Synthesis of [7-14C]bergapten

Article abstract of DOI:10.1002/jlcr.3172

Bergapten (1) is a furocoumarin natural product and currently employed to treat skin disorders. Since past attempts to radiolabel 1 with ¹⁴C were limited to only its 5-methoxy group, a synthesis of the required ring [7-¹⁴C]1 is now described. The literature reported precursor 4-methoxy-6-hydroxybenzofuran-5-carboxaldehyde (3) was Wittig reacted with stabilized [carbonyl-¹⁴C]methoxycarbonylmethylenetriphenylphosphorane (4) to obtain [7-¹⁴C]1 in 47% radiochemical yield, with the desired product being characterized by thin-layer chromatography, HPLC, m.p. and proton NMR.

Full text of DOI:10.1002/jlcr.3172

Protocols and Methods
Received 20 October 2013,
Revised 10 November 2013,
Accepted 12 November 2013
Published online 10 January 2014 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.3172
14
Synthesis of [7- C]bergapten
*
Crist N. Filer and Thomas Rodgers
Bergapten (1) is a furocoumarin natural product and currently employed to treat skin disorders. Since past attempts to
radiolabel 1 with 14C were limited to only its 5-methoxy group, a synthesis of the required ring [7-14C]1 is now described.
The literature reported precursor 4-methoxy-6-hydroxybenzofuran-5-carboxaldehyde (3) was Wittig reacted with stabilized
[carbonyl-14C]methoxycarbonylmethylenetriphenylphosphorane (4) to obtain [7-14C]1 in 47% radiochemical yield, with the
desired product being characterized by thin-layer chromatography, HPLC, m.p. and proton NMR.
Keywords: Bergapten; carbon-14; furocoumarin; Wittig reaction
pyrone ring scission and culminating in a McFadyen–Stevens
Introduction
(acylbenzenesulfonylhydrazide pyrolysis) aldehyde construction.7
The natural product bergapten (1) is a member of the
furocoumarin (psoralen) family and easily obtained from
This route also appears to be the only published synthesis of
aldehyde 3. Conversion of intermediate 3 to bergapten was then
numerous plant sources, many of which are still utilized in
accomplished by malonic acid condensation (with simultaneous
traditional Chinese medicine. It and related toxic botanical
pyrone ring closure) followed by decarboxylation.
substances are also classified as phytoalexins, exploited by some
The preparation of 3 worked just as described, quickly
plants as defensive agents against pests such as fungi and
resulting in significant quantities of the aldehyde. However, for
insects.1 In recent years, bergapten and other psoralens have
also been successfully employed in photochemical therapy to
effectively treat skin conditions such as psoriasis and vitiligo.
Single crystal X-ray diffraction studies have elucidated the
product of this photolysis as a cycloaddition adduct between
the psoralen pyrone ring and the DNA base thymine, thereby
disrupting pathogenic cellular function.2 Also, quantum
mechanical calculations have indicated that bergapten is one
of the more efficient photosensitizers in its structural class.3
Despite the biological interest in this substance, there appears
to be just a single description of radiolabeling bergapten with
14C and only at its peripheral 5-methoxy position.4 For a
particular study, we were required to embed 14C in the
bergapten pyrone ring and specifically at the 7-position
carbonyl group. We now describe a rather efficient method to
accomplish that.
our 14C synthesis, we decided to modify the published path to
bergapten for several reasons. Not only was malonic acid (the
potential 14C reagent) used in large excess, but unless both of
its carbonyls were radiolabeled, the resulting product specific
activity would be significantly diluted as well. Utilizing such a
[dual carboxyl-14C]malonic acid would be both tedious to
prepare and ultimately result in loss of volatile [14C]carbon
dioxide at the final (decarboxylation) step. To circumvent these
complications, our plan was instead to treat aldehyde 3 with a
14C-labeled stabilized Wittig ylide followed by concomitant
lactone ring closure in a single step. Recognizing that a mixture
of intermediate olefin geometrical isomers would likely form
during the Wittig reaction, we employed refluxing diethylaniline
to thermally promote ring closure.8 As encouraging precedent
for this Wittig reaction strategy was our prior successful synthesis
of structurally related [3-14C]coumarin from 2-hydroxybenzaldehyde
using these same experimental conditions.9 The synthesis of [7-14C]
bergapten (5) is displayed in Scheme 1. Wittig phosphorane 4 was
prepared as described in the literature10 and reacted with aldehyde
3 while monitored by thin-layer chromatography (TLC). After
conventional workup, purification of 5 was accomplished by flash
chromatography followed by recrystallization, giving extremely
pure product with its HPLC, proton NMR and m.p. in exact
agreement with that of authentic 1. Although the radiochemical
yield of 47% for the final step conversion of 3 to 5 can be termed
modest, this small-scale transformation compared well with the
Results and discussion
Because of its convenient isolation from diverse plant sources,
there has been relatively little effort directed toward bergapten
total synthesis over the past 60 years. Furthermore, few of these
scattered reports provided a facile route, which could be applied
to 14C labeling. However, in reviewing the literature, one paper
emerged with a useful strategy to both readily assemble the
pyrone ring as well as access an appropriately functionalized
furan containing unlabeled precursor. In 1970, Nour El-Din and
Safwat reported the conversion of visnagin (2) to bergapten.5
Visnagin, also a furocoumarin isolated from the flowering plant
Ammi visnaga, has been utilized in Middle Eastern herbal
medicine to treat kidney stones6 and is commercially available.
The authors described the four-step transformation of visnagin
to key aldehyde intermediate 3, starting with an oxidative
PerkinElmer Life Sciences & Technology Inc., 940 Winter Street, Waltham,
MA 02451, USA
*Correspondence to: Crist N. Filer, PerkinElmer Life Sciences & Technology Inc.,
549 Albany Street, Boston, MA 02118, USA.
E-mail: crist.filer@perkinelmer.com
J. Label Compd. Radiopharm 2014, 57 102–103
Copyright © 2014 John Wiley & Sons, Ltd.

C. N. Filer and T. Rodgers
water were added. The mixture was vigorously stirred with the addition
of 0.35 mL of 2 M aqueous sodium hydroxide solution, and stirring was
continued under nitrogen at ambient temperature for 1 h. The reaction
was diluted with 10 mL of diethyl ether. The organic layer was separated,
dried over sodium sulfate, filtered and evaporated to afford 4 (125 mg,
0.37 mmol,18.7 mCi, and 82% radiochemical yield) as an off-white solid.
[7-14C]Bergapten (5)
A solution of aldehyde 3 (80 mg and 0.42 mmol) and Wittig reagent 4
(125 mg, 0.37 mmol, and 18.7 mCi) in 4 mL of N,N-diethylaniline was
heated at 220 °C under nitrogen, and the reaction was monitored by
TLC (hexane : ethyl acetate [4:1]). After 20 min, the reaction was complete.
It was cooled and diluted with 20 mL of diethyl ether and washed
sequentially with 10 mL portions of water and 1 N aqueous hydrochloric
acid. The ether layer was rotary evaporated to a small volume and
purified by silica gel flash chromatography eluted with a solution of
hexane : ethyl acetate (4:1). The progress of the flash purification was
monitored by TLC (same system as previously mentioned) with liquid
scintillation counting of elution fractions. Appropriate fractions were
pooled and evaporated. The resulting residue was crystallized from
7 mL of methanol to afford 38 mg (m.p. 188–190 °C; lit5 188–191 °C) of
5 (8.8 mCi, 47% radiochemical yield based on 4), which was 99%
radiochemically pure on TLC (hexane : ethyl acetate [4:1]) as well as
reverse phase HPLC (eluted with acetonitrile : water (1:1)). Product 5 also
coeluted with authentic 1 in both of these chromatographic systems. The
specific activity of 5 was measured to be 50.5 mCi/mmol by gravimetric
assay. Proton NMR (CDCl3) δ 8.17 (d, 1, J = 9.5 Hz), 7.60 (d, 1, J = 2 Hz),
7.14 (s, 1), 7.03 (d, 1, J = 2 Hz), 6.28 (d, 1, J = 9.5 Hz), and 4.26 ppm (s, 3).
Scheme 1. Synthesis of [7-14C]bergapten (5).
comparable two-step literature yield of 17% for 1 (based on malonic
acid) and afforded pure mCi amounts of product 5.
Experimental
General
All chemicals used were reagent grade. Evaporations were carried out on
a Buchi rotary evaporator at bath temperatures less than 40 °C. Analytical
and preparative TLC were performed on Analtech silica gel glass plates.
Analytical HPLC was accomplished on a PerkinElmer instrument, and
peak detection was performed simultaneously by UV and an IN/US
Systems Beta RAM Model 3 radioactivity detector. Solution assays were
performed with a PerkinElmer Tri-Carb 3100TR instrument. NMR spectra
were obtained on a Bruker 300 MHz instrument and chemical shift values
are expressed in parts per million (ppm) downfield from internal
tetramethylsilane. M.p.s were done on a Thomas Hoover apparatus and
are uncorrected.
Acknowledgement
We would like to acknowledge the contribution of Dr. Puliyer
Srinivasan of PerkinElmer Life Sciences
obtaining the NMR spectra.
& Technology in
Conflict of Interest
The authors did not report any conflict of interest.
4-Methoxy-6-hydroxybenzofuran-5-carboxaldehyde (3)
This intermediate aldehyde was prepared in comparable yield as
previously reported5 and recrystallized from methanol to give pale
yellow crystals (m.p. 121–124 °C, lit5 125 °C); proton NMR (CDCl3) δ
10.30 (s), 7.45 (s), 6.90 (s), 6.60 (s) and 4.25 ppm (s).
References
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[Carbonyl-14C]methoxycarbonylmethylenetriphenylphosphorane (4)
Precursor [1-14C]bromoacetic acid (62 mg, 0.45 mmol, and 22.7 mCi) was
dissolved in 10 mL of diethyl ether and treated with a diethyl ether
solution of diazomethane until a yellow color persisted. The solution
was stirred for 1 h at ambient temperature, and the diethyl ether was
removed by rotary evaporation. The residue was dissolved in 5 mL of
benzene to which was added triphenylphosphine (160 mg and
0.61 mmol). It was refluxed under nitrogen for 1 h, cooled to ambient
temperature, and allowed to stir overnight. The solid that formed was
filtered and washed with two 5 mL portions of diethyl ether. It was
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