Chemistry Reference
In-Depth Information
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with three cyclodextrins and a linked pyridine ring; no extra pyridine was needed
with this catalyst [194]. As a bonus, this catalyst hydroxylated the C-6 position with
2000 turnovers. Catalyst
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was synthesized through intermediate
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, with a nitro-
phenyl group, and this performed the C-6 hydroxylation with 3000 turnovers (although
it still needed the added pyridine in solution). Such very high turnovers make these
catalytic hydroxylations potentially practical.
While the selective hydroxylations at C-6 are certainly examples of geometrically di-
rected functionalizations, imitating the enzyme cytochrome P-450, C-6 is not the most
attractive target for such selectivity. We wanted to perform a selective multi-turnover
hydroxylation of C-9 in the steroid. This would imitate the hydroxylations that take
place in fermentations, and would let us convert the product into the 9(11) olefin,
as we had done in our directed chlorinations described above.
Computer molecular modeling indicated that the hydroxylations at C-6 occurred
because, with only two binding points for the substrate (into the two cyclodextrins
on opposite sides of the porphyrin), the substrate could rotate so as to point its
edge toward the Mn=O species that performs the hydroxylation. To solve this, we
