Protective groups, removal selective

The benzyl—nitrogen bond is not so easily cleaved as the benzyl—oxygen bond, unless the O-benzyl group is sterically hindered. This difference in activity allows the selective removal of the O-benzyl function in a molecule containing both N-benzyl and O-benzyl protecting groups. The selectivity can be reversed if the amine is protected by the Cbz group. If a small amount of amine (e g., butylamine) is added, then the selective removal of N-benzyl amines can be achieved.292... 

First, the protecting group is selectively removed from the primary Solution... 

N-l acetyl derivative. A much more powerful synthesis of the tert-butylamides was the Ritter reaction of the corresponding nitrile, which allowed direct production of N-l-acylated tetrahydropyrazines [18]. These in turn could be selectively further functionalized at N-4 so that selective protective group removal was possible, giving access to the final N-4 Boc substitution pattern [2], Some examples of the results obtained in the original screening are given in Tab. 7. 

Deprotonation of 141 with t-Buli in the presence of HMPA [99] followed by the addition of epoxide 137 (Scheme 25) afforded the coupling product which after deprotection of the thioketal (NCS, AgNOj, 2,4,6-collidine) [100] produced ketone 142. A selective reduction of ketone 142 to the anti-diol (NaBH(OAc)3, MeCN/AcOH) [51, 101], removal of the protecting O-silyl protecting group and selective MnC>2 oxidation furnish (-)-pironetin (Scheme 27). 

N-Debenzylation of ammonium salts, quaternary 14,404 O-Debenzylation 24, 723 -, selective 24, 8, 17, 43 Deblocking s. Protective groups, removal... 

In a competitive hydrogenolysis reaction, the benzyl protecting group was selectively removed leaving the dimethylbenzyl ether intact (Scheme 95). The dimethylbenzyl ether could be removed after 15 h under 4 atm of hydrogen. 

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