Nucleophilic fluorination of β-ketoester derivatives with HBF4.

Treating readily available α-diazo-β-ketoesters with HBF(4) results in nucleophilic fluorination by the usually inert and stable tetrafluoroborate anion. The resulting α-fluoro-β-ketoesters are highly versatile synthetic intermediates, for example in the preparation of fluoro-heterocycles, as illustrated by the direct formation of fluoro-pyrimidines, -pyrazoles and -coumarins in a single step.