Esters interesting examples

An interesting example of accelerating a reaction when high pressure is applied is the synthesis of a series of highly functionalized 4a,5,8,8a-tetrahy-dro-l,4-naphthalenediones 10 by cycloaddition of p-benzoquinone (8) with a variety of electron-poor dienic esters 9 at room temperature (Equation 5.2) reported by Dauben and Baker [6]. Using conventional methods, these heat-sensitive cycloadducts are difficult to synthesize free of the isomeric hydroquin-ones. When the reactions were carried out under thermal conditions, the primary cycloadducts were mostly converted into the corresponding hydroqui-nones. 

While in the uncatalyzed reaction of l-ethyleneimino-2-hydroxy-3-butene in THF, refluxing for four hours was necessary to produce 70% of the ester, in the presence of NaNH2 a 90% yield was achieved at room temperature after only five minutes.[13 14] An especially interesting example of the use of the imidazolide method for ester synthesis is illustrated by the total synthesis of actinomycin C3.[15],[16] Working with N-protected L-TV-methylvaline and CDI, esterification of the hydroxyl group on the threonine residue proved successful whereas this could not be accomplished by any of the conventional methods. 

Trimethylsilyloxyfurans were employed as C5-dianion equivalents in the spiroannulation with bi-functional ortho-esters to provide spirobutenolide derivatives <060L3705>. When the second step was performed under radical conditions, instead of using base, r is-fused bicyclo[3.n.0]lactones were formed <06CC1200>. An interesting example is... 

An interesting example of participation from a benzoyl group in the ester [29], having a conformationally rigid benzobicyclo[3.3.1]nonene structure, was reported in 1966 (Ghatak and Chakravarty, 1966). The ester [29] was rapidly hydrolysed in 1% methanolic potassium hydroxide at reflux. The esters [30], without a carbonyl group, and [31], isomeric to [29], were... 

Most published examples of prodrugs of relevance in the present context contain an a-amino acyl moiety. A number of reasons may explain this fact, such as the lack of toxicity of these natural compounds, the large differences in lipophilicity and other properties between amino acids, and the variability afforded by A-substituents. Interesting examples are provided by salicylic acid and metronidazole. Thus, the hydrolysis of tyrosine and methionine prodrugs of salicylic acid (8.104 and 8.105, respectively) was examined in rabbits after intraduodenal and intracecal administration [134], The former ester, but not the latter, was hydrolyzed in the mucosa of the small intestine. In addition, both prodrugs underwent marked hydrolysis by intestinal microflora. 

An exceptionally interesting example of the electrohydrocyclization reaction involves the use of allenes which are tethered to a,/S-unsaturated esters (Table 3) [19]. The chemistry takes place in a manner wherein the new carbon-carbon bond forms between the central carbon of the allene and the /S-carbon of the unsaturated ester. Of particular value is the preservation of one of the double bonds of the original allene, thereby providing functionality for further elaboration. It is important to carry out these transformations in an undivided cell, as the use of a divided cell led to hydrogenation of the olefins instead of cyclization. 

An interesting example of the effectiveness of different reagents for 0-nitration can be seen during the synthesis of neo-inositol-based nitrate ester explosives. l,4-Dideoxy-l,4-dinitro-neo-inositol (15), a compound readily prepared from the condensation of nitromethane and glyoxal in the presence of base,undergoes conversion to the tetranitrate ester (16) on... 

An interesting example of biocatalysis and chemical catalysis is the synthesis of a derivative of y-aminobutyric acid (GABA) that is an inhibitor for the treatment of neuropathic pain and epilepsy (Scheme 10.4). The key intermediate is a racemic mixture of cis- and trons-diastereoisomer esters obtained by a hydrogenation following a Horner-Emmons reaction. The enzymatic hydrolysis of both diaste-reoisomers, catalyzed by Candida antarctica lipase type B (CALB), yields the corresponding acid intermediate of the GABA derivative. It is of note that both cis- and trans-diastereoisomers of the desired enantiomer of the acid intermediate can be converted into the final product in the downstream chemistry [10]. 

An interesting example shows the stability of a 1,2,4-trioxolane ring hydrogen to radical abstraction relative to that on a 1,3-dioxolane. Reaction of (117) with A-bromosuccinimide under radical conditions gave selective formation of the bromoethyl ester (118) (Equation (21)) <94TL1743>. 

Let us consider now the case of a specific ionic polysaccharide. The unique properties of complexes of the cationic chitosan with non-ionic sorbitan esters provides an interesting example. Grant and co-workers (2006) have established that mixtures of chitosan and surfactant form emulsion-like solutions and/or creams, where the surfactant component is present as droplets or micelle-like particles and the chitosan solution acts as the system s continuous phase. It was established that the length and the degree of saturation of the surfactant hydrocarbon chain have a significant impact on the development of the chitosan-surfactant complexes. Moreover, an optimal distance between the chitosan s protonated amine groups is required for effective interactions to occur between the polysaccharide and the sorbitan esters. 

When one attempts E2 reactions with a-halo ketones using strong bases such as alkoxides, an interesting rearrangement pathway may occur called the Favorskii rearrangement. In this reaction, the a-halo ketone is converted to an ester. For example, 2-chlorocyclohexanone is converted to the methyl ester of cyclopentanecarboxylic acid by treatment with sodium methoxide in ether ... 

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