Thallium trinitrate-mediated ring contraction of 1,2-dihydronaphthalenes: An approach to the synthesis of indans

Article abstract of DOI:10.1016/S0040-4020(00)01174-1

Oxidation of 1,2-dihydronaphthalenes with thallium trinitrate was studied. 1,2-dihydronaphthalene, 1-methyl-1,2-dihydronaphthalene, 6- and 8-methoxy-1,2-dihydronaphthalenes gave rise to the respective ring contraction products in good yields, whereas the

Full text of DOI:10.1016/S0040-4020(00)01174-1

TETRAHEDRON
Pergamon
Tetrahedron 57 (2001) 1709±1713
Thallium trinitrate-mediated ring contraction of
,2-dihydronaphthalenes: an approach to the synthesis of indans
1
p
Helena M. C. Ferraz, Luiz F. Silva, Jr. and Tiago O. Vieira
Instituto de Qu Âõ mica, Universidade de S aÄ o Paulo, CEP 05508-900, S aÄ o Paulo SP, Brazil
Received 13 November 2000; accepted 21 December 2000
AbstractÐOxidation of 1,2-dihydronaphthalenes with thallium trinitrate was studied. 1,2-dihydronaphthalene, 1-methyl-1,2-dihydro-
naphthalene, 6- and 8-methoxy-1,2-dihydronaphthalenes gave rise to the respective ring contraction products in good yields, whereas the
rearrangement was not observed using 4-methyl-1,2-dihydronaphthalene and 1-n-butyl-4-methyl-1,2-dihydronaphthalene as substrates.
q 2001 Elsevier Science Ltd. All rights reserved.
Thallium(III) salts oxidize a wide variety of different
organic functionalities. A useful transformation promoted
very good yield (Table 2, entry 1). Notably, the reaction
occurred with complete diastereoselectivity, giving rise to
the trans diastereoisomer, as deduced by NMR analysis and
1
by these salts is the ring contraction of simple cyclic
cyclo-
2
ole®ns, such as cyclobutene, cyclohexenes,
3
4,5
12
comparison with other indans data. The trans-1,3 substi-
3
,5
3,5
heptene and cyclooctene. Although this kind of trans4-
formation is well-known since the pioneer work of Kabbe,
the behavior of dihydronaphthalene derivatives toward
thallium(III) salts has never been studied. The most similar
substrates already studied were a few chromenes, which
furnished either ring contraction products, in poor to
tuted indan system, which is not easily achieved by standard
methods, is present in some natural products, such as
mutisianthol and jungianol.
1
3
Reacting 6- and 8-methoxy-1,2-dihydronaphthalenes (7 and
8, respectively) with TTN in methanol, the corresponding
ring contraction products and glycolic derivatives were
isolated, by column chromatography, in reasonable yields
(entries 2 and 4). Since temperature lowering seems to favor
the rearrangement (compare entries 1 and 2, Table 1), the
reaction of 7 was also performed at 2788C, but the same
6
7
moderate yields, or glycolic derivatives.
In our program directed towards the application of
thallium(III) salts in the synthesis of natural and/or bio-
8
,9
logically active compounds, we decided to investigate
the reaction of a series of 1,2-dihydronaphthalenes with
thallium trinitrate (TTN). Such a reaction could lead, in a
single step, to indanic systems, which have the potential to
Table 1. Reaction of 1,2-dihydronaphthalene (1) with TTN
1
0
be biologically active.
This study was initiated by performing the reaction of 1,2-
dihydronaphthalene (1) with TTN in methanol at room
temperature. The ring contraction product 2 was obtained
in 60% yield, together with 8% of trans and cis-1,2-
dimethoxy-1,2,3,4-tetrahydronaphthalenes (3a and 3b)
(
Table 1, entry 1). Acetal 2 was obtained in 77% yield at
lower temperature (entry 2), which corroborates obser-
1
1
vations by Sekizaki et al. Oxidation of 1 with TTN in
ethanol also led to the rearrangement product (4) in good
yield (entry 3).
Under the same reaction conditions, 1-methyl-1,2-dihydro-
naphthalene (6) led to the ring contraction product 11 in
Keywords: oxidation; thallium trinitrate; ring contraction; indans.
Corresponding author. Fax: 11-3815-5579.; e-mail: hmferraz@iq.usp.br
a
p
ca. of 10%, determined by GC, in the crude product.
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)01174-1

1
710
H. M. C. Ferraz et al. / Tetrahedron 57 (2001) 1709±1713
Table 2. Reaction of 1,2-dihydronaphthalenes with TTN
product ratio was observed. Nevertheless, running the
reaction of 7 and 8 with TTN in trimethylorthoformate
Finally, the methodology described above showed to be
useful for directly access a dioxolane derivative. Thus, the
reaction of 1 with TTN in ethyleneglycol furnished 19 as a
single product, in 65% yield as shown in Scheme 1.
(
TMOF) at 08C, made it possible to increase the ratio of
ring contraction product to glycolic derivative (entries 3
and 5).
In conclusion, this work shows that TTN can ef®ciently
promote the ring contraction of 1,2-dihydronaphtha-
lenesÐexceptions are substrates possessing trisubstituted
double bondsÐleading to functionalized indans in very
good yields.
Contrary to the aforementioned examples, substrates
possessing trisubstituted double bonds did not undergo
ring contraction. Thus, naphthalene 9 gave rise to the
dimethoxylated diols 16a and 16b, while 10 led to 17 and
18, together with signi®cant amounts of unidenti®ed by-
products (entries 6 and 7). These results are somewhat
unexpected, because the ring contraction of other ole®ns
bearing an alkyl group at the double bond has already
Herein is demonstrated one more example that thallium
(III)-mediated ring contraction can be an interesting tool
3
,5,11,14
been described. An alternative method for performing
the ring contraction of 1 and 9 has been recently described
1
by Hara et al. using p-Tol-IF . The naphthalene derivative
5
2
18 is presumably formed through an allylic oxidation of 10,
followed by elimination. It is noteworthy that the allylic
oxidation of cyclohexene promoted by thallium(III) salts
1
1,16,17
has been reported in diverse works.
Scheme 1. a: TTN´3H O, HOCH CH OH, room temperature 15 min.
2
2
2

H. M. C. Ferraz et al. / Tetrahedron 57 (2001) 1709±1713
1711
to construct cyclopentane moities from readily available and
inexpensive six-membered ring substrates.
7.0 Hz, 3H), 1.19 (t, Jˆ7.0 Hz, 3H), 1.90±2.01 (m, 1H),
2.09±2.16 (m, 1H), 2.72±2.79 (m, 1H), 2.84±2.90 (m,
1
H), 3.38 (q, Jˆ7.4 Hz, 1H), 3.42±3.50 (m, 2H), 3.57±
3
7.00±7.14 (m, 3H), 7.36±7.37 (m, 1H); C NMR
.65 (m, 1H), 3.66±3.72 (m, 1H), 4.37 (d, Jˆ7.5 Hz, 1H),
1
3
1
. Experimental
(
62.4, 105.5, 124.3, 125.6, 126.0, 126.8, 143.1, 144.9; MS
125 MHz, CDCl ) d 15.3, 15.4, 27.5, 31.4, 48.3, 61.1,
3
Warning. Thallium salts are toxic and must be handled with
care.
1
m/z (%) 220 (M , 0.02), 175 (14), 147 (10), 129 (19), 117
(46), 103 (100), 91 (15), 75 (82), 65 (5), 47 (98). Anal. Calcd
for C H O : C, 76.33; H, 9.15. Found: C, 76.30; H, 8.96.
Substrates 1, 6, 7 and 8 were prepared from the correspond-
ing commercially available 1-tetralones, by reduction
with NaBH (or LiAlH ) followed by dehydration with
1
4
20
2
4
4
1.1.2. trans-1,1-Dimethoxymethyl-3-methyl-2,3-dihydro-
1H-indene (11). A mixture of 1-methyl-1,2-dihydro-
naphthalene (0.113 g, 0.784 mmol), TTN´3H O (0.38 g,
H PO . The naphthalenes 9 and 10 were obtained through
3
4
1
9
Grignard reaction of 1-tetralone and 4-methyl-1-tetralone,
respectively, followed by HCl in situ dehydration.
Thallium(III) nitrate was purchased from Aldrich and
was used as received. Column chromatography was
performed using silica gel Acros 230-400 mesh. TLC
analyses were performed with silica gel plates Merck,
2
0.86 mmol) and MeOH (6 mL) was stirred for 5 min at
08C, affording 11 (0.139 g, 0.670 mmol, 85%) as a colorless
oil; IR (®lm): 2957, 2923, 2869, 2830, 1477, 1459, 1374,
2
1
1
1192, 1125, 1101, 1060, 987, 754 cm ; H NMR
(200 MHz, CDCl ) d 1.24 (d, Jˆ6.5 Hz, 3H), 1.69±1.83
3
1
13
using vanilline solution for visualization. H and
C
(m, H), 2.22±2.34 (m, 1H), 3.20±3.27 (m, 1H), 3.33 (s,
3H), 3.38 (s, 3H), 3.44±3.50 (m, 1H), 4.26 (d, Jˆ7.4 Hz,
NMR spectra were recorded on Bruker spectrometers. IR
spectra were measured on a Perkin±Elmer 1750-FT. Gas
chromatography analyses were performed in a HP-6890
series II.
1
H), 7.13±7.38 (m, 4H); C NMR (50 MHz, CDCl ) d 20.5,
3
3
36.2, 37.7, 46.2, 52.8, 54.4, 107.0, 123.2, 125.6, 126.1,
127.0, 142.2, 149.3; MS m/z (%) 206 (M , 0.05), 175 (4),
1
1
59 (2), 143 (5), 128 (6),115 (8), 91 (5), 75 (100), 63 (1), 47
H O : C, 75.69; H, 8.80. Found: C,
18 2
1.1. General procedure for the oxidation of 1,2-dihydro-
naphthalenes with TTN
(11). Anal. Calcd for C13
75.42; H, 8.54.
To a stirred solution of 1 (0.213 g, 1.64 mmol) in MeOH
(
1.1.3. Reaction of 8-methoxy-1,2-dihydronaphthalene (7)
2
0
5 mL) was added TTN´3H O (0.80 g, 1.1 mmol) at 258C.
2
in MeOH at 08C. A mixture of 7 (0.150 g, 0.934 mmol),
TTN´3H O (0.46 g, 1.0 mmol) and MeOH (5 mL) was
The reagent promptly dissolved. The mixture was stirred for
min and an abundant precipitation was observed. The
resulting suspension was ®ltered through a silica gel pad
2
1
stirred for 5 min at 08C. Puri®cation by ¯ash chroma-
tography (10% EtOAc in hexanes), gave 12 (0.150 g,
0.673 mmol, 72%) and 13 (0.0306 g, 0.138 mmol, 15%).
1,1-Dimethoxymethyl-4-methoxy-2,3-dihydro-1H-indene
(12): colorless oil; IR (®lm): 2942, 2834, 1633, 1593,
(
70-230 mesh, ca. 10 cm), using CH Cl as eluent. The
2 2
®
anhydrous MgSO . The solvent was concentrated under
ltrate was washed with H O, with brine and dried over
2
4
2
1 1
reduced pressure. The residue was puri®ed by column
chromatography (silica gel 230-400 mesh, gradient elution,
1474 cm ; H NMR (200 MHz, CDCl ) d 1.92±2.28 (m,
3
2H), 2.68±2.99 (m, 2H), 3.35 (s, 3H), 3.41 (s, 3H), 3.80 (s,
0
0
3
±50% EtOAc in hexanes), affording
.983 mmol, 60%), 3a (0.010 g, 0.052 mmol, 3%) and
2
(0.189 g,
2H), 3.44±3.52 (m, 1H), 4.31 (d, Jˆ7.4 Hz, 1H), 6.69 (d,
1
8
13
Jˆ7.3 Hz, 1H), 7.01±7.18 (m, 2H); C NMR (50 MHz,
CDCl ) d 26.9, 27.8, 47.9, 52.8, 54.1, 55.0, 107.1, 108.3,
3
1
8
b
(0.017 g, 0.088 mmol, 5%). 1,1-Dimethoxymethyl-
1
indane (2): colorless oil; IR (®lm): 2949, 2937, 2829,
1
117.7, 127.5, 132.3, 144.7, 155.7; MS m/z (%) 222 (M , 2),
191 (8), 159 (6), 147 (7), 131 (2),115 (9), 103 (4), 91 (6), 75
(100), 63 (2), 47 (16). 1,2,5-Trimethoxy-1,2,3,4-tetrahydro-
naphthalene (13): colorless oil; IR (®lm): 2934, 2893, 2828,
21 1
191, 1153, 1123, 1097, 1078, 1058, 750 cm ; H NMR
(
200 MHz, CDCl ) d 1.87±2.05 (m, 1H), 2.10±2.28 (m,
3
1
3
7
3
1
H), 2.74±3.02 (m, 2H), 3.36 (s, 3H), 3.42 (s, 1H), 3.40±
.51 (m, 1H), 4.33 (d, Jˆ7.4 Hz, 1H), 7.12±7.20 (m, 3H),
2
1 1
1589, 1466 cm ; H NMR (200 MHz, CDCl ) d 1.79±2.11
3
1
3
.41 (t, Jˆ4.4 Hz, 1H); C NMR (50 MHz, CDCl ) d 27.3,
(m, 2H), 2.64±2.72 (m, 2H), 3.44 (s, 3H), 3.50 (s, 3H),
3.66±3.73 (m, 1H), 3.79 (s, 3H), 4.25 (d, Jˆ5.1 Hz, 1H),
6.74 (d, Jˆ8.1 Hz, 1H), 6.97 (d, Jˆ7.3 Hz, 1H), 7.13±7.23
3
1.3, 47.4, 52.8, 54.1, 107.1, 124.3, 125.4, 126.0, 126.8,
42.7, 144.7; MS m/z (%) 192 (M , 0.7), 161 (11), 129
1
1
3
(17), 115 (22), 91 (8), 75 (100), 63 (4), 47 (24). HRMS
Calcd for C H O 192.1150, found 192.1149. In an ana-
logous preparation, a mixture of 1 (0.118 g, 0.906 mmol),
MeOH (6 mL) and TTN´3H O (0.44 g, 1.0 mmol) was
stirred for 5 min at 08C, to afford 2 (0.133 g, 0.692 mmol,
77%).
(m, 1H); C NMR (50 MHz, CDCl ) d 19.4, 22.5, 55.2,
3
56.5, 57.3, 77.6, 79.4, 108.8, 121.8, 125.7, 126.3, 135.6;
MS m/z (%) 222 (M , 11), 190 (24), 164 (100), 149
1
2
16
2
1
(58), 134 (10),115 (20), 105 (7), 91 (40), 77 (16), 65 (10),
45 (13). HRMS Calcd for C13
222.1255.
2
H O 222.1256, found
18 3
1
.1.1. 1,1-Diethoxymethylindane (4). A mixture of 1
1.1.4. Reaction of 8-methoxy-1,2-dihydronaphthalene (7)
2
0
(
(
0.115 g, 0.883 mmol), EtOH (5 mL) and TTN´3H O
2
0.43 g, 0.97 mmol) was stirred for 7 min at 08C. Puri®-
in MeOH at 2788C. A mixture of 7
(0.127 g,
0.794 mmol), TTN´3H O (0.39 g, 0.87 mmol) and MeOH
2
cation by ¯ash chromatography (gradient elution, 0±10%
EtOAc in hexanes) gave 4 (0.145 g, 0.658 mmol, 75%) as
a colorless oil; IR (®lm): 2974, 2930, 2884, 1634, 1117,
(6 mL) was stirred for 30 min at 2788C and 5 min at
room temperature. Puri®cation by column chromatography
gave 12 (0.077 g, 0.35 mmol, 44%) and 13 (0.023 g,
0.10 mmol, 13%).
2
1
1
1
062 cm ; H NMR (500 MHz, CDCl ) d 1.12 (t, Jˆ
3

1
712
H. M. C. Ferraz et al. / Tetrahedron 57 (2001) 1709±1713
1.1.5. 1,1-Dimethoxymethyl-4-methoxy-2,3-dihydro-1H-
indene (12). A mixture of 7 (0.113 g, 0.705 mmol),
22.0, 23.0, 26.2, 50.9, 76.1, 81.9, 125.5, 127.1, 127.2,
128.6, 136.2, 138.5.
2
0
TTN´3H O (0.35 g, 0.78 mmol) and TMOF (3.5 mL) was
2
stirred for 5 min at 08C. Puri®cation by ¯ash chroma-
tography (10% EtOAc in hexanes) gave 12 (0.120 g,
1.1.9. Reaction of 4-n-butyl-1-methyl-1,2,3,4-tetrahydro-
naphthalene (10) with TTN at 258C. A mixture of 10
0.540 mmol, 77%) and 13 (0.015 g, 0.069 mmol, 10%).
(0.071 g, 0.35 mmol), TTN´3H O (0.17 g, 0.38 mmol) and
2
MeOH (6 mL) was stirred for 40 min at room temperature.
Puri®cation by column chromatography gave 17 (0.021 g,
0.085 mmol, 23%) and 18. 1-n-Butyl-1,2-dimethoxy-4-
1.1.6. Reaction of 6-methoxy-1,2-dihydronaphthalene (8)
in MeOH. A mixture of 8 (0.123 g, 0.768 mmol),
2
0
TTN´3H O (0.38 g, 0.85 mmol) and MeOH (4 mL) was
2
methyl-1,2,3,4-tetrahydronaphthalene (17): colorless oil;
) d 0.87 (t, Jˆ7.0 Hz, 3H),
1
stirred for 5 min at 08C. Puri®cation by ¯ash chroma-
tography (gradient elution, 10±20% EtOAc in hexanes)
gave 14 (0.116 g, 0.522 mmol, 68%) and 15 (0.014 g,
H NMR (200 MHz, CDCl
0.96±1.42 (m, 2H), 1.38 (d, Jˆ7.0 Hz, 3H), 1.83±2.23
3
(m, 6H), 2.83±2.95 (m, 1H), 3.08 (s, 3H), 3.46 (s, 3H),
1
3
0.063 mmol, 8%). 1-Dimethoxymethyl-6-methoxy-2,3-
dihydro-1H-indene (14): colorless oil; IR (®lm): 2940,
3.47 (dd, Jˆ3.8 and 11.3 Hz, 1H), 7.15±7.40 (m, 4H);
NMR (50 MHz, CDCl ) d 14.0, 22.9, 23.2, 26.5, 30.0, 31.9,
32.2, 50.7, 57.1, 78.6, 79.6, 125.2, 127.6 (2x), 134.9, 142.7.
C
3
2
709, 1612, 1492, 1281 cm ; H NMR (200 MHz,
1
1
1
CDCl ) d 1.87±2.05 (m, 1H), 2.11±2.29 (m, 1H), 2.68±
2
2,23
1
1-n-Butyl-4-methylnaphthalene
NMR (200 MHz, CDCl
) d 0.96 (t, Jˆ7.0 Hz, 3H), 1.39±
1.76 (m, 5H), 2.67 (s, 3H), 3.04 (t, Jˆ7.3 Hz, 2H), 7.24 (d,
(18): colorless oil; H
3
2
3
8
1
5
.95 (m, 2H), 3.36 (s, 3H), 3.40 (m, 1H), 3.43 (s, 3H),
.77 (s, 3H), 4.31 (d, Jˆ7.3 Hz, 1H), 6.73 (dd, Jˆ2.2 and
.5 Hz, 1H), 7.01 (d, Jˆ2.2 Hz, 1H), 7.09 (d, Jˆ8.8 Hz,
3
1
3
Jˆ7.3 Hz, 2H), 7.47±7.55 (m, 2H), 7.99±8.09 (m, 2H);
C
1
3
H); C NMR (50 MHz, CDCl ) d 27.8, 30.3, 47.5, 52.5,
NMR (50 MHz, CDCl ) d 14.0, 19.4, 22.9, 32.8, 33.1,
3
3
4.0, 55.2, 107.0, 110.9, 112.6, 124.5, 136.6, 158.4; MS m/z
124.5, 124.8, 125.1, 125.2, 125.5, 126.3, 131.9, 132.2,
137.1.
1
(
%) 222 (M , 6), 191 (9), 159 (8), 147 (9), 131 (3),115 (9),
103 (5), 91 (6), 75 (100), 63 (2), 47 (16). Anal. Calcd for
C H O : C, 70.24; H, 8.16. Found: C, 69.87; H, 7.86.
1.1.10. 1-Dioxolanyl-2,3-dihydro-1H-indene (19).
mixture of 1 (0.267 g, 2.05 mmol), TTN´3H O (1.00 g,
A
1
3
18
3
1,2,7-Trimethoxy-1,2,3,4-tetrahydronaphthalene (15): color-
less oil; IR (®lm): 2934, 2831, 1613, 1501, 1463, 1264,
2
2.26 mmol) and diethyleneglycol (5 mL) was stirred for
15 min at room temperature, affording pure 19 (0.254 g,
1.34 mmol, 65%) as a colorless oil; IR (®lm): 2947, 2885,
2
1
250, 1110, 1087 cm ; H NMR (200 MHz, CDCl ) d
1
1
1
1
3
.73±1.96 (m, 1H), 2.04±2.19 (m, 1H), 2.62±2.87 (m,
H), 3.46 (s, 3H), 3.53 (s, 3H), 3.67±3.74 (m, 1H), 3.79
2
1
1
1478, 1131, 753 cm ; H NMR (300 MHz, CDCl ) d
3
(
m, 1H), 4.24 (d, Jˆ5.2 Hz, 1H), 6.77 (dd, Jˆ2.9 and
2.05±2.12 (m, 1H), 2.17±2.30 (m, 1H), 2.81±3.05 (m,
2H), 3.37±3.43 (m, 1H), 3.84±3.98 (m, 4H), 4.97 (d, Jˆ
8
1
5
1
.1 Hz, 1H), 6.90 (d, Jˆ2.2 Hz, 1H), 7.02 (d, Jˆ8.1 Hz,
1
H); C NMR (50 MHz, CDCl ) d 23.9, 24.8, 55.3,
3
13
5.3 Hz, 1H), 7.15±7.23 (m, 3H), 7.40±7.43 (m, 1H);
C
3
7.5, 78.0, 80.1, 114.0, 114.2, 129.0, 129.4, 135.7,
57.8.
NMR (75 MHz, CDCl
106.6, 124.5, 125.4, 126.1, 127.1, 142.1, 144.9; MS, m/z
) d 26.7, 31.6, 48.9, 64.9, 65.3,
3
1
%) 190 (M , 2), 129 (3), 115 (16), 91 (5), 73 (100).
(
1.1.7. 1,1-Dimethoxymethyl-6-methoxy-2,3-dihydro-1H-
indene (14). A mixture of 8 (0.141 g, 0.768 mmol),
Anal. Calcd for C12
75.70; H, 7.37.
H O : C, 75.76; H, 7.42. Found: C,
14 2
2
0
TTN´3H O (0.43 g, 0.97 mmol) and TMOF (4.5 mL) was
2
stirred for 5 min at 08C. Puri®cation by ¯ash chroma-
tography (gradient elution, 10±20% EtOAc in hexanes)
gave 14 (0.144 g, 0.648 mmol, 74%) and 15 (0.013 g,
Acknowledgements
0.057 mmol, 6%).
The authors wish to thank FAPESP, CNPq and CAPES for
®nancial support. We also thank Matthew Tremblay for his
1.1.8. Reaction of 4-methyl-1,2-dihydronaphthalene (9)
with TTN at 2308C. A mixture of 9
careful help with the text and Prof. Carlos R. D. Correia for
his help in carrying out the HRMS.
2
1
(0.039 g,
.27 mmol), TTN´3H O (0.21 g, 0.47 mmol) and MeOH
0
2
(
5 mL) was stirred for 15 min at 2308C. Puri®cation by
column chromatography gave 16a (0.021 g, 0.10 mmol,
8%) and 16b (0.017 g, 0.082 mmol, 31%). cis-1,2-
References
3
Dimethoxy-1-methyl-1,2,3,4-tetrahydronaphthalene (16a):
colorless oil; H NMR (200 MHz, CDCl ) d 1.47 (s, 3H),
1. For reviews, see: (a) McKillop, A.; Taylor, E. C. In Compre-
hensive Organomet. Chem., Pergamon Press: New York,
1982; Vol. 7, pp 465.
1
3
1
2
1
.69±1.90 (m, 1H), 2.20±2.34 (m, 1H), 2.87 (q, Jˆ4.4 Hz,
3
H), 7.05±7.26 (m, 3H), 7.42±7.47 (m, 1H); C NMR
H), 3.06 (s, 3H), 3.53 (s, 3H), 3.74 (dd, Jˆ3.7 and 7.0 Hz,
(b) Ferraz, H. M. C.; Silva, Jr., L. F.; Vieira, T. O. Synthesis
2001, 1999.
1
(
50 MHz, CDCl ) d 24.5, 24.7, 28.0, 50.1, 57.1, 58.8,
3
2. For a review concerning thallium(III) salts mediated ring con-
traction, see: Ferraz, H. M. C.; Silva, Jr., L. F. Quim. Nova
2000, 23, 216.
77.6, 80.1, 126.2, 126.4, 128.2, 136.5, 139.6. trans-1,2-
Dimethoxy-1-methyl-1,2,3,4-tetrahydronaphthalene (16b):
1
colorless oil; H NMR (200 MHz, CDCl ) d 1.54 (s, 3H),
3
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1
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1