Metal-catalysed azidation of tertiary C–H bonds suitable for late-stage functionalization

Some enzymes are able to selectively oxidize unactivated aliphatic C-H bonds to form alcohols; however biological systems do not possess enzymes that are able to catalyze the analogous amination of a C-H bond.(1,2) The absence of such chemistry is limiting because nitrogen-containing groups are found in therapeutic agents and clinically useful natural products. In one prominent example, the conversion of the ketone of erythromycin to the –N(Me)CH(2)– group in azithromycin leads to a compound that can be dosed once daily with a shorter length of treatment.(3,4) For such reasons, synthetic chemists are very interested in identifying catalysts that can directly convert C-H bonds to C-N bonds. Most currently used catalysts for C-H bond amination are ill suited for the functionalization of complex molecules, because they require excess substrate or directing groups, harsh reaction conditions, weak or acidic C-H bonds, or reagents containing specialized groups on the nitrogen atom.(5-14) Among C-H bond amination reactions, those forming a carbon-nitrogen bond at a tertiary alkyl group would be particularly valuable, because this linkage is difficult to generate enzymatically from ketone or alcohol precursors.(15) In this manuscript, we report a mild, selective, iron-catalyzed azidation of tertiary C-H bonds with substrate as limiting reagent. The reaction tolerates aqueous environments and is suitable for “late-stage” functionalization of complex structures. Moreover, this azidation creates the ability to install a range of nitrogen functional groups, including those from bio-orthogonal Huisgen “click” cycloadditions and the Staudinger ligation.(16-19) For these reasons, we anticipate this methodology will create opportunities to easily modify natural products, their precursors, and their derivatives to analogs that contain distinct polarity and charge from nitrogen-containing groups. It could also be used to help identify targets of biologically active molecules by creating a point of attachment, for example to fluorescent tags or ‘handles’ for affinity chromatography, directly onto complex molecular structures

Site-selective glycosylation of proteins: creating synthetic glycoproteins.

In higher organisms, the functions of many proteins are modulated by post-translational modifications (PTMs). Glycosylation is by far the most diverse of the PTM processes. Natural protein production methods typically produce PTM or glycoform mixtures within which function is difficult to dissect or control. Chemical tagging methods allow the precise attachment of multiple glycosylation modifications to bacterially expressed (bare) protein scaffolds, allowing reconstitution of functionally effective mimics of glycoproteins in higher organisms. In this way combining chemical control of PTM with readily available protein scaffolds provides a systematic platform for creating probes of protein-PTM interactions. This protocol describes the modification of Cys residues in proteins using glycomethanethiosulfonates and glycoselenenylsulfides and the modification of azidohomoalanine residues, introduced by Met replacement using auxotrophic Met(-) Escherichia coli strains, with glycoalkynes and the combination of these techniques for the creation of dual-tagged proteins. Each glycosylation procedure outlined in this protocol can be achieved in half a day