Multiple neighboring groups

Simultaneous Participation of Multiple Neighboring Groups. The strength of a hypervalent 3c-4e bond increases with electronegativity of the apical ligands. In a sulfuranyl radical such as... 

Based on the efficient multiple carbohydrate esterifications and Lattrell-Dax carbohydrate epimerization, novel and convenient double parallel- and double serial inversion methods have been developed, by which methyl p-D-mannosides and methyl P-D-talosides have been efficiently synthesized in very high yields at very mild conditions in few steps. The results also indicate that an ester group can, either in parallel or serially, induce its two neighboring groups in the epimerization reacriom... 

Multiple detection 2+ detectors, proper calibrants No segment-segment interactions, No neighbor-group eflFects Bulk composition and composition distribution. Broad applicability, Intrinsic generation of calibration curve. No additional sample preparation work Statistical copolymers. Densely grafted chains 2, 3, 5, 8... 

The biochemical rationale for incorporating fluorine in the carbohydrate residue is that replacement of a hydroxyl by fluorine would cause only a very minor steric perturbation of the structure or conformation while at the same time would have a profound electronic effect on neighboring groups. The substitution is possible while retaining the capacity of the position as an acceptor in hydrogen bonding. Yet these same attributes make the synthesis of fluorinated carbohydrates difficult. The synthesis of fluorinated carbohydrates offers a particularly fruitful field for the combination of modem chemical and enzymatic synthetic techniques. Total synthesis would be difficult because of the stereochemical control required at the multiple adjacent asymmetric centers of a fluorinated carbohydrate (105). 

It is of interest that these reactions exhibit stronger activation of the substituent at C-5 by a neighboring electron-acceptor group compared with the corresponding ortho-substituted benzene. This is possibly owing either to the effect of the nitrogen hetero atom or to a weaker delocalization of multiple bonds in the heterocyclic nucleus. 

This step is an example of an elimination reaction, a reaction in which two groups or atoms on neighboring carbon atoms are removed from a molecule, leaving a multiple bond (Fig. 18.8). 

Generally these globular dendritic architectures offer several advantages over other kinds of organic polymers, such as the full exposure of the catalytic centers to the environment. In contrast to linear or cross-Hnked polymeric supports, which can partially hide catalytic centers, the functional groups are located on the surface of the dendritic nanoparticle and diffusional Hmitations are less relevant Furthermore the close proximity of the catalytic centers on the surface of the dendritic polymer can enhance the catalytic activity by multiple complexation or even cooperativity. This behavior is described as positive dendritic effect. However, in some cases a negative dendritic effect was observed, which is caused by an undesired interaction or electron transfer between the neighboring catalytic centers on the surface of the dendrimer [70]. 

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