Oxonium ion, chiral

The alkenyl oxonium ion dienophiles generated from dioxolanes can be made diastereoselective by use of chiral diols. For example, acetals derived from anti-pentane-2,4-diol react under the influence of TiCl4/Ti(/-OPr)4 with stereoselectivity ranging from 3 1 to 15 1. 

Entries 6 to 8 demonstrate addition of allyl trimethylsilane to protected carbohydrate acetals. This reaction can be a valuable method for incorporating the chirality of carbohydrates into longer carbon chains. In cases involving cyclic acetals, reactions occur through oxonium ions and the stereochemistry is governed by steric and stereo-electronic effects of the ring. Note that Entry 8 involves the use of trimethylsilyl... 

Table 21 Differential Arrhenius parameters for the competing neighboring-group participation to the unimoplecular H2O loss in the chiral oxonium ions 26 and 28 (A = H)...

The limited extent of intramolecular rearrangements undergone by the chiral oxonium ions 35 and 36 at 720 torr and at 40 °C (Table 22) allows their use for probing the regio- and stereochemistry of the displacement reactions of Scheme 19. In this case, the allylic alcohol, precursor of the chiral oxonium ions 35 and 36, acts as the nucleophile NuH. The relevant results are condensed in Scheme 21. 

Racemization of chiral a-methyl benzyl cation/methanol adducts. The rate of exchange between water and the chiral labeled alcohols as a function of racemization has been extensively used as a criterion for discriminating the Sn2 from the SnI solvolytic mechanisms in solution. The expected ratio of exchange vs. racemization rate is 0.5 for the Sn2 mechanism and 1.0 for a pure SnI process. With chiral 0-enriched 1-phenylethanol in aqueous acids, this ratio is found to be equal to 0.84 0.05. This value has been interpreted in terms of the kinetic pattern of Scheme 22 involving the reversible dissociation of the oxonium ion (5 )-40 (XOH = H2 0) to the chiral intimate ion-dipole pair (5 )-41 k-i > In (5 )-41, the leaving H2 0 molecule does not equilibrate immediately with the solvent (i.e., H2 0), but remains closely associated with the ion. This means that A inv is of the same order of magnitude of In contrast, the rate constant ratio of... 

Crown ethers bind well to organic ammonium cations and oxonium ions by charge-assisted hydrogen bonding interactions, as well as metal cations. In some cases they can be used to resolve chiral ammonium salts. 

A novel strategy for stereoselective glycosylations has been reported in which a chiral auxiliary at the 2-position of a glycosyl donor is used (Scheme 126).189 Upon formation of an oxonium ion, participation of the nucleophilic moiety of the auxiliary is (g) selective and depedent on the chirality. 

Similarly, chromium-complexed benzylic cations are also stabilized and organic reactions based on the benzylic cation species have been developed. For example, planar chiral o-substituted benzaldehyde dimethylacetal chromium complexes 4 were treated with 3-buten-l-ol in the presence of TiCl4 to give tet-rahydropyran derivatives with high diastereoselectivity (Eq. 5) [5]. The chromium-complexed benzylic oxonium ion 6 would be also generated and subsequent intramolecular cyclization afforded the cyclization product 7. Furthermore, the chromium-complexed benzyl alcohol derivative having electron-rich arene ring at the side chain produced tetrahydroisoquinoline skeleton by treatment with Lewis acid with stereochemical retention at the benzylic position (Eq. 6) [6]. 

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