Anomeric effect competition

Praly and Lemieux (66) have estimated the exo anomeric effect to be 1.66 times as strong for j3-anomers ap, sc here competition between anomeric effects cannot operate) as for a ones (sc, sc). In order to evaluate the corresponding relation between anomeric effects in an acyclic system, let us return to dimethoxymethane (11, Scheme 9). The anomeric effect A for the ap, sc conformer of 11, calculated to be 21.1kJ/mol (cf. Scheme 6),... 

An extreme possibility for the anomeric effect is one-directionality. Such a situation occurs when there is no competition between the anomeric effects, that is, when both AG g,g,x AE(b)x equal to zero. Then AE(a-b) = AEU)Y observed anomeric effect for a... 

Mikolajczyk M, Graczyk P, Wieczorek MW, Bujacz G (1991) Conformational preference of 2-triphenylphosphonio-l,3-dithianes competition between steric and anomeric effects. Angew Chem Int Ed Engl 30 578-580... 

Figure 6.54 Competition between gauche and anomeric effects shifted by acceptor ability of the anomeric group X.

However, carbohydrates also possess additional polar substituents that combine with the anomeric effect to yield new stereoelectronic patterns. In fact, the chemistry of carbohydrates provides a great testing ground for the design of cooperative stereoelectronic patterns. In Chapter 6, we briefly discussed the competition between gauche and anomeric effects. We will expand our discussion of the interplay between stereoelectronic effects... 

The procedure is remarkably effective for the coupling with cyclic ketones and provides a fast access to a-C-ketosides of Neu5Ac. The coupling efficiency is however moderate in the synthesis of C-disaccharides, as shown with the D-galactose-derived aldehyde 198 (O Scheme 44). Acetates 213 provide the expected compound 215 but much less efficiently than sulfide 204 (30 versus 82% yield, respectively). The moderate yield using acetates 213 is due to the competitive pinacol coupling of aldehyde 198 because the rate of the reductive metallation of the anomeric acetates is too slow. 

Figure 9.4 Competition between anomeric and fi-Sn effects in stabilization of developing cationic center in the endo- and exo- allyl stannane cyclizatlons.

Cooperativity/competUion among acceptors (armed/disarmed carbohydrates) Cleavage of the C-X bond at the anomeric position proceeds via evolution of the anomeric n o C-X interaction that culminates in the oxocarbenium ion. This carbocation is so strongly stabilized by n j- p interaction that this interaction is commonly described as a dative it-bond. Because oxacarbenium ions are strongly stabilized, their formation is relatively facile. In cross-coupling glycosylalion reactions, however, it is beneficial to differentiate the reactivity of two saccharides for the selectivity reasons. The competition and cooperativity between stereoelectronic effects can be used to fine-tune the reactivity of anomeric systems. 

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