Cyclic esters, formation

The mechanisms of oxidation using bismuthate, periodate or lead tetraacetate, while still not completely understood, are probably similar, involving some type of cyclic ester formation as the first step. ... 

The electrophilic ester carbonyl is susceptible to nucleophilic addition, the most nucleophilic site now available is the oxygen of the oxime. Alcohols and esters react to give esters, and here there is lactone (i.e. a cyclic ester) formation by standard addition-elimination mechanism at an ester. 

Taking advantage of the more traditional cyclic ester formation between boronic acids and 1,2- and 1,3-diols can also lead to macrocyclic structures. It has been shown by NMR, FTIR, and size exclusion chromatography (SEC) that reaction between phenylene-l,3-diboronic acid and pentaerythritol principally produces 6 6, hexameric cycles (Fig. 16a)." As before, the crude reaction mixture was a dynamic mixture of cyclic and oligomeric products, but the... 

The following mechanism for the reaction has been suggested. It postulates formation of an intermediate paraconic ester (A) the irreversible alkoxlde cleavage of this cyclic ester drives the reaction to completion ... 

Catechols can be protected as diethers or diesters by methods that have been described to protect phenols. However, formation of cyclic acetals and ketals (e.g., methylenedioxy, acetonide, cyclohexylidenedioxy, diphenylmethylenedioxy derivatives) or cyclic esters (e.g., borates or carbonates) selectively protects the two adjacent hydroxyl groups in the presence of isolated phenol groups. 

The mechanism of this reaction has been studied by several groups [133,174-177]. The consensus is that interaction of ester with the phenolic resole leads to a quinone methide at relatively low temperature. The quinone methide then reacts rapidly leading to cure. Scheme 11 shows the mechanism that we believe is operative. This mechanism is also supported by the work of Lemon, Murray, and Conner. It is challenged by Pizzi et al. Murray has made the most complete study available in the literature [133]. Ester accelerators include cyclic esters (such as y-butyrolactone and propylene carbonate), aliphatic esters (especially methyl formate and triacetin), aromatic esters (phthalates) and phenolic-resin esters [178]. Carbamates give analogous results but may raise toxicity concerns not usually seen with esters. 

From their structures, it appears that the hydrolytic stability of macrocyclic lactones must necessarily be inferior to macrocyclic polyethers. Ease of synthesis of the cyclic esters is therefore one of the aspects which commend them to interest. It is probably for this reason that such lactones have not been made more often by the interesting approach of Kdgel and Schroder . These workers report the ozonolysis of dibenzo-18-crown-6 in a mixture of methanol and dichloromethane at —20°. Reduction of the ozon-ide at —75° using dimethylsulfide followed by warming and addition of acetone led to formation of 6 in 14% yield. The bis-oxalate had mp 164—165° from acetone, very similar to that of the starting crown. The transformation is illustrated below in Eq. (5.9). 

Cyclic esters, ring-opening polymerization of, 85-87 Cyclic lactams, 174 Cyclic oligoesters, 31 Cyclic oligomers, 63, 542 formation of, 39... 

The mechanism of oxidation probably involves in most cases the initial formation of a glycol (15-35) or cyclic ester,and then further oxidation as in 19-7. In line with the electrophilic attack on the alkene, triple-bonds are more resistant to oxidation than double bonds. Terminal triple-bond compounds can be cleaved to carboxylic acids (RC=CHRCOOH) with thallium(III) nitrate or with [bis(trifluoroacetoxy)iodo]pentafluorobenzene, that is, C6F5l(OCOCF3)2, among other reagents. 

At pH 13 the final products are manganate and cw-diol. The observation of a large degree of transfer of 0 from labelled MnO into oleate ion at pH 12 suggests formation of a cyclic ester, viz. 

One of the more difficult problems encountered in obtaining a valid assay of formic acid is that of formate ester formation. The formate is derived from the cyclic, hemi-acetal structure which is an equilibrium form of many free sugars in solution. For example, the oxidation of one of the cyclic forms of D-glucose can readily be seen to give a formate ester (as well as a C-formyl group) on the atom originally denoted as C5. It... 

Criegee had earlier postulated the formation of an intermediate cyclic ester, but this was later ruled out and now the above mechanism has been established. 

CC Yang, RB Merrifield. The P-phenacyl ester as a temporary protecting group to minimize cyclic amide formation during subsequent treatment of aspartyl peptides with HF. J Org Chem 41, 1032, 1976. 

Sequence inversion and racemization have been associated with uncatalyzed formation of the cyclic dipeptides and has been shown to greatly complicate the kinetics of formation. Cyclic dipeptide formation, by uncatalyzed processes, is rapid enough to pose an apparent threat to the stability of proteins and a possible rationale for the posttranslational N-acetylation of proteins that have been observed in higher organisms. The rate of DKP formation will also depend on the carbonyl ester protecting groups or the structures of the peptide-resin linkage in the solid-phase mode. Furthermore, cyclization is a concentration-independent reaction and demands the use of dilute solutions. ... 

The PET-oxidative cyclization of unsaturated O-alkyl-O-trimethylsilyl ketene acetals 23 and 27 yields cyclic esters 24, 25, and 28, accompanied by the formation of considerable amounts of non-cyclic esters 26 and 29, respectively [89], The cyclization mode is found to be in accordance with free radical cyclizations of the appropriate esters 26 and 29, performed by heating with organic peroxides [90]. Since organic electrochemistry can be used to oxidize... 

KMnC>4 and OsC>4 have long been used to achieve syn hydroxylation of the carbon—carbon double bond. The syn stereoselectivity results from the formation of cyclic ester intermediates formed by addition of the reagent from the less hindered side of the double bond. 

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