Highly diastereoselective Barbier allylation and iminium cyclization: a simple entry to bicyclic and tricyclic piperazinones

Barbier allylation of 5,6-dihydropyrazin-2(1H)-ones leads to a single isomer of 3-allylpiperazin-2-ones in high yields. Further Pictet-Spengler-Grieco cyclization of 3-allylpiperazin-2-ones with aldehydes provides bicyclic and tricyclic piperazinones with high diastereoselectivity.