2670
P. Din e´ r et al. / Tetrahedron: Asymmetry 16 (2005) 2665–2671
filtration, the organic phase was carefully distilled (at
60 mm), in order to remove the solvents and fractions
8. Seki, A.; Asami, M. Tetrahedron 2002, 58, 4655–
663.
4
7
9
. Nilsson Lill, S. O.; Pettersen, D.; Amedjkouh, M.;
containing epoxide with some allylic alcohol being ob-
tained. The epoxide was separated from the allylic alco-
hol by flash chromatography (Silica, 230–400 mesh)
eluating with chloroform (stabilized with 0.6–1.0%
EtOH). The cyclohexene oxide fractions were combined
and careful distillation (at 760 mm) to give pure cyclo-
hexene oxide.
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4
.4. NMR assignment
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1
1
1
1
3. Rickborn, B.; Thummel, R. P. J. Org. Chem. 1969, 3583–
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The assignment of the different signals in cyclohexene
oxide has been performed using NOE measurements
and computational chemistry. For interpretation of the
NOE results, the four conformers of cyclohexene oxide
were optimized at the B3LYP/6-311+G(2d,p) level of
3
2
0
theory: the half-chair conformers 1a and 1a , which
are enantiomers, the exo-boat 1b and the endo-boat 1c
conformers (Fig. 3). The calculated energy differences
indicate that 1 essentially is present as the half-chair
enantiomers simplifying the NMR-assignment.4
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4
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2
8
From the optimized structure of 1a, distances between
the a- and b-protons were obtained with the results
showing the a-protons (H7 and H5) to be closer in space
1
0. Pascal, R. A., Jr.; Baum, M. W.; Wagner, C. K.; Rodgers,
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˚
˚
to the banti-protons (H8–H7: 2.38 A, H10–H5: 2.48 A)
6
482.
˚
than to the b -protons (H9–H7: 2.75 A, H12–H5:
.57 A) (Fig. 3). The NOE measurements show that
syn
21. Pascal, R. A., Jr.; Baum, M. W.; Wagner, C. K.; Rodgers,
L. R. J. Am. Chem. Soc. 1984, 106, 5377–5378.
22. Zhang, B. L.; Mabon, F.; Martin, M. L. J. Phys. Org.
Chem. 1993, 6, 367–373.
2
2
˚
2
the strongest (fastest build-up) NOE to the a-protons
were observed for the more shielded (o 1.49) protons,
while the less shielded (o 1.74) protons show a weaker
NOE. By this analysis, it can be concluded that the
banti-protons are more shielded and the bsyn-protons are
less shielded. The calculated proton chemical shifts for
the banti- and bsyn-protons are consistent with this
assignment. The c-protons show no NOE to the
a-protons. The calculated chemical shifts show that
the canti-protons are more shielded than the canti-protons.
3. Zhang, B. Magn. Reson. Chem. 1988, 26, 955–959.
4. Singleton, D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995,
1
17, 9357–9358.
5. Zhang, B.-L.; Pionnier, S. J. Phys. Org. Chem. 2001, 14,
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2
2
2
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Chem. Soc. 1997, 119, 9907–9908.
27. Singleton, D. A.; Szymanski, M. J. J. Am. Chem. Soc.
999, 121, 9455–9456.
1
2
2
3
3
3
8. Meyer, M. P.; DelMonte, A. J.; Singleton, D. A. J. Am.
Chem. Soc. 1999, 121, 10865–10874.
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29.
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939–8943.
We thank the Swedish Research Council for financial
support and Drs. G o¨ ran Karlsson and Vladislav Orekhov
at the Swedish NMR centre for their kind assistance.
1
8
2. Merrigan, S. R.; Le Gloahec, V. N.; Smith, J. A.; Barton,
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