1,161 research outputs found
Chemoenzymatic asymmetric total synthesis of nonanolide (Z)-cytospolides D, E and their stereoisomers
No full textChemoenzymatic asymmetric total synthesis of the (Z)-isomer of the naturally occurring decanolide cytospolides D, E and six stereoisomers is reported. The main highlight of the synthetic venture involves ring-closing metathesis (RCM) reaction of a suitably functionalized ester compound, which was assembled by the Yamaguchi coupling of the required acid and alcohol fragments. The alcohol fragment was ac- cessed by two alternative chemoenzymatic processes, one being hydroxynitrile lyase mediated hydrocyanation, whereas lipase-catalyzed transesterification was the key sep in the second route. The acid fragment was constructed by an enantioselective enzymatic desymmetrization (EED) of prochiral 2-methyl-1,3-propanediol and Corey-Bakshi-Shibata (CBS) mediated stereoselective carbonyl reduction.http://deepblue.lib.umich.edu/bitstream/2027.42/177670/2/Eur J Org Chem - 2013 - Rej.pdfPublished versio
Generalized TASE-RK methods for stiff problems
No full textA family of Time-Accurate and Stable Explicit (TASE) methods for the numerical integration of Initial Value Problems in stiff Ordinary Differential Equations (ODEs) y'(t) = f (t, y) was recently introduced in [1]. The main idea was to make local extrapolation of a stabilized Euler method. More recently, in [3] a similar approach by considering the stabilization of arbitrary explicit Runge-Kutta methods (TASE-RK) was analyzed. In this case the explicit Runge-Kutta method integrates a transformed ODE obtained by multiplying the vector field f (t, y) by a certain operator which approximates the identity mapping up to a given order p. The main inconvenience of both approaches is that to reach order p the solution of p2 linear systems plus the evaluation of p derivatives are required per integration step.In order to substantially reduce the computational costs of the former approaches in the linear system solution, but maintaining the good accuracy and stability properties, a new family of TASE-RK methods which allow to introduce a few more free parameters are considered. The formulation of the methods was conceived to be implemented not only in sequential mode but it admits parallelism in a straightforward way. Furthermore, since these methods are linearly implicit, connections to the class of W-methods [19] are properly established. The order conditions for the new class of methods are widely studied by using the rooted tree theory. For p = 3, 4, new methods with p sequential stages and order p are derived and compared on semidiscrete 1D and 2D Partial Differential Equations (PDEs) to those in [1,3] and other standard Rosenbrock and W-methods in the literature.(c) 2023 The Author(s). Published by Elsevier B.V. on behalf of IMACS. This is an open access article under the CC BY-NC-ND license (http:// creativecommons .org /licenses /by-nc -nd /4 .0/)
Asymmetric synthesis of dihydroartemisinic acid through intramolecular Stetter reaction
No full textA short and concise formal synthesis of enantiopure dihydroartemisinic acid from (R)-citronellal is described in this article. Intramolecular version of asymmetric Stetter reaction using Rovis aminoindane based NHC catalyst was explored to access the core substituted cyclohexanone framework which on functional group manipulation and late stage ring closing metathesis (RCM) reaction afforded an advanced intermediate for dihydroartemisinic acid.http://deepblue.lib.umich.edu/bitstream/2027.42/177665/2/Asymmetric synthesis of dihydroartemisinic.pdfPublished versio
Exploration of ring rearrangement metathesis reaction: A general and flexible approach for the rapid construction [5,n]-Fused bicyclic systems en route to linear triquinanes
No full textStructurally diverse [5,n] bicyclic systems with cis ring junction stereochemistry were accessed readily through RRM (ring rearrangement metathesis) reaction of properly functionalized [2.2.1]norbornene skeletons. Several bicyclic enones, ketones, alcohols, and ethers acted as the substrates and yielded the respective linearly fused [5,n] bicyclic systems stereoselectively after the RRM reaction. Such [5,5]bicyclic enone scaffolds were then synthetically manipulated to core structural analogue of naturally occurring liner triquinane hirsutene having cis-syn-cis stereochemistry.http://deepblue.lib.umich.edu/bitstream/2027.42/177663/2/Exploration of Ring Rearrangement.pdfPublished versio
The morphological effects of sediment diversions the Lower Mississippi River
No full textThe wetlands in the Mississippi delta (USA) are drastically subsiding and eroding. Many projects and researches are ongoing to determine how this “drowning effect” of the delta can be stopped. One of the solutions that could be feasible is implementing sediment diversions in the levees of the Lower Mississippi River in order to divert sediment into the delta. This thesis addresses the morphological effects of river diversions on the Lower Mississippi River. The main objective is to analyze and optimize trade-offs between delta building and river navigability. For this purpose a 2DH numerical model with Delf3D has been created; the model simulates the hydro- and morphodynamic behavior. The river reach which has been studied is the final 110 km of the river from Point a la Hache at River Kilometer 78 (RK 78) down to the mouth of the river (RK -30), below Head of Passes. The hydrodynamic model has been calibrated and verified with flow and stage data from daily observations on the river. With the available sediment data a calibration has been carried out of the morphological behavior in the river. The model has been used to simulate several scenarios to get insight in the problems in the delta. From analysis of the model results the river bed in the study area can be divided into three categories. Upstream of RK 4 the bed is subject to erosion, around RK 4 the bed is practically in equilibrium and downstream of RK 4 the bed is subject to sedimentation. The reach downstream of RK 4 is the dredging reach; after analyzing the long-term simulation of 20 years it is not expected that the dredging quantities will decrease in the future. Closing off West Bay diversion has a positive effect on the dredging quantities. The best diversion site for this study area is found in the inner bend of the river upstream of Empire (RK 47) at RK 53. This site is most efficient and diverts the largest quantities of sand through the diversion.Hydraulic EngineeringCivil Engineering and Geoscience
Asymmetric synthesis of naturally occurring nonenolide xyolide through cross metathesis and macrolactonization reaction
No full textAsymmetric total synthesis of xyolide, a small ring macrolide is presented in this article. The synthesis is achieved through an 'E' selective cross metathesis (CM) reaction between two appropriate fragments followed by lactonization by Shiina method. One of the fragments containing 7S,8S,9R stereocenters of xyolide is accessed from n-nonanal by adopting an organocatalytic asymmetric α-aminooxylation, Z-selective Ando olefination, and substrate directed dihydroxylation reaction. The other fragments containing 4S stereocenter was prepared by ME-DKR (metal enzyme combined dynamic kinetic resolution) method. © 2014 Elsevier Ltd. All rights reserved.http://deepblue.lib.umich.edu/bitstream/2027.42/177669/2/Asymmetric synthesis of naturally occurring nonenolide xyolide.pdfPublished versio
Asymmetric synthesis of naturally occurring (-)-seimatopolides A and B
No full textAsymmetric total synthesis of polyhydroxylated naturally occurring nonenolide seimatopolide A (3S,6S,7S,9R) and seimatopolide B (3S,6R,9R) is described in this article. An E-selective cross metathesis (CM) reaction between two suitable fragments followed by macrolactonization reaction is the main highlight of our synthesis for the two natural products. The fragment containing 6S,7S,9R stereocenters for seimatopolide A has been synthesized from l-tartaric acid as a chiral pool starting material, by employing (R)-CBS-mediated stereoselective keto reduction reaction. Another fragment, which is common for both the molecules, containing the 3S stereocenter was prepared by ME-DKR (metal enzyme combined dynamic kinetic resolution) method. The fragment having 6R,9R stereocenters for seimatopolide B has been prepared from n-decanal by adopting (R)-CBS-mediated keto reduction and Brown asymmetric allylation reaction. © 2014 Elsevier Ltd. All rights reserved.http://deepblue.lib.umich.edu/bitstream/2027.42/177668/2/Asymmetric synthesis of naturally occurring.pdfPublished versio
Asymmetric synthesis of cytospolides C and D through successful exploration of stereoselective Julia-Kocienski olefination and Suzuki reaction followed by macrolactonization
No full textAbstract Asymmetric total synthesis of two naturally occurring nonenolides cytospolides C (3) and D (4) was accomplished through successful exploration of stereoselective Julia-Kocienski (JK) olefination reaction for the construction of the required E-olefinic geometry (C4-C5) present in the target molecule. In an alternative approach Suzuki cross coupling reaction of an E-vinylic bromide and an alkylborane species was also efficiently applied to access the macrolactonization precursor. Late stage lactonization through Mitsunobu cyclization afforded the target molecules in good yield.http://deepblue.lib.umich.edu/bitstream/2027.42/177667/2/Asymmetric synthesis of cytospolides C.pdfPublished versio
Asymmetric total synthesis of (-)-mangiferaelactone by using an appropriately substituted thiophene as a masked synthon for C-alkyl glycoside
No full textAbstract Asymmetric total synthesis of naturally occurring nonenolide (-)-mangiferaelactone was attempted through RCAM (ring closing alkyne metathesis) reaction. As the attempted RCAM reaction failed, the synthesis was finally achieved by successful exploration of a ring closing metathesis (RCM) reaction. 2-Propylthiophene was used as a masked synthon for n-heptyl glycoside, which served as main source for one of the RCM precursors and accessed by reductive desulfurization (Mozingo type reduction) of an appropriately substituted thiophene ribofuranoside. The other RCM precursor was accessed by applying an enzymatic kinetic resolution/Mitsunobu inversion sequence to an alkyne alcohol.http://deepblue.lib.umich.edu/bitstream/2027.42/177666/2/Asymmetric total synthesis of mangiferaelactone.pdfPublished versio
The Dunston Manuscript
No full textVariously titled 80 Slides and a Memory, Fit for a King and others this is the definitive performed version as an invited reading in 2001 to the Urban Design Group. It describes in blank verse the early stages of the Gateshead Garden Festival illustrated with slide
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