13C- and1H-nmr evidence for all-cis peptide bonds in cyclic tetrapeptidecyclo(L-Pro-Sar)2

Cyclic Peptoids as Topological Templates: Synthesis via Central to Conformational Chirality Induction

Chiral induction was utilized for the synthesis of diastereopure cyclic peptoids containing an N-benzyl alanine residue. Molecular modeling, NMR spectroscopy, single-crystal X-ray diffraction studies, and HPLC with chiral stationary phase demonstrated easy formation of free and sodium/benzylammonium complexed cyclic oligomers through strategic incorporation of a single stereogenic center in the oligomeric backbone. The synthesis of cyclic peptoids with defined conformational chirality and appropriate side chain topology is now possible.

Conformational isomerism in cyclic peptoids and its specification

Conformational chirality is an emerging and neglected property of rigid cyclic peptoids determining structural, catalytic, and biological properties. The present contribution analyzes its impact and sets the configurational rules to define it.

Enantioselective Alkylation of Amino Acid Derivatives Promoted by Cyclic Peptoids under Phase-Transfer Conditions

The effects of substituents and cavity size on catalytic efficiency of proline-rich cyclopeptoids under phase-transfer conditions were studied. High affinity constants (Ka) for the sodium and potassium cations, comparable to those reported for crown ethers, were observed for an alternated N-benzylglycine/L-proline hexameric cyclopeptoid. This compound was found to catalyze the alkylation of N-(diphenylmethylene)glycine cumyl ester in values of enantioselectivities comparable with those reported for the Cinchona alkaloid ammonium salts derivatives (83-96% ee), and with lower catalyst loading (1-2.5% mol), in the presence of a broad range of benzyl, allyl and alkyl halides.

Conformations and conformational interconversions of diastereomeric cyclic tetraprolines

Cyclic tetrapeptides exclusively composed of L- and D-Pro have been studied by theoretical means (conformational searches and molecular mechanics calculations using the CHARMM program) supported by 1H-NMR spectroscopy, X-ray analysis and chiroptical measurements. We explored the entire conformational space of the diastereomers cyclo(LLLL-Pro4) (I), cyclo(LDLD-Pro4) (II) and cyclo(LLDD-Pro4) (III) including the low-energy conformations and the related interconversion paths. The conformational interconversions were found to be restricted to cis/trans isomerisations of the amide bonds. Owing to the polycyclic nature of cyclo(Pro4) most of the cis/trans transitions are hindered by energy barriers higher than 30 kcal/mol (up to 150-200 kcal/mol). A few transitions are characterized by computed energy barriers comparable to those found in linear -Xxx-Pro- sequences (approximately 18 kcal/mol), and are therefore experimentally significant. Experimental evidence has been obtained in the case of cyclo(LDLD-Pro4), where two enantiomers are interconverted by a series of 4 cis/trans isomerisations ctct-->cttt-->tttt-->tctt-->tctc. The Erying activation parameters of this reaction were determined in H2O and in DMF by chiroptical measurements (delta H not equal to = 44 and 28 kcal/mol; delta S not equal to = 59 and 22 cal K-1 mol-1, respectively), and correlated with the calculated barriers. In I and III comparable series of four cis/trans isomerisations relate two main conformations with the peptide bond sequences ctct and tctc. In compound I pseudorotational images are interconverted via ctct-->ccct-->cctt-->cctc-->tctc.(ABSTRACT TRUNCATED AT 250 WORDS)

Empirical rules predicting conformation of cyclic tetrapeptides from primary structure

A useful set of empirical rules is put forward to predict the conformations of cyclic tetrapeptides and cyclic tetradepsipeptides on the basis of primary structure, briefly presented as follows: A conformation allowing an intramolecular hydrogen bond (IMHB) of gamma-turn is preferred, and an ester bond always adopts a trans form. On a right-handed peptide ring, the carbonyl group acylating a D residue is oriented to the upper side of the main ring. The carbonyl group acylating a D proline or an N-methyl-D-amino acid residue is oriented to the lower side of the ring, forming a cis bond. The LDDL configurational sequence adopts a cis-trans-cis-trans backbone with Ci symmetry. A glycine residue behaves as a D residue in an L-peptide. Conformations of cyclotetrapeptides containing two glycine residues at diametric positions or containing an N-methyl-dehydroamino acid residue are predicted by use of appendices of rule 5. Almost all conformations of cyclic tetrapeptides are predicted by these rules. Energetical rationalization of the rules and prediction of possible new conformations are described. Conformations of cyclo(-L-Pro-L-Leu-D-Tyr(Me)-L-Ile-)(1) and cyclo (-L-Pro-D-Leu-D-Tyr(Me)-L-Ile)(2) are compared. Results of n.m.r. experiments showed that compound 1 adopts a unique cis-trans-trans-trans backbone with a gamma-turn IMHB, and 2 has a cis-trans-cis-trans backbone with Ci symmetry. These observations confirmed the rules described above. Peptides 1 and 2 are the first diastereomeric peptides with trans (LD) and cis (DD) secondary amide bonds.