An expeditious synthesis of (±)-decahydro-5H-dipyrrolo [1,2-a:1',2'-c]pyrimidine-5-carbonitrile: a formal synthesis of (±)-tetraponerines-1, -2, -5, and -6

A short and practical synthesis of (±)-decahydro-5H-dipyrrolo[1,2-a:1',2'-c]pyrimidine-5-carbonitrile (17), a pivotal intermediate in the synthesis of (±)-tetraponerines-1 (5), -2 (6), -5 (7), and -6 (8), in three steps and 24% overall yield, from simple and inexpensive starting materials, is reported. The key step of our synthesis is a one pot stereoselective multistep process, whereupon two molecules of Δ1-pyrroline (9) react with diethyl malonate (10) to afford the tricyclic lactam ester 13, possessing the tetraponerine skeleton. Hydrolysis of the carboethoxy group of 13 followed by decarboxylation yields lactam 15, which is converted into α-aminonitrile 17. Our fruitless efforts to transform lactam 15 into the tetraponerines through reductive alkylation are also discussed.Key words: alkaloids, tetraponerines, stereoselective synthesis, Robinson-Schöpf reaction, lactams, ants.