Amino acids oxazolidinones

Pentafluorobenzyl bromide has been used in the derivatization of mercaptans [55] and phenols [36], m the analysis of prostaglandins [37], and in quantitative GC-MS [5S] 1,3 Dichlorotetrafluoroacetone is used for the derivatization of amino acids to the corresponding cyclic oxazolidinones and allows the rapid analysis of all 20 protein ammo acids [d] Pentafluorophenyldialkylchlorosilane derivatives have facilitated the gas chromatographic analysis of a wide range of functionally substituted organic compounds, including steroids, alcohols, phenols, amines, carboxylic acids, and chlorohydrms [4]... 

The products of the conjugate addidon of W -4-phenyl-2-oxazolidinone to nitroalkenes at converted into o-ct-amino acids v/ith high enandomeric purity fEq 4 30 ... 

In an indirect amination process, acyl halides are converted to amino acids. Reaction of the acyl halide with a chiral oxazolidinone leads to a chiral amide, which reacts with the N=N unit of a dialkyl azodicarboxylate [R"02C—N=N—CO2R ]. Hydrolysis and catalytic hydrogenation leads to an amino acid with good enantioselectivity. ... 

Barbiturates Imides Oxazolidinones Imides Hydantoins N-blocked amino acids... 

The products of the conjugate addition of (f )-4-phenyl-2-oxazolidinone to nitroalkenes are converted into D-a-amino acids with high enantiomeric purity (Eq. 4.30).36... 

Seebach and Brenner have found that titanium enolates of acyl-oxazolidinones are added to aliphatic and aromatic nitroalkenes in high diastereoselectivity and in good yield. The effect of bases on diastereoselectivity is shown in Eq. 4.59. Hydrogenation of the nitro products yields y-lactams, which can be transformed into y-amino acids. The configuration of the products is assigned by comparison with literature data or X-ray crystal-structure analysis. 

The transformation of the cyano group could also introduce a new chiral center under diastereoselective control (Figure 5.13). Grignard-transimination-reduction sequences have been employed in a synthesis of heterocyclic analogues of ephedrine [81]. The preferential formation of erythro-/3-amino alcohols may be explained by preferential hydride attack on the less-hindered face of the intermediate imine [82], and hydrocyanation of the imine would also appear to proceed via the same type of transition state. In the case of a,/3-unsaturated systems, reduction- transimination-reduction may be followed by protection of the /3-amino alcohol to an oxazolidinone, ozonolysis with oxidative workup, and alkali hydrolysis to give a-hydroxy-/3-amino acids [83]. This method has been successfully employed in the synthesis L-threo-sphingosine [84]. 

This imide system can also be used for the asymmetric synthesis of optically pure a,a-disubstituted amino aldehydes, which can be used in many synthetic applications.31 These optically active a-amino aldehydes were originally obtained from naturally occurring amino acids, which limited their availability. Thus, Wenglowsky and Hegedus32 reported a more practical route to a-amino aldehydes via an oxazolidinone method. As shown in Scheme 2 20, chiral diphenyl oxazolidinone 26 is first converted to allylic oxazolidinone 27 subsequent ozonolysis and imine formation lead to compound 28, which is ready for the a-alkylation using the oxazolidinone method. The results are shown in Table 2-6. 

FIGURE 8.17 Preparation of Fmoc-A-methylamino acids by methylation of Fmoc-amino acids.90 Acid-catalyzed reaction of substrate with formaldehyde at elevated temperature with removal of water by azeotropic distillation produces the oxazolidinone, which is then opened and reduced to the A-methylated derivative. 

RM Freidinger, JS Hinkle, DS Perlow, BH Arison. Synthesis of 9-fluorenylmethox-ycarbonyl-protected V-alkyl amino acids by reduction of oxazolidinones. J Org Chem 48, 77, 1983. 

The Bristol group of Christine Willis, in collaboration with Amersham International, developed a procedure for deuterium (or labeling of nonpolar amino acids." In the chemical steps, a selectively methyl-labeled oxazolidinone is converted first into a 2-methyl carboxylic acid and then lengthened by two carbon atoms without racemization to yield an a-keto methyl ester (Scheme 9). 

The utilization of a-amino acids and their derived 6-araino alcohols in asymmetric synthesis has been extensive. A number of procedures have been reported for the reduction of a variety of amino acid derivatives however, the direct reduction of a-am1no acids with borane has proven to be exceptionally convenient for laboratory-scale reactions. These reductions characteristically proceed in high yield with no perceptible racemization. The resulting p-amino alcohols can, in turn, be transformed into oxazolidinones, which have proven to be versatile chiral auxiliaries. Besides the highly diastereoselective aldol addition reactions, enolates of N-acyl oxazolidinones have been used in conjunction with asymmetric alkylations, halogenations, hydroxylations, acylations, and azide transfer processes, all of which proceed with excellent levels of stereoselectivity. 

Oxazolidinone is a versatile functional group and chiral oxazolidinones, readily prepared from amino acids, are premiere auxiliaries with broad utility in synthetic chemistry ". ... 

Further methods [205] successfully employed to synthesize fuUeropyrroUdines include acid-catalyzed [213] or thermal [214] desilylation of trimethylsilyl amino derivatives, tautomerization of a-aminoesters of immonium salts [215] and imines [216, 217], reaction with aldehydes in the presence of aqueous ammonia [218], reaction with oxazolidinone [204] or photochemical reaction with some amino derivatives [219-223], The reaction with amino acids and aldehydes was also carried... 

Scheme 6.81 Transformation of one adduct prepared from the 64-catalyzed asymmetric addition of a-substituted P-keto esters to di-tert-butyl azodicarboxylate (a-hydrazination) into the corresponding oxazolidinone amino acid derivative.

Nonbranched amino acids substituted by a fluoroalkyl chain on a carbon distant at least one methylene from the amino acid function have been prepared as racemates by various methods." Under nonracemic form, co-perfluoroalkyl norvaline and norleucine (Rf = C2F5 or more) have been prepared by bromination of an anion of a fluorinated chiral oxazolidinone (derived from RfCH2CH2C02H). Substitution of the bromine atom by an azide and subsequent reduction yield the desired amino acids (Figure 5.10)." ... 

The amino acid derived chiral oxazolidinone 188 is a very commonly used auxiliary in Diels-Alder and aldol reactions. However, its use in diastereoselective 1,3-dipolar cycloadditions is less widespread. It has, however, been used with nitrile oxides, nitrones, and azomethine ylides. In reactions of 188 (R = Bn, R =Me, R = Me) with nitrile oxides, up to 92% de have been obtained when the reaction was performed in the presence of 1 equiv of MgBr2 (303). In the absence of a metal salt, much lower selectivities were obtained. The same observation was made for reactions of 188 (R = Bn, R = H, R = Me) with cyclic nitrones in an early study by Murahashi et al. (277). In the presence of Znl2, endo/exo selectivity of 89 11 and up to 92% de was observed, whereas in the absence of additives, low selectivities resulted. In more recent studies, it has been shown for 188 (R =/-Pr, R = H, R =Me) that, in the presence of catalytic amounts of Mgl2-phenanthroline (10%) (16) or Yb(OTf)3(20%) (304), the reaction with acyclic nitrones proceeded with high yields and stereoselectivity. Once again, the presence of the metal salt was crucial for the reaction no reaction was observed in their absence. Various derivatives of 188 were used in reactions with an unsubstituted azomethine ylide (305). This reaction proceeded in the absence of metal salts with up to 60% de. The presence of metal salts led to decomposition of the azomethine ylide. 

The (+)- and (—)-4,5-dialkoxy-2-oxazolidinones 173 and 174 prepared through the 5-bromo-4-methoxy- derivatives serve as reliable chiral synthons for the preparation of a wide variety of optically active a-amino acids 176 and a-amino aldehydes 177 (Fig. S.TS)." ... 

The thermal cycloaddition of 3-acyl-2(3/7)-oxazolones 157 to dialkyl azodicar-boxylates 228 proceeds smoothly under mild conditions (at 80 °C) to give the regiocontrolled cycloadducts 229 exclusively, although two other possible addition modes exist neither diazetidines 230 (1,2-addition) nor isoxazolidines 231 (1,3-addition) are detected. In the case of chiral N-substituents diastereoselectivities of up to 72% de have been obtained. Treatment of the chiral cycloadducts 229 with acidic methanol gives tra i-5-hydrazino-4-methoxy-2-oxazolidmone derivatives 232 that are precursors for a variety of optically active a-amino acids 233 and 2-oxazolidinone auxiliaries 234 (Fig. 5.56 Table 5.10, Fig. 5.57)7 ... 

Dialkoxy-2-oxazolidinones 295, which are prepared by reaction of 5-alkoxy-2(3//)-oxazolones 47 with acetals in the presence of Lewis acid catalysts, are hydrolyzed in the presence of a protonic acid to produce a-amino acid esters 296 (Fig. 5.70 Table 5.11, Fig. 5.71 Table 5.12, Fig. 5.72 Table 5.13, Fig. 5.73 Table 5.14, Fig. 5.74). ... 

O) Assignment of the configuration at C-5 of the main isomer of 2-(2-methylpropyl)-5-vinyl-2-oxazolidinone (16) obtained in a cyclization reaction (see p 415), by conversion into the tert-bu-toxycarbonyl derivative of the natural amino acid statin 17127. 

Although the most generally useful heterocycles for the alkylation of simple amino acids by this method are the imidazolidinones84, oxazolidinones have the advantage of being much more readily hydrolyzed. (S)-a-Methylmethionine was prepared by this latter method from the parent amino acid 85,86. The oxazolidinone 1885 [mp 126.4-127.2 °C [a]D +61.8 (e = 1, CHC13)] prepared in 24-30% yield from (S)-methionine, was methylated using LDA and iodomethane in satisfactory yield. 

For the final step involving functionalization at N( ) of 62, anilide deprotonation with lithiated 4-benzyl-2-oxazolidinone as a base and alkylation with benzyl bromides again proved effective. Compared to the results obtained in the benzodiazepine series, the N( 1 )-alkylation reaction was generally found to proceed less smoothly with the 3,4-disubstituted quinox-alinones 62. Good results were obtained only if the resin batches were submitted twice to the alkylation conditions. Figure 3.4 displays a selection of structures (63-65) accessible from this first synthetic approach. In no case was there any evidence for racemization at the a-carbon atom of the amino acid. 

Table 3 Preparation of /V-Alkyl-/V-Fmoc Amino Acids by the Oxazolidinone Method1 011...

As described in Scheme 25, the first method starts with the conversion of an a-amino acid into a p-amino-p-alkyl aldehyde. Then, the chiral aldol condensation between the resulting aldehyde and an oxazolidinone derivative is carried out. In the last step, removal of the oxazolidinone provides the desired product. 

Although the racemization of the a-carbon can now be considered a potential problem, the synthesis of 32-peptides has been achieved in the same way as seen for 33-peptides. As the 32-amino acids cannot be prepared from the analogous a-amino acids, Seebach and co-workers 5,7 opted to use Evans oxazolidinone chemistry to produce enantiomerically pure 32-amino acids. Alkylation of 3-acyloxazolidin-2-ones 17 with A-(chloromethyl)benzamide yielded the products 18 with diastereomeric ratios between 93 7 and 99 1 (Scheme 8). Removal of the chiral auxiliary (Li0H/H202) and debenzoylation (refluxing acid) was followed by ion-exchange chromatography to yield the free 32-amino acids 20 which were converted by standard means into Boc 21 or benzyl ester 22 derivatives for peptide synthesis. 

The reaction of methyl thiosalicylate 187 with 2-oxazolidinone 186 yielded 45% of benzothiazepinone 188. The same procedure has been followed to synthesize chiral thiazepine 190 starting from bicyclic oxazolidinone 189 and methyl thiosalicylate 187 (Scheme 31) <2001T2115>. Related reports about cyclization of amino acid derivatives and analogues of 187 have appeared <2004BMC4459>. 

The application of antibiotics as chiral selectors has resulted in the successful resolution of almost all types of neutral, acidic, and basic racemic molecule. These antibiotics have been used for the enantiomeric resolution of amino acids, their derivatives, peptides, alcohols, and other pharmaceuticals. The selectivities of the most commonly used antibiotic-based (vancomycin, teicoplanin, and ristocetin A) CSPs varied from one racemate to another and are given in Table 1. Vancomycin was used for the chiral resolution of amino acids, amines, amides, imides, cyclic amines, amino alcohols, hydantoins, barbiturates, oxazolidinones, acids, profens, and other pharmaceuticals. Teicoplanin was found to be excellent chiral selector for the enantiomeric resolution of amino acids, amino alcohols, imides, peptides, hydantoins, a-hydroxy and halo acids, and oxazolidinones, whereas ristocetin A is capable of chiral resolution of amino acids, imides, amino... 

Reversed phase Amines, amides, imides, acids, profens a-Hydroxy acids, oxazolidinones, amino acids, peptides a-Hydroxy acids, acids, profens, amino acids, amino esters, hydantoins, peptides... 

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