Highly diastereoselective synthesis of a novel functionalized triepoxytrinaphthylene

Article abstract of DOI:10.24820/ark.5550190.p010.408

The high yielding synthesis of a novel benzocyclotrimer is herein presented. The syn-diastereomer is obtained as major product, presumably in virtue of the presence of an oxa-bridge of the bicylic components. The three oxa-bridges can be used for further functionalization, as well as the six bromine atoms of the three aromatic rings, as demonstrated in the aromatization of a mixture of anti-1 and syn-1 (3:7) leading to trinaphthylene.

Full text of DOI:10.24820/ark.5550190.p010.408

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Organic Chemistry
Highly diastereoselective synthesis of a novel functionalized triepoxytrinaphthylene
Musa Erdoğan,a Selçuk Eşsiz,a Fabrizio Fabris,b and Arif Daştan*,a
a Department of Chemistry, Ataturk University, Faculty of Sciences, Erzurum, Turkey
b Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari di Venezia, Venezia, Italy
E-mail: adastan@atauni.edu.tr
Dedicated to emeritus Professors Metin Balci (Middle East Technical University) and Ottorino De Lucchi (Ca
Foscari University) for their valuable contribution to hydrocarbon chemistry.
Received 11-25-2017
Accepted 01-06-2018
Published on line 01-28-2018
Abstract
The high yielding synthesis of a novel benzocyclotrimer is herein presented. The syn-diastereomer is obtained
as major product, presumably in virtue of the presence of an oxa-bridge of the bicylic components. The three
oxa-bridges can be used for further functionalization, as well as the six bromine atoms of the three aromatic
rings, as demonstrated in the aromatization of a mixture of anti-1 and syn-1 (3:7) leading to trinaphthylene.
Br
Br
Br
Br
O
O
Br
Br
Br
Br
O
+
O
O
O
Br
Br
Br
Br
Br
Br
syn
Br
Br
anti
Keywords: Cyclotrimerization, copper, cup-shaped molecules, stereoselective reactions, aromatic compounds
Introduction
DOI: https://doi.org/10.24820/ark.5550190.p010.408
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Benzocyclotrimers1 are rigid molecules characterized by one or two cavities, which have been successfully
employed in supramolecular chemistry.2-15 Amongst these, benzocyclotrimers bearing aromatic rings are
characterized by large stiff cavities. The cavities are particularly deep and suitable for supramolecular
applications in case of the more symmetrical syn-diastereomer, which is generally obtained in lower yields. In
this report we describe the highly diastereoselective synthesis of the benzocyclotrimer syn-1, displaying one
rigid and functionalized hemi-cavity in the bottom of the structure, concomitantly with three relatively
reactive oxa-bridges (Figure 1). The reactivity of these moieties can be conveniently used for further
functionalization of the less valuable diastereomer anti-1, which furnished the more symmetrical
trinaphtylene 2 (Figure 1).
Figure 1. C3v- and CS-symmetric benzocyclotrimer syn-1 and anti-1, and trinaphtylene 2.
Results and Discussion
The starting material for the synthesis of the benzocyclotrimers syn-1 and anti-1 was 4,5-dibromo-11-
oxatricyclo[6.2.1.02,7]undeca-2(7),3,5,9-tetraene 4, which can be obtained in 85% overall yield from
commercially available 1,2,4,5-tetrabromobenzene 3. The latter reacted with n-butyl lithium to afford 1-
lithium-2,4,5-tribromobenzene, which decomposes to form the 4,5-dibromobenzyne. This reactive
intermediate was captured by furan, acting as dienophile in a [4+2] cycloaddition (Scheme 1).16-25 The resulting
oxabicycle 4 was brominated according to a well-established radical-driven procedure,14 which avoids
Meerwein rearrangement with concomitant opening of the oxa-bridge (Scheme 1).26 The radical bromination
of 4 with 1,2-dibromotetrachloroethane (DBTCE) furnished the trans-dibromide diastereomer 5 in high yields
(85%) (Scheme 1). In order to obtain the alkenyl bromide 6 the dehydrobromination reaction with potassium
tert-butoxide in THF was used,26 to afford the desired product 6 in good yield (90%) (Scheme 1). The latter was
brominated under high temperature conditions, which ensures high yields of dibromination, with limited
amounts of Meerwein rearrangement products,26 to obtain alkenyl tribromides exo-7 and endo-7, in a ratio of
2:1 and 96% overall yield (Scheme 1). The mixture of diastereomers was dehydrobrominated with potassium
tert-butoxide in THF,26 to afford the alkenyldibromide 8 in good yield (93%) (Scheme 1).
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Scheme 1. Synthesis of the key intermediates for the precursor of the cyclotrimerization. (i) n-BuLi; (ii) furan in
toluene, -78 °C to rt (85% for two steps) (iii) DBTCE, in CCl4, hν, (85%), (iv) t-BuOK in THF, reflux. (90%) v) Br2 in
CCl4, reflux. (96%) (vi) t-BuOK in THF, reflux (93%).
The key reagent 9 for the cyclotrimerization was obtained either by proton abstraction of halide 6 with
lithium diisopropylamide (LDA)11,13,27-33 or by removal of one of the bromine atoms of the dibromide 8 with n-
butyl lithium30,31,34,35 followed by a trans-metalation reaction with trimethyltin chloride in 96% yield in both
cases. The high chemoselectivity of removal of the bromine atom in 8 was surprisingly: indeed, the bromine
atoms attached to the aromatic ring remained completely unaffected.
Scheme 2. Two feasible strategies the preparation of the precursor of cyclotrimerization 9. (i) LDA; (ii) Me3SnCl
in THF, -78 °C to rt, (96% for two steps). (iii) n-BuLi; (iv) Me3SnCl in THF, -78 °C to rt, (96% for two steps).
In order to accomplish the cyclotrimerization, the vic-bromostannane 9 was treated with copper(I) 2-
thiophenecarboxylate (CuTC) in dry NMP (N-methyl-2-pyrrolidone) at low temperature.36 The two isomeric
benzocyclotrimers were formed in a highly favourable 7:3 syn to anti ratio and in a very good isolated yield
(92%). The selectivity was opposite to the expected statistical distribution observed in the majority of
cyclotrimerization reactions.1 This behaviour has been previously observed when a ligating group is present in
37,38,11,32
the monomer.
It is likely that the oxa-bridge in the dihydronaphthalene is responsible for the high
reverse selectivity, by means of coordination with copper. The separation of the two isomers was easily
accomplished by chromatography on neutral alumina and the major syn-1 isomer was available for further
functionalization and supramolecular studies.
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Scheme 3. Copper-mediated cyclotrimerization of 9. (i) CuTC in NMP, -20 °C (25% for anti-1 and 67% syn-1
isolated yield).
The cyclotrimers syn-1 and anti-1 are a potential source for the synthesis of polyaromatic rings. In order to
verify the potential of this strategy a mixture of syn-1 and anti-1 have been submitted to reductive oxygen
elimination reactions.39-42 The milder methodologies39-41 based on the use of trimethylsilyl iodide or low valent
titanium(III)chloride furnished complex mixtures of products, containing minor amounts of the expected
2,3,8,9,14,15-hexabromotrinaphthylene. When pure syn-1 or anti-1 (or mixture of syn-1 and anti-1) is treated
with titanium(III)chloride, generated in situ from TiCl4, lithium aluminum hydride (LAH) and triethylamine
(TEA) in refluxing THF,42 a quantitative yield of trinaphthylene 243-50 is obtained.
Br
Br
Br
Br
O
O
i
O
2
Br
Br
1/
syn- anti-
1
Scheme 4. Reductive aromatization of benzocyclotrimer syn-1 and anti-1. (i) TiCl4, LAH, TEA in THF, reflux
(100% yield).
This approach opens up a new possible method for the preparation of polyaromatic systems, based on the
highly chemo- and stereoselective cyclotrimerization reaction.51
Conclusions
In conclusion, a straightforward and high yielding synthesis of a new benzocyclotrimer was developed. The
cyclotrimerization reaction afforded two possible diastereomers syn to anti in a very favourable 7:3 ratio. The
more valuable syn-diastereomer will be considered for supramolecular applications. On the other hand, anti
and syn diastereomers may be conveniently employed for the preparation of polyaromatic structures.
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Experimental Section
General. All reactions were carried out under nitrogen and monitored by TLC and/or 1H-NMR spectroscopy. All
solvents were dried and distilled before use. Column chromatography was performed on silica gel (60 mesh,
Merck) or on neutral alumina. TLC was carried out on silica gel 60 HF254 aluminum plates (Fluka). Melting
points are uncorrected. The 1H- and 13C-NMR spectra were recorded on 400 MHz NMR spectrometers.
Apparent splittings are given in all cases in ppm and coupling constants J in Hz. All new compounds gave
satisfactory elemental analyses and/or HRMS (Q-TOF). All substances reported in this paper are in their
racemic form.
4,5-Dibromo-11-oxatricyclo[6.2.1.02,7]undeca-2(7),3,5,9-tetraene (4). To
a
mixture of 1,2,4,5-
tetrabromobenzene 3 (5.00 g, 12.70 mmol) and furan (10 mL, 138.40 mmol) in toluene (100 mL) was added n-
BuLi (2.5 M in n-hexane, 5.6 ml, 13.97 mmol) at -78 °C. The resulting mixture was stirred for 1 h. The resulting
reaction mixture was allowed to warm up to room temperature and stirred overnight. The mixture was diluted
with saturated solution of ammonium chloride (10 ml) and extracted with EtOAc (3 × 25 mL). Combined
organic layers were washed with brine, dried over Na2SO4 and concentrated. The residue was purified via
column chromatography (silica-gel, 100 g; eluant 1:9 EtOAc/n-hexane) to obtain 6,7-dibromo-1,4-dihydro-1,4-
epoxynaphthalene 4 (3.26 g, 85%) as colourless crystals from CH2Cl2/n-hexane (1:9), M.p. 125-126 °C (lit.19
1
M.p. 125.2-126.6 °C). H NMR (400 MHz, CDCl3): 7.47 (s, 2 H), 6.99 (s, 2 H), 5.65 (s, 2 H). 13C NMR (100 MHz,
CDCl3): 150.2, 142.7, 125.5, 120.6, 81.8.
trans-4,5,9,10-Tetrabromo-11-oxatricyclo[6.2.1.02,7]undeca-2(7),3,5-triene (5). In a 100-mL flask equipped
with a reflux condenser, a solution of 4 (3.00 g, 9.93 mmol), 3.88 g (11.92 mmol) of DBTCE, and AIBN (10 mg)
in CCl4 (40 mL) was irradiated with a 500 W halogen lamp while refluxing for 24 h. Volatile materials were
removed at reduced pressure and the residue was purified via column chromatography (silica-gel, 100 g;
eluant n-hexane) to afford trans-2,3,6,7-tetrabromo-1,2,3,4-tetrahydro-1,4-epoxynaphthalene 5 (3.90 g, 85%)
as colorless crystals from CH2Cl2/n-hexane (1:9), M.p. 112-114 °C. IR (KBr): 3003, 1603, 1566, 1440, 1359,
1
1319, 1217, 1192, 1088, 983. H NMR (400 MHz, CDCl3): 7.64 (s, 1 H), 7.62 (s, 1 H), 5.39 (bs, 1 H), 5.38 (d, J =
4.8, 1 H), 4.49 (dd, J = 4.8 and 2.6, 1 H), 3.77 (d, J = 2.6, 1 H). 13C NMR (100 MHz, CDCl3): 142.4 (2 overlapping
C), 128.5, 125.3, 124.5, 124.1, 86.4, 82.7, 51.9, 49.8. Anal. calc. for C10H6Br4O: C 26.01, H 1.31; found: C 26.07,
H 1.33.
4,5,9-Tribromo-11-oxatricyclo[6.2.1.02,7]undeca-2(7),3,5,9-tetraene (6). Under N2 atmosphere, a solution of
potassium tert-butoxide (0.58 g, 5.20 mmol) in dry THF (10 mL) was added to a stirred solution of 5 (2.00 g,
4.33 mmol) in dry THF (20 mL) in 15 min. The resulting mixture was heated at reflux temperature for 3 h.
Volatile materials were evaporated and the residue was diluted with water and extracted with Et2O (3 × 50
mL). Combined organic extracts were washed with water, dried over MgSO4, and concentrated at the reduced
pressure. The residue was purified by column chromatography (silica-gel, 10 g; eluant EtOAc/n-hexane (1:10))
to furnish 2,6,7-tribromo-1,4-dihydro-1,4-epoxynaphthalene 6 (1.50 g, 90%) as light brown crystals from
1
CH2Cl2/n-hexane (1:3), M.p. 115-116 °C. IR (KBr): 3014, 1573, 1431, 1349, 1318, 1228, 1152, 1084, 992. H
NMR (400 MHz, CDCl3): 7.60 (s, 1 H), 7.49 (s, 1 H), 6.92 (d, J=2.0, 1 H), 5.69 (m, 1 H), 5.40 (bs, 1 H). 13C NMR
(100 MHz, CDCl3): 148.6, 148.1, 139.3, 136.3, 126.1, 125.4, 121.8, 121.1, 86.4, 83.8.
exo/endo-4,5,9,9,10-Pentabromo-11-oxatricyclo[6.2.1.02,7]undeca-2(7),3,5-triene (exo/endo-7).
A
hot
solution of bromine (0.23 g, 1.44 mmol) in CCl4 (5 mL) was added dropwise to a magnetically stirred refluxing
solution of tribromide 6 (0.50 g, 1.31 mmol) in CCl4 (25 ml). The solution was maintained under reflux for 20
min, after cooling to the room temperature the solvent was removed at the reduced pressure to afford a 2:1
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mixture of pentabromides exo-7 and endo-7 (0.68 g, 96% yield), which was used in the following
transformation without further purification. 1H NMR (400 MHz, CDCl3) of exo-7: 7.76 (s, 1 H), 7.62 (s, 1 H), 5.67
(s, 1 H), 5.38 (s, 1 H), 4.36 (s, 1 H). 1H NMR (400 MHz, CDCl3) of endo-7: 7.69 (s, 1 H), 7.59 (s, 1 H), 5.67 (s, 1 H),
5.43 (d, J = 4.7, 1 H), 5.11 (d, J = 4.7, 1 H).
4,5,9,10-Tetrabromo-11-oxatricyclo[6.2.1.02,7]undeca-2(7),3,5,9-tetraene (8). Under N2 atmosphere, a
solution of potassium tert-butoxide (0.18 g, 1.55 mmol) in dry THF (5 mL) was added to a stirred solution of
exo-7 and endo-7 (0.70 g, 1.29 mmol) in dry THF (20 mL) in 15 min. The resulting mixture was maintained at
room temperature overnight. Volatile materials were evaporated and the residue was diluted with water and
extracted with Et2O (3 × 50 ml). Combined organic extracts were washed with water, dried over MgSO4 and
concentrated at reduced pressure. The residue was purified by column chromatography (silica-gel, 20 g; eluant
EtOAc/n-hexane (1:9)) to furnish 2,3,6,7-tetrabromo-1,4-dihydro-1,4-epoxynaphthalene 8 (0.55 g, 93%) as
colourless crystals from CH2Cl2/n-hexane (1:3), M.p. 197-199 °C. IR (KBr): 3014, 1587, 1429, 1345, 1317, 1222,
1155, 1082, 1053, 990. 1H NMR (400 MHz, CDCl3): 7.60 (s, 2 H), 5.52 (s, 2 H). 13C NMR (100 MHz, CDCl3): 146.8,
133.3, 125.9, 122.2, 87.5. Anal. calc. for C10H4Br4O: C 26.12, H 0.88; found: C 26.13, H 0.93.
4,5,9-Tribromo-10-(trimethylstannyl)-11-oxatricyclo[6.2.1.02,7]undeca-2(7),3,5,9-tetraene (9). Method A.
Under N2, A solution of n-BuLi in n-hexane (2.5m, 0.05 ml, 1.25 mmol) was added dropwise to a solution of
tetrabromide 8 (0.55 g, 1.19 mmol) in dry THF (20 mL) at -78 °C and the resulting mixture was stirred at the
same temperature for 1 h. Trimethyltin chloride (0.22 g, 1.43 mmol) was added to the reaction mixture in one
portion and the mixture was stirred 2 h at the same temperature. The resulting mixture was allowed to warm
to room temperature overnight, it was diluted with water (50 mL) and extracted with Et2O (3 × 30 mL).
Combined ethereal extracts were dried over MgSO4, and concentrated in vacuo to afford the bromo-stannyl
olefin 9, as a brown oil (0.68 g, 96%).
Method B. Under N2 atmosphere, a solution of n-BuLi in hexane (2.5 M, 1.3 mL, 2.63 mmol) was added to a
solution of dry diisopropylamine (0.40 mL, 2.88 mmol) in dry THF (5 mL) cooled at -78 °C. After 15 min, a
solution of 6 (0.50 g, 1.31 mmol) in dry THF (15 ml) was added and the mixture was maintained at the same
temperature for 1 h. A solution of trimethyltin chloride (0.31 g, 1.58 mmol) in dry THF (10 mL) was added
dropwise and the temp. was left to rise to r.t. overnight. The final mixture was diluted with water (50 mL) and
extracted with Et2O (3 × 50 mL). Combined ethereal extracts were dried over Na2SO4, and concentrated in
1
vacuo to afford the bromo-stannyl olefin 9, as a brown oil (0.68 g, 96%). H NMR (400 MHz, CDCl3): 7.56 (s, 1
H), 7.38 (s, 1 H), 5.73 (s, 1 H), 5.36 (s, 1 H), 0.27 (s, 9 H). 13C NMR (100 MHz, CDCl3): 151.7, 149.2, 148.4, 146.3,
125.8, 124.7, 121.6, 120.8, 88.7, 86.8, -9.3. Compound 9 resulted pure enough for the following
transformation and the purification was not attempted, because of the poor stability of the vinylstannane
moiety on silica-gel.
syn- and anti-7,8,17,18,27,28-Hexabromo-31,32,33-trioxadecacyclo[22.6.1.14,11.114,21.02,23.03,12.05,10.013,22
.
015,20.025,30]tritriaconta-2,5,7,9,12,15,17,19,22,25,27,29-dodecaene (syn-1) and (anti-1). In a flame dried 50-
ml two-necked round-bottomed flask fitted with a nitrogen inlet, copper(I) 2-thiophenecarboxylate (CuTC)
(0.13 g, 0.69 mmol) was introduced, purging with nitrogen and capping with a rubber septum. The reactor was
cooled to -20 °C and consecutively dry NMP (15 mL) and bromostannyl-olefin 9 (0.25 g, 0.46 mmol) were
1
added via syringe. The reaction evolution was monitored by H-NMR spectroscopy. After 30 min, an aqueous
10% NH3 solution (20 ml) was added and the mixture was stirred until the brown solid disappeared. The
mixture was extracted with diethyl ether (3 × 20 ml) and the combined ethereal extracts were dried over
MgSO4. Volatile materials were removed in vacuo, and the residue was purified by column chromatography on
neutral aluminum oxide with EtOAc/n-hexane (3:7) as eluant.
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First eluate: (anti-1) (34 mg, 25% yield), colorless crystals from CH2Cl2/n-hexane (1:3), M.p. > 350 °C (dec.). IR
1
(KBr): 3008, 1725, 1566, 1435, 1352, 1308, 1264, 1230, 1188, 1158, 1085, 987, 936, 903, 837. H NMR (400
MHz, DMSO-d6): 7.87 (s, 2 H), 7.84 (s, 2 H), 7.82 (s, 2 H), 6.44 (s, 2 H), 6.42 (s, 2 H), 6.40 (s, 2 H)). 13C NMR (100
MHz, DMSO-d6): 149.4, 149.3, 149.3, 137.0, 136.7, 136.4, 126.9 (2 overlapping C), 126.6, 121.9, 121.9, 121.8,
79.9, 79.7, 79.6. Anal. calc. for C30H12Br6O3: C 40.04, H 1.34; found: C 40.09, H 1.36.
Second eluate: (syn-1) (92 mg 67% yield) colorless crystals from CH2Cl2/n-hexane (1:3), M.p. > 350 °C (dec.). IR
1
(KBr): 3008, 1723, 1563, 1429, 1351, 1301, 1261, 1172, 1085, 982. H NMR (400 MHz, CDCl3): 7.44 (s, 6 H),
6.08 (s, 6 H). 13C NMR (100 MHz, CDCl3): 146.9, 135.5, 125.4, 122.7, 79.8. Anal. calc. for C30H12Br6O3: C 40.04, H
1.34; found: C 40.07, H 1.33.
Trinaphthylene (2). In a flame dried 50-ml two-necked round-bottomed flask fitted with argon inlet,
titanium(IV) chloride (0.02 ml, 0.17 mmol) was added to dry THF (5 ml). Lithium aluminumhydride (2.4 mg,
0.06 mmol) and a solution of triethylamine (0.03 mL, 0.22 mmol) in dry THF (0.5 mL) were added consecutively
and the resulting mixture was heated to reflux for 15 min. A solution of anti-1, or a 3:7 mixture of anti-1 and
syn-1, (15 mg, 0.02 mmol) in dry THF (2 mL) was added dropwise and the reaction mixture was refluxed for 12
h. The mixture was cooled and portioned between satd. aq. K2CO3 (10 mL) and CHCl3 (3 × 10 mL). Combined
organic extracts were dried over Na2SO4, concentrated under reduced pressure to afford a solid residue that
was purified by column chromatography (silica-gel, 10 g; eluant EtOAc/n-hexane (1:10)) to furnish
trinaphthylene 2 (6.2 mg, 98%) as colorless crystals from CH2Cl2/n-hexane (1:3), M.p. > 350 °C (lit.44 > 320 °C).
1H NMR (400 MHz, CDCl3): 9.11 (s, 6 H), 8.11-8.07 (m, 6 H), 7.59-7.56 (m, 6 H). 13C NMR (100 MHz, CDCl3):
133.1, 129.2, 128.3, 126.6, 122.9.
Acknowledgements
The authors are indebted to Atatürk University (Project no. BAP 2015/91) for financial support.
Supplementary Material
1H NMR Spectrum of syn-1 anti-1, 2, 4, 5, 6, exo-7 and endo-7, 8, 9; 13C NMR Spectrum of syn-1 and anti-1, 2,
4, 5, 6, 8, 9, Spectrum; and HRMS Spectra 5-6, 8, and 21-23.
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