Expedient deuterolabeling of polyphenols in ionic liquids - DCl/D 2O under microwave irradiation

Article abstract of DOI:10.1021/jo070231p

(Chemical Equation Presented) Postsynthetic regioselective aromatic ring H/D exchanges in polyphenolic compounds are rapidly performed in high yields and isotopic purities in ionic liquid-DCl/D₂O under microwave irradiation. Other C-H bonds, in

Full text of DOI:10.1021/jo070231p

Expedient Deuterolabeling of Polyphenols in
Ionic LiquidssDCl/D2O under Microwave
Irradiation
Ullastiina Hakala and Kristiina Wa¨ha¨la¨*
Laboratory of Organic Chemistry, Department of Chemistry,
UniVersity of Helsinki, P.O. Box 55, 00014,
UniVersity of Helsinki, Finland
kristiina.wahala@helsinki.fi
ReceiVed February 19, 2007
FIGURE 1. Naturally occurring polyphenols.
production of daidzein-d6 required autoclave conditions for 7
days in D3PO4‚BF3/D2O.8 Recently, CF3COOD was reported
to allow deuteration of isoflavones in 15 h under microwave
irradiation.9 Thus, although these methods mostly offer good
yields and high isotopic purity, very long reaction times are
often required, partly due to the poor solubility of polyphenolics
in water and other ordinary polar solvents. In contrast to these
regioselective polydeuterations of aryl rings of phenolic com-
pounds, a recent paper reports the perdeuteration of alkyl-substi-
tuted aromatics.10 For example, all C-H sites in 2-n-propyl-
phenol were indiscriminately deuterated in 24 h by H2 and D2O
at 180 °C with a Pd/Pt catalyst to give the d11 isotopologue.
Use of dielectric microwave heating has been increasingly
exploited in organic synthesis11 including deuteration reactions.12
Using dipolar or ionic solvents microwave energy can be
transferred to the reaction media using two mechanisms, i.e.,
dipole rotation and ionic conduction. Utilizing ionic liquids as
a primary or cosolvent, both mechanisms for energy transfer
can operate, thus making ionic liquids a highly suitable medium
for microwave-assisted organic synthesis.13
Postsynthetic regioselective aromatic ring H/D exchanges in
polyphenolic compounds are rapidly performed in high yields
and isotopic purities in ionic liquidsDCl/D2O under micro-
wave irradiation. Other C-H bonds, including benzylic and
lactone R-carbonyl sites, are not affected.
Introduction
Biologically active polyphenolic compounds are widely found
in plants and food products. In the past years, the possible role
of certain naturally occurring polyphenols, e.g., isoflavonoids
and lignans (Figure 1), in preventing hormone-dependent
diseases has been recognized.1 Synthetic methods for isotopi-
cally labeled analogues of these compounds are therefore
required. The labeled analogues are used as internal standards
for quantitation from biological samples, screening of biological
activities, determining the biosynthesis and metabolic pathways,
as well as elucidation of mass spectral fragmentation.2,3
In general, deuterium-labeling procedures of polyphenols rely
on electrophilic aromatic H/D exchange reactions catalyzed by
acids or occasionally by bases. Examples include matairesinol-
d6, which has been prepared with D3PO4‚BF3 in 1 day4 or
D3PO4 in 3 days.5 Similarly, daidzein-d4 has been produced in
D3PO4‚BF3/D2O in 3 days6 or in CF3COOD in 9 days,7 while
We describe here a much improved H/D exchange method
for several polyphenols utilizing ionic liquids, namely, 1-butyl-
3-methylimidazolium chloride, [bmim]Cl, or [bmim]Br, as a
cosolvent in 35% DCl/D2O, under microwave (MW) irradiation.
In passing, it may be noted that certain substrates that are poorly
(8) Rasku, S.; Wa¨ha¨la¨, K. J. Labelled Compd. Radiopharm. 2000, 43,
849.
(9) Soidisalo, O.; Wa¨ha¨la¨, K. J. Labelled Compd. Radiopharm. 2006,
49, 973.
(10) Ito, N.; Watahiki, T.; Maesawa, T.; Sajiki, H. AdV. Synth. Catal.
(1) (a) Raffaelli, B.; Hoikkala, A.; Leppa¨la¨, E.; Wa¨ha¨la¨, K. J. Chro-
matogr. B 2002, 777, 29. (b) Wa¨ha¨la¨, K.; Rasku, S.; Parikka, K. J.
Chromatogr. B 2002, 777, 111.
(2) Adlercreutz, H.; Kiuru, P.; Rasku, S.; Wa¨ha¨la¨, K.; Fotsis, T. J. Steroid
Biochem. Mol. Biol. 2004, 92, 399.
(3) Heinonen, S.-M.; Hoikkala, A.; Wa¨ha¨la¨, K.; Adlercreutz, H. J. Steroid
Biochem. Mol. Biol. 2003, 87, 285.
(4) Rasku, S; Mazur, W.; Adlercreutz, H.; Wa¨ha¨la¨, K. J. Med. Food
1999, 2, 103.
(5) Adlercreutz, H.; Fotsis, T.; Bannwart, C.; Wa¨ha¨la¨, K.; Brunow, G.;
Hase, T. Clin. Chim. Acta 1991, 199, 263.
(6) Rasku, S.; Wa¨ha¨la¨, K.; Koskimies, J.; Hase, T. Tetrahedron 1999,
55, 3445.
(7) Wa¨ha¨la¨, K.; Ma¨kela¨, T.; Ba¨ckstro¨m, R.; Brunow, G.; Hase, T. J.
Chem. Soc., Perkin Trans. 1 1986, 95.
2006, 348, 1025.
(11) (a) Kappe, O. C. Angew. Chem., Int. Ed. 2004, 43, 6250. (b) Loupy,
A. C. R. Chim. 2004, 7, 103. (c) MicrowaVe Assisted Organic Synthesis;
Tierney, J. P., Lidstro¨m, P., Eds.; Blackwell Publishing: Oxford, 2005. (d)
Bogdal, D. MicrowaVe-assisted Organic Synthesis: one hundred reaction
procedures; Elsevier: Amsterdam, 2005. (e) Kappe, C.O. MicrowaVes in
Organic and Medicinal Chemistry; Wiley-VCH: Weinheim, 2005.
(12) (a) Jones, J. R.; Lu, S-Y. In MicrowaVes in Organic Synthesis;
Loupy, A., Ed.; Wiley-VCH: New York, 2006. (b) Elander, N.; Jones, J.
R.; Lu, S.-Y.; Stone-Elander, S. Chem. Soc. ReV. 2000, 29, 239.
(13) (a) Hoffmann, J.; Nu¨chter, M.; Ondruschka, B.; Wasserscheid, P.
Green Chem. 2003, 5, 296. (b) Leadbeater, N. E.; Torenius, H. M.; Tye,
H. Comp. Chem. High Throughput Screening 2004, 7, 511. (c) Habermann,
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10.1021/jo070231p CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/22/2007
J. Org. Chem. 2007, 72, 5817-5819
5817

TABLE 1. H/D Exchanges in Pre-O-deuterated Daidzein 7 under Microwave/Ionic Liquid Conditions
total
time/min
% of H-3 and H-5
exchanged
% of H-2 and H-6
exchanged
entry
[bmim]Clb/mol equiv
DCl(-D2O)/mol equiv
Tc/°C (P/W)
1
2
3
4
5
6g
7
8
0
4
8
8
40
40
40
40
40
40 + 40
40
120 (40)e
30
30
30
30
10
10 + 10
30
120
45
67
95
0
0
0
120 (40)e
120 (40)e
120 (20)f
65
0
8
170 (70)f
>95
>95
50
70
>95
0
8 + 8
8
8
170 (70)f
with oil bath heating,h 120 ˚C
with oil bath heating,h 120 °C
40
50
0
a After CH/CD exchange, the protic hydroxyls were reinstated by a brief H2O treatment. b [bmim]Br may also be used. c Temperature recorded by IR
1
sensor. d Determined by H NMR of the isolated compound. e With continuous compressed air cooling (power max). f Without continuous cooling (power
max off). g The reaction and isolation procedure was repeated twice. h Preheated.
TABLE 2. Conditions and Results of the Deuteration of Certain Polyphenols
entry
substrate
Ta/°C (P/W)
total time/min
i.p./%b (yield/%)
productc
[3,5,6,8-d4]-daidzein
1
2
3
4
5
6
7
1
1
2
2
4
5
6
120 (40)
170 (70)
120 (40)
170 (70)
120 (50)
120 (40)
70 (20)
30
20
30
40
20
15
40
>85 (94)
>90 (89)
>85 (95)
>78 (80)
>90 (93)
>90 (90)
>90 (91)
[2,3,5,6,6,8-d6]-daidzein
[3,55,8-d4]-4,6,7-trihydroxyisoflavone
[2,3,5,5,6,8-d6]-4,6,7-trihydroxyisoflavone
[2,3,3,5,6,8-d5]-apigenin
[2,3,3,5,5-d5]-O - demethylangolensin
[2,2,3,5,6,6-d6]-matairesinol
a Temperature recorded by IR sensor. b Isotopic purities were determined by ESI-TOF and 1H NMR comparison of the labeled and unlabeled compounds.
c Products were characterized by comparison with published data.3-9,17
soluble in CH2Cl2, a solvent required for iridium-complex-
catalyzed deuteration with D2, undergo partial deuteration under
these conditions if an ionic liquid cosolvent is present to allow
dissolution of the substrate.14
underwent H/D exchange to an extent of more than 90%. As
expected, the pyrone ring 2-H remained completely inactive
under these conditions. For comparison, a test reaction was also
conducted with conventional heating (entry 7 and 8). Under
these conditions, the exchange reaction occurred sluggishly not
only at C-3′ and -5′ but also at C-6 and -8. Without an ionic
liquid cosolvent, only marginal deuteration occurred under
refluxing conditions.
Results and Discussion
Pre-O-deuterated daidzein 7 was used as a model substrate
for screening expedient DCl/D2O-ionic liquid-microwave
conditions (Table 1). In the absence of an ionic liquid cosolvent
(entry 1), low deuterium efficiency resulted due to the low
substrate solubility, while use of 8 mol equiv of [bmim]Cl gave
a good substrate solubility and hence H/D exchange in DCl/
D2O at 120 °C with an initial MW power input of 40 W (entry
3). Under these conditions over 90% exchange took place at
the more active aromatic sites of daidzein, i.e., C-8, -6, -3′, and
-5′, while the less active 2′ and 6′ positions remained unchanged.
Interestingly, when a MW power of 20 W was used to reach a
temperature of 120 °C (entry 4), less H/D exchange reaction
took place. Thus, presumably increased microwave thermal
effects are available with higher energy input, i.e., rapid heating,
volumetric heating, superheating, and hotspots. The less active
aromatic 2′ and 6′ positions started to be slowly exchanged when
the reaction temperature rose over 140 °C, and at 170 °C (70
W of MW power, entry 5 and 6), the 2′ and 6′ hydrogens also
The results of this deuterolabeling procedure applied to a
number of polyphenolic compounds are summarized in Table
2. In glycitein 2, the C-6 methoxy group underwent ether
cleavage resulting in polydeuterated analogues of compound
3. The 4′,6,7-trihydroxyisoflavone-d4 (entry 3) and 4′,6,7-
trihydroxyisoflavone-d6 (entry 4) obtained are of interest in
metabolism studies of daidzein and other nutritional isoflavones.
A high yield and isotopic purity were also gained in the H/D
exchange reaction of apigenin 4 in 20 min and O-demethyl-
angolensin 5 in 30 min to obtain the d5 analogues (entries 5
and 6). Furthermore, our deuterolabeling method was very
suitable for matairesinol 6, in which all six aromatic protons
underwent H/D exchange at 70 °C (entry 7), but interestingly
no benzylic or carbonyl R-H sites were exchanged. When the
same reaction conditions were used without an ionic liquid,
H/D exchange occurred at a level of only about 50%, showing
that the presence of ionic cosolvent was crucial. As expected,
[bmim]Cl is completely resistant to the acidic conditions
employed. It is interesting to note that 2-mono15 and 2,4,5-tri16
(14) Salter, R.; Bosser, I. J. Labelled Compd. Radiopharm. 2003, 46,
489.
5818 J. Org. Chem., Vol. 72, No. 15, 2007

deuterated imidazolium ionic liquids are obtained by D2O/DO-
treatment of the undeuterated materials.
and DCl/D2O (1.5 mL) was heated for the appropriate time and at
the temperature indicated. The product was isolated either by
filtration or by extraction from the reaction mixture. To recover
the remaining ionic liquid, the water phase or filtrate was
concentrated in a rotary evaporator and ionic liquid collected in
90% yield.
The sites and preferred order of exchange in polyphenolic
compounds are governed by the relative electronegativities at
the various positions. We reported6 earlier how Mulliken charges
calculated with the semiempirical AM1 method can be used to
rationalize the electrophilic aromatic deuteration results.
In conclusion, we developed a fast, high-yielding, and aryl-
ring-selective acid-catalyzed deuteration method for representa-
tive polyphenols in ionic liquids using 35% DCl/D2O as a cheap
deuterium source under microwave irradiation. This postsyn-
thetic methodology is expected to be generally applicable and
simple to carry out with reaction times shortened from several
days or 15 h to 20 - 40 min and the ionic solvent easily
recycled.
Acknowledgment. We thank Mr. Antti Hoikkala for running
the ESI-TOF MS(ESI+) and Mr. Heimo Kanerva for skilful
technical assistance. Financial support from the Emil Aaltonen
Foundation (U.H.) and the Finnish Academy (210633) (K.W.)
is gratefully acknowledged.
1
Supporting Information Available: Experimental details, H
NMR spectra, and MS(ESI+ or ESI-) data of all products listed in
Table 2. This material is available free of charge via the Internet
at http://pubs.acs.org.
Experimental Section
JO070231P
General Procedure. A mixture of O-predeuterated (D2O/
acetone) polyphenol (0.39 mmol), [bmim]Cl (0.55 g, 3.14 mmol),
(16) Giernoth, R.; Bankmann, D. Tetrahedron Lett. 2006, 47, 4293.
(17) Rasku, S; Wa¨ha¨la¨, K. Tetrahedron 2000, 56, 913.
(15) Handy, S. T.; Okello, M. J. Org. Chem. 2005, 70, 1915.
J. Org. Chem, Vol. 72, No. 15, 2007 5819