Acyl hydrazone exchange

Carbonic anhydrase [17] Acyl hydrazone exchange <300 ESI-FTICR-MS... 

AChE acyl hydrazone exchange 66 bis-pyridinium structure [24]... 

HPr kinase/phosphatase acyl hydrazone exchange 440 2-aminobenzimidazole m... 

Acyl hydrazone exchange has also been probed as reversible chemistry for the generation of oligotopic and directional carbohydrate DCLs of larger size [45]. 

One can in principle combine different exchange reactions in the same system in order to further increase the structural diversity accessed by the library. However, as this compounds the problem of selectivity (i.e., one now has two or more reactions that must exclusively involve one pair of functional groups), there are very few examples thus far of the practical implementation of this concept. An early, highly intriguing example was described by Lehn and coworkers in 2001 [68]. In this system, imine exchange (acyl hydrazone formation) and reversible metal coordination were employed in library generation. 

The small-molecule-based machine conceived by von Delius, Geertsema, and Leigh [45] is a linear (for reviews, see [46], [100]) motor based on dynamic covalent chemistry [19-24] (forming, breaking, and reforming of dynamic covalent bonds with relatively fast equilibration in response to stimuli), namely on acyl-hydrazone and disulfide exchanges. The motor consists of a track that has four functional groups disposed alternately aldehyde-thiol-aldehyde-thiol which are the positions 1,2, 3, and 4 of the track, a walker NH2-NH-CO-(CH2)5-SH which has the feet A (hydrazide or acyl-hydrazine) and B (thiol), and a placeholder with a foot C of type thiol (Fig. 10). 

This molecular motor is an example that illustrates how the changes in configuration can have an influence on the exchanges of dynamic covalent bonds like -C=N- from acyl hydrazones and -S-S- (disulfides). 

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