79 research outputs found
Alkoxyallenes as building blocks for organic synthesis
Alkoxyallenes are unusually versatile C3 building blocks in organic synthesis.
Hence this tutorial review summarizes the most important transformations,
including subsequent reactions and their applications in the synthesis of
relevant compounds, e.g. natural products. The reactivity patterns involved
and the synthons derived from alkoxyallenes are presented. Often alkoxyallenes
can serve as substitutes of acrolein or acrolein acetals, utilisation of which
has already led to interesting products. Most important is the use of
lithiated alkoxyallenes which smoothly react with a variety of electrophiles
and lead to products with unique substitution patterns. The heterocycles or
carbocycles formed are intermediates for the stereoselective synthesis of
natural products or for the preparation of other structurally relevant
compounds. The different synthons being put into practice by the use of
lithiated alkoxyallenes in these variations will be discussed
The Boekelheide Rearrangement of Pyrimidine-N-oxides as a Case Study of Closed or Open Shell Reactions – Experimental and Computational Evidence for the Participation of Radical Intermediates
In a case study, the acetic anhydride-promoted reaction of a model pyrimidine-N-oxide to the corresponding 4-acetoxymethyl-substituted pyrimidine derivative (Boekelheide rearrangement) was investigated in detail by experiment and quantum chemical calculations. The reaction conditions were varied and several side products formed in low to moderate yields were identified. These experiments indicate that a (pyrimidin-4-yl)methyl radical is one of the key species of the rearrangement. This interpretation was supported by the fact that rearrangements performed in solvents which can easily lose hydrogen atoms, afford considerable quantities of products incorporating the solvent. With TEMPO the key radical could be trapped. Other carboxylic acid anhydrides confirm the conclusion that the Boekelheide rearrangement of the model pyrimidine-N-oxide proceeds, at least in part, via radical intermediates. The high level closed and open shell quantum chemical calculations show that concerted [3,3]-sigmatropic rearrangements or stepwise processes, either via ion pairs or via radicals, are energetically feasible
Iodination of carbohydrate-derived 1,2-oxazines to enantiopure 5-iodo-3,6 -dihydro-2H-1,2-oxazines and subsequent palladium-catalyzed cross-coupling reactions
Iodination of carbohydrate-derived 3,6-dihydro-2H-1,2-oxazines of type 3 using
iodine and pyridine in DMF furnished 5-iodo-substituted 1,2-oxazine
derivatives 4 with high efficacy. The alkenyl iodide moiety of 1,2-oxazine
derivatives syn-4 and anti-4 was subsequently exploited for the introduction
of new functionalities at the C-5 position by applying palladium-catalyzed
carbon–carbon bond-forming reactions such as Sonogashira, Heck, or Suzuki
coupling reactions as well as a cyanation reaction. These cross-coupling
reactions led to a series of 5-alkynyl-, 5-alkenyl-, 5-aryl- and 5-cyano-
substituted 1,2-oxazine derivatives being of considerable interest for further
synthetic elaborations. This was exemplarily demonstrated by the hydrogenation
of syn-21 and anti-24 and by a click reaction of a 5-alkynyl-substituted
precursor
Alkoxyallene-Based Stereodivergent Syntheses of (−)-Hyacinthacine B4 and of Putative Hyacinthacine C5 Epimers: Proposal of Hyacinthacine C5 Structure
Preparativne modifikacije askomicina. V. Dobivanje novih derivata pomoću zamjene cikloheksilvinilidenske podjedinice
Starting from the easily accessible 24-O-tert-butyldimethylsilyl-22(R)-dihydro-28-oxoascomycin, methodologies that allow replacement of the cyclohexylvinylidene moiety of ascomycin by various other substituents are described. In addition, a so far unknown reactivity of the masked tricarbonyl moiety of ascomycin towards a stabilized Wittig reagent is reported.Opisani su postupci koji počinju s lako pristupačnim 25-O-tert-butildimetilsilil-22(R)-dihidro-28-oksoaskomicinom i omogućuju zamjenu askomicinske cikloheksilvinilidenske podjedinice različitim substituentima
Radical Stability vs. Temporal Resolution of EPR-Spectroscopy on Biological Samples
Spin-labeling active compounds is a convenient way to prepare them for EPR spectroscopy with minimal alteration of the target molecule. In this study we present the labeling reaction of dexamethasone (Dx) with either TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or PCA (3-(carboxy)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy) with high yields. According to NMR data, both labels are attached at the primary hydroxy group of the steroid. In subsequent spin-stability measurements both compounds were applied onto HaCaT cells. When the signal of Dx-TEMPO decreased below the detection limit within 3 h, the signal of Dx-PCA remained stable for the same period of time
Indication for the disappearance of reactor electron antineutrinos in the Double Chooz experiment
The Double Chooz Experiment presents an indication of reactor electron
antineutrino disappearance consistent with neutrino oscillations. A ratio of
0.944 0.016 (stat) 0.040 (syst) observed to predicted events was
obtained in 101 days of running at the Chooz Nuclear Power Plant in France,
with two 4.25 GW reactors. The results were obtained from a single 10
m fiducial volume detector located 1050 m from the two reactor cores. The
reactor antineutrino flux prediction used the Bugey4 measurement as an anchor
point. The deficit can be interpreted as an indication of a non-zero value of
the still unmeasured neutrino mixing parameter \sang. Analyzing both the rate
of the prompt positrons and their energy spectrum we find \sang = 0.086
0.041 (stat) 0.030 (syst), or, at 90% CL, 0.015 \sang 0.16.Comment: 7 pages, 4 figures, (new version after PRL referee's comments
Precision muon reconstruction in Double Chooz
We describe a muon track reconstruction algorithm for the reactor anti-neutrino experiment Double Chooz. The Double Chooz detector consists of two optically isolated volumes of the liquid scintillator viewed by PMTs, and an Outer Veto above these made of crossed scintillator strips. Muons are reconstructed by their Outer Veto hit positions along with timing information from the other two detector volumes. All muons are fit under the hypothesis that they are through-going and ultrarelativistic. If the energy depositions suggest that the muon may have stopped, the reconstruction fits also for this hypothesis and chooses between the two via the relative goodness-of-fit. In the ideal case of a through-going muon intersecting the center of the detector, the resolution is ~40mm in each transverse dimension. High quality muon reconstruction is an important tool for reducing the impact of the cosmogenic isotope background in Double Chooz. © 2014 Elsevier B.V
Ortho-positronium observation in the double chooz experiment
The Double Chooz experiment measures the neutrino mixing angle θ13 by detecting reactor ¯νe via inverse beta decay. The positron-neutron space and time coincidence allows for a sizable background rejection, nonetheless liquid scintillator detectors would profit from a positron/electron discrimination, if feasible in large detector, to suppress the remaining background. Standard particle identification, based on particle dependent time profile of photon emission in liquid scintillator, can not be used given the identical mass of the two particles. However, the positron annihilation is sometimes delayed by the orthopositronium (o-Ps) metastable state formation, which induces a pulse shape distortion that could be used for positron identification. In this paper we report on the first observation of positronium formation in a large liquid scintillator detector based on pulse shape analysis of single events. The o-Ps formation fraction and its lifetime were measured, finding the values of 44 % ± 12 % (sys.) ± 5 % (stat.) and 3.68 ns ± 0.17 ns (sys.) ± 0.15 ns (stat.) respectively, in agreement with the results obtained with a dedicated positron annihilation lifetime spectroscopy setup
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