Carboxy esters, preparation

Transesterifications, also termed ester exchange or ester interchange reactions, include hydroxy-ester, carboxy-ester, and ester-ester reactions. Hydroxy-ester reaction is the most important one and is used for many aromatic-aliphatic and wholly aromatic polyester syntheses. Carboxy-ester interchange is restricted to the synthesis of wholly aromatic polyesters while the ester-ester route is rarely used for polyester preparation due to slow kinetics. All these reactions may take place in comparable experimental conditions and can be catalyzed by similar classes of compounds. 

The carboxy-ester interchange melt polyesterification used for the preparation of this wholly aromatic copolyester is followed by a solid-state postpolyesterification (Scheme 2.65). (Reprinted from ref. 439. Copyright 1990 John Wiley Sons, Inc., by permission of the copyright owner.)... 

Transesterification between hydroxyl-esters, carboxy-esters, or two ester groups is one of the most important polyesterification technique used for preparing various aliphatic, aliphatic-aromatic or aromatic polyesters (also known as arylates). Titanium alkoxides are very efficient catalysts for transesterification polymerizations, however in some cases it can cause undesirable discoloration to the resulting polymer [15]. A representative example is the synthesis of a polyester based on dimethyl terephthalate using titanium isopropoxide [16] (Fig. 2.3). 

Transesterifications, also termed as ester exchange or ester interchange reactions, include hydroxy-ester, carboxy-ester, and ester-ester reactions. Hydroxy-ester interchange (called alcoholysis) plays a predominant role in most industrial preparations of aliphatic-aromatic polyesters such as PBT. The original industrial s)mthesis of PBT is based on a relatively expensive two-step melt pol)mierization reaction between dimethyl terephthalate and excess 1,4-butanediol of approximately 30-50 %, in the presence of catalyst [15-20]. V th respect to the catalyst used for the synthesis of PBT, in contrast to the s)mthesis of PET, one and the same metal complex is used for both reaction steps of the process. Usually tetra-alkoxy titanates are applied, often in combination with a particular cocatalyst. The reaction proceeds in two steps transesterification and polycondensation (Figure 5.2). The first, transesterification step, which is generally performed in an inert atmosphere to prevent oxidative side reactions, involves the conversion of the methyl... 

Good Z-selectivities were observed for the reaction of aldehydes with carboxy ylides prepared from the corresponding phosphonium salt and LHMDS. This strategy has been extensively used in the total synthesis of prostaglandins, monohydroxy-eicosatetraenoic acids (HETE), and leukotrienes. Eor instance, the Wittig condensation between aldehyde 5 and car-boxy phosphonium salt 6 in the presence of LHMDS provided 49% yield of the TBDPS-protected 10(5)-HETE methyl ester 7 (eq42). ... 

Reactions. Although carbapenems are extremely sensitive to many reaction conditions, a wide variety of chemical modifications have been carried out. Many derivatives of the amino, hydroxy, and carboxy group of thienamycin (2) have been prepared primarily to study stmcture—activity relationships (24). The most interesting class of A/-derivatives are the amidines which are usually obtained in good yield by reaction of thienamycin with an imidate ester at pH 8.3. Introduction of this basic but less nucleophilic moiety maintains or improves the potency of the natural material while greatiy increasing the chemical stabiUty. Thus /V-formimidoyl thienamycin [64221-86-9] (MK 0787) (18), C 2H yN204S, (25) was chosen for clinical evaluation and... 

The preparation of esters can be classified into two main categories (1) carboxy-late activation with a good leaving group and (2) nucleophilic displacement of a caiboxylate on an alkyl halide or sulfonate. The latter approach is generally not suitable for the preparation of esters if the halide or tosylate is sterically hindered, but there has been some success with simple secondaiy halides and tosylates (ROTs, DMF, K2CO3, 69-93% yield). ... 

Docosanedioic acid has been prepared by Wolff-Kishner reduction of 6,17-diketodocosanedioic acid, formed by reaction of the half-ester acid chloride of adipic acid with the a,co-cadmium derivative of decane (%26 overall yield).3 Reduction of Wolff-Kishner method, followed by simultaneous reduction and desulfurization with Raney nickel of the 2,5-bis(co-carboxyoctyl)thiophene pro-... 

Poly(unsaturated ester)-siloxane segmented copolymers have been prepared by the polycondensation of epoxy-terminated polydimethylsiloxanes and carboxy-terminated poly(ethylene adipate-co-maleate) oligomers 243). Reactions have been conducted in cellosolve solvent, at 140-150 °C, in the presence of 2% by weight potassium hydroxide catalyst. The molecular weights reported were fairly low. The same group has also prepared poly(hexamethylene adipate)-polydimethylsiloxane copolymers con-... 

Hydroxy-L-prolin is converted into a 2-methoxypyrrolidine. This can be used as a valuable chiral building block to prepare optically active 2-substituted pyrrolidines (2-allyl, 2-cyano, 2-phosphono) with different nucleophiles and employing TiQ as Lewis acid (Eq. 21) [286]. Using these latent A -acylimmonium cations (Eq. 22) [287] (Table 9, No. 31), 2-(pyrimidin-l-yl)-2-amino acids [288], and 5-fluorouracil derivatives [289] have been prepared. For the synthesis of p-lactams a 4-acetoxyazetidinone, prepared by non-Kolbe electrolysis of the corresponding 4-carboxy derivative (Eq. 23) [290], proved to be a valuable intermediate. 0-Benzoylated a-hydroxyacetic acids are decarboxylated in methanol to mixed acylals [291]. By reaction of the intermediate cation, with the carboxylic acid used as precursor, esters are obtained in acetonitrile (Eq. 24) [292] and surprisingly also in methanol as solvent (Table 9, No. 32). Hydroxy compounds are formed by decarboxylation in water or in dimethyl sulfoxide (Table 9, Nos. 34, 35). 

Heller, J., Ng, S. Y., and Penhale, D. W. H., Preparation of poly(carboxy-ortho esters) by the reaction of diketene acetals and carboxylic acids. In Preparation. 

The preparation and characterization of five co-ordinate [(triphos)Ir(cat)]Z, H2cat = 9,10-phen-anthrenecatechol, 1,2-naphthalenecatecol, 3,5-di-tert-butylcatechol, 4-methylcatechol, 4-carboxy-catechol Et ester, tetrachlorocatechol, Z=BPh4, PF6, have been reported.244 All compounds... 

Clark et al. [53] subjected primaquine to metabolic studies using microorganisms. A total of 77 microorganisms were evaluated for their ability to metabolize primaquine, of these, 23 were found to convert primaquine to one or more metabolites (thin-layer chromatography analysis). Preparative scale fermentation of primaquine with four different microorganisms resulted in the isolation of two metabolites, identified as 8-(3-carboxy-l-methylpropylamino)-6-methoxyquinoline and 8-(4-acetamido-l-methylbutylamino)-6-methoxyquinoline. The structures of the metabolites were proposed, based primarily on a comparison of the 13C NMR spectra of the acetamido metabolite and the methyl ester of the carboxy metabolite with that of primaquine. The structures of both metabolites were confirmed by direct comparison with authentic samples. 

Piperazine-2,5-diones can be symmetric or asymmetric. Symmetric DKPs are readily obtained by heating amino acid esters,1179-181 whereas asymmetric DKPs are obtained directly from the related dipeptides under basic or, more properly, acid catalysis, or by cyclocondensation of dipeptide esters.1182-185 As an alternative procedure hexafluoroacetone can be used to protect/activate the amino acid for the synthesis of symmetric DKPs or of the second amino acid residue for synthesis of the dipeptide ester and subsequent direct cyclocondensation to DKPs.1186 The use of active esters for the cyclocondensation is less appropriate since it may lead to epimerization when a chiral amino acid is involved as the carboxy component in the cyclization reaction. Resin-bound DKPs as scaffolds for further on-resin transformations are readily prepared using the backbone amide linker (BAL) approach, where the amino acid ester is attached to the BAL resin by its a-amino group and then acylated with a Fmoc-protected amino acid by the HATU procedure, N -deprotection leads to on-resin DKP formation1172 (see Section 6.8.3.2.2.3). 

In order to study the influence of a given chiral group at different positions in the precursor styrylbenzo[c]phenanthrene several carboxy styrylbenzo[c]phenanthrenes were synthesized and esterified with chiral alcohols, and the products were used to prepare hexahelicenes 76 substituted at various positions. The bromo-esters (29) were used as precursors for [6]-helicenes, substituted at position 1. 

Optically pure (M)-(—)-2-formylhexahelicene (76, R = 2-formyl), prepared from the corresponding resolved (—)-carboxy-menthyl ester, was used as a common precursor for the photochemical synthesis of optically pure (M)-[8]-, (M)-[9]-, (M)-... 

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