Allyl as a Protecting Group and its Deprotection

Based on facile formation of 7i-allylpalladium intermediates from various allylic compounds, allyl groups can be used for the protection of carboxylic acids, amines and alcohols. Deprotection can be achieved by two methods using Pd(0) catalysts [128,138]. In one method, the allyl group can be removed as propylene by Pd- 

Another method for deallylation is the Pd-catalyzed transfer of the allyl group to reactive nucleophiles. Sodium 2-ethylhexanoate [141], morpholine [142], dimedone [143] and AA-dimethylbarbituric acid [144] are used as allyl scavengers. It is claimed that benzenesulfinic acid 331 or sodium toluenesulfinate are the best allyl scavengers [145]. 

Allyl phosphates 332 are deprotectd using HCO2H amine [146] or HSnBu3 [147], The method is applied to the protection and deprotection of the amino and phosphate groups in nucleotides synthesized in the solid phase [148], 

Alcohols are protected as allyl ethers, which are difficult to cleave with the Pd catalyst and deprotected by other methods [149]. Alcohols are conveniently converted to allyl carbonates 334 by treatment with allyl chloroformate (333). The allyl carbonates are deprotected using HCO2H [150], and HSnBu3 [151]. This method is called the AOC (allyloxycarbonyl) method. Phenols are protected as allyl phenyl ethers, which can be cleaved with HSnBu3 [152]. 

One protecting method for ketones and aldehydes is the formation of oximes, but sometimes further protection of the oximes is required. For this purpose, the oximes can be protected as the allyl ethers 335, which are deprotected by the Pd-catalysed hydrogenolysis with HCO2H Et3N in boiling dioxane under mild conditions without attacking the acetal group in 335 [153], 

---