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The general structure of an oxocarbenium ion. An oxocarbenium ion (or oxacarbenium ion) is a chemical species characterized by a central sp 2-hybridized carbon, an oxygen substituent, and an overall positive charge that is delocalized between the central carbon and oxygen atoms. [1]
Another class of oxonium ions encountered in organic chemistry is the oxocarbenium ions, obtained by protonation or alkylation of a carbonyl group e.g. R−C= + −R′ which forms a resonance structure with the fully-fledged carbocation R− + −O−R′ and is therefore especially stable:
They hypothesized that the increased lifetime of the intermediate species would allow for over oxidation to occur. After a significant amount of optimization, it was found that an HOAc/Py buffer trapped the oxocarbenium intermediate and prevented overoxidation to exclusively give 2S-hydroxymutilin after hydrolysis of the silyl protecting groups.
The electronic structure is described by two main theories which are used to show how the 4 electron pairs are distributed in a molecule that only has 3 C-O bonds. With valence bond theory the electronic structure of the carbonate ion is a resonance hybrid of 3 canonical forms. In each canonical form there are two single bonds one double bond.
The selectivity in the reaction is due to the stronger electron withdrawing power of the esters compared to the ethers. A stronger electron withdrawing substituent leads to a greater destabilization of the oxocarbenium ion. This slows this reaction pathway, and allows for disaccharide formation to occur with the benzylated sugar.
Anomerization of glycosides typically occurs under acidic conditions. Typically, anomerization occurs through protonation of the exocyclic acetal oxygen, ionization to form an oxocarbenium ion with release of an alcohol, and nucleophilic attack by an alcohol on the reverse face of the oxocarbenium ion, followed by deprotonation.
2-Deoxysugars are unable to form the cyclic cation intermediate 1 because of their missing benzoyl group; instead, under Lewis acidic conditions they form a resonance-stabilized oxocarbenium ion. The diastereoselectivity of nucleophilic attack on this intermediate is much lower than the stereoselectivity of attack on cyclic cation 1 .
In the first step of the mechanism, the glycosyl bromide reacts with silver carbonate upon elimination of silver bromide and the silver carbonate anion to the oxocarbenium ion. From this structure a dioxolanium ring is formed, which is attacked by methanol via an SN 2 mechanism at the carbonyl carbon atom. This attack leads to the inversion.