Synthesis of [3-13C]-2,3-dihydroxy-4-methoxybenzaldehyde

Article abstract of DOI:10.1016/j.tetlet.2015.12.088

An efficient synthesis of [3-¹³C]-2,3-dihydroxy-4-methoxybenzaldehyde, an isotopically labelled probe of a common intermediate used in the synthesis of a number of biologically relevant molecules, has been achieved in 9 steps from an acyclic, n

Full text of DOI:10.1016/j.tetlet.2015.12.088

Accepted Manuscript
Synthesis of [3-13C]-2,3-dihydroxy-4-methoxybenzaldehyde
Rebecca C. Collins, Martyn N. Paley, Gillian M. Tozer, Simon Jones
PII:
S0040-4039(15)30503-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.12.088
TETL 47135
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
27 November 2015
18 December 2015
22 December 2015
Please cite this article as: Collins, R.C., Paley, M.N., Tozer, G.M., Jones, S., Synthesis of [3-13C]-2,3-dihydroxy-4-
methoxybenzaldehyde, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.12.088
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Synthesis of [3-13C]-2,3-dihydroxy-4-methoxybenzaldehyde
Rebecca C. Collins,1 Martyn N. Paley,2 Gillian M. Tozer3 and Simon Jones1
1 Department of Chemistry, Dainton Building, University of Sheffield, Brook Hill, Sheffield. S3 7HF. UK
2 Academic Unit of Radiology, Department of Infection, Immunity & Cardiovascular Disease, The Medical
School, The University of Sheffield, Beech Hill Road, Sheffield. S10 2RX
3 Department of Oncology & Metabolism, The Medical School, The University of Sheffield, Beech Hill Road,
Sheffield. S10 2RX
Abstract: An efficient synthesis of [3-13C]-2,3-dihydroxy-4-methoxybenzaldehyde, an isotopically labelled
probe of a common intermediate used in the synthesis of a number of biologically relevant molecules, has been
achieved in 9 steps from an acyclic, non-aromatic precursor. A 13C label for molecular imaging was introduced
in a linear synthesis from the reaction of [13C]-labelled methyl iodide with glutaric monomethyl ester chloride.
Cyclisation then aromatisation gave 1,3-dimethoxybenzene and an additional methoxy group was introduced by
a formylation/Baeyer-Villiger/hydrolysis/methylation sequence. Subsequent ortho-formylation and selective
demethylation yielded the desired [3-13C]-2,3-dihydroxy-4-methoxybenzaldehyde.
Keywords: Dynamic Nuclear Polarisation; 13C-Labelling; Baeyer-Villiger
2,3-Dihydroxy-4-methoxybenzaldehyde 1 can be used as a key intermediate in a number of natural
products/drugs that have a range of biological activities (Figure 1), including the vascular disrupting agents
CA1P 2 and BNC 105P 3 that are currently in Phase I and II clinical trials, respectively.1–3 Another anti-cancer
compound that can use this compound as a key intermediate is narciclasine 4,4 shown in vivo to be effective
against primary brain cancers and brain metastases.5 Amongst other molecules of interesting biological
properties, thalimonine 5 is a natural product that has been shown to be active against the influenza and herpes
simplex virus,6–8 whilst stenocepflavone 6 exhibits significant inhibitory activity against acetylcholinesterase,
butyrylcholinesterase and lipoxygenases,9 and 2'-hydroxy-3',4',3,4-tetramethoxychalcone 7 shows a topical anti-
inflammatory effect in mice.10 Whilst the biological activity of such compounds is usually easy to obtain, real-
time molecular imaging can provide vital information on molecular targets and the metabolic fates of
 Corresponding author. Tel.: +44 114 222 9483; fax: +44 114 222 9346 e-mail: simon.jones@sheffield.ac.uk
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biologically active species. Dynamic nuclear polarization (DNP) is a hyperpolarization based magnetic
resonance technique that can significantly increase the sensitivity of 13C nuclei;11 when used in conjunction with
specifically labelled 13C samples, the relative sensitivity is further increased, allowing in vivo metabolism to be
investigated.
Figure 1. Potential natural products derived from key intermediate 1
The design criteria for a 13C labelled probe for use in DNP studies is critical, since this needs to be at a site in
the molecule with a long T1 relaxation time (essential for extending the lifetime of hyperpolarization), ideally on
a quaternary centre remote from any neighboring spin ½ nuclei, which cannot undergo any destructive
metabolism that would remove the label from the probe. Furthermore, the label needs to be located at a site
where metabolic activity occurs in order to observe a change in the chemical shift of the enhanced 13C signal
that would be indicative of any new metabolite formed. For example, combretastatin A1P 2 is known to
metabolise into ortho-quinone species 8 and 9 in vivo (Scheme 1), but the biological role of this metabolite has
not yet been established.12 The chemical shift of the highlighted 13C in CA1P 2 is significantly lower than that
on the same site in the ortho-quinone metabolite 9 [c (ppm) 136.7 vs. 178.9],13,14 so by hyperpolarising [13C]-
CA1P 2, signals for both species can easily be distinguished. Thus development of an efficient route to prepare
[3-13C]-2,3-dihydroxy-4-methoxybenzaldehyde 1 would provide an avenue to prepare and investigate real-time
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biological processes for these molecules of potential therapeutic interest. Furthermore, routes to pyrogallol
derivatives of this type with site-specific incorporation of an aromatic 13C label have not been reported.
Scheme 1. Known ortho-quinone metabolites of CA1P 2
The synthesis of [3-13C]-2,3-dihydroxy-4-methoxybenzaldehyde 1 started with the installation of the 13C label
by modifying an existing literature method.15 In this work, Botting and co-workers treated glutaric monomethyl
ester chloride 10 with [13C]-lithium dimethyl cuprate, generated from [13C]-methyl iodide, lithium, and copper(I)
iodide, which resulted in the formation of methyl 5-oxo-[6-13C]-hexanoate 11 in 43% yield. However, in the
route developed herein, an improved yield of 55% for the ester 11 was obtained when the acid chloride 10 was
treated with [13C]-methyl magnesium iodide and copper(I) bromide dimethyl sulfide, where the [13C] labelled
Grignard reagent was accessed from commercially available [13C]-methyl iodide (Scheme 2). This method was
found to be highly reproducible and easier to operate than the original cuprate procedure. Furthermore, this
method circumvents the sometimes difficult generation of [13C]-labelled methyl lithium and uses stoichiometric
quantities of [13C]-methyl iodide, rather than losing an additional unreactive equivalent through the cuprate.
Cyclisation of methyl 5-oxo-[6-13C]-hexanoate 11 to [2-13C]-cyclohexane-1,3-dione 12 was achieved in 61%
yield via an intramolecular Claisen reaction using potassium tert-butoxide as the base. The product was
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observed as a mixture of both keto and enol tautomers in the H NMR spectrum. Aromatisation of [2-13C]-
cyclohexane-1,3-dione 12 to give [2-13C]-resorcinol has previously been achieved by catalytic dehydrogenation
using Pd/C in refluxing xylene or triglyme with yields ranging from 45 – 88%.16–18 However, an alternate, more
efficient route was developed that encompassed both the aromatisation and subsequent methylation steps in a
one-pot reaction. Thus, dione 12 was heated at reflux with iodine in methanol to give the aromatised 1,3-
dimethoxy-[2-13C]-benzene 13 in 67% yield.19 Next, the regioselective addition of a hydroxyl group in the 2-
position was accomplished using a three-step strategy. Regioselective lithiation of 1,3-dimethoxy-[2-13C]-
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benzene 13 with nBuLi at 0 °C, in the presence of N,N,N′,N′-tetramethylethylenediamine, followed by treatment
with dimethylformamide and gave [1-13C]-2,6-dimethoxybenzaldehyde 14 in 89% yield.20 This compound was
then treated with hydrogen peroxide in methanol to affect a Baeyer-Villiger reaction but unfortunately this failed
to yield the desired formate ester, instead giving carboxylic acid 15 in a 2:1 ratio with starting material, formed
by hydride-migration in the rearrangement step of the Baeyer-Villiger reaction. However, when aldehyde 14
was treated with mCPBA, the desired formate ester was formed, which was hydrolyzed using potassium
hydroxide to give phenol 16 in 84% yield over the three-steps from methyl ether 13.
Scheme 2. Synthesis of [2-13C]-2-hydroxy-1,3-methoxybenzene 16
[2-13C]-2,6-Dimethoxyphenol 16 was then methylated using potassium carbonate and methyl iodide to yield [2-
13C]-1,2,3-trimethoxybenzene 17 in 80% yield, that underwent ortho-formylation using Rieche conditions
(titanium tetrachloride and dichloromethyl methyl ether) to give the aldehyde 18 in 71% yield (Scheme 3).
Treatment of [3-13C]-2,3,4-trimethoxybenzaldehyde 18 with two equivalents of boron trichloride led to selective
demethylation of the methyl ethers ortho and meta to the carbonyl group, providing [3-13C]-2,3-dihydroxy-4-
methoxybenzaldehyde 1 in 84% yield. The regiochemistry of this deprotection was confirmed by correlation of
the 13C NMR data with previously reported unlabeled material, with all chemical shifts matching to within 0.3
ppm.21
Scheme 3. Synthesis of [3-13C]-2,3-hydroxy-4-methoxybenzaldehyde 4
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In summary, we have adapted and developed an efficient route to install a 13C label at a strategically important
site of a common intermediate that can be used to construct a number of biologically relevant molecules.
Ongoing work is looking to exemplify the use of such an intermediate in the synthesis of the target molecules
illustrated.
Acknowledgments
We thank University of Sheffield A*STAR programme for funding (RCC), as well as a Programme Grant
C1276/A10345 from Cancer Research UK and EPSRC with additional funding from MRC and Department of
Health (England).
Supplementary data
Supplementary data (experimental procedures and characterization data) associated with this article can be found in
the online version.
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