Thermally initiated cycloadditionreactions of nonsymmetricalallenyl azines 1 with alkynes or other dipolarophiles usually lead to compounds with three fused, five-membered heterocyclic rings. With alkynes with pronounced "push-pull" systems, however, the reaction ends with the formation of substituted pyrrolidino[1,2-b]pyrazoles 4 and, in the case of azines with trifluoromethyl substitution, ring opening