Reactions of 6,7-dimethoxy- 3,4-dihydroisoquinoline with o-quinone methides

Article abstract of DOI:10.1007/s10593-011-0845-6

Products of heterocyclization, 9,10-dimethoxy-12,13-dihydro-7aH,15H- naphtho[1',2':5,6][1,3]oxazino-[2,3-a]isoquinolines, were isolated on condensing 6,7-dimethoxy-3,4-dihydroisoquinoline with 1-dimethylaminomethyl-2-naphthols. In the case of o-hydroxybenzyl alcohols products of a Michael aza reaction, 2-[(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)methyl]phenols were obtained.

Full text of DOI:10.1007/s10593-011-0845-6

Chemistry of Heterocyclic Compounds, Vol. 47, No. 7, October 2011 (Russian original Vol. 47, No. 7, July, 2011)
REACTIONS OF 6,7-DIMETHOXY-
3,4-DIHYDROISOQUINOLINE
WITH o-QUINONE METHIDES
V. A. Osyanin1*, E. A. Ivleva1,
D. V. Osipov1, and Yu. N. Klimochkin1
Products of heterocyclization, 9,10-dimethoxy-12,13-dihydro-7aH,15H-naphtho[1',2':5,6][1,3]oxazino-
[2,3-a]isoquinolines, were isolated on condensing 6,7-dimethoxy-3,4-dihydroisoquinoline with
1-dimethylaminomethyl-2-naphthols. In the case of o-hydroxybenzyl alcohols products of a Michael aza
reaction, 2-[(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)methyl]phenols were obtained.
Keywords: o-hydroxybenzyl alcohols, Mannich bases, 6,7-dimethoxy-3,4-dihydroisoquinoline,
o-quinone methides.
Among the condensed benzo[5,6][1,3]oxazino[2,3-a]isoquinolines highly selective antagonists of mus-
carinic M4 receptors are known [1, 2]. In addition, compounds of this type are of interest as structural analogs of
certain isoquinoline alkaloids, primarly tetrahydroprotoberberines [3].
There are only a few examples in the literature of [4+2] cycloaddition of o-quinone methides to azome-
thines [1, 2, 4–8]. At the same time this reaction opens a route to various polyaminals of heterocyclic series.
We have shown that on interacting Mannich bases of the naphthalene series 1a–f with 6,7-dimethoxy-
3,4-dihydroisoquinoline 2 in boiling ethanol or o-xylene, 9,10-dimethoxy-12,13-dihydro-7aH,15H-naphtho-
[1',2':5,6][1,3]oxazino[2,3-a]isoquinolines 3a–f were formed.
MeO
MeO
R
NMe2
OH
R
N
R
N
MeO
MeO
Ethanol, 78°C
– NHMe2
2
O
O
1a–f
3a–f
a R = H, b R = Ph , c R = 4-MeOC6H4, d R = 2-FC6H4, e R = 3-O2NC6H4, f R = 2-thienyl
_______
*To whom correspondence should be addressed, e-mail: vosyanin@mail.ru.
1Samara State Technical University, 244 Molodogvardeiskaya St., Samara 443100, Russia.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1032–1038, July, 2011. Original
article submitted January 18, 2011.
0009-3122/11/4707-0845©2011 Springer Science+Business Media, Inc.
845

The formation of the cyclic system 12,13-dihydro-7aH,15H-naphtho-[1',2':5,6][1,3]oxazino[2,3-a]iso-
quinoline may be interpreted as the result of a [4+2] cycloaddition reaction with inverted electronic requirements
[9]. o-Quinone methide, generated from the Mannich base, plays the role of diene component and
6,7-dimethoxy-3,4-dihydroisoquinoline – the role of heterodienophile.
The obtained oxazino[2,3-a]isoquinolines 3a–f are high-melting, thermostable crystalline substances,
readily soluble in chloroform and ethyl acetate, and poorly soluble in water and lower alcohols.
In the mass spectra of compounds 3a–f, the peaks for the molecular ions are of low intensity. The main
direction of fragmentation is retro-Diels–Alder reaction, leading to 6,7-dimethoxy-3,4-dihydroisoquinoline and
1
corresponding o-quinone methide. In the H NMR spectra, the protons of the methoxy group were displayed as
singlets in the 3.84–3.95 region, signals were present for the protons of the methylene units at 2.74-4.78, and
also the signals of the methine protons as singlets in the 5.40–5.82 ppm region. The aromatic protons of the
isoquinoline fragment resonate at 6.63–6.98 ppm.
The interaction of 3,4-dihydroisoquinoline and salicylic alcohol described in the literature (170oC,
sealed tube) [7] leads to 4-(2-hydroxybenzyl)isoquinoline, together with 1,2,3,4-tetrahydroisoquinoline. At the
same time in the reaction of 6,7-dimethoxy-3,4-dihydroisoquinoline 2 with o-hydroxybenzyl alcohols 4a,b in
boiling o-xylene, in place of the products of C-alkylation or cycloaddition expected by us, 2-[(6,7-dimethoxy-
3,4-dihydro-2(1H)-isoquinolinyl)methyl]phenols 5a,b were isolated.
OH
HO
MeO
MeO
2
2
OH
N
R
MeO
MeO
O
R
N
4a,b
R
o-Xylene,
145°C
2
MeO
MeO
N
OH
R
4, 5 a R = H, b R = Br
5a,b
It might have been assumed that in boiling xylene, unlike in ethanol, disproportionation takes place initially
of 6,7-dimethoxy-3,4-dihydroisoquinoline into 6,7-dimethoxyisoquinoline and 6,7-dimethoxy-1,2,3,4-tetra-
hydroisoquinoline [10] and then further interaction with o-quinone methide, leading to the formation of phenols
5a,b. However, extended heating of 6,7-dimethoxy-3,4-dihydroisoquinoline (2) in boiling o-xylene in the
absence of o-hydroxybenzyl alcohols does not lead to its disproportionation. Consequently, the mechanism of
the reaction apparently includes initial quaternization of 6,7-dimethoxy-3,4-dihydroisoquinoline 2, as a result of
which its electrophilic character is increased and migration of hydrogen is facilitated, formally corresponding to
a process of intermolecular hydride transfer.
846

O
2
4a,b
–H2O
R
R
R
MeO
MeO
MeO
MeO
MeO
MeO
+
N
N
N
H
–
–
OH
H
OH
5a,b
+
MeO
H
+
MeO
OMe
N
N
MeO
Phenols 5a,b were obtained by a counter synthesis in higher yields from 6,7-dimethoxy-1,2,3,4-tetra-
hydroisoquinoline and o-hydroxybenzyl alcohols 4a,b.
MeO
MeO
MeO
4a,b
O
H
.
.
.
N
NH
o-Xylene,
MeO
145°C
R
5a,b
In the IR spectra of compounds 5a,b, bands were present for the stretching vibrations of the associated
hydrogen bonds of the OH groups in the 2300–3300 cm–1 region, which refutes the formation of a cyclic
structure. In the mass spectra peaks for molecular ions were of low intensity. The main direction of fragmenta-
tion was cleavage of a hydroxybenzyl fragment. In the 1H NMR spectra the protons of the methylene units were
displayed at 2.84–3.88 and signals were present for the protons of the methoxy groups at 3.81–3.86 ppm. Shift
of the signal of the OH group protons to the low field region (>10 ppm) confirms the presence of an
intramolecular hydrogen bond.
EXPERIMENTAL
The IR spectra were described on a Shimadzu FTIR 8400S instrument in KBr disks. The 1H NMR spec-
tra were recorded on a JEOL JNM-ECX 400 spectrometer (at 400 MHz) in CDCl3, internal standard was TMS.
Mass spectra were obtained on a Finnigan Trace DSQ instrument, energy of ionizing electrons was 70 eV.
Elemental analysis was carried out on a EuroVector EA 3000 automatic CHNS analyzer.
The initial Mannich bases 1a–f were obtained from 2-naphthol by the known procedure of [11, 12].
9,10-Dimethoxy-12,13-dihydro-7aH,15H-naphtho[1',2':5,6][1,3]oxazino[2,3-a]isoquinoline (3a). A
mixture of dihydroquinoline 2 (1 g, 5.3 mmol) and Mannich base 1a (1.05 g, 5.2 mmol) in ethanol (10 ml) was
boiled for 1 h, cooled to room temperature, and maintained for 2 h at -20oC. The precipitated solid was filtered
off, washed with ice-cold methanol, and recrystallized from an ethanol–DMF mixture. Compound 3a (1.20 g,
66%) was obtained as colorless crystals, mp 186–187oC (lit., mp 185–187oC [4]). IR spectrum, ν, cm–1: 3001,
2958, 2935, 2885, 2839, 1624, 1601, 1516, 1470, 1393, 1354, 1327, 1269, 1227, 1126, 1107, 1018, 860.
1H NMR spectrum, δ, ppm (J, Hz): 2.91 (2H, br. s, CH2); 3.08 (2H, br. s, CH2); 3.91 (3H, s, CH3); 3.95 (3H, s,
CH3); 4.78 (2H, s, CH2); 5.75 (1H, s, CH); 6.71 and 6.98 (2H, s, H-8,11); 7.11 (1H, d, J = 8.8, H Ar); 7.36–7.41
(1H, m, H Ar); 7.50–7.54 (2H, m, H Ar); 7.68 (1H, d, J = 8.1, H Ar); 7.80 (1H, d, J = 7.3, H Ar). Mass
spectrum, m/z (Irel, %): 347 [M]+ (27), 346 [M-H]+ (9), 330 [M-OH]+ (6), 191 [C11H13NO2]+ (100), 190
847

[C11H12NO2]+ (92), 176 [C10H10NO2]+ (73), 156 [C11H8O]+ (31), 128 [C10H8]+ (62). Found, %: C 76.11; H 6.13;
N 3.98. C22H21NO3. Calculated, %: C 76.06; H 6.09; N 4.03.
9,10-Dimethoxy-15-phenyl-12,13-dihydro-7aH,15H-naphtho[1',2':5,6][1,3]oxazino[2,3-a]isoquino-
line (3b) was obtained analogously to compound 3a from dihydroisoquinoline 2 (1 g, 5.2 mmol) and Mannich
base 1b (1.45 g, 5.2 mmol) in ethanol (15 ml). Yield 1.62 g (73%) of colorless crystals; mp 155–156oC
(2-propanol–DMF). IR spectrum, ν, cm–1: 3063, 3024, 2955, 2916, 2858, 2839, 1620, 1597, 1520, 1466, 1427,
1
1393, 1362, 1327, 1269, 1231, 1126, 1022, 976. H NMR spectrum, δ, ppm (J, Hz): 2.81 (1H, dd, J = 15.4,
J = 2.2, CH2); 3.08 (1H, dd, J = 9.5, J = 5.9, CH2); 3.20–3.41 (2H, m, CH2); 3.88 (3H, s, CH3); 3.89 (3H, s,
CH3); 5.46 (1H, s, CH); 5.63 (1H, s, CH); 6.70 and 6.85 (2H, s, H-8,11); 7.17 (1H, d, J = 8.8, H-6); 7.28–7.34
(7H, m, H Ar); 7.46 (1H, d, J = 7.3, H-1); 7.46–7.83 (2H, m, H Ar). Mass spectrum, m/z (Irel, %): 423 [M]+ (11),
406 [M-OH]+ (4), 232 [C17H12O]+ (35), 231 [C17H11O]+ (100), 202 (28), 191 [C11H13NO2]+ (77), 190
[C11H12NO2]+ (26), 176 [C10H10NO2]+ (35). Found,%: C 79.49; H 6.02; N 3.36. C28H25NO3. Calculated, %:
C 79.41; H 5.95; N 3.31.
9,10-Dimethoxy-15-(4-methoxyphenyl)-12,13-dihydro-7aH15H-naphtho[1',2':5,6][1,3]oxazino-
[2,3-a]isoquinoline (3c) was obtained analogously to compound 3a from dihydroisoquinoline 2 (1 g, 5.2 mmol)
and Mannich base 1c (1.61 g, 5.2 mmol) in ethanol (15 ml). Yield 1.78 g (75%) of colorless crystals;
mp 182-184oC (2-propanol–DMF). IR spectrum, ν, cm–1: 3063, 2928, 2839, 1616, 1512, 1466, 1393, 1273,
1234, 1126, 1026, 976, 852. 1H NMR spectrum, δ, ppm (J, Hz): 2.90 (1H, dd, J = 15.6, J = 2.3, CH2); 3.05–3.09
(1H, m, CH2); 3.21 (1H, td, J = 15.6, J = 5.5, CH2); 3.30–3.36 (1H, m, CH2); 3.76 (3H, s, CH3); 3.88 (6H, s,
2CH3); 5.40 (1H, s, CH); 5.62 (1H, s, CH); 6.68 and 6.83 (2H, s, H-8,11); 6.80 (2H, d, J = 8.5, H Ar); 7.14 (1H,
d, J = 8.8, H Ar); 7.20 (2H, d, J = 7.8, H Ar); 7.29–7.36 (2H, m, H Ar); 7.45 (1H, d, J = 7.8, H Ar); 7.73 (1H, d,
J = 8.8, H Ar); 7.79 (1H, d, J = 7.3, H Ar). Mass spectrum, m/z, (Irel, %): 453 [M]+ (8), 436 [M-OH]+ (1), 262
[C18H14O2]+ (43), 261 [C18H13O2]+ (88), 247 (26), 231 [C17H11O]+ (100), 218 (22), 202 (7), 191 [C11H13NO2]+
(61), 190 [C11H12NO2]+ (28), 176 [C10H10NO2]+ (33). Found, %: C 76.85; H 5.96; N 3.14. C29H27NO4.
Calculated, %: C 76.80; H 6.00; N 3.09.
15-(2-Fluorophenyl)-9,10-dimethoxy-12,13-dihydro-7aH,15H-naphtho[1',2':5,6][1,3]oxazino-
[2,3-a]isoquinoline (3d) was obtained analogously to compound 3a from dihydroisoquinoline 2 (1 g, 5.2 mmol)
and Mannich base 1d (1.55 g, 5.2 mmol) in ethanol (15 ml). Yield 1.29 g (56%), colorless crystals; mp
160-161oC (2-propanol–DMF). IR spectrum.ν, cm–1: 3063, 2951, 2839, 1620, 1601, 1520, 1485, 1466, 1393,
1335, 1269, 1231, 1126, 1022, 856. 1H NMR spectrum, δ, ppm (J, Hz): 2.74 (1H, d, J = 15.6, CH2); 3.12–3.21
(2H, m, CH2); 3.31–3.40 (1H, m, CH2); 3.84 (3H, s, CH3); 3.87 (3H, s, CH3); 5.68 (1H, s, CH); 5.71 (1H, s,
CH); 6.64 and 6.85 (2H, s, H-8,11); 6.88–6.93 (2H, m, H Ar); 7.01–7.14 (2H, m, H Ar); 7.20–7.29 (4H, m,
H Ar); 7.72–7.76 (2H, m, H Ar). Mass spectrum, m/z, (Irel, %): 441 [M]+ (40), 424 [M–OH]+ (10), 250
[C17H11FO]+ (46), 249 [C17H10FO]+ (98), 231 [C17H11O]+ (97), 220 (40), 202 (18), 191 [C11H13NO2]+ (100), 190
[C11H12NO2]+ (78), 176 [C10H10NO2]+ (84). Found, %: C 76.22; H 5.53; N 3.15. C28H24FNO3. Calculated, %:
C 76.17; H 5.48; N 3.17.
9,10-Dimethoxy-15-(3-nitrophenyl)-12,13-dihydro-7aH,15H-naphtho[1',2':5,6][1,3]oxazino[2,3-a]-
isoquinoline (3e) was obtained analogously to compound 3a from dihydroisoquinoline 2 (1 g, 5.2 mmol) and
Mannich base 1e (1.69 g, 5.2 mmol) in ethanol (15 ml). Yield 1.91 g (78%), colorless crystals; mp 203-204oC
(2-propanol). IR spectrum, ν, cm–1: 3063, 2928, 2858, 1620, 1528 (NO2), 1466, 1392, 1350 (NO2), 1269, 1231,
1
1126, 1022, 856. H NMR spectrum, δ, ppm (J, Hz): 2.83 (1H, dd, J = 13.2, J = 2.9) and 3.12–3.44 (3H, m,
2CH2); 3.89 (6H, s, 2CH3); 5.48 (2H, s, 2CH); 6.70 and 6.83 (2H, s, H-8,11); 7.17 (1H, d, J = 8.8, H Ar);
7.27-7.35 (3H, m, H Ar); 7.43 (1H, dd, J = 8.1, J = 7.3, H Ar); 7.58 (1H, d, J = 7.3, H Ar); 7.78–7.84 (2H, m,
H Ar); 8.13 (1H, d, J = 8.1, H Ar); 8.27 (1H, s, H Ar). Mass spectrum, m/z (Irel, %): 468 [M]+ (12), 451 [M-OH]+
(3), 277 [C17H11NO3]+ (15), 276 [C17H10NO3]+ (23), 260 (37), 231 [C17H11O]+ (42), 230 [C17H10O]+ (79), 202
(52), 191 [C11H13NO2]+ (100), 190 [C11H12NO2]+ (47), 176 [C10H10NO2]+ (64). Found, %: C 71.82; H 5.13;
N 6.04. C28H24N2O5. Calculated, %: C 71.78; H 5.16; N 5.98.
848

9,10-Dimethoxy-15-(2-thienyl)-12,13-dihydro-7aH,15H-naphtho[1',2':5,6][1,3]oxazino[2,3-a]iso-
quinoline (3f) was obtained analogously to compound 3a from dihydroisoquinoline 2 (1 g, 5.2 mmol) and
Mannich base 1f (1.48 g, 5.2 mmol) in ethanol (15 ml). Yield 1.82 g (81%), colorless crystals; mp 199-200oC
(ethanol). IR spectrum, ν, cm–1: 3063, 2955, 2843, 1620, 1597, 1520, 1466, 1423, 1389, 1358, 1331, 1269, 1227,
1130, 1018, 976, 856. 1H NMR spectrum, δ, ppm (J, Hz): 2.77 (1H, d, J = 11.9, CH2); 3.02-3.08 (1H, m, CH2);
3.19–3.31 (2H, m, CH2); 3.88 (6H, s, 2CH3); 5.60 (1H, s, CH); 5.82 (1H, s, CH); 6.63 and 6.83 (2H, br. s, H
thiophene); 6.69 and 6.87 (2H, s, H-8,11); 7.10 (1H, d, J = 8.7, H Ar); 7.26 (1H, d, J = 4.6, H thiophene); 7.33
(1H, dd, J = 8.3, J = 6.9, H Ar); 7.41 (1H, dd, J = 7.8, J = 6.9, H Ar); 7.64 (1H, d, J = 8.3, H Ar); 7.73 (1H, d,
J = 8.7, H Ar); 7.79 (1H, d, J = 7.8, H Ar). Mass spectrum, m/z (Irel, %): 429 [M]+ (9), 412 [M-OH]+ (1), 238
[C15H10SO]+ (58), 237 [C15H9SO]+ (100), 209 (12), 208 (22), 191 [C11H13NO2]+ (62), 190 [C11H12NO2]+ (28),
176 [C10H10NO2]+. Found, %: C 72.75; H 5.44; N 3.21; S 7.50. C26H23NO3S. Calculated, %: C 72.70; H 5.40;
N 3.26; S 7.46.
2-[(6,7-Dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)methyl]phenol (5a). A mixture of alcohol 4a
(0.65 g, 5.2 mmol) and dihydroisoquinoline 2 (2 g, 10.4 mmol) (method A) or 6,7-dimethoxy-1,2,3,4-tetra-
hydroisoquinoline (0.98 g, 5.2 mmol) (method B) in o-xylene was boiled for 6 h, the solvent was distilled in
vacuum, the residue was dissolved with heating in methanol (10 ml) and the obtained solution was maintained
for 1 day at -20oC. The precipitated solid was filtered off, washed with a small amount of ice-cold methanol, and
recrystallized from methanol. The product 0.66 g ( 42%, method A) or 0.97 g (62%, method B) was obtained as
colorless crystals; mp 155–156oC (lit. mp 200oC [13]). IR spectrum, ν, cm–1: 2300–3300 (OH), 1609, 1589
(C=C), 1520, 1462, 1420, 1381, 1366, 1350, 1296, 1254, 1227, 1122, 1084, 1014, 987, 964, 860, 768. 1H NMR
spectrum, δ, ppm (J, Hz): 2.87 (4H, br. s, 2CH2); 3.68 (2H, s, CH2); 3.82 (3H, s, CH3); 3.85 (3H, s, CH3); 3.87
(2H, s, CH2); 6.50 and 6.61 (2H, s, H-5',8'); 6.79–6.86 (2H, m, H-4,6); 7.03 (1H, d, J = 7.3, H-3); 7.20 (1H, td,
J = 7.3, J = 1.4, H-5); 10.91 (1H, br. s, OH). Mass spectrum, m/z (Irel, %): 299 [M]+ (33), 298 [M-H]+ (35), 192
[C11H14NO2]+ (67), 176 [C10H10NO2]+ (11), 164 (100), 149 (17), 121 (17), 107 [C7H7O]+ (30). Found, %:
C 72.27; H 7.02; N 4.73. C18H21NO3. Calculated, %: C 72.22; H 7.07; N 4.68.
4-Bromo-2-[(,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)methyl]phenol (5b) was obtained
analogously to compound 5a from alcohol 4b (1.06 g, 5.2 mmol) and dihydroisoquinoline 2 (2 g, 10.4 mmol,
method A) or 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (0.98 g, 5.2 mmol, method B) in o-xylene (30 ml).
Yield 0.97 g (49%) (method A) or 1.36 g (69%) (method B), colorless crystals; mp 154–155oC (methanol). IR
spectrum, ν, cm–1: 2300–3100 (OH), 1609, 1582 (C=C), 1516, 1474, 1450, 1346, 1300, 1258, 1231, 1126, 1084,
1
1018, 964, 825. H NMR spectrum, δ, ppm (J, Hz): 2.87 (4H, br. s, 2CH2); 3.67 (2H, br. s, CH2), 3.83 (3H, s,
CH3); 3.84 (2H, s, CH2); 3.86 (3H, s, CH3); 6.51 and 6.62 (2H, s, H-5',8'); 6.73 (1H d, J = 8.8, H-6); 7.15 (1H, d,
J = 2.2, H-3); 7.27 (1H, dd, J = 8.8, J = 2.2, H-5); 10.82 1H, br. s, OH). Mass spectrum for 79Br isotope, m/z (Irel,
%): 377 [M]+ (32), 376 [M-H]+ (28), 298 [M-Br]+ (2), 192 [C11H14NO2]+ (82), 185 [C7H6BrO]+ (12), 177 (17),
164 (100), 149 (24), 121 (28). Found, %: C 57.20; H 5.35; N 3.66. C18H20BrNO3. Calculated, %: C 57.16;
H 5.33; N 3.70.
The work was carried out with the financial support of the Grants Council of the President of the RF
(program of state support for young Russian students, grant MK-3368.2011.3) and with the use of equipment of
the Collective Use Center "Investigation of the Physicochemical Properties of Substances and Materials".
REFERENCES
1.
2.
3.
T. M. Böhme, C. E. Augelli-Szafran, H. Hallak, T. Pugsley, K. Serpa, and R. D. Schwarz, J. Med.
Chem., 45, 3094 (2002).
C. E. Augelli-Szafran, J. C. Jaen, D. W. Moreland, C. B. Nelson, J. R. Penvose-Yi, and R. D. Schwarz,
Bioorg. Med. Chem. Lett., 8, 991 (1998).
J. Cheng, L. Fu, C. Ling, and Y. Yang, Heterocycles, 81, 2581 (2010).
849

4.
M. von Strandtmann, M. P. Cohen, and J. Shavel Jr, US Patent 3649626; Chem. Abstr., 76, 140844
(1972).
5.
6.
M. von Strandmann, M. P. Cohen, and J. Shavel, Jr., J. Heterocycl. Comp., 6, 429 (1969).
U. Berger, G. Dannhardt, R. Obergrusberger, and W. Wiegrebe, Arch. Pharm. (Weinheim), 314, 365
(1981).
7.
U. Berger, T. Burgemeister, G. Dannhardt, K. K. Mayer, and W. Wiegrebe, Tetrahedron, 40, 215
(1984).
8.
9.
10.
11.
12.
13.
M. Tomasulo and F. M. Raymo, Org. Lett., 7, 4633 (2005).
R. W. Van De Water and T. R. R. Pettus, Tetrahedron, 58, 5367 (2002).
C. I. Brodrick and W. F. Short, J. Chem. Soc., 2587 (1949).
W. R. Brode and J. B. Littman, J. Am. Chem. Soc., 53, 1531 (1931).
S. A. Monti and G. D. Castillo, J. Org. Chem., 56, 3764 (1970).
V. I. Vinogradova and M. S. Yunusov, Khim. Prir. Soed., 331 (1984).
850