Dynamic covalent chemistry [1] relates to the study of chemical reactions carried out under thermodynamic control. Labile coordinative bonds associated with certain metal-ligand interactions, [2] ring-opening/ring-closing metathesis reactions, [3] and protocols for the formation of imines [4] and disulfides [5] have all been exploited in the strict self-assembly of catenanes and rotaxanes.[6] A wide range of other functionalities [7][8][9][10][11] have also been explored in the creation of dynamic combinatorial libraries (DCLs). We describe here the efficient and selective acid-catalyzed formation of a chiral macropolycyclic polyether constituted of multiple [24]crown-8 frameworks, which incorporate either two d-or two lthreitol residues as bicyclic diacetals, by using dynamic template-directed approaches [12] to amplify the production of the most thermodynamically preferred complex(es).Carbohydrates command a unique status [13] in the realm of dynamic covalent chemistry.[1] Acid-catalyzed formation [13,14] of cyclic acetals from alditols and aldehydes or ketones provides a well-known reaction [15] in which covalent (C À O) bonds are made and broken with varying degrees of ease under thermodynamic control. The constitutions of the configurationally isomeric erythro-and threo-1,2,3,4-butanetetraols result [16] in their undergoing three different kinds of acetal ring closures with aldehydes and ketones: 1) 1,3:2,4-diacetal formation yields "6/6" bicycles, 2) 1,2:3,4-diacetal formation affords "5/5" bicycles, and 3) 1,4:2,3-diacetal formation yields "5/7" bicycles. With aldehydes, "6/6" or 1,3,5,7-tetraoxadecalin (TOD) formation usually predominates at equilibrium, whereas with ketones, "5/5" bicycles are invariably the major products. The TOD-forming reactions involving aldehydes (RCHO) are completely diastereospecific, that is, erythritol gives trans-TOD and threitol affords cis-TOD. The latter has long attracted the interest of stereochemists because of its highly distinctive stereoelectronic properties.

A systematic computational ab initio study of the conformational dependent proton affinities of methoxymethoxide, tetrahydropyran 2-oxide, methoxymethanol, dimethoxymethane, 1,3-dioxane, and tetrahydropyran has been carried out at the MP2/6-31+G* level of theory. In addition, methoxide, propoxide and methanol, propanol, and dimethyl ether have been computed at the same level as reference systems. Methoxymethoxide and tetrahydropyran 2-oxide exhibit a strong anomeric effect, e.g., the equatorial oxide is a stronger base than the axial one and all are weaker bases than the simple alkoxides. Axial (n π ) protonation is preferred over equatorial (n σ ) by 2-3 kcal/mol. The COCOC acetals are stronger bases (at the acceptor O) then the simple ethers. The structural changes between bond lengths and bond angles for different conformers correlate well with the On π -σ* C-O lone pair delocalization interactions. Thus, the anomeric effect plays an important role in the charged species and in the process of their formation.

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