8,943 research outputs found
Structure Determination of 8-Benzyl-5-phenyl-3-oxa-4,8-diazatricyclo[5.2.1.02,6]dec-4-ene and 1-(9-Ethoxy-5-phenyl-3-oxa-4,8-diaza-tricyclo[5.2.1.02,6]dec-4-en-8-yl)-ethanone: Their Synthesis, Chemical Relationship and Comparison with Similar Compounds
No full textThe molecular and crystal structures of 8-benzyl-
5-phenyl-3-oxa-4,8-diaza-tricyclo[5.2.1.02,6]dec-4-ene 11a
and 1-(9-ethoxy-5-phenyl-3-oxa-4,8-diaza-tricyclo[5.2.1.02,6]
dec-4-en-8-yl)-ethanone 13a have been XRD determined.
The compounds crystallize in the monoclinic system with
space groups P21/n and P21/c, respectively. The synthesis
and chemical relationship between the two compounds are
reported and their structures are compared with others of the
aza-norbornene and isoxazoline families
Andrea Bacová
No full textAndrea Bacová focuses on research and teaching in the field of residential architecture. Her work includes systematic research on residential buildings and their urban context. She actively participates in promoting Slovak architecture and is the author of several publications and exhibitions
A quad-core 15GHz BiCMOS VCO with -124dBc/Hz phase noise at 1MHz offset, -189dBc/Hz FOM, and robust to multimode concurrent oscillations
No full textThe relentless development of next-generation communication and radar systems sets increasingly stringent requirements on the spectral purity of local oscillators. Decreasing phase noise is crucial to support efficient modulation formats with large symbol constellations, as well as to enable innovative radar applications, e.g., anti-collision, gesture recognition, and medical imaging. To minimize phase noise, bipolar transistors offer some advantages over ultra-scaled CMOS: higher supply voltage (thus larger oscillation amplitudes), lower 1/f noise, higher-Q passives (due to higher resistivity substrate and, possibly, thicker metals), and higher f T , f max for a given technology node, which results in a cost advantage for a variety of medium-volume applications (e.g., infrastructure transceivers). For a given supply voltage, a tank showing a smaller resistance at resonance yields lower phase noise. As a result, the minimum phase noise achievable by a single voltage-controlled oscillator (VCO) is ultimately bounded by the smaller realizable inductor displaying the highest Q. To achieve significantly lower phase noise levels, bilaterally coupling N oscillators [1-3] is a viable option. However, to fully preserve the 10log(N) phase-noise advantage, while avoiding undesired multi-tone concurrent oscillations, the coupling network must be carefully designed. This work presents a quad-core bipolar VCO achieving phase noise as low as -124dBc/Hz at 1MHz offset from the 15GHz carrier, -189dBc/Hz figure-of-merit (FOM), and 16% tuning range. Insights are given into the design of the resistive network employed to couple the four oscillators, a key element in achieving the reported performance
Palladium(0)-catalyzed syntheses of cyclopentenyl-nucleoside analogues
No full textThe N-benzoylamino-2-cyclopenten-1-ol, derived from the nitrosocarbonyl hetero Diels-Alder cycloaddition reaction followed by the mild reductive N-O bond cleavage, allows for the preparation of cyclopentenyl-nucleoside analogues through the palladium(0)-catalyzed addition of both purine and pyrimidine heterobases to the allylic acetate derivative with retention of the starting material stereochemistry. A systematic study is performed on different heterobases to test the general application of the protocol
PHOTOCHEMICAL GENERATION OF NITROSOCARBONYL INTERMEDIATES ON SOLID PHASE: SYNTHONS TOWARD HETERO-DIELS-ALDER AND ENE ADDUCTS THROUGH PHOTOCLEAVAGE
No full textThe synthesis of 1,2,4-oxadiazole-4-oxides on polystyrenic solid phase docked at the position 3 of the
heterocyclic ring has been performed through the cycloaddition of stable supported nitrile oxides to
amidoximes. The photochemical cycloreversion of these heterocycles afforded the free nitrosocarbonyl
intermediates that were trapped by suitable dienes or enes. The method is proposed as a clean and
environmental friendly approach to the fleeting nitrosocarbonyl intermediates, which afford valuable adducts
for various synthetic applications. The isomeric heterocycles docked at the position 5 of the ring have also
been obtained by cycloaddition of nitrile oxides to supported amidoximes. Their photolysis afforded resinbound
nitrosocarbonyls that were trapped with dienes affording valuable supported adducts suitable for
further elaboration on solid-phase chemistry
The Remarkable Cis Effect in the Ene Reactions of NitrosocarbonylIntermediates
No full textNitrosocarbonyls are fleeting and highly reactive intermediates that undergo ene reactions in a two-step
fashion. The addition steps are rate and product determining and lead to polarized diradicals that readily
enter the H-abstraction step yielding the ene products. The addition TSs are reached early, and the
stabilizing CH· · ·O contacts drive the reactions to the cis adducts. B3LYP calculations alone do not
describe the correct ordering of addition TSs in the ene reaction with trimethylethylene and (E)- and
(Z)-3-methyl-2-pentenes. Only at the MPWB1K level of treatment, medium-range noncovalent interactions
are successfully recovered, accounting satisfactorily for the experimental selectivities. The more stable
and isolable ArNOs exhibit late addition TSs, and distortion energies become dominant driving the reaction
exclusively to the Markovnikov adducts
From 1,3-cyclohexadiene through nitrosocarbonyl chemistry, the synthesis of pyrimidine isoxazoline-carbocyclic nucleosides
No full textN-Benzoyl-2-oxa-3-azabicyclo[2.2.2]oct-5-ene undergoes cycloaddition with benzonitrile oxide affording a mixture of syn and anti regioisorneric cycloadducts. The anti cycloadclucts were easily elaborated to stereodefined isoxazoline-carbocyclic arninols that served as synthons for the linear construction of pyrimidine nucleosides, while the syn cycloadclucts do not enter the same synthetic route
Fast active thermal cloaking through PDE-constrained optimization and reduced-order modelling
No full textIn this paper, we show how to efficiently achieve
thermal cloaking from a computational standpoint in
several virtual scenarios by controlling a distribution
of active heat sources. We frame this problem in the
setting of PDE-constrained optimization, where the
reference field is the solution of the time-dependent
heat equation in the absence of the object to cloak.
The optimal control problem then aims at actuating
the space–time control field so that the thermal field
outside the obstacle is indistinguishable from the
reference field. In particular, we consider multiple
scenarios where material’s thermal diffusivity, source
intensity and obstacle’s temperature are allowed to
vary within a user-defined range. To tackle the
thermal cloaking problem in a rapid and reliable
way, we rely on a parametrized reduced order model
built through the reduced basismethod, thus entailing
huge computational speedups compared with a highfidelity,
full-order model exploiting the finite-element
method while dealing both with complex target
shapes and disconnected control domains
From cyclopentadiene to isoxazoline-carbocyclic nucleosides: a rapid access to biological molecules through aza-Diels-Alder reactions
No full textA rapid access to carbocyclic nucleosides containing a fused isoxazoline ring is proposed through the Grieco cycloaddition of cyclopentadiene to iminium salts. The prolific elaboration of the isoxazoline cycloadducts allowed preparation of the target aminols through the unmasking of the hydroxymethylene group at the C3 level of the azanorbornene structure. The heterocyclic aminols are readily converted into nucleosides via the linear construction of purine heterocycles
Variable Markovnikov orientation and cis effect in ene reactions of nitrosocarbonyl intermediates
No full textNitrosocarbonyl intermediates, generated at room temperature
by the mild oxidation of nitrile oxides, undergo clean ene
reactions with trisubstituted olefins. Allylic hydrogens on
the more congested side of the alkene are exclusively
abstracted (the “cis effect”), thus resembling singlet oxygen
behavior. Nitrosocarbonyl benzene follows a Markovnikov
orientation and abstracts preferentially the twix hydrogens
over the lone ones. With the more sterically demanding
nitrosocarbonyl mesitylene, the Markovnikov directing effect
is relieved, and comparable twix and lone abstraction are
observed
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