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44 results on '"Ragains, Justin R."'

1. Attochemistry Regulation of Charge Migration

Author

Folorunso, Aderonke S., Mauger, François, Hamer, Kyle A., Jayasinghe, Denawakage D, Wahyutama, Imam, Ragains, Justin R., Jones, Robert R., DiMauro, Louis F., Gaarde, Mette B., Schafer, Kenneth J., and Lopata, Kenneth

Subjects
Physics - Chemical Physics
Abstract

Charge migration (CM) is a coherent attosecond process that involves the movement of localized holes across a molecule. To determine the relationship between a molecule's structure and the CM dynamics it exhibits, we perform systematic studies of para-functionalized bromobenzene molecules (X-C$_6$H$_4$-R) using real-time time-dependent density functional theory. We initiate valence-electron dynamics by emulating rapid strong-field ionization leading to a localized hole on the bromine atom. The resulting CM, which takes on the order of 1 fs, occurs via an X localized to C$_6$H$_4$ delocalized to R localized mechanism. Interestingly, the hole contrast on the acceptor functional group increases with increasing electron donating strength. This trend is well-described by the Hammett sigma value of the group, which is a commonly used metric for quantifying the effect of functionalization on the chemical reactivity of benzene derivatives. These results suggest that simple attochemistry principles and a density-based picture can be used to predict and understand CM., Comment: 5 pages, 3 figures

Published
2022

5. Attochemistry Regulation of Charge Migration

Author

Folorunso, Aderonke S., primary, Mauger, François, additional, Hamer, Kyle A., additional, Jayasinghe, Denawakage D., additional, Wahyutama, Imam S., additional, Ragains, Justin R., additional, Jones, Robert R., additional, DiMauro, Louis F., additional, Gaarde, Mette B., additional, Schafer, Kenneth J., additional, and Lopata, Kenneth, additional

Published
2023
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16. Total Synthesis of an All-1,2-cis-Linked Repeating Unit from the Acinetobacter baumanniiD78 Capsular Polysaccharide

Author

Njeri, Dancan K. and Ragains, Justin R.

Abstract

Chemical synthetic efforts have resulted in the preparation of the assigned tetrasaccharide repeating subunit from the Acinetobacter baumanniiKL4-associated capsular polysaccharide. A convergent synthetic strategy hinging on a 1,2-cis-selective [2+2] glycosylation to generate the fully protected tetrasaccharide was key to the success of this synthesis.

Published
2022
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17. On the relevance of glycosyl oxonium ions to 1,2-<italic>cis</italic>-selective <italic>O</italic>-glycosylation in ether solvents.

Author

Agarkar, Varad, Hart, Ava E., and Ragains, Justin R.

Subjects
*OXONIUM ions, *CHEMISTS, *STEREOSELECTIVE reactions, *IONS, *SOLVENTS
Abstract

AbstractSince no later than the 1970s, organic chemists have speculated on the role of glycosyl oxonium ions in chemical O-glycosylation. Such species result from the attack of ethers on glycosyl oxocarbenium ions and are invoked to explain 1,2-cis-selectivity in ether solvents. However, a systematic study to probe this phenomenon appears to be lacking in the chemical literature. Herein, we study the effects of solvent, counteranion, protecting group electron-withdrawing effects, and acceptor on O-glycosylation stereoselectivity with D-glucosyl trichloroacetimidate donors. While many of these transformations proceed with 1,2-cis-selectivity, our results suggest that glycosyl oxonium ions play minimal, if any, role in O-glycosylation. [ABSTRACT FROM AUTHOR]

Published
2024
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29. O-Glycosylation Enabled by Remote Activation.

Author

Spell, Mark L., Deveaux, Kristina, Bresnahan, Caitlin G., and Ragains, Justin R.

Subjects
OLIGOSACCHARIDE synthesis, GLYCOSYLATION, VISIBLE spectra, CHARTS, diagrams, etc.
Abstract

O-Glycosylation is a critically important and recurring step in the synthesis of oligosaccharides and other natural and non-natural products. While many approaches to O-glycosylation have been reported, those strategies involving remote activation are distinguished by the mildness and orthogonality that they often engender. As a result, O-glycosylation using remote activation strategies has been utilized successfully in the synthesis of complex molecules that include oligosaccharides and macrolides. Herein, we discuss a number of contributions that have been made to this area since the 1970s. This includes our own recent contribution involving the visible-light activation of 4-pmethoxyphenyl- 3-butenylthioglycosides toward O-glycosylation in the presence of Umemoto's reagent. 1 Introduction 2 Fraser-Reid's Development of n-Pentenylglycosides as Glycosyl Donors 3 Hanessian's Remote Activation of 2-Pyridylthioglycosides 4 Gold-Catalyzed O-Glycosylation (Hotha and Yu) 5 Wan's 'Interrupted Pummerer' Approach to O-Glycosylation 6 O-Glycosylation with 4-p-Methoxyphenyl-3-butenylthioglycosides 7 Conclusions. [ABSTRACT FROM AUTHOR]

Published
2017
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30. Cover Feature: Total Synthesis of a Pentasaccharide O‐Glycan from Acinetobacter baumannii (Eur. J. Org. Chem. 16/2023).

Author

Njeri, Dancan K. and Ragains, Justin R.

Subjects
*ACINETOBACTER baumannii, *SCORPIONS
Abstract

We can see that the bacteria bear our target pentasaccharide (depicted here using "sugar code" notation) and that one of these pentasaccharides adorns the scorpion's stinger. Nevertheless, we see the shadow of the stinger looming ominously over the scorpion and the legion of I A. baumannii i . B The Cover Feature b shows a scorpion amongst a colony of I A. baumannii i bacteria. [Extracted from the article]

Published
2023
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33. Single-particle kinetics of influenza virus membrane fusion

Author

Harrison, Stephen C., Floyd, Daniel L., Ragains, Justin R., Skehel, John J., Oijen, Antoine M. van, Harrison, Stephen C., Floyd, Daniel L., Ragains, Justin R., Skehel, John J., and Oijen, Antoine M. van

Abstract

Membrane fusion is an essential step during entry of enveloped viruses into cells. Conventional fusion assays are generally limited to observation of ensembles of multiple fusion events, confounding more detailed analysis of the sequence of the molecular steps involved. We have developed an in vitro, two-color fluorescence assay to monitor kinetics of single virus particles fusing with a target bilayer on an essentially fluid support. Analysis of lipid- and content-mixing trajectories on a particle-by-particle basis provides evidence for multiple, long-lived kinetic intermediates leading to hemifusion, followed by a single, rate-limiting step to pore formation. We interpret the series of intermediates preceding hemifusion as a result of the requirement that multiple copies of the trimeric hemagglutinin fusion protein be activated to initiate the fusion process.

Published
2008

34. Single-particle kinetics of influenza virus membrane fusion

Author

Floyd, Daniel L, Ragains, Justin R, Skehel, John J, Harrison, Stephen C, van Oijen, Antoine M, Floyd, Daniel L, Ragains, Justin R, Skehel, John J, Harrison, Stephen C, and van Oijen, Antoine M

Abstract

Membrane fusion is an essential step during entry of enveloped viruses into cells. Conventional fusion assays are generally limited to observation of ensembles of multiple fusion events, confounding more detailed analysis of the sequence of the molecular steps involved. We have developed an in vitro, two-color fluorescence assay to monitor kinetics of single virus particles fusing with a target bilayer on an essentially fluid support. Analysis of lipid- and content-mixing trajectories on a particle-by-particle basis provides evidence for multiple, long-lived kinetic intermediates leading to hemifusion, followed by a single, rate-limiting step to pore formation. We interpret the series of intermediates preceding hemifusion as a result of the requirement that multiple copies of the trimeric hemagglutinin fusion protein be activated to initiate the fusion process.

Published
2008

43. A Visible-Light-Promoted O-Glycosylation with a Thioglycoside Donor.

Author

Spell ML, Deveaux K, Bresnahan CG, Bernard BL, Sheffield W, Kumar R, and Ragains JR

Abstract

Visible-light irradiation of 4-p-methoxyphenyl-3-butenylthioglucoside donors in the presence of Umemoto's reagent and alcohol acceptors serves as a mild approach to O-glycosylation. Visible-light photocatalysts are not required for activation, and alkyl- and arylthioglycosides not bearing the p-methoxystyrene are inert to these conditions. Experimental and computational evidence for an intervening electron donor-acceptor complex, which is necessary for reactivity, is provided. Yields with primary, secondary, and tertiary alcohol acceptors range from moderate to high. Complete β-selectivity can be attained through neighboring-group participation., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Published
2016
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44. Total synthesis of novel 6-substituted lavendamycin antitumor agents.

Author

Seradj H, Cai W, Erasga NO, Chenault DV, Knuckles KA, Ragains JR, and Behforouz M

Subjects
Catalysis, Indicators and Reagents, Molecular Structure, Structure-Activity Relationship, Antibiotics, Antineoplastic chemical synthesis, Streptomyces chemistry, Streptonigrin analogs & derivatives, Streptonigrin chemical synthesis
Abstract

[structure: see text] Novel 6-substituted lavendamycins have been synthesized for the first time. The key step in these syntheses is a Pictet-Spengler condensation (Scheme 1). Efficient methods for the synthesis of each compound, including a novel reaction for the facile introduction of alkylamino groups at the C-6 position of the lavendamycin system, are discussed. Possible mechanisms for these reactions are also presented.

Published
2004
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