Synthesis of 2,6-bridged piperazine-3-ones by N-acyliminium ion chemistry.

Several 2-substituted and 2,5-disubstituted piperazine-3,6-diones were synthesized starting from readily available alpha-amino acids. After activation of a lactam carbonyl via introduction of a methoxycarbonyl group onto nitrogen, this carbonyl was selectively reduced. Treatment of the resulting urethane with protic acid generated the corresponding N-acyliminium ion, which was trapped by a nucleophilic C2-side chain to provide 2,6-bridged piperazine-3-ones. Several aromatic, heteroaromatic, and nonaromatic side chains were used as pi-nucleophiles. In addition, the effect of the presence of a C5-methyl group on the stereochemical outcome of the cyclization was examined.

Synthesis of the cyclobutanone core of solanoeclepin A intramolecular allene butenolide photocycloaddition.

The compact tricyclic substructure of solanoeclepin A containing the cyclobutanone ring was prepared by using as the key step a highly regioselective intramolecular [2 + 2]-photocycloaddition reaction between one of the [small pi]-bonds of an allene and the CC double bond of a butenolide.