Stereoselective hetero-Diels–Alder reaction of 3-(polyfluoroacyl)chromones with enol ethers. Novel synthesis of 2-RF-containing nicotinic acid derivatives

3-(Polyfluoroacyl)chromones undergo heterodiene cycloaddition to 3,4-dihydro-2H-pyran, 2,3-dihydrofuran and ethyl vinyl ether under mild conditions, producing novel fused pyrans with high stereoselectivity and in good yields. Some of these pyrans were transformed into 2-RF-containing pyridines on treatment with ammonium acetate in ethanol.