| Abstract
| - The 3-ylidenepiperazine-2,5-diones 16 and 39 and 5-acyloxy-2(1H)-pyrazinones 17 can serve asstarting materials for the Diels−Alder reactions of alkenes and alkynes to the piperazine ring,under acidic conditions or in the presence of acetyl chloride, to afford tricyclic piperazine-2,5-diones19, 20, 23−25, 27, 44, and 45. Intramolecular cycloadditions occur if 3-ylidenepiperazine-2,5-diones30 and 32 are used as the starting materials. This procedure is a convenient path to bridged bicyclo[2.2.2]diazaoctane ring systems such as 31 and 33, the former being found in biologically activesecondary mold metabolites, such as VM55599 (1) or brevianamide A (5), which have been isolatedfrom various fungi. The synthesis of the indole compound 31 provided evidence for the proposedbiochemical pathway with a Diels−Alder reaction as key step. Quantum chemical calculations haverevealed that piperazinones with a cationic azadiene moiety are the most reactive species in Diels−Alder cycloadditions.
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