Selective protection of hydroxyl groups

The selective protection of hydroxyl groups is obviously most frequent in carbohydrate synthesis and, in fact, photosensitive protecting groups have been used to this effect in oligosaccharide synthesis, nucleotide synthesis, and saccharide modification. Here, as well as in other Sections, special attention will be devoted to 2-nitrobenzyl derivatives, whose re-... 

It should be noted that there are some limitations to the use of the tert-butyldimethylsilyl group (and, presumably, other trialkylsilyl groups) for the selective protection of hydroxyl groups. Isomerization between 0-2 and 0-3 occurs with 2 - and 3 -0-(tert-butyldimethyl-silyl) derivatives of nucleosides when they are in prolonged contact with silica gel,309 or when stored in aqueous pyridine, aqueous triethyl-... 

Chemical synthesis, an approach limited by complex synthetic sequences using protection of hydroxyl groups to achieve regio- and stereo-selectivity... 

Protection of hydroxyl groups (11, 166).- 3.4-Dimethoxybcnzyl ethers are oxidized by DDQ more readily thanp-methoxybenzyl ethers. Moreover, the dimethoxybenzyl ethers of secondary alcohols can be selectively oxidized in the presence of the corresponding ethers of primary alcohols. Benzyl, p-methoxybenzyl, and 3,4-dimethoxybenzyl ethers all undergo hydrogenolysis catalyzed by Pt/C or Pd/C, but selective hydrogenolysis of benzyl ethers is possible with W-2 Raney Ni. 

Selective Protection Methodologies (Regioselective Protection of Hydroxyl Groups)... 

SELECTIVE PROTECTION METHODOLOGIES (REGIOSELECTIVE PROTECTION OF HYDROXYL GROUPS)... 

Selective activation of hydroxyl groups by the stannylation procedure is an established practice to efficiently manipulate diols and polyols [53, 54]. Among the various reagents and methods, activation through dialkylstannylene acetals is noteworthy. This two-step protection method involves the conversion of dihydroxy substrates into cyclic dialkyl dioxastan-nanes which, in turn, are treated with suitable nucleophiles, to regioselectively afford the corresponding mono alkyl, silyl or acyl derivatives under neutral conditions. 

A suspension of chloracetylglycylglycine and thiourea warmed 1 hr. at 60-65° in ethanol, the resultant soln. refluxed 40 min., ethanol removed by rotary evaporation, water added, and boiled 90 sec. -> glycylglycine hydrochloride monohydrate. Y 75%. F. e., also selective removal, s. M. Masaki et al.. Am. Soc. 90y 4508 (1968) also protection of hydroxyl groups as chloracetates s. A. Fontana and E. Scoffone, G. 95, 1261 (1968). 

In the synthesis of methyl-2,3-0-isopropylidene-4-0-(methoxymethyl)-6-methylene-a-D-mannopyranoside (13), the hydroxyl groups of methyl-a-o-mannopyranosides are selectively protected 6-hydroxyl group as silyl ether 2,3-hydroxyl groups as acetonides and 4-hydroxyl group as methoxymethyle-ther. The silyl group at the C-6 position is then selectively cleaved to get primary alcohol. Oxidation of the alcohol to aldehyde followed by Wittig methylenation yields the desired olefin (13, Scheme 31.13). 

On completed aldol reactions in the first two steps, the nitro group is reduced before the introduction of the second one in the aromatic ring. To this aim, protection of hydroxyl groups is needed and can be completed by acetylation. Nitration is followed by deprotection of all acetyl groups since selective acetylation of amino groups can be performed by dichloroacetyl chloride in the presence of tert-amine. 

Protection of Hydroxyl Groups. Triisopropylsilyl chloride (TIPSCl) continues to be widely used as the means of introducing the TIPS protecting groups onto alcohols. The steric bulk of the three isopropyl groups on the silicon atom provides considerable stability under acidic and basic conditions and allows selective removal of smaller, more labile silyl groups in the presence of TIPS ethers.i2.i3... 

Some of the original work in the carbohydrate area in particular reveals extensive protection of carbonyl and hydroxyl groups. For example, a cyclic diacetonide of glucose was selectively cleaved to the monoacetonide. A summary describes the selective protection of primary and secondary hydroxyl groups in a synthesis of gentiobiose, carried out in the 1870s, as triphenylmethyl ethers. 

The base-catalyzed reaction of thiothreitol with methyl dithiobenzoate selectively protects a thiol group as an 5-thiobenzoyl derivative in the presence of a hydroxyl group.-... 

The reactivity of various steroid alcohols decreases in the order primary > secondary (equatorial) > secondary (axial) > tertiary. The only systematic investigation relating to the selective protection of steroidal hydroxyl functions has been carried out with the cathylate (ethyl carbonate) group. Since only equatorial hydroxyl groups form cathylates this ester has been used as a diagnostic tool to elucidate the configuration of secondary alcohols. 

Bu2SnO, benzene BnBr, DMF, heat, 80% yield. This method has also been used to protect selectively the anomeric hydroxyl in a carbohydrate derivative. The replacement of Bu2SnO with Bu2Sn(OMe)2 improves the process procedurally. The use of stannylene acetals for the regioselective manipulation of hydroxyl groups has been reviewed. ... 

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