Synthesis of 3‐Fluorofuran‐2(5 H )‐ones Based on Z / E Photoisomerisation and Cyclisation of 2‐Fluoro‐4‐hydroxybut‐2‐enoates

Mixtures of some ( E )‐ and ( Z )‐2‐fluoroalk‐2‐enoates prepared from the corresponding 2‐hydroxycarbonyl compounds and ethyl 2‐(diethoxyphosphoryl)‐2‐fluoroacetate have been transformed in high conversions into the target 3‐fluorofuran‐2(5 H )‐ones by an efficient Z / E photoisomerisation of noncyclisable Z isomers followed by acid‐catalysed cyclisation. In contrast, the acid‐catalysed deprotection of ethyl ( E )‐ and ( Z )‐4‐[ tert ‐butyl(dimethyl)silyloxy]‐2‐fluoro‐4‐phenylbut‐2‐enoates resulted in the displacement of vinylicfluorine, affording ethyl ( E )‐2‐oxo‐4‐phenylbut‐3‐enoate. 3‐Fluoro‐4‐phenylfuran‐2(5 H )‐one was transformed into 2‐[ tert ‐butyl(dimethyl)silyloxy]‐3‐fluoro‐5‐methylfuran as a novel fluorinated building block.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)