Regioselective synthesis of antiparallel loops on a macrocyclic scaffold constrained by oxazoles and thiazoles

An azobenzene container showing a definite folding - synthesis and structural investigation

The combination of photo-switchable units with macrocycles is a very interesting field in supramolecular chemistry. Here, we present the synthesis of a foldable container consisting of two different types of Lissoclinum macrocyclic peptides which are connected via two azobenzene units. The container is controllable by light: irradiation with UV light causes a switching process to the compact cis,cis-isomer, whereas by the use of visible light the stretched trans,trans-isomer is formed. By means of quantum chemical calculations and CD spectroscopy we could show that the trans→cis isomerization is spatially directed; that means that one of the two different macrocycles performs a definite clockwise rotation to the other, caused by irradiation with UV light. For the cis→trans isomerization counterclockwise rotations are found. Furthermore, quantum chemical calculations reveal that the energy of the cis,cis-isomer is only slightly higher than the energy of the cis,trans-isomer. This effect can be explained by the high dispersion energy in the compact cis,cis-isomer.

Total synthesis, structure, and oral absorption of a thiazole cyclic peptide, sanguinamide A

The first total synthesis and three-dimensional solution structure are reported for sanguinamide A, a thiazole-containing cyclic peptide from the sea slug H. sanguineus. Solution phase fragment synthesis, solid phase fragment assembly, and solution macrocyclization were combined to give (1) in 10% yield. Spectral properties were identical for the natural product, requiring revision of its structure from (2) to (1). Intramolecular transannular hydrogen bonds help to bury polar atoms, which enables oral absorption from the gut.

Cyclooligomerization of a helix-bearing template into macrocycles bearing multiple helices

Cyclooligomerization was investigated for separating and spatially arranging helical peptides as discontinuous surfaces. Tetrapeptide H-[Ile-Ser-Lys(Ox)]-OH, containing a turn-inducing oxazole constraint, was connected through its lysine side chain via a beta-alanine linker to the C-terminus of a two-turn helical nonapeptide Ac-(cyclo-4,8)-LRL [KARAD](Aib). The resulting helix-appended template was self-condensed and cyclized to a library of macrocycles ( n = 2-6) containing multiple (2-6) helices. An NMR structure shows retention of alpha helicity in the cyclotrimer ( n = 3).

Marine metabolites: metal binding and metal complexes of azole-based cyclic peptides of marine origin

Azole-based cyclic peptides found in ascidians ("sea squirts") of the genus Lissoclinum have a high propensity to chelate metal ions. This Highlight summarises the current evidence for marine cyclic peptide-metal congruence, and the structural and stereochemical features in cyclic peptides which seem necessary to facilitate metal complexation. The biological relevance of the metal ions in these associations, including their possible role in the assembly of cyclic peptides in the marine milieu, is also briefly considered. Finally, the synthesis of natural, and some novel non-natural, azole-based cyclic peptides from the cyclooligomerisation and assembly of azole-based amino acids, including in the presence of metal ions, is presented.

Organolithium-Mediated Diversification of Peptide Thiazoles

[Reaction: see text] We report a one-step, racemization-free method for the diversification of peptide thiazoles via direct lithiation of the thiazole ring. The method is compatible with N-Boc, N-trityl, carboxylic ester, and carboxamide protecting groups and has been used to directly functionalize the thiazole ring of cyclopeptide natural products.