Anomeric effect quasi

The influence of the classical anomeric effect and quasi-anomeric effect on the reactivity of various radicals has been probed. The isomer distribution for the deu-teriation of radical (48) was found to be selective whereas allylation was non-selective (Scheme 37). The results were explained by invoking a later transition state in the allylation, thus increasing the significance of thermodynamic control in the later reactions. Radical addition to a range of o -(arylsulfonyl)enones has been reported to give unexpected Pummerer rearrangement products (49) (Scheme 38).A mechanism has been postulated proceeding via the boron enolate followed by elimination of EtaBO anion. 

Most furanoses prefer the envelope conformation and it appears that a quasi-equatorial exocyclic side chain and a quasi-axial C(l)—0(1) bond (anomeric effect) are equally important stabilising factors (Figure 1.8). 

Figure 3.21 Time courses of the hydrolysis of an isotopic quasi-racemate in two acid-catalysed hydrolyses of glycosides. Both figures, taken from Ref. 107, by kind permission of Prof. A. J. Bennet and the American Chemical Society, refer to a mixture of labelled d-sugar and unlabelled l-sugar (10mgmL each), show the least-squares best fit and display only every fourth data point for clarity, (a) Ring effect for methyl (3-xylopy-ranoside (b) anomeric effect for methyl 5-thio-oc-xylopyranoside. See Table 3.3 for numerical values of the effects, which are in opposite senses.

The anomeric effect was also found to play a significant role in determining the conformation of methyl 2,3-anhydro-a- and -/3-L-ribopyranosides and the corresponding 2,3-unsaturated derivatives. Both anomers adopt that half-chair conformation having the bond to the 1-methoxyl group quasi-a.xia.1. 

The conformational effects arising from the endoanomeric effect are for furanoses much less profound and as a result relatively little research has been performed in this area. The puckering of the furanose ring of an a and a P anomer usually adjusts the anomeric substituent in a quasi-axial orientation and hence both anomers experience a similar stereoelectronic effect. On the other hand, the conformational preference of the exocyclic C—O bond is controlled by the exoanomeric effect in the usual way. 

The stereochemistry of 3-C-nitro glycals has been studied in some detail [210]. For example, when nitroanhydroglucitol 106 was subjected to reaction with triethylamine, it gave the elimination product 107, which was then rearranged to an equilibrated mixture of glycals 108 and 109 (O Scheme 36) [211,212]. Some researchers have tried to explain this equilibrium shift by arguing that the quasi-equatorial anomeric proton is made more acidic by the stereoelectronic effect [213]. 

Consideration of the anomeric equilibria of various acetylated, 2,3-un saturated, pyranoid sugars led to the conclusion that polar, allylic substituents favor the quasi-axial orientation (0.8 kcal.mole for the acetoxyl group). Such an allylic effect was found to exert significant control over the conformational tendency and configurational equilibria of unsaturated, pyranoid sugar derivatives. Thus, tri-O-acetyl-2-hydroxy-D-xylal (41) adopts the H (d) conformation. In... 

The two anomeric, unsaturated deoxyhexopyranuronates IV and VIII exist, however, in different conformations, as proved by n.m.r. spectroscopy. The characterization of these two reaction products was performed by study of their u.v., o.r.d., and c.d. spectra, shown in Fig. 2. As these compounds have an alkenic (enolacetal) linkage between C-4 and C-5, the conformation-stabilizing effect of the substituent on C-5 is no longer valid, because the alkoxycarbonyl group of the two anomeric, unsaturated deoxyhexopyranuronates IV and VIII has, in both anomeric compounds, a quasi-equatorial disposition. Therefore, the anomeric... 

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