Nucleophilic fluorination of beta-ketoester derivatives with HBF4

Treating readily available alpha-diazo-beta-ketoesters with HBF4 results in nucleophilic fluorination by the usually inert and stable tetrafluoroborate anion. The resulting alpha-fluoro-beta-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.