2-Lithio-3-bromofuran

Directive effects on lithiation have also been studied. The regiospecific /3-metallation of A-methylpyrrole derivatives and 2-substituted furans has been effected by employing the directive effect of the oxazolino group (82JCs(Pl)1343). 2-Substituted furans and thiophenes are metallated in the 5-position. The formation of 2-lithio-3-bromofuran on treatment of... 

Directive effects on lithiation have also been studied. Regiospecific -metallation of iV-methylpyrrole derivatives and 2-substituted furans can be effected by employing the on o-directing effect of, for example, an oxazolino group 2-lithio-3-bromofuran is the result of treatment of 3-bromofuran with lithium diisopropylamide (LDA) at 80C in THF. 

Several 2-lithio-3-bromofurans (206), prepared either by halogen-metal interconversion from the 2,3-dibromofurans (207) or by metallation of the 3-bromofurans (208), show typical properties of aryllithium reagents and give no evidence for the intermediacy of 2,3-didehydrofuran (180). 

Grignard reagents are difficult to prepare in both the furan and benzo[f ]furan series and their use has been superseded by the more tractable lithio compounds. Bromofurans are best converted into the Grignard reagents by treatment with a copper-magnesium alloy in THF (80JOC3125). 

The classical transformations of 18 with electrophilic reagents (e.g., carboxylation with CO2, alkylation with R-X, acylation with R-COX, halogenation with X2) allow the introduction of other functionalities at the furan system. The preference of a-metalation at furan is demonstrated by the fact, that 3-lithiofuran (19) - obtained by halogen-metal exchange of 3-bromofuran with RLi at — 78°C - equilibrates to the more stable 2-lithio furan (18) on raising the temperature to —40°C. 

---