Synthesis of differentially protectedN-acylated reduced pseudodipeptides as building units for backbone cyclic peptides

Abstract

Backbone cyclization has become an important method for generating or stabilizing the bioactive conformation of peptides without affecting the amino acid side-chains. Up to now, backbone cyclic peptides were mostly synthesized with bridges between N-amino- and N-carboxy-functionalized peptide bonds. To study the influence of a more flexible backbone on the biological activity, we have developed a new type of backbone cyclization which is achieved via the N-functionalized moieties of acylated reduced peptide bonds. As described in our previous publications, the formation of N-functionalized dipeptide units facilitates the peptide assembly compared with the incorporation of N-alkyl amino acids. Besides the racemization-free synthesis of Fmoc-protected pseudodipeptide esters with reduced peptide bonds, the new type of backbone modification allows the use of a great variety of omega-amino- and alpha,omega-dicarboxylic acids differing in chain length and chemical properties. Best results for the coupling of the omega-amino- and alpha,omega-dicarboxylic acids to the reduced peptide bond were obtained by the formation of mixed anhydrides with alkyl chloroformates. Whereas the protecting group combination of Z/OBzl in the dipeptide unit and Boc/OtBu for the N-functionalized moiety leads to the formation of 2-ketopiperazine during hydrogenation, the combination of Fmoc/OtBu and Alloc/OAll is very suitable for the synthesis of backbone cyclic peptides on solid support.

Cyclization-activated Prodrugs

“…Intramolecular cyclization has been demonstrated to improve biological properties of bioactive peptides [28][29][30][31][32][33][34][35][36][37][38], in many cases allowing one to improve selectivity for a given receptor and/or metabolic stability [39][40][41]. Cyclic peptides can be divided into homodetic and heterodetic, the first being obtained by formation of an intramolecular peptide (amide) bond, whereas heterodetic refers to all cyclic peptides where the intramolecular bond newly formed is other than an amide (e.g., lactone, ether, thioether and, most commonly, disulphide bridges).…”

Synthesis and Characterization of Octapeptide Somatostatin Analogues with Backbone Cyclization: Comparison of Different Strategies, Biological Activities and Enzymatic Stabilities

“…All materials and solvents were of reagent grade and were used without further purification with the following exceptions: DMF was first dried over molecular sieves and distilled from phthalic anhydride, DCM was stored over molecular sieves. The N-functionalized dipeptide building units were prepared as described elsewhere [10,11].…”

“…These peptides differ in the type of backbone modification containing either an N-alkylated peptide bond or an N-acylated reduced peptide bond [10,11] and due to the different chain length of the N-functionalities of the modified phenylalanines in the ring size and in the direction of the lactam bridge.…”

“…The dipeptide building units were thus prepared carrying the Fmoc-group as the N-terminal protection [10,11].…”

“…A variety of N-functionalized amino acids have been prepared and incorporated in peptide sequences [5 -9]. To circumvent difficult coupling steps during peptide synthesis using the solid phase methodology we have developed a method to introduce different types of N-functionalized dipeptide building units which are prepared in solution before their use in SPPS [10,11]. We were further interested in the application of the method of backbone cyclization to somatostatin analogues to probe our synthetic approach of incorporating preformed di- 1 ) peptide units as well as to produce analogues with bioactivity, receptor subtype selectivity and enzymatic stability.…”