NMR studies of hindered rotation. The Diels-Alder adduct of 4-methyl-1,2,4-triazoline-3,5-dione with phencyclone: Restricted motion of unsubstituted bridgehead phenyls.

4-Methyl-1,2,4-triazoline-3,5-dione was produced by lead tetraacetate oxidation of 4-methylurazole and allowed to react with phencyclone, (1) under bar. The resulting Diels-Alder adduct, (2) under bar, has been characterized by one- and two-dimensional H-1 and C-13 NMR at 300 and 75 MHz, respectively, at ambient temperatures in different solvents. The NMR data are consistent with hindered rotation of the bridgehead unsubstituted phenyl groups about the C(sp(2))-C(sp(3)) bonds, based on numbers of absorption signals in the H-1 and C-13 NMR aryl region, together with magnetic anisotropic effects in the H-1 spectrum. The spectral simplicity suggests further that stereochemistry at the ring junction nitrogens involves only a single isomer or very rapidly interconverting ''exo''/''endo'' isomers (if the ring junction nitrogens are pyramidalized).