Sequential Functionalization of Pyrazole 1-Oxides via Regioselective Metalation:  Synthesis of 3,4,5-Trisubstituted 1-Hydroxypyrazoles

A range of 3,5-diarylated and 3,4,5-triarylated 2-(4-methoxybenzyl)pyrazole 1-oxides have been prepared by regioselective deprotonation at C-5 or bromine−magnesium exchange at C-3 or C-4 followed by transmetalation with ZnCl2 and palladium(0)-catalyzed cross-coupling. Furthermore, the metalated pyra...

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Bibliographic Details
Published in:Journal of organic chemistry 2002-05, Vol.67 (11), p.3904-3907
Main Authors: Paulson, Anna Sif, Eskildsen, Jørgen, Vedsø, Per, Begtrup, Mikael
Format: Article
Language:English
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Summary:A range of 3,5-diarylated and 3,4,5-triarylated 2-(4-methoxybenzyl)pyrazole 1-oxides have been prepared by regioselective deprotonation at C-5 or bromine−magnesium exchange at C-3 or C-4 followed by transmetalation with ZnCl2 and palladium(0)-catalyzed cross-coupling. Furthermore, the metalated pyrazole 1-oxides could be trapped with electrophiles. The sequential metalation/functionalization of the pyrazole 1-oxides may follow the order C-5, C-3, C-4, or alternatively the order C-3, C-5, C-4. The 4-methoxybenzyl group of the functionalized 2-(4-methoxybenzyl)pyrazole 1-oxides could be removed by treatment with TFA and i-Pr3SiH in CH2Cl2, providing the corresponding functionalized 1-hydroxypyrazoles.
ISSN:0022-3263
1520-6904
DOI:10.1021/jo015997i