Yohan Chan | University of East Anglia (original) (raw)
Papers by Yohan Chan
European Journal of Organic Chemistry, 2006
The measurement of the pK a of racemic tetramethoxyresorcin[4]arenes explains the failure to obta... more The measurement of the pK a of racemic tetramethoxyresorcin[4]arenes explains the failure to obtain good yields in attempted Mannich reactions of these substrates under classical reaction conditions. The failure is related to the lack of adequate concentrations of the iminium ions that results from the reduced acid strength of tetraalkoxyresorcin[4]arenes compared with that of the parent octahydroxyresorcin[4]arenes. However, the preparation of a series of Mannich bases derived from racemic tetraalkoxyresorcin[4]arenes was accomplished under microwave-assisted aprotic reaction conditions and the use of preformed iminium ion intermediates. When the reactions were carried out with the use of chiral bis(aminol) ethers, mixtures of diastereomers were obtained that could be separated by flash chromatography. The abso-
![Research paper thumbnail of Mannich andO-Alkylation Reactions of Tetraalkoxyresorcin[4]arenes – The Use of Some Products in Ligand-Assisted Reactions](https://attachments.academia-assets.com/40233686/thumbnails/1.jpg)
](European Journal of Organic Chemistry, 2006
The measurement of the pK a of racemic tetramethoxyresorcin[4]arenes explains the failure to obta... more The measurement of the pK a of racemic tetramethoxyresorcin[4]arenes explains the failure to obtain good yields in attempted Mannich reactions of these substrates under classical reaction conditions. The failure is related to the lack of adequate concentrations of the iminium ions that results from the reduced acid strength of tetraalkoxyresorcin[4]arenes compared with that of the parent octahydroxyresorcin[4]arenes. However, the preparation of a series of Mannich bases derived from racemic tetraalkoxyresorcin[4]arenes was accomplished under microwave-assisted aprotic reaction conditions and the use of preformed iminium ion intermediates. When the reactions were carried out with the use of chiral bis(aminol) ethers, mixtures of diastereomers were obtained that could be separated by flash chromatography. The abso-
European Journal of Organic Chemistry
The first examples of asymmetric epoxidation of dihydroquinoline substrates using iminium salt or... more The first examples of asymmetric epoxidation of dihydroquinoline substrates using iminium salt organocatalysts are reported. The 3,4-epoxytetrahydroquinoline products are obtained in good yields and with moderate to good enantioselectivities.
The Journal of Organic Chemistry, 2015
A range of new biphenylazepinium salt organocatalysts effective for asymmetric epoxidation has be... more A range of new biphenylazepinium salt organocatalysts effective for asymmetric epoxidation has been developed incorporating an additional substituted oxazolidine ring, and providing improved enantiocontrol in alkene epoxidation over the parent structure. Starting from enantiomerically pure aminoalcohols, tetracyclic iminium salts were obtained as single diastereoisomers through an atroposelective oxazolidine formation.
Tetrahedron, 2014
A new family of carbohydrate-based dihydroisoquinolinium salts has been prepared and tested for p... more A new family of carbohydrate-based dihydroisoquinolinium salts has been prepared and tested for potential as asymmetric catalysts for the epoxidation of unfunctionalized alkene substrates, providing up to 57% ee in the product epoxides.
Synlett, 2013
Aminimines derived in situ from a number of enantiomerically pure binaphthalene-based tertiary am... more Aminimines derived in situ from a number of enantiomerically pure binaphthalene-based tertiary amines have been used for the asymmetric aziridination of chalcone, providing N-unprotected aziridines with ee values of up to 43%. A chiral hydrazinium salt has been isolated for the first time and shown to provide similar yield and enantioselectivity to the in situ process in reaction with chalcone.
Synlett, 2013
ABSTRACT The first reported use of benzeneperseleninic acid as a catalytic mediator for oxaziridi... more ABSTRACT The first reported use of benzeneperseleninic acid as a catalytic mediator for oxaziridinium ion catalysed epoxidation is described, providing reaction rates and ee values (up to 85%) similar to those reported when using oxone as the stoichiometric oxidant. A dual catalytic cycle is proposed, in which diphenyl diselenide is initially converted into the perseleninic acid, which in turn oxidises an iminium ion to the corresponding oxaziridinium species, thus facilitating asymmetric oxygen transfer to an alkene.
The Journal of Organic Chemistry, 2013
The first reported examples of kinetic resolution in epoxidation reactions using iminium salt cat... more The first reported examples of kinetic resolution in epoxidation reactions using iminium salt catalysis are described, providing up to 99% ee in the epoxidation of racemic cis-chromenes.
European Journal of Mass Spectrometry, 2013
Electrospray ionisation mass spectrometry (ESI-MS) has been used to study the relative gas-phase ... more Electrospray ionisation mass spectrometry (ESI-MS) has been used to study the relative gas-phase proton and alkali metal (Li, Na, K and Cs) binding affinities of three different resorcin[4]arenes using the kinetic method. Collision-induced dissociation (CID) was used to study the fragmentation of resorcin[4]arene heterodimer sandwich complexes, allowing the relative binding affinity order to be established. All the alkali metal cations have the same gas-phase binding affinity order with the resorcin[4]arene host molecules. At collision energies of > or = 13eV, one of the [resorcin[4]arene+Metal]+, (Metal = Li, Na, K) ions fragmented through break-up of the resorcin[4]arene, whilst the other host resorcin[4]arene remained intact, causing an apparent change in binding affinity at high collision energy. This effect was not observed with caesium, since all complex ions dissociated readily under CID by displacement of the caesium cation. The binding affinity for the protonated resorcin[4]arenes was found to be different from the alkali metal cation binding affinity because of the higher proton affinity of the nitrogen-containing resorcin[4]arenes. It is shown that resorcin[4]arenes containing an oxazine ring can be converted into a ring-opened derivative via an Eschweiler-CLarke reaction in the presence of formic acid. A second ring-opening process also occurs, including a hydrolysis reaction that results in apparent Losses of 12 mass units from the intact resorcin[4]arene. Both these reactions occur in solution before mass spectrometric investigation and cannot be achieved by CID. This observation was confirmed by inducing the Eschweiter-CLarke reaction in a model benzoxazine compound.
ELECTROPHORESIS, 2013
Two flat gold electrodes are placed vis-à-vis with an epoxy spacer layer that is etched out to gi... more Two flat gold electrodes are placed vis-à-vis with an epoxy spacer layer that is etched out to give a ca. 100 μm-deep electrochemically active trench. A water-insoluble oil phase, here the redox system N,N-diethyl-N'N'-didodecyl-phenylenediamine (DDPD) in 4-(3-phenylpropyl)-pyridine (PPP), is immobilized into the trench to allow anion transfer upon oxidation of DDPD (oil) to DDP⁺ (oil). In "mono-potentiostatic mode" quantitative transfer/expulsion of anions into the trench oil phase occurs. However, in "bi-potentiostatic mode" feedback currents dominated by rapid plate-to-plate diffusion normal to the electrode surfaces are observed. Comparison of "normal" diffusion and "lateral" diffusion shows that the rate of diffusion-migration charge transport across the oil film is anion hydrophobicity dependent.
Electrochemistry Communications, 2010
Electrochemical processes at liquid-liquid-electrode interfaces involve simultaneous ion transfer... more Electrochemical processes at liquid-liquid-electrode interfaces involve simultaneous ion transfer and electron transfer. When driven at triple phase boundary electrode systems, electron and ion transfer occur in the same interfacial reaction zone. In this report, preliminary work with a novel electrode system based on two coupled triple phase boundary reaction zones is described. An interdigitated gold band array with 7 lm gold bands separated by 13 lm gaps is employed immersed in aqueous electrolyte with a water-immiscible solution of the redox system N,N-diethyl-N 0 N 0 -didodecyl-phenylenediamine (DDPD) in 4-(3-phenylpropyl)-pyridine (PPP) immobilized on the surface. Well-defined generator-collector feedback currents are observed which depend on the volume of deposit, the concentration of the redox system, and the nature of the aqueous electrolyte.
The Journal of …, Jan 1, 2012
ntroduction of a pseudoaxial substituent at a stereogenic center adjacent to the nitrogen atom in... more ntroduction of a pseudoaxial substituent at a
stereogenic center adjacent to the nitrogen atom in
binaphthyl- and biphenyl-derived azepinium salt organocatalysts
affords improved enantioselectivities and yields in
the epoxidation of unfunctionalized alkenes. In the biphenylderived
catalysts, the atropoisomerism at the biphenyl axis is
controlled by the interaction of this substituent with the chiral
substituent at nitrogen.
Oxidation is one of the most important types of transformation in chemistry, and practical mild o... more Oxidation is one of the most important types of transformation in chemistry, and practical mild oxidation without added reagents or solvents has been a long-standing challenge. We have developed a highly practical solvent-free (biphasic) electrochemically driven oxidation system for the selective conversion of sulfides to the corresponding sulfoxides, and alkenes to the corresponding epoxides, in a very simple reactor system. Excellent yields are obtained for a variety of substrates, and neither over-oxidation to sulfone nor formation of diol by-products from alkene oxidation is observed. This simple system has excellent potential for scale-up, requires no storing or transport of stoichiometric oxidants, no heavy metals, and can be carried out with just water, sodium carbonate, hydrochloric acid, and an applied potential in a single cell. The electrolyte solution can be recycled and reused with no loss in activity.
The Journal of …, Jan 1, 2012
We report the first enantioselective total synthesis of (+)-scuteflorin A in 14% overall yield, e... more We report the first enantioselective total synthesis of (+)-scuteflorin A in 14% overall yield, employing a chiral
iminium salt to effect an organocatalytic asymmetric epoxidation of xanthyletin in >99% ee as the key step.
![Research paper thumbnail of C4‐Symmetric Alkoxyresorcin [4] arene Triflates: The Use of Palladium‐Catalyzed Reactions in the Synthesis of Axially Chiral Derivatives with Amino‐and/or Alkoxy‐ …](https://attachments.academia-assets.com/25721393/thumbnails/1.jpg)
](European Journal of …, Jan 1, 2011
The conversion of axially chiral tetraalkoxyresorcin[4]arenes (cyclochiral resorcinarenes) into t... more The conversion of axially chiral tetraalkoxyresorcin[4]arenes
(cyclochiral resorcinarenes) into the related tetrakis(triflates)
in high yields is described; this provides an efficient source
of chiral materials for use in palladium-catalyzed transforma-
tions, including the reductive removal of triflate groups and
the synthesis of compounds that are formally derived from
3-aminophenol, providing axially chiral derivatives that are
nitrogen-substituted on the upper rim.
![Research paper thumbnail of The Diastereoselective Formation of Tetraalkoxy [4] resorcinarenes Derived from (–)‐(2R)‐2‐Methoxy‐2‐phenylethanol and Proof of Absolute Configurations](https://attachments.academia-assets.com/25721536/thumbnails/1.jpg)
](… Journal of Organic …, Jan 1, 2011
The preparation of optically pure (–)-3-[(2R)-2-methoxy-2- phenylethoxy]phenol from resorcinol mo... more The preparation of optically pure (–)-3-[(2R)-2-methoxy-2-
phenylethoxy]phenol from resorcinol monobenzoate and its
conversion into diastereoisomeric tetraalkoxyresorcin[4]-
arenes together with proof of the absolute configurations of
the products is reported. The results of the study indicate that
diastereoselective ring closure of linear tetrameric intermediates
is controlled by the steric demand of the alkyl group in
the precursor 3-alkoxyphenol
Environmental …, Jan 1, 2010
A bacterium in the genus Halomonas that grew on dimethylsulfoniopropionate (DMSP) or acrylate as ... more A bacterium in the genus Halomonas that grew on
dimethylsulfoniopropionate (DMSP) or acrylate as
sole carbon sources and that liberated the climatechanging
gas dimethyl sulfide in media containing
DMSP was obtained from the phylloplane of the macroalga
Ulva. We identified a cluster that contains
genes specifically involved in DMSP catabolism
(dddD, dddT) or in degrading acrylate (acuN, acuK) or
that are required to break down both substrates
(dddC, dddA). Using NMR and HPLC analyses to trace
13C- or 14C-labelled acrylate and DMSP in strains of
Escherichia coli with various combinations of cloned
ddd and/or acu genes, we deduced that DMSP is
imported by the BCCT-type transporter DddT, then
converted by DddD to 3-OH-propionate (3HP), liberating
dimethyl sulfide in the process. As DddD is a
predicted acyl CoA transferase, there may be an
earlier, unidentified catabolite of DMSP. Acrylate is
also converted to 3HP, via a CoA transferase (AcuN)
and a hydratase (AcuK). The 3HP is predicted to be
catabolized by an alcohol dehydrogenase, DddA, to
malonate semialdehyde, thence by an aldehyde dehydrogenase,
DddC, to acyl CoA plus CO2. The regulation
of the ddd and acu genes is unusual, as a
catabolite, 3HP, was a co-inducer of their transcription.
This first description of genes involved in
acrylate catabolism in any organism shows that
the relationship between the catabolic pathways of
acrylate and DMSP differs from that which had been
suggested in other bacteria.
Electrochemistry …, Jan 1, 2010
Electrochemical processes at liquid–liquid–electrode interfaces involve simultaneous ion transfer... more Electrochemical processes at liquid–liquid–electrode interfaces involve simultaneous ion transfer and electron transfer. When driven at triple phase boundary electrode systems, electron and ion transfer occur in the same interfacial reaction zone. In this report, preliminary work with a novel electrode system based on two coupled triple phase boundary reaction zones is described. An interdigitated gold band array with 7 lm gold bands separated by 13 lm gaps is employed immersed in aqueous electrolyte with
a water-immiscible solution of the redox system N,N-diethyl-N0N0-didodecyl-phenylenediamine (DDPD) in 4-(3-phenylpropyl)-pyridine (PPP) immobilized on the surface. Well-defined generator–collector feedback currents are observed which depend on the volume of deposit, the concentration of the redox system, and the nature of the aqueous electrolyte.
Organic …, Jan 1, 2009
Concise highly enantioselective three-step syntheses are described for (-)-(3′S)-lomatin and (+)-... more Concise highly enantioselective three-step syntheses are described for (-)-(3′S)-lomatin and (+)-(3′S,4′R)-trans-khellactone from 7-hydroxycoumarin
in 97% ee and in 57% and 58% overall yields, respectively, using nonaqueous enantioselective epoxidation by an iminium salt as
the key step.
Journal of Physical …, Jan 1, 2009
A microfluidic double channel device is employed to study reactions at flowing liquid–liquid junc... more A microfluidic double channel device is employed to study reactions at flowing liquid–liquid junctions in contact with
a boron-doped diamond (BDD) working electrode. The rectangular flow cell is calibrated for both single-phase liquid
flow and biphasic liquid–liquid flow for the case of (i) the immiscible N-octyl-2-pyrrolidone (NOP)–aqueous electrolyte
system and (ii) the immiscible acetonitrile–aqueous electrolyte system. The influence of flow speed and liquid
viscosity on the position of the phase boundary and mass transport-controlled limiting currents are examined. In
contrast to the NOP–aqueous electrolyte case, the acetonitrile–aqueous electrolyte system is shown to behave close to
ideal without ‘undercutting’ of the organic phase under the aqueous phase. The limiting current for three-phase
boundary reactions is only weakly dependent on flow rate but directly proportional to the concentration and the
diffusion coefficient in the organic phase. Acetonitrile as a commonly employed synthetic solvent is shown here to
allow effective three-phase boundary processes to occur due to a lower viscosity enabling faster diffusion.
N-butylferrocene is shown to be oxidised at the acetonitrile–aqueous electrolyte interface about 12 times faster
when compared with the same process at the NOP–aqueous electrolyte interface. Conditions suitable for clean
two-phase electrosynthetic processes without intentionally added supporting electrolyte in the organic phase are
proposed.
European Journal of Organic Chemistry, 2006
The measurement of the pK a of racemic tetramethoxyresorcin[4]arenes explains the failure to obta... more The measurement of the pK a of racemic tetramethoxyresorcin[4]arenes explains the failure to obtain good yields in attempted Mannich reactions of these substrates under classical reaction conditions. The failure is related to the lack of adequate concentrations of the iminium ions that results from the reduced acid strength of tetraalkoxyresorcin[4]arenes compared with that of the parent octahydroxyresorcin[4]arenes. However, the preparation of a series of Mannich bases derived from racemic tetraalkoxyresorcin[4]arenes was accomplished under microwave-assisted aprotic reaction conditions and the use of preformed iminium ion intermediates. When the reactions were carried out with the use of chiral bis(aminol) ethers, mixtures of diastereomers were obtained that could be separated by flash chromatography. The abso-
European Journal of Organic Chemistry, 2006
The measurement of the pK a of racemic tetramethoxyresorcin[4]arenes explains the failure to obta... more The measurement of the pK a of racemic tetramethoxyresorcin[4]arenes explains the failure to obtain good yields in attempted Mannich reactions of these substrates under classical reaction conditions. The failure is related to the lack of adequate concentrations of the iminium ions that results from the reduced acid strength of tetraalkoxyresorcin[4]arenes compared with that of the parent octahydroxyresorcin[4]arenes. However, the preparation of a series of Mannich bases derived from racemic tetraalkoxyresorcin[4]arenes was accomplished under microwave-assisted aprotic reaction conditions and the use of preformed iminium ion intermediates. When the reactions were carried out with the use of chiral bis(aminol) ethers, mixtures of diastereomers were obtained that could be separated by flash chromatography. The abso-
European Journal of Organic Chemistry
The first examples of asymmetric epoxidation of dihydroquinoline substrates using iminium salt or... more The first examples of asymmetric epoxidation of dihydroquinoline substrates using iminium salt organocatalysts are reported. The 3,4-epoxytetrahydroquinoline products are obtained in good yields and with moderate to good enantioselectivities.
The Journal of Organic Chemistry, 2015
A range of new biphenylazepinium salt organocatalysts effective for asymmetric epoxidation has be... more A range of new biphenylazepinium salt organocatalysts effective for asymmetric epoxidation has been developed incorporating an additional substituted oxazolidine ring, and providing improved enantiocontrol in alkene epoxidation over the parent structure. Starting from enantiomerically pure aminoalcohols, tetracyclic iminium salts were obtained as single diastereoisomers through an atroposelective oxazolidine formation.
Tetrahedron, 2014
A new family of carbohydrate-based dihydroisoquinolinium salts has been prepared and tested for p... more A new family of carbohydrate-based dihydroisoquinolinium salts has been prepared and tested for potential as asymmetric catalysts for the epoxidation of unfunctionalized alkene substrates, providing up to 57% ee in the product epoxides.
Synlett, 2013
Aminimines derived in situ from a number of enantiomerically pure binaphthalene-based tertiary am... more Aminimines derived in situ from a number of enantiomerically pure binaphthalene-based tertiary amines have been used for the asymmetric aziridination of chalcone, providing N-unprotected aziridines with ee values of up to 43%. A chiral hydrazinium salt has been isolated for the first time and shown to provide similar yield and enantioselectivity to the in situ process in reaction with chalcone.
Synlett, 2013
ABSTRACT The first reported use of benzeneperseleninic acid as a catalytic mediator for oxaziridi... more ABSTRACT The first reported use of benzeneperseleninic acid as a catalytic mediator for oxaziridinium ion catalysed epoxidation is described, providing reaction rates and ee values (up to 85%) similar to those reported when using oxone as the stoichiometric oxidant. A dual catalytic cycle is proposed, in which diphenyl diselenide is initially converted into the perseleninic acid, which in turn oxidises an iminium ion to the corresponding oxaziridinium species, thus facilitating asymmetric oxygen transfer to an alkene.
The Journal of Organic Chemistry, 2013
The first reported examples of kinetic resolution in epoxidation reactions using iminium salt cat... more The first reported examples of kinetic resolution in epoxidation reactions using iminium salt catalysis are described, providing up to 99% ee in the epoxidation of racemic cis-chromenes.
European Journal of Mass Spectrometry, 2013
Electrospray ionisation mass spectrometry (ESI-MS) has been used to study the relative gas-phase ... more Electrospray ionisation mass spectrometry (ESI-MS) has been used to study the relative gas-phase proton and alkali metal (Li, Na, K and Cs) binding affinities of three different resorcin[4]arenes using the kinetic method. Collision-induced dissociation (CID) was used to study the fragmentation of resorcin[4]arene heterodimer sandwich complexes, allowing the relative binding affinity order to be established. All the alkali metal cations have the same gas-phase binding affinity order with the resorcin[4]arene host molecules. At collision energies of > or = 13eV, one of the [resorcin[4]arene+Metal]+, (Metal = Li, Na, K) ions fragmented through break-up of the resorcin[4]arene, whilst the other host resorcin[4]arene remained intact, causing an apparent change in binding affinity at high collision energy. This effect was not observed with caesium, since all complex ions dissociated readily under CID by displacement of the caesium cation. The binding affinity for the protonated resorcin[4]arenes was found to be different from the alkali metal cation binding affinity because of the higher proton affinity of the nitrogen-containing resorcin[4]arenes. It is shown that resorcin[4]arenes containing an oxazine ring can be converted into a ring-opened derivative via an Eschweiler-CLarke reaction in the presence of formic acid. A second ring-opening process also occurs, including a hydrolysis reaction that results in apparent Losses of 12 mass units from the intact resorcin[4]arene. Both these reactions occur in solution before mass spectrometric investigation and cannot be achieved by CID. This observation was confirmed by inducing the Eschweiter-CLarke reaction in a model benzoxazine compound.
ELECTROPHORESIS, 2013
Two flat gold electrodes are placed vis-à-vis with an epoxy spacer layer that is etched out to gi... more Two flat gold electrodes are placed vis-à-vis with an epoxy spacer layer that is etched out to give a ca. 100 μm-deep electrochemically active trench. A water-insoluble oil phase, here the redox system N,N-diethyl-N'N'-didodecyl-phenylenediamine (DDPD) in 4-(3-phenylpropyl)-pyridine (PPP), is immobilized into the trench to allow anion transfer upon oxidation of DDPD (oil) to DDP⁺ (oil). In "mono-potentiostatic mode" quantitative transfer/expulsion of anions into the trench oil phase occurs. However, in "bi-potentiostatic mode" feedback currents dominated by rapid plate-to-plate diffusion normal to the electrode surfaces are observed. Comparison of "normal" diffusion and "lateral" diffusion shows that the rate of diffusion-migration charge transport across the oil film is anion hydrophobicity dependent.
Electrochemistry Communications, 2010
Electrochemical processes at liquid-liquid-electrode interfaces involve simultaneous ion transfer... more Electrochemical processes at liquid-liquid-electrode interfaces involve simultaneous ion transfer and electron transfer. When driven at triple phase boundary electrode systems, electron and ion transfer occur in the same interfacial reaction zone. In this report, preliminary work with a novel electrode system based on two coupled triple phase boundary reaction zones is described. An interdigitated gold band array with 7 lm gold bands separated by 13 lm gaps is employed immersed in aqueous electrolyte with a water-immiscible solution of the redox system N,N-diethyl-N 0 N 0 -didodecyl-phenylenediamine (DDPD) in 4-(3-phenylpropyl)-pyridine (PPP) immobilized on the surface. Well-defined generator-collector feedback currents are observed which depend on the volume of deposit, the concentration of the redox system, and the nature of the aqueous electrolyte.
The Journal of …, Jan 1, 2012
ntroduction of a pseudoaxial substituent at a stereogenic center adjacent to the nitrogen atom in... more ntroduction of a pseudoaxial substituent at a
stereogenic center adjacent to the nitrogen atom in
binaphthyl- and biphenyl-derived azepinium salt organocatalysts
affords improved enantioselectivities and yields in
the epoxidation of unfunctionalized alkenes. In the biphenylderived
catalysts, the atropoisomerism at the biphenyl axis is
controlled by the interaction of this substituent with the chiral
substituent at nitrogen.
Oxidation is one of the most important types of transformation in chemistry, and practical mild o... more Oxidation is one of the most important types of transformation in chemistry, and practical mild oxidation without added reagents or solvents has been a long-standing challenge. We have developed a highly practical solvent-free (biphasic) electrochemically driven oxidation system for the selective conversion of sulfides to the corresponding sulfoxides, and alkenes to the corresponding epoxides, in a very simple reactor system. Excellent yields are obtained for a variety of substrates, and neither over-oxidation to sulfone nor formation of diol by-products from alkene oxidation is observed. This simple system has excellent potential for scale-up, requires no storing or transport of stoichiometric oxidants, no heavy metals, and can be carried out with just water, sodium carbonate, hydrochloric acid, and an applied potential in a single cell. The electrolyte solution can be recycled and reused with no loss in activity.
The Journal of …, Jan 1, 2012
We report the first enantioselective total synthesis of (+)-scuteflorin A in 14% overall yield, e... more We report the first enantioselective total synthesis of (+)-scuteflorin A in 14% overall yield, employing a chiral
iminium salt to effect an organocatalytic asymmetric epoxidation of xanthyletin in >99% ee as the key step.
European Journal of …, Jan 1, 2011
The conversion of axially chiral tetraalkoxyresorcin[4]arenes (cyclochiral resorcinarenes) into t... more The conversion of axially chiral tetraalkoxyresorcin[4]arenes
(cyclochiral resorcinarenes) into the related tetrakis(triflates)
in high yields is described; this provides an efficient source
of chiral materials for use in palladium-catalyzed transforma-
tions, including the reductive removal of triflate groups and
the synthesis of compounds that are formally derived from
3-aminophenol, providing axially chiral derivatives that are
nitrogen-substituted on the upper rim.
… Journal of Organic …, Jan 1, 2011
The preparation of optically pure (–)-3-[(2R)-2-methoxy-2- phenylethoxy]phenol from resorcinol mo... more The preparation of optically pure (–)-3-[(2R)-2-methoxy-2-
phenylethoxy]phenol from resorcinol monobenzoate and its
conversion into diastereoisomeric tetraalkoxyresorcin[4]-
arenes together with proof of the absolute configurations of
the products is reported. The results of the study indicate that
diastereoselective ring closure of linear tetrameric intermediates
is controlled by the steric demand of the alkyl group in
the precursor 3-alkoxyphenol
Environmental …, Jan 1, 2010
A bacterium in the genus Halomonas that grew on dimethylsulfoniopropionate (DMSP) or acrylate as ... more A bacterium in the genus Halomonas that grew on
dimethylsulfoniopropionate (DMSP) or acrylate as
sole carbon sources and that liberated the climatechanging
gas dimethyl sulfide in media containing
DMSP was obtained from the phylloplane of the macroalga
Ulva. We identified a cluster that contains
genes specifically involved in DMSP catabolism
(dddD, dddT) or in degrading acrylate (acuN, acuK) or
that are required to break down both substrates
(dddC, dddA). Using NMR and HPLC analyses to trace
13C- or 14C-labelled acrylate and DMSP in strains of
Escherichia coli with various combinations of cloned
ddd and/or acu genes, we deduced that DMSP is
imported by the BCCT-type transporter DddT, then
converted by DddD to 3-OH-propionate (3HP), liberating
dimethyl sulfide in the process. As DddD is a
predicted acyl CoA transferase, there may be an
earlier, unidentified catabolite of DMSP. Acrylate is
also converted to 3HP, via a CoA transferase (AcuN)
and a hydratase (AcuK). The 3HP is predicted to be
catabolized by an alcohol dehydrogenase, DddA, to
malonate semialdehyde, thence by an aldehyde dehydrogenase,
DddC, to acyl CoA plus CO2. The regulation
of the ddd and acu genes is unusual, as a
catabolite, 3HP, was a co-inducer of their transcription.
This first description of genes involved in
acrylate catabolism in any organism shows that
the relationship between the catabolic pathways of
acrylate and DMSP differs from that which had been
suggested in other bacteria.
Electrochemistry …, Jan 1, 2010
Electrochemical processes at liquid–liquid–electrode interfaces involve simultaneous ion transfer... more Electrochemical processes at liquid–liquid–electrode interfaces involve simultaneous ion transfer and electron transfer. When driven at triple phase boundary electrode systems, electron and ion transfer occur in the same interfacial reaction zone. In this report, preliminary work with a novel electrode system based on two coupled triple phase boundary reaction zones is described. An interdigitated gold band array with 7 lm gold bands separated by 13 lm gaps is employed immersed in aqueous electrolyte with
a water-immiscible solution of the redox system N,N-diethyl-N0N0-didodecyl-phenylenediamine (DDPD) in 4-(3-phenylpropyl)-pyridine (PPP) immobilized on the surface. Well-defined generator–collector feedback currents are observed which depend on the volume of deposit, the concentration of the redox system, and the nature of the aqueous electrolyte.
Organic …, Jan 1, 2009
Concise highly enantioselective three-step syntheses are described for (-)-(3′S)-lomatin and (+)-... more Concise highly enantioselective three-step syntheses are described for (-)-(3′S)-lomatin and (+)-(3′S,4′R)-trans-khellactone from 7-hydroxycoumarin
in 97% ee and in 57% and 58% overall yields, respectively, using nonaqueous enantioselective epoxidation by an iminium salt as
the key step.
Journal of Physical …, Jan 1, 2009
A microfluidic double channel device is employed to study reactions at flowing liquid–liquid junc... more A microfluidic double channel device is employed to study reactions at flowing liquid–liquid junctions in contact with
a boron-doped diamond (BDD) working electrode. The rectangular flow cell is calibrated for both single-phase liquid
flow and biphasic liquid–liquid flow for the case of (i) the immiscible N-octyl-2-pyrrolidone (NOP)–aqueous electrolyte
system and (ii) the immiscible acetonitrile–aqueous electrolyte system. The influence of flow speed and liquid
viscosity on the position of the phase boundary and mass transport-controlled limiting currents are examined. In
contrast to the NOP–aqueous electrolyte case, the acetonitrile–aqueous electrolyte system is shown to behave close to
ideal without ‘undercutting’ of the organic phase under the aqueous phase. The limiting current for three-phase
boundary reactions is only weakly dependent on flow rate but directly proportional to the concentration and the
diffusion coefficient in the organic phase. Acetonitrile as a commonly employed synthetic solvent is shown here to
allow effective three-phase boundary processes to occur due to a lower viscosity enabling faster diffusion.
N-butylferrocene is shown to be oxidised at the acetonitrile–aqueous electrolyte interface about 12 times faster
when compared with the same process at the NOP–aqueous electrolyte interface. Conditions suitable for clean
two-phase electrosynthetic processes without intentionally added supporting electrolyte in the organic phase are
proposed.