P-nitrophenylcholine carbonate

When the guest used is p-nitrophenylcholine carbonate (PNPCC) the Lewis acid zinc(n) activates the well-positioned carbonyl group in the P PCC Zn-cavitand towards reactions with external nucleophiles. The energy minimized structure of the PNPCC Zn-cavitand complex shows that cation-n interactions and C —O -Zn coordination bond occurs simultaneously. 

FIGURE 7 Left Top view of the CAChe-minimized structure of Rebek s Zn-cavitand and p-nitrophenylcholine carbonate (PNPCC) (27). The cavitand is shown in CPK representation and the PNPCC (yellow) in stick. The C=0 group is shown in ball and stick representation. Note the proximity between the oxygen of the C=0 group and the Zn center. Right Molecular structures of the species involved in the complex. (See Color Insert in the back of this book.)... 

The combination of a calixarene subunit and the bicyclic guanidinium in macrocycle 15 demonstrates affinities for dicatonal-L-x-phosphatidylcholine (Kn = 7.5 x 10" M ) and acetylcholine (ATa = 7.3 x 10 M ) in chloroform. These affinities are reduced in CDCI3/ CD3OD (99 1). Kinetic studies were performed on the hydrolysis of p-nitrophenylcholine carbonate in the presence of 15, which showed a 149-fold rate enhancement over methanolysis. The guanidinium is thought to piay a role in the stabilization of the BAc2-type transition state. This provides an example of an artificial acetylcholinesterase. 

A remarkable example of the use of the calixarene cavity for substrate recognition is provided by calix[6]arenes 42 developed by de Mendoza et al. [37] These ditopic receptors, featuring a guanidinium unit at the catalytic site, have been reported as effective turnover catalysts of the basic methanolysis of p-nitrophenylcholine carbonate (PNPCC), (Eq. 26.7). 

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