Reduction oxonium ions

Surprisingly, methyl esters are also suitable substrates whereby intramolecular cyclization occurs with concomitant loss of methyl iodide. Larock used internal alkynes as coupling partners for lactone synthesis (Scheme 2.33) [74]. The proposed mechanism involves oxidative addition of Pd(0) to the aryl iodide, followed by addition across the alkyne and cyclization of the carbonyl O of the ester to form an oxonium ion. Reductive elimination followed by loss of the methyl group then yields the product [74]. Shen and coworkers also reported a variant utilizing o-2,2-dibromovinylbenzoates (Scheme 2.34) [79]. 

Coe et al. reported an efficient modification for the preparation of /-substituted indole analogs for biology screening in good yield. The intermediate P-nitrostyrene 44, prepared from the condensation of 43 with DMFDMA, underwent methanolysis and reduction to provide the aniline acetal intermediate 45. Alkylation of amine 45 was carried out employing standard conditions of reductive alkylation to provide A-alkyl analogs represented by 46. The indole 47 was generated by formation of the oxonium ion (from 46) under acidic conditions, followed by cyclization, accompanied by loss of methanol. 

These reactions presumably proceed by catalytic cycles in which the carbonyl component is silylated. The silyl ether can then act as a nucleophile, and an oxonium ion is generated by elimination of a disilyl ether. The reduction of the oxonium ion regenerates the silyl cation, which can continue the catalytic cycle. 

The T-conformation had two prevalent conformers 88 and 89 where the steric bulk of the 4-fluorophenyl group blocks the P-face of the oxonium ions (Scheme 7.27). Reduction occurs from the less hindered a-face on each conformer leading to the diastereomeric mixture of products. Since there was only a O kcalmoT1 energy difference in these lowest energy conformers 88 and 89, this nicely supported the observed product distribution of approximately 3 1 of 18 19 under the best reaction conditions. 

Attempts were made to quantitatively treat the elementary process in electrode reactions since the 1920s by J. A. V. Butler (the transfer of a metal ion from the solution into a metal lattice) and by J. Horiuti and M. Polanyi (the reduction of the oxonium ion with formation of a hydrogen atom adsorbed on the electrode). In its initial form, the theory of the elementary process of electron transfer was presented by R. Gurney, J. B. E. Randles, and H. Gerischer. Fundamental work on electron transfer in polar media, namely, in a homogeneous redox reaction as well as in the elementary step in the electrode reaction was made by R. A. Marcus (Nobel Prize for Chemistry, 1992), R. R. Dogonadze, and V. G. Levich. 

Unlike cyclohexene, its oxa analog, 3,4-dihydro-2//-pyran, undergoes facile reduction to tetrahydropyran in yields ranging from 70 to 92% when treated with a slight excess of triethylsilane and an excess of either trifluoroacetic acid or a combination of hydrogen chloride and aluminum chloride (Eq. 69).146 This difference in behavior can be understood in terms of the accessibility of the resonance-stabilized oxonium ion intermediate formed upon protonation. 

N-Boc-cyclododecylamine, reductive Boc protection, 128 Oxirane, iodoalkane iodoreduction, cyclohexyl iodide, 136 Oxonium ions, alkene to alkane... 

We should also expect stereoelectronic control when the hydroxyl group is replaced by another nucleophile in the reaction with cyclic oxonium ions. A recent report (110) shows that hydride transfer to cyclic oxonium ion is subject to stereoelectronic control. Tricyclic spiroketal 140 (Fig. 19) undergoes an acid-catalyzed oxidation-reduction reaction to give the equatorial bicyclic aldehyde 147 stereospecifically. Similarly, spiroketals 148 and M9 gave the corresponding equatorial bicyclic ketone 150. 

This rationalization indicates that internal delivery of a hydride is not a requisite for the observed stereospecificity. Reduction of the oxonium ion with an external hydride reagent should also give equatorially oriented bicyclic ether only. Accordingly (112), reduction of tricyclic spiroketal 145 with sodium cyanoborohydride at pH =3-4 yields only the equatorial bicyclic ether alcohol (J47, CHO=CH2OH). Eliel and co-workers (113) have previously suggested that the orientation of the electron pairs of oxygen atoms influence the course of the reduction of 2-alkoxytetrahydropyran with lithium aluminium hydride-aluminium trichloride. 

Phenyl 3-phenyl-2-propenyl ethers 448, which feature a latent oxonium ion functionality, cyclize upon treatment with acid to afford the chroman 449 as a mixture of diastereomers. Reduction of the chroman 449 with Raney nickel affords a single diastereomeric product 450 (Scheme 102) <2005TL3719>. 

The usual acid-sensitivity of acyclic acetals was similarly of no avail in the related system studied by Myers and co-workers [Scheme 2.53].111 In this case, treatment of the dimethyl acetal 55.1 with trichloroacetic acid generated an oxonium ion intermediate, that was rapidly and efficiently intercepted with hydrogen peroxide - a reagent which is much more nucleophilic than water. The resultant a-methoxyhydroperoxide 53 was then reductively cleaved under neutral conditions to produce a hemiacetal, which lost methanol to give the desired aldehyde 53.3. 

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