ISSN:0018-019X    

DOI:10.1002/hlca.19920750602

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe feasibility of the titled reactions for the rapid, enantioselective synthesis ofcis‐tricyclo[9.3.1.03,8]pentadecane precursors to taxusin and taxol has been examined. The catalysts most well suited to inducing the appropriate 1,2‐shifts have been identified. To a great extent, the rearrangement products are formed as a direct consequence of appropriate structural features (kinetic phenomenon) and strain minimization (thermodynamic driving force). Complementary MM2 calculations of the global minimum in each series provided indications that were completely in line with the experimental observations. Sophisticated NMR studies and X‐ray crystallographic determinations were coordinated to remove any ambiguity of product structure and solid‐state confo

ISSN:0018-019X    

DOI:10.1002/hlca.19920750603

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe key elements associated with the synthetic elaboration of functionalizedtrans‐tricyclo‐[9.3.1.03,8]pentadecanes carrying either a bridgehead H or OH substituent are detailed. Starting with12, a ketone available in two steps from (R)‐2‐oxo‐7,7‐dimethyl‐l‐vinylbicyclo[2.2.1]heptane, it proved possible to introducetrans‐B/C ring juncture configuration as in16in five steps. This advanced intermediate constitutes the point of bifurcation. The pathway to taxusin precursor23was attained by stereospecific osmylation, reduction, and pinacol‐like 1,2‐Wagner‐Meerweinrearrangement within acetoxy mesylate22c.Still more abbreviated is the route to32, which again takes advantage of the osmylation step but proceeds, forward without reduction of the rear carbonyl group. Once hydroxy diketone31is produced, equilibration in the presence of (t‐BuO)3Al results in complete conversion to32. The many stereoselective transformations developed in the course of this study, in combination with the several thermodynamic questions that have been clarified, are expected to be highly serviceable as more advanced thrusts toward taxu

ISSN:0018-019X    

DOI:10.1002/hlca.19920750604

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe reaction of FeO+with toluene in the gas phase occurs at collision rate (kr= 1.36 × 10−9cm3molecule−1s−1), and labeling experiments demonstrate that the total products due to CH bond activation involve to>92% the benzylic position. In the ‘hydride’ abstraction process (formation of FeOH and C7H 7+), the H‐atom originates elusively from the benzylic position to generate a benzyl cation, and an intramolecular kinetic isotope effectkH/kD= 1.75 has been obtained. There is no evidence for the existence of isotopically sensitive branching (‘metabolic switching’) i

ISSN:0018-019X    

DOI:10.1002/hlca.19920750605

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

Di(tert‐butyl)diazomethane: Thermal Decomposition and One‐Electron Redox Reactions.Di(tert‐butyl)diazomethane is a potential precursor for the still unknown, presumably sterically overcrowded tetrakis(tert‐butyl)ethane and, therefore, re‐investigated. Its (Hel) photoelectron spectrum exhibits a low first vertical ionization energy of only 7.45 eV. Based on the ionization pattern, both the thermal decomposition above 600 K under nearly unimolecular conditions as well as the N2elimination at the surface of contacts, [Nix/C]∞, [Rh4(CO)12/SiO2]∞, [Rhx/SiO2]∞, and [Ag2CO3]∞are analyzed in a flow‐system. Heterogeneously catalyzed, N2is split off already at room temperature, but in contrast to results for sterically less shielded diazo compounds, no dimer is formed, and only mixtures of known di(tert‐butyl)carbene‐isomerization products are isolated. Cyclic voltammetry at 233 K using a glassy carbon electrode proves a reversible oxidation followed by N2elimination at higher temperatures and an irreversible reduction. On chemical oxidation, however, no paramagnetic species can be detected, whereas chemical reduction at a potassium metal mirror in a THF solution containing (2.2.2)cryptand, yields the radical anion characterized by ESR spectroscopy. Without a cation‐chelating ligand, the radical anion of a hitherto unknown dimer, ((CH3)3C)2CNNNNC(C(CH3)3)2'⊖, is generated, which dissociates at higher temperature, forming ((CH3)3C)2N2'⊖. This one‐electron reduction product of di(tert‐butyl)diazomethane can also be detected after quickly warming up a solution containing presumably the radical anion of the triphenylphosphane adduct ((CH3)3C)2CNNPPh3'⊖. In one of t

ISSN:0018-019X    

DOI:10.1002/hlca.19920750606

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

Structural Changes on Reduction of 9‐(Diphenylmethylidene)fluorene by Sodium Metal to Sheets of (R2O)Na⊕‐ Connected Hydrocarbon DianionsThe single‐crystal structure of 9‐(diphenylmethylidene)fluorene (monoclinic, space groupP21/n,Z= 4) proves a CC bond length of 135 pm and a twisting of the two Ph rings out of the methylidene‐fluorene plane by 70° and 83°. Its reduction in Et2O solution by a Na mirror under Ar yields dark green crystals with a metallic luster. Their structure, determined in a N2flow cooled to 223 K (monoclinic, space groupC2/c,Z= 4), reveals sheets of hydrocarbon dianions, connected by sodium(diethylether) links. The Na centers are sixfold coordinated as half‐sandwiches to the fluorene benzo rings and threefold allylic to the Ph rings. In the dianion of 9‐(diphenyl‐methylidene)fluorene, the central CC bond is stretched by 13 pm to 148 pm length, and the two molecular cyanine‐like halves are twisted by 51° relative to each other. Both the contact‐ion multiple structures as well as the structural changes on twofold reduction are discussed by way of comparison with literature examples and based o

ISSN:0018-019X    

DOI:10.1002/hlca.19920750607

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

Reaction of 2‐Diazopropane and 1,3‐Thiazole‐5(4H)‐thionesAt 0°, 1,3‐thiazoles1and 2‐diazopropane in pentane or MeOH undergo a smooth 1,3‐dipolar cycloaddition to give spirocyclic 2,4‐dihydro‐1,3,4‐thiadiazoles6(Scheme 3). Elimination of N2occurs already at 20°, yielding spirocyclic thüranes7. At room temperature, the latter decompose slowly to give 4,5‐dihydro‐5‐isopropylidene‐1,3‐thiazoles8. The desulfurization7 → 8can be accelerated by heating or by addition of trimethyl phosphite. The intermediate of the N2elimination of6is a thiocarbonyl ylideDwhich can be trapped with the reactive dipolarophiles thiobenzophenone and fumarodinitrile (Scheme 4). In the absence of these trapping reagents,Dundergoes ring closure to thüranes7. The cycloaddition ofDand a second molecule of1, the so‐cal

ISSN:0018-019X    

DOI:10.1002/hlca.19920750608

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

Synthesis of (±)‐MuscopyridineviaC‐ZIP Ring EnlargementTreatment of 4‐(1‐nitro‐2‐oxocyclododec‐1‐yl)butanal (1) and of its methyl derivative5with pentylamine in EtOH at room temperature gave the ring‐enlarged aminomethylidene derivatives6and7, respectively (Scheme 1). After hydrolysis of the aminomethylidene group in6and7and deformylation followed by a reductiveNef‐type reaction, the macrocyclic diketones10and11, respectively, were obtained. They were transformed by a modifiedHantzschprocedure to the title compound (±)‐muscopyridine (13) and normuscopyri

ISSN:0018-019X    

DOI:10.1002/hlca.19920750609

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractSyntheses of thecis‐configurated isopenam9(Scheme 1), isocephem14(Scheme 2), and isocephem19(Scheme 3) are described. The key step in the preparation of14and19involved aPummerer‐type rearrangement of the corresponding sulfoxides12and18. These β‐lactams were found to possess biological activity against several pathogenic microorganisms invitro. The electronic activation of the lactam moiety of19remarkably enhanced its biological activity. A retinoic moiety was attached to19viaan amino linker. The resultant retinamido‐β‐lactam21showed significant cytostatic activity in tracheal organ cultures obtained from vitamin‐A‐def

ISSN:0018-019X    

DOI:10.1002/hlca.19920750610

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractEight (Z)‐isomers of lycopene were prepared by stereocontrolled syntheses and fully characterized by1H‐NMR,13C‐NMR, mass, and UV/VIS spectroscopy: (5Z)‐, (7Z)‐, (15Z)‐, (5Z,5′Z)‐, (7Z,7′Z)‐, (7Z,9Z)‐, (9Z,9′Z)‐, and (7Z,9Z,7′Z,9′Z)‐lycopene. Six additional (Z)‐isomers, namely (9Z)‐, (13Z)‐, (5Z,9′Z)‐, (9Z,13′Z)‐, (5Z,9Z,5′Z)‐, and (5Z,13Z,5′Z)‐lycopene, were isolated in small quantities from isomer mixtures by sem

ISSN:0018-019X    

DOI:10.1002/hlca.19920750611

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY