Electron-withdrawing protecting groups

A chiral auxiliary-based approach has been developed for the preparation of chiral, non-racemic cyclopropyhnethylamines that are not protected with electron-withdrawing groups. The cyclopropanation of ally he tertiary amines bearing a -hydroxide occurred... 

To avoid any loss of benzyl ester protection during acidolytic removal of the benzyloxy-carbonyl and ferf-butoxycarbonyl groups, electron-withdrawing substituents were used to destabilize the intermediate benzyl cation and thus to increase the acid stability. In addition to the very useful 4-nitrobenzyl esters (vide infra), the picolyl ester (see Section 2.2.1.2.2.3) as well as halo-P l or cyano-P°°l substituted benzyl esters have been reported, the latter being rarely used for a-carboxy protection. Conversely, an increase in sensitivity toward acids can be achieved by introduction of electron-releasing substituents, such as methoxy or methyl groups. Addition of scavengers to quench intermediate carbocations and to prevent electrophilic substitutions at sensitive amino acid side chains is beneficial in the deprotection of such esters. 

Pyrrole Carboxylic Acids and Esters. The acids are considerably less stable than benzoic acid and often decarboxylate readily on heating. However, electron-withdrawing substituents tend to stabilize them toward decarboxylation. The pyrrole esters are important synthetically because they stabilize the ring and may also act as protecting groups. Thus, the esters can be utilized synthetically and then hydrolyzed to the acid, which can be decarboxylated by heating. Often P-esters are hydrolyzed more easily than the a-esters. 

In carbohydrates in which benzyl groups are used extensively for protection, the stability of the benzyl groups toward electrophilic reagents is increased by the presence of electron-withdrawing groups in the ring." ... 

Several methods for synthesizing IV-protected (usually with electron-withdrawing groups) 2-and 3-haloindoles have been developed and the resulting haloindoles are much less prone to decomposition than the unsubstituted compounds. Bromination of A/-(phenylsulfonyl)indole (3), which is readily available via lithiation [9, 10] or phase-transfer chemistry [11, 12], affords 3-bromo-l-(phenylsulfonyl)indole (4) in nearly quantitative yield [12],... 

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