Hydrofuranol ring

If the disposition of hydroxyl groups is such that either an ethylene oxide or a hydrofuranol ring could be formed, then it is the three-membered anhydro ring that is preferentially established. Thus, if 6-tosyl-isopropylidene-D-glucofuranose (XII), in which there are present free hydroxyls at C3 and C5, is submitted to alkaline hydrolysis, it is the 5,6-anhydride VI alone that is formed7 the 3,6-anhydride appears only if the hydroxyl at C5 is protected by substitution as in X. 

The inference is that the hydrofuranol ring of XL can never be directly formed by the saponification of a 3-tosyl ester of D-glucose, but only indirectly by the intermediate formation and scission of an anhydro ring of the ethylene oxide type. The sequence of reactions involved in the conversion of methyl 3-tosyl-jS-D-gIueoside into methyl 3,6-anhydro-n-glucoside is shown by XXXVI to XL. 

When this paper11 was published (in 1938), the evidence indicated that the hydrofuranol ring compound isolated was the pyranoside form (XLVI). The later work of Haworth, Owen, and Smith,28 which is discussed in section V-l, corrected this impression and proved that the compound actually isolated by Peat and Wiggins was methyl 3,6-anhydro-/3-D-glueofuranoside (XLVIII). This correction does not invalidate the above argument, for it is known that XLVI is unstable and a trace of acid suffices to transform it into the stable furanoside. 

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