Imidates transesterification

Thus, the best compromises for Boc and Fmoc chemistries seem to be cyclohexyl and 2,4-dimethylpent-3-yl (Dmpn), which is of intermediate stability, and the removal of which by trifluoromethanesulfonic acid with the aid of thioanisole (see Section 6.22) leads to minimal imide formation (see Section 6.13). Points to note are that acidolysis of esters by hydrogen fluoride can lead to fission at the oxy-car-bonyl bond instead of the alkyl-oxy bond, thus generating acylium ions that can react with nucleophiles (see Sections 6.16 and 6.22), and that benzyl esters may undergo transesterification if left in methanol. The side reactions of cyclization (see Section 6.16) and acylation of anisole (see Section 6.22) caused by acylium ion formation do not occur at the side chain of aspartic acid.47-51... 

Alcohols can also be prepared from support-bound carbon nucleophiles and carbonyl compounds (Table 7.4). Few examples have been reported of the a-alkylation of resin-bound esters with aldehydes or ketones. This reaction is complicated by the thermal instability of some ester enolates, which can undergo elimination of alkoxide to yield ketenes. Traces of water or alcohols can, furthermore, lead to saponification or transesterification and release of the substrate into solution. Less prone to base-induced cleavage are support-bound imides (Entry 2, Table 7.4 see also Entry 3, Table 13.8 [42]). Alternatively, support-bound thiol esters can be converted into stable silyl ketene acetals, which react with aldehydes under Lewis-acid catalysis (Entries 3 and 4, Table 7.4). 

In reactions with acyclic dienes, the stereoselection with the (S)-valinol-derived cro-tonate imides is only about 3 1, but diastereoselection is high with N-acyl oxazolidines derived from phenylalanine. The chiral auxiliary is cleaved by transesterification with lithium benzyloxide in 85-95% yield. ... 

The different properties of ILs, with regard to their polarity, hydrophobicity, and solvent miscibility behavior through combination with different anions, are the reason for the different biocatalyst activities. Good to excellent activity of CALB was observed with a decrease in polarity and hydrophobicity and a viscosity increase of the ILs. In [bmim][PF6] a conversion of (R)-l-phenylethanol into the ester of 48.9% and an ee of 95.6% were achieved after 5h and 100% of (R)-l-phenylethanol was converted into the enantiopure (R)-l-phenylethyl acetate after a 1-day reaction. Immobilized CALB exhibited excellent stability, activity, and selectivity towards the (R)-enantiomer of 1-phenylethanol in [bmim][PF6]. In some research bis(trifluoromethylsulfonyl)imide-based ILs have been regarded as very suitable media for biocatalysis [39, 46, 50]. On the contrary, in the present work, lower suitability of the same IL was demonstrated. Since immobilized CALB catalyzed both hydrolytic and transesterification reactions, its enantioselectivity for long reaction times was lower. 

These results in Table I can be explained by reactions in Scheme 2. As shown in Figures 2b and 2c, peaks at 1270 cm due to C-O stretching band in conjugated ester disappeared, while peaks at 1730 cm due to ester C=0 groups remained. These results indicate that the transesterification proceeded in the presence of alcohols and diols. In case of diol, the transesterification at both ends would afford re-crosslinking. A similar trend was observed for other difunctional additives such as diamine and amino acid. When amino compounds were added, recovered polymers had a peak at 1700 cm as shown in Figures 2d and 2e due to cyclic imide moieties. 

Transesterification of imidates is a well-known reaction, which is catalyzed preferably by alkoxides. In 0-phosphorylated compounds (247 equation 136) such an exchange occurs with ease. From imidates and 1,2- and 1,3-amino alcohols 1,3-oxazolines " and 1,3-oxazines, respectively, were synthesized. 

The vast majority of recipes existing in the patent literature contains ethylene glycol (1). The reason is that poly(ester-imide)s used as thermosets are curing by a transesterification, so that ethylene glycol is formed and eliminated from the film by diffusion and evaporation. The boiling points of other diols are too... 

Patents on poly(ester-imide) resins where one of the components is ethylene carbonate [54, 55] or an imide modified linear poly(ethylene carbonate) [56] also exist. In another case the carbonate group is introduced into the molecule using diphenylcarbonate via transesterification reaction [57]. 

The curing of the poly(ester-imide) resins in the ovens of the enameling machines occurs via a transesterification reaction. Ethylene glycol is formed and evaporated. This reaction is catalysed by titanium esters like butyl or polycresyl titanates. Combinations are known of butyl titanate with toluensulfonic acid... 

In 1982, Evans reported that the alkylation of oxazolidinone imides appeared to be superior to either oxazolines or prolinol amides from a practical standpoint, since they are significantly easier to cleave [83]. As shown in Scheme 3.17, enolate formation is at least 99% stereoselective for the Z(0)-enolate, which is chelated to the oxazolidinone carbonyl oxygen as shown. From this intermediate, approach of the electrophile is favored from the Si face to give the monoalkylated acyl oxazolidinone as shown. Table 3.6 lists several examples of this process. As can be seen from the last entry in the table, alkylation with unactivated alkyl halides is less efficient, and this low nucleophilicity is the primary weakness of this method. Following alkylation, the chiral auxiliary may be removed by lithium hydroxide or hydroperoxide hydrolysis [84], lithium benzyloxide transesterification, or LAH reduction [85]. Evans has used this methology in several total syntheses. One of the earliest was the Prelog-Djerassi lactone [86] and one of the more recent is ionomycin [87] (Figure 3.8). 

The factors affecting the competition between transesterification and olefination in reactions of phosphonoalkylcarboxylates (111) with aldehydes and potassium carbonate in alcohols under heterogeneous conditions have been investigated.57 The reaction of the carbanion of I-cyano-1-fluoromethanephosphonate, prepared in situ from diethyl cyanomethane-phosphonate (112) and N-fluorobis(trifluoromethanesulphonyl)imide, with aldehydes and ketones provides a synthesis of a-fluoro-a,3-unsaturated nitriles (113) in moderate yield (Scheme 8).5 Wadsworth-Emmons... 

The analysis of polymerization processes in nanofiller presence does not differ principally from the one for transesterification model reaction [1]. In the present chapter some important aspects of such polymerization will be studied, mainly on the example of solid-phase imidization. 

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