Aglycone group

In Table I, the more stable anomer is listed first. In each of the four cases, this anomer is the Irans-C 1 -C2-aldofuranoside, having a Irons relationship of the aglycon group to the hydroxyl group at C-2. Thus, the 0-d anomers of methyl D-ribofuranoside and methyl D-xylofuranoside are listed first, even though the interactions for the latter are equal. Later in this Chapter, the effect of this conformational stability will be discussed in relation to formation of products At present, it suffices to mention that there are two factors to be considered, the C-l and C-2 interactions, and the overall interactions (C-l and C-2, C-2 and C-3, C-3 and C-4). The... 

In the case of the methyl D-fructofuranosides, there is a relationship to the D-arabinofuranosides, with a different type of C-l, C-2 interaction. Here, the hydrogen atom on C-l has been replaced by a hydroxymethyl group, and the difference in the C-2 interactions with the C-3 hydroxyl group should be small for the anomers the effect of the aglycon group (OMe) and the hydroxymethyl group will probably be similar. 

In almost every case, the preponderant anomer is the trans-l, 2-glycoside the interaction between its aglycon group and the 02 hydroxyl group is much less than for the m-1,2 anomer. Conformational stabilities for the aldopentofuranosides have already been discussed (see Section II and Table I). To a certain extent, the conformational stabilities of the aldo-hexofuranosides are comparable to those of the corresponding pento-furanosides but the former have a bulkier group at CM to cause interactions that lower the stability. 

The stability of the furanoid ring may be altered in three ways. First, the polar nature of the groups at C-2 and C-3 can be altered. Secondly, another five-membered ring can be introduced into the molecule. Finally, the oxygen atom in the ring or attached to the aglycon group can be replaced by a different atom. Any or all of these transformations may raise or lower the stability of the aldofuranoside. 

The roles of nucleophilic assistance and stereoelectronic control in determining endo-versus exo-cyclic cleavage of pyranoside acetals have been investigated for a series of a- and j8-anomers.15 Exocyclic cleavage of a-anomers, via a cyclic oxocarbenium ion, is predicted by the theory of stereoelectronic control, and was found exclusively for the cases studied. The endocyclic route, with an acyclic ion, is predicted for the /1-structures, and a measurable amount was found in all cases, but its extent was dependent on temperature, solvent, and the nature of the aglycone group. 

Anomerization may occur by migration (without complete dissociation) of the aglycon group (A) from one side of the ring to the other, by way of an intermediate, molecular complex. The following formulation is given for purposes of illustration only. 

Anomerization may proceed by way of a bimolecular replacement process, under conditions where the environment can supply the aglycon group. 

Figure 1. Reaction mechanism of retaining b-glycosidases. R aglycon group. R = H, hydrolysis. R = alcohol, transglycosylation.

Figure 2. Azide rescue of an acid/base catalyst (A) and a nucleophile (B) mutant of a retaining b-glycosidase. R aglycon group.

Figure 11.3 shows the result on the direct observation by NMR of the interaction between duplex DNA and esperamicin. Thus, the planer aglycone group E in esperamicin intercalates between Cl - -G2 and C6 - -G3 of the duplex DNA, and the... 

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