2,045 research outputs found
STUDIES TOWARD TETRAHYDROFURAN-CONTAINING NATURAL PRODUCTS: TOTAL SYNTHESIS OF AMPHIDINOLIDE C AND OXYLIPIDS
Tetrahydrofurans (thf) and tetrahydropyrans (thp) are important structural motifs found in a broad array of biologically relevant natural products such as polyether antibiotics and acetogenins. Classic methods used for thf- or thp-ring synthesis have their own advantages and disadvantages which tend to be substrate specific. Therefore, development of new and efficient methods is imperative especially to address the issue of diastereoselectivity when attainment of both cis- and trans-2,5-thf (or 2,6-thp) is highly desirable. Herein, a strategy of cross-metathesis and Pd(0)-catalyzed cyclization, useful for obtaining both cis- and trans-2,5-disubstituted thf (or 2,6-disubstituted thp), will be discussed along with its application in the synthesis of amphidinolide C and oxylipids. Amphidinolide C, a complex macrolide isolated from Okinawan marine dinoflagellates, exhibits in vitro cytotoxicity against human skin cancer cells at a low nano-molar range. Oxylipids isolated from Australian brown algae show strong activity against parasitic nematodes. This dissertation embodies investigations, synthetic efforts, and insights in achieving the total synthesis of these marine natural products
Micro-macro transition and simplified contact models for wet granular materials
Wet granular materials in a quasi-static steady state shear flow have been
studied with discrete particle simulations. Macroscopic quantities, consistent
with the conservation laws of continuum theory, are obtained by time averaging
and spatial coarse-graining. Initial studies involve understanding the effect
of liquid content and liquid properties like the surface tension on the
macroscopic quantities. Two parameters of the liquid bridge contact model have
been studied as the constitutive parameters that define the structure of this
model (i) the rupture distance of the liquid bridge model, which is
proportional to the liquid content, and (ii) the maximum adhesive force, as
controlled by the surface tension of the liquid. Subsequently a correlation is
developed between these micro parameters and the steady state cohesion in the
limit of zero confining pressure. Furthermore, as second result, the
macroscopic torque measured at the walls, which is an experimentally accessible
parameter, is predicted from our simulation results as a dependence on the
micro-parameters. Finally, the steady state cohesion of a realistic non-linear
liquid bridge contact model scales well with the steady state cohesion for a
simpler linearized irreversible contact model with the same maximum adhesive
force and equal energy dissipated per contact
Inferring Concept Prerequisite Relations from Online Educational Resources
The Internet has rich and rapidly increasing sources of high quality
educational content. Inferring prerequisite relations between educational
concepts is required for modern large-scale online educational technology
applications such as personalized recommendations and automatic curriculum
creation. We present PREREQ, a new supervised learning method for inferring
concept prerequisite relations. PREREQ is designed using latent representations
of concepts obtained from the Pairwise Latent Dirichlet Allocation model, and a
neural network based on the Siamese network architecture. PREREQ can learn
unknown concept prerequisites from course prerequisites and labeled concept
prerequisite data. It outperforms state-of-the-art approaches on benchmark
datasets and can effectively learn from very less training data. PREREQ can
also use unlabeled video playlists, a steadily growing source of training data,
to learn concept prerequisites, thus obviating the need for manual annotation
of course prerequisites.Comment: Accepted at the AAAI Conference on Innovative Applications of
Artificial Intelligence (IAAI-19
Graded components of local cohomology modules of -monomial ideals in characteristic zero
Let be a commutative Noetherian ring of characteristic zero and be a polynomial ring over with the standard
-grading. Let be an ideal which can be generated
by elements of the form where (possibly nonunit) and is a
monomial in 's. We call such an ideal as a `-monomial
ideal'. Local cohomology modules supported on monomial ideals gain a great deal
of interest due to their applications in the context of toric varieties. It was
observed that for , their
components depend only on which coordinates of are negative. In
this article, we show that this statement holds true in our general setting,
even for certain invariants of the components. We mainly focus on the Bass
numbers, injective dimensions, dimensions, associated primes, Bernstein-type
dimensions, and multiplicities of the components. Under the extra assumption
that is regular, we describe the finiteness of Bass numbers of each
component and bound its injective dimension by the dimension of its support.
Finally, we present a structure theorem for the components when is the ring
of formal power series in one variable over a characteristic zero field
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