Deprotection alcohol/phenol groups

The first asymmetric synthesis of (-l-)-abresoline was achieved from the chiral piperidine derivative 153, which upon treatment of its hydroxy side-chain substituent with carbon tetrabromide, triphenylphosphine, and triethyl-amine cyclized to the frarcr-quinazolidine 154. Deketalization and silyl protection of the phenolic group, followed by stereoselective reduction with lithium tri-t -butylborohydride (L-Selectride ), gave an alcohol, which after acylation and deprotection furnished (-l-)-abresoline 155 (Scheme 25) <2005TL2669>. 

Among a wide range of other applications, the combination of alumina-supported reactions and microwave irradiation was successfully applied to the cleavage of esters, a commonly used strategy to deprotect alcoholic groups in multi-step organic synthesis. Deacylation of alcoholic and phenolic ac-... 

Another hybrid protecting group is the p-[(trimethylsilyl)ethoxy]methoxy benzyl (p-SEM-benzyl) ether354 Phenolate anions generated by treatment of p-SEM-benzyl ethers with TBAF in DMF at 80 °C eliminate to give the deprotected alcohol as illustrated in Scheme 4.303 p-SEM-benzyl ethers are compatible with many of the standard manipulations in oligosaccharide synthesis and they arc orthogonal to benzyl and p-methoxybenzyl ethers. 

Tetrohydropyranylation. Alcohols and phenols are converted into tetrahydro-pyranyl ethers by reaction with 3,4-dihydro-2//-pyrane catalyzed by a strongly acidic ion-exchange resin such as Amberlyst H-15. Yields are >90%. Deprotection of the group can be effected by methanolysis catalyzed by the same resin or by treatment with acid-washed Dowex-50-W-X8. ... 

Protection of some reactive functional groups is possible during polymerization, using known organic chemical methods, such as silanalion of alcohol or phenol groups. This expedient and the subsequent deprotection step will, of course, complicate the overall polymerization. 

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. 

Protection and deprotection of functional groups is an important strategy in organic S3Tithesis. Particularly, trimethylsilyl ether (TMS), methotymethyl acetal (MOM) and tert-butoty carbamate (Boc) are three frequently used effective reagents in the protection of alcohols, phenols, amines, carbotylic... 

Silicon Bond Formation/Cleavage. The vast majority of applications of silicon intermediates in organic synthesis involve tert5)orary installation and removal of silicon atoms. While there are many well-established examples of classical, backside, 8 2 displacements at tetravalent silicon, it is useful for the purpose of this handbook to artificially consider that aU silicon bond formations and cleavages occur via siliconate [Si(V)] intermediates. This key reaction extends across the periodic table especially including groups 4, 5, 6, and 7. Well-known examples include the often used alcohol, phenol, and carboxylic acid protections (and selective deprotections) via the formation of silicon-oxygen derivatives (eq 1). ... 

The above section already introduced the influence of leaving groups at the benzylic position that eliminate to form and regenerate QM3, and the trend extends beyond adducts formed by the deoxynucleosides as expected. The standard benzylic acetate of QMP4 eliminates completely from the deprotected phenol under neutral aqueous conditions and ambient temperature within approximately 20 h, while an equivalent benzyl bromide eliminates completely within 5 min.48 Benzylic phosphates are also extremely labile, and, if the phosphate backbone of DNA is able to trap QM, the resulting products are likely to be too labile for standard detection.53,54 In contrast, amines and thiols are much less susceptible to elimination from the benzylic position and require forcing conditions to regenerate the parent QM.26,30 The benzylic alcohol derivative also appears stable under almost all thermal conditions and only eliminates routinely to form a QM after photochemical excitation.55... 

Notes This alcohol protecting is easily attached and readily removed by Lewis acids such as zinc bromide and titanium tetrachloride. Phenols can be protected (reaction of the sodium salt with MEMC1) and deprotected with TFA. More easily removed than the MOM group. 

Proticlion of hydroxyl groups. f-Butoxymethyl ethers of alcohols can be obtained by rcaclion with <-butyl chloromethyl ether and triethylamine in THF at room tempcralure. The acetals are obtained in 55-80% yield. Yields are poor in the case of phenols. Deprotection is accomplished with TFA at room temperature. The acetals are stable to hot acetic acid therefore selective deprotection of other acid-sensitivc groups is possible. ... 

---