Protecting group selection

Although successfully carried out with model compounds, the cyclopentyl i dene acetal here could not be hydrolyzed selectively in the presence of the other protecting groups. Selective deprotection occurred only upon addition of trinitrotoluene, which us a more electron-deficient aromalic system forms a charge-transfer complex with the electron rich aromalic portion of the PMPh acetal. The charge-transfer complex decreases the electron density of the acetal and thus increases its stability, presumably due to reduced stabilization of cation 38 In this way it proved possible to release the alcohol functions at C-12 and C-13 selectively with acid. 

The NSEC group is stable to glycosylation conditions using glycosyl phosphates, and it is not affected under deprotection conditions that facilitate the removal of Lev, Fmoc, allyl, and PMB groups. For ester-type protecting groups selective deprotection in the presence of NSEC derivatives is not possible. 

One form of protecting group selectivity is selectivity for a single enantiomer of a racemic alcohol. A number of catalytic systems have been developed that give good to excellent results for the selective acylation of a single... 

Ferric chloride Protection of quinones as pivaloyloxymethoxy derivatives Removal of the protective groups Selective O-deacylation Quinones from quinol monoethers... 

Mercuric acetate/formic acid Benzylthiomethyl as S-protective group Selective removal of the protective group... 

Hydrogen chloride (s. a. under H2O2) Dibenzosuberyl as protective group Selective removal... 

Conditions must be chosen to free one protecting group selectively so that the sequence can be repeated. 

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