Thio-sugars. Part 2. Glycosides from acid-catalysed reactions of 1-O-acetyl-2,3-di-O-acyl-4-thiotetrofuranoses
Alcohols and thiols in presence of toluene-p-sulphonic acid readily displace the 1-O-acetyl group from the title compounds, now available by Pummerer rearrangement of the appropriate sulphoxides, thus providing a very convenient synthesis of these thio-sugar glycosides. Glycosyl bromides can also be prepared. The advantages of the phenylboronate function described in Part 1 are further illustrated. O-Glycosides are formed with a high degree of stereoselectivity, S-glycosides are not. Toluene-ω-thiols with electron-repelling substituents react normally, but the corresponding alcohols are acetylated rather than glycosylated. A disaccharide (glycosyl glycoside) has been prepared. Other esters give lower yields of glycosides than the phenylboronates. The reactions with hydrazides of the substituted 3-heteraglutaraldehydes from periodate oxidation of some of these glycosides and related nucleosides were compared with earlier examples.