Search results
Results from the WOW.Com Content Network
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]
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.
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:
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.
Second, while oxidation occurred from the desired convex face of the silyl enol ether, the authors saw a significant number of overoxidation products that they attributed to the stability of the oxocarbenium ion intermediate under sodium bicarbonate buffered conditions.
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.
Get AOL Mail for FREE! Manage your email like never before with travel, photo & document views. Personalize your inbox with themes & tabs. You've Got Mail!
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.