Isocyanide-Based Three-Component Synthesis of Highly Substituted 1,6-Dihydro-6,6-dimethylpyrazine-2,3-dicarbonitrile, 3,4-Dihydrobenzo[g]quinoxalin-2-amine, and 3,4-Dihydro-3,3-dimethylquinoxalin-2-amine Derivatives.