Oxonium stereoselectivity

Treatment of phenylchalkogen substituted alkenyl alcohols with /-BuOK provided useful tetrahydrofurans stereoselectively <96JOC8200>. A concise synthesis of cis- and trans-theaspirones via oxonium ion-initiated pinacol ring expansion was developed <96JOCl 119>. 

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. 

Selective Transformation Glycosidic bond formation is complicated by the potential for both a- and P-epimeric products. As most of these reactions proceed through an oxonium ion intermediate, the a/p ratio is derived from substituents on the pyranose ring. The number of required building blocks doubles even with the oversimplification that the stereoselectivity is derived solely from the nature of C2 substituent. 

A -tritylaziridine-2-(5)-carboxaldehyde. The application of a novel, sequential, trans-acetalation oxonium ene cyclization has delivered a stereoselective synthesis of the C-aromatic taxane skeleton, and a combinatorial sequence of the regioselective propiolate-ene, catalytic enantioselective epoxidation and carbonyl-ene cyclization reactions has been used to complete the synthesis of the A-ring of a vitamin D hybrid analogue. 

The mechanistic rationale for the high stereoselectivity is provided by the intermediate production of the trans-iodonium species 279 and its collapse to the bridged oxonium species 280 prior to the introduction of the toxyloxy ligand. 

Stereoselective oxonium ylide reaction, in particular the asymmetric catalysis, has been a problem of considerable challenge in this field. Since the first report by McKervey and co-workers in the asymmetric induction in metal carbene-mediated ylide formation/[2,3]-sigmatropic rearrangement in 1992, there have been efforts being directed... 

Stereoselective reaction with ketones. The reaction of ketone 1 with methyl-lithium, trimethylaluminum, and lithium letramethylaluminate shows no stcrco-specificity. The reaction with mcthylmagncsium bromide gives the two possible adducts in the ratio 2.4 1. The best stereospccificity is observed with dimethylsulf-oxonium methylide, which converts 1 into 2 and 3 in a ratio about 5 1. Reduction of the epoxides with lithium triethylborohydride gives the desired tertiary alcohols. This reaction was used in a synthesis of ( ) stemodin (4).2... 

The internally stabilized silyl cation (36), formed by hydride transfer as shown, has a 2-silanorbomyl cation structure, and is not coordinated to die solvent or the counterion.78 NMR chemical shifts calculated on the basis of die bridged structure shown are in agreement with the experimental values. The autiiors describe it as free but internally re-stabilized .78 The silicon-stabilized oxonium ion (37) shows considerable stereoselectivity in its reactions (38) is the preferred product isomer by a 92 8 ratio, and (39) by 98 2.79... 

Simpkins et al. used an intramolecular variation of this allylsilane addition to oxonium cations for the synthesis of eight-membered rings [14]. Allylsilane 22, containing the oxonium ion precursor (acetal function) is transformed upon treatment with EtAlCl2 into the medium-sized ring 23 in moderate yield and stereoselectivity (Scheme 13.10). 

The stereoselective formation of spiroketals 242 can be explained in terms of the thermodynamic stability of the three possible products. Oxonium cation 245, formed by the condensation of ortholactone 244b and allylsilyl ether 106a, is in equilibrium with the starting materials. Spiroketal 242 also equilibrates under the reaction conditions with the other anomers. The thermodynamically more stable product 242b, stabilized by a double anomeric effect, is obtained as the only product of the reaction (Scheme 13.89) as the substituents attempt to occupy equatorial positions in the newly generated tetrahydropyran ring. 

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. 

Allylsilanes are commonly employed as nucleophiles in the synthesis of tetrahydrofurans, as exemplified by Scheme 64. Oxidative formation of an oxonium intermediate 49 is followed by a highly stereoselective spontaneous cyclization with the tethered allylsilane moiety <2000JOG3252>. 

Unsaturated amides are easily converted to imidates by 0-alky la ting reagents such as trimethyl-oxonium telrafluoroborate or trimethylsilyl triflate. Nucleophilic attack by the thus formed imino group on an halonium ion followed by hydrolysis gives a lactam. Thus an unsaturated dimethyl imidate was treated with bromine in dichloromethane at 0 °C and cyclized with total regio- and stereoselectivity to give 1, since the tertiary cation forces the cyclization towards the formation of a six-membered ring. The trans addition to the double bond was shown by successive conversion to a m-aziridine 2126. 

On treatment with diethylaluminum trimethylsilylacetylide in the presence of BF3 OEt2, a-acetoxy ether 94 gave the rran5-l,3-dioxane 95 in good yield with excellent stereoselectivity, as reported by Rychonovsky and Dahanukar [91]. The trans alkyne adduct has the configuration expected from axial addition to a cyclic oxonium ion (Sch. 59). 

Vanecko, J. A., West, F. G. A Novel, Stereoselective Silyl-Directed Stevens [1,2]-Shift of Ammonium Ylides. Org. Lett. 2002,4, 2813-2816. Marmsaeter, F. P., Murphy, G. K., West, F. G. Cyclooclanoid Ring Systems from Mixed Acetals via Heteroatom-AssIsted [1,2]-Shlft of Oxonium Ylides. J. Am. Chem. Soc. 2003,125,14724-14725. 

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