Effect of Protecting Groups

In a 4,6-benzylidene group, the conformation about C5-C6 is constrained to tg, so that the positive end of the C6-06 dipole points towards the developing positive charge on 05. Protection by cyclic acetals which enforce this conformation is thus strongly deactivating. 

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JP Tam, MW Rieman, RB Merrifield. Mechanisms of aspartimide formation the effects of protecting groups, acid, base, temperature and time. Pept Res 1, 6, 1988. 

The nucleophilicity of the N-7 atom of guanosine, and some of its O - and N -substituted derivatives, has been studied by following the N chemical shifts of the protonated species.The effect of protecting groups of the nucleobases during... 

Scheme 6.3 Tuning the reactivity of glycosylations electronic effect of protecting groups.

Scheme 2 Effect of protecting groups on oxidative hydrolysis of NPGs...

Although the effect of protecting groups on reactivity had been noted [20], it was Fraser-Reid who described, in 1988, a new manner by which the differential... 

Because they are readily available from natural sources in enantiomerically pure form, carbohydrates are very useful starting materials for the synthesis of other enantiomerically pure substances. However, the high number of similar functional groups present in the carbohydrates requires versatile techniques for protection and deprotection. Show how appropriate manipulation of protecting groups and other selective reactions could be employed to effect the following transformations. 

Indicate one or more satisfactory oxidants for effecting the following transformations. Each molecule poses issues of selectivity or the need to preserve a sensitive functional group. Select oxidants that can avoid the installation of protecting groups. In most cases, a one-pot reaction is possible, and in no case is a sequence of more than three steps required. Explain the reason for your choice of reagent(s). 

While the distinct amino acid residues have mostly only a modulating effect (see Table 1.9) (e.g., FMOC-protected amino acids), the type of protection group or derivative formed decides on the molecular and chiral recognition mechanism and hence on the obtained elution order as well as the level of enantiomer recognition (i.e., magnitudes of a-values) that can be afforded. From a practical point of view, we may distinguish between two groups of IV-derivatives ... 

The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their veiy high reactivity. Substitution tends to occur at ortho- and para-positions. If we have to prepare monosubstituted aniline derivative, how can the activating effect of -NH group be controlled This can be done by protecting the -NH group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine. 

The best yields are obtained with phenols bearing electron-withdrawing substituents such as nitro, chloro, trifluoromethyl, or ammonium. The deactivating effect of these groups protects the aromatic nucleus from attack by any intermediates.227... 

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