The regiospecific 1,3-dipolarcycloaddition reaction of 1,2-dihydropyridines1with organic azides2afford 2,7-diazabicyclo[4.1.0]hept-4-enes4, whereas solutions of1also containing lithium hydroxide give predominately4along with the diazo product6. In contrast, the dimeric product7was obtained from the reaction of1awith hydrazoic acid. Catalytic hydrogenation of4gives rise to a tautomeric mixture of9and10; when R2is methanesulfonyl orp-aminobenzenesulfonyl only9was obtained. Treatment of4with neutral alumina gives rise to the 1,2,5,6-tetrahydropyridylidenes8which are likewise reduced to a mixture of9and10or9.