J. Peptide Res. 51,1998,116-120 Printed in the United Stares ofArnerrra zyxwvutsrqpon - zyxwvutsrqponmlkji all rights reserved Copyright zyxw 0 Munksgaard. 1998 zyx JOURNAL OF zyxw PEPTIDE RESEARCH ISSN 1397-002X Synthesis and conformational analysis of two 2-oxopiperazine- containing tetrapeptide analogues ADRIANA POHLMANN,* DOMINIQUE GUILLAUME, JEAN-CHARLES QUIRION AND HENRI-PHILLPPE HUSSON Laboratoire de Chimie The'rapeutique, FacultP des Sciences Pharmaceutiques et Biologiques, Paris, France Received 9 July, revised 12 August, accepted for publication zyxwv 14 September 1997 One unsubstituted and one stereoselectively prepared zyxwvu 3-(S)-substituted-2-oxopiperazine have been used as dipeptide templates to generate tetrapeptide analogues. NMR analysis shows that these tetrapeptide analogues present an inverse y-turn conformation in chloroform. 0 Munksgaard 1998. zyxw Key words: 2-oxopiperazine; conformational analysis; dipeptide template Peptidomimetics were designed to circumvent the physical-chemical properties and metabolic stability that peptides lack and are considered essential for thera- peutics. An easy way of freezing the flexibility of pep- tides by short-range cyclization is to bridge two consecutive nitrogen atoms by an appropriate linker. This method has rarely been used to produce peptido- mimetics, probably because of the very few methods allowing this transformation. Until recently, the only available strategies to produce N,-N, + ,-ethylene- bridged dipeptides implied either the cyclization of a symmetric ethylene-bridged linear dipeptide (1-3) or complex multi-step procedures that needed to be re- peated for each desired amino acid side-chain (4, 5). Recently, we reported an efficient method for the asym- metric C-3 alkylation (de > 95%) of 2-oxopiperazines and the easy transformation of the resulting adducts into dipeptide templates (6). With this method, the chirality was induced by almost any optically pure a-amino al- cohol (we initially used D-phenylglycinol(7,8) and for this study we used L-leucinol) which will generate the i + 1 residue of the template and whose nitrogen atom is the 2-oxopiperazine N-1 atom. Subsequently, the side-chain of the i residue was introduced by stereo- selective alkylation of the anion of the 2-oxopiperazine (t-BuLi) with a selected alkyl halide. X-ray crystallo- graphic data of 3-substituted adducts established that if the initial optically pure a-amino alcohol comes from a natural amino acid, the i residue configuration is iden- ~ *On leave of absence from Insituto de Quimica, Universidade Fed- eral do Rio Grande do Sul, Port0 Alegre, Brazil. tical with the natural series after alkylation zy (6-8). This article reports on the inclusion of this template in a tet- rapeptide analogue together with the conformational analysis of two tetrapeptide analogues obtained (la, b). RESULTS AND DISCUSSION Peptide synthesis Two strategies, depicted in Fig. 1, corresponding to an initial elongation from the N (route A) or C (route B) extremity of the 2-oxopiperazine 3 could be proposed for the synthesis of la-b. Because we had already synthesised 2b (6), we decided to explore route A. Cou- pling, in CH2C12, of the free acid generated by saponifi- cation of the ester function of 2b (1 N NaOH) with LeuOMe furnished the expected tetrapeptide analogue in poor yield (30%) using the isobutyl chloroformate (IBCF) method. Although numerous effective coupling reagents (benzotriazol- 1 -yloxytris [dimethylaminol- phosphinium hexafluorophosphate [BOP], 0-[7- azabenzotriazol- 1 -yl]-N,N,N ',N '-tetramethyluronium hexafluorophosphate [HATU], 0-benzotriazol-1 -yl- N,N,N',N'-tetramethyluronium hexafluorophosphate [HBTU]) have recently become available, before inves- tigating the necessity of using these somewhat expen- sive reagents, we decided to explore the second possible strategy; we attempted to elongate the peptide chain zy from the C-terminal end of 3. Activation of the carboxylate of 3a-b by IBCF and condensation with LeuOMe fur- nished 4a-b in 75-81% yield. After deprotection of 4a-b (HClgas/CHZC12), coupling with Z- Ala was accom- plished in yields similar to those obtained for the syn- thesis of 2a-b (6). This demonstrated that la-b can be obtained easily using the IBCF-activating approach. We 116