DOI: 10.1002/chem.200600815 Total Solid-Phase Synthesis of the Azathiocoraline Class of Symmetric Bicyclic Peptides Nfflria Bayó-Puxan, [a] Ariadna Fernµndez, [a] Judit Tulla-Puche, [a] Estela Riego, [a] Carmen Cuevas, [b] Mercedes lvarez, [a, c] and Fernando Albericio* [a, d] Introduction Thiocoraline is a potent new antitumor agent that has been isolated from the marine organism Micromonospora sp. [1] It exhibits a complex depsipeptide structure that shows biolog- ical activity of interest, and the research efforts of many sci- entific groups and pharmaceutical companies have conse- quently focused on this peptide in order to improve knowl- edge about its mode of action and to study its potential ap- plication in cancer therapy. Thiocoraline shares several common motifs with a family of antitumor peptide antibio- ACHTUNGTRENNUNGtics, which includes BE-22179, triostin A and echinomycin. [2] This group of peptides is characterized by: a) bicyclic struc- tures, b) C 2 symmetry, c) DNA intercalation chromophore moieties, d) ester or thioester linkages at the terminal part of the peptide chains, [3] e) disulfide or analogous bridges in the middle of the peptide chain, f) the presence of several N-methyl amino acids, and g) non-natural amino acids of d configuration. The N-terminal amino function in thiocora- line is thus capped with 3-hydroxyquinaldic acid, which acts as an intercalating chromophore group, whilst the two pep- tide chains are bridged by thioester and disulfide linkages from Cys residues, these being the components that afford the disulfide N-methylated bridge and d configuration, as well as the two Cys(Me) residues. All these characteristics confer on this family of peptides the capacity to bind with DNA by bisintercalation and thus to alter the vital cell cycle. Thiocoraline inhibits DNA elongation by DNA poly- merase a at a concentration that blocks cell cycle progres- sion and clonogenicity. [4] Peptide synthesis now faces the challenge of obtaining this family of molecules: a task that is crucial if they are to be used for therapeutic applications. Here we discuss two solid-phase syntheses of aza ana- logues of thiocoraline (Scheme 1), in which the thioester bridges have been replaced by amide ones with the aim of improving pharmacokinetics. Azathiocoraline should be re- sistant to enzyme degradation and more soluble than the natural peptide. Abstract: Thiocoraline is a potent anti- tumor agent isolated from the marine organism Micromonospora sp. This symmetric bicyclic depsipeptide binds the minor groove of DNA. Here we report two solid-phase strategies for the syntheses of azathiocoraline and its analogues. The thioester linkage was replaced by an amide bond to improve the compound)s pharmacokinetic prop- erties. The first strategy is based on a convergent (4+4) approach, whilst the second is a stepwise synthesis, cycliza- tions in both approaches occurring on the solid support. These two strategies were designed to overcome problems caused by the presence of consecutive noncommercial N-methyl amino acids, to avoid epimerization during cycliza- tion and/or fragment condensation, and to form the disulfide bridge under solid-phase conditions. The heterocyclic moiety was added in the last step of the synthesis to assist the preparation of libraries of new compounds with po- tential therapeutic applications. Keywords: bisintercalators · cou- pling reagents · cyclic peptides · natural products · on-resin cycliza- tion · peptides [a] N. Bayó-Puxan, A. Fernµndez, Dr. J. Tulla-Puche, Dr. E. Riego, Prof. Dr. M. lvarez, Prof. Dr. F. Albericio Barcelona Biomedical Research Institute, University of Barcelona Barcelona Science Park (PCB), 08028 Barcelona (Spain) Fax: (+ 34)934-037-126 E-mail: albericio@pcb.ub.es [b] Dr. C. Cuevas PharmaMar, S.A., 28770-Colmenar Viejo (Spain) [c] Prof. Dr. M. lvarez Department of Pharmacology and Therapeutic Chemistry University of Barcelona, 08028 Barcelona (Spain) [d] Prof. Dr. F. Albericio Department of Organic Chemistry, University of Barcelona 08028 Barcelona (Spain) Fax: (+ 34)934-037-126 Chem. Eur. J. 2006, 12, 9001–9009 # 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 9001 FULL PAPER