Conformations and Chemistry of Oxocarbenium Ion

While the confoimational analysis of stable compounds has been studied in detail, determining the conformational preferences of reactive intermediates is much more difficult. For example, knowledge of the three-dimensional structures of cyclic oxocarbenium ions is very important for imderstanding both uncatalyzed and enzymatic reactirms of carbohydrates involving the anomeric carbon, since these reactions often involve oxocarbenium ion intermediates [1-5], Since the charged intermediates are generally much too reactive, it is impossible to directly observe oxocarbenium ions, particularly in aqueous envirorunents [6, 7]. 

Consideration of the preference for alkoxy groups to adopt axial conformers at certain positions, as shown in Fig. 4.1, explains the relative rates of monosaccharide hydrolysis [8, 11]. For example, the reactivity pattern exhibited by methyl p5U an-osides 3-5 (Fig. 4.2) showed a trend that the more axial hydroxyl groups are present in the acetal, the faster the rate of hydrolysis (Fig. 4.2) [14, 15]. 

Since the transition state structures of exocyclic C-O bond cleavage resemble the oxocarbenium ion intermediates, favorable interactions that stabilize the charged intermediates would facilitate hydrolysis [16]. The p5U anoside 5 bearing axial substituents at the C3 and C4 carbons reacted at the highest rate via an 

Miljkovic, Electrostatic and Stereoelectronic Effects in Carbohydrate Chemistry, 

When the corresponding alkyl-substituted cation was prepared, the alkyl group was equatorially orientated both in solution and in the solid state. 

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The substitution chemistry at ring-fused oxocarbenium ions was accomplished with good product yield and stereoseleetivity in the reactions of allyl silanes with the silyl-protected 2-deoxyribose system (95). Thus, Lewis-acid-catalyzed ionization provides the oxocarbenium ion, which exists in two basic conformations (96 and 97). DFT calculations B3LYPI6-31G level) estimate that the diaxial conformer (97) is 5.2 kcal mol more stable than the diequitorial conformer (96). In accordance with previous studies, nucleophilic attack occurs from the inside of the envelope-shaped oxocarbenium ion (97), leading to substimtion product (98) (Scheme 22). 

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