Generation of 1-azapentadienyl anion from N-(tert-butyldimethylsilyl)-3-buten-1-amine

A Straightforward and Versatile Synthetic Approach to 1-Azabicyclic Alkaloids

A very straightforward route to 1-azabicyclo alkaloid scaffolds with several ring sizes is reported. The final bicyclic structures were built through a synthetic scheme that involved (i) the construction of dienic 4-piperidone systems by an imino-Diels-Alder reaction between aminotrienes and N-omega-vinylimines, in the presence of Yb(OTf)(3), and (ii) the ring-closing metathesis reaction of these cyclic dienes, under the influence of the first-generation Grubbs' Ru-complex catalyst. During this investigations, various polysubstituted azabicyclic ring skeletons, including several examples of the quinolizidine alkaloids, are reported, and their relative stereochemistry is adequately discussed.

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.