New synthetic approaches to naturally occurring and unnatural pyranoflavones

Article abstract of DOI:10.1002/hlca.201200278

Total synthesis of biologically interesting natural and unnatural pyranoflavones has been accomplished starting from readily available 2,4-dihydroxyacetophenone or 2,4-dihydroxy-6-methoxyacetophenone in three steps, i.e., benzopyran formation, condensatio

Full text of DOI:10.1002/hlca.201200278

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Helvetica Chimica Acta – Vol. 96 (2013)
New Synthetic Approaches to Naturally Occurring and Unnatural
Pyranoflavones
by Likai Xia and Yong Rok Lee*
School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Korea
(phone: þ 82-53-8102529; fax: þ 82-53-8104631; e-mail: yrlee@yu.ac.kr)
Total synthesis of biologically interesting natural and unnatural pyranoflavones has been
accomplished starting from readily available 2,4-dihydroxyacetophenone or 2,4-dihydroxy-6-methoxy-
acetophenone in three steps, i.e., benzopyran formation, condensation, and cyclization reaction.
Introduction. – Pyranoflavones, an abundant subclass of flavonoids, are widely
distributed in nature [1]. They have been associated with a wide variety of biological
properties such as antimutagenic, antimicrobial, anti-ulcer, and antitumor activities,
and some plants containing these compounds are used in traditional medicines in China
and Europe [2]. Among these, pyranoflavones 1 – 3 (Fig.) were isolated from
Lonchocarpus subglaucescens [3] and L. montanus [4]. Isopongaflavone (4) was
isolated from Tephrosia tunicate [5] and T. egregia [6], and exhibited potent antifeedant
activities against Maruca testualis and Eldana saccharina [7]. Natural pyranoflavone 5
was isolated from Pongamia pinnata [8]. Unnatural atalantoflavone dimethyl ether (6)
also exhibited significant insecticidal activity [7]. This wide range of biological
properties has attracted interest in the synthesis of natural and unnatural pyrano-
flavones. Although synthetic approaches for compounds 1, 2, and 4 – 6 have been
reported [9][10], simpler and more concise synthetic routes are still needed.
Figure. Selected naturally occurring pyranoflavones 1 –5 and unnatural atalantoflavone dimethyl ether (6)
Recently, we have reported a new methodology for synthesizing a variety of
benzopyrans by ethylenediamine diacetate-catalyzed and base-mediated reactions of
1,3-dicarbonyl compounds or resorcinols with a,b-unsaturated aldehydes [11]. We also
reported convergent synthetic routes for pyranoflavones 1 and 2 starting from
cyclohexane-1,3-dione through five steps in 46 – 48% overall yield (Scheme 1) [12]. As
ꢀ 2013 Verlag Helvetica Chimica Acta AG, Zꢁrich

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645
Scheme 1
an extension of our previous work, we report herein more concise and efficient
synthetic approaches to naturally occurring pyranoflavones 1 – 5 and the unnatural
atalantoflavone dimethyl ether (6) through three-step reactions.
Results and Discussion. – The retrosynthetic strategy for the synthesis of
pyranoflavones 1 – 6 is outlined in Scheme 2. These compounds could be prepared
via cyclization of b-hydroxypyranochalcones 11 – 16. These key intermediates could be
generated from benzopyrans 9 and 10 through base-mediated condensations. The latter
could be prepared from readily available compounds 7 and 8 through benzopyran
formation reactions.
Scheme 2. Retrosynthetic Analysis for the Synthesis of Naturally Occurring Pyranoflavones 1 – 5 and
Unnatural 6
The concise total synthesis of 1 – 6 was carried out as depicted in Scheme 3.
Recently, we developed a new and useful methodology for preparing a variety of
benzopyrans using Ca(OH)2-mediated reactions of 1,3-dihydroxyxanthen-9-one to a,b-
unsaturated aldehydes [13]. By this methodology, reactions of 7 and 8 with 3-
methylbut-2-enal in the presence of 1.0 equiv. of Ca(OH)2 in refluxing MeOH for 5 –
6 h gave 9 [2e][11n] and 10 [14] in 70 and 90% yield, respectively. Reaction of 9 with an
excess of lithium diisopropylamide (LDA) in THF at ꢀ 788, followed by addition of

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Scheme 3
PhCOCl (BzCl), gave the desired compound 11 in 75% yield. Similarly, treatment of 9
and 10 with the corresponding acid chlorides afforded products 12 – 16 in 80, 72, 80, 85,
and 78% yield, respectively. Interestingly, the compounds 12 – 16 occur as two
tautomers, an enol and a keto form. The presence of the tautomeric chalcones was
readily identified by analysis of their spectroscopic data. In addition, compound 14 was
previously isolated as a natural product from the roots of Tephrosia tunicate [5], and
compound 16 was previously synthesized by Banerji et al. [15]. Several oxidative
cycloadditions of b-hydroxy chalcones to afford flavones have been already reported by
other groups utilizing several catalysts and reagents such as silica-PCl5 [16], Ga(OTf)3
[17], CuCl2/MW [18], and KHSO4 [19]. However, these reactions have a limitation due
to harsh reaction conditions and unsatisfactory yields. Recently, we have developed a
novel and efficient methodology for the synthesis of a variety of flavones by InCl3-
catalyzed or -mediated cyclization of 1,3-diketones [20]. By this methodology, we
attempted the cyclization reactions of 11 – 16 to produce 1 – 6. Treatment of 11 in the
presence of InCl3 (0.5 equiv.) in refluxing toluene for 4 h provided the desired product 1
in 95% yield. Similarly, reactions of 12 – 16 afforded 2 – 6 in 93, 94, 88, 90, and 91%
yield, respectively. The spectroscopic data of the synthetic materials 1 – 5 agreed well
with those reported for natural products in the literature [2 – 5][8].
In conclusion, a concise total synthesis of naturally occurring pyranoflavones 1 – 5
and unnatural 6 was accomplished starting from readily available 2,4-dihydroxyace-
tophenone (7) and 2,4-dihydroxy-6-methoxyacetophenone (8) via benzopyran for-
mation, followed by base-mediated condensation and subsequent oxidative cycliza-
tions. These synthetic approaches provide rapid routes to biologically interesting
pyranoflavones.

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Experimental Part
General. All experiments were carried out under N2. Anal. TLC: Merck precoated silica-gel plates
(SiO2; Art. 5554) with a fluorescent indicator. Flash column chromatography (FC): SiO2 9385 (Merck).
1
IR Spectra: BioRad FTS 3000 spectrophotometer; ˜n in cmꢀ1. H- and 13C-NMR spectra: Bruker Model
ARX (300 and 75 MHz, resp.) spectrometer in CDCl3 or (D6)DMSO as the solvent; d in ppm rel. to
Me4Si as internal standard, J in Hz. HR-MS: Jeol JMS 700 spectrometer at the Korea Basic Science
Institute; in m/z.
General Procedure for the Synthesis of Benzopyrans 9 and 10. To a mixture of a ketone 7 or 8
(10.0 mmol) and 3-methylbut-2-enal (11.0 mmol) in anh. MeOH (30 ml), Ca(OH)2 (30.0 mmol) was
added at r.t. The mixture was heated to reflux for 10 h, and the progress was monitored by TLC. After
removal of the solvent, 2n HCl (20 ml) was added to the residue, and the mixture was extracted with
AcOEt (3 ꢁ 30 ml). The combined org. layer was washed with H2O (30 ml) and dried (MgSO4).
Removal of solvent and purification by CC (SiO2; hexane/AcOEt 7:1) gave benzopyrans 9 and 10.
Demethylisoencecalin (¼1-(5-Hydroxy-2,2-dimethyl-2H-1-benzopyran-6-yl)ethanone; 9) [2e][11n].
Yield: 70%. M.p. 102 – 1038.
Isoevodionol (¼1-(5-Hydroxy-7-methoxy-2,2-dimethyl-2H-1-benzopyran-6-yl)ethanone; 10) [14].
Yield: 90%. M.p. 128 – 1298. IR (KBr): 2924, 2855, 1620, 1464, 1362, 1269, 1206, 1159, 1124, 891, 831, 731.
1H-NMR (CDCl3): 14.28 (s, 1 H); 6.62 (d, J ¼ 10.0, 1 H); 5.85 (s, 1 H); 5.38 (d, J ¼ 10.0, 1 H); 3.81 (s,
3 H); 2.56 (s, 3 H); 1.41 (s, 6 H). 13C-NMR (CDCl3): 203.0; 162.8; 161.7; 160.0; 125.2; 115.9; 105.5; 102.5;
90.5; 78.0; 55.4; 32.9; 28.2; 28.2. HR-MS: 248.1047 (Mþ, C14H16O4þ ; calc. 248.1049).
General Procedure for Synthesis of Compounds 11 – 16. To a mixture of 9 or 10 (1.0 mmol) in anh.
THF (10 ml) at ꢀ 788 was added LDA (3.0 mmol), and the mixture was allowed to warm to ꢀ 258 for
1 h. After cooling to ꢀ 788, corresponding benzoyl chloride (1.2 mmol) in dry THF (2 ml) was added
during 20 min. The resulting mixture was allowed to warm to 08 during 3 h, and a sat. aq. NH4Cl soln.
(20 ml) was added. The mixture was extracted with AcOEt (20 ml ꢁ 3). The combined org. layers were
washed with H2O (30 ml), dried (MgSO4), and the solvent was removed in a rotary evaporator. The
residue was purified by FC (SiO2; hexane/AcOEt 9 :1) to give 11 – 16.
(2Z)-3-Hydroxy-1-(5-hydroxy-2,2-dimethyl-2H-1-benzopyran-6-yl)-3-phenylprop-2-en-1-one (11).
Yield: 75%. M.p. 65 – 678. IR (KBr): 3061, 2975, 2929, 1602, 1571, 1488, 1336, 1270, 1124, 1077, 896,
767, 690, 626. 1H-NMR (CDCl3): enol form: 15.36 (s, OH); 12.81 (s, OH); 7.89 (d, J ¼ 7.8, 2 H); 7.55 (d,
J ¼ 8.7, 1 H); 7.52 – 7.42 (m, 3 H); 6.73 (d, J ¼ 10.2, 1 H); 6.67 (s, 1 H); 6.36 (d, J ¼ 8.7, 1 H); 5.57 (d, J ¼
9.9, 1 H); 1.45 (s, 6 H); keto form: 12.99 (s, OH); 7.99 (d, J ¼ 7.8, 2 H); 7.55 (d, J ¼ 8.7, 1 H); 7.52 – 7.44 (m,
3 H); 6.71 (d, J ¼ 10.2, 1 H); 6.31 (d, J ¼ 8.7, 1 H); 5.56 (d, J ¼ 10.2, 1 H); 4.51 (s, 2 H); 1.43 (s, 6 H).
13C-NMR (CDCl3): enol form: 194.5; 175.6; 159.5; 159.4; 133.8; 132.0; 129.5; 128.6; 128.6; 128.1; 126.5;
126.5; 115.9; 112.4; 109.6; 108.5; 91.8; 77.6; 28.3; 28.3; keto form: 202.6; 193.8; 159.8; 159.5; 136.1; 133.7;
131.9; 128.8; 128.2; 128.2; 126.5; 126.5; 115.7; 115.5; 108.8; 108.2; 77.8; 49.8; 28.2; 28.2. HR-MS: 322.1202
(Mþ, C20H18Oþ4 ; calc. 322.1205).
(2Z)-3-(1,3-Benzodioxol-5-yl)-3-hydroxy-1-(5-hydroxy-2,2-dimethyl-2H-1-benzopyran-6-yl)prop-2-
en-1-one (12). Yield: 80%. M.p. 114 – 1158. IR (KBr): 3076, 2973, 2922, 1734, 1674, 1584, 1486, 1447, 1329,
1
1246, 1117, 1042, 804, 780. H-NMR (CDCl3): enol form: 15.34 (s, OH); 12.54 (s, OH); 7.37 (d, J ¼ 9.0,
1 H); 7.33 (d, J ¼ 9.3, 1 H); 7.17 (s, 1 H); 6.69 (d, J ¼ 8.4, 1 H); 6.60 (d, J ¼ 9.9, 1 H); 6.39 (s, 1 H); 6.22 (d,
J ¼ 9.0, 1 H); 5.88 (s, 2 H); 5.45 (d, J ¼ 9.9, 1 H); 1.34 (s, 6 H); keto form: 12.67 (s, OH); 7.48 – 7.43 (m,
2 H); 6.69 (d, J ¼ 8.4, 1 H); 6.55 (d, J ¼ 10.2, 1 H); 6.39 (s, 1 H); 6.21 (d, J ¼ 9.0, 1 H); 5.89 (s, 2 H); 5.44
(d, J ¼ 9.9, 1 H); 4.30 (s, 2 H); 1.32 (s, 6 H). 13C-NMR (CDCl3): enol form: 193.7; 175.2; 159.3; 159.1;
150.9; 148.0; 131.9; 129.2; 127.9; 121.9; 115.8; 112.3; 109.4; 108.3; 108.1; 106.4; 101.7; 90.7; 77.4; 28.1; 28.1;
keto form: 198.0; 191.5; 160.1; 159.8; 152.3; 148.2; 130.8; 128.1; 127.5; 125.5; 115.4; 113.4; 109.0; 108.7;
108.1; 107.8; 101.9; 77.8; 49.5; 28.2; 28.2. HR-MS: 366.1105 (Mþ, C21H18O6þ ; calc. 366.1103).
(2Z)-3-Hydroxy-1-(5-hydroxy-2,2-dimethyl-2H-1-benzopyran-6-yl)-3-(4-methoxyphenyl)prop-2-en-
1-one (13). Yield: 72%. M.p. 83 – 848. IR (KBr): 3056, 2972, 2932, 1734, 1673, 1598, 1509, 1488, 1442,
1
1339, 1267, 1241, 1175, 1118, 1075, 1031, 897, 803, 781, 695, 585. H-NMR (CDCl3): enol form: 15.48 (s,
OH); 12.69 (s, OH); 7.82 (d, J ¼ 8.4, 2 H); 7.51 (d, J ¼ 8.1, 1 H); 6.89 (d, J ¼ 8.1, 2 H); 6.72 (d, J ¼ 9.9,
1 H); 6.56 (s, 1 H); 6.34 (d, J ¼ 8.4, 1 H); 5.55 (d, J ¼ 9.9, 1 H); 3.81 (s, 3 H); 1.34 (s, 6 H); keto form: 12.85

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(s, OH); 7.94 (d, J ¼ 8.4, 2 H); 7.54 (d, J ¼ 7.8, 1 H); 6.89 (d, J ¼ 8.1, 2 H); 6.66 (d, J ¼ 9.9, 1 H); 6.31 (d,
J ¼ 7.8, 1 H); 5.54 (d, J ¼ 9.6, 1 H); 4.42 (s, 2 H); 3.81 (s, 3 H); 1.42 (s, 6 H). 13C-NMR (CDCl3): enol
form: 193.8; 175.7; 162.7; 159.3; 159.0; 131.1; 129.2; 128.4; 128.4; 125.8; 115.9; 113.9; 113.9; 112.4; 109.5;
108.3; 90.4; 77.4; 55.3; 28.1; 28.1; keto form: 198.2; 191.9; 163.9; 160.1; 159.8; 132.0; 129.1; 128.1; 128.0;
128.0; 115.5; 113.8; 113.8; 113.5; 109.0; 108.7; 77.8; 55.4; 49.5; 28.2; 28.2. HR-MS: 352.1307 (Mþ,
C21H20Oþ5 ; calc. 352.1311).
(2Z)-3-Hydroxy-1-(5-hydroxy-7-methoxy-2,2-dimethyl-2H-1-benzopyran-6-yl)-3-phenylprop-2-en-
1-one (14) [21]. Yield: 80%. M.p. 122 – 1238. IR (KBr): 3069, 2975, 2936, 1596, 1568, 1462, 1426, 1389,
1274, 1199, 1149, 1119, 832, 766. 1H-NMR (CDCl3): enol form: 15.41 (s, OH); 13.65 (s, OH); 7.86 (d, J ¼
6.6, 2 H); 7.46 – 7.44 (m, 3 H); 7.30 (s, 1 H); 6.67 (d, J ¼ 9.9, 1 H); 5.93 (s, 1 H); 5.45 (d, J ¼ 10.2, 1 H); 3.90
(s, 3 H); 1.44 (s, 6 H); keto form: 13.90 (s, OH); 8.05 (d, J ¼ 8.4, 2 H); 7.44 – 7.37 (m, 3 H); 6.58 (d, J ¼ 9.9,
1 H); 5.71 (s, 1 H); 5.37 (d, J ¼ 9.9, 1 H); 4.46 (s, 2 H); 3.35 (s, 3 H); 1.35 (s, 6 H). 13C-NMR (CDCl3): enol
form: 193.8; 175.5; 161.9; 161.2; 159.6; 134.4; 131.6; 128.6; 128.6; 126.6; 126.6; 125.4; 116.1; 104.2; 103.2;
98.2; 91.8; 78.0; 55.9; 28.3; 28.3; keto form: 198.5; 193.7; 162.0; 160.7; 154.4; 133.7; 130.1; 130.1; 128.7;
128.4; 128.4; 125.4; 115.8; 105.2; 103.0; 91.2; 78.3; 55.4; 54.7; 28.3; 28.3. HR-MS: 352.1309 (Mþ,
C21H20Oþ5 ; calc. 352.1311).
(2Z)-3-(1,3-Benzodioxol-5-yl)-3-hydroxy-1-(5-hydroxy-7-methoxy-2,2-dimethyl-2H-1-benzopyran-
6-yl)prop-2-en-1-one (15). Yield: 85%. M.p. 179 – 1808. IR (KBr): 3059, 2974, 2921, 1718, 1676, 1610,
1585, 1485, 1445, 1366, 1252, 1204, 1149, 1120, 1039, 933, 884, 814, 735, 631, 567. 1H-NMR (CDCl3): enol
form: 15.74 (s, OH); 13.71 (s, OH); 7.70 (d, J ¼ 8.1, 1 H); 7.60 (s, 1 H); 6.78 (d, J ¼ 8.1, 1 H); 6.88 (d, J ¼
9.9, 1 H); 6.09 (br. s, 3 H); 5.84 (s, 1 H); 5.32 (d, J ¼ 9.9, 1 H); 3.96 (s, 3 H); 1.50 (s, 6 H); keto form: 14.03
(s, OH); 7.59 (d, J ¼ 8.1, 1 H); 7.48 (s, 1 H); 6.93 (d, J ¼ 8.1, 1 H); 6.69 (d, J ¼ 9.9, 1 H); 6.11 (s, 2 H); 5.84
(s, 1 H); 5.50 (d, J ¼ 9.9, 1 H); 4.50 (s, 2 H); 3.54 (s, 3 H); 1.48 (s, 6 H). 13C-NMR (CDCl3): enol form:
192.9; 175.3; 166.2; 161.8; 150.7; 128.3; 125.1; 121.9; 116.0; 109.0; 108.2; 108.0; 106.5; 105.2; 102.8; 101.7;
97.1; 91.6; 77.9; 55.8; 28.3; 28.3; keto form: 198.6; 192.7; 161.9; 161.9; 160.6; 151.8; 148.2; 131.3; 125.4;
124.3; 115.7; 107.9; 107.6; 105.2; 102.8; 101.9; 91.2; 78.2; 55.4; 54.4; 28.3; 28.3. HR-MS: 396.1207 (Mþ,
C22H20Oþ7 ; calc. 396.1209).
(2Z)-3-Hydroxy-1-(5-hydroxy-7-methoxy-2,2-dimethyl-2H-1-benzopyran-6-yl)-3-(4-methoxyphen-
yl)-2-propen-1-one (16) [15]. Yield: 78%. M.p. 142 – 1448. IR (KBr): 3058, 2973, 2934, 1675, 1603, 1510,
1460, 1422, 1321, 1292, 1263, 1213, 1167, 1147, 1122, 1028, 830, 735. 1H-NMR (CDCl3): enol form: 15.68 (s,
OH); 13.69 (s, OH); 7.84 (d, J ¼ 9.0, 2 H); 7.21 (s, 1 H); 6.92 (d, J ¼ 9.0, 2 H); 6.65 (d, J ¼ 9.9, 1 H); 5.90 (s,
1 H); 5.43 (d, J ¼ 9.9, 1 H); 3.88 (s, 3 H); 3.85 (s, 3 H); 1.42 (s, 6 H); keto form: 14.04 (s, OH); 7.91 (d, J ¼
8.7, 2 H); 6.95 (d, J ¼ 8.7, 2 H); 6.64 (d, J ¼ 9.9, 1 H); 5.77 (s, 1 H); 5.44 (d, J ¼ 9.9, 1 H); 4.46 (s, 2 H); 3.86
(s, 3 H); 3.42 (s, 3 H); 1.41 (s, 6 H). 13C-NMR (CDCl3): enol form: 192.9; 175.9; 162.6; 160.9; 160.6; 159.2;
129.6; 129.6; 126.5; 126.0; 116.1; 113.8; 113.8; 104.1; 103.2; 96.9; 91.7; 77.9; 55.8; 55.4 ; 26.2; 26.2; keto
form: 198.8; 193.2; 163.5; 161.9; 161.9; 160.5; 130.2; 130.2; 129.4; 125.3; 115.7; 113.7; 113.7; 105.1; 102.7;
91.1; 78.1; 55.3; 55.3; 54.3; 28.2; 28.2. HR-MS: 382.1414 (Mþ, C22H22O6þ ; calc. 382.1416).
General Procedure for Synthesis of Flavones 1 – 6. InCl3 (0.25 mmol) was added to a stirred soln. of
11 – 16 (0.5 mmol) in dry toluene (10 ml). The mixture was heated to reflux for 5 h, and the progress was
monitored by TLC. After removal of the solvent, the residue was purified by FC (SiO2; hexane/EtOAc
1:1) to give 1 – 6.
8,8-Dimethyl-2-phenyl-4H,8H-benzo[1,2-b:3,4-b’]dipyran-4-one (1) [12]. Yield: 95%. M.p. 134 –
1358. IR (KBr): 3055, 2984, 2920, 2851, 1647, 1591, 1576, 1441, 1397, 1381, 1366, 1215, 1190, 1130, 1113,
1080, 1030, 839. 1H-NMR (CDCl3): 7.92 (d, J ¼ 8.7, 1 H); 7.82 – 7.80 (m, 2 H); 7.46 – 7.44 (m, 3 H); 6.85 (d,
J ¼ 9.9, 1 H); 6.79 (d, J ¼ 8.4, 1 H); 6.69 (s, 1 H); 5.70 (d, J ¼ 9.9, 1 H); 1.46 (s, 6 H). 13C-NMR (CDCl3):
177.7; 162.3; 157.3; 152.1; 131.7; 131.3; 130.3; 128.9; 128.9; 125.9; 125.9; 125.8; 117.6; 115.0; 115.0; 109.3;
107.1; 77.6; 28.0; 28.0.
2-(1,3-Benzodioxol-5-yl)-8,8-dimethyl-4H,8H-benzo[1,2-b :3,4-b’]dipyran-4-one (2) [12]. Yield:
93%. M.p. 230 – 2328. IR (KBr): 2982, 2920, 1640, 1584, 1503, 1449, 1395, 1381, 1337, 1294, 1254, 1215,
1115, 1076, 1036, 930, 862. 1H-NMR (CDCl3/(D6)DMSO): 7.77 (d, J ¼ 8.4, 1 H); 7.27 (d, J ¼ 7.2, 1 H); 7.12
(s, 1 H); 6.76 – 6.65 (m, 3 H); 6.43 (s, 1 H); 5.91 (s, 2 H); 5.60 (d, J ¼ 10.2, 1 H); 1.35 (s, 6 H). 13C-NMR
(CDCl3): 177.3; 161.8; 157.0; 151.7; 150.1; 148.1; 130.2; 125.4; 125.4; 120.7; 117.3; 114.7; 114.6; 109.0; 108.4;
105.9; 105.6; 101.6; 77.4; 27.7; 27.7.

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2-(4-Methoxyphenyl)-8,8-dimethyl-4H,8H-benzo[1,2-b :3,4-b’]dipyran-4-one (3). Yield: 94%. M.p.
191 – 1928. IR (KBr): 3072, 2975, 2933, 1629, 1599, 1511, 1429, 1394, 1258, 1181, 1117, 1083, 1032, 832, 739.
1H-NMR (CDCl3/(D6)DMSO): 7.80 (d, J ¼ 8.7, 1 H); 7.73 (d, J ¼ 7.8, 2 H); 6.91 (d, J ¼ 8.1, 2 H); 6.80 (d,
J ¼ 9.9, 1 H); 6.72 (d, J ¼ 8.7, 1 H); 6.52 (s, 1 H); 5.69 (d, J ¼ 9.9, 1 H) 3.77 (s, 3 H); 1.42 (s, 6 H). 13C-NMR
(CDCl3): 176.6; 161.6; 161.4; 156.4; 151.2; 129.7; 129.7; 126.8; 124.7; 123.0; 116.7; 114.2; 114.0; 113.6;
113.6; 108.6; 104.7; 76.8; 54.6; 27.2; 27.2. HR-MS: 334.1201 (Mþ, C21H18O5þ ; calc. 334.1205).
Isopongaflavone (¼ 5-Methoxy-8,8-dimethyl-2-phenyl-4H,8H-benzo[1,2-b:3,4-b’]dipyran-4-one; 4)
[5]. Yield: 88%. M.p. 213 – 2148. IR (KBr): 3073, 2976, 1639, 1585, 1450, 1352, 1203, 1124, 848, 772, 734,
691. 1H-NMR (CDCl3): 7.87 (d, J ¼ 7.2, 2 H); 7.61 – 7.48 (m, 3 H); 7.36 (s, 1 H); 6.76 (d, J ¼ 9.9, 1 H); 6.43
(s, 1 H); 5.75 (d, J ¼ 10.2, 1 H); 4.00 (s, 3 H); 1.51 (s, 6 H). 13C-NMR (CDCl3/(D6)DMSO): 176.4; 159.6;
159.4; 157.2; 153.0; 130.7; 130.5; 128.2; 128.2; 127.4; 127.0; 125.0; 125.0; 114.3; 107.8; 101.9; 95.9; 77.3; 55.6;
27.4; 27.4. HR-MS: 334.1206 (Mþ, C21H18O4þ ; calc. 334.1205).
2-(1,3-Benzodioxol-5-yl)-5-methoxy-8,8-dimethyl-4H,8H-benzo[1,2-b:3,4-b’]dipyran-4-one (5)
[10a]. Yield: 90%. M.p. 241 – 2428. IR (KBr): 2958, 2923, 1673, 1624, 1491, 1447, 1294, 1256, 1113,
1033, 924, 769, 664, 556. 1H-NMR (CDCl3/(D6)DMSO): 7.54 (d, J ¼ 8.1, 1 H); 7.32 (s, 1 H); 6.77 (d, J ¼
8.1, 1 H); 6.49 (d, J ¼ 9.9, 1 H); 5.98 (br. s, 3 H); 5.92 (s, 1 H); 5.41 (d, J ¼ 9.9, 1 H); 3.84 (s, 3 H); 1.35 (s,
6 H). 13C-NMR (CDCl3/(D6)DMSO): 178.0; 165.9; 158.9; 158.9; 157.8; 149.9; 146.2; 124.5; 123.8; 123.5;
114.4; 108.0; 107.9; 106.4; 106.4; 100.4; 100.4; 90.5; 76.4; 55.1; 26.8; 26.8.
5-Methoxy-2-(4-methoxyphenyl)-8,8-dimethyl-4H,8H-benzo[1,2-b :3,4-b’]dipyran-4-one, (6) [15].
Yield: 91%. Mp 206 – 2078. IR (KBr): 3093, 2976, 1634, 1579, 1449, 1360, 1252, 1222, 1123, 1047, 850,
1
734. H-NMR ((D6)DMSO): 7.89 (d, J ¼ 7.2, 2 H); 7.03 (d, J ¼ 7.2, 2 H); 6.81 (d, J ¼ 9.6, 1 H); 6.57 (s,
1 H); 6.38 (s, 1 H); 5.75 (d, J ¼ 9.6, 1 H); 3.79 (br. s, 6 H); 1.40 (s, 6 H). 13C-NMR ((D6)DMSO): 176.03;
162.0; 160.2; 159.6; 157.4; 153.2; 128.4; 127.9; 127.9; 123.1; 114.9; 114.8; 114.8; 108.2; 106.8; 102.3; 96.9;
78.2; 56.5; 55.8; 28.0; 28.0. HR-MS: 364.1307 (Mþ, C22H20O5þ ; calc. 364.1311).
This research was supported by the Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012009620).
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Received May 28, 2012