Xylose dithioacetals, preparation

Enzymic syntheses are considered next. Xylitol is a substrate for sheep-liver L-iditol dehydrogenase, a NAD-linked enzyme. 1-Deoxy-D-xylitol, prepared by Raney nickel reduction of D-xylose diethyl dithioacetal in a 27% overall yield from D-xylose, was also reported31 to be a substrate, although with a higher Km and lower Vmax. The product was assumed to be l-deoxy-D-f/ireo-pentulose because of the appearance of a yellowish fluorescent spot when a chromatogram was sprayed with acidic 3,5-aminobenzoic acid, resembling that formed from 1 -deoxyfructose. There was no more-rigorous characterization. 

Triazoline imino sugar derivatives 297 that are prospective glycosidase inhibitors have been prepared as single diastereomers in high yield via an lAOC reaction of in situ generated azido alkene 296 (Eq. 32) [78]. m-CPBA oxidation of the dithioacetal groups in the 0-acetylated 5-azido-5-deoxydibenzyl dithio-acetal of o-xylose or D-ribose 294 to the bis-sulfone 295, followed by loss of HOAc between C-1 and C-2 provided the lAOC precursor 296. 

Like D-glucose and D-fructose, however, D-xylose can be utilized chemic ly or microbially—to generate a variety of interesting five-ca n c emica s o er than furfural (vide supra) or xylitol, a noncaloric sweetener, both being duectly produced from xylan hydrolysates, that is, without the actual isolation of the sugar. Other readily accessible intermediate products of high preparative utiUty (Scheme 2.14) are the open-chain fixed dithioacetal, the D-xylal, and D-hydroxy-xylal esters, or pyrazol or imidazol A -heterocycles with a hydrophilic trihydroxypropyl side chain. 

The same diamino sugar was also prepared through treatment of 5-azido-5-deoxy-2,3-0-isopropylidene-4-0-(mediylsulfonyl)-D-arabi-nose diethyl dithioacetal with sodium azide. Besides the normal reaction product of Sjv2 substitution, namely 4,5-diazido-4,5-dideoxy-2,3-0-isopropylidene-L-xylose diethyl dithioacetal, there was obtained a small proportion of 4,5-diazido-4,5-dideoxy-2,3-0-isopropyli-... 

The Secrist group also prepared 4 -thio-/3-D-arabinofuranosylpurinc nucleosides 111-114 and the cytosine analogue (4 -thio-ara-C) 116 from L-xylose, which was transformed, by the dithioacetal cyclization method, into precursors... 

Simple elimination did not occur as in the pyranoid series (see p. 232) when 2,3-frans-sulfonic esters were treated with the Tipson-Cohen reagent (sodium iodide and zinc in N,N-dimethylformamide) instead, 2,5-anhydro-3,4-di-0-p-tolylsulfonyl-D-xylose diisobutyl dithioacetal underwent a double elimination72 but see Ref. 76a. An alternative means of preparing 2,5-dihydrofuran derivatives that has its analogy in the pyranoid series (see p. 233) is, however, the treatment of appropriate aziridines with sodium nitrite in acetic acid, and, by this means, compound 41 [R = CH(S-isoBu)2, R = H] has been prepared in high yield.77... 

Horton and coworkers have described an enol thioacetal-forming, elimination procedure starting from 2,3 4,5-di-0-isopropylidene-n-xylose diphenyl dithioacetal (246) leading to 2-deoxy-4,5-0-isopropylidene-D-tfireo-pent-l-enose diphenyl dithioacetal (247), which is, formally, a carbohydrate ketene derivative. Compound 247, obtained in a yield of 62%, is of potential interest for the preparation of various rare sugars. Treatment with acid converts it into derivatives of heteroaromatic compounds (furans and pyrans). 

D-Xylose has been converted to (25)-3-(indol-3-yl)propane-l,2-diol 237 by two different routes, one involving direct Fischer indolization of 238. The dibenzyl-dithioacetal 239 was elaborated to the fused triazoline 240 following reaction with MCPBA. Initial oxidation was followed by elimination of acetic acid allowing intramolecular 1,3-dipolar cycloaddition reaction to construct the triazole ring. The bicyclic iV,S -acetals 242 and 241 were prepared by reaction of the 2,3-0-isopropylidene-D-ribofuranose with 2-aminoethane thiol followed by Mitsunobu reaction. These products are considered analogues of castanosper-mine and australine. ... 

Deoxy-5-seleno-D-arabino-, D-ribo- and D-xylo-selenoformates 8, 9 and 10 have been prepared from D-arabinose and known 2,3,4-tri-O-benzyl-D-ribose and -D-xylose diethyl dithioacetals, respectively/ The thermolysis of these compounds to afford l,5-anhydro-5-deoxy-5-seleno-D-pentitols is covered in Chapter 11. 

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