Synthesis of pterocarpan-type heterocycles via oxidative cycloadditions of phenols and electron-rich arenes

Article abstract of DOI:10.1080/00397910902838961

Oxidation of 4-alkoxyphenols or 4-methoxynaphthol with phenyl iodonium(bis)trifluoroacetate (PIFA) in the presence of electron rich 2H-chromenes or dihydronaphthalenes affords pterocarpans or 5-carbapterocarpans via a formal [3+2] cycloaddition process. A

Full text of DOI:10.1080/00397910902838961

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Synthesis of Pterocarpan-Type
Heterocycles via Oxidative
Cycloadditions of Phenols and
Electron-Rich Arenes
Amber L. Mohr a , Vincent M. Lombardo a , Teresa M.
Arisco a & Gary W. Morrow a
a Department of Chemistry , University of Dayton ,
Dayton, Ohio, USA
Published online: 07 Oct 2009.
To cite this article: Amber L. Mohr , Vincent M. Lombardo , Teresa M. Arisco &
Gary W. Morrow (2009) Synthesis of Pterocarpan-Type Heterocycles via Oxidative
Cycloadditions of Phenols and Electron-Rich Arenes, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 39:21,
3845-3855, DOI: 10.1080/00397910902838961
To link to this article: http://dx.doi.org/10.1080/00397910902838961
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Synthetic Communications1, 39: 3845–3855, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902838961
Synthesis of Pterocarpan-Type Heterocycles
via Oxidative Cycloadditions of Phenols
and Electron-Rich Arenes
Amber L. Mohr, Vincent M. Lombardo,
Teresa M. Arisco, and Gary W. Morrow
Department of Chemistry, University of Dayton, Dayton, Ohio, USA
Abstract: Oxidation of 4-alkoxyphenols or 4-methoxynaphthol with phenyl
iodonium(bis)trifluoroacetate (PIFA) in the presence of electron rich 2H-
chromenes or dihydronaphthalenes affords pterocarpans or 5-carbapterocarpans
via a formal [3 þ 2] cycloaddition process. Acid-catalyzed ionization of quinone
dimethylmonoacetal in the presence of 7-methoxy-2H-chromene gave similar
results, suggesting a phenoxonium ion intermediate in the oxidative process.
Keywords: Carbapterocarpan, cycloaddition, hypervalent iodine reagent,
oxidation, phenol, pterocarpan, quinone monoacetal
INTRODUCTION
Pterocarpans are members of a large family of widely distributed
isoflavanoids produced by plants in response to fungal infections.[1]
Interest in these polyoxygenated heterocycles stems from their unusal
cis-fused dihydrobenzofuran–benzopyran ring juncture (Fig. 1) and a
spectrum of biological properties including COX-2 inhibition,[2] and
antitumor,[3] LDL-antioxidant,[4] anti-HIV,[5] and anti–snake venom
activities.[6] A number of synthetic approaches have been reported over
the years, many of which are multistep and proceed in poor overall
Received December 22, 2008.
Address correspondence to Gary W. Morrow, Department of Chemistry,
University of Dayton, Dayton, OH 45469-2357, USA. E-mail: gary.morrow@
notes.udayton.edu
3845

3846
A. L. Mohr et al.
Figure 1. Numbering for pterocarpan ring system.
yield.[7c] Heck oxyarylation[8] and Ti(IV)-mediated benzoquinone
cycloadditions with 2H-chromenes[9] have been widely employed, though
both can give rise to structurally rearranged products in addition to the
requisite cis-fused pterocarpan skeleton.
Oxidation of phenols with hypervalent iodine reagents in the presence
of electron-rich styrenes has been shown to provide ready access to a
number of dihydrobenzofuran structures in reasonable yield via a formal
[3þ 2] cycloaddition process.[10] We report herein the use of 2H-chromenes
and dihydronaphthalenes as nucleophilic arene components in such
oxidative cycloadditions, leading to rapid and convergent assembly of
several novel pterocarpan-type ring systems in a single step from readily
available phenols under mild conditions.
RESULTS AND DISCUSSION
7-Methoxy-1,2-dihydronaphthalene (1a) and chromenes 1b and 1c for the
study (Table 1) were prepared by literature methods.[11] Initial chemical
oxidation of an equimolar mixture of 1a and 4-methoxyphenol (2a) in
MeCN with phenyl iodonium (bis)trifluoroacetate (PIFA) (1 equiv) at
room temperature for 15 min followed by standard workup and chroma-
1
tographic isolation gave a product whose H and 13C NMR data were
consistent with formation of the corresponding 5-carbapterocarpan
cycloadduct 3a in 61% isolated yield (Entry 1, Table 1).
In multiple trials, no improvement in yield of 3a was obtained
through use of an excess of 1a and 2a or the oxidant. This stands in

3847

3848

Synthesis of Pterocarpan-Type Heterocycles
3849
contrast to the 5:1 styrene-to-phenol molar ratio reportedly required
for optimal yields in similar oxidative cycloadditions, leading to dihydro-
benzofurans.[10] As shown in Table 1, yields of other cycloadducts 3 were
reasonable only when the phenolic component possessed alkoxyl sub-
stitution solely at the para-position. Thus, oxidation of phenols 2a and
2b or 4-methoxynaphthol (2c) in the presence of 1a (entries 1–3) or 1b
(entries 4–6) gave, respectively, 5-carbapterocarpans 3a–c and ptero-
carpans 3d–f in comparable yields (47–62%). Additional oxygenation at
position 2 of the chromene ring, as in 1c, gave similar results (entry 7).
However, sesamol (2d), when oxidized in the presence of 1b, led to a
disappointingly poor yield of naturally occurring (ꢀ)-pterocarpin (3 h)
(entry 8), whereas oxidation of 3,4-dimethoxyphenol (2e) in the presence
of 1b led to a complex mixture containing only a small amount of impure
cycloadduct 3i (entry 9). Similar oxidation using 3-methoxyphenol gave
no recoverable product. Because oxygenation at position 9 is common
to most naturally occuring pterocarpans, these latter results represent
an important limitation of the chemistry.
Structure assignments for cycloadducts 3 were supported by compar-
ison of 1H and 13C NMR and=or melting-point (mp) data with literature
values (3d, h) as well as by elemental analysis for new compounds (3a–c,
3e–g). The expected cis-fused pterocarpan ring junction was also
confirmed by single-crystal x-ray analysis of 3d (Fig. 2).
For the reactions leading to 3 h and 3i, mostly unreacted 1b was
recovered after chromatography, which argued against competing oxida-
tion of products (in spite of their additional oxygenation) as a cause for
poor yields. Nevertheless, because 3 h was rapidly consumed when treated
alone with excess PIFA in MeCN, we briefly examined nonoxidative
methods for generating the phenoxonium ions presumably involved in
these cycloadditions. Thus, stirring of a solution of quinone monoacetal
4a and chromene 1b in MeCN in the presence of acidic montmorillonite
Figure 2. Ortep diagram for 3d.

3850
A. L. Mohr et al.
Scheme 1. Synthesis of 3d via nonoxidative cycloaddition.
K-10 clay led to formation of pterocarpan 3d in 62% yield, comparable to
that obtained via PIFA oxidation of phenol 2a (Scheme 1). This
supported the hypothesis that phenoxonium ions 5 were likely intermedi-
ates in the oxidative cycloaddition process. However, reaction of 1b with
quinone monoacetals 4b or 4c[12] under the same conditions of acid
catalysis gave no recoverable amount of the corresponding pterocarpan
cycloadducts. Although no further studies were done, these results
suggested that poor yields of cycloadducts from phenols 2d or 2e via
the oxidative method may have been due to an inherent instability of
the corresponding phenoxonium ions.
CONCLUSIONS
Pterocarpan-type structures may be prepared in a single step in reason-
able yield via oxidation of 4-alkoxyphenols or naphthols with PIFA in
the presence of electron-rich chromenes or dihydronaphthalenes in
acetonitrile solution at room temperature.
EXPERIMENTAL
All NMR spectra were recorded at 300 MHz (1H) and 125 MHz (13C) on
a Bruker Avance spectrometer and were measured in deuteriochloroform
with tetramethylsilane (TMS) as internal standard. Signals are reported
in d values (ppm) downfield from TMS. Melting points were determined
in open capillary tubes on a Thomas-Hoover apparatus and are uncor-
rected. Elemental analyses were provided by Galbraith Laboratories,
Knoxville, TN. Compounds 1a–c and 4a–c were prepared following
previously published procedures.[11,12] All other compounds and reagents
were obtained from the Aldrich Chemical Co.

Synthesis of Pterocarpan-Type Heterocycles
3851
General Procedure for Oxidative Cycloadditions
Solid PIFA (1.8 mmol) was added to an equimolar mixture of the
requisite phenol (1.8 mmol) and chromene or dihydronaphthalene
(1.8 mmol) in CH3CN (5 mL) all at once. After stirring 15 min, the
reaction was quenched with sat. NaHCO3(2 mL). Ether (25 mL) was
added, the layers were separated, and the organic phase was extracted
with brine (2 ꢁ 10 mL), dried over anhydrous CaSO4, and concentrated
in vacuo. Chromatography of the residue on silica gel (10% EtOAc–
hexane) afforded the indicated cycloadducts 3a–i as crystalline solids.
Data
(ꢀ)-5-Carba-3,8-dimethoxypterocarpan 3a
Yield: 0.310 g (61%); mp 89–91ꢂC; 1H NMR: d 1.78–1.87 (m, 1 H),
1.97–2.04 (m, 1 H), 2.60–2.69 (m, 2 H), 3.56–3.63 (m, 1 H), 3.76 (s, 3 H),
3.78 (s, 3 H), 5.61 (d, J ¼ 8.3 Hz, 1 H), 6.65–6.69 (m, 3 H), 6.81–6.84
(m, 2 H), 7.43 (d, J ¼ 8.2 Hz, 1 H); 13C NMR: d 27.91, 27.92, 41.57,
55.30, 56.02, 81.96, 109.41, 110.78, 112.64, 112.89, 113.30, 125.86,
131.47, 132.54, 140.45, 153.50, 154.17, 159.41. Anal. calcd. for
C18H18O3: C, 76.57; H, 6.43. Found: C, 76.70; H, 6.54.
(ꢀ)-8-Benzyloxy-5-carba-3-methoxypterocarpan 3b
1
Yield: 0.322 g (50%); mp 101.5–103ꢂC; H NMR: d 1.75–1.88 (m, 1 H),
1.95–2.10 (m, 1 H), 2.58–2.80 (m, 2 H), 3.60–3.70 (m, 1 H), 3.80 (s, 3 H),
5.01 (s, 2 H), 5.63 (d, J ¼ 8.4 Hz, 1 H), 6.67–6.78 (m, 3 H), 6.86
(dd, J ¼ 8.4 Hz, J ¼ 2.6 Hz, 1 H) 6.91 (d J ¼ 2.6 Hz, 1 H), 7.31–7.49
(m, 6 H); 13C NMR: d 27.87, 27.88, 41.53, 55.31, 71.08, 81.99, 109.43,
111.97, 112.63, 113.30, 114.14, 125.84, 127.61 (2C), 127.94, 128.59 (2C),
131.46, 132.50, 137.38, 140.45, 153.37, 153.73, 159.40. Anal. calcd. for
C24H22O3: C, 80.42; H, 6.19. Found: C, 80.57; H, 6.23.
(ꢀ)-9,10-Benzo-5-carba-3,8-dimethoxypterocarpan 3c
1
Yield: 0.239 g (40%); mp 127–129ꢂC; H NMR: d 1.82–1.98 (m, 1 H),
2.03–2.17 (m, 1 H), 2.57–2.78 (m, 2 H), 3.81 (s, 3 H), 3.80–3.85 (m,
1 H), 3.99 (s, 3 H), 5.82 (d, J ¼ 8.7 Hz, 1 H), 6.69 (d, J ¼ 2.3 Hz, 1 H),
6.78 (s, 1 H), 6.88 (dd, J ¼ 8.4 Hz, J ¼ 2.5 Hz, 1 H), 7.39–7.43 (m, 2 H),
7.56 (d, J ¼ 8.4 Hz, 1 H), 7.87–7.89 (m, 1 H), 8.16–8.19 (m, 1 H);

3852
A. L. Mohr et al.
13C NMR: d 27.86, 28.03, 42.64, 55.29, 56.10, 82.13, 100.98, 112.43,
113.29, 121.56, 122.36, 122.76, 124.98, 125.51, 125.84, 126.24, 131.58,
140.90, 148.61, 150.13, 159.41, 166.17. Anal. calcd. for C22H20O3: C,
79.50; H, 6.06. Found: C, 79.74; H, 6.06.
(ꢀ)-3,8-Dimethoxypterocarpan 3d
1
Yield: 0.251 g (49%); mp 118–119ꢂC (lit.[13] mp 115–116ꢂC); H NMR: d
3.55–3.66 (m, 1 H), 3.73 (t, J ¼ 10.9 Hz, 1 H), 3.78 (s, 3 H), 3.79 (s, 3 H),
4.27 (dd, J ¼ 10.9 Hz, J ¼ 5.0 Hz, 1 H), 5.48 (d, J ¼ 6.7 Hz, 1 H), 6.46 (d,
J ¼ 2.5 Hz, 1 H), 6.63 (dd, J ¼ 8.5 Hz, J ¼ 2.5 Hz, 1 H), 6.67–6.80 (m,
2 H), 6.83 (d, J ¼ 2.5 Hz, 1 H), 7.41 (d, J ¼ 8.6 Hz, 1 H); 13C NMR: d
40.61, 55.39, 56.02, 66.23, 77.84, 101.61, 109.19, 110.15, 111.10, 112.37,
113.85, 128.10, 131.84, 153.37, 154.32, 156.50, 161.02.
(ꢀ)-8-Benzyloxy-3-methoxypterocarpan 3e
1
Yield: (0.305 g (47%); mp 135–136ꢂC; H NMR: d 3.59–3.53 (m, 1 H),
3.69 (t, J ¼ 10.9 Hz, 1 H), 3.80 (s, 3 H), 4.25 (dd, J ¼ 10.9 Hz, J ¼ 4.6 Hz,
Hz, 1 H), 5.01 (s, 2 H), 5.48 (d, J ¼ 6.7 Hz, 1 H), 6.47 (d, J ¼ 2.5 Hz, 1 H),
6.64 (dd, J ¼ 8.5 Hz, J ¼ 2.5 Hz, 1 H), 6.74–6.82 (m, 2 H), 6.91 (d,
J ¼ 2.5 Hz, 1 H), 7.33–7.45 (m, 6 H); 13C NMR: d 40.61, 55.38, 66.23,
71.09, 77.88, 101.64, 109.20, 110.16, 112.31, 112.38, 115.11, 127.53
(2C), 127.96, 128.11, 128.58, 131.84 (2C), 137.20, 153.53, 153.63,
156.53, 161.04. HRMS m=z 360.1367; calcd. for C23H20O4 360.1362.
(ꢀ)-9,10-Benzo-3,8-dimethoxypterocarpan 3f
Yield: 0.373 g (62%); mp 163–164.5ꢂC; 1H NMR: d 3.80 (s, 3 H),
3.83–3.73 (m, 2 H), 3.99 (s, 3 H), 4.37 (dd, J ¼ 9.6 Hz, J ¼ 3.6 Hz, 1 H),
5.67 (d, J ¼ 6.6 Hz, 1 H), 6.49 (d, J ¼ 2.5 Hz, 1 H), 6.69 (dd, J ¼ 8.5 Hz,
J ¼ 2.5 Hz, 1 H), 6.77 (s, 1 H), 7.43–7.48 (m, 2 H), 7.56 (d, J ¼ 8.5 Hz,
1 H), 7.94 (dd, J ¼ 6.1 Hz, J ¼ 2.3 Hz, 1 H), 8.20 (dd, J ¼ 7.2 Hz,
J ¼ 1.1 Hz, 1 H); 13C NMR: d 41.71, 55.40, 56.06, 66.66, 78.07, 100.63,
101.66, 109.19, 112.80, 118.93, 121.51, 121.25, 122.53, 125.45, 125.89,
126.18, 132.03, 148.95, 150.36, 156.70, 161.02. Anal. calcd. for
C21H18O4: C, 75.43; H, 5.43. Found: C, 75.81; H, 5.49.
(ꢀ)-8-Methoxy-3,4-methylenedioxypterocarpan 3g
Yield: 0.263 g (49%); mp 161–163ꢂC; 1H NMR: d 3.54–3.67 (m, 2 H), 3.77
(s, 3 H), 4.23 (dd, J ¼ 10.5 Hz, J ¼ 4.3 Hz, 1 H), 5.42 (d, J ¼ 6.7 Hz, 1 H),

Synthesis of Pterocarpan-Type Heterocycles
3853
5.86, (dd, J ¼ 9.2, 1.1 Hz, 2 H), 6.46 (s, 1 H), 6.69–6.76 (m, 2 H), 6.83 (d,
J ¼ 2.4 Hz, 1 H), 6.93 (s, 1 H); 13C NMR: d 40.63, 56.01, 66.46, 78.22,
98.85, 101.29, 108.79, 110.10, 111.09, 111.94, 113.88, 127.94, 142.60,
148.76, 150.85, 153.20, 154.35. Anal. calcd. for C17H14O5: C, 68.45; H,
4.73. Found: C, 68.22; H, 4.69.
(ꢀ)-3-Methoxy-8,9-methylenedioxypterocarpan 3h ((ꢀ)-pterocarpin)
Yield: 0.091 g (17%); mp 183–185ꢂC (lit. mp 168–169ꢂC[14a] and 185–
1
186ꢂC[14b]); H NMR: d 3.48–3.52 (m, 1 H), 3.66 (t, J ¼ 10.9 Hz, 1 H),
3.79 (s, 3 H), 4.23 (dd, J ¼ 10.9 Hz, J ¼ 4.9 Hz, 1 H), 5.49 (d, J ¼ 6.9 Hz,
1 H), 5.90 (dd, J ¼ 8.6 Hz, J ¼ 1.3 Hz, 2 H), 6.43 (s, 1 H), 6.47 (d, J ¼
2.5 Hz, 1 H), 6.64 (dd, J ¼ 8.5 Hz, J ¼ 2.5 Hz, 1 H), 6.72 (s, 1 H), 7.40
(d, J ¼ 8.6 Hz, 1 H); 13C NMR: d 40.20, 55.38, 66.50, 78.50, 93.83,
101.29, 101.63, 104.73, 109.17, 112.34, 117.94, 131.75, 141.70, 148.09,
154.27, 156.58, 161.05.
(ꢀ)-3,8-Dimethoxypterocarpan 3d via Nonoxidative Cycloaddition
Powdered montmorillonite K-10 clay (0.050 g) was added to a solution of
7-methoxy-2H-chromene 1b (0.090 g, 0.56 mmol) and quinone dimethyl-
monoacetal 4a (0.043 g, 0.28 mmol) in dry CH3CN (2 mL), and the
resulting suspension was stirred for 15 min. Concentration in vacuo
and chromatography of the residue on silica gel (10% EtOAc=hexane)
afforded 3d (0.050 g, 62% based on 4a).
ACKNOWLEDGMENTS
We gratefully acknowledge the donors of the Petroleum Research Fund,
administered by the American Chemical Society, for partial support of
this work. Also acknowledged are Dr. Albert V. Fratini for his assistance
in obtaining the x-ray crystal structure data and Mr. Justin Williams for
obtaining high-resolution mass spectral data.
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