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The general structure of an oxocarbenium ion. In organic chemistry, an oxocarbenium ion (alternatively spelled oxacarbenium) is a chemical species characterized by a central sp 2-hybridized atom of carbon, a substituent atom of oxygen, and an overall positive charge that is delocalized between the central carbon and oxygen atoms (R 2 [CO] + R). [1]
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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:
A carbenium ion is a positive ion with the structure RR′R″C +, that is, a chemical species with carbon atom having three covalent bonds, and it bears a +1 formal charge. Carbenium ions are a major subset of carbocations , which is a general term for diamagnetic carbon-based cations.
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Upon activation of the glycosyl donor group (Y) (usually SR, OAc, or Br group) in the next step, the tethered aglycon traps the developing oxocarbenium ion at C-1, and is transferred from the same face as OH-2, forming the glycosidic bond stereospecifically. The yield of this reaction drops as the bulkiness of the alcohol increases.
They have the structure – O[(C=O)–O] n – and the molecular formula [C n O 2n+1] 2–. Whereas the carbonate dianion itself is well known, as found in many salts , many organic compounds containing esters of it have been made, and the parent carbonic acid is also well-known, higher homologs are substantially less 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.