Natural Thioglycosides

Another glycoside belonging in the same category as sinalbin is sinigrin, which is the potassium salt of a complex acid, myronic acid. It 

Schneider and Wrede, using potassium methoxide as the hydrolytic agent in place of myrosin, isolated thioglucose and merosinigrin. The latter results by closure of a new ring, after removal of the sulfuric acid group. 

Schneider and Wrede assumed a furanose ring for the merosinigrin in conformity with the general practice of that time, but appreciated that the second ring need not necessarily be in position (2). This structure does not appear to have been further investigated or confirmed. 

Sinalbin undergoes hydrolysis by myrosin to give sinapin (as the bisulfate), p-hydroxybenzyl isothiocyanate and n-glucose. Sinapin, in turn, hydrolyzes readily to choline and sinapinic acid. The structure of the latter was ascertained by oxidation to known reference substances. 

On the basis of these and other data, sinalbin was assigned the structure  

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From their method of preparation these compounds are evidently 0-pyranosides, so it is of interest that, unlike all the natural thioglycosides, they are not hydrolyzed by the enzyme myrosin. The latter, therefore, must be either an a-thiopyranosidase or a thiofuranosidase. 

Wong and coworkers showed that 5-azido sialyl donors protected with O-acetyl ester are useful for a-selective glycosylations of primary hydroxyls (Scheme 4.6e) [169]. It was proposed that the linear and electron-withdrawing nature of the C-5 azido moiety stabilizes the reactive axial acetonitrile adduct to allow the incoming nucleophile to approach the a-face in an SN2-like fashion. In addition, a chemose-lective glycosylation method has been developed for the synthesis of NeuAca-(2 —> 9) NeuAc as thioglycoside donor for use in the subsequent glycosylations [169]. 

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