2- phenyl protection

Barbas III and co-workers reported organocatalyzed Mannich reactions between p-methoxy-phenyl-protected a-imino ethyl glyoxolate and ketones (O Scheme 30) [157,158,159]. 

TEA, Pyr, 20°C, 2h. These conditions were developed for use with 2-chloro-phenyl protection at the internucleotide junctions. 

Boon (1949) prepared the l,5-bis(methyl- or ethylamino)-3-oxapentanes by two methods as shown. The initial diamino ether contained phenyl protecting groups that required a two-step method for removal. Boon also isolated... 

Phenylaziridine, 559 A -Phenylbis(2-chloroethyl)amine, 488 o-Phenylenediamine, 90,91, 152 A -Phenylmorpholine, 42 A -Phenyl protecting groups. 41 Phosphate ion complexes, 14 Phosphinaaza-crown macrocycles, 487-492... 

Evans phenyl- protected alanine-derived amino-alcohol auxiliary... 

Sodeoka and co-workers subsequently reported a Pd"-catalyzed asymmetric oxidative Mannich-type reaction of malonate 5 with THIQ 4 using DDQ as the stoichiometric oxidant. The reactive iminium intermediate from the N-Boc-protected THIQ 4 could be generated in situ by the slow addition of DDQ. The desired product 6 was obtained in 82% yield with 86% ee (Scheme 2.3). It is notable that an easily removable protecting group (Boc) can be used in this reaction, whereas Af-phenyl-protected substrates have been used in the previously reported CDC reactions with alkynes. More detailed mechanistic studies of this oxidative Mannich-type reaction were presented in a subsequent report from the Sodeoka group. Under the optimized reaction conditions, they examined the generality of the reaction. This oxidative Mannich-type protocol was also suitable for... 

Unsaturated phosphatidylinositol has been synthesized by Molotkovskii and Bergelson (1971) using an optically active glycerol derivative but a racemic myo-inositol moiety which was added from an acetylated inositol phosphate benzyl ester. Klyash-chitski et ah (1971) prepared a completely optically active dipalmitoylphosphatidylinositol. However, because they used benzyl and phenyl protecting groups their method is not suitable for unsaturated phosphatidylinositols. 

The procedure for phenyl Mooyanate is similar, but great care must be taken to protect both the reagent and the reaction mixture from moistvu-e. 

Phenyl isocyanate is a colourless liquid, b.p. 164° or 55°/13 mm. its vapour is lacluymatory. The liquid reacts readily with water, yielding diphenyl urea, m.p. 241°, and hence must be protected from atmospheric moisture ... 

Protection Against Flex Cracking. Most antioxidants including waxes provide oxidation protection under static conditions. However, most of them are not effective in mbber products subjected to dynamic flexing, eg, sidewall compounds in tires. The best dynamic protection is provided by either /V-alkyl-/V-phenyl or diaryl-/)-phenylene diamines. 

As in dry compounding, acid acceptors must be incorporated into neoprene latices because of the wide use of these latices in coating fabrics and metals. The hydrochloric acid that forms during service life has a particularly destmetive effect on coated cotton fabrics that are not adequately protected. High zinc oxide concentration (ca 15 parts) and use of 0.4 parts AJ-phenyl-AT(p-toluenesulfonyl)-/)-phenylenediamine (Aranox, Uniroyal) as an antioxidant provides adequate protection. 

Ai,Af-Disubstituted- -phenylenediamines, such as A/-phenyl-Af -(l,3-dimethylbutyl)-/)-phenylenediamine [793-24-8] (10), are used in greater quantities than other classes of antioxidants. These products protect unsaturated elastomers against oxidation as well as ozone degradation (see Antiozonants). 

The best oxidation inhibitors are not usually the best antio2onants (qv). A disubstituted i ra-phenylenediamine such as AJ-isopropyl-AT-phenyl- -phenylenediamine is often selected for that purpose. -Phenylenediamine derivatives iaterfere with cure chemistry and scorchiness, and can stain objects ia contact with the vulcani2ate (114). On balance, /V-(1,3-dimethy1buty1)-/V-phenyl- -phenylenediamine and phenyl /to1y1- -pheny1enediamines have the best combination of properties. They are less scorchy and provide excellent o2one and heat resistance. Additional protection is gained ia blends with a small amount of EPDM mbber (126). 

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 tritylone ether is used to protect primary hydroxyl groups in the presence of secondary hydroxyl groups. It is prepared by the reaction of an alcohol with 9-phenyl-9-hydroxyanthrone under acid catalysis (cat. TsOH, benzene, reflux, 55-95% yield).It can be cleaved under the harsh conditions of the WolfT-Kishner reduction (H2NNH2, NaOH, 200°, 88% yield), " and by electrolytic reduction (-1.4 V, LiBr, MeOH, 80-85% yield). It is stable to 10% HCl, 55 h. ... 

The SEM ester was used to protect a carboxyl group where DCC-mediated esterification caused destruction of the substrate. It was formed from the acid and SEM chloride (THF, 0°, 80% yield) and was removed solvolytically. The ease of removal in this case was attributed to anchimeric assistance by the phosphate group. Normally SEM groups are cleaved by treatment with fluoride ion. Note that in this case the SEM group is removed considerably faster than the phenyl groups from the phosphate. ... 

Phenyl esters can be prepared from A-protected amino acids (PhOH, DCC, CH2CI2, -20° 20°, 12 h, 86% yield PhOH, BOP, Et3N, CH2CI2, 25°, 2 h. 

The / -(methylmercapto)phenyl ester has been prepared from an /-protected amino acid and 4-tH3SC6H40H (DCC, CH2CI2, 0°, 1 h 20°, 12 h, 60-70% yield). The p-(methylmercapto)phenyl ester serves as an unactivated ester that is activated on oxidation to the sulfone (H2O2, AcOH, 20°, 12 h, 60-80% yield) which then serves as an activated ester in peptide synthesis. ... 

This derivative is prepared from an A-protected amino acid and the anthrylmethyl alcohol in the presence of DCC/hydroxybenzotriazole. It can also be prepared from 2-(bromomethyl)-9,10-anthraquinone (Cs2C03). It is stable to moderately acidic conditions (e.g., CF3COOH, 20°, 1 h HBr/HOAc, / 2 = 65 h HCl/ CH2CI2, 20°, 1 h). Cleavage is effected by reduction of the quinone to the hy-droquinone i in the latter, electron release from the —OH group of the hydroqui-none results in facile cleavage of the methylene-carboxylate bond. The related 2-phenyl-2-(9,10-dioxo)anthrylmethyl ester has also been prepared, but is cleaved by electrolysis (—0.9 V, DMF, 0.1 M LiC104, 80% yield). ... 

The phenyl group became a practical protective group for carboxylic acids when Sharpless published a mild, effective one-step method for its conversion to a carboxylic acid. It has recently been used in a synthesis of the amino acid statine, where it served as a masked or carboxylic acid equivalent. ... 

Although a sulfhydryl group generally is not converted to an S-phenyl thioether, thiophenol can be used to introduce sulfur into molecules, and thus the phenyl group serves as a suitable protective group that can be removed by electrolysis (-2.7 V, DMF, R4N+X ). ... 

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