A high-yield synthesis of 3-carboethoxy-4-trifluoromethylfuran and some Diels-Alder reactions of this furoate with acetylenic dienophiles

Abstract

The reaction of furan with ethyl 4,4,4-trifluorobut-2-ynoate ( 1) (1.2:1 molar ratio) in vacuo at 40 °C gives the 1:1 adduct oxanorbornadiene ( 3) (70%) and the 2:1 adduct ( 5) as the endo,exo ( 5a) (16%) and exo,exo ( 5b) (10%) isomers; at 20 °C, using an excess of ester 1, the yield of 1:1 adduct 3 is increased to 93%. Catalytic hydrogenation ( Pd C/Me 2CO ) of1:1 adduct 3 with control of the hydrogen uptake affords either the oxanorbornane 9 (89%) or the oxanorbornene 10 (90%), and flow pyrolysis of compound 10 at 400 °C in nitrogen gives 3-carboethoxy-4-trifluoromethylfuran ( 2) (89%). Diels-Alder cycloaddition reactions of furan 2 with the alkynes CF 3CCCF 3 (100 °C), 1 (100 °C) and MeO 2CCCCO 2Me (120 °C) in solvent CH 2Cl 2 afford the 1:1 adducts 13 (95%), 14 (47%) plus 15 (45%), and 16 (92%), respectively, but reaction with ethyl propynoate does not take place at or below 120 °C and at 150 °C a complex product mixture is obtained. The 1:1 adduct 3 undergoes reaction with triflic acid in solvent CH 2Cl 2 heated under reflux to give 2-carboethoxy-3-trifluoromethylphenol ( 23) (31%) and 3-carboethoxy-2-trifluoromethylphenol ( 24) (48%) and with concentrated sulfuric acid under the same conditions to afford phenols 23 (21%) and 24 (32%), but the oxanorbornadiene 13 does not undergo reaction with triflic acid at temperatures up to 80 °C.