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Application of Two-Directional Carbonyl Ene Reaction to The Synthesis of Complex Tetrahydropyrans, and Studies Toward the Synthesis of C26-C39 Subunit of Spirastrellolide B.
Get PDFThe use of exocyclic enol ethers as nucleophiles in carbon-carbon bond forming reactions has been underutilized in organic synthesis. This work describes the use of 6-iodomethyl-2-methylenetetrahydropyran as a bis-exocyclic enol ether equivalent for the synthesis of pyran derivatives. Use of this linchpin allows for bidirectional elaboration of the pyran core by two directional carbonyl ene. The development of a convergent, modular synthesis of 2,6-disubstituted-4H-pyrans in a three-step sequence starting from 6-iodomethyl-2-methylenetetrahydropyran is described. Initial carbonyl ene reaction of 6-iodomethyl-2-methylenetetrahydropyran with aldehydes or activated ketones was followed by elimination of iodide to give cross oriented bis-enol ethers that contain both endocyclic and an exocyclic enol ethers. A second carbonyl ene reaction then resulted in formation of the target 2,6-disubstituted-4H-pyrans. This approach provided 2,6-disubstituted-4H-pyrans in synthetically useful yields and allows for rapid diversification about the central pyran core. Studies to evaluate the scope of the second carbonyl ene reaction revealed that the use of electron deficient aldehydes is required. The extended conjugation present in the cross oriented exocyclic enol ethers is thought to reduce their reactivity. Several strategies were explored to reduce conjugation in the cross-oriented substrates to enhance the reactivity of the exocyclic enol ether in the second carbonyl ene reaction. Functionalization of the endocyclic bond by spiroketalization, reduction, and nucleophilic addition resulted in higher product yields and expanded scope in the subsequent carbonyl ene reactions. These modifications also provided for a rapid increase in complexity in the pyran products available by this two directional sequence. Application of the bidirectional carbonyl ene methodology toward the synthesis of the C26-C39 bis-spiroketal of spirastrellolide B is also described. This work has resulted in the enantioselective preparation of starting materials and their use in the synthesis of an advanced spiroketal intermediate via a carbonyl ene/spiroketalization/elimination sequence; the second carbonyl ene reaction was also carried out, though initial attempts to convert the resulting ene product to the desired bis-spiroketal skeleton were not successful. This work demonstrates the utility of exocyclic enol ethers as neutral nucleophiles for the formation of carbon-carbon bonds. Application of this chemistry using the bi-directional carbonyl ene methodology provides access to functionally complex pyran derivatives that can be utilized as intermediates for the preparation of biologically active natural products
An Optimization Framework For Data Enrichment: A First Principles Approach To Designing Optimal Smoothers and Splines
Get PDFWe develop a general framework for state estimation in systems modeled with stochastic dynamics and noisy measurements. Our approach is based on maximum likelihood estimation and employs a variety of techniques from optimization theory including the calculus of variations and discrete optimization to derive optimality conditions for many different types of problems. We make no rigid assumptions on the form of the mapping from measurements to state-estimate or on the distributions of the random processes, making the framework more general than existing techniques such as splines, Kalman smoothing or Gaussian Process Regressions, which are subject to limiting restrictions and modeling challenges. The framework is first applied to a discrete fluid dynamical system where the position of a particle moving in a turbulent flow is estimated. The optimal solution is naturally interpreted as a digital filter and its performance is compared to related methods in the literature to demonstrate efficacy. In a second application the framework is applied to general continuous time systems in which the optimal solution that arises is interpreted as a continuous time spline, the structure and temporal dependency of which adapts to the system dynamics and the distributions of the process and measurement noise automatically. We demonstrate the utility and generality of our splines via illustrative examples that render both linear and nonlinear data filters depending on the particular system. In a final application multi-parameter continuous systems are analyzed to generate state estimates on high dimensional system domains. The optimal solution arising in the final investigation is generated using a superposition of physically motivated basis functions, both derived and fit to measurements using our framework, and the solution is interpreted as an adaptive B-spline techniques. Following the initial general treatment, the multi-parameter solution is used to produce state estimates on real open-source population data measured in Ghana over latitude and longitude as an illustrative example
“MAKING A BRIDGE BETWEEN COUNTRIES”: KAZAKH STUDENTS’ SOCIALIZATION AND PROFESSIONAL DEVELOPMENT WITHIN AMERICAN UNIVERSITIES
No full textThe concept of educational migration links skilled and educational migration to the assumption that migration is an instrument for pursuing professional and intellectual goals (Czaika & Reinprecht, 2022). Although education is one of the factors influencing migration, the extent to which education affects migration decisions persistently has been drawing vigorous scholarly attention. Scholars seek to explore the relationship between education and migration, as it is a complex phenomenon from both theoretical and empirical perspectives, which is not comprehensively understood (Dustmann & Glitz, 2011). Perhaps a migrant\u27s educational background, the degree to which acquired skills are convertible to the education and labor landscape of the host country, and the effort migrants make in obtaining new knowledge and skills upon arriving in a new setting all play a critical role in determining the migrant\u27s overall success in the host country. This research attempts to contribute to the field by addressing the relationship between education and migration and focusing on the factors facilitating Kazakh students’ choice of American education and how they evolve during integration into the US education and social environment. Using empirical analyses based on a sample of Kazakh students enrolled in American universities, the study explores the role of institutional, social, and economic structures in the migration decision-making process. In particular, the current dissertation work assembles empirical research relevant to the educational migration dynamics among Kazakh students, covering experiences at various points in their migration process and how they have changed their stay or move decisions in response to institutional, structural, and other changes. The dissertation consists of three main chapters developed as separate research articles exploring the nature of the educational relocation of Kazakh youth to the United States. Each chapter answers the main research questions: What factors affect Kazakh students’ college and destination choice decisions? What challenges do Kazakh students face while integrating into the American education system? What does the migration decision process look like? To answer these questions, the study triangulates qualitative and quantitative methods. Specifically, it conducts an online survey with 121 Kazakh students at American universities and semi-structured interviews with 28 participants discussing their socialization and migration decision-making experiences. At the individual micro-level, the study reveals the unique socialization challenges faced by Kazakh students, and at the macro-level, it supports the consistency of previous research findings on the migration decision-making of international students. The first chapter traces the motivations and multifaceted reasons behind Kazakh students’ selection of American universities and academic degrees to demonstrate how they relate to migration frameworks. The second chapter then covers the adaptation and socialization challenges faced by Kazakh students in American education and social environments to explain the specifics of their navigation processes in the new social structure. The third chapter explores how Kazakh students decide on their post-graduation destination and what factors they see as pivotal in shaping their decision-making process. The findings demonstrate that the general characteristics and structural context of the host country influence the migration decisions of Kazakh students. Migration aspirations and capabilities play a decisive role in their destination and college choice
Grit\u27s Impact on Academic Impairment in College Students with ADHD Symptoms: An Examination of Racial Differences
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