Rotational Side‐Chain Isomerism And Aggregation Of The Diastereomeric Cyclic Dipeptides Phenylalanine, 4‐Thiapipecolic Acid

Abstract

SUMMARY.The dimerization constants in CDC13 at 25 O C have been measured for the cis-and trans-[K2(cis) = The dimer is of a C -type H-bridge structure. Concomidiketopiperazines (DKP) c/Phe,(S)Pip/ from concentration-dependent proton shifts 4.3 llmol; Kp(trans) = 1.27 l/moll. tantly with aggregation an increasing folding of the aromatic nucleus over the DKP ring is observed at the dependency of the extended-to-N form. The greater stabilization for the folded structure in the *-isomer than in the =-isomer is reflected by the dimerization capacities. The tendency for stacking can be rationalized as the interplay of two phenomena;disrupture of intramolecular NH-s interaction by solute-solute or solute-solvent interaction taken over by side-chainlbackbone (DKP-ring) and side-chainlside-chain attractions. For the latter, a hydrophobic Van der Waa1s'-type interaction and also a specific sulphur-n interaction seems probable. 8INTRODUCTION.

The Crystal Structure of (3R,65)‐3‐Benzyl‐4‐Methyl‐1, 4‐Diaza‐Bicyclo [4.4.0] Decan‐2,5‐Dione

“…the benzene r i n g i s stacked above the DKP r i n g (Figs. 1 and 2), thus confirming p r e d i c t i o n s based on nmr-spectra (Anteunis, Schrooten and Nachtergale, 1980). Distances between the center o f the benzene nucleus and atoms i n the DKP r i n g ( F i g .…”