Crown ether effect

Addition of 15-crown-5 to the higher-order cuprate led to a reagent that is totally unrcac-tive towards 2-phenylpropanal even at room temperature18. If, however, boron trifluoride-diethyl ether complex was added as additional ingredient, the reactivity was restored and, furthermore, the Cram selectivity increased to 90 10 (Table 4). Analogous results could be obtained by placing the crown-ether effect within the cuprate itself, as in reagent 10. 

Eliminations from cyclic compounds are more sensitive to the crown-ether effect. Potassium t-butoxide-promoted elimination of HX from [178] gives the cis- and trans-isomers of [179] and [180] (36), the cis isomers being formed... 

Shinbo, T., Yamaguchi, T., Yanagishita, H., Sakaki, K., Kitamoto, D., and Sugiura, M., Supported liquid membranes for enantiose-lective transport of amino acid mediated by chiral crown ether—effect of membrane solvent on transport rate and membrane stability. J. Membr. Sci., 1993, 84 241-248. 

Demini S, Palmas S, Polcara AM, Maronglu B, Extraction and transport of sodium ion and potassium ion in a liquid membrane containing crown ethers Effect of the mixed solvent. J. Chem. Eng. Data 1992 37, 281-284. 

Addition of -butylmagnesium bromide to 624 followed by Swem oxidation affords the ketone 642. Zinc borohydride addition occurs with almost exclusive anri-selectivity (>99 1), leading to 646 in accordance with an a-coordinated transition-state model in which the r -face of the carbonyl is exposed to the reagent. Presumably the MOM-ethers display a crown ether effect to facilitate a-chelation. In marked contrast, L-Selectride shows excellent 5y -selectivity to provide 645 (92 8), consistent with a j5-chelation and/or Felkin— Anh model. The a ri-adduct 646 is converted in five steps to ketone 647, which undergoes a similar highly selective hydride reduction with zinc borohydride to yield the anti,syn,syn-alcohol 648 (96 4). This product is converted in six steps to the r n5-(2i ,57 )-pyrroline 649, which undergoes a Wacker oxidation followed by catalytic reduction to (— )-indolizidine 195B (650) and its C-5 epimer (86 14) (Scheme 142). 

Anomeric O-Alkylation of Mannofuranose with Primary Triflates The Crown Ether Effect... 

This is a classic crown ether effect. The 18-C-6 sequesters potassium ions, carrying them, and the acetate counterions, into the acetonitrile. Thus, part of the effect derives from the solubilizing of the reagent. Additionally, the 18-C-6 breaks up the potassium acetate ion pairs, leaving the acetate naked and hence enhancing its nucleophilicity in the 5 2 substitution reaction. 

Reiser, A., Photoreactive Polymers — The Science and Technology of Resists, Wiley, New York, 1989. Anzai, J., Suzuki, Y., Ueno, A., and Osa, T, Cation transport through Hquid membranes mediated by photoreactive crown ethers. Effects of alkali metal cations on their photoreactivities and transporting properties, Isr. J. Chem., 26, 60, 1985. 

Alternatively, the reaction may be carried out in a mixture of two immiscible solvents. The contact area between the phases may be increased by agitation. Phase transfer agents, in particular quaternary ammonium compounds, are useful aids in many two-phase reactions. Also, crown ethers are very effective in overcoming phase contact problems however, their usefulness is limited by their high price. (Open chain polyoxyethylene compounds often give a crown ether effect and may constitute practically interesting alternative phase transfer agents.)... 

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