Oxonium ions, formation

Conformation Groups that favor a flattening of the pyranose, a consequence of oxonium ion formation, increase the rate of reactivity. Ley uses 3,4-spiroketals to enforce chairlike structure on the pyranose ring, thus deactivating them toward reaction. 

Scheme 11.3 Using a C2-protecting group for arming/disarming of an n-pentenyl glycoside for oxonium ion formation.

For this 2° alcohol, oxonium-ion formation is followed by loss of HjO to give a 2° carbocation that rearranges to a 3 carbocation. Both carbocations react by two pathways they form bonds to Br to give alkyl bromides or they lose H to yield alkenes. 

Bawn, Bell, Fitzsimmons, and Ledwith (20) have suggested that transfer reaction in the bulk polymerization of THF must involve either hydride ion abstraction from the alpha methylene of THF or of tetra-methyleneoxy units in the polymer, or degradative oxonium ion formation with the ethereal oxygen atoms of polymers of the type discussed in Section IIID4. 

Of equal importance with these two reactions, but unfortunately much less clear, is the mechanism of oxonium ion formation since it will determine the importance of these intermediates in the polymerizations. The scheme proposed by Meerwein and described above was based entirely on careful product analysis of reactions carried out without extreme precautions to exclude moisture. While it seems reasonable enough, it may require modification as more information becomes available. The question of the role of water in the formation may perhaps be an academic one in the case of the epoxides because even rigorously dry ethylene oxide appears to react with boron fluoride at — 80° (4) to form compounds of the type... 

If reactions (a) and (b) cannot account for the increasing rate with molecular weight, then the problem is a difficult one for, as we have already seen, oxonium ion formation seems to lead to relatively slow... 

The molecular weight of the polyoxacyclobutane through any one polymerization may rise to a maximum and then decrease (19) but in similar experiments with ethylene oxide the observed maximum was no greater than the experimental error of the weight measurements. This point is of considerable interest and deserves more attention than it has received, for it raises once again the question of monomer-polymer equilibrium and suggests that such an equilibrium may be obscured in the epoxides by the non-equilibrium depolymerization to dioxane. It could also mean however that oxonium ion formation is much slower with oxacyclobutane than with epoxides so that depolymerization becomes important only towards the end of the reaction. 

In order to synthesize 17-a-hydroxyaldosterone 2a,12 a side chain protection was needed. Otherwise the 18-carbon radical tended to furnish a methylene group and open the 13-17 bond in ring D. A convenient protection for the corticoid side chain is the A/.v-ketal derived from formaldehyde. Thus Msmethyl-enedioxy-1-dehydrohydrocortisone 21 was converted to its nitrite and photolyzed in the usual way. The only product (60 % isolated yield) was the desired isomer 22. When this was treated with nitrous acid, an unusual sequence of reactions took place. The normal nitrosation intermediate underwent ring closure to 24 followed by oxonium ion formation 25 as indicated. The oxonium ion 25 then opened one of the adjacent methylenedioxy groups (25 -> 26) which on hydration with water afforded the isolated product 27. Acetylation of 27 with acetic anhydride and acid catalysis gave the triacetate 28 in 80 % overall yield from the oxime 22. Mild alkaline hydrolysis... 

C. Denekamp and Y. Sandlers, Anomeric distinction and oxonium ion formation in acetylated glycosides, J. Mass Spectrom., 40 (2005) 765-771. 

According to Olah s investigations the conversion of methyl alcohol over bifunctional acidic-basic catalyst after initial acid-catalyzed dehydration to dimethyl ether involves oxonium ion formation catalyzed also by the acid functionality of the catalyst. This is followed by basic site catalyzed deprotonation to a reactive surface-bound oxonium ylide, which is then immediately methylated by excess methyl alcohol or dimethyl ether leading to the crucial - 2 conversion step. The ethyl methyl oxonium ion formed subsequently eliminates ethylene. All other hydrocarbons are derived from ethylene by known oligomerization-fragmentation chemistry. Propylene is formed via a cyclopropane intermediate. The overall reaction sequence is depicted in Scheme 19. 

Selective reduction of benzylidene acetals to form mono-benzyl protected diols was discussed in section 2.3.1, Benzyl ethers but differentiation of the oxygen atoms within benzylidene derivatives can also be achieved in the widely used Hanessian procedure. In this process, benzylic bromination, oxonium ion formation, and nucleophilic attack by bromide ion at the (1°-) 6-position leads to the 4-O-benzoyl-6-deoxy-6-bromo derivative. Figure 2.62 [83]. 

The synthesis and structural correlations of (448) and the reference compound (449) have been described, and arguments presented that the effect of ionization of the endo-carboxylic acid group on the hydrolysis of (448) represents a reasonable model for the effect of aspartic acid-52 of lysozyme on the hydrolysis of saccharides by the enzyme. However, on the basis of kinetic studies of the hydrolysis, it is concluded that electrostatic facilitation of general acid-catalysed a-oxonium ion formation is not significant, and hence is probably unimportant in lysozyme. ... 

A similar ring contraction was obtained under Lewis acid catalysis, when the elongated bicycles type 67, with 6R or 65 configuration, were debenzylated in the presence of boron trichloride (Scheme 4.10). The cleavage of the endocyclic O—Cl bond of the pyranose ring results in the oxonium ion formation that suffers then the nucleophilic attach of OH to generate the p-furanoside 70 in a good yield [21]. 

Braddock, D.C., Millan, D.S., Perez-Euertes, Y, Pouwer, R.H., Sheppard, R.N., Solanki, S., and White, A.J.P. (2009) Bromonium ion induced transarmular oxonium ion formation-fragmentation in... 

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