Cleavages in stable tetrahedral intermediates' bonds and the role of water

Abstract

Using two‐dimensional energy diagrams, results on the formation and cleavage of detectable tetrahedral intermediates are used to predict the position of transition states involved in the cleavages of unstable and stable tetrahedral intermediates (STI). A late transition state (anti‐Hammond) for the former and synchronous cleavage for the latter are predicted and supported with experimental results. For instance, in the case of STI, intrinsic leaving abilities in the order CO > CNHCO ≫ CNH were experimentally observed, confirming the departure of these groups without full heteroatom protonation. The STI designed herein mimics serine proteases enzymes due to the thermoneutral relationship between the enzyme substrate (ES) and enzyme‐acyl tetrahedral intermediate (EATI) analogs. In the STI models, a dramatic change in pKa is observed when changing the lactam ring size due to the strain in the 3‐fused‐ring system of the STI‐H2O complex formed and exo‐anomeric effect. A plot of ΔSo (STI formation) versus pKa provides evidence for the STI–water interaction. Since only one water molecule is required to produce the effect it is also detected when using ‘pure’ CDCl3 as the solvent and therefore may also play a role in the active sites of serine proteases. Copyright © 2010 John Wiley & Sons, Ltd.

Cyclols Revisited: Facile Synthesis of Medium‐Sized Cyclic Peptides

“…However, for compounds 2 e , f , allowing the reactions to proceed at room temperature resulted in clean and complete conversion to the desired medium‐sized rings in just under 2 h. Neither the ring size nor the aromatic substituents adversely affected the reaction outcome, which showed promise in extending this methodology to more complex systems. Having secured a reliable route to medium‐sized rings from simple lactams, we investigated the possibility of using an α‐amino imide fragment . Using glycine‐derived α‐amino imide 3 as our model substrate we calculated the probability of forming a 9‐membered ring however, computational analysis predicted that the cyclol form of this system would be thermodynamically favored over the ring expanded product (see Supporting Information).…”