NMR-Spectra and Solution Structures of Some Piperazine-2,5-Diones. X-Ray Crystal-Structure of Threo-1-Acetyl-3-(Alpha-Bromobenzyl)-3-Methoxy-Piperazine-2,5-Dione

Abstract

3-(α- Bromobenzyl )-3-methoxypiperazine-2,5-diones are prepared by the reaction of 3-benzylidenepiperazine-2,5-diones with N- bromosuccinimide in methanol. The crystal structure of the threo-1-acetyl compound shows the side chain in the E_N conformation, with the bromine above the heterocyclic ring, the latter ring system being almost planar. This conformer predominates in CDCl₃ solution also, but in (CD₃)₂SO the piperazine-2,5-dione ring adopts a boat conformation. Both the erythro isomer and an N(4)-methyl derivative exist in solution predominantly as folded conformers. The importance of repulsive interactions in predicting the conformation of substituted piperazine-2,5-diones is discussed.