Esters substitution

The Chugaev reaction, or thermal decomposition of the substituted esters of the xanthates, gives olefins without rearrangement (35,36). For example ... 

The separation of Hquid crystals as the concentration of ceUulose increases above a critical value (30%) is mosdy because of the higher combinatorial entropy of mixing of the conformationaHy extended ceUulosic chains in the ordered phase. The critical concentration depends on solvent and temperature, and has been estimated from the polymer chain conformation using lattice and virial theories of nematic ordering (102—107). The side-chain substituents govern solubiHty, and if sufficiently bulky and flexible can yield a thermotropic mesophase in an accessible temperature range. AcetoxypropylceUulose [96420-45-8], prepared by acetylating HPC, was the first reported thermotropic ceUulosic (108), and numerous other heavily substituted esters and ethers of hydroxyalkyl ceUuloses also form equUibrium chiral nematic phases, even at ambient temperatures. 

Ester hydrolysis is the most studied and best understood of all nucleophilic acyl substitutions. Esters are fairly stable in neutral aqueous media but are cleaved when heated with water in the presence of strong acids or bases. The hydrolysis of esters in dilute aqueous acid is the reverse of the Eischer esterification (Sections 15.8 and 19.14) ... 

The simplification of the local anesthetic phaimacophore of cocaine to an aryl substituted ester of ethanolamine has been described previously. Atropine (S2) is a structurally closely related natural product whose main biologic action depends on inhibition of the parasympathetic nervous system. Among its many other actions, the compound exerts useful spasmolytic effects. 

The slower rate of hydrolysis of alkyl substituted esters in the presence of the cation exchange resin can be explained by the assumption that the alkyl groups interfere more in the formation of the intermediate complex on the resin surface than in the homogeneous system. The efficiency of the resin q was less than unity... 

Using an Alkylation Reaction to Prepare a Substituted Ester... 

The addition of tributylphosphine-stabilized alkylcopper to boron trifluoride coordinated (E)-enoatcs 4 gave, after aqueous workup, / -substituted esters in good yields and with 94-98 % de29. 

The (S )-valine based bislacdm ether adds regioselectively in a 1,6-fashion to a,/ -y,<5-unsat-urated -substituted esters with both simple and induced diastereoselectivity exceeding 99 1. This provides, after hydrolysis, virtually enantiomerically pure dimethyl ( )-2-amino-3-hep-tene-l,7-dioates 206. 

As the final proton transfer cannot occur when a-substituted esters are used, such compounds do not condense under the normal reaction conditions, but this limitation... 

Barrett and coworkers have explored hetero-substituted nitroalkenes in organic synthesis. The Michael addition of nucleophiles to 1-alkoxynitroalkenes or 1-phenylthionitroalkenes followed by oxidative Nef reaction (Section 6.1) using ozone gives a-substituted esters or thiol esters, respectively.41 As an alternative to nucleophilic addition to l-(phenylthio)-nitroalkenes, Jackson and coworkers have used the reaction of nucleophiles with the corresponding epoxides (Scheme 4.4).42 Because the requisite nitroalkenes are readily prepared by the Henry reaction (Chapter 3) of aldehydes with phenylthionitromethane, this process provides a convenient tool for the conversion of aldehydes into ot-substituted esters or thiol esters. 

This differentiation between meta- and para-substituted esters, independent of electronic effects, must be the manifestation of a steric effect associated with the interaction of the esters with the cycloamyloses specifically, inclusion of the ester within the cycloamylose cavity prior to hydro-... 

Values of /c2 and Kd for the reactions of the cycloamyloses with a variety of phenyl acetates are presented in Table IV. The rate constants are normalized in the fourth column of this table to show the maximum accelerations imposed by the cycloamyloses. These accelerations vary from 10% for p-f-butylphenyl acetate to 260-fold for m-f-butylphenyl acetate, again showing the clear specificity of the cycloamyloses for meta-substituted esters. Moreover, these data reveal that the rate accelerations and consequent specificity are unrelated to the strength of binding. For example, although p-nitrophenyl acetate forms a more stable complex with cyclohexa-amylose than does m-nitrophenyl acetate, the maximal rate acceleration, h/kan, is much greater for the meta isomer. 

An alternative method to prepare (Mormyl esters uses different building blocks to assemble the 1,4-dicarbonyl system and is complementary in many cases.10 Base-catalyzed addition of nitromefhane to a, J-unsaturated esters, followed by a variation of the Nef reaction, provides y-dialkoxy-substituted esters. The scope of this sequence has not yet been explored. Another approach involves cuprate additions to norephedrine-derived 2-alkenyloxazolidines this process allows small-scale synthesis of several p-formyl esters in optically active form (ee up to 95%).11... 

Newman and his co-workers (Newman and Hishida, 1962 Newman and Leegwater, 1968) have studied the effects of 6-methyl and 6-chloro substituents on the rates of alkaline hydrolysis of methyl 2-benzoyl- and 2-acetylbenzoates [17]. The 6-substituted esters hydrolysed faster than the... 

The chiral hydrosilylation of -substituted a,/3-unsaturated esters to their saturated counterparts is the subject of reports by two groups. The combination of triphenylphosphinecopper hydride and () )-DTBM-SEGPHOS is reported to give excellent yields of the -substituted esters (Eq. 353).598 Comparable yields, but with lower ee values, are reported for this transformation.599 600... 

Hata, T., Nakajima, M., and Sekine, M., Facile synthesis of fS-alkyl-substituted esters from a, 3-unsaturated aldehydes, Tetrahedron Lett., 2047, 1979. 

Alkylmanganese reagents were used in three-component, one-pot coupling reactions to afford a-substituted esters and amides.293 The reported reactions, where benzaldehyde or -hexanal is used as electrophile, are of particular relevance to this chapter, and examples are given in Schemes 65 and 66. 

This hydroxycyclopropanation of a terminal double bond also works perfectly well in an intramolecular situation, e. g. with terminally alkenyl-substituted esters, to yield substituted l-hydroxybicyclo[n.l.O]alkanols [85a,85b,100], aminobicyclo[n.l.O]alkanols [85c], and heterocyclic analogues [99a,99b,100], in which five- and six-membered rings are formed (Table 11.7). 

Silicone polymers which were developed in 1930s became commercially important during and after the World War II. The starting material for producing silicone polymers are alkyl chlorosilanes, arychlorosilanes or substituted esters of ortho-silicic acid. 

The cleavage of phenyl acetates by /3-CD shows the same general features as that by a-CD (Table A5.1), although there are quantitative differences that must arise from the larger cavity size of /3-CD. Generally, the mem-substituted esters are not cleaved as well as by a-CD and the para-substituted esters are cleaved better. Thus, the distinction between the kinetic parameters for two series of esters is less dramatic for /3-CD, presumably because of the looser fit of substituted phenyl groups in /3-CD. This trend is continued with the two entries for y-CD (which has a still larger cavity) where the differences between the meta and para isomers of t-butylphenyl acetate are quite small (Tables A5.1). Nevertheless, the... 

Only limited precedent exists for the stereoselective enolization and subsequent condensation of a-heteroatom-substituted esters 48a and 48b (eq. [29]). Ireland has examined the enolization process for a-amino ester derivatives where the Claisen rearrangement (chair-preferred transition states) was employed to ascertain enolate geometry (Scheme 10) (43). These results imply that 48a [X = N(CH2Ph)2 ] exhibits only modest selectivity for ( )-enoIate formation under the... 

Extensive investigations have been directed toward the development of chiral ester enolates that might exhibit practical levels of aldol asymmetric induction. Much of the early work in this area has been reviewed (111). In general, metal enolates derived from chiral acetate and propionate esters exhibit low levels of aldol asymmetric induction that rarely exceed 50% enantiomeric excess. The added problems associated with the low levels of aldol diastereoselection found with most substituted ester enolates (cf. Table 11) further detract from their utility as effective chiral enolates for the aldol process. Recent studies have examined the potential applications of the chiral propionates 121 to 125 in the aldol condensation (eq. [94]), and the observed erythro-threo diastereoselection and diastere-oface selection for these enolates are summarized in Table 31. For the six lithium enolates the threo diastereoselection was found to be... 

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