No abstract

In Tabellen werden unter Auswertung von zahlreichen Lit.‐ Stellen nucleophile Substitutionsreaktionen mit Boranat beschrieben, die z.B. zur Umwandlung von Halogeniden oder Sulfonaten (I) in Kohlenwasserstoffe, zur Eliminierung einer Methylgruppe aus den aus (IV) hergestellten Ammonium‐Verbindungen (V) zu den tert. Aminen (VI) oder auch zu einer Spaltung von Disulfonylimiden (VII) unter Bildung der Kohlenwasserstoffe (VIII) führen können.

The internal cyclopropanation of several diversely substituted dienic diazo esters is described. Thermolysis of the resulting vinylcyclopropanes yielded cyclopentene-annulated lactones in good yields. Depending on the choice of the dienyl unit, either guaiane or pseudoguaiane substitution patterns of the cyclopentene portion were obtained. Stereochemical assignments based on 13C NMR data are provided for all of these lactones. Subsequent transformations of the bicyclic lactones to differentially functionalized cyclopentenes are described. The potential of these synthons in the synthesis of perhydroazulene sesquiterpenes and several monoterpene cyclopentanoid natural products is addressed.

No abstract

Conformational preferences of alkyl substituents at various positions in the 1,3-dithiane system have been determined by acid-catalyzed equilibration of diastereoisomeric diand trialkyldithianes. The results are supported by evidence from nuclear magnetic resonance spectroscopy. For alkyl groups at positions 2 and 4, free-energy differences (-AG°values) between equatorial and axial locations are of the same magnitude as in cyclohexane. In contrast, the preference of alkyl substituents at position 5 for the equatorial orientation is much less than in cyclohexane. The equatorial-axial free-energy differences (in kilocalories per mole) are Me-2, 1.77; Me-4, 1.69; Me-5, 1.04; Et-2, 1.54; Et-5,0.77; /-Pr-2, 1.95; /-Pr-5, 0.85; /-Bu-2, 5= 2.7. The chair-skew boat enthalpy and entropy differences for 2,5-di-/-butyl-l,3-dithiane have been determined to be 3.42 kcal/mol and 5.3 cal/deg mol, respectively, by a study of the dependence of equilibrium upon temperature. The results are interpreted in terms of the known geometry of the 1,3-dithiane ring, the relative space requirement of sulfur compared with a methylene group and the known barrier to rotation about C-S bonds.During the past several years, much effort has been devoted to studies of conformational equilibria (Scheme I) in alicyclic molecules with special emphasis on substituted cyclohexanes. A number of methods and techniques have been devised to measure the extent to which a particular substituent X prefers the equatorial over the axial orientation.4 Such studies have Scheme I X <=± x^^vx -&G°=RT In Kprovided -AG°values or "conformational energies" for a wide variety of groups5 67and have led to at least a qualitative understanding of the forces and interactions which determine conformation and reactivity in substituted cyclohexanes. Moreover, theoretical treatments of cycloalkanes have also progressed to the point where energy differences between various conformations of substituted cyclohexanes can be calculated using appropriate potential functions; such treatments provide values in reasonable agreement with experiment.6-10

No abstract

No abstract

hexen-l-yloxy)trimethylsilane, 6651-36-1; (1-cyclopenten-1-yloxy)trimethylsilane, 19980-43-9; trimethyl[ (1 -phenylethenyl)oxy ]silane, 13735-81-4; l,l'-[[l,2-bis(methylene)-l,2-ethanediyl]bis(oxy)bis(trimethyl)silane, 31411-71-9; trimethyl[ [(1 -phenylmethyl)etheny 1]oxy]silane, 59021-31-7; trimethyl[(1-methyl-2-phenylethenyl)oxy]silane, 43108-63-0; trimethyl[(6-methyl-l-cyclohexen-l-yl)oxy]silane, 19980-33-7; trimethyl[(2-methyl-l-cyclohexen-l-yl)oxy]silane,