Deprotection of alcohol

Selective Deprotection of Alcoholic and Phenolic TBDMS Ethers (14)... 

Finally, achiral phosphonium salts have been applied as Lewis acid catalysts in some other reactions. The examples will be listed here but not discussed in more detail. Phosphonium salts have been used as catalysts for the A,A-dimethylation of primary aromatic amines with methyl alkyl carbonates giving the products in good yields [123]. In addition acetonyltriphenylphosphonium bromide has been found to be a catalyst for the cyclotrimerization of aldehydes [124] and for the protection/ deprotection of alcohols with alkyl vinyl ethers [125, 126]. Since the pK of the salt is 6.6 [127-130], the authors proposed that, next to the activation of the phosphonium center, a Brpnsted acid catalyzed pathway is possible. 

The sensitivity of some alcohol protecting groups to the acidic conditions of Jones oxidation allow the operation of one-pot reactions, in which deprotection of alcohols is followed by in situ oxidation to ketones. Some interesting synthetic applications of this principle are listed bellow ... 

The SmI2-II20 amine system has also been used for the selective cleavage of allyl ether-protected alcohols (Scheme 8.14). This method shown to be useful in the deprotection of alcohols and carbohydrates.16... 

Photoenolization [45], one of the well-documented [46] photochemical rearrangements of o-alkylphenacyl groups (Scheme 17.15) can be used for the deprotection of alcohols [47], carboxylic acids [48] and phosphates [49]. Klan chose the 2,5-dimethylphenacyl (DMP) chromophore [30] as a photoremovable protecting group, and its photolysis with >280 nm UV radiation results in an almost quantitative yield of deprotection. 

Table 1. Deprotection of Alcohols and Carboxylic Acids using Pd(OAc)2/TPPTS catalyst... 

Protection and deprotection of alcohols are important steps in organic synthesis and can be used to obtain selectivity - particularly in the chemistry of carbohydrates (for extended considerations, see Chapter 12), where the presence of several hydroxyl groups makes it difficult to obtain the desired selectivity. Microwave irradiation has been used for selective protection and deprotection of alcohols in several systems. 

The utility of recyclable alumina as a viable support surface for deacylation reactions is described by Varma and his colleagues [83] wherein the orthogonal deprotection of alcohols is possible under solvent-free conditions on a neutral alumina surface using MW irradiation (Scheme 2.2-23). Interestingly, chemoselectivity between alcoholic and phenolic groups in the same molecule has been achieved simply by varying the reaction time phenolic acetates are deacetylated faster than alcoholic analogues. 

FIGURE 3.22 Synthesis of functional and branched polyethylene core by chain walking polymerization (CWP) (a NaBAr b deprotection of alcohol). Source Popeney et al. [21]. Reproduced with permission from American Chemical Society. 

Deprotection of Silylated Ethers. TBBDS is also an efficient catalyst for the deprotection of alcoholic and phenolic silyl ethers. In the presence of an aprotic solvent such as dichloromethane and some drops of H2O, the alcohols are isolated in good to excellent yields (eq 10). The reaction is highly chemoselective bacause no halogenation of the aromatic ring occurs, and no overoxidation of the newly liberated alcohol to the corresponding aldehyde is observed. For this reaction again, the... 

Torii S, Inokuchi T, Kondo K, Ito H (1985) Electrogenerated acid as an efficient catalyst for the protection and deprotection of alcohols with dihydropyran and transesterification of glyceride. Bull Chem Soc Jpn 58 1347-1348. doi 10.1246/ bcsj.58.1347... 

Where the acetals are used as temporary protecting groups, they may be removed with aqueous acid to recover the ketone or aldehyde. Deprotection of alcohols is done in aqueous or alcoholic acid. The methoxyethoxyethers (Eq. 6.35) can be removed specifically by cerium (III) chloride in acetonitrile, conditions that leave other acetals intact [59]. This allows concurrent protection of two different alcohol functions in a molecule, and the selective deprotection of one of them at an appropriate stage in a synthesis. 

TBAF Tetrabutyl ammonium fluoride. Used for deprotection of alcohols with silyl protecting groups. 

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