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 Mechanisms1. Host-Guest (H-G) Associations2. Guest Exchange Ion-Molecule Reactions3. Chiral Recognition Based on Complex Dissociation

IV. Application of Chiral Recognition Organocatalytic Asymmetric Conjungate Addition of Aldehydes to Nitroolefins

V. Summary

2

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)

Chiral analysis(Quality control)

Chiral RecognitionMass SpectrometryMass Spectrometry

Introduction

Ranking of the top 10 best-selling US pharmaceutical products in 2011 was obtained from webpage: http://www.imshealth.com/.

7 of the 10 best-selling US pharmaceutical products are single enantiomer.

3

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)

•…

Introduction

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

H. Awad, A. EI-Aneed, Mass Spectrom Rev, 2013, 32, 466–483

4

• 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)

Coupling of chiral sensitive analytical tools with MS

Hyphenated MS techniques

New detector: Mass Spectrometer (MS)

Sensitive Accurate Speed High throughput

Advantages: Limitations: 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

H. Awad, A. EI-Aneed, Mass Spectrom Rev, 2013, 32, 466–483

5

Two options for chiral analysis using hyphenated MS techniques:

Hyphenated MS techniques

Indirect approach:

Analysis of covalent

diastereomeric complexes

Separated by conventional

methods

CS: chiral derivatization reagent

Direct approach:

Analysis of noncovalent

diastereomeric complexes

CS: chiral mobile phase additives (CMPAs) chiral stationary phases (CSPs)

Indirect approach:Need more time for the reaction step

Direct approach is preferred

H. Awad, A. EI-Aneed, Mass Spectrom Rev, 2013, 32, 466–483 6

Derivatized by the CS to form covalent complexes

Form transient bond with CS

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.

7

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

Chiral Recognition Mechanisms

8

Chiral Recognition Mechanisms

1. Host-Guest (H-G) Associations

One of the two enantiomers (guest) tagged with deuterium atomsCS (host)

Ion abundance ratio:The affinity of each enantiomer towards the CS

J. Kim, et al, Bull. Korean. Chem. Soc. 2008, 29, 1069-1072.9

2. Guest Exchange Ion-Molecule Reactions

Unlabeled analyte enantiomers (guest) react with the CS (host) forming identical diastereomeric complexes

Principle:Depends on the different exchange behavior of enantiomers with a foreign reagent R

Chiral Recognition Mechanisms

J. Ramirez, et al, J. Am. Chem. Soc. 1998, 120, 7387–7388. G. Grigorean, et al, Anal. Chem. 2001, 73, 1684–1691.

10

Can’t be separated in a single stage MS

The complex ions are mass selected and allowed to react with a neutral gas-phase reagent R

Different intensity ratio

Relative abundances based on two factors:the enantiomeric ratio of the used chiral analytethe time of the exchange reaction

Solely varying the enantiomeric ratios of the chiral analytes

Chiral Recognition Mechanisms2. Guest Exchange Ion-Molecule Reactions

J. Ramirez, et al, J. Am. Chem. Soc. 1998, 120, 7387–7388. G. Grigorean, et al, Anal. Chem. 2001, 73, 1684–1691.

11

Chiral Recognition Mechanisms3. 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

W. A. Tao, R. G. Cooks, Anal. Chem. 2003, 25-31. 12

3. Chiral Recognition Based on Complex Dissociation

Chiral Recognition Mechanisms

IR ISIref*(1) Iref*(2)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.

13

Chiral selectivity Rchiral is defined as

14

Mass Spectrometric Chiral Recognition Mechanisms

3. Chiral Recognition Based on Complex Dissociation

Chiral Recognition Mechanisms

1. Host-Guest (H-G) Associations 2. Guest Exchange Ion-Molecule Reactions

Application of Chiral Recognition

15B. Florian, et al, Angew. Chem. Int. Ed. 2013 , 52 ,1–6

Enamine mechanism•Widely accepted•Not been validated experimentally

Application of Chiral Recognition

16

Possible mechanisms of amine catalyzed reaction of aldehyde with electrophiles

Z. G. Hajos, D. R. Parrish, J. Org. Chem. 1974, 39, 1615 – 1621.

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

Problem: Enamine mechanism not been validated experimentally

Experimental proof of enamine mechanism:Detect an enamine intermediate by ESI-MS

Application of Chiral Recognition

17

Enamine mechanism

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

Application of Chiral Recognition

18

The stereoselectivity 2/ent-2’(= k1/k2) is determined by ΔΔG≠ of the transition state.

If En/En’ ratio (back reaction) = 2/ent-2’ ratio (forward reaction), it will provides strong evidence to enamine mechanism.

Application of Chiral Recognition

19R=k1/k2= IEn/IEn’ = eΔΔG≠/RT

ent-2’

2

Im

Im’

En

En’

ΔΔG≠

Back reaction

Back-reaction screening and enantioselectivity of the forward reaction in DMSO

Application of Chiral Recognition

20

En/En’ (back reaction) = 2/ent-2’ (forward reaction):Enamine mechanism Stereomeric determining step is En to Im.

Additional organocatalysts investigated in this study

Application of Chiral Recognition

Catalyst Screening

21

I. Chirality is significant

Summary

22

II. Concepts of hyphenated MS techniquesIII. Mass Spectrometric Chiral Recognition Mechanisms

Host-Guest (H-G) Associations

Guest Exchange Ion-Molecule Reactions Chiral Recognition Based on Complex Dissociation

Hyphenated MS techniques

IV. An example that using chiral recognition to solve mechanistic problem

What can we do by using MS?

Mass Spectrometry

23

Studying Reaction Mechanism

24

Interesting reaction systems

Propose reaction mechanism

Combine MS with DFT calculation

Catalysts Screening by Mass Spectrometry

Simultaneous screening of a mixture of five catalysts

C. Markert, A. Pfaltz, Angew. Chem. Int. Ed. 2004, 116, 2552-2554

25

Thanks for your attention!Thanks for your attention!

26

Mass Spectrometry to study reaction mechanism

ESI-MS to capture reaction intermediates

Propose reaction mechnism

Combined with DFT calculation

27

H. Guo, et al, J. Am. Chem. Soc. 2005, 127, 13060-13064

Chiral Recognition Mechanisms3. Chiral Recognition Based on Complex Dissociation

W. A. Tao, R. G. Cooks, Anal. Chem. 2003, 25-31. 28

Metal: Cu2+

Ref: two L-TrpAnalytes: (+)-ephedrine (–)-ephedrine

Cu2+ (L-Trp)2 (+)-ephedrine

Cu2+ (L-Trp)2 (-)-ephedrine

Chiral selectivity Rchiral :

I+/I ref*(1) = 3.8I-/I ref*(2) = 0.91R chiral = 4.7

-A

-A

-Ref

-Ref

The interaction between (+)-ephedrine and ref* is stronger

Chiral Drug

Asymmetric Synthesis

Chiral Analysis

Chiral Resolution

Chiral Recognition

Mass Spectrometry

Introduction

PPT from Xinhao

Catalyst screening

Quality control

Chromatography

29

Catalysts Screening by Mass Spectrometry

Screening Methodology

Mass Spectrometric Screening of Their Racemic Forms

30

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

T. R. Brandon, J. Am. Soc. Mass. Spectrom. 2004, 15, 870-878

31

Structural Characterization of Oligomer-Aggregates of β-Amyloid Polypeptide

ESI-mass spectra (LC-MS) of Aß(1–40)

32

3. Chiral Recognition Based on Complex Dissociation

Chiral Recognition Mechanisms

Quantitative chiral analysis

The relative rates of the two competitive dissociations (kA and kref) can be expressed as the relative abundance ratio:

Calibration curves for chiral analysis

Different ratio of AR and AS

W. A. Tao, R. G. Cooks, J. Am. Chem. Soc., 2000, 122, 10598-10609

△ [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

33

3. Chiral Recognition Based on Complex Dissociation

Chiral Recognition Mechanisms

Quantitative chiral analysis

W. A. Tao, R. G. Cooks, J. Am. Chem. Soc., 2000, 122, 10598-10609 34