Synthesis of 2‘-Substituted 4-Bromo-2,4‘-bithiazoles by Regioselective Cross-Coupling Reactions
作者:Thorsten Bach、Stefan Heuser
DOI:10.1021/jo025661o
日期:2002.8.1
which underwent a second regioselectivecross-coupling with another equivalent of 2,4-dibromothiazole (2). The Negishicross-coupling gave high yields of the 2'-alkyl-4-bromo-2,4'-bithiazoles 1a-g (88-97%). The synthesis of the 2'-phenyl- and 2'-alkynyl-4-bromo-2,4'-bithiazoles 1h-j required a Stille cross-coupling that did not proceed as smoothly as the Negishicross-coupling (58-62% yield). The title
Cross-coupling reactions occur on 2,4-dibromothiazole preferentially at the more electron-deficient 2-position. This fact can be favorably used to prepare 2-substituted 4-bromothiazoles, which serve as precursors for 2,4-disubstituted thiazoles. Protocols for regioselective Negishi and Stille cross-coupling reactions are provided. Alternatively, the title compound can be metalated in 2-position by a halogen-metal exchange reaction. As a supplement to the well-established bromine-lithium exchange, the regioselective bromine-magnesium exchange reaction is presented.