Intramolecular Photocycloaddition of N-Alkenyl Substituted Maleimides: A Potential Tool for the Rapid Construction of Perhydroazaazulene Alkaloids

UV irradiation of a number of N-alkenyl-substituted maleimide derivatives leads to the formation of complex perhydroazaazulenes in excellent yields. The overall process can be considered as a formal intramolecular [5+2] cycloaddition. Substrates were prepared by Mitsunobu coupling of the appropriate alkenols with various maleimides. Methyl substitution of the alkenyl side chain gave the cycloadducts 13a−g in good yields, with moderate to high stereoselectivity being observed for 13e and 13g, respectively. Use of cyclic alkene side chains led to the formation of tri- and tetracyclic products with high degrees of stereoselectivity in most cases. Some of the polycyclic ring systems that were prepared constitute the core skeleton of a number of complex alkaloids. The substrate 29 underwent an unexpected [2+2] photocycloaddition to yield the unusual cyclobutane 31.