Diacyl asymmetrical

Figure 2.40 illustrates some of the methods which can be employed for the preparation of diacyl peroxides [(RC02)2], both symmetrical and asymmetrical.120 122... 

Asymmetric Alkylation. 4-Pseudoephedrine ([IS, 2S]-(+)) is a commodity chemical employed in over-the-counter medications with annual worldwide production in excess of 300 metric tons. The enantiomer, /-pseudoephedrine, is also readily available in bulk and is inexpensive. Pseudoephedrine has been shown to be highly effective as a chiral auxiliary in asymmetric alkylation reactions. Treatment of either enantiomer of pseudoephedrine with carboxylic acid chlorides and anhydrides leads to efficient and selective iV-acylation to form the corresponding tertiary amide derivatives (Table 1). Typically, the only by-product in the acylation reactions is a small amount (<5%) of the A,0-diacylated product, which is easily removed by crystallization or flash column chromatography. Because intramolecular 0- -N acyl transfer within pseudoephedrine 3-amino esters occurs rapidly, and because the A-acyl form is strongly favored under neutral or basic conditions, products arising from (mono)acylation on oxygen rather than nitrogen are not observed. 

Acylation of 3,4,7,8-tetramethylglycoluril by the method of Sun and Harrison [34] leads to the monoacyl derivatives, which can be further selectively acylated with lithium diisopropylamide (LDA) and acyl chlorides. The resulting symmetrical or asymmetrical diacyl compounds undergo a base-catalyzed acyl transfer reaction. Consequently, the glycoluril acts as a temporary template that facilitates a condensation between acyl units. The absence of O-acylation products is explained by chelation of the lithium by the intermediate enolate. 

The enolates of //-acylimidazolidinones derived from 1.131 generated from ephedrine 1.61 (R = H) are useful in asymmetric alkylations [447, 448] and aldol reactions [449, 450] and cuprate additions to the a,p-unsaturated acyl analogs have recently been described [451], These chiral auxiliaries are cleaved by MeONa/MeOH or LiEtBHj. Recently, Davies and coworkers have suggested the use of symmetrical AyV-diacyl-1,3-imidazolidin-2-ones 1.132, formed from diamines having a C2 axis of symmetry [452], for asymmetric aldol reactions [449]. Juaristi [453] has used peihydropyrimidin-4-ones for related purposes. 

Asymmetric hydrogenation of 1,2-diacyl- and 1,2-bis(alkoxycarbonyl)-2,3,4,5-tetrahydro-1 diazepine-7-carboxylic acids (80), using optically active ruthenium diphenylphosphine catalysts, for example (81), proceeds in high yields and optical purities (Equation (16)) in a process that has been applied to the synthesis of an intermediate to the antihypertensive drug, cilazapril <93EUP570764>. 

The choline- and ethanolamine-containing phospholipids of human platelets can be subdivided, based on the nature of the linkage to the fatty acid residue at the sn-1 position of the glycerol backbone, into 1,2-diacyl, l-O-alkyl-2-acyl and l-0-alk-l -enyl-2-acyl species (Table 1.2). The majority of arachidonic acid in the phosphatidylcholines is present in the diacyl species while 1-0-alk-l enyl-2-acyl-PE (plasmalogen) contains most of the arachidonic acid in the phosphatidylethanolamines. The amount of arachidonic acid present in the diacyl species of phosphatidylethanolamine is similar to that in diacyl phosphatidylcholine. Ferret et al have shown that the distribution of arachidonate in the phospholipids of the platelet plasma membrane is asymmetric, the majority of it being present in the inner leaflet. 

Symmetrical (two equal fatty acid chains) (Figure 5.4, 1) and asymmetrical (two different fatty acid chains) (Figure 5.4, 2) nonionic double-chain surfactants of the type A ,A -diacyl lysine polyoxyethylene glycol amide compounds, with a structural resemblance to natural lecithin phospholipids, have been reported by the authors lab [35-38] to determine the effect of several structural parameters (hydrophobic chain length, polyoxyethylene (POE) chain length and number of polyoxyethylene chains) on the physicochemical properties and biological performance of these natural mimics. 

DYKAT is a dynamic system closely related to DKR. Although four types of DYKAT have been reported in the literature, all the examples reviewed here are included in DYKAT type 111. They focus on the asymmetric transformation of a diastereomeric mixture of enantiomeric pairs of acyclic and cyclic diols by means of lipase-catalyzed transesterification and epimerization of the chiral centers mediated by the Ru catalyst 3a. The major differences with respect to DKR reside in the formation of chiral intermediates (hydroxyketones) during the metal-catalyzed epimerization as well as the involvement of two successive enzymatic transformations with different selectivities. An additional complexity originates from possible intramolecular acyl-migrations. Nevertheless, if similar requirements to those described for an efficient DKR are fulfilled, the result of these DYKATs could be the formation of only one stereoisomer of the diacylated product. 

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