An approach toward azacycles using photochemical and radical cyclizations of N-alkenyl substituted 5-thioxopyrrolidin-2-ones.

The photochemical reactions of a series of cyclic N-alkenyl-substituted thioimides have been examined. Irradiation of N-3-methylbut-3-enyl-5-thioxo-pyrrolidin-2-one (16) results in intramolecular [2 + 2] cycloaddition to give the highly strained thietane 17, whose structure was confirmed on the basis of its X-ray analysis. Treatment of cycloadduct 17 with dimethyl(methylthio)sulfonium tetrafluoroborate gave 2,5,6,7-tetrahydropyrrolizin-3-one (20) in good yield. Further reduction of 20 with Raney-Ni afforded 5,5-dimethylhexahydro-pyrrolizin-1-one (21). This sequence of reactions demonstrates the facility with which the 2 + 2 photoadduct can be converted into the pyrrolizidine alkaloid core skeleton. The photochemistry of the closely related N-butenyl thioxopyrrolidin-one (22) proceeded in a slightly different fashion and produced 7-mercaptomethyl tetrahydropyrrolizin-3-one (24) in 68% yield. In contrast to the above results, irradiation of the thioxaphthalimido system containing an N-cycloalkenyl group in the side chain gave rise to products derived by gamma-hydrogen abstraction from the n-pi triplet excited state. The photobehavior of the related N-3-alkenyl pyrrolidine-2,5-dithione system (62) was also studied and found to give products derived from both a 2 + 2 cycloaddition (63) and hydrogen atom transfer (64). Finally, the reaction of several N-alkenyl substituted thioimides (71-73) with tributylstannane in the presence of AIBN gave cyclized products derived from transient radical intermediates.