Conformational studies of diastereoisomeric cyclo(alanyl-phenylalanyl) with the coupling constants 1H–1H, 15N–1H, 13C–1H,and 13C–15N

The cyclodipeptides cyclo(L -alanyl-L -phenylalanyl) and cyclo(D -alanyl-L -phenylalanyl) were synthesized with various atoms substituted by the isotopes ¹⁵N and ¹³C. Thus, the coupling constants ¹⁵N–¹H, ¹³C–¹H, and ¹³C–¹⁵N could be obtained, in addition to the commonly used ¹H–¹H constants. The applicability of these coupling constants for obtaining conformational information on side chains and substituted 2,5-piperazinedione rings is discussed.