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Synthesis of an Isotopically Labeled Naphthalene Derivative That
Supports a Long-Lived Nuclear Singlet State
Joseph T. Hill-Cousins,*Ionut-Alexandru Pop, Giuseppe Pileio, Gabriele Stevanato, Pa
rHa
kansson,
Soumya S. Roy, Malcolm H. Levitt, Lynda J. Brown, and Richard C. D. Brown*
Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, U.K.
*
SSupporting Information
ABSTRACT: The synthesis of an octa-alkoxy substituted isotopically labeled
naphthalene derivative, shown to have excellent properties in singlet NMR
experiments, is described. This highly substituted naphthalene system, which
incorporates an adjacent 13C spin pair, is readily accessed from a commercially
available 13C2-labeled building block via sequential thermal alkynyl- and
arylcyclobutenone rearrangements. The synthetic route incorporates a simple
desymmetrization approach leading to a small difference in the chemical shifts of
the 13C spin pair, a design constraint crucial for accessing nuclear singlet order.
Singlet NMR has potential as a diagnostic tool with a
number of potential applications including the study of
molecular diffusion and motion,
1−3
protein−ligand binding,
4
analysis of intrinsically disordered protein domains,
5
and
metabolomics.
6
Furthermore, the combination of nuclear
hyperpolarization and singlet NMR offers opportunities to
develop novel MR imaging techniques.
7−9
Nuclear hyper-
polarization, generated by methods such as dynamic nuclear
polarization (DNP), gives rise to greatly increased NMR signal
intensities;
10−14
theoretically 13C NMR signals can be enhanced
by a factor of 105compared with conventionally thermally
polarized nuclei. However, this technique has been limited by
the short lifetime of hyperpolarized magnetization, which
decays with the spin−lattice relaxation time constant, T1.
Nuclear singlet order is immune to many of the relaxation
mechanisms responsible for T1and decays with a time constant
TSwhich can often be far larger than T1.
15−19
As a result,
nuclear singlet order provides a means to “store”nuclear
hyperpolarization for extended periods of time, paving the way
for a variety of applications.
9
We have recently reported an
octa-alkoxy substituted naphthalene derivative, incorporating a
13C spin pair, which supports a long-lived nuclear singlet state
in both low (TS> 1 h; 0.4 T; acetone-d6) and high magnetic
field (TS≈950 s; 9.4 T; acetone-d6).
20
Herein, we report the
synthetic approach to this target.
Two key aspects of the design of molecular systems that
support long-lived singlet states are the ability to access the
singlet state and attenuating the rate of relaxation of the singlet
state occurring through different mechanisms.
18,19
The criteria
for a suitable molecule can be summarized as follows:
20
(1)
Fundamentally, the molecule must incorporate a strongly
coupled spin-1/2 pair with which the singlet state can be
created. (2) There should be no spin-active nuclei in close
proximity (through bond and through space) to the spin pair,
especially isotopes with strong magnetism such as 1H and 19F.
(3) Nuclei such as 2H with quadrupole moments, while
preferable to 1H, should also be physically remote from the spin
pair. (4) The local molecular environment of the spin pair
should exhibit inversion symmetry. (5) The molecule as a
whole must provide either a small chemical shift difference
between the members of the spin pair or different spin−spin
couplings between the members of the spin pair and other
magnetic nuclei. (6) The local molecular environment of the
spin pair should be conformationally inflexible. (7) The spin
pair should be shielded against close approach of paramagnetic
molecules, such as molecular oxygen.
Based upon these design criteria, we considered that a
naphthalene 7, with a central 13C spin pair and fully deuterated
side chains (R1and R2), would provide a suitable candidate
(Scheme 1). Such an aromatic system is rigid and incorporates
a local inversion center, and all other spin-active nuclei are at
least four bonds away from the spin pair as well as an optimal
distance through space. A small chemical shift difference may
be provided by asymmetric substitution.
An additional challenge posed for the synthesis of such
molecules lies in the availability of isotopically labeled starting
materials. Starting materials incorporating 13Catomsare
generally limited to small-molecule building blocks, and as
such syntheses must be designed around these. We have
previously utilized commercially available ethynyltrimethyl-
silane-13C2(1) to synthesize a series of acetylene-based
compounds that support long-lived nuclear singlet states,
9,21
and considered this fragment to be convenient for construction
of the requisite naphthalene system 7. Our approach was
designed around sequential thermal alkynyl- and arylcyclobu-
tenone rearrangements to construct each ring of the bis-
aromatic framework.
22−29
By this method the central 13C pair
would be derived from the acetylene building block 1and
Received: March 13, 2015
Published: April 21, 2015
Letter
pubs.acs.org/OrgLett
© 2015 American Chemical Society 2150 DOI: 10.1021/acs.orglett.5b00744
Org. Lett. 2015, 17, 2150−2153
This is an open access article published under a Creative Commons Attribution (CC-BY)
License, which permits unrestricted use, distribution and reproduction in any medium,
provided the author and source are cited.
asymmetry could be easily introduced using an unsymmetrically
substituted squarate fragment 5.
The synthetic route was optimized using unlabeled materials,
primarily to reduce costs as well as to simplify analysis of the
intermediates. The left-hand side of the naphthalene ring
system was constructed first, beginning with squaric acid (8,
Scheme 2). One of the primary requirements for the target
naphthalene system was perdeuteration of the alkoxy
substituents for reasons described above. Ultimately, this
would be achieved through alkylation of the disilver salt of
squaric acid using deuterated iodomethane (99.5 atom % D).
30
Thus, for the unlabeled synthesis, treatment of squaric acid (8)
with AgNO3and Et3N, followed by reaction with CH3Iin
refluxing Et2O for 18 h, afforded dimethyl squarate (9) in 68%
yield. Alkynylation of dimethyl squarate (9) with the lithium
salt of ethynyltrimethylsilane proceeded smoothly to afford
cyclobutenone 10 in 92% yield. Subsequent silyl-deprotection
of 10 with TBAF gave cyclobutenone 11 in 84% yield,
providing the substrate for the first thermal rearrangement.
Thermal rearrangement of cyclobutenone 11 was initially
conducted under reflux in toluene, affording quinone 12 as the
only isolated product in 56% yield after 2 h of heating (Table 1,
entry 1). The remaining material consisted of intractable
baseline components. We considered that the moderate yield of
the quinone 12 could be attributed to prolonged heating under
reflux, and consequently, alternative reactor technologies were
explored (Table 1). Harrowven and co-workers have recently
demonstrated a series of highly efficient arylcyclobutenone
rearrangements in a flow reactor proceeding with short
residence times and excellent yields.
28
On application of similar
conditions to the rearrangement of acetylenyl-substituted
cyclobutenone 11, the yield of quinone 12 was improved to
62% (Table 1, entry 2). Further improvement was achieved
under microwave irradiation, delivering quinone 12 in 72%
yield after 20 min in MeCN at 130 °C (Table 1, entry 3).
Reduction of quinone 12 by treatment with NaBH4afforded
the dihydroquinone (Scheme 3), which was immediately
dimethylated under basic conditions to afford the tetramethoxy-
benzene 13 in 69% over the two steps.
The second ring required an unsymmetrical squarate
fragment 14, obtained in 60% yield by reaction of sodium
isopropoxide with dimethyl squarate (9). The relatively high
yield of 14 is perhaps quite surprising as we had anticipated
rapid equilibration to a statistical mixture of both symmetrical
esters and the unsymmetrical ester 14. Indeed, on extension of
Scheme 1. Synthesis Plan
Scheme 2. Synthesis of Cyclobutenone 11 as a Precursor for
the First Cyclobutenone Rearrangement
Table 1. Optimization of the Rearrangement of
Cyclobutenone 11
entry conditions yield
a
of 12
1A: PhMe, reflux, 2 h 56%
2B: Flow reactor, dioxane, 130 °C, tR= 30 min 62%
3C: Microwave irradiation, MeCN, 130 °C, 20 min 72%
a
Isolated yields of purified compounds are quoted.
Scheme 3. Synthesis of Unlabeled Naphthalene System 16
Organic Letters Letter
DOI: 10.1021/acs.orglett.5b00744
Org. Lett. 2015, 17, 2150−2153
2151
the reaction time to 20 min, the yield of squarate 14 was
reduced to 51%. In any case this method of desymmetrization
would again permit the introduction of the required
perdeuterated isopropoxy side chain during the labeled
synthesis.
The squarate and tetramethoxybenzene fragments, 14 and
13, were combined through an ortho-lithiation
31
coupling
sequence, affording an inseparable mixture of regioisomers 15a
and 15b (∼1:1, 1H NMR) in 76% overall yield. Upon thermal
rearrangement under microwave irradiation the mixture of
isomers 15a and 15b converged upon a common, naphthalene-
1,4-diol intermediate. Despite the reaction solution being
purged with N2gas prior to heating, small amounts of the
naphthalene-1,4-dione were present in the crude product.
Attempts to reduce the quinone present in the crude reaction
mixture proved unsuccessful; NaBH4and Na2S2O4were both
incompatible, ultimately leading to degradation of the products.
Consequently, following the thermal rearrangement, the crude
reaction mixture was immediately treated with K2CO3and
Me2SO4in refluxing acetone, allowing isolation of naphthalene
16 in 50% yield over the two steps.
Gratifyingly, the asymmetry of naphthalene 16, resulting
from the presence of a single isopropoxy group, achieved a
suitable chemical shift difference of 0.08 ppm between the two
central carbons in the 13C NMR spectrum of 16 (acetone-d6).
Such near-equivalence of the spin pair in the labeled system is
necessary for a long-lived singlet state that is stable in high
magnetic field, while the small measure of asymmetry enables
initial creation of the singlet state.
21
With an optimized route to the unlabeled naphthalene
system 16 established, the labeled synthesis was subsequently
performed (Scheme 4). Alkylation of squaric acid (8) with
CD3I via the disilver salt afforded perdeuterated squarate 17 in
88% yield over the two steps. Introduction of the 13C spin pair
proceeded smoothly via deprotonation of ethynyltrimethyl-
silane-13C2(1) with n-BuLi and subsequent reaction with
squarate 17,toafford cyclobutenone 18 in 96% yield. Silyl
deprotection of 18 with TBAF gave cyclobutenone 19 in 95%
yield. The 1H NMR spectrum of 19 displays some interesting
second-order effects, with the signal for the alkyne proton
appearing as a well-defined X portion of an ABX spin system.
32
The small chemical shift difference between the 13C labels and
the large difference between 1JCH and 2JCH for this system
presents a case in which all six spectral lines are clearly visible
(see Supporting Information).
Cyclobutenone 19 was submitted to the previously
optimized conditions for thermal rearrangement, affording
quinone 20 in 65% yield. Reduction of quinone 20 and
subsequent alkylation of the intermediate hydroquinone, using
CD3I, delivered the labeled tetra-alkoxybenzene 21 in 73% yield
over the two steps. Quinone 20 and tetra-alkoxybenzene 21
also both display interesting 1H NMR spectra with well-
resolved signals for the XX′portion of an AA′XX′spin system,
arising from magnetic nonequivalence (Figure 1).
32
As a
consequence of the adjacent 13C labels, a rare occasion is
presented in which the coupling constants (JHH,JCC,1JCH, and
2JCH) of these AA′XX′systems can be easily determined. The
corresponding AA′portions of the spectra for 20 and 21 were
observed as singlets due to proton decoupling during 13C NMR
Scheme 4. Synthesis of Isotopically Labeled Naphthalene 24
Figure 1. 1H NMR spectra for compounds 20 and 21 (400 MHz,
CDCl3).
Organic Letters Letter
DOI: 10.1021/acs.orglett.5b00744
Org. Lett. 2015, 17, 2150−2153
2152
data acquisition. Furthermore, as anticipated the 1H NMR
spectrum of tetra-alkoxybenzene 21 confirmed very high levels
of deuterium incorporation (>99%, 1H NMR) into the alkoxy
substituents of the left-hand fragment.
The unsymmetrical squarate 22 was prepared from squarate
17 in 60% yield, using isopropanol-d8to achieve perdeuteration
of the fragment. The left-hand and right-hand fragments were
coupled as described above, affording a mixture of regioisomers
23a and 23b in an 85% overall yield. Following thermal
rearrangement of the mixture of regioisomers 23a and 23b,
sequential alkylation of the intermediate hydroquinone, using
Me2SO4-d6,afforded isotopically labeled naphthalene 24 in 47%
yield for the two steps.
In summary, we have synthesized an isotopically labeled
naphthalene derivative 24, incorporating an adjacent 13C spin
pair and perdeuterated alkoxy substituents. As reported
elsewhere, this compound supports a long-lived nuclear singlet
state with a lifetime exceeding 1 h in room-temperature
solution.
20
The target naphthalene 24 was synthesized on a 1−
4 mmol scale from commercially available starting materials in
10 linear steps (11 steps in total) with a 15% overall yield for
the linear sequence.
■ASSOCIATED CONTENT
*
SSupporting Information
Experimental details and procedures; compound character-
ization data; copies of 1H, 2H, and 13C NMR spectra for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
■AUTHOR INFORMATION
Corresponding Authors
*E-mail: r.c.brown@soton.ac.uk.
*E-mail: J.Hill-Cousins@sygnaturediscovery.com.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The authors acknowledge EPSRC (EP/I036141/1 and EP/
K039466/1), ERC, the European Regional Development Fund
(ERDF) for funding the AI-Chem project through the
INTERREG IVa program 4061, and the Royal Society
(L.J.B.) for a Dorothy Hodgkin fellowship. Additionally the
authors would like to thank Dr. Neil J. Wells (University of
Southampton) for assistance obtaining 2H NMR data.
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