Oxonium ions, ring opening

Step 2 Nucleophilic attack by water on carbon of the oxonium ion The carbon-oxygen bond of the ring is broken in this step and the ring opens... 

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

There is no published mechanistic study on the Auwers flavone synthesis. The mechanism may involve the nucleophilic addition of oxonium 7, derived from 1, with hydroxide to give 8. Base-promoted ring opening of 8 could provide the putative intermediate 9, which then could undergo an intramolecular Michael addition to form 10. Expulsion of bromide ion from 10 would then give flavonol 2. 

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

The protonated epoxides, i.e. the oxonium ions, could not be characterized as minima on the respective potential energy surfaces, as in every case the epoxide ring opened by a barrierless process upon O-protonation. Charge delocalization maps are shown in Figure 9, and some selected NPA-derived charges for the carbocations are displayed in Table 4. 

In the initial step of the polymerization, a cyclic oxonium ion is formed by transfer of an alkyl group from the initiator to the cyclic ether. Propagation occurs by SN2 attack of a monomer molecule at a ring a-methylene position of the cyclic tertiary oxonium ion, followed by opening of the oxonium ring and formation of a new cyclic oxonium ion. 

Ring-opening polymerizations are generally initiated by the same types of ionic initiators previously described for the cationic and anionic polymerizations of monomers with carbon-carbon and carbon-oxygen double bonds (Chap. 5). Most cationic ring-opening polymerizations involve the formation and propagation of oxonium ion centers. Reaction... 

Initiation consists of protonation of monomer followed by subsequent reaction with monomer to form the tertiary oxonium ion LXXXI (Eq. 7-109). Propagation for the ring-opening... 

THF can be polymerized only with cationic initiators, for example, boron trifluoride or antimony pentachloride. The initial step consists of the formation of a cyclic oxonium ion one of two activated methylene groups in the a-position to the oxonium ion is then attacked by a monomer molecule in an S 2-reaction, resulting in the opening of the ring. Further chain growth proceeds again via tertiary oxonium ions and not, as formerly assumed, via free carbonium ions ... 

Dihydroartemisinin (DHA) is the active metabolite of acetalic derivatives of artemisinin (artemether, artesunate). Oxidation by cytochrome P450 enzymes or/and hydrolysis provides DHA, which is itself poorly stable in vivo. Indeed, the corresponding oxonium ion, a precursor of inactive metabolites by ring opening or by glucuronidation, can easily be formed (Figure 4.15). 

Even more stable salts than those already mentioned can be prepared from oxonium, R30+, sulphonium, R3S+, and ammonium, R4N+ ions. Not surprisingly, however, these are in general too stable to initiate vinyl polymerisations though Et30+BF4 has been reported to polymerise alkyl vinyl ethers (96). Both oxonium and strained sulphonium ion salts are very efficient initiators of ring opening polymerisations as we shall see later (Section IV). 

---