Sugars thioacetals

Allylstannanes are much better nucleophiles for dithioacetals than allylsilanes, and dimethyl(methyl-thio)sulfonium fluoroborate is a particularly good catalyst in this reaction. The reaction has been used tellingly in a macrocyclization (Scheme 56). ° Monothioacetals also react with allylstannanes, with cleavage of either the carbon-oxygen or the carbon-sulfur bond, depending upon the choice of Lewis acid (Scheme 57).Sugar thioacetals react in the same way (Scheme 58), but with the opposite anomeric stereoselectivity from that of the corresponding radical chain substitution. ... 

The isostere in which sulfur replaces the methylene group at the 4 position in the furan ring of customary sugars also shows good activity as an inhibitor of HIV reverse transcriptase. The initial step consists of formation of the thioacetal (53-3) from glyoxal benzoate (53-1) and the methyl acetal of thioglyoxal (53-2). Reaction of... 

The use of alkyl and alkaryl thioacetals for the identification of ribose57,68 as well as for the preparation of ribose derivatives90,102,153 has already been mentioned. D-Ribose appears to react with thiols somewhat more rapidly than the majority of sugars, a fact which may in part be attributable to its greater solubility in hydrochloric acid.58... 

Further Experiments on the Oxidation of Sugar Acetals and Thioacetals by Aceto-bacter suboxydans, D. T. Williams and J. K. N. Jones, Can. J. Chem., 45 (1967) 741-744. 

Thio-D-xylopyranose has been the sugar of this type most thoroughly investigated. Its synthesis was reported in 1961, independently and simultaneously from three different laboratories. With sodium thiocyanate, l,2-0-isopropylidene-5-0-p-tolylsulfonyl-a-D-xylofuranose (211) gives a thiocyanate that reacts with sodium sulfide to give l,2-0-isopropylidene-5-thio-a-D-xylofuranose (212). A better synthesis is the treatment of 211 with sodium thiosulfate, followed by reduction of the resultant Bunte salt with sodium boro-hydride. The same compound 212 is obtained by nucleophilic replacement of the 5-sulfonyloxy group in 211 by treatment with potassium thioacetate followed by deacetylation to 212, or by the reaction of 211 with sodium a-toluenethioxide, followed by scission of the resultant S-benzyl compound with sodium in liquid ammonia. 

A different type of sulfur-containing. sugar derivative had been obtained by Fischer many years earlier by the interaction of various aldoses with mercaptans. The sugars react, as aldehydes, with two moles of mercaptan and the mercaptal or thioacetal is formed. (These compounds will not be reviewed herein in detail.) In 1916 Schneider and Sepp made the important observation that these mercaptals would... 

The electrochemical deprotection of carbonyl compounds proved to be a useful method especially in cases where alternative chemical reactions are unsuccessful, a-Keto- and a-hydroxythioacetals, when oxidized in MeCN-HoO (9 1 v/v) on a Pt anode, are transformed into the corresponding a-diones and a-ketols [142]. Diethyl dithioacetals of sugars were anodically oxidized in MeCN-H20 (5% H2O) on Pt electrode, and the substrates were successfully deprotected producing the correspondent carbohydrates in 65-85% yield [143]. It is noteworthy that protected hydroxy groups as esters or cyclic acetals were not affected. Selective deprotection to carbonyl compounds electrooxidizing mixtures of thioacetals, like a ketone and an aldehyde thioacetal, the former being preferentially deprotected, was described [144]. 

The first total synthesis of the nucleoside antibiotic herbicidin B was accomplished in the laboratory of A. Matsuda. The key step was a novel aldol-type C-glycosidation reaction promoted by Sml2 between a 1-phenylthio-2-ulose derivative and a 1- 3-D-xylosyladenine-5 -aldehyde derivative. During the preparation of the phenylthio sugar subunit, the Moffatt oxidation was applied to convert the primary alcohol to the corresponding aldehyde, which was immediately oxidized with PDC in DMF/MeOH to the methyl ester. The reaction conditions were completely compatible with the silyl protecting group as well as the thioacetal functionality. 

The words acetal and thioacetal are written after the name of the parent sugar as in D-glucose dimethyl acetal 4.33 and D-glucose dimethyl dithioacetal 434. 

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