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The best Lewis structure for an oxocarbenium ion contains an oxygen–carbon double bond, with the oxygen atom attached to an additional group and consequently taking on a formal positive charge. In the language of canonical structures (or "resonance"), the polarization of the π bond is described by a secondary carbocationic resonance form ...
Oxocarbenium ions [ edit ] 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= + O −R′ which forms a resonance structure with the fully-fledged carbocation R− + C −O−R′ and is therefore especially stable:
This group, under the reaction conditions, is activated and via the formation of an oxocarbenium is eliminated leaving an electrophilic anomeric carbon. A glycosyl acceptor is a sugar with an unprotected nucleophilic hydroxyl group which may attack the carbon of the oxocarbenium ion formed during the reaction and allow for the formation of the ...
(Left) Lewis acid-promoted acetal substitution may proceed via either the S N 1 or the S N 2 mechanism. (Right) Denmark's model system for acetal substitution mechanism. If the S N 1 mechanism is operative for the acetal substitution, the two reactions shown here should proceed via the same oxocarbenium ion and give similar stereochemical ...
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.
The tert-butyl cation is a relatively stable carbenium ion. [1]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.
Synthesis of nucleosides involves the coupling of a nucleophilic, heterocyclic base with an electrophilic sugar. The silyl-Hilbert-Johnson (or Vorbrüggen) reaction, which employs silylated heterocyclic bases and electrophilic sugar derivatives in the presence of a Lewis acid, is the most common method for forming nucleosides in this manner.
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.