Endo-binding

The system has been further elaborated preparing rosettes bearing pyridyl residues attached to the external periphery of the assembly, able to make hydrogen bonding with dicarboxylic acids. This class of rosettes enabled exclusive endo binding of anthraquinone guests allocated within the cavity and exo binding of up to three... 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

In his original paper, Lehn recognized the interesting possibility of exo-endo isomerism. This kind of isomerism is shown in Eq. (8.2) and is similar to that referred to by Simmons and Park as in-out isomerism. Lehn and his coworkers recognized from the very beginning that this new class of molecules had the potential for binding metal cations of many types. Since the molecules were designed to have molec-... 

The chiral catalyst 142 achieves selectivities through a double effect of intramolecular hydrogen binding interaction and attractive tt-tt donor-acceptor interactions in the transition state by a hydroxy aromatic group [88]. The exceptional results of some Diels-Alder reactions of cyclopentadiene with substituted acroleins catalyzed by (R)-142 are reported in Table 4.21. High enantio- and exo selectivity were always obtained. The coordination of a proton to the 2-hydroxyphenyl group with an oxygen of the adjacent B-0 bond in the nonhelical transition state should play an important role both in the exo-endo approach and in the si-re face differentiation of dienophile. 

Ghosh et al. [70] reviewed a few years ago the utihty of C2-symmetric chiral bis(oxazoline)-metal complexes for catalytic asymmetric synthesis, and they reserved an important place for Diels-Alder and related transformations. Bis(oxazoline) copper(II)triflate derivatives have been indeed described by Evans et al. as effective catalysts for the asymmetric Diels-Alder reaction [71]. The bis(oxazoline) Ugand 54 allowed the Diels-Alder transformation of two-point binding N-acylimide dienophiles with good yields, good diastereos-electivities (in favor of the endo diastereoisomer) and excellent ee values (up to 99%) [72]. These substrates represent the standard test for new catalysts development. To widen the use of Lewis acidic chiral Cu(ll) complexes, Evans et al. prepared and tested bis(oxazoHnyl)pyridine (PyBOx, structure 55, Scheme 26) as ligand [73]. 

Fig. 2 Inrnume complex binding enhancing activity of Carbohydrate fragments obtained by endo-polygalacturonase digestion...

By XPS spectra, Endo et al. [96] confirmed that formation of binary structure prevented Pd atoms from oxidation in the AuPd and PtPd bimetallic nanoparticles which exhibited higher catal5hic activity than monometallic ones. Wang et al. [112]. characterized PtCu bimetallic alloy nanoparticles Ijy XPS. XPS revealed that both elements in the nanoparticles are in zero-valence and possess the characteristic metallic binding energy. 

Berthod, H., and A. Pullman. 1978. Electrostatic Molecular Potential Hydration and Cation Binding Scheme of C3-Endo-gg-Ribose. Theor. Chim. Acta 47, 59. 

Cytisine is a tricyclic quinolizidine alkaloid that binds with high affinity and specificity to nicotinic acetylcholine receptors. In principle, this compound can exist in several conformations, but semi-empirical calculations at the AM 1 and PM3 levels have shown that stmctures 19 and 20 are more stable than other possible conformers by more than 50 kcalmol-1. Both structures differ by 3.7 kcalmol 1 at the AMI level and 2.0 kcalmol 1 at the PM3 level, although this difference is much smaller when ab initio calculations are employed <2001PJC1483>. This conclusion is in agreement with infrared (IR) studies and with H NMR data obtained in CDCI3 solution, which are compatible with an exo-endo equilibrium < 1987JP21159>, although in the solid state cytisine has an exo NH proton (stmcture 19) (see Section 12.01.3.4.2). 

The initial steps of the intrinsic pathway are somewhat more complicated. This system requires the presence of clotting factors VIII, IX, XI and XII, all of which, except for factor VIII, are endo-acting proteases. As in the case of the extrinsic pathway, the intrinsic pathway is triggered upon exposure of the clotting factors to proteins present on the surface of body tissue exposed by vascular injury. These protein binding/activation sites probably include collagen. 

Fig. 6.— Schematic Illustration of Two Possibilities for Enzyme-Substrate Complexes of A. niger Endo-D-galacturonanase.133 B, and B2, binding sites A, active groups of endo-D-galacturonanase.]...

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