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Varinder K. Aggarwal - One of the best experts on this subject based on the ideXlab platform.
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practical and highly selective sulfur ylide mediated asymmetric Epoxidations and aziridinations using a cheap and readily available chiral sulfide extensive studies to map out scope limitations and rationalization of diastereo and enantioselectivities
Journal of the American Chemical Society, 2013Co-Authors: Ona Illa, Eoghan M. Mcgarrigle, Mariam Namutebi, Chandreyee Saha, Mehrnoosh Ostovar, Chun C Chen, Mairi F Haddow, Sophie Nocquetthibault, Matteo Lusi, Varinder K. AggarwalAbstract:The chiral sulfide, isothiocineole, has been synthesized in one step from elemental sulfur, γ-terpinene, and limonene in 61% yield. A mechanism involving radical intermediates for this reaction is proposed based on experimental evidence. The application of isothiocineole to the asymmetric Epoxidation of aldehydes and the aziridination of imines is described. Excellent enantioselectivities and diastereoselectivities have been obtained over a wide range of aromatic, aliphatic, and α,β-unsaturated aldehydes using simple protocols. In aziridinations, excellent enantioselectivities and good diastereoselectivities were obtained for a wide range of imines. Mechanistic models have been put forward to rationalize the high selectivities observed, which should enable the sulfide to be used with confidence in synthesis. In Epoxidations, the degree of reversibility in betaine formation dominates both the diastereoselectivity and the enantioselectivity. Appropriate tuning of reaction conditions based on understanding the reaction mechanism enables high selectivities to be obtained in most cases. In aziridinations, betaine formation is nonreversible with semistabilized ylides and diastereoselectivities are determined in the betaine forming step and are more variable as a result.
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practical and highly selective sulfur ylide mediated asymmetric Epoxidations and aziridinations using an inexpensive readily available chiral sulfide applications to the synthesis of quinine and quinidine
Journal of the American Chemical Society, 2010Co-Authors: Ona Illa, Eoghan M. Mcgarrigle, Muhammad Arshad, Abel Ros, Varinder K. AggarwalAbstract:Heating one of the most abundant naturally occurring inorganic chemicals (elemental sulfur) with one of the most readily available homochiral molecules (limonene) gives a one-step synthesis of a chiral sulfide which exhibits outstanding selectivities in sulfur ylide mediated asymmetric Epoxidations and aziridinations. In particular reactions of benzyl and allylic sulfonium salts with both aromatic and aliphatic aldehydes gave epoxides with perfect enantioselectivities and the highest diastereoselectivities reported to date. In addition reactions with imines gave aziridines again with the highest enantioselectivities and diastereoselectivities reported to date. The reactions are scaleable, and the sulfide can be reisolated in high yield. The Epoxidation has been used as the key step in a convergent and stereoselective synthesis of each of the diastereoisomers of the cinchona alkaloids, quinine and quinidine.
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Synthesis and application of easily recyclable thiomorpholines for use in sulfur ylide mediated asymmetric Epoxidation of aldehydes.
Chemistry an Asian journal, 2008Co-Authors: Markus Hansch, Ona Illa, Eoghan M. Mcgarrigle, Varinder K. AggarwalAbstract:Chiral nonracemic thiomorpholines have been synthesized in four to six steps from limonene or achiral alkenes using alpha-methylbenzylamine to control absolute stereochemistry. These aminosulfides have been used to generate sulfur ylides, which have been applied in the asymmetric Epoxidation of aldehydes as easily recoverable catalysts. Excellent yields (up to 98 %), enantioselectivities (up to 97:3 e.r.), and diastereoselectivities (>or=98:2 trans/cis) were achieved in these Epoxidations and the sulfides were easily recovered in high yield (up to 97 %) by simple acid/base extraction.
Ona Illa - One of the best experts on this subject based on the ideXlab platform.
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practical and highly selective sulfur ylide mediated asymmetric Epoxidations and aziridinations using a cheap and readily available chiral sulfide extensive studies to map out scope limitations and rationalization of diastereo and enantioselectivities
Journal of the American Chemical Society, 2013Co-Authors: Ona Illa, Eoghan M. Mcgarrigle, Mariam Namutebi, Chandreyee Saha, Mehrnoosh Ostovar, Chun C Chen, Mairi F Haddow, Sophie Nocquetthibault, Matteo Lusi, Varinder K. AggarwalAbstract:The chiral sulfide, isothiocineole, has been synthesized in one step from elemental sulfur, γ-terpinene, and limonene in 61% yield. A mechanism involving radical intermediates for this reaction is proposed based on experimental evidence. The application of isothiocineole to the asymmetric Epoxidation of aldehydes and the aziridination of imines is described. Excellent enantioselectivities and diastereoselectivities have been obtained over a wide range of aromatic, aliphatic, and α,β-unsaturated aldehydes using simple protocols. In aziridinations, excellent enantioselectivities and good diastereoselectivities were obtained for a wide range of imines. Mechanistic models have been put forward to rationalize the high selectivities observed, which should enable the sulfide to be used with confidence in synthesis. In Epoxidations, the degree of reversibility in betaine formation dominates both the diastereoselectivity and the enantioselectivity. Appropriate tuning of reaction conditions based on understanding the reaction mechanism enables high selectivities to be obtained in most cases. In aziridinations, betaine formation is nonreversible with semistabilized ylides and diastereoselectivities are determined in the betaine forming step and are more variable as a result.
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practical and highly selective sulfur ylide mediated asymmetric Epoxidations and aziridinations using an inexpensive readily available chiral sulfide applications to the synthesis of quinine and quinidine
Journal of the American Chemical Society, 2010Co-Authors: Ona Illa, Eoghan M. Mcgarrigle, Muhammad Arshad, Abel Ros, Varinder K. AggarwalAbstract:Heating one of the most abundant naturally occurring inorganic chemicals (elemental sulfur) with one of the most readily available homochiral molecules (limonene) gives a one-step synthesis of a chiral sulfide which exhibits outstanding selectivities in sulfur ylide mediated asymmetric Epoxidations and aziridinations. In particular reactions of benzyl and allylic sulfonium salts with both aromatic and aliphatic aldehydes gave epoxides with perfect enantioselectivities and the highest diastereoselectivities reported to date. In addition reactions with imines gave aziridines again with the highest enantioselectivities and diastereoselectivities reported to date. The reactions are scaleable, and the sulfide can be reisolated in high yield. The Epoxidation has been used as the key step in a convergent and stereoselective synthesis of each of the diastereoisomers of the cinchona alkaloids, quinine and quinidine.
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Synthesis and application of easily recyclable thiomorpholines for use in sulfur ylide mediated asymmetric Epoxidation of aldehydes.
Chemistry an Asian journal, 2008Co-Authors: Markus Hansch, Ona Illa, Eoghan M. Mcgarrigle, Varinder K. AggarwalAbstract:Chiral nonracemic thiomorpholines have been synthesized in four to six steps from limonene or achiral alkenes using alpha-methylbenzylamine to control absolute stereochemistry. These aminosulfides have been used to generate sulfur ylides, which have been applied in the asymmetric Epoxidation of aldehydes as easily recoverable catalysts. Excellent yields (up to 98 %), enantioselectivities (up to 97:3 e.r.), and diastereoselectivities (>or=98:2 trans/cis) were achieved in these Epoxidations and the sulfides were easily recovered in high yield (up to 97 %) by simple acid/base extraction.
Eoghan M. Mcgarrigle - One of the best experts on this subject based on the ideXlab platform.
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practical and highly selective sulfur ylide mediated asymmetric Epoxidations and aziridinations using a cheap and readily available chiral sulfide extensive studies to map out scope limitations and rationalization of diastereo and enantioselectivities
Journal of the American Chemical Society, 2013Co-Authors: Ona Illa, Eoghan M. Mcgarrigle, Mariam Namutebi, Chandreyee Saha, Mehrnoosh Ostovar, Chun C Chen, Mairi F Haddow, Sophie Nocquetthibault, Matteo Lusi, Varinder K. AggarwalAbstract:The chiral sulfide, isothiocineole, has been synthesized in one step from elemental sulfur, γ-terpinene, and limonene in 61% yield. A mechanism involving radical intermediates for this reaction is proposed based on experimental evidence. The application of isothiocineole to the asymmetric Epoxidation of aldehydes and the aziridination of imines is described. Excellent enantioselectivities and diastereoselectivities have been obtained over a wide range of aromatic, aliphatic, and α,β-unsaturated aldehydes using simple protocols. In aziridinations, excellent enantioselectivities and good diastereoselectivities were obtained for a wide range of imines. Mechanistic models have been put forward to rationalize the high selectivities observed, which should enable the sulfide to be used with confidence in synthesis. In Epoxidations, the degree of reversibility in betaine formation dominates both the diastereoselectivity and the enantioselectivity. Appropriate tuning of reaction conditions based on understanding the reaction mechanism enables high selectivities to be obtained in most cases. In aziridinations, betaine formation is nonreversible with semistabilized ylides and diastereoselectivities are determined in the betaine forming step and are more variable as a result.
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practical and highly selective sulfur ylide mediated asymmetric Epoxidations and aziridinations using an inexpensive readily available chiral sulfide applications to the synthesis of quinine and quinidine
Journal of the American Chemical Society, 2010Co-Authors: Ona Illa, Eoghan M. Mcgarrigle, Muhammad Arshad, Abel Ros, Varinder K. AggarwalAbstract:Heating one of the most abundant naturally occurring inorganic chemicals (elemental sulfur) with one of the most readily available homochiral molecules (limonene) gives a one-step synthesis of a chiral sulfide which exhibits outstanding selectivities in sulfur ylide mediated asymmetric Epoxidations and aziridinations. In particular reactions of benzyl and allylic sulfonium salts with both aromatic and aliphatic aldehydes gave epoxides with perfect enantioselectivities and the highest diastereoselectivities reported to date. In addition reactions with imines gave aziridines again with the highest enantioselectivities and diastereoselectivities reported to date. The reactions are scaleable, and the sulfide can be reisolated in high yield. The Epoxidation has been used as the key step in a convergent and stereoselective synthesis of each of the diastereoisomers of the cinchona alkaloids, quinine and quinidine.
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Synthesis and application of easily recyclable thiomorpholines for use in sulfur ylide mediated asymmetric Epoxidation of aldehydes.
Chemistry an Asian journal, 2008Co-Authors: Markus Hansch, Ona Illa, Eoghan M. Mcgarrigle, Varinder K. AggarwalAbstract:Chiral nonracemic thiomorpholines have been synthesized in four to six steps from limonene or achiral alkenes using alpha-methylbenzylamine to control absolute stereochemistry. These aminosulfides have been used to generate sulfur ylides, which have been applied in the asymmetric Epoxidation of aldehydes as easily recoverable catalysts. Excellent yields (up to 98 %), enantioselectivities (up to 97:3 e.r.), and diastereoselectivities (>or=98:2 trans/cis) were achieved in these Epoxidations and the sulfides were easily recovered in high yield (up to 97 %) by simple acid/base extraction.
Reimund Goss - One of the best experts on this subject based on the ideXlab platform.
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The lipid dependence of diadinoxanthin de-Epoxidation presents new evidence for a macrodomain organization of the diatom thylakoid membrane
Journal of plant physiology, 2009Co-Authors: Reimund Goss, Bernard Lepetit, Jana Nerlich, Susann Schaller, Astrid Vieler, Christian WilhelmAbstract:The present study shows that thylakoid membranes of the diatom Cyclotella meneghiniana contain much higher amounts of negatively charged lipids than higher plant or green algal thylakoids. Based on these findings, we examined the influence of SQDG on the de-Epoxidation reaction of the diadinoxanthin cycle and compared it with results from the second negatively charged thylakoid lipid PG. SQDG and PG exhibited a lower capacity for the solubilization of the hydrophobic xanthophyll cycle pigment diadinoxanthin than the main membrane lipid MGDG. Although complete pigment solubilization took place at higher concentrations of the negatively charged lipids, SQDG and PG strongly suppressed the de-Epoxidation of diadinoxanthin in artificial membrane systems. In in vitro assays employing the isolated diadinoxanthin cycle enzyme diadinoxanthin de-epoxidase, no or only a very weak de-Epoxidation reaction was observed in the presence of SQDG or PG, respectively. In binary mixtures of the inverted hexagonal phase forming lipid MGDG with the negatively charged bilayer lipids, comparable suppression took place. This is in contrast to binary mixtures of MGDG with the neutral bilayer lipids DGDG and PC, where rapid and efficient de-Epoxidation was observed. In complex lipid mixtures resembling the lipid composition of the native diatom thylakoid membrane, we again found strong suppression of diadinoxanthin de-Epoxidation due to the presence of SQDG or PG. We conclude that, in the native thylakoids of diatoms, a strict separation of the MGDG and SQDG domains must occur; otherwise, the rapid diadinoxanthin de-Epoxidation observed in intact cells upon illumination would not be possible.
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the regulation of xanthophyll cycle activity and of non photochemical fluorescence quenching by two alternative electron flows in the diatoms phaeodactylum tricornutum and cyclotella meneghiniana
Biochimica et Biophysica Acta, 2009Co-Authors: Irina Grouneva, Torsten Jakob, Christian Wilhelm, Reimund GossAbstract:Intact cells of diatoms are characterized by a rapid diatoxanthin Epoxidation during low light periods following high light illumination while Epoxidation is severely restricted in phases of complete darkness. The present study shows that rapid diatoxanthin Epoxidation is dependent on the availability of the cofactor of diatoxanthin epoxidase, NADPH, which cannot be generated in darkness due to the inactivity of PSI. In the diatom Phaeodactylum tricornutum, NADPH production during low light is dependent on PSII activity, and addition of DCMU consequently abolishes diatoxanthin Epoxidation. In contrast to P. tricornutum, DCMU does not affect diatoxanthin Epoxidation in Cyclotella meneghiniana, which shows the same rapid Epoxidation in low light both in the absence or presence of DCMU. Measurements of the reduction state of the PQ pool and PSI activity indicate that, in the presence of DCMU, NADPH production in C. meneghiniana occurs via alternative electron transport, which includes electron donation from the chloroplast stroma to the PQ pool and, in a second step, from PQ to PSI. Similar electron flow to PQ is also observed during high light illumination of DCMU-treated P. tricornutum cells. In contrast to C. meneghiniana, the electrons are not directed to PSI, but most likely to a plastoquinone oxidase. This chlororespiratory electron transport leads to the establishment of an uncoupler-sensitive proton gradient in the presence of DCMU, which induces diadinoxanthin de-Epoxidation and NPQ. In C. meneghiniana, electron flow to the plastoquinone oxidase is restricted, and consequently, diadinoxanthin de-Epoxidation and NPQ is not observed after addition of DCMU.
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evidence for a rebinding of antheraxanthin to the light harvesting complex during the Epoxidation reaction of the violaxanthin cycle
Journal of Plant Physiology, 2006Co-Authors: Reimund Goss, Bernard Lepetit, Christian WilhelmAbstract:In the present study, we investigated the Epoxidation reaction of the violaxanthin (Vx) cycle in intact cells of Chlorella vulgaris. Our results show that the overall Epoxidation is slightly slower in darkness compared to the Epoxidation during high light (HL) illumination. The calculation of the rate constants of the two Epoxidation steps revealed that, for both conditions, the first Epoxidation step from zeaxanthin (Zx) to antheraxanthin (Ax) is faster than the second Epoxidation step from Ax to Vx. However, the most noteworthy result of our present study is that Ax, which is transiently formed during the Epoxidation reaction, participates in non-photochemical quenching of chlorophyll fluorescence (NPQ). A correlation between NPQ and the de-epoxidized xanthophyll cycle pigments during the time-course of the Epoxidation reaction can only be achieved when NPQ is plotted versus the sum of Zx and Ax. The accumulation of significant amounts of Ax during the Epoxidation reaction further indicates that Ax-dependent quenching proceeds with a similar efficiency compared to the Zx-mediated NPQ. As the xanthophyll-dependent NPQ relies on the presence of de-epoxidized xanthophylls in the PS II antenna, Ax-dependent NPQ is only possible under the assumption that Ax rebinds to the light-harvesting complex (LHC) II during the Epoxidation reaction.
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Role of hexagonal structure-forming lipids in diadinoxanthin and violaxanthin solubilization and de-Epoxidation
Biochemistry, 2005Co-Authors: Reimund Goss, Dariusz Latowski, Christian Wilhelm, Astrid Vieler, Martin Lohr, Joanna Grzyb, Kazimierz StrzałkaAbstract:In this study, we have examined the influence of different lipids on the solubility of the xanthophyll cycle pigments diadinoxanthin (Ddx) and violaxanthin (Vx) and on the efficiency of Ddx and Vx de-Epoxidation by the enzymes Vx de-epoxidase (VDE) from wheat and Ddx de-epoxidase (DDE) from the diatom Cyclotella meneghiniana, respectively. Our results show that the lipids MGDG and PE are able to solubilize both xanthophyll cycle pigments in an aqueous medium. Substrate solubilization is essential for de-epoxidase activity, because in the absence of MGDG or PE Ddx and Vx are present in an aggregated form, with limited accessibility for DDE and VDE. Our results also show that the hexagonal structure-forming lipids MGDG and PE are able to solubilize Ddx and Vx at much lower lipid concentrations than bilayer-forming lipids DGDG and PC. We furthermore found that, in the presence of MGDG or PE, Ddx is much more solubilizable than Vx. This substantial difference in Ddx and Vx solubility directly affects the respective de-Epoxidation reactions. Ddx de-Epoxidation by the diatom DDE is saturated at much lower MGDG or PE concentrations than Vx de-Epoxidation by the higher-plant VDE. Another important result of our study is that bilayer-forming lipids DGDG and PC are not able to induce efficient xanthophyll de-Epoxidation. Even in the presence of high concentrations of DGDG or PC, where Ddx and Vx are completely solubilized, a strongly inhibited Ddx de-Epoxidation is observed, while Vx de-Epoxidation by VDE is completely absent. This indicates that the inverted hexagonal phase domains provided by lipid MGDG or PE are essential for de-epoxidase activity. We conclude that in the natural thylakoid membrane MGDG serves to solubilize the xanthophyll cycle pigments and furthermore provides inverted hexagonal structures associated with the membrane bilayer, which are essential for efficient xanthophyll de-epoxidase activity.
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the de epoxidase and epoxidase reactions of mantoniella squamata prasinophyceae exhibit different substrate specific reaction kinetics compared to spinach
Planta, 2001Co-Authors: Ruth Frommolt, Reimund Goss, Christian WilhelmAbstract:In vivo the prasinophyceaen alga Mantoniella squamata Manton et Parke uses an incomplete violaxanthin (Vx) cycle, leading to a strong accumulation of antheraxanthin (Ax) under conditions of high light. Here, we show that this zeaxanthin (Zx)-depleted Vx/Ax cycle is caused by an extremely slow second de-Epoxidation step from Ax to Zx, and a fast Epoxidation from Ax back to Vx in the light. The rate constant of Ax Epoxidation is 5 to 6 times higher than the rate constant of Zx formation, implying that Ax is efficiently converted back to Vx before it can be de-epoxidated to Zx. It is, however, only half the rate constant of the first de-Epoxidation step from Vx to Ax, thus explaining the observed net accumulation of Ax during periods of strong illumination. When comparing the rate constant of the second de-Epoxidation step in M. squamata with Zx formation in spinach (Spinacia oleracea L.) thylakoids, we find a 20-fold reduction in the reaction kinetics of the former. This extremely slow Ax de-Epoxidation, which is also exhibited by the isolated Mantoniella violaxanthin de-epoxidase (VDE), is due to a reduced substrate affinity of M. squamata VDE for Ax compared with the VDE of higher plants. Mantoniella VDE, which has a similar K m value for Vx, shows a substantially increased K m for the substrate Ax in comparison with spinach VDE. Our results furthermore explain why Zx formation in Mantoniella cells can only be found at low pH values that represent the pH optimum of VDE. A pH of 5 blocks the Epoxidation reaction and, consequently, leads to a slow but appreciable accumulation of Zx.
Abel Ros - One of the best experts on this subject based on the ideXlab platform.
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practical and highly selective sulfur ylide mediated asymmetric Epoxidations and aziridinations using an inexpensive readily available chiral sulfide applications to the synthesis of quinine and quinidine
Journal of the American Chemical Society, 2010Co-Authors: Ona Illa, Eoghan M. Mcgarrigle, Muhammad Arshad, Abel Ros, Varinder K. AggarwalAbstract:Heating one of the most abundant naturally occurring inorganic chemicals (elemental sulfur) with one of the most readily available homochiral molecules (limonene) gives a one-step synthesis of a chiral sulfide which exhibits outstanding selectivities in sulfur ylide mediated asymmetric Epoxidations and aziridinations. In particular reactions of benzyl and allylic sulfonium salts with both aromatic and aliphatic aldehydes gave epoxides with perfect enantioselectivities and the highest diastereoselectivities reported to date. In addition reactions with imines gave aziridines again with the highest enantioselectivities and diastereoselectivities reported to date. The reactions are scaleable, and the sulfide can be reisolated in high yield. The Epoxidation has been used as the key step in a convergent and stereoselective synthesis of each of the diastereoisomers of the cinchona alkaloids, quinine and quinidine.
