Oxonium ions cleavage

The first step is protonation of the ether oxygen to give an oxonium ion. Cleavage is by an S 2 pathway on the less hindered methyl carbon. 

The protonated azirine system has also been utilized for the synthesis of heterocyclic compounds (67JA44S6). Thus, treatment of (199) with anhydrous perchloric acid and acetone or acetonitrile gave the oxazolinium perchlorate (207) and the imidazolinium perchlorate (209), respectively. The mechanism of these reactions involves 1,3-bond cleavage of the protonated azirine and reaction with the carbonyl group (or nitrile) to produce a resonance-stabilized carbonium-oxonium ion (or carbonium-nitrilium ion), followed by attack of the nitrogen unshared pair jf electrons to complete the cyclization. 

Contained within intermediate 25 is an acid-labile mixed acetal group and it was found that treatment of 25 with camphorsulfonic acid (CSA) results in the formation of dioxabicyclo[3.3.0]octane 26 in 77 % yield. Acid-induced cleavage of the mixed cyclic acetal function in 25, with loss of acetone, followed by intramolecular interception of the resultant oxonium ion by the secondary hydroxyl group appended to C leads to the observed product. Intermediate 26 clearly has much in common with the ultimate target molecule. Indeed, the constitution and relative stereochemistry of the dioxabicyclo[3.3.0]octane framework in 26 are identical to the corresponding portion of asteltoxin. 

The use of iodotrimethylsilane for this purpose provides an effective alternative to known methods. Thus the reaction of primary and secondary methyl ethers with iodotrimethylsilane in chloroform or acetonitrile at 25—60° for 2—64 hours affords the corresponding trimethylsilyl ethers in high yield. The alcohols may be liberated from the trimethylsilyl ethers by methanolysis. The mechanism of the ether cleavage is presumed to involve initial formation of a trimethylsilyl oxonium ion which is converted to the silyl ether by nucleophilic attack of iodide at the methyl group. tert-Butyl, trityl, and benzyl ethers of primary and secondary alcohols are rapidly converted to trimethylsilyl ethers by the action of iodotrimethylsilane, probably via heterolysis of silyl oxonium ion intermediates. The cleavage of aryl methyl ethers to aryl trimethylsilyl ethers may also be effected more slowly by reaction with iodotrimethylsilane at 25—50° in chloroform or sulfolane for 12-125 hours, with iodotrimethylsilane at 100—110° in the absence of solvent, " and with iodotrimethylsilane generated in situ from iodine and trimcthylphenylsilane at 100°. ... 

Description glycosidic cleavage to form an oxonium ion charge retained on nonreducing end positive-ion mode only often referred to as A -type cleavage, because of similarity to one of the cleavages seen in electron impact-mass spectrometry. 

The reactivity of alkynylstannanes toward electrophiles is one element in the oxygen-to-carbon rearrangement of alkynylstannane derivatives of furanyl and pyranyl lactols (e.g., Equation (85)). The cleavage of the anomeric C-O bond is assisted by the Lewis acid to give an oxonium ion, which is trapped in situ by the nucleophilic stannylalkyne. The utility of this process has been demonstrated in the synthesis of the natural product muricatetrocin C, and the drug substance CMI-977.246... 

In case of alkanols, the methylene oxonium ion, CH2=OH, m/z 31, deserves special attention. Resulting from a-cleavage, it undoubtedly marks spectra of primary alkanols, where it either represents the base peak or at least is the by far most abundant of the oxonium ion series (Fig. 6.8). [32] The second important fragmentation route of aliphatic alcohols, loss of H2O, is discussed in Chap. 6.10. 

Both end groups can be determined quantitatively. A second side reaction is the transacetalization. Here a poly(oxymethylene) cation attacks an oxygen of a poly(oxymethylene) chain with formation of an oxonium ion that decomposes. Through continued cleavage and recombination of poly(oxymethylene) chains one obtains polymers which are chemically and molecularly largely homogeneous. For the case of a trioxane/ethylene oxide copolymer the following reaction scheme can be formulated ... 

Cleavage of oxonium ions 0-10 Reaction between carboxylic esters and alkoxide ion... 

Rupture of the C-C bond adjacent to oxygen in ether ROR leads to the formation of oxonium ions, which are stabilised by resonance (Fig. 16.26). A similar situation is encountered in amines (formation of the iminium ion at mass 30 for primary amines CH2 = NH2). Cleavage of the C O bond in ethers is also observed and leads to the cations R + and R +. 

Fragmentation of 2-alkyltetrahydropyrans is dominated by a-cleavage with loss of the substituent, resulting in the formation of an oxonium ion. 

Acidic conditions also can be used for the cleavage of oxacyclopropane rings. An oxonium ion is formed first, which subsequently is attacked by the nucleophile in an SN2 displacement or forms a carbocation in an SN1 reaction. Evidence for the SN2 mechanism, which produces inversion, comes not only from the stereochemistry but also from the fact that the rate is dependent on the concentration of the nucleophile. An example is ring opening with hydrogen... 

Quantum mechanical calculations show that the silyl cation (19) has a twisted structure, and that the a-C02 group provides substantial electrostatic stabilization.58 Isotope effects for its formation reaction are also reported.58 Evidence is provided for the stabilization of incipient oxocarbenium ions by axial electronegative substituents, as in (20) the presence of the most electronegative substituent results in the fastest reaction.59 Lewis acid-promoted cleavage of spirocyclic dioxanes such as (21) involves oxonium ions, and high axial vs equatorial product selectivities are possible with the correct choice of Lewis acid and nucleophile.60 Reactions which lead to 1,3-dioxenium salts have been reviewed.61... 

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