55,932 research outputs found
Instantons on hyperkähler manifolds
No full textAn instanton on a (pseudo-)hyperk\"ahler manifold is a vector bundle associated to a principal -bundle with a connection whose curvature is pointwise invariant under the quaternionic structures of , and thus satisfies the Yang-Mills equations. Revisiting a construction of solutions, we prove a local bijection between gauge equivalence classes of instantons on and equivalence classes of certain holomorphic functions taking values in the Lie algebra of defined on an appropriate -bundle over . Our reformulation affords a streamlined proof of Uhlenbeck's Compactness Theorem for instantons on (pseudo-)hyperk\"ahler manifolds
Glucopyranosylidene-spiro-benzo[<i>b</i>][1,4]oxazinones and -benzo[<i>b</i>][1,4]thiazinones: Synthesis and Investigation of Their Effects on Glycogen Phosphorylase and Plant Growth Inhibition
No full textGlucopyranosylidene-spiro-benzo[b][1,4]oxazinones
were obtained via the corresponding 2-nitrophenyl glycosides obtained
by two methods: (a) AgOTf-promoted glycosylation of 2-nitrophenol
derivatives by O-perbenzoylated methyl (α-d-gluculopyranosyl bromide)heptonate or (b) Mitsunobu-type reactions
of O-perbenzoylated methyl (α-d-gluculopyranose)heptonate
with bulky 2-nitrophenols in the presence of diethyl azodicarboxylate
(DEAD) and PPh3. Catalytic hydrogenation (H2–Pd/C) or partial reduction (e.g., H2–Pd/C,
pyridine) of the 2-nitro groups led to spiro-benzo[b][1,4]oxazinones and spiro-benzo[b][1,4]-4-hydroxyoxazinones
by spontaneous ring closure of the intermediate 2-aminophenyl or 2-hydroxylamino
glycosides, respectively. The analogous 2-aminophenyl thioglycosides,
prepared by reactions of O-perbenzoylated methyl
(α-d-gluculopyranosyl bromide)heptonate with 2-aminothiophenols,
were cyclized in m-xylene at reflux temperature to
the corresponding spiro-benzo[b][1,4]thiazinones. O-Debenzoylation was effected by Zemplén transesterification
in both series. Spiro-configurations were determined by NMR and electronic
circular dichroism time-dependent density functional theory (ECD-TDDFT)
methods. Inhibition assays with rabbit muscle glycogen phosphorylase
b showed (1′R)-spiro{1′,5′-anhydro-d-glucitol-1′,2-benzo[b][1,4]oxazin-3(4H)-one} and (1′R)-spiro{1′,5′-anhydro-d-glucitol-1′,2-benzo[b][1,4]thiazin-3(4H)-one} to be the most efficient inhibitors (27 and 28%
inhibition at 625 μM, respectively). Plant growth tests with
white mustard and garden cress indicated no effect except for (1′R)-4-hydroxyspiro{1′,5′-anhydro-d-glucitol-1′,2-benzo[b][1,4]oxazin-3(4H)-one} with the latter plant to show modest inhibition
of germination (95% relative to control)
Spiro-Conjugated Molecular Junctions: Between Jahn–Teller Distortion and Destructive Quantum Interference
No full textThe
quest for molecular structures exhibiting strong quantum interference
effects in the transport setting has long been on the forefront of
chemical research. We establish theoretically that the unusual geometry
of spiro-conjugated systems gives rise to complete destructive interference
in the resonant-transport regime. This results in a current blockade
of the type not present in meta-connected benzene or similar molecular
structures. We further show that these systems can undergo a transport-driven
Jahn–Teller distortion, which can lift the aforementioned destructive-interference
effects. The overall transport characteristics are determined by the
interplay between the two phenomena. Spiro-conjugated systems may
therefore serve as a novel platform for investigations of quantum
interference and vibronic effects in the charge-transport setting.
The potential to control quantum interference in these systems can
also turn them into attractive components in designing functional
molecular circuits
Monge-Ampere equations and moduli spaces of manifolds of circular type
No full textA (bounded) manifold of circular type is a complex manifold M of dimension n admitting a(bounded) exhaustive real function u, defined on M minus a point x_o, so that: a) it is a smooth solution on M\ {x_o} to the Monge-Amp\`ere equation (d d^c u)^n = 0; b) x_o is a singular point for u of logarithmic type and e^u extends smoothly on the blow up of M at x_o; c) d d^c (e^u) >0 at any point of M\ {x_o}. This class of manifolds naturally includes all smoothly bounded, strictly linearly convex domains and all smoothly bounded, strongly pseudoconvex circular domains of C^n.
A set of of modular parameters for bounded manifolds of circular type is considered. In particular, for each biholomorphic equivalence class of them it is proved the existence of an essentially unique manifold in normal form. It is also shown that the class of normalizing maps for an n-dimensional manifold M is a new holomorphic invariant with the following property: it is parameterized by the points of a finite dimensional real manifold of dimension n^2 when M is a (non-convex) circular domain while it is of dimension n^2 + 2 n when M is a strictly linearly convex domain. New characterizations of the circular domains and of the unit ball are also obtained
Spiro-Lactams as Novel Antimicrobial Agents
No full textIntroduction: Structural modulation of previously identified lead spiro-β-lactams with antimicrobial activity was carried out.
Objective: The main objective of this work was to synthesize and evaluate the biological activity of novel spiro-lactams based on previously identified lead compounds with antimicrobial activity.
Methods: The target chiral spiro-γ-lactams were synthesized through 1,3-dipolar cycloaddition reaction of a diazo-γ-lactam with electron-deficient dipolarophiles. In vitro activity against HIV and Plasmodium of a wide range of spiro-β-lactams and spiro-γ-lactams was evaluated. Among these compounds, one derivative with good anti-HIV activity and two with promising antiplasmodial activity (IC50 < 3.5 µM) were identified.
Results: A novel synthetic route to chiral spiro-γ-lactams has been established. The studied β- and γ- lactams were not cytotoxic, and three compounds with promising antimicrobial activity were identified, whose structural modulation may lead to new and more potent drugs.
Conclusion: The designed structural modulation of biologically active spiro-β-lactams involved the replacement of the four-membered β-lactam ring by a five-membered γ-lactam ring. Although conformational and superimposition computational studies revealed no significant differences between β- and γ- lactam pharmacophoric features, the studied structural modulation did not lead to compounds with a similar biological profile. The observed results suggest that the β-lactamic core is a requirement for the activity against both HIV and Plasmodium.Américo J. S. Alves, Nuno G. Alves, Cátia C. Caratão, Margarida I. M. Esteves, Maria I. L. Soares, Susana M. M. Lopes and Teresa M. V. D. Pinho e Melo, thank to the Coimbra Chemistry Centre (CQC) supported by the Portuguese Agency for Scientific Research, “Fundação para a Ciência e a Tecnologia” (FCT), through Project UID/QUI/00313/2019. Américo J. S. Alves and Nuno G. Alves thank FCT for fellowships SFRH/BD/128910/2017 and PD/BD/135287/2017, respectively. Miguel Prudêncio acknowledges FCT for grant PTDC-SAU-INF-29550-2017 and contract CEEC-2017.info:eu-repo/semantics/publishedVersio
Recent Advances in the Synthesis of Spiro‐β‐Lactams and Spiro‐δ‐Lactams
No full textSpirocyclic molecules are widely recognized
for their complex three-dimensional features
and structural rigidity. Among this class of molecules,
the spiro-lactams subclass stands out, being extensively
explored due to its bioactivity and utility in a
variety of scientific fields such as drug design and
organic synthesis. Given the recognition of spirocyclic
lactams’ broad potential, several efforts have
been engaged on the pursuit of new synthetic
strategies towards these molecules. The present review
gathers advances on the synthesis of both spiro-
β-lactams (spiroazetidin-2-ones) and spiro-δ-lactams
(spiropiperidon-2-ones) reported since 2015. The
work is divided into two distinct parts, each one
dedicated to one of these types of spiro-lactam, with
the approach used for building the spirocyclic system
being extensively discussed, according to the method’s
scope, efficiency, selectivity, and reaction mechanism
Spiro-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Nonradiative Decay for High-Quantum-Efficiency Low-Roll-Off, Organic Light-Emitting Diodes
No full textHerein,
we report the use of spiro-configured fluorene-xanthene scaffolds
as a novel, promising,
and effective strategy in thermally activated delayed fluorescence
(TADF) emitter design to attain high photoluminescence quantum yields
(ΦPL), short delayed luminescence lifetime, high
external quantum efficiency (EQE), and minimum efficiency roll-off
characteristics in organic light-emitting diodes (OLEDs). The optoelectronic
and electroluminescence properties of SFX (spiro-(fluorene-9,9′-xanthene))-based
emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally.
All three emitters exhibited sky blue to green emission enabled by
a Herzberg–Teller mechanism in the excited state. They possess
short excited-state delayed lifetimes (<10 μs), high photoluminescence
quantum yields (ΦPL ∼ 70%), and small singlet–triplet
splitting energies (ΔEST < 0.10
eV) in the doped films in an mCP host matrix. The OLEDs showed some
of the highest EQEs using spiro-containing emitters where maximum
external quantum efficiencies (EQEmax) of 11 and 16% were
obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a
spiro-based emitter coupled with a low efficiency roll-off (19% at
100 cd m–2) was attained with SFX-PO-DPA-Me
Spiro-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Nonradiative Decay for High-Quantum-Efficiency, Low-Roll-Off, Organic Light-Emitting Diodes
No full textHerein,
we report the use of spiro-configured fluorene-xanthene scaffolds
as a novel, promising,
and effective strategy in thermally activated delayed fluorescence
(TADF) emitter design to attain high photoluminescence quantum yields
(ΦPL), short delayed luminescence lifetime, high
external quantum efficiency (EQE), and minimum efficiency roll-off
characteristics in organic light-emitting diodes (OLEDs). The optoelectronic
and electroluminescence properties of SFX (spiro-(fluorene-9,9′-xanthene))-based
emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally.
All three emitters exhibited sky blue to green emission enabled by
a Herzberg–Teller mechanism in the excited state. They possess
short excited-state delayed lifetimes (<10 μs), high photoluminescence
quantum yields (ΦPL ∼ 70%), and small singlet–triplet
splitting energies (ΔEST < 0.10
eV) in the doped films in an mCP host matrix. The OLEDs showed some
of the highest EQEs using spiro-containing emitters where maximum
external quantum efficiencies (EQEmax) of 11 and 16% were
obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a
spiro-based emitter coupled with a low efficiency roll-off (19% at
100 cd m–2) was attained with SFX-PO-DPA-Me
Spiro-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Nonradiative Decay for High-Quantum-Efficiency, Low-Roll-Off, Organic Light-Emitting Diodes
No full textHerein,
we report the use of spiro-configured fluorene-xanthene scaffolds
as a novel, promising,
and effective strategy in thermally activated delayed fluorescence
(TADF) emitter design to attain high photoluminescence quantum yields
(ΦPL), short delayed luminescence lifetime, high
external quantum efficiency (EQE), and minimum efficiency roll-off
characteristics in organic light-emitting diodes (OLEDs). The optoelectronic
and electroluminescence properties of SFX (spiro-(fluorene-9,9′-xanthene))-based
emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally.
All three emitters exhibited sky blue to green emission enabled by
a Herzberg–Teller mechanism in the excited state. They possess
short excited-state delayed lifetimes (<10 μs), high photoluminescence
quantum yields (ΦPL ∼ 70%), and small singlet–triplet
splitting energies (ΔEST < 0.10
eV) in the doped films in an mCP host matrix. The OLEDs showed some
of the highest EQEs using spiro-containing emitters where maximum
external quantum efficiencies (EQEmax) of 11 and 16% were
obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a
spiro-based emitter coupled with a low efficiency roll-off (19% at
100 cd m–2) was attained with SFX-PO-DPA-Me
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