The first isolation and crystal structure of a boron difluoro complex (isoflavone yellow). Biologically active intermediates produced during isoflavone synthesis

Article abstract of DOI:10.1039/a908915b

The isolation and crystal structure of a boron difluoro complex (isoflavone yellow) were studied. The cyclization step involves the treatment of a deoxybenzoine 3 with N,N′-dimethyl(chloromethylene)ammonium chloride in the presence of BF₃·Et₂O, which yielded a mixture of the desired isoflavone 5 and an unstable yellow compound 4. This yellow intermediates showed anticancer, nematicidal and mosquitocidal activities.

Full text of DOI:10.1039/a908915b

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The first isolation and crystal structure of a boron difluoro
complex (isoflavone yellow). Biologically active intermediates
produced during isoflavone synthesis
1
Sreenivasan Balasubramanian,a Donald L. Wardb and Muraleedharan G. Nair*a
a Bioactive Natural Products Laboratory, Department of Horticulture and National Food
Safety and Toxicology Center; b Department of Chemistry, Michigan State University,
East Lansing, MI 48824, USA
Received (in Cambridge, UK) 9th November 1999, Accepted 7th December 1999
The yellow intermediate 4f produced during the reaction of the deoxybenzoin 3f with N,NЈ-dimethyl(chloro-
methylene)ammonium chloride in the presence of BF3ؒEt2O prior to the formation of the corresponding isoflavone was
isolated for the first time and characterized by NMR, MS and single-crystal X-ray diffraction techniques. These
yellow intermediates showed anticancer, nematicidal and mosquitocidal activities.
Introduction
Isoflavones (3-phenyl-4H-1-benzopyran-4-ones) which exist in
plants, fruits, seeds and flowers of species belonging to the
Leguminosae family are known to possess many biological
activities. These compounds are extensively studied for their
estrogenic,1a anticancer,1b antibacterial,1c antifungal1d and
antimicrobial1e activities. We have shown that1f–l the isoflavone
formononetin (Mycoform) is very effective in stimulating the
growth of vesicular arbuscular mycorrhizal (AM) fungi and
enhances the growth of many plant species that are host to AM
fungi. Over the years many synthetic methods, such as the
oxidative rearrangement of chalcones,2a–c the ring closure of
benzyl phenyl ketones,2d–i the rearrangement of flavonone to
isoflavone by hydroxy(tosyloxy)iodobenzene2j and other
methods2k–m have been developed for the synthesis of iso-
flavones. However, these methods are unsuccessful for their
scale-up due to economic reasons. We have recently reported3 a
‘one pot’ and a two-step process for the synthesis of isoflavones.
In order to optimize the reaction conditions, we have studied
the mechanism for the formation of isoflavones by monitoring
the intermediates formed under different conditions using
various reagents. The deoxybenzoin route used for the synthesis
of isoflavones involved the addition of a single carbon at the
benzylic position followed by cyclization. In our reported
method, the cyclization step, which was carried out in the same
vessel, involved the treatment of a deoxybenzoin 3 with N,NЈ-
dimethyl(chloromethylene)ammonium chloride4 in the pres-
ence of BF3ؒEt2O. This reaction, carried out under very mild
conditions, yielded a mixture of the desired isoflavone 5 and an
unstable yellow compound 4. Treatment of this yellow product
with dilute acid converted it into the corresponding isoflavone
5, making it a simple process for the production of isoflavones
in high yields and purity.
Scheme 1
was extremely difficult due to overlapping Rf-values for the iso-
flavones and their respective yellow products. However, genistin
5j and biochanin-A 5k contained very little of the yellow
product and converted rapidly on the column to their corre-
sponding isoflavones. The reaction involving the formation of
4Ј,7-dimethoxyisoflavone 5f yielded a yellow, oily mixture and
further purification yielded a product in pure form.
In the two-step synthesis of isoflavones, a minimum of 3
mole equivalents of BF3ؒEt2O was required for the completion
of the reaction. The yellow product was absent when the reac-
tion was carried out in the absence of BF3ؒEt2O in DMF.
Similarly, the yellow product was not formed when the reaction
was conducted with BF3ؒEt2O and DMF in the absence of
reagent N,NЈ-dimethyl(chloromethylene)ammonium chloride.
Also, extending the reaction time to two hours did not yield the
yellow product. Similar results were obtained for the ‘one-step’
process as well. Our results indicate that N,NЈ-dimethyl(chloro-
methylene)ammonium chloride is required for the reaction and
that the yellow product is the key intermediate in the formation
of isoflavone products.
Results and discussion
The overall procedure consisted of the reaction of a phenyl-
acetic acid 1 and a phenol 2 in the presence of a Lewis acid
(BF3ؒEt2O) to form the corresponding deoxybenzoin 3, and its
treatment with N,NЈ-dimethyl(chloromethylene)ammonium
chloride to form the isoflavone 5 (Scheme 1). Attempts to purify
the yellow product generated during the synthesis of several
isoflavones like formononetin 5d, daidzin 5c, genistin 5j and
biochanin-A 5k using column chromatography were unsuccess-
ful. In the case of formononetin and daidzin, the purification
DOI: 10.1039/a908915b
J. Chem. Soc., Perkin Trans. 1, 2000, 567–569
This journal is © The Royal Society of Chemistry 2000
567

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The structural data for 1-(2Ј-difluoroboryloxy-4Ј-meth-
oxyphenyl)-3-(dimethylamino-2-(4Љ-methoxyphenyl)-prop-2-
enone, 4f, indicated that the bond lengths for F–B and B–O
bonds are similar to those reported for other boron com-
pounds, and the bond angles for F–B–F, F–B–O, O–B–O were
in agreement with the published data.5 Similarly the bond
lengths for C–N, C–C and N–C were in agreement with the
reported data for N,NЈ-dimethyleneamine, (H C) N–CH᎐C,
᎐
3
2
bonds.6
The presence of BF3ؒEt2O in the reaction mixture prior to
the addition of N,NЈ-dimethyl(chloromethylene)ammonium
chloride leads to the existence of a difluoroboron deoxybenzoin
complex followed by the formation of the isoflavone yellow. In
the case of 4Ј,7-dimethoxyisoflavone 5f, the mechanism for the
formation of the isoflavone yellow can be explained by the
initial formation of a difluoroboryl complex of 2-hydroxy-4,4Ј-
dimethoxydeoxybenzoin followed by attack of the electrophilic
reagent N,NЈ-dimethyl(chloromethylene)ammonium chloride
at the benzylic position to yield 4f (Scheme 2). Under acidic
Fig. 1 The structure of 1-(2Ј-difluoroboryloxy-4Ј-methoxyphenyl)-3-
dimethylamino-2-(4Љ-methoxyphenyl)-prop-2-enone 4f. Relevent bond
distances (Å) and bond angles (Њ): F(1)–B(1) 1.387(3), F(2)–B(1)
1.383(2), B(1)–O(2Ј) 1.452(3), B(1)–O(1) 1.501(3), O(1)–C(1) 1.327(2),
C(1)–C(2) 1.411(3), C(1)–C(1Ј) 1.458(3), C(2)–C(3) 1.406(3), C(2)–
C(1Љ) 1.496(3), C(3)–N(3) 1.319(2), C(2Ј)–O(2Ј) 1.350(2); F(2)–B(1)–
F(1) 110.35(17), F(2)–B(1)–O(2Ј) 109.74(17), F(1)–B(1)–O(2Ј)
111.48(17), F(2)–B(1)–O(1) 106.48(16), F(1)–B(1)–O(1) 108.64(16),
O(2Ј)–B(1)–O(1) 110.02(16), C(1)–O(1)–B(1) 123.02(15), O(1)–C(1)–
C(2) 115.93(17), O(1)–C(1)–C(1Ј) 116.91(16), C(2)–C(1)–C(1Ј)
127.17(17), C(3)–C(2)–C(1) 115.43(18), C(3)–C(2)–C(1Љ) 121.59(17),
C(1)–C(2)–C(1Ј) 122.66(17), N(3)–C(3)–C(2) 129.4(2), N(3)–C(3)–H(3)
113.2(12), C(3)–N(3)–C(4) 124.99(18), C(2Ј)–O(2Ј)–B(1) 116.45(15).
1
The H NMR spectrum of the yellow product 4f, isoflavone
yellow, showed peaks similar to those of its corresponding iso-
flavone. Peaks at δ 3.74 and 3.83 were assigned to two methoxy
groups, a doublet of a doublet at δ 5.98 for H-6, a doublet at
δ 6.43 for H-8, a doublet at δ 6.60 for H-5, and two doublets at
δ 6.89 and 7.08 for B-ring protons H-2Ј, -3Ј, -5Ј and -6Ј. In
addition, two broad signals at δ 2.54 and 3.31 integrating for 3
protons each, and a singlet at δ 8.32 integrating for one proton,
observed in its 1H NMR spectrum confirmed the presence of an
N,NЈ-dimethyleneamine moiety in the molecule. A comparison
of 1H NMR chemical shifts with that of the isoflavone 5f
showed appreciable upfield shifts for H-5, H-6 and H-7 of the
A-ring protons as well as the absence of the ubiquitous singlet
around δ 7.8 for the vinylic proton on C-2. The 13C NMR spec-
trum of the yellow product showed peaks at δC 36.25, 44.52 and
159.96 and confirmed the presence of a DMF moiety as part
of the molecule (DMF signals are at δC 31.1, 36.2 and 162.4,
respectively). Another interesting feature in its 13C NMR spec-
trum was the upfield shift of the carbonyl peak. The carbonyl
peak in 4f appeared at δC 161.76, as compared with δC 174.65
for its corresponding isoflavone.
Scheme 2
conditions, the difluoroboryl complex converts to the isoflavone
5f via cyclization.
A comparison of the spectral data (NMR, MS and UV–
visible) for isoflavone yellow and the X-ray diffraction data for
the yellow product 4f confirmed that a difluoroboryl complex
similar to 4f is a common yellow intermediate formed during
the formation of isoflavones. Therefore, it is possible that the
formation of difluoroboryl complex (isoflavone yellow) is cru-
cial for the conversion of deoxybenzoins to isoflavones under
mild conditions in high yield and purity.
Bioassays performed on 4c, 4d and 4f showed nematicidal,7a
mosquitocidal7b and topoisomerase I- and II-inhibitory7c
activities at 250 ppm concentration. Further work is in
progress to study the anticancer activity of these yellow inter-
mediates.
1
The H NMR spectrum for the yellow product 4d isolated
from the reaction involving the formation of formononetin 5d
also showed two broad singlets integrating for three protons
each at δ 2.55 and 3.33, respectively, and a singlet integrating
for one proton at δ 8.33. Again, this confirmed the presence of
an N,NЈ-dimethyleneamine moiety.
Fast-atom bombardment mass spectroscopic (FAB MS)
analysis of 4f gave a molecular ion at m/z 375 and indicated the
presence of additional moieties in the product that were not
revealed by its NMR spectra. The fragment ions at m/z 356,
331, and 199 indicated the loss of a fluorine atom, N(CH3)2,
and C11H14NO (176), respectively. A similar result was obtained
for 4d which gave a molecular ion at m/z 361.
After several attempts to produce single crystals of the yellow
product 4f in different solvent systems, a DMSO–acetone
mixture yielded suitable crystals for X-ray diffraction study. The
structure of 4f, as confirmed by the X-ray diffraction data, is
depicted in Fig. 1.
Experimental
The melting points were determined on a Bristoline micro melt-
ing point apparatus and are uncorrected. H-NMR and 13C-
1
NMR spectra were recorded on a Varian VXR 300 MHz spec-
trometer. The fast-atom bombardment mass spectroscopic
(FAB MS) analysis was carried out on a JEOL JMS-HX110
using a NBA matrix. The UV-Visible analysis was performed
on a Shimadzu UV-260 spectrophotometer. The X-ray diffrac-
tion studies were conducted on a Siemens SMART CCD Dif-
fractometer. The starting materials, solvents and reagents used
were purchased from Aldrich Chemical Company and were
used without further purification.
568
J. Chem. Soc., Perkin Trans. 1, 2000, 567–569

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1-(2Ј-Difluoroboryloxy-4Ј-methoxyphenyl)-3-dimethylamino-2-
(4Љ-methoxyphenyl)prop-2-enone 4f
Acknowledgements
This is a contribution from the Michigan State University
Agricultural Experiment station. Funding for this research was
partly provided by VAMTech L.L.C, Lansing, Michigan.
The NMR data were obtained on instrumentation that was
purchased in part with the funds from NIH grant # 1-S10-
RR04750, NSF grant # CHE-88-00770 and NSF grant # CHE-
92-13241. The X-ray diffraction studies were conducted on a
Siemens SMART CCD Diffractometer provided by NSF grant
CHE-96-34638, Department of Chemistry, MSU. The mass
analysis was done at the MSU–NIH Mass Spectrometry
Facility in the Department of Biochemistry, MSU.
A mixture of 1-(2-hydroxy-4-methoxyphenyl)-2-(4-methoxy-
phenyl)ethanone 3f (0.816 g, 3 mmol) and BF3ؒEt2O (1.2 mL,
9 mmol) was cooled to 10 ЊC and DMF (4.6 mL) was added
dropwise. In another flask, DMF (8 mL) was cooled to 10 ЊC
and PCl5 (0.939 g, 4.5 mmol) was added in small portions.
The mixture was then kept at 55 ЊC for 20 min. The pale
pink solution containing N,NЈ-dimethyl(chloromethylene)-
ammonium chloride was then added to the above reaction
mixture slowly. During the addition the temperature of the
reaction mixture was maintained below 27 ЊC. The mixture
was then stirred at room temperature for 5 min. The orange-
yellow, viscous solution was then poured into aq. NaOAc
(12.5%; 200 mL) and the product was extracted with EtOAc
(stored over K2CO3). The orange layer was washed with brine
and dried over Na2SO4. The solvent was removed using a
rotary evaporator and the crude product was vacuum dried
to yield 1.05 g of crude title product (90%). This product was
purified by column chromatography on silica using EtOAc–
light petroleum (distillation range 68–70 ЊC) (30%) to yield
1.01 g (87%) of 4f as a bright yellow amorphous powder, mp
162–164 ЊC; λmax 417.4 (εmax dm3 molϪ1 cmϪ1 37 050), 291.4
(8694) and 256.8 nm (9526); 1H NMR (CDCl3) δ 8.32
(s, 1H), 7.08 (d, J 8.9 Hz, 2H), 6.89 (d, J 8.9 Hz, 2H), 6.60
(d, J 9.7 Hz), 6.43 (d, J 2.7 Hz, 1H), 5.98 (dd, J 9.2 and 2.8
Hz, 1H), 3.83 (s, 3H), 3.74 (s, 3H), 3.31 (s, 3H), 2.54 (s, 3H);
13C NMR (CDCl3) δ 161.76, 159.96, 155.00, 154.78, 128.60,
128.18, 123.18, 110.20, 103.70, 97.77, 50.94, 50.78, 44.52,
36.25 [Found: m/z 375.1444 (Mϩ). C19H20BF2NO4BF2 requires
M, 375.1457]; m/z 375 (Mϩ), 356, 331, 199, 154 (100%) and
136.
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Crystallographic data for 1-(2Ј-difluoroboryloxy-4Ј-
methoxyphenyl)-3-dimethylamino-2-(4Љ-methoxyphenyl)-
prop-2-enone†
Single crystals of 1-(2Ј-difluoroboryloxy-4Ј-methoxyphenyl)-3-
dimethylamino-2-(4Љ-methoxyphenyl)prop-2-enone 4f, were
recrystallized from DMSO–acetone, mounted in ‘Paratone-N’
and transferred to the cold gas stream of the diffractometer. The
structure was solved using direct methods and refined by full
matrix least-squares on F2.
C19H20BF2NO4, 375.17, orthorhombic, space group Pbca,
a = 14.056(3), b = 13.921(3), c = 18.250(4) Å, V = 3571.3(12) A3,
T = 173 K, Z = 8, Dc = 1.396 Mg mϪ3, F(000) = 1568, absorp-
tion coefficient = 0.110 mmϪ1, crystal size = 0.08 × 0.10 × 0.36
mm, 30962 reflections measured, 3147 unique (Rint = 0.0658)
which were used in all calculations. The final wR(F2) was 0.095
(all data).
† CCDC reference number 207/386. See http://www.rsc.org/suppdata/
p1/a9/a908915b for crystallographic files in .cif format.
Paper a908915b
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