Design and synthesis of cyclic depsipeptide containing triazole ring (#23)
Cyclic peptides comprise a potent class of naturally occurring bioactive molecules. The cyclic peptides are a large and diverse class of natural products, whose backbones may contain both proteinogenic and nonproteinogenic amino acids, and whose equally diverse range of biological activities makes them and their analogues important targets for organic synthesis.1 Considering the vast application of cyclic peptides and its analogues in synthetic as well as in medicinal chemistry; cyclic peptide remains an important goal for both academic and industrial laboratories. Head-to-tail or cyclization via peptide bond formation poses significant challenges; like oligomerization a possible side reaction, but ring strain in the transition state leading to the cyclic product may prohibit cyclization together.2 In spite of this, another analogue of cyclic peptide i.e. triazole containing cyclic peptide came into existence. These triazoles have atom placement and electronic properties similar to those of a peptide bond,3 and are accessible in one step via CuI-catalyzed alkyne-azide cyclo addition.4 Further, the increased ring size of triazole analogue and the apparent “ring contraction” mechanism of CuI-catalyzed alkyne-azide cycloaddition5 may help to promote cyclization.6 Herein, we report the synthesis of cyclic depsipeptide containing triazole (CDPT) ring by the utility of click chemistry. 1,3-dipolar cyclization via CuI-catalyzed alkyne-azide coupling affords the CDPT in 60-70% yield.7 The designed CDPT is a hybrid of sansalvamide A8 and the triazole analogue of natural cyclic peptide.7 To the best of our knowledge no synthesis of such class of cyclic peptide has been reported; where amide, ester and triazole were placed in one ring. We have synthesized a library of CDPT and the structural evidences were obtained from IR, ESI-MS, NMR, COSY, DEPT, TOCSY and HSQC and X-ray crystallography. We also anticipate that the proposed protocol can be useful for the synthesis of various diversified cyclic peptide.
SOLID PHASE 2013*