Thiols acceptor

Kusumoto and coworkers have found that the treatment of hemiacetal 1 with trifluoro- or trichloroacetic anhydride 94 (1 equiv) and trimethylsilyl perchlorate (0.2 equiv) selectively provides the corresponding anomeric ester intermediate 91 [152], Hemiacetal acylation occurs even in the presence of the alcohol acceptor. With Lewis acid assistance, the glycosyl ester intermediate is displaced to provide disaccharide products in good yields. This transformation allowed the synthesis of disaccharides 98 (81%) and 99 (91%). In some cases, acetic anhydride has been used as the electrophilic activator of hemiacetal donors and the reaction with thiol acceptors yields S-linked glycosides [153,154],... 

Figure 15.10 (a) The three components involved in the B12-dependent methyltransferases. (b) The MT2 enzymes have a thiol group which activates the thiol acceptor. (From Banneijee and Ragsdale, 2003. Reprinted with permission from Annual Reviews.)... 

Trichloroacetimidates function well as donors also with saccharide thiol acceptors. A number of thiodisaccharides have accordingly been prepared using this method [169,170,171,172,173], Possible by-products are various elimination products and orthoester formation. The stereochemical outcome is not easy to predict or control and, as so often in carbohydrate chemistry, an optimization of conditions is usually necessary for each glycosylation. For example, the use of an acetylated o -trichloroacetimidate glucose donor can yield not only an almost exclusively -linked disaccharide product but also preferentially an a-linked product, depending on the acceptor and conditions used ( Scheme 53) (see also Sect. 3.3). 

The second approach to thiooligosaccharides follows that outlined in O Scheme 2, i. e., an anomeric thiol (or thiolate) is first constructed and then reacted with a saccharide electrophile to give the thiooligosaccharide through an Sn2 displacement reaction. Problems to consider are the stereospecific introduction of the anomeric sulfur group, its (preferably selective) conversion into a thiol, and the nature of the electrophile. Side-reactions encountered in the couplings are elimination of the electrophile and anomerization of the thiol. In a model study of the s)uithesis of methyl 4-thiocellobioside, this approach was found to be superior to the one using thiol acceptors [179]. 

In a subsequent reaction, the thiol group of mercaptopyruvate is transferred to a thiol acceptor to form pyruvate and a disulfide. An enzyme supporting the latter reaction (mercaptopyruvate sulfotransferase) which uses dithioerythritol, mercaptoethanol, and mercaptopyruvate as acceptors has been reported in several higher plants, Chlorella, and Synechococcus (Schmidt, 1984). However, information on the formation of mercaptopyruvate in plants is required before... 

All the individual steps are catalyzed by enzymes NAD" (Section 15 11) is required as an oxidizing agent and coenzyme A (Figure 26 16) is the acetyl group acceptor Coen zyme A is a thiol its chain terminates m a sulfhydryl (—SH) group Acetylation of the sulfhydryl group of coenzyme A gives acetyl coenzyme A... 

Aliphatic Alcohols and Thiols. Ahphatic alcohols on reaction with chloroformates give carbonates and hydrogen chloride. Frequendy, the reaction proceeds at room temperature without a catalyst or hydrogen chloride acceptor. However, faster reactions and better yields are obtained in the presence of alkaU metals or their hydroxides, or tertiary amines. Reactions of chloroformates with thiols yield monothiolocarbonates (14). 

Mono-substitution occurs most readily in the stepwise replacement of the halogen substituents of 2,4,6-trichloro-s-triazine with aqueous methanol and sodium bicarbonate (30°, 30 min), the monomethoxy derivative (324) is obtained on heating (65°, 30 min), the disubstitu-ted derivative is formed and on brief heating (65°) with the more basic sodium carbonate or methanolic sodium hydroxide (25°, 3 hr) complete methoxylation (320) occurs. Ethanolic ethoxide (25°, 1 hr) or sodium carbonate (35°) is sufficient to give complete ethoxy-dechlorination. The corresponding phenoxy derivatives are obtained on treatment with one (0°), two (15°, 1 hr), or three equivalents (25-70°, 3 hr) of various sodium phenoxides in aqueous acetone. The stepwise reaction with phenols, alcohols, or thiols proceeds in better yield in organic solvents (acetone or chloroform) with collidine or 2,6-lutidine as acid acceptors than in aqueous sodium bicarbonate. ... 

Quite a number of asymmetric thiol conjugate addition reactions are known [84], but previous examples of enantioselective thiol conjugate additions were based on the activation of thiol nucleophiles by use of chiral base catalysts such as amino alcohols [85], the lithium thiolate complex of amino bisether [86], and a lanthanide tris(binaphthoxide) [87]. No examples have been reported for the enantioselective thiol conjugate additions through the activation of acceptors by the aid of chiral Lewis acid catalysts. We therefore focussed on the potential of J ,J -DBFOX/ Ph aqua complex catalysts as highly tolerant chiral Lewis acid catalyst in thiol conjugate addition reactions. 

Unsaturated acyl derivatives of oxazolidinones can be used as acceptors, and these reactions are enantioselective in the presence of chiral to-oxazoline catalysts.321 Silyl ketene acetals of thiol esters are good reactants and the stereochemistry depends on the ketene acetal configuration. The Z-isomer gives higher diastereoselectivity than the Zf-isomer. 

Alkylidenemalonate esters are also good acceptors in reactions with silyl ketene acetals of thiol esters under very similar conditions.322... 

Yu et al. prepared 48, bound to an Au(lll) surface by a thiol on the biphenyl end, then deprotected on the bipyrimidine end to expose a thiol to bind an Au nanoparticle. Rectification was measured by scanning tunneling spectroscopy (STS), and it could be reversed by the addition of acid, which protonated the pyrimidine rings, converting them from donor to acceptor [121]. Yu s group has also inverted the attachment of a rectifier, as shown with 49 and 50, and confirmed that the rectification direction reversed [122]. This was recently verified by STM-BJ measurements [123]. 

Gow et al. [45] proposed that NO may directly react with thiols because the formation of nitrosothiols is possible under anaerobic conditions in the presence of another dioxygen electron acceptor. 

Another free radical, which is supposedly formed in biological systems is the nitric dioxide N02. This radical is much more reactive than nitric oxide its rate constants with thiols, urate, and Trolox C are about 107—10s lmol-1 s 1 [88,89] (Table 21.1). It has been proposed [88] that thiols are dominant acceptors of N02 in cells and tissues while urate is a major scavenger... 

Gow and Stamler (Gow and Stamler, 1998) looked more carefully at Hb-SNO chemistry under more physiological conditions i.e. at NO to Hb ratios ranging from 1 100 to 10 1. They reported that at NO Hb ratios of -1 20 NO is bound to the hemes. As 02 is slowly introduced the NO is transferred from the Heme to the )G93 thiol and 02Hb-SNO is converted to the R state. They also presented spectral evidence that in the absence of oxygen, at low NO Hb ratios (< 1 20) the major product is nitrosyl Hb, whereas metHb is produced at intermediate NO Hb ratios (1 20 to 1 2) and nitrosylH b is once again produced at ratios of > 1 2. The authors proposed that when Heme-Fe(II)-NO is in close proximity to [5G93-S11, NO is directly transferred to yield SNO H. They further proposed that 02 would then act as an electron acceptor to yield RSNO plus superoxide. 

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