Diels-alder reactions with ethyl 1-benzofuran-3-carboxylates

Article abstract of DOI:10.1134/S1070428013060134

Substituted salicylaldehydes reacted with ethyl diazoacetate to give ethyl 1-benzofuran-3-carboxylates containing various substituents in the aromatic ring. The Diels-Alder reaction of these compounds with Danishefsky's diene was regioselective, and it pr

Full text of DOI:10.1134/S1070428013060134

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2013, Vol. 49, No. 6, pp. 872–885. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © A.S. Kil’met’ev, E.E. Shul’ts, M.M. Shakirov, T.V. Rybalova, G.A. Tolstikov, 2013, published in Zhurnal Organicheskoi Khimii,
2013, Vol. 49, No. 6, pp. 888–900.
Diels–Alder Reactions with Ethyl 1-Benzofuran-3-carboxylates
A. S. Kil’met’ev, E. E. Shul’ts, M. M. Shakirov, T. V. Rybalova, and G. A. Tolstikov
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia
e-mail: schultz@nioch.nsc.ru
Received August 29, 2012
Abstract—Substituted salicylaldehydes reacted with ethyl diazoacetate to give ethyl 1-benzofuran-3-carbox-
ylates containing various substituents in the aromatic ring. The Diels–Alder reaction of these compounds with
Danishefsky’s diene was regioselective, and it provided an effective method for the construction of the
heterocyclic skeleton of hexahydrodibenzo[b,d]furan-7-one or tetrahydrodibenzo[b,d]furan-7-one. The adducts
were found to undergo rearrangement to substituted 4′-hydroxybiphenyl-2-yl methyl carbonates during column
chromatography on silica gel.
DOI: 10.1134/S1070428013060134
Tricyclic hexa- or tetrahydrodibenzo[b,d]furan frag-
ment is a structural element of many pharmacologi-
cally important substances, e.g., plant alkaloids
morphine [1], galanthamine [2], lycoramine [2], and
lunarine [3], melanin biosynthesis inhibitor (±)-lin-
derol A isolated from Lindera umbellata [4], and
a number of selective estrogen receptor β agonists [5].
attracts interest not only from the viewpoint of the
design of rational schemes for total synthesis of natural
compounds; it is also important for the synthesis of
their structural analogs [12].
In the present communication we describe synthetic
approaches to ethyl 7-oxo-5a,6,7,8,9,9a-hexahydrodi-
benzo[b,d]furan-9a-carboxylate, ethyl 7-oxo-5a,6,7,9a-
tetrahydrodibenzo[b,d]furan-9a-carboxylate, and their
derivatives on the basis of the Diels–Alder reaction of
substituted ethyl 1-benzofuran-3-carboxylates with
Danishefsky’s diene (1-methoxy-3-trimethylsilyloxy-
buta-1,3-diene). Diels–Alder reactions with 3-formyl-
and 3-(2-oxoacetyl)-1-benzofurans as dienophiles were
reported in [13, 14]; 3-ethoxycarbonyl-substituted de-
rivatives were not brought into cycloaddition reactions.
In the past decade, classical procedures for building
up hexa(tetra)hydrodibenzo[b,d]furan skeleton, such as
oxidative coupling of phenols [2, 6] and radical cy-
clization reactions [2], have been successfully supple-
mented by approaches based on the Birch reaction–
Cope rearrangement sequence [7], Claisen rearrange-
ment [8], intramolecular cyclization of functionalized
bis-aryl compounds [9], intramolecular oxa-Michael
reactions [10], and intramolecular cross-couplings
[2, 5, 11].
Ethyl 1-benzofuran-3-carboxylate and its deriva-
tives were synthesized by reaction of substituted
salicylaldehydes Ia–Ij with ethyl diazoacetate (II)
[15]. Treatment of aldehydes Ia–Ij in methylene
Development of methods for the selective construc-
tion of a hexa- or tetrahydrodibenzo[b,d]furan skeleton
Scheme 1.
R1
R1
R1
4
COOEt
COOEt
R2
R3
CHO
OH
R2
R3
R2
R3
3
HBF4·Et2O
CH2Cl2, 20°C
5
6
3a
7a
H2SO4
2
+
N2CHCOOEt
OH
O
O
7
R4
R4
IIIa–IIIj
R4
Ia–Ij
II
IVa–IVj
R1 = R2 = R3 = R4 = H (a); R1 = R2 = R3 = H, R4 = MeO (b); R1 = R2 = R4 = H, R3 = MeO (c); R1 = R3 = R4 = H, R2 = MeO (d);
R1 = MeO, R2 = R3 = R4 = H (e); R1 = R3 = H, R2 = R4 = t-Bu (f); R1 = R3 = R4 = H, R2 = O2N (g); R1 = R2 = R4 = H, R3 = Et2N (h);
R1 = R3 = R4 = H, R2 = Br (i); R1 = R3 = H, R2 = I, R4 = MeO (j).
872

DIELS–ALDER REACTIONS WITH ETHYL 1-BENZOFURAN-3-CARBOXYLATES
873
chloride with a catalytic amount of tetrafluoroboric
acid in diethyl ether (Et2O·HBF4, 0.1 equiv) and
subsequent reaction with excess diazo ester II gave
ethyl 2-hydroxy-2,3-dihydro-1-benzofuran-3-carbox-
ylates IIIa–IIIj which were subjected to dehydration
by the action of concentrated sulfuric acid. As a result,
ethyl 1-benzofuran-3-carboxylates IVa–IVj were
isolated in 40–92% yield (Scheme 1). Intermediate
compounds IIIa–IIIj were not isolated. We failed to
achieve complete conversion of initial compounds Ia–
Ij when the synthesis of IVa–IVj was carried out ac-
cording to the procedure described in [15]. Therefore,
we changed some reaction conditions. Increase of the
amount of Et2O·HBF4 from 10 to 20 mol % did not
affect the yield of IV to an appreciable extent, whereas
increase of the amount of ethyl diazoacetate (II) from
1.6 to 3.2 equiv improved the yield of IV. Further rise
in the amount of II had no effect. Reduction of the
amount of sulfuric acid by half (down to 0.5 ml per
0.005 mol of the initial aldehyde) led to incomplete
dehydration and formation of mixtures of III and IV.
While optimizing the conditions, in one experiment we
obtained a mixture of compounds IIIb and IVb, from
which 2,3-dihydro-1-benzofuran IIIb was isolated by
column chromatography on silica gel.
Compounds IVa–IVj turned out to be highly reac-
tive in the Diels–Alder cycloaddition with Danishef-
sky’s diene (V) (Scheme 2). The reactions were carried
out in toluene in two versions: (a) in a sealed ampule
at a bath temperature of 165–170°C and (b) under
microwave irradiation (190°C). The conversion of
benzofuran derivatives IVa–IVj was 84–100%. The
regioselectivity of the reaction did not depend on the
thermal activation mode, and in both cases the ratios of
stereoisomeric cycloadducts A with (9S) and (9R)
1
configuration were similar (according to the H NMR
data). Primary adducts A were decomposed by treat-
ment with (c) ammonium fluoride in methanol [14],
(d) p-toluenesulfonic acid in methanol [16], (e) tri-
fluoroacetic acid in methylene chloride [17], or
(f) p-toluenesulfonic acid in boiling toluene.
Decomposition of adducts A with ammonium
fluoride, followed by chromatographic separation,
afforded stereoisomeric ethyl (9S)- and (9R)-methoxy-
7-oxo-5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-
carboxylates VIa–VIj (7–50%) and VIIa–VIIj (15–
49%). In this case, no tetrahydrodibenzo[b,d]furan de-
rivatives VIII were isolated; according to the 1H NMR
data, their concentration in the hydrolysis product
mixture was 2–8%. Instead, we isolated biphenylyl
Scheme 2.
R1
O
OEt
OMe
R2
MeO
OSiMe3
CH2
i or ii
IVa–IVj
+
R3
OSiMe3
O
R4
H
V
A
R1
O
R1
O
R1
O
OEt
OEt
O
OEt
OMe
OMe
R2
R2
R3
R2
R3
1
iii–vi
2
9
8
+
+
9a
5a
9b
7
R3
O
O
O
3
6
5
4a
4
O
O
R4
R4
R4
H
H
H
VIa–VIj
VIIa–VIIj
VIIIa–VIIIj
OH
R1
6
1'
R2
R3
5
4
Silica
1
2
VIa–VIj
+
VIIa–VIIj
+
O
O
3
R4
OEt
IXa, IXb, IXd, IXe
R1 = R2 = R3 = R4 = H (a); R1 = R2 = R3 = H, R4 = MeO (b); R1 = R2 = R4 = H, R3 = MeO (c); R1 = R3 = R4 = H, R2 = MeO (d); R1 =
MeO, R2 = R3 = R4 = H (e); R1 = R3 = H, R2 = R4 = t-Bu (f); R1 = R3 = R4 = H, R2 = O2N (g); R1 = R2 = R4 = H, R3 = Et2N (h); R1 =
R3 = R4 = H, R2 = Br (i); R1 = R3 = H, R2 = I, R4 = MeO (j); i: PhMe, sealed ampule, 165–170°C, 48–52 h; ii: PhMe, microwave
irradiation, 190°C, 4 h; iii: NH4F–MeOH, –15 to 20°C; iv: TsOH–MeOH, 20°C; v: CF3CO2H–CH2Cl2, 20°C; vi: TsOH–PhMe, 110°C.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 6 2013

KIL’MET’EV et al.
874
(a)
(b)
C13
C13
C12
C12
O4
C10
C10
O4
O7
N1
O3
O2
C9
O2
O3
C11
C11
C1
C2
C1
C2
C9
O6
C9a
C9a
C5a
C9b
C4a
C8
C9b
C8
C7
C3
O1
C7
C3
C5a
C6
O1
C6
C4a
C4
O5
O5
C4
Fig. 1. Structures of the molecules of (a) ethyl (5aS,9S,9aS)-9-methoxy-7-oxo-5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-carbox-
ylate (VIa) and (b) ethyl (5aS,9R,9aS)-9-methoxy-2-nitro-7-oxo-5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-carboxylate (VIIg)
according to the X-ray diffraction data.
carbonates IXb, IXd, and IXe (13–41%) which were
likely to be formed during chromatographic separation
on silica gel (the 1H NMR spectra of the reaction mix-
tures contained no signals assignable to compounds
like IX). Analogous results were obtained when ad-
ducts A were decomposed with p-toluenesulfonic acid
in methanol or with trifluoroacetic acid in methylene
chloride (methods d and e) at 20°C. By column chro-
matography on silica gel we isolated stereoisomeric
hexahydrodibenzo[b,d]furans VIa, VId, VIf, and VIj
(10–35%) and VIIa, VIIb, VIId, VIIf, and VIIj (10–
40%), as well as biphenylyl carbonates IXa, IXb, and
IXd (13–44%).
Decomposition of adduct A with p-toluenesulfonic
acid in toluene (110°C) gave ethyl (5aS,9aR)-4-me-
thoxy-7-oxotetrahydrodibenzo[b,d]furan-9a-carbox-
ylate (VIIIb) as the major product, the ratio
(VIb+VIIb)/VIIIb being 1:7. It is seen that treatment
with TsOH at elevated temperature favors formation of
unsaturated tricyclic ketones VIII.
C5
C6
C4
The structure of the synthesized compounds was
confirmed by their IR, UV, 1H and 13C NMR, and mass
spectra. The regioselectivity of the Diels–Alder reac-
tion of substituted ethyl 1-benzofuran-3-carboxylates
IV with Danishefsky’s diene follows from the 1H NMR
spectra of compounds VI–VIII. Tricyclic ketones VI
C2′
C3′
C1
C3
C4′
C1′
C8
C2
C6′
O6″
O7
O7′
C5′
O1″
1
and VII characteristically displayed in the H NMR
C2″
O3″
spectra signals from protons on C6 and C8 as doublets
of doublets in the region δ 1.93–3.03 ppm (2J = 17–18,
3J = 2–12 and 3.5–8 Hz for 8-H and 6-H, respectively).
In the spectra of esters VIII, distinguishing signals are
those from 5a-H, 8-H, and 9-H. The 5a-H signal was
a double doublet of doublets at δ 5.42–5.48 ppm (3J =
C4″
C5″
4
4, J= 2, 2 Hz). The 8-H and 9-H protons resonated as
doublets of doublets at δ 6.65–6.75 and 6.80–
Fig. 2. Structure of the molecule of ethyl 4′-hydroxy-3-me-
thoxybiphenyl-2-yl carbonate (IXb) according to the X-ray
diffraction data.
4
6.90 ppm, respectively (3J = 10.0–10.4, J = 1–2 Hz).
The vicinal coupling constant (10.0–10.4 Hz) unam-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 6 2013

DIELS–ALDER REACTIONS WITH ETHYL 1-BENZOFURAN-3-CARBOXYLATES
875
biguously indicates Z configuration of the double
plane of the neighboring molecule; the latter alternates
with the pair interaction O
C8=C9 bond.
···π with the distances
between O6 and the centroid of the C1′C2′C3′C4′C4a′9b′
benzene ring 3.837(2) and 3.771 Å and between O6′
and the centroid of the C1C2C3C4C4aC9b benzene ring
3.863(2) and 3.777 Å. The stacks are linked to form
a three-dimensional structure through shortened C···O
and C–H · · · O contacts, the shortest of which are
O3···C7′ 2.933(2) and H9′···O3 2.41Å.
The structure of VIa, VIIg, and IXb was proved by
X-ray analysis (Figs. 1, 2). The geometric parameters
of two independent molecules of compound VIIg al-
most coincide. The bond lengths in VIIg are close to
the corresponding bond lengths in VIa, the largest dif-
ference being observed for the C9–C9a bonds [1.564(2)
and 1.566(2) Å in the two independent molecules of
VIIg and 1.542(2) Å in VIa]. The conformation of the
tricyclic fragment of VIa is similar to that of exo-N,N-
diethyl-2-[9-methoxy-7-oxo-6,7,8,9-tetrahydrodibenzo-
[b,d]furan-9a(5aH)-yl]-2-oxoacetamide [14], and the
conformation of VIIg, to that of (4aS,9bS)-8-hydroxy-
1,2,3,4,4a,9b-hexahydrodibenzofuran-3-one [18]. The
furan ring has an envelope conformation with the C5a
atom deviating from the plane formed by the other ring
atoms; the folding angles are 12.9 and 14.3° for VIIg
and 18.9° for VIa. The cyclohexane fragment in VIIg
adopts a distorted boat conformation, whereas its con-
formation in molecule VIa is twist. There are also dif-
ferences in the orientation of the ester group. The ester
group in VIa is eclipsed by the C5a–C9a bond, and the
intramolecular contact C5a–H···O3, 2.33Å (C5a···O3
2.792(2)Å, ∠CHO 107°) is appreciably shortened (the
sums of the van der Waals radii of the oxygen and
hydrogen and oxygen and carbon atoms are 2.68 and
3.35 Å, respectively [19]), and the C13H3 methyl group
is located above the cyclohexane ring. In both indepen-
dent molecules of VIIg the carboxylate group resides
above the cyclohexane fragment, and the ethyl group
appears in the trans position with respect to the C11–O4
bond. Different configurations of C9 in molecules VIa
and VIIg should also be noted. The nitro group in
VIIg is planar, and it forms with the benzene ring
plane a dihedral angle of –3.9(3) and 15.3(3)°
(O6N1C2C3).
Figure 2 shows the structure of molecule IXb in
crystal. The aromatic rings C1C2C3C4C5C6 and
C1′C2′C3′C4′C5′C6′ are planar within ±0.0065(9) and
±0.0129(8) Å, respectively, and their planes form
a dihedral angle of 51.22(4)°. The O1″C2″O6″O3″C4″
fragment is planar within ±0.0157(6) Å, and it is
turned through an angle of 80.56(3)° with respect to
the C1C2C3C4C5C6 benzene ring. The hydroxy group
deviates from the benzene ring plane, the torsion angle
C3′C4′O7′H being 13°. The methoxy group is oriented
almost in one plane with the C1C2C3C4C5C6 benzene
ring [the torsion angle C2C3O7C8 is 175.9(1)°]. Mole-
cules IXb in crystal are linked by intermolecular
hydrogen bonds O7′–H···O6″ [O–H 0.84, H···O 1.94,
O···O 2.767(1) Å, ∠OHO 170°] to form zigzag chains
along the b axis. In addition, hydrogen bonds
C4″–H · · · O6″ [C–H 0.99, H · · · O 2.40, C · · · O
3.354(2) Å, ∠CHO 162°] connecting the chains to
layers parallel to the (a+c, b) plane were detected.
Thus the Diels–Alder reaction of ethyl substituted
1-benzofuran-3-carboxylates IVa–IVj with Danishef-
sky’s diene (V) regioselectively yields stereoisomeric
ethyl (9S)- and (9R)-9-methoxy-7-oxohexahydrodi-
benzo[b,d]furan-9a-carboxylates VI and VII or ethyl
7-oxotetrahydrodibenzo[b,d]furan-9a-carboxylates
VIII. Biphenylyl carbonates IXa, IXb, IXd, and IXe
were also isolated as a result of transformation of tri-
cyclic compounds VIII during chromatographic sepa-
ration of the products on silica gel. Presumably, this
transformation is initiated by acid or basic centers on
the silica surface (Scheme 3); it is likely to involve
opening of the furan ring with formation of dienone
intermediate B. The latter undergoes rearrangement
through migration of the ethoxycarbonyl group, which
is analogous to the fast step in the retro-Fries rear-
rangement [20], and it smoothly affords biphenylyl
carbonates IX. Partial isomerization of cycloaddition
products derived from Danishefsky’s diene (V), e.g., of
methyl 6-methyl-4-oxocyclohex-2-ene-1-carboxylate
into methyl 6-methyl-4-oxocyclohex-1-enecarbox-
ylate, on silica gel was reported in [21]; an example of
The three-dimensional crystalline structure of com-
pound VIa is formed due to C9–H···π(C1–C9b) interac-
tions (the distance to the plane and to the center of
the phenyl group is 2.84Å) and numerous C–H···O
contacts, the shortest of which are C3–H···O5 and
C10–H · · · O3 (2.44–2.46Å), C · · · O 3.370(2) and
3.390(2) Å, ∠CHO 158 and 168°, respectively. In the
crystalline structure of VIIg alternating stacks parallel
to the b axis are formed via π-stacking interactions of
phenyl groups in the two independent molecules with
an interplanar angle of 6.2°, an intercentroid distance
of 3.834(1) Å, and distances of 3.4353(8) and
3.5672(8) Å from the centroid to the benzene ring
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 6 2013

KIL’MET’EV et al.
876
Scheme 3.
R1
O
R1
O
OEt
OEt
R2
R3
R2
R3
Silica (H+)
VIII
B
VIII
–H+
O
O
O
O
R4
OEt
H
R4
H
H
O
O–
OH
R1
R1
R1
H+
R2
R2
R2
O
R3
O
R3
OC(O)Et
R3
OC(O)Et
R4
R4
R4
B
IX
furan ring opening in 9a-substituted 5a,9a-dihydroben-
zo[b,d]furans with migration of the substituents was
described in [22].
Silica gel from Acros Organics (0.035–0.070 mm)
was used for column chromatography. The solvents
(toluene, methylene chloride, methanol, diethyl ether,
petroleum ether) were purified by standard methods.
Danishefsky’s diene, tetrafluoroboric acid (50%) in
diethyl ether, and ethyl diazoacetate were commercial
products (from Alfa Aesar).
EXPERIMENTAL*
1
The H and 13C NMR spectra were recorded on
Bruker AV-300 [300.13 (1H) and 75.48 MHz (13C)],
AV-400 [400.13 (1H) and 100.61 MHz (13C)], and
AV-600 spectrometers [600.30 (1H) and 150.95 MHz
(13C)] from solutions in CDCl3. The chemical shifts
were measured relative to the residual proton and
carbon signals of the solvent (CHCl3, δ 7.24 ppm,
CDCl3, δC 76.90 ppm). Signals were assigned using
Microwave-activated syntheses were carried out in
CEM Discovery (300 W) and Anton Paar Monowave
(850 W) single mode microwave ovens.
Ethyl 1-benzofuran-3-carboxylates IVa–IVj
(general procedure). Substituted 2-hydroxybenzalde-
hyde Ia–Ij, 5 mmol, was dissolved in 10 ml of methy-
lene chloride, 0.08 ml of tetrafluoroboric acid in
diethyl ether (0.5 mmol of HBF4) was added dropwise
under stirring, and a solution of 2.0 ml (16 mmol) of
ethyl diazoacetate (II) in 10 ml of methylene
chloride was then added dropwise under stirring. The
mixture was stirred until nitrogen no longer evolved
and for 20–30 min more and concentrated, and 1.0 ml
of concentrated sulfuric acid was added under stirring.
After 10 min, the mixture was diluted with 10 ml of
methylene chloride and neutralized with solid sodium
hydrogen carbonate. The precipitate was filtered off
and washed with a 9:1 mixture of petroleum ether and
diethyl ether. The filtrate was subjected to double
column chromatography on silica gel using petroleum
ether–diethyl ether (9:1 and 19:1) as eluent. Removal
of the solvent from the eluate gave esters IVa–IVj.
1
one-dimensional H and 13C NMR experiments and
1
13
two-dimensional H–1H and C–1H shift correlation
techniques (COSY, COXH, COLOC, BSMS). The
molecular weights and elemental compositions were
determined from the high-resolution mass spectra
which were obtained on a DFS Thermo Scientific mass
spectrometer (electron impact, 70 eV; vaporizer tem-
perature 180–250°C). The IR spectra were measured
on a Bruker Vector spectrometer with Fourier trans-
form from samples prepared as KBr pellets (crystalline
products) or thin films (liquids). The UV spectra were
recorded on a Hewlett Packard HP 8453-Vis spectro-
photometer from solutions in chloroform or ethanol.
The melting points were determined using a Stuart
SMP melting point apparatus. X-Ray analysis of single
crystals of compounds VIa, VIIg, and IXb was
performed on a Bruker APEX diffractometer (MoKα
irradiation, graphite monochromator, CCD detector,
2θmax = 56.6, 54.2, and 53.5°, respectively).
1
Ethyl 1-benzofuran-3-carboxylate (IVa). The H
and 13C NMR spectra of IVa coincided with those
given in [15]. Mass spectrum, m/z (Irel, %): 190.1
(49.5) [M]+, 175.0 (2.34), 162.0 (24.0), 145.0 (100),
118.0 (5.06), 89.0 (15.1). Found: m/z 190.0624 [M]+.
C11H10O3. Calculated: M 190.0625.
* Analytical and spectral studies were carried out at the Shared
Chemical Service Center of the Siberian Branch of the Russian
Academy of Sciences.
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DIELS–ALDER REACTIONS WITH ETHYL 1-BENZOFURAN-3-CARBOXYLATES
877
Ethyl 7-methoxy-1-benzofuran-3-carboxylate
6.93 d.d (1H, 6-H, J = 9.0, 2.7), 7.38 d (1H, 7-H, J =
9.0), 7.50 d (1H, 4-H, J = 2.7), 8.19 s (1H, 2-H).
13C NMR spectrum, δC, ppm: 14.30 (CH2CH3), 55.76
(OCH3), 60.40 (OCH2), 103.63 (C4), 112.12 and
114.40 (C6, C7), 114.52 (C3), 125.32 (C3a), 150.41
(C7a), 151.40 (C2), 156.86 (C5), 163.43 (C=O). Mass
spectrum, m/z (Irel, %): 220.1 (100) [M]+, 192.1 (28.9),
177.1 (19.4), 175.1 (79.4), 160.1 (7.28), 148.1 (15.9),
119.1 (7.66). Found: m/z 220.0728 [M]+. C12H12O4.
Calculated: M 220.0730.
1
13
(IVb). mp 41–42°C. The H and C NMR spectra of
IVb coincided with those given in [15]. IR spectrum,
ν, cm–1: 3149, 3107, 2981, 2843, 1718, 1626, 1592,
1498, 1277, 1153, 1111, 1042. UV spectrum, λmax, nm
(logε): 242 (3.65), 265 (3.60). Mass spectrum, m/z
(Irel, %): 220 (100), 192 (43), 175 (82.5), 160 (16), 148
(33), 119 (3.7). Found: m/z 220.0729 [M]+. C12H12O4.
Calculated: M 220.0730.
In one experiment during optimization of the reac-
tion conditions, we isolated compound IVb and ethyl
2-hydroxy-2,3-dihydro-1-benzofuran-3-carboxylate
(IIIb) (from Ib and II). Yield of IIIb 0.40 g (5.6%),
transparent oily substance. IR spectrum, ν, cm–1: 3430,
2980, 2938, 2840, 1737, 1620, 1494, 1462, 1283,
1191, 1102. UV spectrum, λmax, nm (logε): 242 (3.37),
Ethyl 4-methoxy-1-benzofuran-3-carboxylate
(IVe). Yield 0.57 g (52%), oily substance. IR spectrum,
ν, cm–1: 3145, 3086, 2982, 2840, 1739, 1597, 1558,
1501, 1465, 1374, 1291, 1136, 1082, 1046. UV spec-
trum, λmax, nm (logε): 243 (3.90), 265 (3.74). 1H NMR
spectrum, δ, ppm (J, Hz): 1.37 t (3H, CH2CH3, J =
7.1), 3.95 s (3H, OCH3), 4.35 q (2H, OCH2, J = 7.1),
6.75 d.d (1H, 5-H, 7-H, J = 8.1, 0.7), 7.13 d.d (1H,
7-H, J = 8.3, 0.7), 7.27 d.d (1H, 6-H, J = 8.3, 8.1),
1
279 (3.44). H NMR spectrum, δ, ppm (J, Hz): 1.23 t
(3H, CH2CH3, J = 7.1), 3.86 s (3H, OCH3), 4.10 d (1H,
3-H, J = 2.9), 4.18 q (2H, OCH2, J = 7.1), 6.08 s (1H,
2-H, J = 2.9), 6.80 d.d (1H, 6-H, J = 8.0, 0.9), 6.85 d.d
(1H, 5-H, J = 8.0, 8.0), 6.96 d.d (1H, 4-H, J = 8.0, 0.9).
13C NMR spectrum, δC, ppm: 14.03 (CH2CH3), 54.69
(OCH3), 55.95 (C3), 61.44 (OCH2), 108.17 (C2);
112.42, 117.24, 121.47 (C4, C5, C6), 122.23 (C3a),
146.71 (C2a, C7), 169.52 (C=O).
13
8.13 s (1H, 2-H). C NMR spectrum, δC, ppm: 14.27
(CH2CH3), 55.88 (OCH3), 60.58 (OCH2), 104.71 and
105.13 (C5, C7), 113.84 and 115.00 (C3, C3a), 126.20
(C6), 150.12 (C2), 154.40 (C4), 157.19 (C7a), 162.65
(C=O). Mass spectrum, m/z (Irel, %): 220 (100) [M]+,
192 (6.00), 175 (90.0), 160 (21.7), 145 (42.9), 117
(9.29), 105 (9.86), 89 (28.0). Found: m/z 220.0729
[M]+. C12H12O4. Calculated: M 220.0730.
Ethyl 6-methoxy-1-benzofuran-3-carboxylate
(IVc). Yield 0.62 g (56%), mp 54–55°C; published
data [23]: mp 55–56°C. IR spectrum, ν, cm–1: 3132,
3086, 2992, 2841, 1709, 1624, 1558, 1501, 1437,
1379, 1273, 1103, 1051. UV spectrum, λmax, nm
Ethyl 5,7-di-tert-butyl-1-benzofuran-3-carbox-
ylate (IVf). Yield 1.4 g (92%), light yellow crystals,
mp 80–81°C. IR spectrum, ν, cm–1: 3138, 3097, 2964,
2872, 1712, 1603, 1570, 1483, 1363, 1302, 1227,
1124. UV spectrum, λmax, nm (logε): 241 (3.55), 267
(3.61). 1H NMR spectrum, δ, ppm (J, Hz): 1.38 s (9H,
t-Bu), 1.40 t (3H, CH2CH3, J = 7.1), 1.47 s (9H, t-Bu),
4.38 q (2H, OCH2, J = 7.1), 7.30 d.d (1H, 6-H, J =
2.0), 7.93 d (1H, 4-H, J = 2.0), 8.22 s (1H, 2-H).
13C NMR spectrum, δC, ppm: 14.34 (CH2CH3), 29.80
[C(CH3)3], 31.80 [C(CH3)3], 34.42 (7-C), 34.99 (5-C),
60.29 (OCH2), 114.30 (C3), 115.81 and 119.82 (C4,
C6), 124.75 (C3a), 134.24 (C7), 147.06 (C5), 150.12
(C2), 152.30 (C7a), 163.74 (C=O). Mass spectrum, m/z
(Irel, %): 302 (15.4) [M]+, 287 (100), 257 (6.75), 203.1
(4.56), 185.1 (3.84), 128.1 (4.57), 115.1 (3.83), 57.0
(21.8). Found: m/z 302.1869 [M]+. C19H26O3. Calculat-
ed: M 302.1876.
1
(logε): 242 (3.65), 296 (3.63). H NMR spectrum, δ,
ppm (J, Hz): 1.39 t (3H, CH2CH3, J = 7.1), 3.83 s (3H,
OCH3), 4.37 q (2H, OCH2, J = 7.1), 6.96 d.d (1H, 6-H,
J = 8.6, 2.2), 7.01 d.d (1H, 7-H, J = 2.2, 0.5), 7.88 d.d
(1H, 4-H, J = 8.6, 0.5), 8.14 s (1H, 2-H). 13C NMR
spectrum, δC, ppm: 14.33 (CH2CH3), 55.63 (OCH3),
60.45 (OCH2), 95.86 (C7), 113.19 (C5), 114.59 (C3),
117.78 (C3a), 122.05 (C4), 149.88 (C2), 156.52 (C6),
158.48 (C7a), 163.49 (C=O). Mass spectrum, m/z
(Irel, %): 220.2 (100) [M]+, 205.1 (9.12), 192.1 (44.2),
177.1 (52.4), 175.1 (47.8), 160.1 (3.21), 148.1 (9.54),
119.1 (18.3). Found: m/z 220.0728 [M]+. C12H12O4.
Calculated: M 220.0730.
Ethyl 5-methoxy-1-benzofuran-3-carboxylate
(IVd). Yield 0.93 g (85%), mp 55.5–56.0°C. IR spec-
trum, ν, cm–1: 3144, 3098, 2982, 2835, 1720, 1616,
1562, 1481, 1373, 1267, 1204, 1128, 1049. UV spec-
trum, λmax, nm (logε): 251 (3.74), 297 (3.60). 1H NMR
spectrum, δ, ppm (J, Hz): 1.39 t (3H, CH2CH3, J =
7.1), 3.85 s (3H, OCH3), 4.38 q (2H, OCH2, J = 7.1),
Ethyl 5-nitro-1-benzofuran-3-carboxylate (IVg).
Yield 0.66 g (56%), colorless crystals, mp 129–130°C.
IR spectrum, ν, cm–1: 3143, 3095, 2985, 1712, 1595,
1564, 1525, 1448, 1344, 1250, 1169, 1115, 1086, 1039.
UV spectrum, λmax, nm (logε): 239 (4.00), 276 (3.56).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 6 2013

KIL’MET’EV et al.
878
1H NMR spectrum, δ, ppm (J, Hz): 1.44 t (3H,
CH2CH3, J = 7.1), 4.44 q (2H, OCH2, J = 7.1), 7.62 d
(1H, 7-H, J = 9.1), 8.28 d.d (1H, 6-H, J = 9.1, 2.3),
1470, 1431, 1294, 1271, 1196, 1173, 1115, 1039, 933,
820, 769, 739, 623. UV spectrum: λmax 216 nm (logε
1
4.17). H NMR spectrum, δ, ppm (J, Hz): 1.39 t (3H,
13
8.95 d (1H, 4-H, J = 2.3), 8.37 s (1H, 2-H). C NMR
CH2CH3, J = 7.1), 3.97 s (3H, OCH3), 4.38 q (2H,
OCH2, J = 7.1), 7.11 d (1H, 6-H, J = 1.5), 7.99 d (1H,
4-H, J = 1.5), 8.17 s (1H, 2-H). 13C NMR spectrum, δC,
ppm: 14.32 (CH2CH3), 56.31 (OCH3), 60.78 (OCH2),
87.95 (C5), 114.24 (C3), 116.38 (C6), 123.09 (C4),
128.13 (C3a), 144.58 and 145.72 (C7, C7a), 151.10 (C2),
162.78 (C=O). Mass spectrum, m/z (Irel, %): 346 (100)
[M]+, 318 (18.2), 301 (30.4), 274 (26.1), 191 (9.2), 173
(16.7), 147 (6.21), 75 (15.2). Found: m/z 345.9699
[M]+. C12H11IO4. Calculated: M 345.9697.
spectrum, δC, ppm: 14.32 (CH2CH3), 61.19 (OCH2),
112.27 (C7), 115.70 (C3), 118.82 (C4), 121.13 (C6),
125.21 (C3a), 145.12 (C5), 153.29 (C2), 157.92 (C7a),
162.14 (C=O). Mass spectrum, m/z (Irel, %): 235 (61.3)
[M]+, 207 (57.8), 190 (100), 174 (18.7), 143 (43.8), 88
(14.9). Found: m/z 235.0476. C11H9NO5. Calculated:
M 235.0475.
Ethyl 6-(diethylamino)-1-benzofuran-3-carbox-
ylate (IVh). Yield 0.52 g (40%), yellow oily liquid. IR
spectrum, ν, cm–1: 3149, 3094, 2974, 2932, 1718,
1632, 1563, 1509, 1450, 1357, 1258, 1217, 1101,
1047. UV spectrum, λmax, nm (logε): 245 (4.20), 336
Diels–Alder reaction of ethyl 1-benzofuran-3-
carboxylates IVa–IVj with Danishefsky’s diene (V)
(general procedures). a. A glass ampule was filled with
argon and charged with a solution of 0.0005 mol of
compound IVa–IVj and 0.33 ml (0.0015 mol) of
Danishefsky’s diene (V) in 4.0 ml of toluene. The
ampule was sealed and heated for 48–52 h at 165–
170°C on an oil bath. The ampule was cooled and
opened, the reaction mixture was transferred into
a round-bottom flask, the ampule was washed with
methylene chloride (3×5 ml), the solvent was removed
under reduced pressure, and the residue was subjected
to desilylation.
1
(3.78). H NMR spectrum, δ, ppm (J, Hz): 1.17 t [6H,
N(CH2CH3)2, J = 7.0], 1.39 t (3H, OCH2CH3, J = 7.2),
3.38 q (4H, NCH2, J = 7.0), 4.36 q (2H, OCH2, J =
7.2), 6.74 d.d (1H, 7-H, J = 7.0, 0.6), 6.77 d.d (1H,
5-H, J = 8.6, 2.3), 7.78 d.d (1H, 4-H, J = 8.6, 0.6),
13
8.03 s (1H, 2-H). C NMR spectrum, δC, ppm: 12.41
[N(CH2CH3)2], 14.35 (OCH2CH3), 44.91 (NCH2),
60.24 (OCH2), 94.04 (C7), 110.68 (C5), 113.61 and
114.59 (C3, C3a), 121.87 (C4), 146.92 (C6), 148.33 (C2),
157.98 (C7a), 163.91 (C=O). Mass spectrum, m/z
(Irel, %): 261 (46.3) [M]+, 246 (100), 218 (18.3), 190
(9.92), 172 (3.76). Found: m/z 261.1358 [M]+.
C15H19NO3. Calculated: M 261.1360.
b. A test tube was filled with argon and charged
with 0.0005 mol of compound IVa–IVj and
0.0025 mol (0.55 ml) of diene V in 2.5 ml of toluene.
The test tube was placed in a microwave oven, and the
mixture was heated to 190°C and stirred for 4 h at that
temperature. The test tube was then withdrawn from
the oven, the reaction mixture was transferred into
a round-bottom flask, and the test tube was washed
with methylene chloride (2×5 ml). If necessary, the
solvent was removed under reduced pressure.
Ethyl 5-bromo-1-benzofuran-3-carboxylate
(IVi). Yield 0.70 g (52%), mp 80–81°C. IR spectrum,
ν, cm–1: 3082, 2982, 1709, 1562, 1441, 1373, 1340,
1279, 1227, 1163, 1122, 1063, 1040. UV spectrum,
1
λmax, nm (logε): 239 (4.00), 276 (3.56). H NMR
spectrum, δ, ppm (J, Hz): 1.41 t (3H, CH2CH3, J =
7.1), 4.39 q (2H, OCH2, J = 7.1), 7.38 d (1H, 7-H, J =
8.8), 7.44 d.d (1H, 6-H, J = 8.8, 2.0), 8.18 d (1H, 4-H,
J = 2.0), 8.22 s (1H, 2-H). 13C NMR spectrum, δC,
ppm: 14.36 (CH2CH3), 60.78 (OCH2), 113.12 (C7),
114.39, 117.51 (C3, C5), 124.84 (C4), 126.54 (C3a),
128.33 (C6), 151.78 (C2), 154.30 (C7a), 162.82 (C=O).
Mass spectrum, m/z (Irel, %): 270 (71.9) [M + 2]+, 269
(9.73) [M + 1]+, 268 (75.3) [M]+, 242 (36.6), 240
(38.2), 225 (99.3), 223 (100), 198 (5.88), 197 (6.14),
196 (5.25), 195 (4.94), 169 (17.0), 167 (17.2), 143
(15.7), 88 (22.4). Found: m/z 267.9736 [M]+.
C11H9BrO3. Calculated: M 267.9730.
c. The residue obtained after evaporation of the
solvent was treated with 5 ml of methanol, the solution
was cooled to –15°C, and ammonium fluoride in
equimolar amount to initial diene V was added under
stirring. The mixture was stirred for 18 h, gradually
allowing it to warm up to room temperature, the
solvent was distilled off, and the residue was subjected
to column chromatography on silica gel.
d. The residue obtained after evaporation of the
solvent was treated with 6 ml of methanol and 10 mg
of p-toluenesulfonic acid monohydrate, the mixture
was stirred for 18 h at 20°C and evaporated, 10 ml of
methylene chloride was added to the residue, and 5 ml
of a saturated aqueous solution of sodium hydrogen
Ethyl 5-iodo-7-methoxy-1-benzofuran-3-carbox-
ylate (IVj). Yield 1.00 g (58%), mp 128–129°C. IR
spectrum, ν, cm–1: 3020, 2981, 1699, 1620, 1578,
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DIELS–ALDER REACTIONS WITH ETHYL 1-BENZOFURAN-3-CARBOXYLATES
879
carbonate was then added. The organic phase was
separated, the aqueous phase was extracted with
methylene chloride (3×4 ml), the extracts were com-
bined with the organic phase, dried over magnesium
sulfate, and evaporated, and the residue was subjected
to column chromatography on silica gel.
(OCH2), 81.26 (C5a), 81.66 (C9), 110.19 (C4), 121.10
(C2), 125.23 (C1), 125.67 (C1a), 130.52 (C3), 159.03
(C4a), 170.13 (9a-C=O), 206.40 (C7). Mass spectrum,
m/z (Irel, %): 290.2 (13.3) [M]+, 217.2 (4.18), 190.1
(100), 162.1 (28.4), 145.1 (36.6), 131.1 (26.8), 100.1
(66.8). Found: m/z 290.1147 [M]+. C16H18O5. Calculat-
ed: M 290.1149.
e. The residue obtained after evaporation of the
solvent was treated with 6 ml of methylene chloride
and 0.03 ml of trifluoroacetic acid, the mixture was
stirred for 18 h at 20°C and transferred into a sepa-
ratory funnel containing 5 ml of a saturated aqueous
solution of NaHCO3. The organic phase was separated,
the aqueous phase was extracted with methylene
chloride (3×4 ml), the extracts were combined with
the organic phase, dried over magnesium sulfate, and
filtered, the solvent was distilled off, and the residue
was subjected to column chromatography on silica gel.
Ethyl (5aS,9S,9aS)-4,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIb). Yield 10% (c), 7% (d); oily substance.
1H NMR spectrum, δ, ppm (J, Hz): 1.35 t (3H,
CH2CH3, J = 7.1), 2.06 d.d (1H, 8-H, J = 17.6, 1.8),
2.64 d.d (1H, 8-H, J = 17.6, 3.8), 2.92 d.d (1H, 6-H,
J = 17.9, 3.5), 2.99 d.d (1H, 6-H, J = 17.9, 2.7), 3.29 s
(3H, 9-OCH3), 3.84 s (3H, 4-OCH3), 4.08d.d (1H, 9-H,
J = 3.8, 1.8), 4.31 q (2H, OCH2, J = 7.1), 5.80 d.d (1H,
5a-H, J = 3.5, 2.7), 6.81 d.d (1H, 3-H, J = 8.0, 1.2),
6.87 d.d (1H, 2-H, J = 8.0, 7.6), 7.12 d.d (1H, 1-H, J =
7.6, 1.2). 13C NMR spectrum, δC, ppm: 14.22
(CH2CH3), 38.17 (C8), 41.11 (C6), 56.00 (4-OCH3),
57.32 (9-OCH3), 59.86 (C9a), 61.70 (OCH2), 82.11
(C5a), 81.48 (C9) 112.96 (C3), 117.00 (C1), 121.76 (C2),
126.75 (C1a), 144.64 (C4), 147.68 (C4a), 170.00
(9a-C=O), 206.05 (C7). Mass spectrum, m/z (Irel, %):
320.2 (19.9) [M]+, 247.2 (3.05), 220.2 (100), 192.1
(14.5), 175.1 (14.0), 161.1 (17.1), 100.1 (35.4). Found:
m/z 320.1256 [M]+. C17H20O6. Calculated: M 320.1254.
f. The reaction mixture was transferred from the
ampule into a round-bottom flask, and the ampule was
washed with toluene (3×2 ml) for complete transfer.
p-Toluenesulfonic acid monohydrate, 38 mg
(0.0001 mol), was added to the mixture, and the mix-
ture was heated for 3 h under reflux with stirring. The
mixture was treated with 5 ml of a 5% aqueous
solution of sodium hydrogen carbonate, the organic
phase was separated, the aqueous phase was extracted
with methylene chloride (3×3 ml), the extracts were
combined with the organic phase and dried over
magnesium sulfate, and the solvent was removed under
reduced pressure.
Ethyl (5aS,9S,9aS)-3,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIc). Yield 25% (c), oily substance.
1H NMR spectrum, δ, ppm (J, Hz): 1.35 t (3H,
CH2CH3, J = 7), 2.03 d.d (1H, 8-H, J = 17.8,1.8),
2.63 d.d (1H, 8-H, J = 17.8, 3.6), 2.87 d.d (1H, 6-H,
J = 17.7, 3.0), 2.94 d.d (1H, 6-H, J = 17.7, 3.3), 3.29 s
(3H, 9-OCH3), 3.74 s (3H, 3-OCH3), 4.03 d.d (1H,
9-H, J = 3.6, 1.8), 4.31 q (2H, OCH2, J = 7.1), 5.75 d.d
(1H, 5a-H, J = 3.3, 3.0), 6.32 d (1H, 4-H, J = 2.3),
6.45 d.d (1H, 2-H, J = 8.6, 2.3), 7.35 d (1H, 1-H, J =
8.6). 13C NMR spectrum, δC, ppm: 14.26 (CH2CH3),
37.97 (C8), 41.46 (C6), 55.52 (3-OCH3), 57.31
(9-OCH3), 58.76 (C9a), 61.61 (OCH2), 81.69 and 82.15
(C5a, C9), 96.00 (C4), 107.55 (C2), 117.63 (C1a), 125.42
(C1), 160.40 (C3), 162.15 (C4a), 170.38 (9a-C=O),
206.49 (C7). Mass spectrum, m/z (Irel, %): 320.2 (38.7)
[M]+, 274.1 (15.9), 247.1 (8.69), 220.1 (100), 192.1
(22.6), 175.1 (10.8), 161.1 (42.5), 100.1 (35.5). Found:
m/z 320.1249 [M]+. C17H20O6. Calculated: M 320.1254.
By column chromatography we isolated in succes-
sion compounds IVa–IVj (eluent petroleum ether or
hexane–diethyl ether, 95:5 to 90:10), VIa–VIj [petro-
leum ether (or hexane)–diethyl ether, 80:20], VIIa–
VIIj, and IXa, IXb, IXd, or IXe [methylene chloride–
petroleum ether (hexane), 70:30 to 100:0].
Ethy l (5a S, 9 S, 9a S) - 9 - m e t ho x y - 7 - o x o-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIa). Yield 35% (e), mp 79–80°C (from
1
petroleum ether). H NMR spectrum, δ, ppm (J, Hz):
1.35 t (3H, CH2CH3, J = 7.1), 2.00 d.d (1H, 8-H, J =
17.8, 1.8), 2.63 d.d (1H, 8-H, J = 17.8, 3.7), 2.91 d.d
(1H, 6-H, J = 17.8, 2.9), 2.94 d.d (1H, 6-H, J = 17.8,
3.3), 3.30 s (3H, OCH3), 4.06 d.d (1H, 9-H, J = 3.7,
1.8), 4.32 q (2H, OCH2, J = 7.1), 5.74 d.d (1H, 5a-H,
J = 3.3, 2.9), 6.75 d.d (1H, 4-H, J = 8.1, 1.0),
6.90 d.d.d (1H, 2-H, J = 7.6, 7.5, 1.0), 7.21 d.d.d (1H,
3-H, J = 8.1, 7.5, 1.3), 7.51 d.d (1H, 1-H, J = 7.6, 1.3).
13C NMR spectrum, δC, ppm: 14.24 (CH2CH3), 37.99
(C8), 41.31 (C6), 57.33 (OCH3), 59.29 (C9a), 61.71
Ethyl (5aS,9S,9aS)-2,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VId). Yield 25% (c), 7% (d); oily substance.
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KIL’MET’EV et al.
880
1H NMR spectrum, δ, ppm (J, Hz): 1.35 t (3H,
CH2CH3, J = 7.1), 2.04 d.d (1H, 8-H, J = 17.8, 1.8),
2.63 d.d (1H, 8-H, J = 17.9, 3.8), 2.87 d.d (1H, 6-H,
J = 17.7, 3.0), 2.92 d.d (1H, 6-H, J = 17.7, 3.3), 3.29 s
(3H, 9-OCH3), 3.75 s (3H, 3-OCH3), 4.05 d.d (1H,
9-H, J = 3.8, 1.8), 4.31 q (2H, OCH2, J = 7.1), 5.71 d.d
(1H, 5a-H, J = 3.3, 3.0), 6.66 d (1H, 4-H, J = 8.7),
6.76 d.d (1H, 2-H, J = 8.7, 2.7), 7.08 d (1H, 1-H, J =
(C9), 81.85 (C5a), 119.21 and 124.23 (C1, C3), 125.43
(C1a), 132.76 (C4), 143.75 (C2), 154.75 (C4a), 170.49
(9a-C=O), 206.97 (C7). Mass spectrum, m/z (Irel, %):
402.3 (23.5) [M]+, 387.3 (16.9), 370.3 (6.24), 329.3
(3.14), 315.2 (5.98), 302.2 (40.4), 287.2 (100), 243.2
(10.4), 100.0 (35.3). Found: m/z 402.2404 [M]+.
C24H34O5. Calculated: M 402.2401.
Ethyl (5aS,9S,9aS)-9-methoxy-2-nitro-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIg). Yield 25% (c), mp 145–145.5°C (from
13
2.7). C NMR spectrum, δC, ppm: 14.26 (CH2CH3),
38.06 (C8), 41.32 (C6), 56.02 (3-OCH3), 57.33
(9-OCH3), 59.65 (C9a), 61.70 (OCH2), 81.45 (C5a),
81.64 (C9), 110.24 (C4), 111.10 (C1), 115.76 (C3),
126.44 (C1a), 153.16 (C4a), 154.37 (C2), 169.98
(9a-C=O), 206.36 (C7). Mass spectrum, m/z (Irel, %):
320.2 (33.0) [M]+, 247.2 (4.21), 220.2 (100), 192.1
(9.06), 175.1 (9.73), 161.1 (18.3), 100.1 (33.7). Found:
m/z 320.1255 [M]+. C17H20O6. Calculated: M 320.1254.
1
hexane). H NMR spectrum, δ, ppm (J, Hz): 1.39 t
(3H, CH2CH3, J = 7.1), 1.93 d.d (1H, 8-H, J = 18.0,
1.9), 2.70 d.d (1H, 8-H, J = 18.1, 3.6), 2.95 d.d (1H,
6-H, J = 17.8, 3.0), 2.99 d.d (1H, 6-H, J = 17.8, 3.4),
3.33 s (3H, 9-OCH3), 4.11 d.d (1H, 9-H, J = 3.6, 1.9),
4.37 q (2H, OCH2, J = 7.1), 5.92 d.d (1H, 5a-H, J =
3.4, 3.0), 6.85 d (1H, 4-H, J = 8.9), 8.19 d.d (1H, 3-H,
J = 8.9, 2.5), 8.43 d (1H, 1-H, J = 2.5). 13C NMR
spectrum, δC, ppm: 14.24 (CH2CH3), 37.81 (C8), 40.98
(C6), 57.53 (9-OCH3), 58.60 (C9a), 62.44 (OCH2),
81.35 (C9), 83.94 (C5a), 110.36 (C4), 122.11 (C3),
127.17 (C1a), 127.70 (C1), 142.41 (C2), 163.96 (C4a),
168.90 (9a-C=O), 204.77 (C7). Mass spectrum, m/z
(Irel, %): 335.2 (1.15) [M]+, 262.1 (4.77), 235.1 (63.1),
207.1 (40.8), 190.0 (21.7), 176.1 (16.8), 143.0 (10.6),
130.0 (11.1), 100.0 (100). Found: m/z 335.0997 [M]+.
C16H17NO7. Calculated: M 335.0999.
Ethyl (5aS,9S,9aS)-1,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIe). Yield 7% (c), oily substance. 1H NMR
spectrum, δ, ppm (J, Hz): 1.27 t (3H, CH2CH3, J =
7.1), 2.08 d.d (1H, 8-H, J = 18.3, 2.3), 2.60 d.d (1H,
8-H, J = 18.3, 3.3), 2.71 d.d (1H, 6-H, J = 16.9, 2.0),
3.03 d.d (1H, 6-H, J = 16.9, 5.3), 3.36 s (3H, 9-OCH3),
3.80 s (3H, 3-OCH3), 4.23 q (2H, OCH2, J = 7.1),
4.77 d.d (1H, 9-H, J = 3.3, 2.33), 5.81 d.d (1H, 5a-H,
J = 5.3, 2.0), 6.38 d.d (1H, 4-H, J = 8.2, 0.8), 6.38 d.d
(1H, 2-H, J = 8.2, 0.8), 7.12 d.d (1H, 3-H, J = 8.2, 8.2).
13C NMR spectrum, δC, ppm: 14.26 (CH2CH3), 38.70
(C8), 43.54 (C6), 55.39 (3-OCH3), 57.59 (9-OCH3),
59.97 (C9a), 61.43 (OCH2), 76.69 (C9), 82.88 (C5a),
103.56 and 103.66 (C2, C4), 111.78 (C1a), 131.70 (C3),
157.49 (C1), 160.79 (C4a), 170.87 (9a-C=O), 207.27
(C7). Mass spectrum, m/z (Irel, %): 320.2 (30.6) [M]+,
274.1 (10.1), 247.2 (4.19), 220.1 (100), 192.1 (1.83),
175.1 (13.2), 161.1 (16.9), 100.1 (14.8). Found:
m/z 320.1253 [M]+. C17H20O6. Calculated: M 320.1254.
Ethyl (5aS,9S,9aS)-3-diethylamino-9-methoxy-7-
oxo-5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-
carboxylate (VIh). Yield 45% (c), oily substance.
1H NMR spectrum, δ, ppm (J, Hz): 1.12 t [6H,
N(CH2CH3)2, J = 7.1], 1.34 t (3H, OCH2CH3, J = 7.1),
2.10 d.d (1H, 8-H, J = 17.7, 1.9), 2.61 d.d (1H, 8-H,
J = 17.7, 3.8), 2.86 d.d (1H, 6-H, J = 17.6, 3.0),
2.92 d.d (1H, 6-H, J = 17.6, 3.5), 3.28 q [4H,
N(CH2CH3)2, J = 7.1], 3.28 s (3H, 9-OCH3), 4.01 d.d
(1H, 9-H, J = 3.8, 1.9), 4.28 q (2H, OCH2, J = 7.1),
5.69 d.d (1H, 5a-H, J = 3.5, 3.0), 6.04 d (1H, 4-H, J =
2.3), 6.20 d.d (1H, 2-H, J = 8.6, 2.3), 7.25 d (1H, 1-H,
J = 8.6). 13C NMR spectrum, δC, ppm: 12.49
[N(CH2CH3)2], 14.28 (OCH2CH3), 38.13 (C8), 41.58
(C6), 44.60 [N(CH2CH3)2], 57.25 (9-OCH3), 58.72
(C9a), 61.34 (OCH2), 81.56 and 81.79 (C5a, C9), 92.82
(C4), 104.66, (C2) 111.83 (C1a), 125.36 (C1), 150.20
(C3), 160.75 (C4a), 170.80 (9a-C=O), 207.15 (C7).
Mass spectrum, m/z (Irel, %): 361.3 (45.9) [M]+, 346.2
(11.9), 330.2 (1.66), 315.2 (2.35), 288.2 (19.6), 274.2
(61.5), 261.2 (64.9), 246.2 (100), 218.1 (16.4), 202.2
(31.6), 158.1 (8.20). Found: m/z 361.1882 [M]+.
C20H27NO5. Calculated: M 361.1884.
Ethyl (5aS,9S,9aS)-2,4-di-tert-butyl-9-methoxy-
7-oxo-5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-
carboxylate (VIf). Yield 50% (c), 10% (d); oily sub-
1
stance. H NMR spectrum, δ, ppm (J, Hz): 1.27 s and
1.28 s (9H each, t-Bu), 1.36 t (3H, CH2CH3, J = 7.1),
2.00 d.d (1H, 8-H, J = 17.8, 1.8), 2.61 d.d (1H, 8-H,
J = 17.8, 3.7 Hz), 2.94 d.d (1H, 6-H, J = 17.8, 3.2),
2.94 d.d (1H, 6-H, J = 17.8, 2.9), 3.30 s (3H, 9-OCH3),
4.03 d.d (1H, 9-H, J = 3.7, 1.8), 4.31 q (2H, OCH2, J =
7), 5.71 d.d (1H, 5a-H, J = 3.2, 2.9), 7.17 d (1H, J =
2.1), 7.38 d (1H, J = 2.1). 13C NMR spectrum, δC,
ppm: 14.29 (CH2CH3), 29.18 and 31.73 [C(CH3)3],
34.32 and 34.62 [C(CH3)3], 38.08 (C8), 41.54 (C6),
57.27 (9-OCH3), 59.19 (C9a), 61.43 (OCH2), 80.72
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 6 2013

DIELS–ALDER REACTIONS WITH ETHYL 1-BENZOFURAN-3-CARBOXYLATES
881
Ethyl (5aS,9S,9aS)-2-bromo-9-methoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIi). Yield 13% (c), transparent light brown
6.76 d.d.d (1H, 4-H, J = 8.0, 1.0, 0.6), 6.93 d.d.d (1H,
2-H, J = 7.6, 7.5, 1.0), 7.21 d.d.d (1H, 3-H, J = 8.0,
7.5, 1.5), 7.46 d.d.d (1H, 1-H, J = 7.6, 1.5, 0.6).
13C NMR spectrum, δC, ppm: 14.09 (CH2CH3), 40.52
(C8), 41.98 (C6), 57.88 (9-OCH3), 59.59 (C9a), 62.08
(OCH2), 77.96 (C9), 82.21 (C5a), 109.50 (C4), 120.93
(C2), 123.58 (C1a), 128.75 (C1), 129.90 (C3), 159.66
(C4a), 172.74 (9a-C=O), 205.89 (C7). Mass spectrum,
m/z (Irel, %): 290.1 (18.1) [M]+, 217.2 (6.24), 190.1
(100), 162.0 (26.7), 145.0 (33.7), 131.0 (26.4), 100.1
(66.8). Found: m/z 290.1146 [M]+. C16H18O5. Calculat-
ed: M 290.1149.
1
oily liquid. H NMR spectrum, δ, ppm (J, Hz): 1.36 t
(3H, CH2CH3, J = 7.1), 1.99 d.d (1H, 8-H, J = 17.9,
1.8), 2.66 d.d (1H, 8-H, J = 17.9, 3.8), 2.88 d.d (1H,
6-H, J = 17.8, 3.0), 2.94 d.d (1H, 6-H, J = 17.8,
3.4), 3.29 s (3H, 9-OCH3), 4.03 d.d (1H, 9-H, J = 3.8,
1.8), 4.33 q (2H, OCH2, J = 7.1), 5.75 d.d (1H,
5a-H, J = 3.4, 3.0), 6.65 d (1H, 4-H, J = 8.6), 7.31 d.d
(1H, 2-H, J = 8.6, 2.2), 7.61 d (1H, 1-H, J = 2.2).
13C NMR spectrum, δC, ppm: 14.24 (CH2CH3), 37.95
(C8), 41.12 (C6), 57.40 (9-OCH3), 59.20 (C9a),
62.01 (OCH2), 81.54 (C9), 82.04 (C5a), 111.84 (C4),
112.80 (C1a), 127.94 (C2), 128.28 (C3), 133.42 (C1),
158.18 (C4a), 169.52 (9a-C=O), 205.73 (C7). Mass
spectrum, m/z (Irel, %): 370.1 (12.2) [M + 2]+, 369.1
(2.29) [M + 1]+, 368.1 (11.9) [M]+, 297.0 (2.91), 295.0
(2.86), 270.0 (49.8), 268.0 (49.2), 242.0 (11.8), 240.0
(12.8), 225.0 (14.4), 223.0 (13.0), 211.0 (13.3), 209.0
(13.5), 100.0 (100). Found: m/z 368.0258 [M]+.
C16H17BrO5. Calculated: M 368.0254.
Ethyl (5aS,9R,9aS)-4,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIIb). Yield 23% (c), 32% (d); transparent
1
oily liquid. H NMR spectrum, δ, ppm (J, Hz): 1.27 t
(3H, CH2CH3, J = 7.2), 2.12 d.d (1H, 8-H, J = 17.7,
11.2), 2.57 d.d (1H, 8-H, J = 17.7, 3.4), 2.76 d.d (1H,
6-H, J = 17.3, 4.0), 3.03 d.d (1H, 6-H, J = 17.3, 4.1),
3.33 s (3H, 9-OCH3), 3.84 s (3H, 4-OCH3), 4.26 d.d
(1H, 9-H, J = 11.2, 3.4), 4.26 q (2H, OCH2, J = 7.2),
5.28 d.d (1H, 5a-H, J = 4.1, 4.0), 6.81 d.d (1H, 3-H,
J = 8.2, 1.3), 6.89 d.d (1H, 2-H, J = 8.2, 7.5), 7.07 d.d
Ethyl (5aS,9S,9aS)-2-iodo-4,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
13
(1H, 1-H, J = 7.5, 1.3). C NMR spectrum, δC, ppm:
14.08 (CH2CH3), 40.61 (C8), 41.88 (C6), 55.86
(4-OCH3), 57.94 (9-OCH3), 60.19 (C9a), 62.12
(OCH2), 77.93 (C9), 83.02 (C5a), 112.25 (C3), 120.35
(C1), 121.42 (C2), 124.83 (C1a), 144.19 (C4), 148.33
(C4a), 172.53 (9a-C=O), 205.68 (C7). Mass spectrum,
m/z (Irel, %): 320.2 (14.6) [M]+, 274.2 (2.63), 247.2
(2.15), 220.1 (100), 192.1 (11.1), 175.1 (10.0), 161.1
(13.8), 148.1 (10.1), 100.1 (26.9). Found:
m/z 320.1253 [M]+. C17H20O6. Calculated: M 320.1254.
1
boxylate (VIj). Yield 32% (c), 14% (d). H NMR
spectrum, δ, ppm (J, Hz): 1.35 t (3H, CH2CH3, J =
7.1), 2.04 d.d (1H, 8-H, J = 17.7, 1.8), 2.66 d.d (1H,
8-H, J = 17.7, 3.9), 2.90 d.d (1H, 6-H, J = 17.9, 3.4),
2.97 d.d (1H, 6-H, J = 17.9, 2.8), 3.28 s (3H, 9-OCH3),
3.81 s (3H, 4-OCH3), 4.04 d.d (1H, 9-H, J = 3.9, 1.8),
4.32 q (2H, OCH2, J = 7.1), 5.79 d.d (1H, 5a-H, J =
3.4, 2.8), 7.06 d (1H, J = 1.6), 7.43 d (1H, J = 1.6).
13C NMR spectrum, δC, ppm: 14.20 (CH2CH3), 38.08
(C8), 40.89 (C6), 56.26 (4-OCH3), 57.36 (9-OCH3),
59.51 (C9a), 61.95 (OCH2), 81.27 (C9), 82.06 (C2),
82.54 (C5a), 122.01 (C3), 126.00 (C1), 128.70 (C1a),
145.36 (C4), 147.79 (C4a), 169.42 (9a-C=O), 205.41
(C7). Mass spectrum, m/z (Irel, %): 369.1 (2.29)
[M + 1]+, 446.1 (40.5) [M]+, 373.0 (3.01), 346.0 (90.0),
318.0 (8.77), 301.0 (6.76), 287.0 (19.9), 274.0 (14.6),
220.1 (19.7), 100.1 (100). Found: m/z 446.0218 [M]+.
C17H19IO6. Calculated: M 446.0221.
Ethyl (5aS,9R,9aS)-3,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIIc). Yield 25% (c), oily substance.
1H NMR spectrum, δ, ppm (J, Hz): 1.28 t (3H,
CH2CH3, J = 7.1), 2.03 d.d (1H, 8-H, J = 17.9, 11.6),
2.58 d.d (1H, 8-H, J = 17.9, 3.6), 2.74 d.d (1H, 6-H,
J = 17.1, 3.6), 2.92 d.d (1H, 6-H, J = 17.1, 3.8), 3.34 s
(3H, 9-OCH3), 3.75 s (3H, 3-OCH3), 4.22 d.d (1H,
9-H, J = 11.6, 3.6), 4.25 q 2H, CO2CH2CH3, J = 7.1),
5.20 d.d (1H, 5a-H, J = 3.8, 3.6), 6.32 d (1H, 4-H,
J = 2.3), 6.49 d.d (1H, 2-H, J = 8.5, 2.3), 7.33 d
(1H, 1-H, J = 8.5). 13C NMR spectrum, δC, ppm: 14.09
(CH2CH3), 40.52 (C8), 42.05 (C6), 55.42 (3-OCH3),
57.83 (9-OCH3), 59.08 (C9a), 61.99 (OCH2), 78.02
(C9), 83.02 (C5a), 95.46 (C4), 107.34 (C2), 115.38 (C1a),
128.95 (C1), 160.96 (C3), 161.94 (C4a), 173.02 (9a-
C=O), 205.94 (C7). Mass spectrum, m/z (Irel, %): 320.2
(12.6) [M]+, 274.2 (4.91), 247.2 (3.79), 220.2 (100),
Ethy l (5a S, 9 R, 9a S) - 9 - m e t ho x y - 7 - o x o-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIIa). Yield 39% (e), transparent oily
1
liquid. H NMR spectrum, δ, ppm (J, Hz): 1.28 t (3H,
CH2CH3, J = 7.1), 2.04 d.d (1H, 8-H, J = 17.9, 11.5),
2.58 d.d (1H, 8-H, J = 17.9, 3.6), 2.76 d.d (1H, 6-H,
J = 17.0, 3.7), 2.95 d.d (1H, 6-H, J = 17.0, 4.0), 3.34 s
(3H, OCH3), 4.26 d.d (1H, 9-H, J = 11.5, 3.6), 4.26 q
(2H, OCH2, J = 7.1), 5.20 d.d (1H, 5a-H, J = 4.0, 3.7),
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 6 2013

KIL’MET’EV et al.
882
192.1 (11.9), 175.1 (3.92), 161.2 (19.3), 148.1 (1.54),
100.1 (8.54). Found: m/z 320.1257 [M]+. C17H20O6.
Calculated: M 320.1254.
J = 7.1), 5.19 d.d (1H, 5a-H, J = 3.6, 4.0), 7.17 d (1H,
J = 2.2), 7.32 d (1H, J = 2.2). C NMR spectrum, δC,
13
ppm: 14.11 (CH2CH3), 29.23 and 31.72 [C(CH3)3],
34.15 and 34.48 [C(CH3)3], 40.71 (C8), 42.09 (C6),
57.82 (9-OCH3), 59.39 (C9a), 61.85 (OCH2), 78.09
(C9), 81.63 (C5a), 123.23 and 123.50 (C1, C3), 123.30
(C1a), 131.66 (C4), 143.04 (C2), 155.38 (C4a), 173.13
(9a-C=O), 206.41 (C7). Mass spectrum, m/z (Irel, %):
402.3 (52.1) [M]+, 387.3 (26.6), 370.3 (1.50), 329.3
(3.04), 315.3 (2.75), 302.2 (56.4), 287.2 (100), 243.2
(10.9), 100.1 (45.1). Found: m/z 402.2397 [M]+.
C24H34O5. Calculated: M 402.2401.
Ethyl (5aS,9R,9aS)-2,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIId). Yield 25% (c), 10% (e); oily sub-
1
stance. H NMR spectrum, δ, ppm (J, Hz): 1.29 t (3H,
CH2CH3, J = 7.0), 2.06 d.d (1H, 8-H, J = 17.7, 11.7),
2.59 d.d (1H, 8-H, J = 17.7, 3.7), 2.74 d.d (1H, 6-H,
J = 17.1, 3.7), 2.92 d.d (1H, 6-H, J = 17.1, 3.7), 3.34 s
(3H, 9-OCH3), 3.76 s (3H, 3-OCH3), 4.24 d.d (1H,
9-H, J = 11.7, 3.7), 4.26 q (2H, OCH2, J = 7.0),
5.18 d.d (1H, 5a-H, J = 3.7, 3.7), 6.66 d (1H, 4-H,
J = 8.7), 6.76 d.d (1H, 2-H, J = 8.7, 2.8), 7.03 d (1H,
1-H, J = 2.8). 13C NMR spectrum, δC, ppm: 14.11
(CH2CH3), 40.49 (C8), 42.04 (C6), 55.97 (3-OCH3),
57.84 (9-OCH3), 59.95 (C9a), 62.08 (OCH2), 77.97
(C9), 82.51 (C5a), 109.40 (C4), 114.52 (C1), 115.38
(C3), 124.47 (C1a), 153.95 (C4a), 154.13 (C2), 172.65
(9a-C=O), 205.87 (C7). Mass spectrum, m/z (Irel, %):
320.2 (21.5) [M]+, 247.2 (2.67), 220.1 (100), 192.1
(10.8), 175.1 (11.8), 161.1 (18.1), 147.1 (1.59), 100.1
(31.2). Found: m/z 320.1253 [M]+. C17H20O6. Calculat-
ed: M 320.1254.
Ethyl (5aS,9R,9aS)-9-methoxy-2-nitro-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIIg). Yield 32% (c), mp 113–113.5°C
1
(from hexane). H NMR spectrum, δ, ppm (J, Hz):
1.30 t (3H, CH2CH3, J = 7.1), 1.98 d.d (1H, 8-H, J =
18.2, 11.4), 2.65 d.d (1H, 8-H, J = 18.1, 3.8), 2.81 d.d
(1H, 6-H, J = 17.1, 4.0), 2.99 d.d (1H, 6-H, J = 17.1,
3.9), 3.36 s (3H, OCH3), 4.29 d.d (1H, 9-H, J = 11.4,
3.8), 4.30 q (2H, OCH2, J = 7.1), 5.36 d.d (1H, 5a-H,
J = 4.0, 3.9), 6.82 d (1H, 4-H, J = 9.0), 8.18 d.d (1H,
3-H, J = 9.0, 2.6), 8.36 d (1H, 1-H, J = 2.6). 13C NMR
spectrum, δC, ppm: 14.06 (CH2CH3), 40.22 (C8), 41.79
(C6), 57.89 (9-OCH3), 59.00 (C9a), 62.73 (OCH2),
77.52 (C9), 84.32 (C5a), 109.60 (C4), 125.24 (C1a),
125.57 (C3), 127.02 (C1), 142.46 (C2), 164.45 (C4a),
171.45 (9a-C=O), 204.42 (C7). Mass spectrum, m/z
(Irel, %): 335.2 (0.78) [M]+, 262.1 (4.27), 235.1 (77.4),
207.1 (52.4), 190.0 (31.0), 176.1 (18.7), 143.0 (14.3),
130.0 (13.4), 100.0 (100). Found: m/z 335.1000 [M]+.
C16H17NO7. Calculated: M 335.0999.
Ethyl (5aS,9R,9aS)-1,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIIe). Yield 15% (c), oily substance.
1H NMR spectrum, δ, ppm (J, Hz): 1.19 t (3H,
CH2CH3, J = 7.1), 2.57 d.d (1H, 8-H, J = 18.1, 3.4),
2.62 d.d (1H, 8-H, J = 18.1, 6.2), 2.77 d.d (1H, 6-H,
J = 16.4, 5.1), 2.96 d.d (1H, 6-H, J = 16.4, 7.9), 3.26 s
(3H, 9-OCH3), 3.80 s (3H, 1-OCH3), 4.19 q (2H,
OCH2, J = 7.1), 4.55 d.d (1H, 9-H, J = 6.2, 3.4),
5.19 d.d (1H, 5a-H, J = 7.9, 5.1), 6.46 d.d (1H, 4-H,
J = 8.2, 0.7), 6.46 d.d (1H, 2-H, J = 8.0, 0.7), 7.17 d.d
Ethyl (5aS,9R,9aS)-3-diethylamino-9-methoxy-7-
oxo-5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-
carboxylate (VIIh). Yield 42% (c), oily substance.
1H NMR spectrum, δ, ppm (J, Hz): 1.12 t [6H,
N(CH2CH3)2, J = 7.1], 1.29 t (3H, OCH2CH3, J = 7.2),
2.09 d.d (1H, 8-H, J = 17.8, 11.8), 2.57 d.d (1H, 8-H,
J = 17.8, 3.6), 2.72 d.d (1H, 6-H, J = 17.1, 3.6),
2.91 d.d (1H, 6-H, J = 17.0, 3.7), 3.28 q [4H,
N(CH2CH3)2, J = 7.1], 3.35 s (3H, 9-OCH3), 4.18 d.d
(1H, 9-H, J = 11.8, 3.6), 4.24 q (2H, OCH2, J = 7.2),
5.14 d.d (1H, 5a-H, J = 3.7, 3.6), 6.05 d (1H, 4-H, J =
2.3), 6.25 d.d (1H, 2-H, J = 8.6, 2.3), 7.23 d (1H, 1-H,
J = 8.6). 13C NMR spectrum, δC, ppm: 12.55
[N(CH2CH3)2], 14.15 (OCH2CH3), 40.66 (C8), 42.98
(C6), 44.50 [N(CH2CH3)2], 57.85 (9-OCH3), 59.04
(C9a), 61.80 (OCH2), 78.34 (C9), 82.56 (C5a), 92.20
(C4), 104.89 (C2), 109.32 (C1a), 128.85 (C1), 149.85
(C3), 161.40 (C4a), 173.61 (9a-C=O), 206.62 (C7).
Mass spectrum, m/z (Irel, %): 361.3 (84.6) [M]+, 346.2
13
(1H, 3-H, J = 8.2, 8.0). C NMR spectrum, δC, ppm:
14.01 (CH2CH3), 41.10 (C8), 42.75 (C6), 55.38
(1-OCH3), 58.14 (9-OCH3), 59.66 (C9a), 61.78
(OCH2), 75.96 (C9), 83.64 (C5a), 103.14 and 103.68
(C2, C4), 113.58 (C1a), 130.93 (C3), 157.36 (C1), 160.75
(C4a), 172.26 (9a-C=O), 206.62 (C7). Found:
m/z 320.1256 [M]+. C17H20O6. Calculated: M 320.1254.
Ethyl (5aS,9R,9aS)-2,4-di-tert-butyl-9-methoxy-
7-oxo-5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-
carboxylate (VIIf). Yield 29% (c), 10% (d); oily sub-
stance. 1H NMR spectrum, δ, ppm (J, Hz): 1.28 s (9H,
t-Bu), 1.30 t (3H, CH2CH3, J = 7.1), 1.30 s (9H, t-Bu),
2.05 d.d (1H, 8-H, J = 17.8, 11.3), 2.55 d.d (1H, 8-H,
J = 17.7, 3.6), 2.77 d.d (1H, 6-H, J = 16.9, 3.6),
2.96 d.d (1H, 6-H, J = 16.9, 4.0), 3.31 s (3H, OCH3),
4.23 d.d (1H, 9-H, J = 11.3, 3.6), 4.26 q (2H, OCH2,
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DIELS–ALDER REACTIONS WITH ETHYL 1-BENZOFURAN-3-CARBOXYLATES
883
(20.1), 330.2 (0.73), 315.2 (3.76), 288.2 (36.7), 274.2
(95.6), 261.2 (85.1), 246.1 (100), 218.1 (8.76), 202.1
(54.5), 158.1 (11.4). Found: m/z 361.1882 [M]+.
C20H27NO5. Calculated: M 361.1884.
J = 17.8, 2.6, 1.0), 3.85 s (3H, 4-OCH3), 4.33 q (2H,
OCH2, J = 7.1), 5.48 d.d.d (1H, 5a-H, J = 4.1, 2.6,
1.9), 6.70 d.d (1H, 9-H, J = 10.1, 1.9), 6.83 d.d (1H,
3-H, J = 10.2, 1.9), 6.85 d.d (1H, 8-H, J = 10.2, 1.0),
6.93 d.d (1H, 2-H, J = 7.9, 7.6), 7.08 d.d (1H, 1-H,
J = 7.6, 1.0). 13C NMR spectrum, δC, ppm: 14.16
(CH2CH3), 37.86 (C6), 56.08 (4-OCH3), 62.53 (C9a),
62.73 (OCH2), 82.77 (C5a), 112.98 (C3), 115.22
(C2), 122.55 (C1), 129.72 (C8), 133.76 (C1a), 142.54
(C9), 145.04 (C4), 147.14 (C4a), 165.29 (9a-C=O),
196.71 (C7).
Ethyl (5aS,9R,9aS)-2-bromo-9-methoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIIi). Yield 23% (c), oily substance.
1H NMR spectrum, δ, ppm (J, Hz): 1.29 t (3H,
CH2CH3, J = 7.1), 2.01 d.d (1H, 8-H, J = 17.9, J =
11.6), 2.61 d.d (1H, 8-H, J = 18.0, 3.7), 2.75 d.d (1H,
6-H, J = 17.1, 3.8), 2.93 d.d (1H, 6-H, J = 17.1, 3.8),
3.34 s (3H, 9-OCH3), 4.23 d.d (1H, 9-H, J = 11.6, 3.4),
4.28 q (2H, OCH2, J = 7.1), 5.20 d.d (1H, 5a-H, J =
3.8, 3.8), 6.64 d (1H, 4-H, J = 8.5), 7.31 d.d (1H, 3-H,
J = 8.5, 2.2), 7.56 d (1H, 1-H, J = 2.2). 13C NMR
spectrum, δC, ppm: 14.09 (CH2CH3), 40.37 (C8), 41.92
(C6), 57.85 (9-OCH3), 59.62 (C9a), 62.35 (OCH2),
77.75 (C9), 82.85 (C5a), 111.04 (C4), 112.82 (C1a),
125.88 (C2), 131.72 (C3), 132.79 (C1), 158.83 (C4a),
172.17 (9a-C=O), 205.33 (C7). Mass spectrum, m/z
(Irel, %): 370.1 (8.76) [M + 2]+, 369.1 (1.60) [M + 1]+,
368.1 (8.61) [M]+, 297.1 (1.90), 295.1 (1.99), 270.0
(50.0), 268.0 (52.4), 242.0 (14.5), 240.0 (15.2), 225.0
(17.4), 223.0 (16.0), 211.0 (13.9), 209.0 (13.4), 189.1
(2.19), 100.1 (100). Found: m/z 368.0258 [M]+.
C16H17BrO5. Calculated: M 368.0254.
Ethyl 4′-hydroxybiphenyl-2-yl carbonate (IXa).
1
Yield 13% (e). H NMR spectrum, δ, ppm (J, Hz):
1.18 t (3H, CH2CH3, J = 7.1), 4.11 q (2H, OCH2, J =
7.1), 5.11 br.s (1H, OH), 6.84 m (2H, 3′-H, 5′-H),
7.18 d.d (1H, J = 7.0, 2.1), 7.29 d.d (1H, J = 7.0, J =
2.1), 7.31 d.d (1H, J = 7.0, 2.0), 7.32 m (2H, 2′-H,
13
6′-H), 7.36 d.d (1H, J = 7.0, 2.0). C NMR spectrum,
δC, ppm: 14.07 (CH2CH3), 64.71 (OCH2), 115.28 (C3′,
C5′), 122.29, 126.56, 128.17, 129.67, 130.26 (C2′, C6′),
130.82, 134.31, 148.10, 153.26, 155.13. Mass spec-
trum, m/z (Irel, %): 258.1 (40.9) [M]+, 214.1 (16.8),
199.1 (12.6), 186.1 (100), 171.1 (3.72), 157.1 (35.3),
139.1 (10.4), 128.1 (22.5). Found: m/z 258.0889 [M]+.
C16H16O5. Calculated: M 258.0887.
Ethyl 4′-hydroxy-3-methoxybiphenyl-2-yl car-
bonate (IXb). Yield 25% (c), 44% (d); mp 136.5–
Ethyl (5aS,9R,9aS)-2-iodo-4,9-dimethoxy-7-oxo-
5a,6,7,8,9,9a-hexahydrodibenzo[b,d]furan-9a-car-
boxylate (VIIj). Yield 49% (c), 40% (d); oily sub-
1
137°C (from CH2Cl2). H NMR spectrum, δ, ppm
1
(J, Hz): 1.26 t (3H, CH2CH3, J = 7.1), 3.88 s (3H,
OCH3), 4.20 q (2H, OCH2, J = 7.1), 5.06 br.s (1H,
OH), 6.84 m (2H, 3′-H, 5′-H), 6.94 d.d (1H, 4-H, J =
8.3, 1.0), 6.96 d.d (1H, 6-H, J = 7.8, 1.0), 7.24 d.d (1H,
stance. H NMR spectrum, δ, ppm (J, Hz): 1.28 t (3H,
CH2CH3, J = 7.1), 2.10 d.d (1H, 8-H, J = 17.9, 11.2),
2.59 d.d (1H, 8-H, J = 17.8, 3.5), 2.75 d.d (1H, 6-H,
J = 17.1, 4.0), 3.00 d.d (1H, 6-H, J = 17.2, 4.0), 3.33 s
(3H, 9-OCH3), 3.81 s (3H, 4-OCH3), 4.21 d.d (1H,
9-H, J = 11.2, 3.5), 4.26 q (2H, OCH2, J = 7.1),
5.26 d.d (1H, 5a-H, J = 4.0, 4.0), 7.06 d (1H, 3-H, J =
1.6), 7.43 d (1H, 1-H, J = 1.6). 13C NMR spectrum, δC,
ppm: 14.08 (CH2CH3), 40.47 (C8), 41.86 (C6), 56.17
(4-OCH3), 57.92 (9-OCH3), 60.07 (C9a), 62.36
(OCH2), 77.77 (C9), 82.09 (C2), 83.34 (C5a), 121.45
(C3), 126.84 (C1a), 129.29 (C1), 144.91 (C4), 148.55
(C4a), 171.99 (9a-C=O), 205.13 (C7). Mass spectrum,
m/z (Irel, %): 446.1 (31.0) [M]+, 346.0 (100), 318.0
(9.98), 301.0 (8.00), 287.0 (24.0), 274.0 (11.5), 220.1
(2.77), 100.1 (77.2). Found: m/z 446.0223 [M]+.
C17H19IO6. Calculated: M 446.0221.
13
5-H, J = 8.3, 7.8), 7.33 m (2H, 2′-H, 6′-H). C NMR
spectrum, δC, ppm: 14.09 (CH2CH3), 56.13 (OCH3),
64.83 (OCH2), 111.00, 115.21 (C3′, C5′), 122.24,
126.58, 129.38, 130.27 (C2′, C6′), 135.49, 137.32,
151.66, 153.10, 155.22. Mass spectrum, m/z (Irel, %):
288.2 (16.1) [M]+, 244.2 (23.9), 229.1 (7.77), 216.1
(100), 200.1 (24.8), 184.1 (23.2), 173.1 (17.4), 155.1
(7. 28), 144. 1 (12. 6), 115. 1 (12. 4). Found:
m/z 288.0994 [M]+. C16H16O5. Calculated: M 288.0992.
Ethyl 4′-hydroxy-5-methoxybiphenyl-2-yl car-
bonate (IXd). Yield 16% (d), oily substance. 1H NMR
spectrum, δ, ppm (J, Hz): 1.17 t (3H, CH2CH3, J =
7.1), 3.80 s (3H, OCH3), 4.10 q (2H, OCH2, J = 7.1),
5.95 br.s (1H, OH), 6.80 m (2H, 3′-H, 5′-H), 6.82 d.d
(1H, 4-H, J = 8.8, 3.0), 6.88 d (1H, 6-H, J = 3.0),
7.08 d (1H, 3-H, J = 8.8), 7.29 m (2H, 2′-H, 6′-H).
13C NMR spectrum, δC, ppm: 14.03 (CH2CH3), 55.61
(OCH3), 64.70 (OCH2), 113.16, 115.28 (C3′, C5′),
Ethyl (5aS,9aR)-4-methoxy-7-oxo-5a,6,7,9a-
tetrahydrodibenzo[b,d]furan-9a-carboxylate
1
(VIIIb). Yield 56% (f), oily substance. H NMR spec-
trum, δ, ppm (J, Hz): 1.35 t (3H, CH2CH3, J = 7.1),
2.91 d.d (1H, 6-H, J = 17.8, 4.1), 3.17 d.d.d (1H, 6-H,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 6 2013

KIL’MET’EV et al.
884
115.69, 122.98, 129.25, 130.08 (C2′, C6′), 135.21,
141.74, 153.75, 155.58, 157.55.
according to the riding model. The coordinates of
atoms and bond lengths and angles in structures VIa,
VIIg, and IXb were deposited to the Cambridge Crys-
tallographic Data Centre (entry nos. CCDC 860246,
860247, and 824884, respectively).
Ethyl 4′-hydroxy-6-methoxybiphenyl-2-yl car-
1
bonate (IXe). Yield 41% (c), oily substance. H NMR
spectrum, δ, ppm (J, Hz): 1.15 t (3H, CH2CH3, J =
7.1), 3.74 s (3H, OCH3), 4.07 q (2H, OCH2, J = 7.1),
5.58 br.s (1H, OH), 6.81 m (2H, 3′-H, 5′-H), 6.82 d.d
(1H, 5-H, J = 8.2, 1.0), 6.96 d.d (1H, 3-H, J = 8.3, 1.0),
7.17 m (2H, 2′-H, 6′-H), 7.29 d.d (1H, 4-H, J = 8.3,
8.2). 13C NMR spectrum, δC, ppm: 14.08 (CH2CH3),
56.04 (OCH3), 64.76 (OCH2), 109.09, 114.38, 114.95
(C3′, C5′), 123.98, 125.49, 128.40, 131.54 (C2′, C6′),
149.29, 153.33, 155.03, 157.83. Mass spectrum, m/z
(Irel, %): 288.2 (84.9) [M]+, 244.2 (21.1), 229.2 (14.7),
216.2 (100), 200.1 (34.9), 184.1 (28.4), 173.1 (11.1),
155.1 (10.9), 144.1 (11.3), 115.1 (14.1). Found:
m/z 288.0989 [M]+. C16H16O5. Calculated: M 288.0992.
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 12-03-00 535) and by the Program for Support of
Leading Scientific Schools (project no. NSh-
3986.2012.3).
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