- Facile synthesis of enol ethers by cleavage of α-bromoacetals and α-bromoketals mediated by SmI2
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α-Bromocyclicacetals, α-bromocyclicketals and α-bromocyclicthioketals, derived from cyclic and acyclic ketones and aldehydes, reacted with samarium diiodide at -78°C in THF to furnish the corresponding enol ethers or thioenol ethers containing hydroxy or thiol moieties in high yields.
- Park, Heui Sul,Kim, Seong Ho,Park, Min Young,Kim, Yong Hae
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p. 3729 - 3732
(2007/10/03)
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- 5. Formation of cyclic ketals from hydroxyalkyl enol ethers, a stereoelectronically controlled endo-trig-cyclizatitm process
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Acid-catalyzed cyclic ketal formation vs. hydrolysis of a series of hydroxyalkyl cyclic enol ethers in the presence of 1 equiv. of H2O, and acid-catalyzed cyclic-ketal formation (same ketals as above) vs. methanolysis of a series of mixed pent-4-enyl hydroxyalkyl ketals with N-bromosuccinimide in the presence of 4 equiv. of MeOH led to the same result: the intramolecular cyclization processes occur at similar rates as the intermolecular H2O or MeOH attacks independently of the size of the rings formed (five-, six-, or seven-niembered), by cyclizations. These results can be explained by the facts that, due to stereoelectronic effects which impose a torsional strain to the sp2 hybridized O-atom, the cyclization activation enthalpy decreases, as the length of the hydroxyalkyl chain increase (ease of cyclization: 7 > 6 > 5), whereas the entropy factor favors the cyclization in the reverse fashion (ease of cyclization: 5 > 6 > 7). The various reaction pathways have been examined using the semi-empirical Hamiltonian AM1, and the results obtained confirm that large-ring formation is enthalpically much favored over the cyclization processes leading to small rings (ease of cyclization: 7 > 6 > 5).
- Deslongchamps, Pierre,Dory, Yves L.,Li, Shigui
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- Stereospecific Cyclization of Vinyl Ether Alcohols. Facile Synthesis of (-)-Lardolure
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An efficient and stereospecific ring formation from unsaturated alcohols, which in turn are prepared by regio- and stereospecific ring opening of acetals, is described.
- Kaino, Makoto,Naruse, Yuji,Ishihara, Kazuaki,Yamamoto, Hisashi
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p. 5814 - 5815
(2007/10/02)
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- The 9-Decalyl and Related Cations. VII Solvolysis of 3-(Cyclohex-1'-enyloxy)propyl p-Nitrobenzenesulfonate
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The solvolysis of 3-(cyclohex-1'-enyloxy)propyl p-nitrobenzenesulfonate (5) in ethanol buffered separately with sodium ethoxide and triethylzmine and 2,2,2-trifluoroethanol buffered with triethylamine has been investigated.Kinetic determinations and product studies have been carried out.In ethanol buffered with sodium ethoxide, ?-bond participation in the above ester occurs to the extent of 30percent; this is raised to 84percent when triethylamine is used as the buffering agent.With buffered trifluoroethanol as solvent, ?-bond participation in the ester is complete; kunsat/ksat = 920 and a quantitative yield of cyclized products is obtained.Kinetic evidence indicates a lack of significant involvement of a lone pair on oxygen (enol ether system) in the solvolysis of the sulfonate ester; in trifluoroethanol, the compound solvolyses only 1.15 times more rapidly than does 4-(cyclohex-1'-enyl)butyl p-nitrobenzenesulfonate (2), its carbon analogue.
- Ferber, Peter H.,Gream, George E.,Stoneman, Terry I.
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p. 699 - 711
(2007/10/02)
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