5-Nitro-6-substituted-2-norbornenes: determination of Diels-Alder isomer ratios from nuclear magnetic resonance spectra

The nuclear magnetic resonance spectra of representative 5-nitro-6-substituted-2-norbornenes (II–VII) are described. From the spectra, ratios of the two possible endo–exo trans stereoisomers derived from the Diels–Alder reaction of cyclopentadiene with the appropriate trans nitroölefin are estimated as shown in Table II. For all cases, there was predominance (3:1 to 9:1) of the 5-endo-nitro trans stereoisomer. The synthesis of the 6-(p-nitrophenyl) (V), 6-(2-furyl) (VI), and 6-(2-thienyl) (VII) derivatives is reported and the 6-ethyl derivative (III) has been characterized