Thio esters, conversion

The biosynthesis of menaquinone is outlined in Fig. 44. Isomerization of chorismate to isochorismate followed by condensation with a-ketogluta-rate and aromatization gives o-succinylbenzoic acid. Conversion of 238 to the CoA thio ester, followed by cyclization, prenylation and methylation completes the biosynthesis. The biosynthesis of the prenyl side chain follows the alternative terpene biosynthetic pathway described for ubiquinone. 

Mercuric trifluoroacetate [Hg(OCOCF3)2] is an effective reagent for the cyclization of hydroxy thio-esters. 5 This cyclization procedure was used by Masamune et al. for the conversion of 225 to 227 (in 90% yield) in a synthesis of zearalenone dimethyl ether. Masamune developed this procedure to complete the total synthesis of methymycin, where one step involved treatment of a thioester with mercuric salts. 2,96 Mercury(II) has an affinity for bivalent sulfur and it binds the thioester unit as well as the terminal hydroxyl... 

Scheme 1.2) [2, 9, 10], (A minor pathway is from the a-amino acid, leucine.) The initial steps involve reactive thio-esters (Co-A esters) and lead to )8-hydroxy-)8-methylglutaryl-CoA [(4.12) HMG-CoA]. The conversion of (4.12) into mevalonic acid (4.13) is irreversible the mevalonic acid has no metabolic future except in the formation of terpenes and steroids. 

The standard method for the conversion of a-thio-esters into a,/3-unsaturated esters involves oxidation to the corresponding sulphoxide followed by pyrolysis. An alternative approach, which avoids the use of an oxidant, is by formation of a sulphonium salt using CFjSOaCHzSiMea followed by ylide formation using caesium fluoride and rearrangement (Scheme 44). This method can run into difficulties if the starting thio-ester already contains an olefinic bond. 

Masamune has discussed the use of the benzothiazole derivative (132) in a high yield conversion of carboxylic acids into amides or thio esters (Scheme 48). It appears that both the acid chloride and acid anhydride are formed during the reaction, the chloride giving the desired product while the anhydride is hydrolysed to the corresponding acid which is recycled. 

In the reaction with 2-octanol catalyzed by C. antarctica lipase B, Ohmer et al. compare four acyl donors differing only in (heir leaving groups [7] (Table 2). In the case of octanoic acid, its low compared with the other acyl donors makes it beneficial at high conversions since it competes well with the product ester for the free enzyme (Scheme 5). However, the initial rate is lower than for other acyl donors, resulting in a slightly longer reaction time than for the thio ester and vinyl ester. 

Notes A combination of phosphorus pentasulfide and hexamethyldisiloxane can be used to prepare thionoesters and thionolactones in comparable yields and with a simplified workup.1 A solvent-free conversion of ketones, lactones, esters and amides, to the corresponding thio analogs with Lawesson s Reagent.2 See also Belleau s and Daw ,v Reagents. 

Conformational factors in furanoid sugars can dictate the stability of a frans-benzylthio-sulfonate system toward formation of episulfonium ion. Christensen and Goodman noted that the 3-)S-benzyl-3-thio-D-arabinoside ester (311) requires vigorous conditions for conversion into the mixture of azides (313) that arises by opening of the intermediate ion... 

A popular reaction type in recent years has been the conversion of alcohols into other compounds by use of an organophosphorus compound in the presence of a source of a nucleophile [reaction (8)]. PhsP plus diethyl azodicarboxylate has been a particularly popular form of organophosphorus reagent, and the product is formed with inversion of configuration. In 1977 there appeared a number of variations on this general theme, for the synthesis of halides, thiocyanates, thio-ethers, azides, and esters from alcohols.Similar systems have been used to convert amides and thioamides into nitriles. ... 

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