A-Arylpropanoic acids

Carbenoid imidazolidene ligands such as C can also be used in conjunction with Pd(dba)2, and this method has been applied to a-arylpropanoic acids (NSAIDS) such as naproxen.175... 

Hydrocarbonylation. The hydrocarbonylation reaction can be applied to the synthesis of a-arylpropanoic acids of the NSAIDS type.239 For this synthesis to be effective, selective carbonylation of the more-substituted sp2 carbon is required. Although many carbonylation conditions are unselective, PdCl2(PPh3)2 with /2-toluenesulfonic acid and LiCl achieves excellent selectivity. The selectivity is thought to involve the formation of a benzylic chloride intermediate. 

Production of enantiomerically pure a-arylpropanoic acids, also known as profens, is of critical importance to the pharmaceutical industry because they constitute a major class of antiinflammatory agents. One of the most practical approaches to preparing optically pure a-arylpropanoic acids is by resolution with chiral amines. Notable examples include brucine, quinidine, cinchonidine, morphine, ephedrine, and a-(l-naphthyl)ethylamine. For instance, (.Sj-a-methylbenzylaminc and... 

An enantioselective route to 1,3-dithiane 1-oxide (33) (R = R = H) was subsequently developed [69]. It involves asymmetric oxidation of (32) (R = pivaloyl, R = H) by cumene hydroperoxide in presence of the chiral titanium complex. The syn/anti mixture (around 90% ee for each diastereoisomer) is recrystallized and then deacylated, giving the desired product in 80% yield. A recent application of this chemistry is the asymmetric synthesis of enantiopure (R)-(-)-2,6-dimethylheptanoic acid in two steps from (33) (R = C(0)Et, R = Et) [70]. The reaction involves a fully stereoselective methylation in the a-position of the keto group, followed by basic deacylation, which also regenerates enantiopure 2-ethyl-l,3-dithiane 1-oxide (33) (Ri = H, R = Et). A range of a-arylpropanoic acids have since been prepared by similar routes in high ee s. [162]... 

Asymmetric Hydroformylation of Vinylarenes a-Arylpropanals, the products of asymmetric hydroformylation of vinylarenes, serve as useful intermediates for pharmaceutical drugs. For example, (5)-2-arylpropanals can be oxidized to the corresponding (5)-2-arylpropanoic acids, such as (5)-ibuprofen (Ar = 4-isobutylphe-nyl), (5)-naproxen (Ar = 6-methoxynaphthalen-2-yl), and (5)-suprofen (Ar = 4-(2-thienylcarbonyl)phenyl) (see later in chapter. Scheme 4.4). Styrene is thus one of the most popular substrates used to test new catalyst systems. Representative ligands and their use as Pt or Rh complexes in the asymmetric hydroformylation are summarized in Figure 4.1 and Table 4.1. (See also Scheme 4.3.)... 

An alternative approach to the synthesis of isocoumarins which probably proceeds through the intermediacy of 2-carboxybenzyl ketones is based on the oxidative cleavage of indan-1-ones (76JCS(P1)1438). Although ozonolysis of the silyl enol ether (505) leads to the 2-hydroxy-2-methylindan-l-one (506), periodate oxidation of which gives the isocoumarin, a more convenient and direct route involves ozonolysis of the enol trifluoroacetate (Scheme 182). This synthesis is especially attractive for the preparation of isotopically labelled isocoumarins, since the precursors of the indanones, arylpropanoic acids or acrylophenones, are readily available bearing labels at specific sites. 

This conversion of a 1,2-diol to an epoxide has been used as an approach to 2-arylpropanoic acids, members of the nonsteroidal antiinflammatory drugs (NSAIDS) family of drugs.163 However, the sequence can be shortened by a selective hydrogenolysis, as illustrated for naproxen (20) (Scheme 9.24).164... 

Isse, A.A., Ferlin, M.G. and Gennaro, A. (2005a) Electrocatalytic reduction of arylethyl chlorides at silver cathodes in the presence of carbon dioxide Synthesis of 2-arylpropanoic acids. J. Electroanal. Chem. 581, 38 15. 

Arylpropanoic acids. The ethylene kctals (2) of aryl a-phenylselenoethyl ketones, prepared by reaction of aryl a-bromo ketals (1) with diphenyl diselenide and sodium wire, are converted into the hydroxycthyl esters (3) of 2-arylpropanoic acids (4) on oxidation with excess w-chloroperbenzoic acid. The reaction probably involves a selenone intermediate, in which the rearrangement of the aryl group occurs. The acetal group of 2 is essential for this rearrangement. 

In the case of 2-arylpropanoic acids, although the (5)-enantiomer (9) is available by a terminal oxidation, the alternative (/ )-enantiomer (11) can be prepared by the more extensive oxidative degradation of the alkylbenzene (10 equation 3) by Rhodococcus spp. (BPM 1613). In this case the optical induction is due to oxidative kinetic resolution of intermediates the recovered substrate is racemic. 

In addition, OPEN is an efficient chiral solvating agent for determination of the enantiomeric excess in the H NMR analysis of various chiral mono- and dicarboxylic acids including a-arylpropanoic and a-halo carboxylic acids. The chemical-shift non-equivalence (S A) in certain diastereomeric complexes is greater than 0.05 ppm. A DPEN/Pd(II) complex can be used for determination of enantiomeric excess of the non-protected chiral amino acids by H and C NMR analysis. For example, Pd[(S,S)-dpen](D20)2 and racemic alanine with a base forms the square-planar complex (eq 14). The 5 A of H-NMR resonance in the diastereomeric complexes in D2O is 0.056 ppm, while this complex hardly dissolves in D2O. 

Combination of the processes of the C-O bond cleavage in 1-naphthylethyl esters with CO insertion catalyzed by palladium complexes in the presence of a formate salt affords a new route to 2-arylpropanoic acids [67]. 

Hydrovinylation reactions of vinylarenes, Eq. (1), have been investigated most extensively because of the importance of 3-aryl-1-butenes as potential intermediates for widely used anti-inflammatory 2-arylpropanoic acids [4]. Since the first report of a high pressure (1000 atm) ethylene/styrene codimerization in the presence of RhClj [5] various metals such as Ru [6], Co [7],Pd [8],andNi [9,... 

Long-chain aliphatic olefins give only insufficient conversion to the acids due to low solubility and isomerization side reactions. In order to overcome these problems the effect of co-solvents and chemically modified /i-cyclodextrins as additives was investigated for the hydrocarboxylation of 1-decene [23], Without such a promoter, conversion and acid selectivity are low, 10% and 20% respectively. Addition of co-solvents significantly increases conversion, but does not reduce the isomerization. In contrast, the addition of dimethyl-/i-cyclodextrin increased conversion and induced 90% selectivity toward the acids. This effect is rationalized by a host/ guest complex of the cyclic carbohydrate and the olefin which prevents isomerization of the double bond. This pronounced chemoselectivity effect of cyclodextrins is also observed in the hydroformylation and the Wacker oxidation of water-insoluble olefins [24, 25]. More recent studies of the biphasic hydrocarboxylation include the reaction of vinyl aromatic compounds to the isomeric arylpropanoic acids [29, 30], and of small, sparingly water-soluble alkenes such as propene [31]. 

Thus, considerable effort is necessary to achieve a wide and synthetically useful application of this method. Nevertheless, the first examples of interesting target molecules obtainable via asymmetric hydroformylation have appeared (amino acids, arylpropionic acids)180. Thus, if appropriate catalytic systems and reaction conditions can be found, even industrial applications might be realized within the near future. Thus, asymmetric hydroformylation is considered to be a powerful tool for the preparation of a large number of different chiral products to be used as precursors of several organic compounds endowed with therapeutical activity180. Examples are the essential and non-essential amino acids, 2-arylpropanoic acids, aryloxypropyl-and /1-phenylpropylamines. modified /1-phenylethylamines, pheniramines and others180. 

Determination of enantiomeric purity. Three mL of the solution was diluted in 50 mL of acetone and treated with 0.3 g of potassium permanganate and 0.32 g magnesium sulfate to effect oxidation of the product aldehydes to their respective acids. The mixture was stirred at room temperature for 30 minutes after which time the solvent was removed under reduced pressure. The residue was extracted three times with 50 mL of hot water. The three aqueous solutions were then combined, filtered, and washed with 50 mL of chloroform. The aqueous layer was acidified with hydrochloric acid to a pH of 2 and then extracted with 50 mL of chloroform. The chloroform was removed in vacuo and the resulting residue dissolved in 0,5 mL of toluene. This solution was analyzed by GC on a chiral /3-cyclodextrin column which separated the two enantiomers of the resulting 2-arylpropanoic acid. This analysis indicated a 91 9 ratio of the S and R enantiomers for an ee of 82%. 

As the previous chapters have demonstrated, chiral auxiliaries have found a widespread application in the asymmetric synthesis of lignans. Among them, chiral oxazolidinones have been used extensively due to their ability to produce excellent diastereoselectivities in aldol as well as in numerous other reactions. For example, Kise et al. reported the use of (5)-4-isopropyl-3-(phenylacetyl)-2-oxazolidinone (141) in oxidative homocoupling reactions and its application in the asymmetric synthesis of dibenzylbutyrolactones and dibenzylbutandiols, Scheme (26) [86,87]. Treatment of 3-arylpropanoic acid derivative 142 with LDA in the presence of TiCU yielded a mixture of the dimeric compounds 143 in a ratio of 85 15 to 87 13. The major product having (R,R) configuration was converted into dibenzylbutyrolactones 145 in a three step sequence... 

Addition to multiple bonds. A water-soluble Pd catalyst (picolinic acid as one of the ligands) is applicable to the synthesis of 2-arylpropanoic acids from styrenes by carbonylation. ... 

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