ABSTRACT.The diketopiperazines c/L-(S)Pip,L-Phe/ and c/L-(S)Pip,D-Phe/ have been studied by IH nmr spectroscopy at 360 MHz and the complete analyses have disclosed their solution conformations.Both isomers possess a pronouncedly flattened boat DKP-ring (puckering of about loo), and the aromatic side-chain in both diastereomers are pronouncedly stacked over this ring.the cis-L,L isomer is almost exclusively folded (and to a higher degree as in the Pip-congener) this happens f o r the trans-D,L-isomer only in hydrophilic surroundings.well with the expected torsions around the (S)Pip-DKP annelations for a (S)Pip-chair, and with the idea that next to peptide-n interactions, hydrophobic interresidue-stabilisation and s u l -
ABSTRACT.Both 3-0x0-(2) and 4-0x0-thiane-1-oxide [,l) exist in the chair form with axial sulfoxide bonds. The axial preference is retained in the dimethyl acetal of 1 b u t not in the dimethyl acetal of k. Beside steric strain, local electrostatic stabilization identified as a gauche 1.4-or a 1,5(g-,g+)-interaction between opposite residual charges rationalize the conformational preferences.
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 isomeric assignments and respective conformations in solution of the oxidation products of N-Me-L-Thiazolidine-4-carboxylic acid hydantoin are described. The cis-sulfoxide isomer occurs mainly in an envelope with C6 as the flap down (6-).what more deformed with a conformation close to the &-.
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