Journal article
The N-vinyl group as a protection group of the preparation of 3(5)-substituted pyrazoles via bromine-lithium exchange
Treatment of 3,4,5-tribromopyrazole with 1,2-dibromoethane and triethylamine gave 3,4,5-tribromo-1-vinylpyrazole, which underwent regioselective bromine–lithium exchange at the 5-position. Subsequent addition of an electrophile gave 5-substituted 3,4-dibromo-1-vinylpyrazoles. These underwent bromine–lithium or bromine–magnesium exchange predominantly at the 4-position, with the regioselectivity between the 3- and 4-positions being influenced by the nature of the metal and the 5-substituent.
The 5-substituted products were de-vinylated by mild treatment with KMnO4 affording 3-substituted pyrazoles. Alternatively, the 1-vinyl group could be used in ring-closing metathesis. Thus, 5-allylthio-1-vinylpyrazole produced 5H-pyrazolo[5,1-b][1,3]thiazine upon treatment with Grubbs' second-generation catalyst.
Language: | English |
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Year: | 2007 |
Pages: | 56-61 |
ISSN: | 14645416 and 00404020 |
Types: | Journal article |
DOI: | 10.1016/j.tet.2006.10.009 |