Pyrazole chemistry. Part 4. Directed lithiation of 4-bromo-1-phenyl-sulphonylpyrazole: a convenient approach to vicinally disubstituted pyrazoles
Abstract
4-Bromo-1 -phenylsulphonylpyrazole (10), obtained from 4-bromopyrazole (5) and benzenesulphonyl chloride, can be metallated regioselectively by phenyl-lithium to give the 5-lithio derivative (15) which upon quenching with appropriate electrophiles leads to the 4-bromo-1 -phenylsulphonyl-5-substituted pyrazoles (16)–(23), (25), and (26). Compounds (22), (23), and (25) were found to undergo isomerisation to afford the thermodynamically more stable 4-bromo-1 -phenylsulphonyl-3-substituted pyrazoles (11), (24), and (13) under the reaction conditions applied. The phenylsulphonyl protecting group then can be removed readily under alkaline conditions to yield the corresponding 4-, bromo-3(5)-substituted 1H-pyrazoles (6), (9), (28), (29), and (30).
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