Rings pyranoid, stability

Studies on the rates of hydrolysis of di-O-isopropylidene derivatives of ketoses indicated that the relative order of stability of 2,2-di-methyl-l,3-dioxolane rings is in the order cis-fused to a furanoid ring > sptro-fused to a pyranoid ring > cts-fused to a pyranoid ring > sptro-fused to a furanoid ring.8 Thus, partial hydrolysis of l,2 3,4-di-0-isopropylidene-/3-D-en/fhro-2-pentulofuranose (1) to the 3,4-acetal (2)... 

Evidence of complexing was obtained with the anomeric methyl 3,6-anhydro-D-glucopyranosides (see Table II), which suggests that the oxygen atoms on the pyranoid ring can play a role in ester stabilization (45) but, with l,6-anhydro-/3-D-glucopyranose (46), no complex was... 

As may be seen from Tables IV and V, Fischer and Delbriick s iso-trehalose is probably identical with /3,j8-trehalose. The a,j8-trehalose claimed first in 1918 reduced neither Fehling solution nor neutral permanganate. It was formulated as containing two D-glucopyranose residues, and its specific rotation (compared to that of a,a-trehalose) indicated that it was probably the a,fi isomer. The stability of a,a-trehalose to hydrolysis by N hydrochloric acid (compared to the lability of sucrose) favored the pyranoid ring for the two moieties. 

There are four main types of pyranoid and furanoid sugars bearing a single, enolacetal double bond in an endocyclic or exocyclic position. Secondary efiFects on the ring stability, for example, the anomeric effect, will not be considered here. Furthermore, only preparative, experimental observations of a qualitative nature are discussed. More-exact quantitative measurements have not yet been made. 

Conversely, FucA will produce diastereomers at low de, because of the more balanced stability relationships for pyranoid products sharing a as-diol substitution pattern. With racemic 2-hydroxylated aldehydes, thermodynamic control in FruA- or RhuA-catalyzed reactions favors (2R) configured enantiomers but also with lower discrimination (de up to 55%), because of the higher fiexibility of the corresponding ketofuranose rings [25, 206]. 

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