were synthesized from xanthates 5 and 6, respectively, by reaction
with diethylamine in petroleum ether based on methods already
described.12
1
INETI, Instituto Nacional de Engenharia, Tecnologia e Inovac¸ a˜ o, I.P.,
Estrada do Pac¸ o do Lumiar, 1649-038 Lisboa, Portugal
2
´
Universidade da Beira Interior, Departamento de Quımica and Unidade
´
de Materiais Teˆ xteis e Papeleiros, Rua Marqueˆ s d’ Avila e Bolama,
6201-001 Covilha˜ , Portugal
3
University College of London, Department of Chemistry, 20 Gordon
Street, London, WC1H OJA, UK
The structural analysis and 13C NMR spectral assignment
(Table 1) for all cholestanes presented here were, in general, based
on the data previously reported for cholestane backbone,13 together
with calculated increment values for cyclohexane substituted with
hydroxyl and benzoyloxy groups in axial and equatorial position,13
after 13C NMR assignment for cholestane derivative 8 performed by
a complete analysis of the 1H NMR, 13C NMR, DEPT135, COSY and
HMQC spectral data.
Received 22 April 2005; revised 2 June 2005; accepted 12 June 2005
13C NMR chemical shift assignments of five a- and b-
epimeric pairs of cholestanes functionalized at C-3 are
presented. Empirical increment estimations proved to be
a valuable tool for the unequivocal structural elucidation
when compared with the chemical shift values of
cholestanes derivatized by introduction of N- and S-
containing groups at C-3 in equatorial and axial positions.
Moreover, the possibility is demonstrated to anticipate
the effect of –OC(S)R substituents at neighboring carbon
atoms of the ring A backbone. Copyright 2005 John
Wiley & Sons, Ltd.
13C and DEPT135 studies afforded the identification of all
quaternary carbons along with all signals in B, C and D rings
as well as in the side chain. The C-3 assignment for all cholestane
derivatives 1–10 was easily determined, through evidence of the 13C-
chemical shifts expected to a sp3-hybridized carbon substituted by
oxygen containing groups, namely, hydroxyl and benzoyloxy.13 The
assignment of the remaining carbons in ring A for the compounds
1–4 was accomplished by analogy of the experimental values
with the estimated increment values using calculated values for
cyclohexane substituted in axial and equatorial position.13
The experimental chemical shifts [υCi(exp.)], thus obtained
for compounds 1–4, were compared with the calculated values
[υCi(calc.)], which were estimated by adding increments outlined
for hydroxy [υꢀi(OH)] and benzoyloxy substituents [υꢀiOBz)]
in axial and equatorial position of a cyclohexane to the respective
chemical shifts values of the unsubstituted cholestane.13 In general,
the experimental and the calculated values correlate fairly well
(within š1 ppm) although some noticeable differences up to
j3.3j ppm exist. These discrepancies may be explained by the
influence of two additional alkyl groups present in the ring A
of the cholestanes, which have been not considered in the model
cyclohexane.
The overall ring A signal assignments of 5–10 compounds were
deduced on the basis of the previous assignment of 1–4 taking
the axial versus equatorial position of the 3-substituted groups as
reference.
Table 2 summarizes all increments υꢀi(group) [D υCi(chole-
stane) ꢀ υCi(exp.)] calculated for the carbons in the ring A of
cholestane derivatives 1–10 due to the presence of an axial and
equatorial 3-substituent by different groups. They are presented
relative to the corresponding values of the reference compound
cholestane.
KEYWORDS: 1H NMR; 13C NMR; DEPT; COSY; HMQC;
cholestane derivatives; substituent increments
INTRODUCTION
Hemisynthetic derivatives of cholestane, 1–10 (Scheme 1), have been
our subject of interest as model precursors in an ongoing study
seeking novel strategies for the functionalization of inactivated
carbons in terpenes in the presence of ferric catalysts through
Patin chemistry.1 For this purpose, groups containing N- and S-
heteroatoms had to be introduced at C-3 position in cholestanol.2
These hemisynthetic cholestane derivatives could potentially be
useful since the insertion of these functional groups not introduced
by biosynthetic processes awards new biological activities. Following
this idea, a structure-activity relationship study of the influence of ˛-
and ˇ- configuration of substituents at C-3 in relation to antimicrobial
activities2 was performed based on the series of C-3 epimer pairs
synthesized.
The 13C NMR spectral assignment of the five functionalized
cholestanes 3˛-/3ˇ- epimer pairs 1–10 based on the common
cholestane skeleton are presented here. It is our belief that the present
study can contribute in further assignments of all carbons present
in ring A backbone of related compounds possessing equatorial and
axial groups in C-3 position.
From Table 2 it can be concluded that the deshielding effect
of any O-substituted functional group in an equatorial position is
generally stronger in relation to the axial position with differences
of increment values, for about 3–4 ppm. The thiocarbonyl ˛-
increment values are always higher than those of the carbonyl
group, irrespective of axial or equatorial position. Despite both
C-2 and C-4 being Cˇ carbons, the increment value of the first
is lower whenever a C-3 substituent is in axial or equatorial
position. The influence of the C-3 substituent on the increment
value of carbon Cυ (C-10) is quite negligible due to its distant
position.
Moreover, although cholestan-3˛-ol (1),3 cholestan-3ˇ-ol (2)3
have already been described with 13C NMR data fully assigned,
and the 3˛-benzoate (3)4 has been characterized although with
interchange between C-12 and C-16 carbon chemical shifts, we could
not find the full assignment of ˇ-derivatives 4, 6, 8, and 10 in the
literature and, to the best of our knowledge, the ˛-derivatives 7 and
9 have never been reported previously. The synthesis of compound
5 has been reported by Barton et al.5 without any NMR assignment.
In conclusion, the increment values presented in Table 2 allow
the estimation of the chemical-shift value of all the carbons present
in ring A of any cholestane analogue with a –OC(S)R substituents in
C-3 position.
ŁCorrespondence to: M. C. Costa, INETI, DTIQ, Estrada Pac¸o do Lumiar,
Edifıcio F, 1649-038 Lisboa, Portugal. E-mail: ceu.costa@ineti.pt
´
Copyright 2005 John Wiley & Sons, Ltd.
Ramos
