Chiral Recognition detected by
Mass Spectrometry
CHEN Ping
2013.12.06
1
Outline
I. Introduction
II. Hyphenated Mass Spectrometric Techniques for Chiral Analysis
III. Mass Spectrometric Chiral Recognition Mechanisms
1. Host-Guest (H-G) Associations
2. Guest Exchange Ion-Molecule Reactions
3. Chiral Recognition Based on Complex Dissociation
IV. Application of Chiral Recognition
Organocatalytic Asymmetric Conjungate Addition of Aldehydes to Nitroolefins
V. Summary
2
Introduction
More than half of the currently approved drugs are chiral molecules.
Develop single enantiomer drugs
 Reducing the required dose
 Increasing the potency
 Improving the safety profile
Asymmetric synthesis
(Catalysts screening)
7 of the 10 best-selling US pharmaceutical
products are single enantiomer.
Mass Spectrometry
Chiral Recognition
Chiral analysis
(Quality control)
Ranking of the top 10 best-selling US pharmaceutical products in 2011 was obtained from webpage:
http://www.imshealth.com/.
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Introduction
Traditionally, MS has been considered a “chiral-blind” technique
Enantiomers: Same mass and show identical mass spectra
Two strategies to differentiate a pair of enantiomers with MS
1. Coupling of chiral sensitive analytical tools with MS
• Liquid Chromatography-Mass Spectrometry (LC-MS)
• Gas Chromatography-Mass Spectrometry (GC-MS)
• …
2. MS is used solely in chiral analysis based on different methods
of chiral recognition
• Host-Guest (H-G) Associations
• Guest Exchange Ion-Molecule Reactions
• Chiral Recognition Based on Complex Dissociation
4
H. Awad, A. EI-Aneed, Mass Spectrom Rev, 2013, 32, 466–483
Hyphenated MS techniques
Coupling of chiral sensitive analytical tools with MS
• Liquid Chromatography-Mass Spectrometry (LC-MS)
• Gas Chromatography-Mass Spectrometry (GC-MS)
• Capillary Electrophoresis-Mass Spectrometry (CE-MS)
• Capillary Electrochromatography-Mass Spectrometry (CEC-MS)
• Supercritical Fluid Chromatography-Mass Spectrometry (SFC-MS)
New detector: Mass Spectrometer (MS)
Advantages:




Limitations:

Sensitive

Accurate
Speed
High throughput 
Nonpolar solvents were incompatible with ESI or APCI
Salts and other nonvolatile compounds in the mobile
phase were incompatible with ESI
Choosing chiral stationary phase is a daunting task
5
H. Awad, A. EI-Aneed, Mass Spectrom Rev, 2013, 32, 466–483
Hyphenated MS techniques
Two options for chiral analysis using hyphenated MS techniques:
Indirect approach:
Direct approach:
 Analysis of covalent
 Analysis of noncovalent
diastereomeric complexes
diastereomeric complexes
 Separated by conventional
methods
Direct approach is preferred
Derivatized by
the CS to form
covalent
complexes
Indirect approach:
Need more time for
the reaction step
Form transient
bond with CS
CS: chiral derivatization reagent
CS: chiral mobile phase additives (CMPAs)
chiral stationary phases (CSPs)
H. Awad, A. EI-Aneed, Mass Spectrom Rev, 2013, 32, 466–483
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Hyphenated MS techniques
The HPLC-MS chromatograms of (S ,R) ifosfamide (IF)
R. V. Oliveira, et al, J. Pharm. Biomed. Anal. 2007, 45, 295–303.
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Chiral Recognition Mechanisms
Two strategies to differentiate a pair of enantiomers with MS
1. Coupling of chiral sensitive analytical tools with MS
• Liquid Chromatography-Mass Spectrometry (LC-MS)
• Gas Chromatography-Mass Spectrometry (GC-MS)
• …
2. MS is used solely in chiral analysis based on different methods
of chiral recognition
• Host-Guest (H-G) Associations
• Guest Exchange Ion-Molecule Reactions
• Chiral Recognition Based on Complex Dissociation
8
Chiral Recognition Mechanisms
1. Host-Guest (H-G) Associations
Ion abundance ratio:
The affinity of each
enantiomer towards the CS
CS (host)
One of the two enantiomers (guest)
tagged with deuterium atoms
9
J. Kim, et al, Bull. Korean. Chem. Soc. 2008, 29, 1069-1072.
Chiral Recognition Mechanisms
2. Guest Exchange Ion-Molecule Reactions
Unlabeled analyte enantiomers
(guest) react with the CS (host)
forming identical
diastereomeric complexes
Can’t be separated in a single stage MS
Different intensity ratio
Principle:
Depends on the different
exchange behavior of
enantiomers with a foreign
reagent
R
The complex ions are mass selected and allowed to react
with a neutral gas-phase reagent R
J. Ramirez, et al, J. Am. Chem. Soc. 1998, 120, 7387–7388.
G. Grigorean, et al, Anal. Chem. 2001, 73, 1684–1691.
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Chiral Recognition Mechanisms
2. Guest Exchange Ion-Molecule Reactions
Relative abundances based on two factors:
 the enantiomeric ratio of the used chiral analyte
 the time of the exchange reaction
Solely varying the enantiomeric ratios of the chiral analytes
J. Ramirez, et al, J. Am. Chem. Soc. 1998, 120, 7387–7388.
G. Grigorean, et al, Anal. Chem. 2001, 73, 1684–1691.
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Chiral Recognition Mechanisms
3. Chiral Recognition Based on Complex Dissociation
The chiral analyte and chiral reference compound (ref*) are complexed with a
transition-metal ion (M) to generate high-order metal ion-bound cluster ions
12
W. A. Tao, R. G. Cooks, Anal. Chem. 2003, 25-31.
Chiral Recognition Mechanisms
3. Chiral Recognition Based on Complex Dissociation
Chiral selectivity Rchiral is defined as
Iref*(1) IR
Iref*(2) IS
R chiral =1 : no chiral discrimination
R chiral is more different from 1, the
chiral recognition ability is higher
W. A. Tao, R. G. Cooks, Anal. Chem. 2003, 25-31.
R. Berkecz, et al, J. Mass. Spectrom. 2010,45, 1312–1319.
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Chiral Recognition Mechanisms
Mass Spectrometric Chiral Recognition Mechanisms
1. Host-Guest (H-G) Associations
2. Guest Exchange Ion-Molecule Reactions
3. Chiral Recognition Based on Complex Dissociation
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Application of Chiral Recognition
B. Florian, et al, Angew. Chem. Int. Ed. 2013 , 52 ,1–6
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Application of Chiral Recognition
Enamine mechanism
• Widely accepted
• Not been validated
experimentally
Z. G. Hajos, D. R. Parrish, J. Org. Chem.
1974, 39, 1615 – 1621.
Possible mechanisms of amine catalyzed
reaction of aldehyde with electrophiles
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Application of Chiral Recognition
Addition reaction between aldehydes and
nitroolefins catalyzed by H-d-Pro-Pro-Glu-NH2
 Excellent yields and stereoselectivities
 Catalyst loadings lower than 1 mol %
Proposed catalytic cycle
Enamine mechanism
Problem:
Enamine mechanism not been
validated experimentally
Experimental proof of enamine mechanism:
Detect an enamine intermediate by ESI-MS
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Application of Chiral Recognition
Methodology: ESI-MS back-reaction screening
A pair of mass-labeled quasienantiomeric
conjugate addition products
Concept: Host-Guest (H-G) Associations
Host (Chiral Selector)
Guests
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Application of Chiral Recognition
If En/En’ ratio (back reaction) = 2/ent-2’ ratio (forward reaction), it will
provides strong evidence to enamine mechanism.
ΔΔG≠
Im’
En’
ent-2’
En
Im
Back reaction
2
The stereoselectivity 2/ent-2’(= k1/k2) is determined by ΔΔG≠ of
the transition state.
R=k1/k2= IEn/IEn’ = eΔΔG≠/RT
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Application of Chiral Recognition
Back-reaction screening and enantioselectivity of the forward reaction in DMSO
En/En’ (back reaction) = 2/ent-2’ (forward reaction):
 Enamine mechanism
 Stereomeric determining step is En to Im.
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Application of Chiral Recognition
Catalyst Screening
Additional organocatalysts investigated in this study
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Summary
I. Chirality is significant
II. Concepts of hyphenated MS techniques
III. Mass Spectrometric Chiral Recognition Mechanisms
IV. An example that using chiral recognition to solve mechanistic problem
Hyphenated MS techniques
Host-Guest (H-G) Associations
Guest Exchange Ion-Molecule Reactions
Chiral Recognition Based on
Complex Dissociation
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Mass Spectrometry
What can we do by using MS?
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Studying Reaction Mechanism
Interesting reaction systems
Propose reaction mechanism
Combine MS with DFT calculation
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Catalysts Screening by Mass Spectrometry
Simultaneous screening of a mixture of five catalysts
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C. Markert, A. Pfaltz, Angew. Chem. Int. Ed. 2004, 116, 2552-2554
Thanks for your attention!
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Mass Spectrometry to study reaction mechanism
ESI-MS to capture reaction intermediates
Propose reaction mechnism
Combined with DFT calculation
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H. Guo, et al, J. Am. Chem. Soc. 2005, 127, 13060-13064
Chiral Recognition Mechanisms
3. Chiral Recognition Based on Complex Dissociation
Cu2+ (L-Trp)2 (+)-ephedrine
-Ref
-A
Metal: Cu2+
Ref: two L-Trp
Analytes: (+)-ephedrine
(–)-ephedrine
Chiral selectivity Rchiral :
Cu2+ (L-Trp)2 (-)-ephedrine
-Ref
-A
I+/I ref*(1) = 3.8
I-/I ref*(2) = 0.91
R chiral = 4.7
The interaction between (+)-ephedrine and ref* is stronger
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W. A. Tao, R. G. Cooks, Anal. Chem. 2003, 25-31.
Introduction
Asymmetric
Synthesis
Catalyst screening
Chiral
Drug
Chiral
Analysis
Chiral
Recognition
Quality control
Chiral
Resolution
Chromatography
Mass Spectrometry
PPT from Xinhao
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Catalysts Screening by Mass Spectrometry
Screening Methodology
Mass Spectrometric Screening of
Their Racemic Forms
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Conformation Analysis by Ion Mobility Spectrometry-Mass Spectrometry
Drift time versus m/z plot measured by
Mass Spectrometer
Conformers produced for cyclic peptide from
Molecular Dynamics simulations
Plot of Normalized MD energy versus collision
cross-section from the simulated annealing
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T. R. Brandon, J. Am. Soc. Mass. Spectrom. 2004, 15, 870-878
Structural Characterization of Oligomer-Aggregates of β-Amyloid Polypeptide
ESI-mass spectra (LC-MS) of Aß(1–40)
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Chiral Recognition Mechanisms
3. Chiral Recognition Based on Complex Dissociation
Quantitative chiral analysis
△ [CuII(Pro)2(Tyr)-H]+ complex, Pro as the analyte
 [CuII(Phe)2(Ile)-H]+ complex, Phe as the analyte
 [CuII(Trp)2(Met)-H]+ complex, Trp as the analyte
The relative rates of the two competitive
dissociations (kA and kref) can be expressed as
the relative abundance ratio:
Different ratio of AR and AS
Calibration curves for chiral analysis
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W. A. Tao, R. G. Cooks, J. Am. Chem. Soc., 2000, 122, 10598-10609
Chiral Recognition Mechanisms
3. Chiral Recognition Based on Complex Dissociation
Quantitative chiral analysis
34
W. A. Tao, R. G. Cooks, J. Am. Chem. Soc., 2000, 122, 10598-10609