A CHIRAL TAGGING STRATEGY FOR DETERMINING ABSOLUTE CONFIGURATION AND ENANTIOMERIC EXCESS - PowerPoint Presentation

Slide1

A CHIRAL TAGGING STRATEGY FOR DETERMINING ABSOLUTE CONFIGURATION AND ENANTIOMERIC EXCESS BY MOLECULAR ROTATIONAL SPECTROSCOPY

Luca Evangelisti and Walther CaminatiDipartmento di Chimica “Giacomo Ciamician”, Universita Di BolognaDavid PattersonDepartment of Physics, Harvard UniversityYunjie Xu and Javix ThomasDepartment of Chemistry, University of AlbertaChanning West and Brooks H. PateDepartment of Chemistry, University of Virginia

ISMS 2017 RG03Slide2

Acknowledgements

This work supported by the National Science Foundation (CHE 1531913) and The Virginia Biosciences Health Research Corporation (Frank Gupton: VCU, Justin Neill: BrightSpec)Special thanks for work on chiral tag rotational spectroscopy:Luca EvangelistiNathan Seifert, Lorenzo Spada

Dave Patterson, Walther Caminati, Yunjie Xu,

Javix

Thomas, David Pratt,

Smitty

Grubbs, Galen Sedo, Mark Marshall, Helen Leung, Kevin Lehmann, Justin Neill

Frank Marshall, Marty Holdren, Kevin Mayer, Taylor Smart, Reilly

Sonstrom

, Channing West

Ellie Coles,

Elizabeth Franck, John Gordon, Julia

Kuno

, Pierce

Eggan

, Victoria Kim, Ethan Wood, Megan Yu

Slide3

Unmet Needs in Analytical Chemistry: Routine Analysis of Molecules with Multiple Chiral Centers

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Supelco/Posters/1/T413140H_HPLC_Chiral_Multiple_Centers.pdf

Many traditional pharmaceutical compounds exhibit multiple chiral centers, requiring methods that can at least separate the potential enantiomers and diastereomers from the API. Even more desirable is a method that can separate each of the potential isomeric impurities for accurate quantitation;

however, this is rarely accomplished. Slide4

Chiral Analysis: The Search For a Universal Tool

Image Credit: http://doktori.bme.hu/bme_palyazat/2013/honlap/Bagi_Peter_en.htm

Enantiomers: Mirror images of each other that are not superimposable and have opposite configurations at their stereocenters

Diastereomers: Distinct compounds that have different configurations at one or more, but not all of the stereocenters

For “N” chiral centers

2

N

isomers

2

N-1

unique diastereomers

2 enantiomers per diastereomer

Need for

universally applicable

chiral analysis methods

Quantitative ratios of

all stereoisomers

Complex

mixture

analysis

Rapid

monitoring

Molecules with multiple chiral centers pose an issue for current techniquesSlide5

The sign of the product of dipole vector components are opposite for enantiomers

Rotational Spectroscopy for Chiral Analysis: Three Wave Mixing for Enantiomers

D. Patterson, M. Schnell, and J.M Doyle, Nature

497

, 475- 478 (2013).

D. Patterson and J.M. Doyle, Phys. Rev.

Lett

.

111

, 023008 (2013).

J.U.

Grabow

,

Angew

. Chem. 52, 11698 (2013).

V.A Shubert, D. Schmitz, D. Patterson, J.M Doyle, and M. Schnell,

Angew

. Chem. 52, (2013).

m

b

Simon Lobsiger, Cristobal Perez, Luca Evangelisti, Kevin K. Lehmann, Brooks H. Pate, “Molecular Structure and Chirality Detection by Fourier Transform Microwave Spectroscopy”, J. Phys. Chem. Lett. 6, 196-200 (2015).

m

a

m

b

m

c

(-)

m

a

m

b

m

c

(+)Slide6

Determination of Absolute Configuration by Chiral Tag Rotational Spectroscopy:

Enantiomers-to-DiastereomersEnantiomers of molecules have identical rotational spectra

Complexes of enantiomers with an enantiopure “chiral tag” form diastereomers that have different rotational spectra

Heterochiral Complex

A = 975.3 MHz

m

a

= 3.3 D

B = 320.1 MHz

m

b

= 1.6 D

C = 301.6 MHz

m

c

= 0.9 D

Homochiral Complex

A = 1038.6 MHz

m

a

= 3.1 D

B = 294.6 MHz

mb = -1.9 D C = 278.5 MHz mc

= - 0.1 D Lowest Energy Isomers: B3LYP D3BJ def2TZVPSlide7

Determination of Enantiomeric Excess using Chiral Tag

Enantiopure Chiral Tag ((S)-3-butyn-2-ol)

Heterochiral Spectrum

Homochiral Spectrum

Enantiomer populations converted to different

diastereomers with distinct spectra

Analogy to Chromatography

Different enantiomers gives signals in distinct detection windows

Background free detection permits determination of high EE when peaks are highly resolved

Rotational spectroscopy can be used to identify which enantiomer gives each signal

Rotational spectroscopy has the potential for significant decreases in analysis time

(Chiral GC Example:

30 min

) Slide8

Rotational Spectroscopy for Chiral Analysis: Diastereomers

Chirped-Pulse FTMW Spectroscopy

Extreme sensitivity

to changes in mass distribution

Agreement with Theory:

“Library-Free” Diastereomer Identification

Low Frequency (2-8 GHz):

Peak Transition Intensity of Large Molecules

High Resolution + Broadband Coverage:

Mixture Analysis

C. Perez, S.

Lobsiger

, N. A. Seifert, D. P.

Zaleski

, B.

Temelso

, G.C. Shields, Z.

Kisiel

, B. H. Pate, Chem. Phys. Lett.

571

, 1 (2013).Slide9

Isopulegol

Carlos Kleber Z. Andrade*, Otilie E. Vercillo, Juliana P. Rodrigues and Denise P. Silveira

,

J. Braz. Chem. Soc

.,

15

,

813-817, 2004.

Isopulegol

has three chiral centers with four diastereomers.

Isopulegol

is an intermediate in the synthesis of menthol which is produced at 3,000 tons per year.

A single diastereomer is needed because only a single diastereomer of menthol has the desired flavor.

Ryōji

Noyori

shared the 2001 Nobel Prize in Chemistry for the stereoselective synthesis of menthol (94%

ee

).Nomenclature for Stereoisomers

RSS: Methyl: R Hydroxyl: S Isopropenyl

: S(-)-isopulegol: RRS(+)-isopulegol: SSR(+)-neoisopulegol: RSS(-)-

neoisopulegol: SRRCyclohexyl Ring Conformation:Isopropenyl in the equatorial position is the most stable chair isomerIsopulegol and

neoisopulegol allow isopropenyl and methyl groups to be equatorial.

RSSRRS

RSR

RRRSlide10

Analysis of TCI America Sample

Optical Rotation of Enantiopure (-)-isopulegol: -22

o

(neat)

(Reference Optical Rotation from Aldrich CoA: Chiral GC + Optical Rotation)

At 94% pure isopulegol, but with a -6.1

o

specific rotation, the expected enantiomeric excess is ~30% with assumption of achiral impurities.

Pierce

Eggan

, Victoria Kim, Ethan Wood, Megan Yu

, Luca

EvangelistiSlide11

Measurement Methodology

Broadband Rotational Spectrum of the Monomer Used for diastereomer analysisBroadband Rotational Spectrum with Racemic Tag: Propylene Oxide Forms both homochiral and heterochiral complexes

Isolate complexes by cutting the monomer spectrum

Broadband Rotational Spectrum with

Enantiopure

Tag

Forms either homochiral or heterochiral complex

Isolate one set of complexes by cutting the monomer spectrum

Isolate the

other set

by cutting this spectrum from the racemic tagSlide12

Is (-)-isopulegol (RRS) the predominant enantiomer?

Spectroscopic Analysis of Complex with (S)-propylene oxide:Assessment of Absolute Configuration of Isopulegol from Spectroscopy:

Experiment: A = 1070.92 MHz Theory RRS: A = 1096.85 MHz (

-2.4%

) Theory SSR: A = 952.27 MHz

B = 290.54 MHz B = 293.20 MHz (

-0.6%

) B = 309.77 MHz

C = 249.63 MHz C = 251.05 MHz (

-1.1%

) C = 266.39 MHz

B3LYP D3BJ def2TZVP

(

-0.1%

)

(

-1.4%

)

(

-1.1%

)Slide13

Is (-)-isopulegol (RRS) the predominant enantiomer?

High Confidence Determination of Absolute Configuration from Substitution Structure(Analogous to Internal Chiral Reference X-ray Crystallography)

Experiment is known to use

(S)-propylene oxide as the tag

Structure of RRS Isopulegol with Correct StereochemistrySlide14

What is the EE for (-)-isopulegol?

The expectation from the Certificate of Analysis is an EE of 35% (assumes pure isopulegol)

(S)-Propylene Oxide: (

ee

Tag

) = 0.998

Uses 25 transitions for each complex

EE=74.7%

75% EE Reference Sample from Enantiopure

(-)-isopulegol and Racemic isopulegol mixture

EE=74.6%Slide15

Is there an impurity that could be affecting the optical rotation measurement?

(+) –

Neoisopulegol

is formed in this reaction with a specific rotation of +36

o

Identification of

Neoisopulegol

by Comparison to Quantum Chemistry Rotational Constants

Species

Theory

Experiment

Percent Error

Isopulegol

1948.4

1949.96485(299)

-0.39

700.5

700.16883(113)

-0.81

591.1

591.63903(111)

-0.23

Neo isopulegol

2151.4

2138.08622(207)

-0.62

678.9

676.60098(76)

-0.34

630.0

625.97792(73)

-0.64Slide16

Are the chiral properties of the stereoisomer impurity consistent with the (presumed) reaction chemistry?

EE of reaction products set by the EE of the citronellal reagent.

R

SS

R

RS

Assessment of Absolute Configuration of

Neoisopulegol

from Spectroscopy:

Experiment: A = 803.97 MHz Theory

R

SS: A = 815.94 MHz (-1.5%) Theory

S

RR: A = 781.40 MHz

B = 348.51 MHz B = 350.47 MHz (-0.6%) B = 359.93 MHz

C = 303.09 MHz C = 306.49 MHz (-1.1%) C = 289.42 MHz

B3LYP D3BJ def2TZVP

R

RS

EE=74.7%

R

SS

EE=74.1%Slide17

Is the analysis by rotational spectroscopy consistent with the optical rotation characterization?

Chiral Tag Rotational Spectroscopy Analysis ResultsStereoisomer Percent Optical Rotation (Pure)(-)-isopulegol RRS 67.4 -22o(+)-isopulegol SSR 9.6 +22o(+)-neoisopulegol RSS 20.1 +36o(-)-

neoisopulegol

SRR 2.9 -36

o

Net Optical Rotation: -6.5

o

TCI Certificate of Analysis: -6.1

oSlide18

Conclusions

Chiral tag rotational spectroscopy has the potential to be a quantitative analytical tool for routine analysis of molecules with multiple chiral centersCoupled with complementary three-wave mixing capabilities, rotational spectroscopy offers the full range of chiral analysis capabilitiesPotential for high speed monitoring of stereoisomers using cavity-enhanced spectrometers (Balle-Flygare)There is much to validate in the methodologyDevelopment of sampling methods for large molecules is a high prioritySlide19

Unmet Needs in Analytical Chemistry: Routine Analysis of Molecules with Multiple Chiral Centers

Chiral Analysis by Molecular Rotational Spectroscopy

High-Sensitivity

Broadband Spectrometers

(CP-FTMW)

High-Speed

Cavity-Enhanced Spectrometers

(Balle-Flygare)

Automated

Spectral Assignments

(Autofit, PGOPHER, JB95, AABS)

Molecular Structures from Isotopologue Analysis (Kraitchman,R

0

,r

m

)

Conformational Properties of Molecules

Structures of Weakly Bound Complexes

Pulse Sequences for FTMW Spectroscopy

(Three Wave Mixing)

Design of Nozzle Sources and Properties of Pulsed Jets (Cooling)

Validation of Accurate Methods in Quantum Chemistry

Hamiltonians for Molecular Rotation

(Watson Hamiltonian, SPCAT,BELGI)