- Diastereoselectivity in the epoxidation of substituted cyclohexenes by dimethyldioxirane
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Three series of compounds based on the cyclohexene framework have been epoxidized by dimetbyldioxirane. A pronounced dependence of epoxide diastereoselectivity on substituent has been observed. In addition there is a solvent influence on this stereoselectivity. The results have been explained by invoking steric, H-bonding, and dipole - dipole influences on the epoxide stereochemistry.
- Murray, Robert W.,Singh, Megh,Williams, Brian L.,Moncrieff, Hazel M.
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p. 1830 - 1841
(2007/10/03)
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- Dehydration of secondary alcohols via thermolysis of in situ generated alkyl diphenyl phosphates: An inexpensive and environmentally compatible method for the preparation of alkenes
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Secondary alcohols are converted into diphenyl phosphate esters by the action of triphenyl phosphate or diphenyl phosphorochloridate in high-boiling, water-miscible solvents in the presence of base. The alkyl diphenyl phosphates undergo thermolysis to afford high yields of alkenes which distill from the reaction mixtures. Purification of the products is achieved by extraction with dilute sulfuric acid which removes traces of solvent and base that may have codistilled. The ratios of 2- (14) and 3-menthene (15), obtained from menthol (13) and neomenthol (16), and the formation of rearranged alkenes by 1,2-shifts from 3, 6, and 3,3-dimethylbutan-2-ol are consistent with ionic intermediates of the elimination reaction. The novel dehydration method offers distinct and important advantages over the existing methods.
- Quast,Dietz
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p. 1300 - 1304
(2007/10/02)
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- Solvolysis Reactions and Force Field Calculations with Epimeric Cyclohexane Derivatives
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Reactions of the epimeric 4-tert-butylcyclohexyl tosylates in hexafluoroisopropyl alcohol (HFIP) (1e, 1a) allow for the first time to observe SN1-type non-stereospecific substitution, whereas conventional solvents including even trifluoroethanol yield SN2-type inversion by solvent assisted pathway (*ks).Large differences are found between the epimers in the solvent participation, measured kinetically by the Schleyer-Bentley equation.This, as well as the even enhanced elimination with the equatorial isomer 1e as compared to 1a (98percent vs. 96percent in HFIP) points to the occurence of non-chair intermediates from 1e-derivatives, and to more E2- than E1-type reactions.Kinetic measurements, including those of cis-3,5-dimethylcyclohexane esters (2a, 2e) and 3α/3β-(tosyloxy)androstanes (3a, 3e) show little differences between the equatorial esters in agreement with MM2 calculations, which establish small strain energy variations between the differently substituted twist-boat intermediates.Large differences, however, of up to 500percent are measured between the axial esters (1a, 2a, 3a), although the alkyl substituents are remote from the leaving group and do not alter the chair geometry.These variations, which demonstrate the severe limitations of the Winstein-Holness equation for solvolysis reactions, are explained by MM2 calculated significant strain differences between the educts and the corresponding substituted cyclohexenes.
- Schneider, Hans-Joerg,Schmidt, Guenter
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- A New Reducing System: Calcium Metal in Amines. Reduction of Aromatic Hydrocarbons
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A new reducing system consisting of calcium dissolved in a mixture of amines (methylamine-ethylenediamine) is described.Representative aromatic hydrocarbons have been reduced by this new reagent largely to monoalkenes.Hydrocarbons like tetralin, m- and p-xylene, and indan are reduced in excellent yields by the calcium system to a crude product containing 88percent or better of a single alkene.A new technique involving oxymercuration-demercuration is used to purify two of the monoalkene isomer mixtures obtained in these reductions.Unexpectedly, durene is reduced by the calcium reagent to 1,2,4,5-tetramethyl-1,4-cyclohexadiene in excellent yield.Likewise anthracene is reduced in one step to 1,2,3,4,5,6,7,8,9,10-decahydroanthracene.Experiments designed to elucidate why the calcium system does not reduce durene or anthracene to monoalkenes are described.Similarities and differences between the calcium-amine and the lithium-amine reducing systems are discussed.
- Benkeser, Robert A.,Belmonte, Frank G.,Kang, Jahyo
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p. 2796 - 2802
(2007/10/02)
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- vic-Iodothiocyanates and Iodoisothiocyanates. Part 3. Further Preperations, and the Formation of 2-Alkoxy-2-thiazolines
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The reactions of (E)-hex-3-ene, 4-t-butylcyclohexene, and 3-t-butylcyclohexene with iodine and potassium thiocyanate are examined.The action of iodine and thiocyanogen with (E)-hex-3-ene gives different results from those reported in the literature.Isomerization of the vic-iodothiocyatates and formation of 2-alkoxy-2-thiazolines from the latter adducts are reported.
- Cambie, Richard C.,Chambers, David,Rutledge, Peter S.,Woodgate, Paul D.,Woodgate, Shella D.
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