Diels-alder cycloadditions of N-substituted-1,2-dihydropyridines with 1,2,4-triazoline-3,5-diones and maleimides

The reaction of N-substituted-1,2-dihydropyridines 1 with 1,2,4-triazoline-3.5-diones 2 and maleimides 9 proceeds stereospecifically to afford endo cycloaddition products. N-Acetyl-1,2-dihydropyridines react with 2 to afford a stereo isomeric mixture of 3 and 4 whereas those possessing a N-ethoxycarbonyl, methoxycarbonyl, methanesulfonyl or benzenesulfonyl substituent yield 3 exclusively: similar results are also obtained in reactions employing maleimides. Stereochemistry was assigned on the basis of nmr data and use was made of the anisotropic effects of the 7,8 unsaturation on the R₁ and R₂ substituents.