Abstract Nitroso-arene dienophiles 1 react regiospecifically with the conjugated dienals 2 and 3a/3b, and lead thereby to the unstable Diels-Alder cycloadducts 4 and 5. These undergo two types of cascade reactions which give pyridinium betaines 6 and pyrrolo- indoles 8 as the major reaction products. The one-pot syntheses of pyrroloindoles 8 represent a new and easy access to the basic skeleton of mitomycins 10. The scope and ...