A HIGH-YIELD SYNTHESIS OF 3-CARBOETHOXY-4-TRIFLUOROMETHYLFURAN AND SOME DIELS-ALDER REACTIONS OF THIS FUROATE WITH ACETYLENIC DIENOPHILES

The reaction of furan with ethyl 4,4,4-trifluorobut-2-ynoate (1) (1.2:1 molar ratio) in vacuo at 40 degrees 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 degrees C, using an excess of ester 1, the yield of 1:1 adduct 3 is increased to 93%. Catalytic hydrogenation (Pd/C/Me(2)CO) of 1:1 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 degrees C in nitrogen gives 3-carboethoxy-4-trifluoromethylfuran (2) (89%). Diels-Alder cycloaddition reactions of furan 2 with the alkynes CF3C=CCF3 (100 degrees C), 1 (100 degrees C) and MeO(2)CC=CCO(2)Me (120 degrees C) in solvent CH2Cl2 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 degrees C and at 150 degrees C a complex product mixture is obtained. The 1:1 adduct 3 undergoes reaction with triflic acid in solvent CH2Cl2 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 degrees C.