The Experts below are selected from a list of 282 Experts worldwide ranked by ideXlab platform
William S Johnson - One of the best experts on this subject based on the ideXlab platform.
-
the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations total synthesis of dl dammarenediol1
Journal of Organic Chemistry, 1999Co-Authors: William S Johnson, William R Bartlett, Boris A Czeskis, Arnaud Gautier, Remy C Lemoine, Eric J Leopold, And Gregory R Luedtke, Katherine J BancroftAbstract:Dammarenediols I (1a) and II (1b) were prepared by an efficient nonenzymatic biomimetic polyene tetracyclization route. The cyclization substrate, pentaenol 3, contains a tetramethylallylic alcohol initiator, an allyltrimethylsilane terminating group, and a Fluorine Atom at pro-C-13 to serve as a cation-stabilizing (C-S) auxiliary controlling the regiochemistry of the C/D ring juncture. The synthesis of 3 employed lithium−halogen exchange to create alcohols 10 and 19. The Z-fluoroalkene in 3 was introduced stereoselectively via the Trost palladium-catalyzed alkylation of allylic acetate 11 (Z/E: 4.6/1). The cyclization of 3 was most efficient (62% isolated yield) when it was added as a dilute solution in dichloromethane to trifluoroacetic acid at −45 °C to afford tetracyclic fluoro diene 24 possessing the trans−anti−trans−anti−trans ring stereochemistry of the dammaranes. Replacement of the Fluorine Atom of 24 with hydrogen with complete retention of configuration was accomplished using the Ohsawa−Oishi r...
-
cation stabilizing auxiliaries in polyene cyclizations 4 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 1 background and exploratory experiments
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, Balan Chenera, Fook S Tham, Rudolph K KullnigAbstract:With the aim of testing the Fluorine Atom as a potential cation-stabilizing (C-S) auxiliary for enhancing polyene cyclizations, new methodology has been developed for the stereoselective synthesis of polyenic cyclization substrates in which the vinyl H of an E-trisubstituted olefinic bond is replaced by F
-
cation stabilizing auxiliaries in polyene cyclizations 5 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 2 asymmetric synthesis of a steroid
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, William R Bartlett, Balan Chenera, Fook S Tham, Vernon R Fletcher, Rudolph K KullnigAbstract:The use of the Fluorine Atom an a cation-stabilizing (C-S) auxiliary for enhancing polyene cyclization has been improved in synthetic effectiveness by the demonstration that an acetal 5 can be converted to a racemic fluorocycle, bearing the natural backbone configuration of the steroids
-
cation stabilizing auxiliaries in polyene cyclizations 6 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 3 use to effect regiospecific control
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, William R Bartlett, Fook S Tham, Robert A Buchanan, Rudolph K KullnigAbstract:The Fluorine Atom substituted at the pro-C-13 position (steroid numbering) has been shown to be an effective cation-stabilizing (C-S) auxiliary in the acid-catalyzed cyclization of polyene substrates. The preparation of these polyenes employed the known fragmentation of cyclic epoxy hydrazones to construct the methylacetylenic and gem-dimethyl groups. The fluoroolefinic ketal Claisen and cyclopropylcarbinol rearrangements established the 3(E) and 11 (E) trisubstituted alkenes
-
cation stabilizing auxiliaries in polyene cyclizations 7 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 4 total synthesis of dl beta amyrin
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, Mark S Plummer, Pulla S Reddy, William R BartlettAbstract:A total synthesis or dl-β-amyrin (1) is reported, utilizing as the key step a cyclization or a polyolefin having a Fluorine Atom as the cation-stabilizing (C-S) auxiliary. Thus polyene substrate 7, upon acid-catalyzed cyclization, gave fluoropentacycle 8, a compound having five fused rings and bearing six or the eight chiral centers round in the natural product β-amyrin (1). The preparation or polyene 7 required the development or stereoselective methods for introducing the three alkene bonds of the cyclopentenol side chain
William R Bartlett - One of the best experts on this subject based on the ideXlab platform.
-
the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations total synthesis of dl dammarenediol1
Journal of Organic Chemistry, 1999Co-Authors: William S Johnson, William R Bartlett, Boris A Czeskis, Arnaud Gautier, Remy C Lemoine, Eric J Leopold, And Gregory R Luedtke, Katherine J BancroftAbstract:Dammarenediols I (1a) and II (1b) were prepared by an efficient nonenzymatic biomimetic polyene tetracyclization route. The cyclization substrate, pentaenol 3, contains a tetramethylallylic alcohol initiator, an allyltrimethylsilane terminating group, and a Fluorine Atom at pro-C-13 to serve as a cation-stabilizing (C-S) auxiliary controlling the regiochemistry of the C/D ring juncture. The synthesis of 3 employed lithium−halogen exchange to create alcohols 10 and 19. The Z-fluoroalkene in 3 was introduced stereoselectively via the Trost palladium-catalyzed alkylation of allylic acetate 11 (Z/E: 4.6/1). The cyclization of 3 was most efficient (62% isolated yield) when it was added as a dilute solution in dichloromethane to trifluoroacetic acid at −45 °C to afford tetracyclic fluoro diene 24 possessing the trans−anti−trans−anti−trans ring stereochemistry of the dammaranes. Replacement of the Fluorine Atom of 24 with hydrogen with complete retention of configuration was accomplished using the Ohsawa−Oishi r...
-
cation stabilizing auxiliaries in polyene cyclizations 5 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 2 asymmetric synthesis of a steroid
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, William R Bartlett, Balan Chenera, Fook S Tham, Vernon R Fletcher, Rudolph K KullnigAbstract:The use of the Fluorine Atom an a cation-stabilizing (C-S) auxiliary for enhancing polyene cyclization has been improved in synthetic effectiveness by the demonstration that an acetal 5 can be converted to a racemic fluorocycle, bearing the natural backbone configuration of the steroids
-
cation stabilizing auxiliaries in polyene cyclizations 6 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 3 use to effect regiospecific control
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, William R Bartlett, Fook S Tham, Robert A Buchanan, Rudolph K KullnigAbstract:The Fluorine Atom substituted at the pro-C-13 position (steroid numbering) has been shown to be an effective cation-stabilizing (C-S) auxiliary in the acid-catalyzed cyclization of polyene substrates. The preparation of these polyenes employed the known fragmentation of cyclic epoxy hydrazones to construct the methylacetylenic and gem-dimethyl groups. The fluoroolefinic ketal Claisen and cyclopropylcarbinol rearrangements established the 3(E) and 11 (E) trisubstituted alkenes
-
cation stabilizing auxiliaries in polyene cyclizations 7 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 4 total synthesis of dl beta amyrin
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, Mark S Plummer, Pulla S Reddy, William R BartlettAbstract:A total synthesis or dl-β-amyrin (1) is reported, utilizing as the key step a cyclization or a polyolefin having a Fluorine Atom as the cation-stabilizing (C-S) auxiliary. Thus polyene substrate 7, upon acid-catalyzed cyclization, gave fluoropentacycle 8, a compound having five fused rings and bearing six or the eight chiral centers round in the natural product β-amyrin (1). The preparation or polyene 7 required the development or stereoselective methods for introducing the three alkene bonds of the cyclopentenol side chain
Takahisa Ohno - One of the best experts on this subject based on the ideXlab platform.
-
theoretical study on diffusion mechanism of Fluorine Atom adsorbed on si 111 reconstructed surface
Surface Science, 2011Co-Authors: Yusuke Asari, Jun Nara, Takahisa OhnoAbstract:Abstract We study diffusion mechanisms of Fluorine Atom adsorbed on Si(111) reconstructed surface by means of the first-principles density functional calculation. Recently, it was revealed experimentally that the Fluorine diffusion is assisted by extra silicon Atoms diffusing freely on the surface, whereas the detailed mechanism of the Si assist has not been understood. We first investigate F Atom adsorption structures with and without a Si Atom and find that extra Si Atoms do not affect them. Next, we investigate the diffusion mechanism, considering four models in which extra silicon Atoms act a different role, and find that SiF-complex diffusion model with activation energy of 1.34 eV is appropriate to explain the experiment. In this model, the Si–F bond is kept during diffusion, while in the others, it repeats breaking and re-bonding. This diffusion mechanism is also understood as the Si diffusion with carrying an F Atom. We analyze why this diffusion mechanism is preferable to the others and find that two important features play roles. One is the strong Si-F bond that favors to keep the complex form, and the other is the existence of an extra silicon Atom, which takes over the adAtom position to passivate dangling bonds left behind after the SiF-complex removal.
Rudolph K Kullnig - One of the best experts on this subject based on the ideXlab platform.
-
cation stabilizing auxiliaries in polyene cyclizations 4 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 1 background and exploratory experiments
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, Balan Chenera, Fook S Tham, Rudolph K KullnigAbstract:With the aim of testing the Fluorine Atom as a potential cation-stabilizing (C-S) auxiliary for enhancing polyene cyclizations, new methodology has been developed for the stereoselective synthesis of polyenic cyclization substrates in which the vinyl H of an E-trisubstituted olefinic bond is replaced by F
-
cation stabilizing auxiliaries in polyene cyclizations 5 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 2 asymmetric synthesis of a steroid
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, William R Bartlett, Balan Chenera, Fook S Tham, Vernon R Fletcher, Rudolph K KullnigAbstract:The use of the Fluorine Atom an a cation-stabilizing (C-S) auxiliary for enhancing polyene cyclization has been improved in synthetic effectiveness by the demonstration that an acetal 5 can be converted to a racemic fluorocycle, bearing the natural backbone configuration of the steroids
-
cation stabilizing auxiliaries in polyene cyclizations 6 the Fluorine Atom as a cation stabilizing auxiliary in biomimetic polyene cyclizations 3 use to effect regiospecific control
Journal of the American Chemical Society, 1993Co-Authors: William S Johnson, William R Bartlett, Fook S Tham, Robert A Buchanan, Rudolph K KullnigAbstract:The Fluorine Atom substituted at the pro-C-13 position (steroid numbering) has been shown to be an effective cation-stabilizing (C-S) auxiliary in the acid-catalyzed cyclization of polyene substrates. The preparation of these polyenes employed the known fragmentation of cyclic epoxy hydrazones to construct the methylacetylenic and gem-dimethyl groups. The fluoroolefinic ketal Claisen and cyclopropylcarbinol rearrangements established the 3(E) and 11 (E) trisubstituted alkenes
S T Ceyer - One of the best experts on this subject based on the ideXlab platform.
-
Fluorine Atom abstraction by si 100 ii model
Journal of Chemical Physics, 2000Co-Authors: M R Tate, D Gosalvezblanco, D P Pullman, A A Tsekouras, Y L Li, S T CeyerAbstract:A model is developed to describe the kinetics of the three scattering channels—unreactive scattering and dissociative chemisorption via single Atom abstraction and two Atom adsorption—that are present in the interaction of F2 with Si(100). The model provides a good description of the non-Langmuirian coverage dependence of the probabilities of single Atom abstraction and two Atom adsorption, yielding insight into the dynamics of the gas–surface interaction. The statistical model is based on the premise that the two dissociative chemisorption channels share a common initial step, F Atom abstraction. The subsequent interaction, if any, of the complementary F Atom with the surface determines if the overall result is single Atom abstraction or two Atom adsorption. The results are consistent with the orientation of the incident F2 molecular axis with respect to the surface affecting the probability of single Atom abstraction relative to two Atom adsorption. A perpendicular approach favors single Atom abstractio...
-
Fluorine Atom abstraction by si 100 i experimental
Journal of Chemical Physics, 1999Co-Authors: M R Tate, D Gosalvezblanco, D P Pullman, A A Tsekouras, Y L Li, J J Yang, K B Laughlin, S C Eckman, Massimo F Bertino, S T CeyerAbstract:In the interaction of low energy F2 with Si(100) at 250 K, a dissociative chemisorption mechanism called Atom abstraction is identified in which only one of the F Atoms is adsorbed while the other F Atom is scattered into the gas phase. The dynamics of Atom abstraction are characterized via time-of-flight measurements of the scattered F Atoms. The F Atoms are translationally hyperthermal but only carry a small fraction (∼3%) of the tremendous exothermicity of the reaction. The angular distribution of F Atoms is unusually broad for the product of an exothermic reaction. These results suggest an “attractive” interaction potential between F2 and the Si dangling bond with a transition state that is not constrained geometrically. These results are in disagreement with the results of theoretical investigations implying that the available potential energy surfaces are inadequate to describe the dynamics of this gas–surface interaction. In addition to single Atom abstraction, two Atom adsorption, a mechanism anal...