Cyclo-(L-asparagyl-L-asparagyl)[(3S,6S)-3,6-bis(carbamoylmethyl)-piperazine-2,5-dione]: preparation and crystal structure
Two methods are described for the preparation of optically pure cyclo-(L-asparagyl-L-asparagyl)[(3S,6S)-bis(carbamoylmethyl)piperazine-2,5-dione]: (i) by self-condensation of (S)-3-aminopyrrolidine-2,5-dione in refluxing acetonitrile, and (ii) by self-condensation of L-asparagine methyl ester at room temperature; the latter method is more efficient. The identity of this previously uncharacterised piperazinedione was confirmed by analytical and spectroscopic data, and by a crystal structure determination, which showed the conformation of the piperazinedione ring to be a bowsprit boat (i.e. quasiequatorial carbamoylmethyl substituents). The angle between the amide units of the piperazinedione ring is 18°. Intramolecular hydrogen bonds are absent from the crystal structure, the packing of which is dominated by intermolecular hydrogen bonds from each piperazinedione CO to the carbamoyl NH2(each NH of which is engaged in hydrogen bonding to a different molecule of piperazinedione).