Structural Elucidation of a Diterpene Derivative from Stemodia Maritima Eugene E. Kwan and William...
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Transcript of Structural Elucidation of a Diterpene Derivative from Stemodia Maritima Eugene E. Kwan and William...
![Page 1: Structural Elucidation of a Diterpene Derivative from Stemodia Maritima Eugene E. Kwan and William F. Reynolds April 2003.](https://reader036.fdocuments.in/reader036/viewer/2022081504/56649f255503460f94c3c369/html5/thumbnails/1.jpg)
Structural Elucidation of a Diterpene Derivative from
Stemodia MaritimaBr
H
BrH
O
Eugene E. Kwan and William F. Reynolds
April 2003
![Page 2: Structural Elucidation of a Diterpene Derivative from Stemodia Maritima Eugene E. Kwan and William F. Reynolds April 2003.](https://reader036.fdocuments.in/reader036/viewer/2022081504/56649f255503460f94c3c369/html5/thumbnails/2.jpg)
BackgroundPurpose: determine the structure of a natural product via NMR and MS.
Q: Why study natural products?A: New pharmaceuticals.
e.g., quinine, digitalis, cyclosporin, penicillin…
- very difficult to design pharmaceuticals
- takes advantage of traditional folk knowledge
- even animals known to use plants for medicine
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HistoryStructural elucidation of natural products used to be very hard and take forever.
Strychnine alkaloid toxin Past: H. Leuchs workedon structure for 40 yearsuntil R. Woodward beathim to it.
Today: <1 mg sampleneeded; a weekend wouldbe enough.
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Nuclear Magnetic Resonance- Modern structural elucidation relies on NMR.
- Nuclear spin energies: quantized.
- Can observe transitions in magnetic field.
- Transition energy depends on field strength.
- Each nucleus experiences a local magnetic field, which is slightly different from the bulk magnetic field. This difference reveals different chemical environments.
-A molecule has many different nuclei, each with different magnetic resonance frequencies.
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The NMR Experiment-Modern NMR uses the “FT pulse” technique.
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Outlineidentify a promising plant:
stemodia maritima
extract the interestingorganic material
separate via HPLC
analyze the fractionsmyjob
traditional treatment for venereal disease
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Analysispreliminary analysis
detailed spectroscopicinvestigation
work out fragments
guess at structure
check for consistency;assign peaks
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Preliminary ResultsMass Spectrometry MW = 444
bromine present
13C NMR (1D) 20 carbons
ketone present (C=O)
1H NMR (1D) & 30 hydrogensHSQC NMR (2D)
Deduced Molecular Formula: C20H30Br2O
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1H Proton SpectrumOne Dimensional NMR
- one dimensional = one frequency domain- complicated spectrum; many overlapping peaks
(1) Each proton has a peak.
(2) Each peak has a splitting pattern.
(3) Splitting pattern = H-H spin coupling.
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Two Dimensional NMRTwo Dimensional NMR
- two dimensional = two frequency domains- can show H-H or H-C interactions through space or through bonds
Key ExperimentsA. tROESY
tells if two protons are close in space
1D proton spectra appear on x and y axes (“f1, f2”)
if two protons are near each other, “cross peak”
helps determine absolute stereochemistry
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“diagonal peak”
“off-diagonal cross peak”
projection on axes = 1D spectrum
tROESY: H-H spatial proximity
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Two Dimensional NMRB. HSQC (spectrally edited)
connects each proton to its adjacent carbon
separates overlapping peaks
distinguishes between CH2, and CH3/CH
C. HMBCshows C-H connections over more than one bond
D. COSYshows H-H coupling mostly over one bond
often shows peaks from coupling over multiple bonds
interactions governed by coupling constants J
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projection on axes = 1D spectrum
CH
CH2
CH3
proton axis
carb
on a
xis
HSQC: C-H connections
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Working Out Fragments I“fragment” = part of molecule
C20 molecule from plant: suggests common diterpene fragment
analysis of data: consistent with this structure
CH3
H3C
CH3
H
O ?
common trans-decalin fragment
?
probable
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Working Out Fragments II
- further analysis suggested:
- what was the rest? what are the possibilities?
- molecular formula and lack of double bonds in NMR suggestsfour rings:
- but…trouble! nothing fit!
O
O O
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Clever Thinking – A Guess- much thought + some help produced a complete structure:
- funny: ring system too complicated to be named by computer
- stereochemistry ambiguous at bromines
- parts of tROESY “fuzzy”, needed better technique: 1D NOE
O Br
Br
stereochemistryambiguous!
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The NOE ExperimentHow can we distinguish between:
“Nuclear Overhauser Effect” (NOE) Experiment
H
H R
R' R
H H
R'vs. ?
H
H R
R'R
H H
R'
excite thisproton
resonanceseen here
vs.
noresonance
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time
pulse
Fourier
Transform
frequency
time
"shaped" pulse
Fourier
Transform
frequency
Accomplishing the PulseIn general, signals are very close together, maybe 0.01 ppm!!How do we only “ping” one proton?
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The Answer
Br
H
BrH
OConnolly solvent
accessible surface;PM3 geometry optimization
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AcknowledgementsProf. Reynolds: product sample, spectral acquisition &
processing, help with structural elucidation
Prof. Reese, University of the West Indies: sample acquisition and extraction; preliminary analyses
Tim Burrow: NMR spectrometer help
Jordan Dinglasan: T.A.
Br
H
BrH
O