Novel Diels-Alder cycloaddition involving two α,β-unsaturated esters in the formation of isocoumarins from the reaction of ethly propynoate with 3,4-bis(trifluoromethy)furan

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Reaction of ethyl propynoate with 3,4-bis(trifluoromethyl)furan ( 4) at 150°C affords a mixture of ethyl 6,7-bis(trifluoromethy)isocoumarin-3- carboxylate ( 7) and the corresponding 4-carboxylate ( 8) in the ratio 52:9, produced by Diels-Alder cycloaddition of the alkyne to the α,β-unsaturated ester function of the initially-formed 1:1 adduct, the oxanorbornadiene ( 9), followed by opening of the oxygen bridge and elimination of ethanol. The corresponding reaction involving dimethyl acetylenedicarboxylate gives the expected oxanorbornadiene ( 5). Thermal reactions of furan and its derivatives with acetylenic esters have been widely investigated since that between furan and dimethyl acetylenedicarboxylate (DMAD) was first reported in 1931 [1]. Products from the DMAD reactions are temperature and reactant ratio dependent, e.g. Diels-Alder 1:1, 2:1, and 3:1 furan : DMAD adducts have been isolated and characterised [1,2]. Reactions involving ethyl propynoate (EP) are less facile than those with DMAD and the only product isolated from the reaction with furan at 130°C was the 1:2 adduct ( 1) [3]; 2,5-dimethylfuran afforded the analogous compound ( 2) as the major product [ 4]. Compounds ( 1) and ( 2) were considered to arise by dimerisation of the initially-formed 1:1 adduct, the oxanorbornadiene ( 3), by a π2 s + π2 s + π2 s mechanism followed by retrocleavage of furan or 2,5-dimethylfuran [4] (Scheme 1). In the present work the reactions of 3,4-bis(trifluoromethyl)furan ( 4) [5,6] with DMAD and EP have been carried out as part of a general investigation of the chemistry of this furan derivative.