cesium fluoride in acetonitrile T=20°C; 24 h;

1: N-Boc-4-oxo-L-proline methyl ester With sodium hexamethyldisilazane in tetrahydrofuran T=-78°C; 0.5 h; 2: N,N-diphenyltrifluoromethanesulfonamide in tetrahydrofuran T=-78°C; 4 h;