Side chain-to-side chain cyclizations represent a strategy to select a family of bioactive conformations by reducing the entropy and enhancing the stabilization of functional ligand-induced receptor conformations. This structural manipulation contributes to increased target specificity, enhanced biological potency, improved pharmacokinetic properties, increased functional potency, and lowered metabolic susceptibility. The CuI-catalyzed azide–alkyne 1,3-dipolar Huisgen’s cycloaddition, the prototypic click reaction, presents a promising opportunity to develop a new paradigm for an orthogonal bioorganic and intramolecular side chain-to-side chain cyclization. In fact, the proteolytic stable 1,4- or 4,1-disubstituted [1,2,3]triazolyl moiety is isosteric with the peptide bond and can function as a surrogate of the classical side chain-to-side chain lactam forming bridge. Herein we report the design, synthesis, conformational analysis, and functional biological activity of a series of i-to-i+5 1,4- and 4,1-disubstituted [1,2,3]triazole-bridged cyclopeptides derived from MT-II, the homodetic Asp5 to Lys10 side chain-to-side chain bridged heptapeptide, an extensively studied agonist of melanocortin receptors.

I n d i u m -P r o m o t e d O r g a n i c R e a c t i o n sAbstract: This review deals with organic reactions which are promoted by indium metal or indium salts, with a focus on recent advances in stoichiometric and catalytic pathways. Applications to transmetalations, cross-coupling reactions and carboindation, in which an organoindium species may be postulated, are highlighted, as well as the reactions in which a radical is the key intermediate. Special attention is placed on the role of indium metal as a reducer, and on the Lewis acidity of indium salts in catalytic processes.

Carbonated analogues of ribavirin were synthesized from ethyl C-ribosylpropiolate obtained by an alkynylation reaction mediated by indium(0). The C-ribosides were then engaged in a Huisgen 1,3-dipolar cycloaddition reaction under a micellar catalysis. In these conditions, formation of 1,2,3-triazoles with control of the regioselectivity was observed. The regiochemistry of the adducts was determined by HMBC 2D-NMR analysis.