Synthesis of thienyl-substituted dihydrofuran compounds promoted by manganese(III) acetate

Article abstract of DOI:10.1002/hlca.201200098

Radical cyclization reactions of both aliphatic 1,3-diones 1a and 1b and of cyclic 1,3-diones 1c-1e with 2-thienyl- and 3-thienyl-substituted alkenes 2a-2d in the presence of manganese(III) acetate were investigated. Thienyl-substituted dihydrofurans 3 were obtained with moderate to high yields (Table 1-3). Also, the favorable effect of the thienyl substituent on the intermediate carbocation stability was evaluated by comparison with a phenyl substituent. Copyright

Full text of DOI:10.1002/hlca.201200098

Helvetica Chimica Acta – Vol. 96 (2013)
135
Synthesis of Thienyl-Substituted Dihydrofuran Compounds Promoted by
Manganese(III) Acetate
by Emre BiÅer*a), Mehmet Yılmaz*b), E. Vildan Burgazc), and A. Tarık Pekelc)
a) TUBITAK Marmara Research Center, Energy Institute, 41470, Gebze/Kocaeli, Turkey
(phone: þ 90-262-6772811; e-mail: bicer_emre@yahoo.com)
b) Department of Chemistry, Faculty of Arts and Sciences, Kocaeli University, 41380 Umuttepe/Kocaeli,
Turkey (phone: þ 90-262-3032058; e-mail: mehmet.yilmaz@kocaeli.edu.tr)
c) Department of Chemistry, Faculty of Science, Ankara University, 06100 Tandogan/Ankara, Turkey
Radical cyclization reactions of both aliphatic 1,3-diones 1a and 1b and of cyclic 1,3-diones 1c – 1e
with 2-thienyl- and 3-thienyl-substituted alkenes 2a – 2d in the presence of manganese(III) acetate were
investigated. Thienyl-substituted dihydrofurans 3 were obtained with moderate to high yields (Table 1 –
3). Also, the favorable effect of the thienyl substituent on the intermediate carbocation stability was
evaluated by comparison with a phenyl substituent.
Introduction. – Previously, we have reported the formation of furan [1] and
dihydrofuran [2] derivatives resulting from radical cyclizations with various alkenes
and alkynes. Moreover, the cyclization reactions of 4-hydroxycoumarins (¼4-hydroxy-
2H-1-benzopyran-2-ones), 2-hydroxy-1,4-naphthoquinones [3] and 3-oxopropaneni-
triles [4] with alkenes have been studied, and 4,5-dihydrofurans-3-carbonitriles have
shown antibacterial and antifungal activities [5].
In the course of our studies, we found that the radical cyclization reactions are
possible with 1,3-diones 1 and thienyl-substituted alkenes 2a – 2d mediated by
[Mn(OAc)3]. Thienyl-substituted dihydrofuran compounds were achieved as a result
of these reactions. We conceive the effect of heteroaromatic substituents in radical
cyclization reactions. The impact of this effect on the yields is also demonstrated in this
article.
Results and Discussion. – At first, we attempted to opimize the radical cyclization
reaction of acetylacetone (¼ pentane-1,3-dione; 1a) and 2-[(1E)-1-methyl-2-phenyl-
ethenyl]thiophene (2a). For this purpose, we varied the reaction temperature and the
amounts of the reagents along with that of [Mn(OAc)3]. When we modified the amount
the alkene 2a and [Mn(OAc)3], the yields of 3a differed with the amount of each
reagent (Table 1). Also the reaction temperature influenced the yield of the product 3a.
Finally, the highest yield of 3a was obtained with 2.0 equiv. of 1a and 1.0 equiv. of 2a in
the presence of 3.0 equiv. of [Mn(OAc)3] in AcOH at 608 (Table 1, Entry 3).
The mechanism we propose for the formation of 4,5-dihydrofurans involves the
interaction of [Mn(OAc)3] with 1,3-diones 1a – 1e resulting in a [MnIII(enolato)]
complex A (Scheme). Radical B is formed while MnIII is reduced to MnII. Addition of B
to alkene 2b might take place by the two Pathways i and ii. If the reaction follows
ꢀ 2013 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Helvetica Chimica Acta – Vol. 96 (2013)
Table 1. Manganese(III)-Mediated Reaction of Acetylacetone (1a) with [(1E)-1-Methyl-2-phenylethe-
nyl]thiophene (2a) under Various Conditions
Entry
Reaction Conditions [mmol]
Temperature [8]
Yield of 3a [%]
1a
2a
[Mn(OAc)3]
1
2
3
4
1.0
1.0
2.0
2.0
1.2
1.2
1.0
1.0
2.0
2.0
3.0
3.0
60
80
60
80
43
33
55
33
Pathway i, radical intermediate F is generated and finally product G is obtained. On the
other hand, if Pathway ii is followed, radical intermediate C is generated, which is then
oxidized to carbocation D with 1 equiv. of MnIII. Thereafter, the intramolecular ring
closure leads to dihydrofuran E.
Scheme. Mechanism of the Formation of 4,5-Dihydrofurans
Thus, according to the mechanism, dihydrofurans E and G might be observed
depending on the addition to the alkene. However, only dihydrofuran product E was
obtained. Other cyclization products were not found in isolable amounts. Differ-

Helvetica Chimica Acta – Vol. 96 (2013)
137
entiation of structures E and G was realized by 1H-NMR and HMBC measurement of
4,5-dihydro-5-(2-thienyl)furan derivative 3e (Table 2, Entry 5). The signal of HꢀC(5)
of 3e was observed at lower field than that of HꢀC(4), because HꢀC(5) is next to the
ether O-atom. Moreover, C(4) correlated with the Hortho atoms of the phenyl group of
3e. This indicated that the phenyl group was attached to C(4) and the 2-thienyl group to
C(5) of the dihydrofuran moiety. These results showed unambiguously that the isolated
compound possessed structure E and that the addition of B to alkene 2b followed
Pathway ii.
Treatment of 1a – 1c with 2a gave 4,5-dihydrofuran derivatives 3a – 3c, while 2-
[(1E)-phenylethenyl]thiophene (2b) gave 3d, 3e, and 3g in slightly lower yields
(Table 2, Entries 1 – 3 vs. Entries 4, 5, and 7). The higher efficiency of 2a as compared to
2b is due to the additional Me group of 2a, which increases the stability of the
intermediate carbocation of type D. Moreover, to determine the effect of the thienyl
substituent on the carbocation stability, ethyl acetoacetate (¼ethyl 3-oxobutanoate;
1b) was treated with 2b and (E)-stilbene (¼1,1’-[(1E)-ethene-1,2-diylbis[benzene]; 2c)
leading to the formation of 3e and 3f in yields of 62 and 45%, respectively (Table 2,
Entry 5 vs. Entry 6). Thus, the 2-thienyl group was stabilizing the intermediate
carbocation D more efficiently than the phenyl group. On the other hand, although the
phenyl and 2-thienyl substituents are in trans-position in alkene 2b, the phenyl and 2-
thienyl moieties turned in cis-position in 4,5-dihydro-4-phenyl-5-(2-thienyl)furans 3d,
3e, and 3g – 3i. Also in the case of 3f, the two phenyl groups were cis-positioned. These
results were established by the NOESY spectra.
The 3-[(1E)-1-methyl-2-phenylthenyl]thiophene (2d) used additionally in this
study resembles 2a, the main difference being that 2d is a 3-substituted instead of a 2-
substituted thiophene. The effect of the position of the S-atom on the stability of the
intermediate carbocation (see Scheme) was thereby investigated. Radical cyclization
reaction of 1a with 2d gave 3j in 42% yield (Table 3). Similarly, treatment of 2d with
ethyl acetoacetate (1b) and dimedone (1c) produced 4,5-dihyro-4-phenyl-5-(3-
thienyl)furans 3k (58%) and 3l (70%), respectively. The less electron-releasing nature
of the 3-thienyl than of the 2-thienyl group [6] causes the decrease in the yields of 3j – 3l
compared with those of 3a – 3c.
Conclusions. – Electron-rich thienyl-substituted alkenes 2a – 2d were used in the
cyclization reactions with 1,3-diones 1a – 1e promoted by [Mn(OAc)3]. Consequently,
we established that the thienyl substituent of the alkenes affects the stability of the
intermediate carbocation of type D via the vicinal heteroatom of the thiophene moiety
thus increasing the yields of the products 3. In other words, carbocations were formed
adjacent to the electron-rich heteroaromatic subtituent (see D in Scheme). Also, (2-
thienyl)- and (3-thienyl)-substituted alkenes 2a and 2d were compared to understand
the involvement of the position of the S-atom on carbocation stability. Hence, the
position of the substitution at the thiophene moiety of alkenes 2 influences the yields of
the products 3.

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Helvetica Chimica Acta – Vol. 96 (2013)
Table 2. Reaction of 2-[(1E)-1-Methyl-2-phenylethenyl]thiophene (2a), 2-[(1E)-2-Phenylethenyl)thio-
phene (2b), and (E)-Stilbene (2c) with 1,3-Diones 1
Entry
1
1,3-Dionesa)
Alkeneb)
Product
Yield [%]
55
1a
2a
3a
2
3
1b
1c
2a
2a
3b
3c
64
76
4
5
6
7
1a
1b
1b
1c
2b
2b
2c
2b
3d
3e
3f
3g
53
62
45
71
8
9
1d
1e
2b
2b
3h
3i
56
66
a
)
,
,
,
,
1a
1b
1c
1d
1e
b
)
,
,
2a
2b
2c

Helvetica Chimica Acta – Vol. 96 (2013)
139
Table 3. Reaction of 3-[(1E)-1-Methyl-2-phenylethenyl]thiophene (2d) with 1,3-Diones
1,3-Dionea)
Alkeneb)
Product
Yield [%]
42
1a
2d
3j
1b
1c
2d
2d
3k
3l
58
70
b
a) See Footnote a of Table 2.
)
2d
Experimental Part
General. Acetylacetone (¼ pentane-2,4-dione; 1a), ethyl acetoacetate (¼ethyl 3-oxobutanoate; 1b),
dimedone (¼ 5,5-dimethylcyclohexane-1,3-dione; 1c), 5-phenylcyclohexane-1,3-dione (1d), and cyclo-
hexane-1,3-dione (1e) are commercially available and were purchased from Merck. The 2-[(1E)-1-
methyl-2-phenylethenyl]thiophene (2a) [7], 2-[(1E)-1-phenylethenyl)thiophene (2b) [8], (E)-stilbene
(2c), and 3-[(1E)-1-methyl-2-phenylethenyl]thiophene (2d) were prepared as described in [9] (see
below). All conjugated alkenes were freshly prepared before being used in the radical cyclizations.
Column chromatography (CC): silica gel 60 (SiO2, 40 – 60 mm; Merck). Prep. TLC: silica gel PF254–366
(SiO2; Merck), 2 mm thickness on 20 ꢁ 20 cm plates. TLC: aluminium-packed silica gel plates (SiO2;
Merck). M.p.: Gallenkamp capillary melting point apparatus. IR Spectra (KBr disc): Matson-1000 FT-IR
spectrometer; 400 – 4000 cmꢀ1 range with 4 cmꢀ1 resolution; ˜n in cmꢀ1 1H- and 13C-NMR Spectra:
.
Bruker-DPX-400 MHz high performance digital FT-NMR spectrometer; at 400 (1H) and 100 MHz (13C);
in CDCl3; d in ppm rel. to Me4Si as internal standard, J in Hz; Tph ¼ thienyl. MS: Micromass-UK-
Platform-II spectrophotometer; in m/z (rel. %). Elemental analyses: Leco 932 CHNS-O instrument.
Substituted Alkenes 2a – 2d: General Procedure. NaH (22 mmol, in 60% mineral oil) was added to a
soln. of benzyltriphenylphosphonium bromide (21 mmol) in anh. THF. This mixture was kept at 608 for
1 h, and then the corresponding ketone or aldehyde (20 mmol) was added dropwise under stirring and
cooling in ice maintaining the temp. between 58 and 108. The temp. was then rapidly raised to reflux temp.
for 2 h and the mixture kept overnight at r.t. The precipitated NaBr was then removed by filtering
through a funnel. The filtrate was concentrated, and the residue was extracted with hexane (5 ꢁ 20 ml).
The org. phase was dried (Na2SO4), and concentrated, and the crude product purified by CC (hexane).
Dihydrofurans: General Procedure. Oxidative cyclization reactions were carried out with 1/2/
[Mn(OAc)3] · 2 H2O in a molar ratio of 2 :1:3. [Mn(OAc)3] · 2 H2O (98%) was prepared electrochemi-
cally according to [10].
Thus, [Mn(OAc)3] · 2 H2O (0.83 g, 3 mmol) in AcOH (20 ml) was heated under N2 to 808 until it
dissolved. Thereafter, the soln. was cooled to 608, and a soln. of 1 (2 mmol) and 2 (1 mmol) in AcOH
(5 ml) was added. The reaction was completed when the initial dark brown soln. had changed to red. H2O
(20 ml) was added and the mixture extracted with CHCl3 (3 ꢁ 20 ml). The combined org. phase was
neutralized with sat. NaHCO3 soln., dried (Na2SO4) and concentrated and the crude product purified by
CC or prep. TLC (SiO2, hexane/AcOEt).

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Helvetica Chimica Acta – Vol. 96 (2013)
1-[4,5-Dihydro-2,5-dimethyl-4-phenyl-5-(2-thienyl)furan-3-yl]ethanone (3a): Yield 164 mg (55%).
Yellow oil. IR: 3063, 3027 (arom. CꢀH), 2981, 2932 (aliph. CꢀH), 1699 (C¼O), 1649 (C¼C). 1H-NMR:
7.26 – 7.41 (m, 5 arom. H, HꢀC(5) of Tph); 7.07 (d, J ¼ 3.6, HꢀC(3) of Tph); 7.02 (dd, J ¼ 5.0, 3.6, HꢀC(4)
of Tph); 4.56 (s, HꢀC(4)); 2.47 (d, J ¼ 1.3, MeꢀC(2)); 1.88 (s, MeCOꢀC(3)); 1.30 (s, MeꢀC(5)).
13C-NMR: 195.6 (C¼O); 168.3 (C(2)); 151.9; 139.3; 129.0; 127.9; 127.0; 124.4; 122.7; 115.4 (C(3)); 90.4
(C(5)); 61.2 (C(4)); 29.7; 26.1; 15.4. MS: 299 (9, [M þ H]þ), 298 (23, Mþ), 280 (6, [M ꢀ H2O]þ), 255 (10,
[M ꢀ C2H3O]þ), 172 (9, [M ꢀ C4H3S ꢀ C2H3O]þ), 111 (17, C5H3OSþ), 91 (6, C6H5CH2þ ), 83 (3, C4H3Sþ),
77 (13, C6H5þ). Anal. calc. for C18H18O2S (298.40): C 72.5, H 6.1, S 10.8; found: C 72.3, H 6.0, S 10.6.
Ethyl 4,5-Dihydro-2,5-dimethyl-4-phenyl-5-(2-thienyl)furan-3-carboxylate (3b): Yield 210 mg
(64%). Yellow oil. IR: 3063 (arom. CꢀH), 2988, 2928 (aliph. CꢀH), 1670 (C¼C), 1598 (C¼O).
1H-NMR: 7.25 – 7.37 (m, 5 arom. H, HꢀC(5) of Tph); 7.08 (d, J ¼ 3.6, HꢀC(3) of Tph); 7.02 (dd, J ¼ 5.0,
3.6, HꢀC(4) of Tph); 4.54 (s, HꢀC(4)); 3.93 – 4.08 (m, MeCH2O); 2.45 (d, J ¼ 1.3, MeꢀC(2)); 1.31 (s,
MeꢀC(5)); 1.02 (t, J ¼ 7.1, MeCH2O). 13C-NMR: 167.9 (C¼O); 165.8 (C(2)); 152.2; 139.7; 128.8; 128.5;
127.4; 127.0; 124.3; 122.6; 106.5 (C(3)); 90.1 (C(5)); 60.4; 59.6; 46.3; 26.2; 14.6; 14.3. MS: 329 (7, [M þ
H]þ), 328 (22, Mþ), 313 (2, [M ꢀ Me]þ), 285 (8, [M ꢀ C3H5O2]þ), 239 (12, [M ꢀ C7H5]þ), 128 (19,
C13H12Sþ), 111 (12, C5H3OSþ), 77 (13, C6H5þ), 43 (100, C3H5Oþ2 ). Anal. calc. for C19H20O3S (328.42): C
69.5, H 6.1, S 9.7; found: C 69.3, H 6.0, S 9.6.
3,5,6,7-Tetrahydro-2,6,6-trimethyl-3-phenyl-2-(2-thienyl)benzofuran-4(2H)-one (3c): Yield 257 mg
1
(76%). Yellow oil. IR: 3022 (arom. CꢀH), 2955, 2922 (aliph. CꢀH), 1641 (C¼O). H-NMR: 7.28 – 7.35
(m, 4 arom. H); 7.13 – 7.15 (m, 1 arom. H, HꢀC(5) of Tph); 7.07 (d, J ¼ 3.6, HꢀC(3) of Tph); 7.02 (dd, J ¼
5.0, 3.6, HꢀC(4) of Tph); 4.60 (s, HꢀC(3)); 2.57 (dd, J ¼ 16.5, 2.0, HꢀC(7)); 2.32 (s, 2 H); 1.35 (s,
MeꢀC(2)); 1.28 (s, MeꢀC(6)); 1.19 (s, MeꢀC(6)). MS: 339 (26, [M þ H]þ), 338 (100, Mþ), 323 (20, [M ꢀ
Me]þ), 320 (6, [M ꢀ H2O]þ), 254 (14, [M ꢀ C4H4S]þ), 212 (5, [M ꢀ C6H6O]þ), 178 (10, [M ꢀ C4H3S ꢀ
C6H5]þ), 152 (12, [M ꢀ C12H10S]þ), 91 (18, C6H5CH2þ ), 83 (60, C4H3Sþ), 77 (25, C6H5þ). Anal. calc. for
C21H22O2S (338.46): C 74.5, H 6.6, S 9.5; found: C 74.4, H 6.7, S 9.4.
1-[4,5-Dihydro-2-methyl-4-phenyl-5-(2-thienyl)furan-3-yl]ethanone (3d): Yield 151 mg (53%). Yel-
low oil. IR: 3082, 3028 (arom. CꢀH), 2922 (aliph. CꢀH), 1754, 1716, 1670 (C¼O). 1H-NMR: 7.12 – 7.27
(m, 5 arom. H, HꢀC(5) of Tph); 6.92 (d, J ¼ 3.1, HꢀC(3) of Tph); 6.89 (dd, J ¼ 4.8, 3.6, HꢀC(4) of Tph);
5.42 (d, J ¼ 5.3, HꢀC(5)); 4.36 (d, J ¼ 5.3, HꢀC(4)); 2.33 (s, MeꢀC(2)); 1.81 (s, MeCOꢀC(3)). 13C-NMR:
194.9 (C¼O); 168.1 (C(2)); 143.4; 142.5; 129.1; 127.5; 127.3; 125.8; 125.2; 115.2 (C(3)); 87.7 (C(5)); 58.1
(C(4)); 29.6; 15.1. MS: 285 (1, [M þ H]þ), 284 (2, Mþ), 241 (2, [M ꢀ C2H3O]þ), 184 (1, [M ꢀ C5H8O2]þ),
139 (0.5, [M ꢀ C10H4Oþ]), 91 (1, C6H5CHþ2 ), 83 (1, C4H3Sþ), 77 (3, C6H5þ), 43 (100, C2H3Oþ). Anal. calc.
for C17H16O2S (284.37): C 71.8, H 5.7, S 11.3; found: C 71.7, H 5.6, S 11.2.
Ethyl 4,5-Dihydro-2-methyl-4-phenyl-5-(2-thienyl)furan-3-carboxylate (3e): Yield 195 mg (62%).
Yellow oil. IR: 3055, 3022 (arom. CꢀH), 2922, 2853 (aliph. CꢀH), 1741 (C¼O), 1702, 1648 (C¼C).
1H-NMR: 7.16 – 7.29 (m, 5 arom. H, HꢀC(5) of Tph); 6.98 (d, J ¼ 3.3, HꢀC(3) of Tph); 6.93 (dd, J ¼ 4.9,
3.3, HꢀC(4) of Tph); 5.54 (d, J ¼ 5.5, HꢀC(5)); 4.36 (d, J ¼ 5.5, HꢀC(4)); 3.90 – 4.00 (m, MeCH2O); 2.33
(s, MeꢀC(2)); 0.99 (t, J ¼ 7.1, MeCH2O). MS: 315 (8, [M þ H]þ), 314 (3, Mþ), 268 (11, [M ꢀ C2H5OH]þ),
128 (7, [M ꢀ C6H8O3]þ), 91 (4, C6H5CHþ2 ), 83 (4, C4H3Sþ), 77 (10, C6H5þ). Anal. calc. for C18H18O3S
(314.40): C 68.8, H 5.8, S 10.2; found: C 68.7, H 5.7, S 10.1.
Ethyl 4,5-Dihydro-2-methyl-4,5-diphenylfuran-3-carboxylate (3f): Yield 139 mg (45%). Yellow oil.
1H-NMR: 7.20 – 7.40 (m, 10 arom. H); 5.40 (d, J ¼ 5.6, HꢀC(5)); 4.20 (dd, J ¼ 5.6, 1.6, HꢀC(4)); 3.90 – 4.00
(m, MeCH2O); 2.40 (s, MeꢀC(2)); 1.00 (t, J ¼ 7.2, MeCH2O). MS: 309 (100, [M þ H]þ). Anal. calc. for
C20H20O3 (308.37): C 77.9, H 6.5; found: C 78.0, H 6.6.
3,5,6,7-Tetrahydro-6,6-dimethyl-3-phenyl-2-(2-thienyl)benzofuran-4(2H)-one (3g): Yield 230 mg
(71%). Yellow oil. IR: 3053, 3020 (arom. CꢀH), 2953, 2872 (aliph. CꢀH), 1655 (C¼O), 1631 (C¼C).
1H-NMR: 7.14 – 7.29 (m, 5 arom. H, HꢀC(5) of Tph); 7.01 (d, J ¼ 3.4, HꢀC(3) of Tph); 6.95 (dd, J ¼ 5.0,
3.5, HꢀC(4) of Tph); 5.68 (d, J ¼ 5.4, HꢀC(2)); 4.43 (d, J ¼ 5.4, HꢀC(3)); 2.41 (dd, J ¼ 12.0, 2.0, HꢀC(7));
2.23 (s, 2 H); 1.14 (s, MeꢀC(6)); 1.11 (s, MeꢀC(6)). 13C-NMR: 193.6 (C¼O); 175.6 (C(7a)); 143.1; 141.8;
129.1; 127.5; 127.3; 127.2; 126.5; 125.9; 114.9 (C(3a)); 90.6 (C(2)); 54.1; 51.5; 46.3; 38.3; 34.5; 29.0; 28.9.
MS: 325 (1, [M þ H]þ), 324 (3, Mþ), 306 (2, [M ꢀ H2O]þ), 291 (1, [M ꢀ H2O ꢀ CH3]þ), 233 (7, [M ꢀ
C6H5CH2]þ), 164 (3, [M ꢀ C6H5 ꢀ C4H3S]þ), 111 (14, C5H3OSþ), 91 (11, C6H5CHþ2 ), 83 (100, C4H3Sþ).
Anal. calc. for C20H20O2S (324.44): C 74.0, H 6.2, S 9.8; found: C 73.9, H 6.1, S 9.7.

Helvetica Chimica Acta – Vol. 96 (2013)
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3,5,6,7-Tetrahydro-3,6-diphenyl-2-(2-thienyl)benzofuran-4(2H)-one (3h): Yield 209 mg (56%). Pale
yellow oil. IR: 3082, 3053 (arom. CꢀH), 2914, 2822 (aliph. CꢀH), 1635 (C¼O). 1H-NMR: 7.36 – 7.40 (m, 5
arom. H); 7.29 – 7.33 (m, 5 arom. H); 7.21 (d, J ¼ 7.2, HꢀC(5) of Tph); 7.13 (d, J ¼ 3.2, HꢀC(4) of Tph);
7.06 (dd, J ¼ 3.2, 1.5, HꢀC(3) of Tph); 5.80 (d, J ¼ 5.5, HꢀC(2)); 4.59 (d, J ¼ 5.5, HꢀC(3)); 3.58 – 3.63 (m,
HꢀC(6)); 2.69 – 2.90 (m, 4 H). 13C-NMR: 193.2 (C¼O); 175.6 (C(7a)); 142.7; 141.6; 141.3; 129.1; 127.6;
127.4; 127.3; 127.2; 127.0; 126.6; 126.3; 126.1; 116.3 (C(3a)); 90.8 (C(2)); 53.9; 44.5; 40.6; 32.0. MS: 373
(31, [M þ H]þ), 372 (100, Mþ), 354 (2, [M ꢀ H2O]þ), 281 (6, [M ꢀ C6H5CH2]þ), 226 (13, [M ꢀ
C10H10O]þ), 186 (2, C12H10Sþ), 91 (21, C6H5CHþ2 ), 83 (2, C4H3Sþ), 77 (32, C6H5þ). Anal. calc. for
C24H20O2S (372.48): C 77.4, H 5.4, S 8.6; found: C 77.3, H 5.3, S 8.5.
3,5,6,7-Tetrahydro-3-phenyl-2-(2-thienyl)benzofuran-4(2H)-one (3i): Yield 196 mg (66%). Pale
yellow oil. IR: 3026 (arom. CꢀH), 2945, 2888 (aliph. CꢀH), 1652 (C¼C), 1633 (C¼O). 1H-NMR:
7.10 – 7.25 (m, 5 arom. H, HꢀC(5) of Tph); 6.98 (d, J ¼ 3.4, HꢀC(3) of Tph); 6.92 (dd, J ¼ 4.9, 3.4, HꢀC(4)
of Tph); 5.61 (d, J ¼ 5.7, HꢀC(2)); 4.40 (d, J ¼ 5.7, HꢀC(3)); 2.40 – 2.60 (m, 2 H); 2.24 – 2.37 (m, 2 H);
2.07 – 2.01 (m, 2 H). MS: 297 (22, [M þ H]þ), 296 (100, Mþ), 278 (5, [M ꢀ H2O]þ), 213 (8, [M ꢀ
C4H3S]þ), 184 (17, [M ꢀ C5H4OS]þ), 110 (22, C12H10Sþ), 91 (14, C6H5CHþ2 ), 77 (28, C6H5þ). Anal.
calc. for C18H16O2S (296.38): C 72.9, H 5.4, S 10.8; found: C 72.8, H 5.3, S 10.7.
1-[4,5-Dihydro-2,5-dimethyl-4-phenyl-5-(3-thienyl)furan-3-yl]ethanone (3j): Yield 125 mg (42%).
Yellow oil. 1H-NMR: 7.30 – 7.34 (m, 5 arom. H, HꢀC(5) of Tph); 7.18 (dd, J ¼ 3.1, 1.5, HꢀC(2) of Tph);
7.11 (dd, J ¼ 5.1, 1.5, HꢀC(4) of Tph); 4.40 (s, HꢀC(4)); 2.47 (d, J ¼ 1.2, MeꢀC(2)); 1.81 (s, MeCOꢀC(3));
1.19 (s, MeꢀC(5)). 13C-NMR: 195.6 (C¼O); 168.5 (C(2)); 149.0; 139.7; 129.0; 127.8; 127.0; 125.4; 124.9;
119.5 (C(3)); 92.1; 90.6; 60.2 (C(4)); 29.7; 25.4; 15.4. Anal. calc. for C18H18O2S (296.38): C 72.5, H 6.1, S
10.7; found: C 72.4, H 6.2, S 10.6.
Ethyl 4,5-Dihydro-2,5-dimethyl-4-phenyl-5-(3-thienyl)furan-3-carboxylate (3k): Yield 190 mg
(58%). Pale yellow oil. 1H-NMR: 7.15 – 7.36 (m, 5 arom. H, 2 H of Tph); 7.10 (d, J ¼ 4.8, HꢀC(4));
4.38 (s, HꢀC(4)); 3.85 – 4.03 (m, MeCH2O); 2.44 (s, MeꢀC(2)); 1.20 (s, MeꢀC(5)); 0.96 (t, J ¼ 7.2,
MeCH2O). Anal. calc. for C19H20O3S (328.42): C 69.5, H 6.1, S 9.8; found: C 69.4, H 6.2, S 9.7.
3,5,6,7-Tetrahydro-2,6,6-trimethyl-3-phenyl-2-(3-thienyl)benzofuran-4(2H)-one (3l). Yield 237 mg
(70%). Yellow oil. 1H-NMR: 7.16 – 7.39 (m, 5 arom. H, HꢀC(5) of Tph); 7.11 (d, J ¼ 3.8, HꢀC(2) of Tph);
7.08 (dd, J ¼ 5.2, 1.6, HꢀC(4) of Tph); 4.44 (s, HꢀC(3)); 2.55 (d, J ¼ 17.6, HbꢀC(7)); 2.46 (dd, J ¼ 17.6, 2.4,
HaꢀC(7)); 2.27 (s, 2 H); 1.25 (s, MeꢀC(2)); 1.24 (s, MeꢀC(6)); 1.15 (s, MeꢀC(6)). 13C-NMR: 194.1
(C¼O); 175.8 (C(7a)); 148.3; 138.4; 128.7; 128.6; 127.5; 127.1; 124.9; 119.6 (C(3a)); 95.0 (C(2)); 94.1
(C(3)); 56.2; 51.5; 38.4; 34.3; 29.2; 29.1; 25.2. Anal. calc. for C21H22O2S (338.46): C 74.5, H 6.6, S 9.5;
found: C 74.4, H 6.7, S 9.4.
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Received February 28, 2012