Asymmetric anhydride

Anilino vinyl derivatives of thiazolium (30, R = H) or acetanilido (30, R = C0CH3), as well as formyl methylene 30b (methods E-G), give asymmetrical dyes when condensed with a methyl reactive group of another species (Scheme 42). Mesosubstituted symmetrical or unsymmet-rical thiazolocyanines are obtainable via /S-alkylmercaptovinyl thiazolium derivatives (32) (methods H and I) (Scheme 43). a or /S carbon atoms of the trimethine chain can be substituted by acetyl when a dye is treated with acetic anhydride (method L). The hydrolysis of neocyanines lead to trimethine cyanine by fractional elimination of a composant chain (method K). 

Unsyimnetiical rhodainines can be piepaied by the condensations of one mole of a yW-aminophenol with phthahc anhydride to give an o-benzoyl benzoic acid (27) which is then further condensed with a different yW-aminophenol to give the requited product, Rhodamine 3GO (28) (2). A general route to asymmetrical acid xanthenes has been patented (39). 

The original procedure for the trifluoroacetylation of amino acids used trifluoroacetic anhydride [Acetic acid, trifluoro-, anhydride].4 This reagent, although inexpensive and readily available, has certain disadvantages it is a highly reactive compound and thus has caused undesired reactions such as the cleavage of amide or peptide bonds,5 unsymmetrical anhydrides are formed between the newly formed A-trifluoroacetylamino acids and the by-product trifluoroacetic acid, and excess trifluoroacetic anhydride has caused racemization of asymmetric centers. 

Very low asymmetric induction (e.e. 0.3-2.5%) was noted when unsymmetrical sulphides were electrochemically oxidized on an anode modified by treatment with (— )camphoric anhydride or (S)-phenylalanine methyl ester299. Much better results were obtained with the poly(L-valine) coated platinum electrodes300. For example, t-butyl phenyl sulphide was converted to the corresponding sulphoxide with e.e. as high as 93%, when electrode coated with polypyrrole and poly(L-valine) was used. 

As shown, an asymmetric carboxylic-sulfonic acid anhydride is formed, but the cellulose attack occurs on the C = O group, since a nucleophilic attack on sulfur is slow, and the tosylate moiety is a much better leaving group than the carboxylate group [193]. Similar to other acylation reactions, there is a large preference for tosylation at the 5 position, and cellulose tosylates... 

Enantioenriched alcohols and amines are valuable building blocks for the synthesis of bioactive compounds. While some of them are available from nature s chiral pool , the large majority is accessible only by asymmetric synthesis or resolution of a racemic mixture. Similarly to DMAP, 64b is readily acylated by acetic anhydride to form a positively charged planar chiral acylpyridinium species [64b-Ac] (Fig. 43). The latter preferentially reacts with one enantiomer of a racemic alcohol by acyl-transfer thereby regenerating the free catalyst. For this type of reaction, the CsPhs-derivatives 64b/d have been found superior. 

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

The synthesis in Scheme 13.40 features a catalytic asymmetric epoxidation (see Section 12.2.1.2). By use of me30-2,4-dimethylglutaric anhydride as the starting material, the proper relative configuration at C(4) and C(6) is ensured. The epoxidation directed by the (+)-tartrate catalyst controls the configuration established at C(2) and C(3) by the epoxidation. Although the epoxidation is highly selective in... 

Stepwise addition of the aniline moieties in analogy to Scheme 7 (Route A) allows the preparation of asymmetrically substituted derivatives.55,56 The use of pyridine-3,4-dicarboxylic acid anhydride similarly results in the formation of mixtures of 5- and 6-azaphthalides.57 Quinoline-2,3-dicarbo-xylic anhydride has also been converted into the corresponding azaphthal-ides in a similar manner.58 Pyrazine-2,3-dicarboxylic acid anhydride yields... 

Acetylene dicarboxylate and maleic anhydride failed to react with simple methylene cyclopropenes, but reacted readily with calicene derivatives, as shown by Prinz-bach293. Thus ADD combined with benzocalicene 497 to give the dimethyl tri-phenylene dicarboxylate 499, whose formation can be rationalized via (2 + 2) cycloaddition across the semicyclic double bond as well as (4 + 2) cycloaddition involving the three-membered ring (498/501). The asymmetric substitution of 499 excludes cycloaddition of ADD to the C /C2 triafulvene bond (500), which would demand a symmetrical substituent distribution in the final triphenylene derivative. 

Both compounds were tested for their catalytic activity in asymmetric aziridination using p-toluenesullbnic anhydride (TS2O) to activate the nitridomanganese complex. As shown in Scheme 4-60, the aziridination generally gave poor results, while addition of pyridine A-oxide improved both the yield and the enantiomeric excess of the products. 

Asymmetric esterification. me.vo-Cyclohexanedicarboxylic anhydride (1) undergoes a highly stereoselective esterification with the diphenylboric ester (2) of (R)-2-methoxy-l-phenylethanol in the presence of diphenylboryl triflate to provide,... 

Gosselin and colleagues169 prepared Karahana ether (264), starting with an asymmetric Diels-Alder reaction between chiral diene 261 and maleic anhydride. This reaction yielded diastereomers 262 and 263 in a 1 4 ratio (equation 72). 

Stereoselective ring cleavage and monoesterification of chiral Meldrum s acid derivatives has been achieved in high yield with a 34% enantiomeric excess under phase-transfer catalytic conditions in the presence of A-benzylquininium chloride [29]. A similar asymmetric ring-opening of prochiral (meso) acid anhydrides with... 

Reaction of exo-carbinol ( ) — 16 with racemic a-phenylbutyric anhydride afforded the dextrorotatory acid hence the chirality center has the absolute configuration (R). The behavior of the endo isomer 15 was also in accord with this finding the resolved levorotatory acid indicates that the asymmetric C atom has the absolute configuration (S). 

The essential role of our concept of ligand in the proper functioning of the Sequence Rule becomes apparent on examining an example taken from the paper by Cahn et al. (4). The authors state that C(3) of their anhydride 25 (15) is symmetric, as in the free acid, and hence receives no label. Both molecules lack symmetry beyond the trivial and ubiquitous one of C,. The center of C(3) of the anhydride is symmetric only in the sense that it is not linked to four different ligands and therefore is not an asymmetric carbon atom as defined by van t Hoff. However, this observation can be made only if the ring ligands are viewed as open-chain structures, as we are defining them, because only these... 

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