Structure assignment and conformations of the four diastereomeric cyclic dipeptides of phenylalanine, S‐methyl‐4‐thiazolidinium‐2‐carboxylic ACII

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Abstract
The title compounds were obtained each time as a mixture of the cis‐S‐methyl and trans‐S‐methyl diastereomers by methylation of the respectively cis‐ and trans‐diketopiperazines (DKP), c/L‐Phe, L‐Thz/ and c/D‐Phe, L‐Thz/ (Thz = 4‐thiazolidine‐2‐carboxylic acid = 4‐thiaproline).Although these methylthianium compounds were unstable in protic solvents and could therefore not be separated into the single diastereomers, an 1H NMR study on the pairwise mixtures enabled the identification of the configurations and an insight upon their respective conformations was possible. In D2O, the aromatic ring is folded for about 75% over the DKP ring, as well in all four onium derivatives as in the parent sulfide‐DKP's.The two compounds having the S⊕‐Me substitution trans with respect to the thiazolidine carboxamide grouping (cis‐2F and trans‐2F, figures 1 and 2) possess a thiazolidine fragment in the envelope α−, with Cα as the flap down. The five membered ring in the trans‐2S isomer is best characterized as S+ (envelope with sulfur as the flap up), while in the cis2S isomer a pronounced admixture of the γ− and/or SγT is probably present.The DKP‐rings in the onium derivatives are rather flat (buckle <15°) except in trans‐2S where the puckering may perhaps amount to 40°.

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