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| - Regioselective Introduction of Electrophiles in the 4-Position of1-Hydroxypyrazole via Bromine−Lithium Exchange
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| - Two protocols for introduction of electrophiles at the 4-position of 1-hydroxypyrazoles have beendeveloped. The first is deprotonation of 4-bromo-1-[(tert-butyldiphenylsilyl)oxy]pyrazole (6) withLDA to produce the 5-lithio derivative in which the silyl group migrates spontaneously from oxygento C-5 affording 4-bromo-5-(tert-butyldiphenylsilyl)-1-lithoxypyrazole (8). Subsequent treatment witht-BuLi causes bromine−lithium exchange to give 5-tert-butyldiphenylsilyl-4-lithio-1-lithoxypyrazolewhich is trapped with electrophiles affording 4-substituted 5-(tert-butyldiphenylsilyl)-1-hydroxypyrazoles 9a−e in a one-pot sequence. The second is treatment of 4-bromo-1-hydroxypyrazole (5)with excess LDA and trimethylsilyl chloride to produce 3,5-bis(trimethylsilyl)-4-bromo-1-hydroxypyrazole (12), which upon sequential metalation with n-BuLi and reaction with electrophiles affords4-substituted 3,5-bis(trimethylsilyl)-1-hydroxypyrazoles 13a−e. The tert-butyldiphenylsilyl groupof 9a and the trimethylsilyl groups of 12 can be removed selectively by treatment withtetrabutylammonium fluoride in THF in the presence of trifluoroacetic acid.
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