Rhodium(I)-Catalyzed Arylative Annulation of β-Alkynyl Ketones for Preparation of Fused Aromatics

Article abstract of DOI:10.1002/ejoc.201901481

Rhodium(I)-catalyzed addition of arylboronic acids to β-(arylethynyl) ketones proceeds through 1,4-rhodium migration to afford 4-(arylmethylene)tetralin-1-ols, which upon treatment with triflic anhydride furnish (arylmethyl)naphthalenes through sequential dehydration–aromatization. Fused aromatic compounds with three to five rings have been prepared using the arylative annulation strategy.

Full text of DOI:10.1002/ejoc.201901481

DOI: 10.1002/ejoc.201901481
Full Paper
Fused Aromatic Compounds
Rhodium(I)-Catalyzed Arylative Annulation of ꢀ-Alkynyl Ketones
for Preparation of Fused Aromatics
Takanori Matsuda,*[a] Takashi Izutsu,[a] and Masaru Hashimoto[a]
Abstract: Rhodium(I)-catalyzed addition of arylboronic acids to through sequential dehydration–aromatization. Fused aromatic
ꢀ-(arylethynyl) ketones proceeds through 1,4-rhodium migra-
tion to afford 4-(arylmethylene)tetralin-1-ols, which upon treat-
ment with triflic anhydride furnish (arylmethyl)naphthalenes
compounds with three to five rings have been prepared using
the arylative annulation strategy.
Introduction
ynyl ketones which furnish fused aromatic compounds with
three to five rings.
Rhodium(I)-catalyzed addition of arylboron compounds is a
useful C–C bond-forming reaction with broad substrate scope
and reasonable functional group tolerance, with remarkable
progress achieved in this area over the past two decades.[1]
Rhodium-, iridium-, and cobalt-catalyzed arylative annulations
of alkynyl ketones with arylboron compounds are efficient
methods for synthesis of four- to seven-membered cyclic alco-
hols and ketones.[2–4] The substrate scope of ꢀ-alkynyl ketones
employed thus far in the rhodium(I)-catalyzed arylative annula-
tion reactions is limited to the use of α,α-disubstituted sub-
strates,[3] and ꢀ-alkynyl ketones bearing at least one hydrogen
at the α position have not been examined as substrates for this
reaction.[5,6] Continuing our interest in the synthesis of aromatic
compounds by annulation–aromatization sequences,[7] we ex-
amined the possibility of transforming the annulation products
of rhodium(I)-catalyzed arylation of the ꢀ-alkynyl ketones to
fused aromatic compounds.
Scheme 1. Postulated arylative annulation of ꢀ-alkynyl ketones involving 1,4-
rhodium migration.
For such an annulation–dehydration–aromatization se-
quence forming a benzene structure to be feasible, the forma-
tion of a six-membered ring via annulation of ꢀ-alkynyl ketones
is necessary (Scheme 1). In this regard, the addition of an aryl-
rhodium(I) species to the alkyne moiety of a ꢀ-alkynyl ketone
followed by 1,4-rhodium migration[8,9] would generate a nu-
cleophilic arylrhodium(I) species requisite for the six-membered
ring formation. To circumvent the regioselectivity issues of the
arylrhodation to the alkyne, we chose to use the arylethynyl
group, which would direct the addition to the desired regio-
selectivity.[10] The resulting six-membered cyclic alcohols
through arylative annulation are amenable to sequential de-
hydration–aromatization, thus affording fused aromatics. We re-
port herein, rhodium(I)-catalyzed arylative annulation of ꢀ-alk-
Results and Discussion
We began the methodology development with 6-phenyl-5-
hexyn-2-one (1a) as the substrate, which was allowed to react
with phenylboronic acid (2a, 1.5 equiv.) in the presence of
[Rh(OH)(cod)]2 (2.5 mol-%, 5 mol-% Rh) and rac-BINAP (5 mol-
%) in 1,4-dioxane at 100 °C for 3 h. The expected arylative annu-
lation involving 1,4-rhodium migration occurred to afford te-
tralin alcohol 3a in 90 % yield (Scheme 2). We had previously
reported that silica gel is effective for inducing dehydration–
aromatization of similar substrates,[7a] and we attempted this
approach by addition of silica gel to the reaction mixture con-
taining 3a after annulation, followed by heating the reaction
mixture at the same temperature. However, no reaction oc-
curred under these conditions, and 3a was recovered in 77 %
yield. The use of molecular sieves 4Å or Na2SO4 was also ineffec-
tive for the conversion.
[a] Department of Applied Chemistry, Tokyo University of Science,
1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
E-mail: mtd@rs.tus.ac.jp
http://www.rs.tus.ac.jp/mtd
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201901481.
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Table 1. Rhodium(I)-catalyzed arylative annulation/dehydration of 1a with 2a.
Scheme 2. Rhodium(I)-catalyzed reaction of 6-phenyl-5-hexyn-2-one (1a) with
phenylboronic acid (2a).
Entry
Deviation from standard conditions
Yield[a] [%]
1
2
3
4
5
6
7
8
standard conditions
10 mol-% rac-BINAP instead of 5 mol-%
no rac-BINAP
DPPBZ instead of rac-BINAP
toluene, 110 °C
chlorobenzene, 100 °C
diglyme, 160 °C
without K2CO3
84
86
54
78
64
75
74
83
To our delight, the addition of triflic anhydride (1.2 equiv.) to
the same reaction mixture after annulation, followed by addi-
tional heating (1 h) resulted in clean conversion to the desired
1-benzyl-4-methylnaphthalene (4a),[11] which was isolated in
84 % yield (Table 1, Entry 1). With the initial conditions in hand,
we carried out a series of reactions to optimize the reaction
conditions. While increasing the ligand loading to 10 mol-% of
rac-BINAP delivered 4a in excellent yield (Entry 2), the reaction
efficiency was largely reduced in the absence of the ligand
(Entry 3).[12] The use of 1,2-bis(diphenylphosphino)benzene
(DPPBZ) as the ligand instead of rac-BINAP furnished 4a in a
slightly lower 78 % yield. However, no reaction was observed
when 1,2-bis(diphenylphosphino)ethane (DPPE) was employed
as the ligand. Next, we evaluated the use of other solvents such
as toluene, chlorobenzene, and diglyme, and found them com-
patible with the reaction (Entries 5–7). While the absence of
K2CO3 allowed the reaction of 1a with 2a to 4a in 83 % yield
(Entry 8), further studies revealed that the presence of the base
is crucial for obtaining reproducible results for all substrates.
[a] Isolated yield.
tricyclic products, anthracene, phenanthrene, and benzo-fused
thiophenes, as well as naphthalenes. The annulation of 1a with
phenylboronic acids bearing para substituents (2b–d) provided
the corresponding 1,4,6-trisubstituted naphthalenes 4b–d in
61–80 % yields. In the reactions using 3-methylphenyl-, 3-nitro-
phenyl-, 3,4-dimethylphenyl-, and 2-naphthylboronic acids
(2e–h), the 1,4-rhodium migration occurred regioselectively at
the less sterically hindered site on the aromatic ring and fur-
nished the products (4e–h) in 70–79 % yields. The use of 2-
methylphenyl- and 1-naphthylboronic acids (2i and 2j) afforded
Having optimized and established the protocol for annula- the products in low yields of 4 likely due to the steric hindrance
tion–dehydration–aromatization sequence, we examined the of the boronic acids.[13,14] Thiophenes 2k and 2l were tolerated,
scope of this transformation (Table 2). We evaluated a variety leading to the products 4k and 4l, respectively.[15] In the reac-
of arylboronic acids 2b–l for the rhodium(I)-catalyzed arylative tion using 2k, benzo[c]thiophene 4k was obtained through the
annulation of ꢀ-alkynyl ketone 1a. The reactions furnished the selective migration of rhodium to the 4 position of the thio-
Table 2. Arylative annulation of ꢀ-alkynyl ketones 1 with arylboronic acids 2.[a]
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phene ring. The attempted reaction with (E)-styrylboronic acid for the arylative annulation and produced 1-benzyl-4-ethyl-
was unsuccessful, giving a complex mixture of products. Other naphthalene (4p) and 1-benzyl-4-phenylnaphthalene (4q), re-
6-aryl-5-hexyn-2-ones 1b–d also underwent the arylative annu- spectively.[16]
lation to generate the corresponding substituted naphthalene
products 4m–o, respectively. Particularly, the 2-methylphenyl
and 1-naphthyl derivatives delivered the products in high
yields, which could be attributed to the increased regioselecti-
vity of the initial phenylrhodium(I) addition across C–C triple
bond of 1 due to the presence of sterically demanding aryl
substituents. Furthermore, 7-phenyl-6-heptyn-3-one (1e) and
1,5-diphenyl-4-pentyn-1-one (1f) were viable coupling partners
We then studied the reactions of various α-(arylpropargyl)
cyclic ketones 1g–o in reaction with phenylboronic acid (2a),
which afforded the corresponding fused naphthalenes
(Table 3). The reaction of 2-(3-phenylpropargyl)-1-indanone de-
rivatives 1g–m furnished the corresponding tetracyclic prod-
ucts 4r–x, respectively, with the benzo[c]fluorene core (entries
1–7). Additionally, the six-membered ring ketones, cyclohex-
anone 1n, and 1-tertalone 1o underwent the arylative annula-
tion with 2a to afford tetrahydrophenanthrene 4y, and dihydro-
benzo[c]phenanthrene 4z, respectively (entries 8 and 9).
Having prepared a variety of bicyclic, tricyclic, and tetracyclic
ring systems using the developed methodology, we sought to
extend the arylative annulation to the synthesis of pentacyclic
products (Table 4). Arylative annulation of 1-indanone 1g with
2-naphthylboronic acid (2g) and benzo[b]thiophen-2-ylboronic
acid (2l) produced indeno[1,2-a]anthracene 4A in 33 % and
benzo[b]fluoreno[4,3-d]thiophene 4B in 27 % yields, respec-
tively. Dihydrobenzo[a]tetraphene 4C and dihydrobenzo-
[c]chrysene 4D were prepared from the reaction of 1-tetralone
1o with 2h and 2j, respectively. Similarly, the reaction between
1o and 2l afforded dihydrobenzo[b]phenanthro[4,3-d]thio-
phene 4E, showcasing the efficiency of the methodology for
rapid synthesis of fused aromatics.
Table 3. Arylative annulation of α-(arylpropargyl) cyclic ketones with 2a.[a]
Table 4. Synthesis of pentacyclic products.[a]
We studied the derivatization of the prepared 1-benzyl-6-
bromo-4-methylnaphthalene (4d) using cross-coupling reac-
tions (Scheme 3). Suzuki–Miyaura coupling of 4d with phenyl-
boronic acid (2a) afforded 1-benzyl-4-methyl-6-phenylnaphth-
alene (5) in 96 % yield. Sonogashira coupling of 4d with
phenylacetylene provided 1-benzyl-4-methyl-6-(phenylethynyl)-
naphthalene (6) in 77 % yield.
[a] Alkynyl ketone 1, arylboronic acid 2 (1.5 equiv.), [Rh(OH)(cod)]2 (5.0 mol-
% Rh), rac-BINAP (10 mol-%), and K2CO3 (2 equiv.) were reacted at 100 °C in
1,4-dioxane (0.10 M). After 3 h, Tf2O (1.2 equiv.) was added, and the reaction
mixture was heated further for 1 h. [b] Isolated yield. [c] 5.0 mol-% rac-BINAP
was used. [d] 73 % yield in diglyme at 160 °C.
Scheme 3. Product derivatizations.
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(E)-5-Benzylidene-5,6,6a,7-tetrahydro-11bH-benzo[c]fluoren-
11b-ol (3r): The title compound was obtained as a yellow oil in
68 % yield when the reaction was performed without treatment
Conclusion
In summary, we developed a rhodium(I)-catalyzed arylative an-
nulation of ꢀ-(arylethynyl) ketones that proceeds through 1,4-
rhodium migration. The resulting tetralin alcohols undergo de-
hydration–aromatization upon treatment with Tf2O, providing
access to a variety of fused aromatic compounds, up to penta-
cyclic systems bearing a variety of substituents.
1
with Tf2O. H NMR (500 MHz, CDCl3): δ = 7.91 (dd, J = 8.0, 1.0 Hz,
1H), 7.66–7.61 (m, 2H), 7.43–7.33 (m, 5H), 7.32–7.15 (m, 5H), 7.14 (s,
1H), 3.02 (dd, J = 15.5, 7.0 Hz, 1H), 2.97 (ddd, J = 13.7, 4.8, 1.5 Hz,
1H), 2.91–2.81 (m, 2H), 2.66 (dd, J = 15.8, 7.3 Hz, 1H), 2.38 (s, 1H);
13C NMR (126 MHz, CDCl3): δ = 147.2, 142.0, 138.6, 137.8, 135.5,
129.3, 128.4, 128.3, 128.2, 127.8, 126.8, 126.7, 126.4, 125.2, 124.7,
124.1, 81.1, 49.4, 34.5, 28.1; IR (neat): ν = 3331, 1473, 1443 cm–1
;
˜
HRMS (ESI): m/z calcd. for C24H20O+Na+: 347.1406 [M + Na]+, found
Experimental Section
347.1402.
General Procedure for the Rhodium(I)-Catalyzed Arylative An-
nulation of ꢀ-Alkynyl Ketones 1 with Arylboronic Acids 2: A
Schlenk tube containing a magnetic stirring bar was charged with
arylboronic acid 2 (0.150 mmol), [Rh(OH)(cod)]2 (1.1 mg, 2.5 μmol),
and rac-BINAP (3.1 mg, 5.0 μmol or 6.2 mg, 10 μmol) under nitrogen
atmosphere. The solvent 1,4-dioxane (1.0 mL) and ꢀ-alkynyl ketone
1 (0.100 mmol) were added through the septum using a syringe,
and the mixture was stirred at 100 °C. After 3 h, Tf2O (0.120 mmol)
was added to the reaction mixture, and the mixture was heated
further for 1 h at 100 °C. After cooling to room temperature, the
reaction mixture was filtered through a plug of Florisil® eluting with
hexane/AcOEt (2:1). The filtrate was concentrated, and the residue
was purified by preparative thin-layer chromatography (TLC) to fur-
nish the naphthalene 4.
6-Benzyl-8H-indeno[1,2-a]anthracene (4A): The general proce-
dure was followed using 1g (49.3 mg, 0.200 mmol), 2h (51.6 mg,
0.300 mmol), [Rh(OH)(cod)]2 (2.3 mg, 5.0 μmol), rac-BINAP (12.5 mg,
20.1 μmol), K2CO3 (55.3 mg, 0.400 mmol), 1,4-dioxane (2.0 mL), and
Tf2O (67.9 mg, 0.241 mmol). Purification by preparative TLC (CHCl3/
hexane = 2:1) yielded 4A (43.0 mg, 0.121 mmol, 60 %) as an orange
1
solid. M.p. 143–148 °C; H NMR (500 MHz, CDCl3): δ = 9.35 (s, 1H),
8.68 (s, 1H), 8.58 (d, J = 7.5 Hz, 1H), 8.15 (d, J = 9.0 Hz, 1H), 7.99 (d,
J = 8.5 Hz, 1H), 7.66 (d, J = 7.5 Hz, 1H), 7.59–7.45 (m, 4H), 7.40–7.20
(m, 6H), 4.66 (s, 2H), 4.00 (s, 2H); 13C NMR (126 MHz, CDCl3): δ =
144.1, 143.1, 141.8, 140.5, 136.4, 134.7, 131.6, 130.75, 130.71, 128.9,
128.7, 128.5, 128.2, 126.9, 126.2, 125.8, 125.40, 125.36, 124.7, 124.6,
124.3, 122.8, 122.5, 39.8, 38.2; HRMS (ESI): m/z calcd. for C28H20+Na+:
379.1457 [M + Na]+, found 379.1458.
1-Benzyl-4-methylnaphthalene (4a): The general procedure was
followed using 1a (17.2 mg, 0.100 mmol), 2a (18.3 mg, 0.150 mmol),
[Rh(OH)(cod)]2 (1.1 mg, 2.4 μmol), rac-BINAP (6.2 mg, 10 μmol),
K2CO3 (27.7 mg, 0.200 mmol), 1,4-dioxane (1.0 mL), and Tf2O
(33.8 mg, 0.120 mmol). Purification by preparative TLC (hexane/
AcOEt = 10:1) yielded 4a (19.9 mg, 0.086 mmol, 86 %) as an orange
1-Benzyl-4-methyl-6-phenylnaphthalene (5): An oven-dried
Schlenk tube containing a magnetic stirring bar was charged with
4d (31.2 mg, 0.100 mmol), PhB(OH)2 (24.4 mg, 0.200 mmol),
Pd(PPh3)4 (5.8 mg, 5.0 μmol), and K2CO3 (34.6 mg, 0.250 mmol). THF
(2.5 mL) and H2O (125 μL) were added successively through the
septum with a syringe, and the mixture was stirred at 80 °C. After
9 h, the reaction mixture was passed through a plug of Florisil®
eluting with hexane/AcOEt (4:1). The filtrate was concentrated, and
the residue was purified by preparative TLC (hexane/AcOEt =
10:1 × 2, then hexane) to afford 5 (29.8 mg, 0.097 mmol, 96 %) as
a pale yellow solid. M.p. 84–87 °C; 1H NMR (500 MHz, CDCl3): δ =
8.25–8.23 (m, 1H), 8.10 (d, J = 8.5 Hz, 1H), 7.77–7.73 (m, 3H), 7.55–
7.49 (m, 2H), 7.44–7.37 (m, 1H), 7.35–7.29 (m, 3H), 7.28–7.21 (m, 4H),
4.48 (s, 2H), 2.78 (s, 3H); 13C NMR (126 MHz, CDCl3): δ = 141.4, 140.9,
138.0, 134.6, 133.4, 133.3, 131.3, 128.8, 128.7, 128.4, 127.5, 127.3,
127.2, 126.8, 126.0, 125.4, 125.2, 122.8, 39.0, 19.6; HRMS (ESI) calcd.
for C24H20+Na+: 331.1457 [M + Na]+, found 331.1460.
1
oil. H NMR (500 MHz, CDCl3): δ = 8.07–8.00 (m, 2H), 7.55–7.45 (m,
2H), 7.31–7.25 (m, 3H), 7.24–7.18 (m, 4H), 4.45 (s, 2H), 2.71 (s, 3H);
13C NMR (126 MHz, CDCl3): δ = 140.9, 134.7, 133.2, 133.0, 132.1,
128.7, 128.4, 127.1, 126.3, 125.9, 125.6, 125.4, 124.8, 124.7, 39.0, 19.5;
HRMS (ESI): m/z calcd. for C18H16+Na+: 255.1144 [M + Na]+, found
255.1145.
(E)-4-Benzylidene-1-methyltetralin-1-ol (3a): The title compound
was obtained as a pale yellow oil in 90 % yield when the reaction
was performed without treatment with Tf2O. 1H NMR (500 MHz,
CDCl3): δ = 7.70–7.64 (m, 2H), 7.41–7.37 (m, 4H), 7.36–7.25 (m, 3H),
7.06 (s, 1H), 3.04–2.97 (m, 1H), 2.85–2.77 (m, 1H), 2.02 (ddd, J = 12.9,
6.6, 4.9 Hz, 1H), 1.94 (ddd, J = 13.0, 10.3, 5.0 Hz, 1H), 1.83 (s, 1H),
1.56 (s, 3H); 13C NMR (126 MHz, CDCl3): δ = 143.3, 137.8, 135.2,
136.4, 129.3, 128.2, 128.1, 127.6, 126.7, 125.3, 124.8, 124.3, 70.8, 39.2,
1-Benzyl-4-methyl-6-(phenylethynyl)naphthalene (6): An oven-
dried Schlenk tube containing a magnetic stirring bar was charged
with 4d (31.2 mg, 0.100 mmol), phenylacetylene (12.7 mg,
0.124 mmol), PdCl2(PPh3)2 (2.8 mg, 4.0 μmol), and CuI (0.4 mg,
2.1 μmol). (iPr)2NH (0.50 mL), THF (1.0 mL) were added successively
through the septum via a syringe, and the mixture was stirred at
80 °C. After 6 h, the reaction mixture was passed through a plug of
Florisil® eluting with hexane/AcOEt (4:1). The filtrate was concen-
trated, and the residue was purified by preparative TLC (hexane/
AcOEt = 10:1, then hexane × 2) to afford 6 (25.6 mg, 0.077 mmol,
77 %) as a pale yellow solid. M.p. 97–99 °C; 1H NMR (500 MHz,
CDCl3): δ = 8.27 (s, 1H), 7.99 (d, J = 8.5 Hz, 1H), 7.64–7.55 (m, 3H),
7.42–7.34 (m, 3H), 7.33–7.27 (m, 3H), 7.25–7.19 (m, 4H), 4.44 (s, 2H),
2.73 (s, 3H); 13C NMR (126 MHz, CDCl3): δ = 140.6, 134.7, 133.2,
29.4, 25.3; IR (neat): ν = 3347, 1597, 1492, 1444 cm–1; HRMS (ESI):
˜
m/z calcd. for C18H18O+Na+: 273.1250 [M + Na]+, found 273.1249.
5-Benzylbenzo[c]fluorene (4r): The general procedure was fol-
lowed using 1g (24.6 mg, 0.100 mmol), 2a (18.3 mg, 0.150 mmol),
[Rh(OH)(cod)]2 (1.1 mg, 2.4 μmol), rac-BINAP (3.1 mg, 5.0 μmol),
K2CO3 (27.6 mg, 0.200 mmol), 1,4-dioxane (1.0 mL), and Tf2O
(33.9 mg, 0.120 mmol). Purification by preparative TLC (hexane/
AcOEt = 10:1) yielded 4r (21.1 mg, 0.069 mmol, 69 %) as a brown
solid. M.p. 140–142 °C; 1H NMR (500 MHz, CDCl3): δ = 8.84 (d, J =
9.0 Hz, 1H), 8.41 (d, J = 8.0 Hz, 1H), 8.14 (d, J = 9.0 Hz, 1H), 7.68–
7.62 (m, 2H), 7.55–7.48 (m, 3H), 7.36 (t, J = 7.2 Hz, 1H), 7.33–7.20
(m, 5H), 4.55 (s, 2H), 3.99 (s, 2H); 13C NMR (126 MHz, CDCl3): δ =
144.1, 142.8, 142.0, 140.7, 136.1, 135.2, 131.9, 130.1, 128.8, 128.5, 132.8, 131.65, 131.61, 128.6, 128.5, 128.44, 128.36, 128.3, 128.1,
126.9, 126.1, 126.08, 125.5, 125.4, 125.1, 125.0, 124.8, 124.3, 122.7,
39.5, 37.8; HRMS (ESI): m/z calcd. for C24H18+Na+: 329.1301 [M +
Na]+, found 329.1296.
127.9, 127.0, 126.1, 125.0, 123.3, 120.0, 90.0, 89.6, 38.9, 19.4; HRMS
(ESI): m/z calcd. for C26H20+Na+: 355.1457 [M + Na]+, found
3355.1452.
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Acknowledgments
This work was supported by JSPS, Japan (Grant-in-Aid for
Scientific Research (C) No. 16K05783).
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[12] It was determined later that the use of 10 mol-% of the ligand affords
superior activity in majority of other reactions examined in Table 2,
Table 3, and Table 4.
Keywords: Annulation · Homogeneous catalysis ·
Hydrocarbons · Polycycles · Rhodium
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[13] Alkynyl ketone 1a was completely consumed during the reaction. The
low yields of 4i and 4j might be attributed to a side reaction.
[14] The reaction of 1a with 9-phenanthreneboronic acid afforded the corre-
sponding annulation product 1-benzyl-4-methyltriphenylene in yields
below 10 %.
[15] In the case with 2k, 26 % of alkynyl ketone 1a was recovered.
[16] When 5-phenylpent-4-ynal was treated with 2a, 4-benzylidene-1-tetral-
one was obtained in 20 % yield through ꢀ-hydride elimination of the
corresponding rhodium(I) tetralin-1-olate.
Received: October 8, 2019
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© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Fused Aromatic Compounds
T. Matsuda,* T. Izutsu,
M. Hashimoto ...................................... 1–6
Rhodium(I)-Catalyzed Arylative An-
nulation of ꢀ-Alkynyl Ketones for
Preparation of Fused Aromatics
Rhodium(I)-catalyzed arylative annula- aromatization sequence, giving rise to
tion of ꢀ-(phenylethynyl) ketones with (arylmethyl)naphthalenes.
Aromatic
arylboronic acids affords 4-(arylmeth- compounds with as many as five
ylene)tetralin-1-ols, treatment of which cycles can be prepared using this
with Tf2O resulted in a dehydration– method.
DOI: 10.1002/ejoc.201901481
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© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim