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413
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As for the rigid ꢀ5-steroidal substrates, these events
concern the allylic C-7 of the molecules rather than C-4,
which is the other conformationally inflexible site of allylic
hydrogen (selective oxidation). It is to be expected that ax-
ial hydrogens will be preferentially abstracted to equatorial
hydrogens because of the more favorable stereoelectronic
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[49].
In this case, the axial hydrogen of C-4 lies above the
plane of the steroid molecule, and the angular methyl group
at C-10 would hinder the approach of the oxidizing species.
The axial hydrogen at C-7, which is below the plane of
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copper and a phase-transfer catalyst in a two-phase system
of water/DCM where the peroxide is added periodically. The
reaction time does not exceed 4 h, and for most of the sub-
strates, the yield of the oxidation process is greater than 85%.
Acknowledgments
This research was supported by the “K. Karatheodori”
grant No 2456, Research Committee of the University of
Patras, Greece. The NMR spectra and the elemental analyses
were carried out in the Center of Instrumental Analysis of
the University of Patras, and the authors are indebted to
Dr. D. Vachliotis for performing these measurements.
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