[11, 161, 162]. The reaction of furans with ethyl 2,2-difluoro-1-diethylaminocarboxyacrylate generates fluorinated 7-oxanorbornenes [163]. These cycloadducts
undergo alkene catalytic hydrogenation (or other reactions) into the corresponding
7-oxabicyclo[2.2.1]heptanes. Examples have been presented with the synthesis of
norcantharidin, 5, with the synthesis of the 2-epimer of rac-3
0 ,6
0 -epoxyauraptene,
32 (Scheme 3), and with the synthesis of thromboxane mimetics (Scheme 8). In
1929, Diels and Alder first reported the reaction of furan with maleic anhydride that
produces at room temperature the exo-adduct 132 [164]. In 1962, Anet found that at
low temperature, the reaction produces first the endo-adduct 133 in agreement with
the endo Alder rule [165]. At 25
C and in MeCN, the exo-adduct 132 is more stable
by 1.9 kcal/mol compared with 133 [166] (Scheme 20).
Berson and Swidler have shown that the Diels–Alder reaction of furan with
maleic acid in water gives first the endo-adduct 60 that reacts with bromine to give
Scheme 20 Diels–Alder reaction of furan with maleic anhydride: kinetic (endo Alder rule) and
thermodynamic control
Scheme 19 Warrener et al.’s synthesis of 7-oxabicyclo[2.2.1]heptane-derived molecular devices
162
A.J. Moreno-Vargas and P. Vogel