P. Lhot a´ k et al. / Tetrahedron Letters 44 (2003) 7333–7336
7335
In conclusion, we have demonstrated that tetraalkylated
thiacalix[4]arenes 5 can be transformed into the corre-
sponding tetrasulfoxides 6 which are novel building
blocks with many potential applications in supramolec-
ular chemistry. The cone stereoisomers of type A are
accessible by direct stereoselective oxidation of the start-
ing materials using NaNO in trifluoroacetic acid. Stere-
3
ochemical outcomes of this new oxidation agent for other
thiacalixarene conformers (partial cone, 1,3-alternate)
are currently under investigation.
Acknowledgements
This research was partially supported by the Grant
Agency of the Czech Republic (GA 203/03/0926).
Figure 2. ORTEP drawing of derivative 6a.
References
ilar solid-state structures were found for the tetraacetate
compound 6d and for the tetramethoxy-tetrasulfoxide 6c.
1
. For books on calixarenes, see: (a) Calixarenes 2001;
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Very recently, oxidation of the tetrabenzyloxy derivative
19
of thiacalix[4]arene using NaBO ·4H O was described.
3
2
The oxidation of the corresponding cone conformer leads
to the same stereoisomers as in the case of our immo-
bilised derivatives 6b and 6d, e.g. the rccc configuration.
To compare the stereochemical outcomes of both oxida-
tion systems, we carried out several experiments using the
19
reaction conditions reported (CHCl /acetic acid, 50°C,
3
overnight, 1.05 equiv. of NaBO ·4H O per S atom).
3
2
Using 5d as the starting material, the sulfoxide 6d
exhibiting the same stereochemistry (A) was isolated in
high yield (94%). On the other hand, the tetramethoxy
derivative 5a which is conformationally mobile led to a
very complicated reaction mixture where we were unable
to identify stereoisomer 6A. This indicated that the
stereochemical preferences of both systems are different
in the case of non-immobilised compounds.
2
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6
a are connected with each other via CH-p interactions
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of the methoxy group with the aromatic subunits of
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1
1
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1. Tetraalkylated derivatives of classical calix[4]arene were
nitrated using the NaNO /CF COOH system in 60%
3
3
yield. See: Arduini, A.; Mirone, L.; Paganuzzi, D.;
Pinalli, A.; Pochini, A.; Secchi, A.; Ungaro, R. Tetra-
hedron 1996, 52, 6011.
12. General procedure for the oxidation of thiacalixarenes.
Figure 3. Crystal packing of 6a.
Sodium nitrate (3.37 mmol, 12 equiv.) was added to the