Synthesis of
2-(methylthio)isoflavones in the presence of tri(3,6-dioxaheptyl)amine
Lei Yingjie, Liu Fude, Shi Xiaofeng
(Department of Chemistry & Chemical Engineering, Tianjin University of Technology,
Tianjin 300384)
Abstract: A convenient
method to synthesize four 2-(methylthio)isoflavones (5a-d) in one-pot procedure by means
of a mixture of carbon disulfide, iodomethane, and the deoxybenzoin compounds, prepared by
a Friedel-Crafts acylation of
resorcinol with arylacetic acids followed by Mitsunobu reaction, at the NaHCO3/KCl
alkaline solutions in presence of the phase transfer catalyst tri (3,6-dioxaheptyl)amine
(TDA-1) was discussed. The formation of the corresponding isoflavones was confirmed by IR,
1HNMR and elemental analysis studies, yielding in within 85-92% respectively.
Keywords: Deoxybenzoin; 2-(Methylthio)isoflavone; Synthesis; Tri (3,6-dioxaheptyl) amine.
1 INTRODUCTION
The isoflavones, mainly occurring in
species of the leguminosae family, have demonstrated numerous biological activities such
as antiviral, antiinflammatory, antiallergic, antimutagenic and anticarcinogenic
activities [1-2]. Among all the possible and potential sites of the isoflavone
molecules, the 2-postion is expected to be an ideal site for the introduction of major
diversity in the library in that a substituent could be easily introduced as a nucleophile
by means of Michael addition, even though most natural isoflavones possess no substituent
but proton or rarely a hydroxyl group at this position. Therefore, substituents with
alkylthio groups on the 2-postion, or 2-(alkylthio)isoflavones, as potential drug
candidates for hormone-dependent breast cancer have obtained much attention[3].
However, only a few
methods have been reported for the synthesis of 2-(alkylthio) isoflavones[4].
Further more, these methods suffer several disadvantages such as low yields, multiple
steps, and harsh conditions. Lee GH and Pak CS reported a one-pot synthesis of
2-methylthio isoflavones from 2'-hydroxy acetophenone analogues by using ketene
dithioacetal as a base [5]. Nevertheless, this method also requires skillful
handling of the base, low temperature and anhydrous reaction conditions. Kim YW et al.
described a convenient conversion of deoxybenzoins into 2-(alkylthio)isoflavones with
aqueous NaOH solution at room temperature in the presence of tetrabutylammonium
hydrogensulfate [6]. Anyway, it is still desirable to have new, efficient
methods for the syntheses of these compounds.
Tri(3,6-dioxaheptyl)amine(TDA-1),being
recognized as environmentally benign media, have been widely applied in many reactions as
catalysts, such as alkylation, esterification, oligomerization and carbene complexing[7-8].
Therefore, in the present study, we introduce TDA-1 as a phase transfer catalyst and
sodium bicarbonate as a base, to carry out the convenient synthesis of
2-(methylthio)-isoflavones from a mixture of carbon disulfide, iodomethane and a selected
deoxybenzoin compound, in order to supply an alternative approach to introduce of
alkylthio groups on the 2-postion within isoflavone molecules (Scheme 1).
2 EXPERIMENTAL
2.1 Materials and Instruments
All chemicals were obtained commercially
and used without further purification. Melting points were determined with an X-4
apparatus and were uncorrected. Infrared spectra were recorded with KBr pellets on a
Nicolet Inpact 170S FT-IR spectrometer in the 4000- 400 cm-1 region. Elemental
analyses for C and H were performed on a Perkin Elmer 240 analytical apparatus. 1H-NMR
spectra were measured on a Bruker DPX 300MHz in DMSO solutions with TMS as internal
standard.
Scheme 1 The synthetic route for the compounds of 5a-d
2.2 Synthesis of
2',4'-dihydroxydeoxybenzoins(3a-c)
To freshly molten anhydrous ZnCl2 (18mmol), an appropriate phenylacetic acid
(15mmol) was added with vigorous stirring and heating at about 120ºC followed by the slow addition of resorcinol (15mmol) for 3-4
hours.The reaction mixture was cooled and poured into ice water. The separated oil was
extracted with ethyl acetate twice(2×30 mL). The combined organic layer was washed with
water and then brine, dried over magnesium sulfate (MgSO4), and concentrated
under reduced pressure. The residue was purified by recrystallization from ethanol to give
the corresponding product (3a-c).
2.3 Synthesis of 4'-alkoxy-2'-hydroxydeoxybenzoins (4a-d)
To a solution of 2-aryl-1-(2,4-dihydroxy phenyl) ethanone (3a-c) (10mmol) and an
alcohol (10.5mmol) in THF (50mL), was added triphenyl phosphine (2.62g,10mmol) and
followed by diisopropyl azodicarboxylate (2mL,10mmol). The resulting yellow solution was
kept at 0ºC and stirred for 20 min. The solvent was removed under
reduced pressure, and the residue was purified by silica gel column chromatography
(eluting with ethyl acetate: petroleum ether 1:2,v/v) to give the desired deoxybenzoin (4a-d).
2.4 Synthesis of 2-(methylthio) isoflavone (5a-d)
To a solution of deoxybenzoin (4a-d) (5mmol)
in a mixture of 1mol·L-1 NaHCO3 with the same equivalents
of KCl (20 mL), was added TDA-1 (0.32 mL, 1mmol) and carbon disulfide (3 mL, 50mmol) and
followed by iodomethane (10mmol). The resulting mixture was stirred and slightly heated at
40ºC for 6 hours. The reaction mixture was poured into water (100mL) and
extracted with ethyl acetate twice (2×30 mL). The separated organics were washed with
aqueous sulfuric acid ,water and then with brine, dried over MgSO4, and
concentrated under reduced pressure. The residue was purified by silica gel column
chromatography (eluting with ethyl acetate: petroleum ether 1:1, v/v) to give the product (5a-d).
3 RESULTS AND DISCUSSION
3.1 Physical characterization of the products
The specific details of the products are listed below.
Compound 3a: using
the previous procedure and starting from phenylacetic acid, a white solid, y=85%. mp.
112-114ºC[lit[9]115ºC]. Compound 3b: using the previous procedure and
starting from p-methylphenylacetic acid, a white solid, y=88%. mp. 111-113ºC[lit[9]115-116ºC].
Compound 3c: using the previous procedure and starting from p-methoxyphenylacetic
acid, a white solid, y=87%. mp.158-160ºC[lit[9]
163ºC].
Compound 4a: using the previous procedure and starting from
1-(2,4-dihydroxyphenyl)-2-phenylethan-one and methanol, a white solid, y=88%. mp. 87-89ºC [lit[9] 92ºC]. Compound 4b: using the
previous procedure and starting from 1-(2,4-dihydroxyphenyl)-2-(4-methyl phenyl)ethanone
and methanol, a white solid, y=92%. mp. 68-70ºC[lit[9] 71-72ºC]. Compound 4c: using the previous procedure and starting
from 1-(2,4-dihydroxyphenyl)-2- (4-methoxy phenyl)ethanone and methanol, a white solid,
y=82%. mp.101-103℃[lit[9]104ºC]. Compound 4d: using the
previous procedure and starting from 1-(2,4-dihydroxyphenyl)-2-phenylethan- one and benzyl
alcohol, a white solid, y=90%. mp.102-104ºC [lit[6]104-105ºC].
Compound 5a: using the previous procedure and starting from
1-(2-hydroxy-4- methoxyphenyl)-2- phenyl ethanone, a white solid, y=90%. mp.133-135ºC[lit[6]136-139ºC]. IR(KBr) n/cm-1: 1629, 1584, 1499,
1373, 1252, 1201, 1017, 835;1H-NMR(DMSO-d6)
d: 8.15(1H,d,J=8.8Hz,5-H), 7.44-7.32(5H,m,ArH), 6.92
(1H,dd,J=8.8,2.4Hz, 6-H), 6.83(1H,d,J=2.4Hz,8-H), 3.91(3H,s,OCH3), 2.53(3H,s,
CH3). Anal. Calcd. For C17H14O3S: C, 68.44; H,
4.73. Found: C, 68.40; H, 4.71. Compound 5b: using the previous procedure and
starting from 1-(2-hydroxy -4-methoxyphenyl)- 2-(4- methylphenyl) ethanone, a white solid,
y=92%. mp. 184-186ºC[lit[6] 188-189ºC]. IR(KBr) n/cm-1: 1632, 1612, 1544,
1433, 1368, 1287, 1196, 1067, 841;1H-NMR(DMSO-d6)
d: 8.14(1H,d,J=8.8Hz,5-H), 7.38 (2H,d,J=8.0Hz,2',6'-H), 7.20(2H, d,J=8.0Hz, 3',5'-H), 6.95(1H,dd, J=8.9,2.4Hz,
6-H),6.83(1H, d,J=2.4Hz, 8-H), 3.90(3H,s,OCH3), 2.53(3H,s,CH3),
2.37(3H,s, CH3). Anal. Calcd. For C18H16O3S:
C, 69.21; H,5.16. Found: C,69.18; H,5.13. Compound 5c: using the previous procedure
and starting from 1-(2-hydroxy- 4-methoxyphenyl)-2-(4- methoxyphenyl)ethanone, a white
solid, y=92%. mp. 165-167ºC[lit[6] 169-170ºC]. IR(KBr) n/cm-1: 1616, 1585, 1540,
1437, 1374, 1286, 1198, 1022, 844;1H-NMR(DMSO-d6)
d: 8.12(1H,d, J=8.8Hz,5-H), 7.27 (2H,d,J=8.0 Hz,2', 6'-H), 7.02(2H,d, J=8.0Hz, 3',5'-H), 6.93(1H,dd,J=8.9,2.4Hz,6-H),
6.83 (1H,d,J= 2.3Hz, 8-H), 3.90(3H,s,OCH3), 3.82 (3H,s, OCH3), 2.52
(3H, s,CH3). Anal. Calcd. For C18H16O4S: C,
65.84; H, 4.91. Found: C, 65.82; H,4.89. Compound 5d: using the previous
procedure and starting from 1-[2-hydroxy -4-(phenylmethoxy)phenyl]- 2-phenylethanone, a
white solid, y=91%.mp.127-
129ºC[lit[6] 130-132ºC]. IR(KBr) n/cm-1: 1613, 1541, 1502, 1458, 1364, 1267,
1196, 1030, 943;1H-NMR(DMSO-d6)
d:
8.14(1H,d,J=8.8Hz,5-H),7.45-7.32(10H,m,ArH),7.04 (1H,dd,J=8.8, 2.3Hz, 6-H),
6.93(1H,d,J=2.3Hz,8-H), 5.15(2H,s,ArCH2), 2.52(3H,s, CH3). Anal.
Calcd. For C23H18O3S: C,73.77; H,4.85. Found: C,73.74;
H,4.84.
3.2 Synthesis approach to the title compounds
The synthesis of the target compounds was carried out, as outlined in Scheme 1.
Compound 3(a-c) was prepared from resorcinol and phenylacetic acid in the presence of zinc
chloride in good yield, which is no less than that of the boron trifluoride diethyl
etherate as the lewis acid in literature [6]. In addition, deoxybenzoins
exhibit a considerable difference in reactivity on their hydroxyl groups. Therefore, the
4-hydroxyl group of compound 3(a-c) was selectively protected with a benzyl group under
Mitsunobu reaction conditions to give a monoalkyl ether 4(a-d) and the yields were
excellent.
Cyclized reaction of the deoxybenzoins to 2-(methylthio)isoflavones was
reported to be carried out in a THF aqueous NaOH solution in the presence of
tetrabutylammonium hydrogensulfate [6]. During the course of our research, we
investigated this procedure by using sodium bicarbonate as a base and TDA-1 as a phase
transfer catalyst since we envisioned that in this condition the active methylene group
within deoxybenzoins may promote the intermediate O-aryl-S-alkyl dithiocarbonates[10]
to undergo further cyclization reaction. As expected, the reaction apparently occurred in
a stepwise manner as monitored by TLC. Interestingly, the formation of dithioacetal
intermediate is very fast but the subsequent cyclization is the rate-determining step, and
thereby requires a longer reaction time to be completed.
Anyway, total yields of the desired products 5(a-d) are
within 85-92% respectively, which is in accordance with or somewhat better than that of
the literature[6]. Therefore, this method proved to be efficient to prepare the
2-(methylthio)isoflavones, which can be used as useful intermediates to introduce various
nucleophiles at the 2-position.
4 CONCLUSION
In summary, we have described a convenient
phase transfer catalysis procedure allowing the efficient conversion of deoxybenzoins into
2-(methylthio) isoflavones in a single step at the sodium bicarbonate solution in presence
of TDA-1. This method can be very useful to generate a number of isoflavones from easily
available deoxybenzoins in a short period of time. Further studies on reaction scopes and
applications of this method are currently underway.
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相转移催化合成2-甲硫基异黄酮
雷英杰,刘福德,史小凤
(天津理工大学化学化工学院,天津,300384)
摘要 以间苯二酚和取代苯乙酸生成的脱氧安息香为原料,三(3,6-二氧杂庚基)胺
(TDA-1)为相转移催化剂, NaHCO3/KCl体系为碱性介质,研究了Mitsunobu反应制备的4′-烷氧基-2-羟基脱氧安息香与二硫化碳和碘甲烷一步法制备四种2-甲硫基异黄酮类化合物的合成工艺,其结构经IR和1HNMR以及元素分析测试技术加以确证,合成收率为85-92%。
关键词 脱氧安息香;2-甲硫基异黄酮;合成;三(3,6-二氧杂庚基)胺
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