Enantiomers carboxylic acid-containing

Carboxylic acids containing double bonds are easily converted to saturated acids by catalytic hydrogenation over common catalysts. If a new chiral center is generated in the reduction process, homogeneous hydrogenation over a chiral catalyst gave 40-45% enantiomeric excess of one enantiomer [19],... 

Aziridines can be detected by a color reaction with 4-(4 -nitrobenzyl)pyridine that was originally developed for the detection and/or determination of a wide range of alkylating substances. 58 It can be adapted to TLC. 59 60 Upon acid hydrolysis of aziridine-2-carboxylic acid containing peptides, as required for amino acid analysis, this amino acid is recovered as serine in varying yields. 47 Configurational characterization of aziridine-2-carboxylates is performed by NMR spectroscopy 47 51 61-63 and X-ray analysis. 61 In addition, optical rotation of the related benzyl Wtritylaziridine-2-carboxylate is also used 64 with [a]D —95.5 (c 1, THF) for the 25-enantiomer 44 as reference. 

Schreiber and co-workers (436) prepared a library calculated to contain 2.18 million polycyclic compounds through the 1,3-dipolar cycloaddition of a number of nitrones with alkenes supported on TentaGel S NH2 resin (Scheme 1.83). (—)-Shikimic acid was converted into the polymer bound epoxycyclohexenol carboxylic acid 376 (or its enantiomer), coupled to the resin via a photolabile linker developed by Geysen and co-workers (437) to allow release of the products from the resin in the presence of live cells by ultraviolet (UV)-irradiation. A range of iodoaromatic nitrones (377) was then reacted with the ot,p-unsaturation of the polymer-bound amide in the presence of an organotin catalyst, using the tandem esterification/ dipolar cycloaddition methodology developed by Tamura et al. (84,85) Simultaneous cyclization by PyBrop-mediated condensation of the acid with the alcohol... 

Beginning with the enantiomer of 27, the reaction sequence outlined in Fig. 4 was repeated, leading to compound 14 (63). Selective oxidation of 14 afforded the carboxylic acid 23(63). The arsenic-containing nucleoside 25 was obtained by treating 5 -chloro-5 -deoxyadenosine with excess dimethylarsinosodium and oxidation of the resultant arsine (63). 

Peptide a-oxo esters 1 (R4= alkyl, substituted alkyl) and a-oxoamides 2 can be prepared by treatment of a-substituted carboxylic acid derivatives (X1 = H, X2=OH X1, X2=CH2, N2) with an oxidizing agent (Scheme 3).Pa6,s,9] The final peptidyl products can contain either a mixture of enantiomers at C2 in PI of the peptide or be optically pure depending on the method employed for the preparation of the key intermediates or peptide a-substituted carboxylic acid derivatives, or the choice of the oxidizing agent in the final step. 

More recently, a facile synthesis of 126 and pseudo-a-L-glucopyranose (137) has been described [32], When the bromolactone 38 was heated with glacial acetic containing hydrogen bromide and subsequently acetylated, DL-(l,3,5/2,4)-2,3-diacetoxy-4,5-dibromocyclohexane-l-carboxylic acid (130) was obtained [31], Resolution of 130 with optically active a-methylbenzylamines provided the enantiomer (131), m.p. 

In the copolymerisation of D- and L-enantiomers of alanine A-carboxylic acid anhydride in systems containing an excess of the L-enantiomer with respect to the D-enantiomer and with diisobutylaluminium A, /V-d ietli y I ami de as the catalyst, the content of L-enantiomer units in the polymer formed was always higher than the content of this enantiomer in the monomer feed [173], This fact shows the enantiomer present in excess in the starting monomer mixture to have been polymerised preferentially throughout the copolymerisation [75],... 

Many organosulfur compounds can be resolved into optically active forms (enantiomers) owing to the presence of a chiral (asymmetric) sulfur atom 5 important examples include sulfoxides and sulfonium salts. Chiral sulfoxides containing amino or carboxylic acid groups have been resolved by formation of the diastereoisomeric salts with d-camphor-10-sulfonic acid or d-brucine. The salts can then be separated by fractional crystallisation and the free optically isomeric sulfoxides liberated by acid hydrolysis. However, a more convenient synthetic procedure for the preparation of chiral sulfoxides of high optical purity is Andersen s method (see p. 30). 

BINAP was first introduced by Noyori [80]. It has been particularly explored for reduction with ruthenium catalysts. BINAP is an atropisomeric ligand because rotation aroimd the central C-C bond is blocked. Accordingly BINAP exists in two enantiomers. Complexes of Ru(II) with BINAP are extremely powerful catalysts for enantioselective hydrogenations of prochiral a,p- and P,Y-unsaturated carboxylic acids, enamides, allylic and homoallylic alcohols, imines etc. [83]. In many cases, the hydrogenation is quantitative with enantiomeric excesses of over 95%. A wide variety of vitamins, terpenes, P-lactam antibiotics, etc. are accessible by the use of catalysts containing the BINAP stereogenic element. An example for 3-oxo carboxylic esters is shown in reaction (1) of Fig. 6.32. 

The Diels-Alder reaction between furan and maleic anhydride is reversible and gives the more stable exo-adduct 100 (also commercially available). This compound contains the bicyclic ring system of ifetroban but is achiral and the key problem is to disrupt symmetry of the adduct 100 in a controlled way. The original synthesis used to make the drug17 converted the adduct 100 into the menthyl acetal 101 as a single enantiomer in four steps and then into the carboxylic acid 102 in another six steps. This strategy amounts to a resolution as each menthol adduct could be isolated by crystallisation of a diastereoisomeric mixture in only about 30% yield. 

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