Thioesters, radical carbonylations

Scheme 43 Formation of thioesters through radical carbonylations...

The mechanism of these reactions was proposed to take place by initial electron transfer to the carbonyl group of the thioester, generating a ketyl-like radical anion such as 103 (Scheme 7.43). Subsequent radical addition to the electron-deficient alkene (acrylamide or acrylate), possibly guided by pre-complexation to a Sm(III) metal ion, generates a new radical centre, which is reduced to the corresponding Sm(III) enolate by a second equivalent of Sml2. Protonation of this enolate and hydrolysis of the hemithioacetal upon work-up then lead to the y-ketoamide or ester. 

This enzyme s role in humans is to assist the detoxification of propionate derived from the degradation of the amino acids methionine, threonine, valine, and isoleucine. Propionyl-CoA is carboxylated to (5 )-methylmalonyl-CoA, which is epimerized to the (i )-isomer. Coenzyme Bi2-dependent methylmalonyl-CoA mutase isomerizes the latter to succinyl-CoA (Fig. 2), which enters the Krebs cycle. Methylmalonyl-CoA mutase was the first coenzyme B -dependent enzyme to be characterized crystallographically (by Philip Evans and Peter Leadlay). A mechanism for the catalytic reaction based on ab initio molecular orbital calculations invoked a partial protonation of the oxygen atom of the substrate thioester carbonyl group that facilitated formation of an oxycyclopropyl intermediate, which connects the substrate-derived and product-related radicals (14). The partial protonation was supposed to be provided by the hydrogen bonding of this carbonyl to His 244, which was inferred from the crystal structure of the protein. The ability of the substrate and product radicals to interconvert even in the absence of the enzyme was demonstrated by model studies (15). 

S -Phenyl chlorothioformate was used as a radical trap in the radical-mediated carboxylation approach (Scheme 18) [40b]. Among several carbonyl derivatives including phosgene and bis(thiophenyl) carbonate, 5-phenyl chlorothioformate gives the best result and works with primary, secondary, and tertiary alkyl iodides. In addition to the desired thioester, a small amount of the corresponding sulfide is isolated as a by-product. 

Section 15.4 contains anion radicals from nitro compounds. No subdivision has been made into compounds containg one, two or more nitro groups. In the case of a dianion that follows die monoanion directly. Section 15.5 contains tables of magnetic data obtained from anion radicals wifli carbonyl functionality and their sulphur analogs. The data have been divided into subsections consisting of esters and thioesters aldehydes, ketones and their thio analogs semidiones and acid anhydrides. These subsections have been furdier subdivided, for example the subsection Esters and thioesters has been subdivided into arylesters, fliioe-sters and oxocarbothioate and dithioate esters. 

Antioxidants (Newton, 1986 Regan, 1986) - Antioxidants inhibit polymer oxidation. They are normally used with UV absorbers since they often they exhibit a synergistic effect. Antioxidants can be divided in primary and secondary types. The primary antioxidants are able to trap free radicals formed by oxidation or UV reaction processes. Aromatic amines and substituted phenols are of this type. Care should be taken when using aromatic amines as these can stain light colored sealants. Secondary antioxidants can break down any hyperoxides that may be formed, preventing carbonyl group formahon and free radical initiahon. Thioesters and organophosphites function as secondary antioxidants. 

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