Strain-promoted alkyne azide cycloaddition for the functionalization of poly(amide)-based dendrons and dendrimers

Strain-promoted alkyne-azide cycloadditions (SPAAC) reveal new features of glycoconjugate biosynthesis

We have shown that 4-dibenzocyclooctynol (DIBO), which can easily be obtained by a streamlined synthesis approach, reacts exceptionally fast in the absence of a Cu(I) catalyst with azido-containing compounds to give stable triazoles. Chemical modifications of DIBO, such as oxidation of the alcohol to a ketone, increased the rate of strain promoted azide-alkyne cycloadditions (SPAAC). Installment of a ketone or oxime in the cyclooctyne ring resulted in fluorescent active compounds whereas this property was absent in the corresponding cycloaddition adducts; this provides the first example of a metal-free alkyne-azide fluoro-switch click reaction. The alcohol or ketone functions of the cyclooctynes offer a chemical handle to install a variety of different tags, and thereby facilitate biological studies. It was found that DIBO modified with biotin combined with metabolic labeling with an azido-containing monosaccharide can determine relative quantities of sialic acid of living cells that have defects in glycosylation (Lec CHO cells). A combined use of metabolic labeling/SPAAC and lectin staining of cells that have defects in the conserved oligomeric Golgi (COG) complex revealed that such defects have a greater impact on O-glycan sialylation than galactosylation, whereas sialylation and galactosylation of N-glycans was similarly impacted. These results highlight the fact that the fidelity of Golgi trafficking is a critical parameter for the types of oligosaccharides being biosynthesized by a cell. Furthermore, by modulating the quantity of biosynthesized sugar nucleotide, cells might have a means to selectively alter specific glycan structures of glycoproteins.

Copper-free click conjugation of methotrexate to a PAMAM dendrimer platform

The synthesis of a generation 5 (G5) poly(amidoamine) (PAMAM) dendrimer platform having cyclooctyne ligands that were subsequently be used for a copper-free Huisgen 1,3-dipolar cycloaddition (click reaction) with azido modified methotrexate is described. The G5 PAMAM dendrimer was first partially (70%) acetylated and then coupled with 20 cyclooctyne ligands through amide bonds. The remaining primary amine groups on the dendrimer surface were neutralized by acetylation. The platform was then "clicked" with different numbers (5, 10, and 17) of γ-azido functionalized methotrexate. The copper-free click reactions were stoichiometric with excellent yields.

Metal-free sequential [3 + 2]-dipolar cycloadditions using cyclooctynes and 1,3-dipoles of different reactivity

Although metal-free cycloadditions of cyclooctynes and azides to give stable 1,2,3-triazoles have found wide utility in chemical biology and material sciences, there is an urgent need for faster and more versatile bioorthogonal reactions. We have found that nitrile oxides and diazocarbonyl derivatives undergo facile 1,3-dipolar cycloadditions with cyclooctynes. Cycloadditions with diazocarbonyl derivatives exhibited similar kinetics as compared to azides, whereas the reaction rates of cycloadditions with nitrile oxides were much faster. Nitrile oxides could conveniently be prepared by direct oxidation of the corresponding oximes with BAIB, and these conditions made it possible to perform oxime formation, oxidation, and cycloaddition as a one-pot procedure. The methodology was employed to functionalize the anomeric center of carbohydrates with various tags. Furthermore, oximes and azides provide an orthogonal pair of functional groups for sequential metal-free click reactions, and this feature makes it possible to multifunctionalize biomolecules and materials by a simple synthetic procedure that does not require toxic metal catalysts.