Protecting group, allyl

The benzyl group can serve as an alcohol-protecting group when acidic conditions for ether cleavage cannot be tolerated. The benzyl C-O bond is cleaved by catalytic hydrogenolysis," or with sodium in liquid ammonia. Allyl ethers are an alternative to benzyl ethers as a protecting group. Allyl ethers may be isomerized quantitatively by potassium r-butoxide in dimethyl sulfoxide to propenyl ethers, which are quite labile to dilute acid. ... 

I will not go through the synthesis of 84 (related to 83) as this can be an exercise for you. Removal of the hT-DBS protecting group, allylic oxidation (Corey-Fleet reagent) of the alkene to enone 86, and a Wittig methylena-tion, provided cyclization substrate 83. 

In all cases examined the ( )-isomers of the allylic alcohols reacted satisfactorily in the asymmetric epoxidation step, whereas the epoxidations of the (Z)-isomers were intolerably slow or nonstereoselective. The eryfhro-isomers obtained from the ( )-allylic alcohols may, however, be epimerized in 95% yield to the more stable tlireo-isomers by treatment of the acetonides with potassium carbonate (6a). The competitive -elimination is suppressed by the acetonide protecting group because it maintains orthogonality between the enolate 7i-system and the 8-alkoxy group (cf the Baldwin rules, p. 316). 

It is widely recognized that an allyl group is a useful protecting group for acids, amines, and alcohols. Facile formation of a Tr-allylpalladium complex from... 

The protected nucleoside-3-phosphoramidite monomer units such as 671 are used in the solid-phase oligonucleotide synthesis. In the 60mer synthesis, 104 allylic protective groups are removed in almost 100% overall yield by the single Pd-catalyze reaction with formic acid and BuNH2[432], N,(9-protection of uridine derivatives was carried out under pha.se-transfer conditions[433]. 

A method for protecting 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 allyl ethers. The oxime ethers ean be eleaved with triethylammonium formate in boiling dioxane[444]. The allyl ether of oximes is eleaved under mild conditions without attaeking the aeetal group in 677. 

An important reaction parameter is the choice of the base and NajCO or NaOAc have been shown to be preferable to EtjN in some systems[2]. The inclusion of NH4CI has also been found to speed reaction[2]. An optimization of the cyclization of A -allyl-2-benzyloxy-6-bromo-4-nitroaniline which achieved a 96% yield found EtjN to be the preferred base[3]. The use of acetyl or inethanesulfonyl as N-protecting groups is sometimes advantageous (see Entries 4 and 5, Table 4.1). 

Three selective methods to remove protective groups are receiving much attention assisted, electrolytic, and photolytic removal. Four examples illustrate assisted removal of a protective group. A stable allyl group can be converted to a labile vinyl ether group (eq. 4) a /3-haloethoxy (eq. 5) or a /3-silylethoxy (eq. 6) derivative is cleaved by attack at the /3-substituent and a stable o-nitro-phenyl derivative can be reduced to the o-amino compound, which undergoes cleavage by nucleophilic displacement (eq. 7) ° ... 

The cleavage proceeds by initial reduction of the nitro groups followed by acid-catalyzed cleavage. The DNB group can be cleaved in the presence of allyl, benzyl, tetrahydropyranyl, methoxy ethoxy methyl, methoxymethyl, silyl, trityl, and ketal protective groups. 

Butyl ethers can be prepared from a variety of alcohols, including allylic alcohols. The ethers are stable to most reagents except strong acids. The /-butyl ether is probably one of the more underused alcohol protective groups, considering its stability, ease and efficiency of introduction, and ease of cleavage. 

Protection for the double bond in the allyl protecting group may be achieved by epoxidation. Regeneration of the allyl group occurs upon... 

The use of various allyl protective groups in complex molecule synthesis has been reviewed. ... 

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of structure. The most useful compounds (not necessarily the simplest structures) are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hydrogenol-ysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and allyl, readily cleaved by Pd-catalyzed isomerization. 

The Paloc group was developed as an amino acid protective group that is introduced with the p-nitrophenyl carbonate (H2O, dioxane, 68-89% yield). It is exceptionally stable to TFA and to rhodium-catalyzed allyl isomerization, but it is conveniently cleaved with Pd(Ph3P)4 (methylaniline, THF, 20°, 10 h, 74-89% yield). ... 

Allylic protection groups and their removal through catalytic palladium rr-allyl methodology in transformations of heterocycles 98T2967. [3- -3]Benzannelation of heteroaromatics consisting of successive nucleophilic... 

Allylic protection groups and their removal through catalytic palladium Tr-allyl methodology in synthesis of heterocycles 98T2967. 

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