Synthesis of (3R,6R)- and (3S,6S)-3,6-dialkylpiperazin-2,5-dione derivatives as useful intermediates to both (R) and (S) α-aminoacids

The alkylation of 2 leads to a diastereomeric mixture where 4 is generally present in greater amounts than 3 . The successive alkylation of 4 affords exclusively the cis derivative 7 , while a cis/trans mixture is obtained from isomer 3 . In alkaline solution, the trans isomer 6 suffers a total conversion into a 1:1 mixture of the cis isomers 5 and 7 easily separated by silica gel chromatography. The configurations of the introduced stereogenic centers at C-3 and C-6 have been assigned on the basis of 1 H-NMR spectroscopic data. Cleavage of heterocyclic ring of 5 and 7 leads to the corresponding ( R )- and ( S )-α-aminoacidsrespectively. Therefore, the substrate 2 appears a very useful chiral template to synthetize enantiomerically pure α-amino acids.