Phenylacetamide group

Molecular structure of a self-immolative AB6 dendron with tryptophan tail units and phenylacetamide group as a trigger. 

FIGURE 5.37 Chemical structure of a molecular probe with UV-Vis and fluorescence outputs for penicillin G amidase activity. The phenylacetamide group (red) is a substrate for PGA. The reporter units, 4-nitrophenol and 6-aminoquinoline, provide a visible signal and a fluorescence signal, respectively, upon release. (See the color version of this figure in Color Plates section.)... 

One of the possibilities to convert the amino nitrile (R,S)-3 to (5)-terMeucine (7) is shown in the reaction sequence of Scheme 25.3. Hydrolysis of the aminonitrile (R,S)-3 to the diamide 5 was performed in concentrated sulfuric acid. Removal of the phenylacetamide group using H2 and Pd-C gave (S)-tert-leucine amide (6). Acidic hydrolysis of the amide 6 yielded (S)-tert-leucine (7) in 73% overall yield for the 3 steps and an enantiomeric excess of >98%. The overall yield for this nonoptimized protocol is 66% based on pivaldehyde. Obviously, other routes to convert the amino nitrile to the amino acid can be envisaged. As an example, by heating the diamide in sulfuric acid, after dilution with water, the diacid can be obtained that, after hydrogenolysis, affords the amino acid in 2 steps from the amino nitrile 3. 

Enzymatic protecting group techniques have recently been extended [10,11] to the synthesis of phosphopeptides, in which the phenylacetamide group is removed from the respective PhAc amides by means of penicillin G acylase (PGA). In the course of the ensuing enzymatic transformations only the phenylacetamide blocking group was attacked the peptide bonds, the C-terminal ester, and the phosphates remained intact (Fig. 3). More importantly, the reaction conditions were so mild that no trace of p elimination was observed [10,11]. 

The phenylacetamide group has been used for the temporary masking of nucleobases in the synthesis of oligonucleotides, both in solution and on a solid support. PhAc-pro-tected oligonucleotides were synthesized on controlled-pore glass beads. After release of... 

An exo-linker according to Fig. 10.1 must contain an enzyme labile group R, which is recognized and attacked by the biocatalyst Possible combinations could be phenylacetamide/penicillin amidase, ester/esterase, monosaccharide/glycosid-ase, phosphate/phosphatase, sulfate/sulfatase and peptides/peptidases [41]. The following systems have been worked out (Tab. 10.2). 

Penicillin acylase catalyzes the hydrolysis of phenylacetamides and has been used in peptide synthesis for the cleavage of protecting groups [46—47]. In linker (40) developed by Flitsch et al. [41—42] (Scheme 10.8) the group -XR represents the alcohol or amine group of the target molecule. Hydrolysis of the phenylaceta-... 

From the aforementioned, it should be evident that the amino group is one of the most reactive functionalities towards dioxirane oxidation consequently, to achieve a chemo-selective oxidation of a multi-functionalized substrate that possesses an amino group, the latter must be protected. This may be accomplished by masking the amino substituent in the form of its ammonium salt ", or BF3 complex ", even better as an amide functionality (iV-phenylacetamide resists TFD oxidation at room temperature""). This will reduce sufficiently the oxidative reactivity of the amino group, such that another less reactive group may be selectively oxidized" ... 

S-Acetamidomethyl groups are introduced by reaction of the thiol with l-(acet-amidomethyl)-benzotriazole69 or N-( hydroxy me thyl)acetamide under acidic conditions.62 70 The latter procedure is also used to introduce the S-phenylacetami-domethyl (Phacm) group [Scheme 5.37].71 Thus, the Phacm group can be introduced onto L-cysteine by reaction of yV-(hydroxymethyl)phenylacetamide 37 1 with cysteine hydrochloride in the presence of trifluoromethanesulfonic add. The crude intermediate is then treated with di-/erf-butyl dicarbonate (or... 

Potassium fluoride can be used to introduce fluorine in the a-position of A-substituted amides via substitution of tosyloxy groups. 2-Fluoro-A-phenylacetamide (mp 76 C) and 2-fluoro-A-phenylpropanamide (mp 62 C) are prepared by heating the corresponding tosyloxy derivatives with potassium fluoride in ethylene glycol at 100 C in 54 and 63% yield, respectively. This method is especially advantageous for the synthesis of higher a-fluoro acid derivatives, because in these cases the products cannot be distilled off from the reaction mixtures and side reactions of simple ester groups with the solvent become more important. 

The liberation of the (3-sulfanyl group of cysteine is achieved by means of PGA-mediated hydrolysis of phenylacetamides.P " °l For this purpose, the thiol group is masked with the phenylacetamidomethyl (Phacm) blocking function. After penicillin acylase catalyzed hydrolysis of the amide incorporated in the acylated thioacetal, the labile 5-aminomethyl compound is formed and immediately liberates the desired thiol. This method has been used in a synthesis of glutathione which is isolated as its disulfide 6 (Scheme 12). 

For the N-terminal deprotection of peptides, the enzyme penicillin G acylase from E. coli has been applied. This attacks phenylacetic acid (PhAc) amides and esters but does not hydrolyze peptide bonds [12-14,25]. The danger of a competitive cleavage of the peptide backbone at an undesired site, which always exists when proteases like trypsin and chymotrypsin are used, is overcome by using the acylase. The penicillin G acylase accepts a broad range of protected dipeptides (27) as substrates, and selectively liberates the N-terminal amino group under almost neutral conditions (pH 7-8, room temperature), leaving the peptide bonds as well as the C-terminal methyl-, allyl-, benzyl-, and tert-butyl ester unaffected (Fig. 8) [25a,bj. On the other hand, the phenylacetamide... 

The application of the non-urethane PhAc blocking group results in ca. 6% racemization during the construction of the phenylacetamido-protected dipeptides by chemical activation of the phenylacetamido amino acids. This disadvantage can be overcome by forming the peptide bonds enzymatically, e.g. with trypsin [26,27], chymotrypsin [26,28], or carboxypeptidase Y [26,29]. An interesting example is the biocatalyzed synthesis of leucine-enkephalin tert-butyl ester [25e], in which phenylacetamides are introduced and cleaved by means of penicillin G acylase, and the elongation of the peptide chain is carried out with papain or a-chymotrypsin. 

It is significant that both groups observed the efficient incorporation of a phenylacetamide derivative, (112) and (113) respectively, and on balance the evidence at this stage of the investigation is weighted in favour of path a. 

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