KAHA ligation

It is important to cite several other recently developed methods for peptide ligation, which include the traceless Staudinger ligations of Bertozzi [62], Raines [63], and their co-workers, the decarboxylative amide ligation (KAHA ligation) developed by Bode et al. [64], and the Ser/Thr ligation of Li et al. [65, 66]. While these methods can be considered complementary to NCL, they have various constraints which limit their general utility. 

Bode et al. devised a unique chemoselective amide ligation by the decarboxylative condensations of a-ketoacids and A -alkylhydroxylamines (KAHA ligation) (Scheme 12) [64, 150-154]. This process requires neither coupling reagents nor catalysts, produces only water and carbon dioxide as by-products, and tolerates unprotected amino acid functional groups. It is also completely orthogonal to NCL and theoretically can be utilized at any junction. However, the limited access to A-alkylhydroxylamines and a-ketoacids restricts broader application of this chemistry. 

The Next Steps. .. Protein Synthesis by KAHA Ligation. 30... 

Scheme 4 Three key precedents for the design and invention of the KAHA ligation...

Scheme 5 Our first attempt at the KAHA ligation (December 2003)...

Scheme 6 Conditions for the formation of simple amides by KAHA ligation...

Scheme 8 An early demonstration that enantiomerically enriched a-ketoacids do not epimerize under the conditions for the KAHA ligation. Actually synthesizing enantiopure peptide...

When we first disclosed the KAHA ligation, the general sentiment was that making the a-ketoacids would be the major problem. In contrast, most chemists did not feel that the peptide hydroxylamines would present a serious obstacle. This intuition was supported by the literature. There were only a few scattered reports of peptide ot-ketoacids but a respectable body of literature, including a Chemical Reviews article, on A-hydroxyamino acids and peptides [26]. 

Scheme 16 Synthesis of human GLP-1 by KARA ligation of two unprotected peptide fragments. This work demonstrated that the KAHA ligation has the potential to serve as a general peptide ligation strategy...

Scheme 18 The two prototypical types of the KAHA ligation. Type I ligations proceed with 0-substituted hydroxylamines and prefer polar aprotic solvents. Type II ligations require 0-substituted hydroxylamines and generally prefer water as the reaction solvent...

For reasons that we did not understand at the time, the ligation of a-ketoacids and 0-unsubstituted hydroxylamines, which we now call type 1 ligations, occurred best in polar aprotic solvents such as DMSO or DMF (Scheme 18). Water was detrimental to the reaction rates and tended to accelerate the decomposition of the peptide hydroxylamine. In contrast, we had identified a number of other variants of the KAHA ligation with 0-substituted hydroxylamines that occurred preferentially in water (type II ligations). Unfortunately, the most effective variants, 0-Bz hydroxylamines, cannot be used for a-peptides due to facile elimination [36]. 

Scheme 19 Mechanistic investigations of the KAHA ligation 0-isotope labels and conversion of oxaziridine acids to amides...

Scheme 20 The three-ring circus mechanism for the type I KAHA ligation. We were very surprised to find that the oxaziridine is involved in the amide formation...

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