ABSTRACT.acid and L-y-thiapipecolic acid with glycine or sarcosine are negatively buckled. of foldin; of the diketopiperazine ring increases in the y-thia-analog. effect of changing the solvent from chloroform to methanol. and of N-methylation of the Gly residue, on the buckle of the DKP ring is attempted.From '€I N M parameters we demonstrate that the cyclic dipeptides derived from L-pipecolic