Enantioselective Synthesis of Distorted π-Extended Chiral Triptycenes Consisting of Three Distinct Aromatic Rings by Rhodium-Catalyzed [2+2+2] Cycloaddition

Article abstract of DOI:10.1002/chem.201905519

The enantioselective synthesis of distorted π-extended chiral triptycenes, consisting of three distinct aromatic rings, has been achieved with high ee value of 87 % by the cationic rhodium(I)/segphos complex-catalyzed enantioselective [2+2+2] cycloaddition of 2,2′-di(prop-1-yn-1-yl)-5,5′-bis(trifluoromethyl)-1,1′-biphenyl with 6-methoxy-1,2-dihydronaphthalene followed by the diastereoselective Diels–Alder reaction and aromatization. Demethoxy derivatives were also synthesized by the C?O bond cleavage. In this synthesis, the use of the electron-deficient diyne and the electron-rich alkene is crucial to suppress the undesired strain-relieving carbocation rearrangement and stabilize the distorted triptycene structure.

Full text of DOI:10.1002/chem.201905519

A Journal of
Accepted Article
Title: Enantioselective Synthesis of Distorted π-Extended Chiral
Triptycenes Consisting of Three Distinct Aromatic Rings by
Rhodium-Catalyzed [2+2+2] Cycloaddition
Authors: Yukimasa Aida, Yu Shibata, and Ken Tanaka
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To be cited as: Chem. Eur. J. 10.1002/chem.201905519
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COMMUNICATION
Enantioselective Synthesis of Distorted π-Extended Chiral
Triptycenes Consisting of Three Distinct Aromatic Rings by
Rhodium-Catalyzed [2+2+2] Cycloaddition
Yukimasa Aida,[a] Yu Shibata,[a] and Ken Tanaka*[a]
Abstract: The enantioselective synthesis of distorted π-extended
chiral triptycenes, consisting of three distinct aromatic rings, has
been achieved with high ee value of 87% by the cationic
rhodium(I)/segphos complex-catalyzed enantioselective [2+2+2]
cycloaddition of 2,2'-di(prop-1-yn-1-yl)-5,5'-bis(trifluoromethyl)-1,1'-
biphenyl with 6-methoxy-1,2-dihydronaphthalene followed by the
dibromoanthracene-9,10-dione followed by the ruthenium(II)-
catalyzed [2+2+2] cycloaddition (Scheme 1a).[9] However, the
enantioselective alkynylation afforded the desired cis isomer as
a minor product with moderate ee value of 58%.
a) Shibata in 2015 (enantioselective alkynylation)
Ph
diastereoselective
Diels-Alder
reaction
and
aromatization.
O
O
Br
Br
OH
Ph
Li
Demethoxy derivatives were also synthesized by the C-O bond
cleavage. In this synthesis, the use of the electron-deficient diyne
and the electron-rich alkene is crucial to suppress the undesired
strain-relieving carbocation rearrangement and stabilize the distorted
triptycene structure.
(+)-sparteine
RT
enantioselective
alkynylation
Br
OH
Br
Ph cis / trans = 1.0:2.7
R2
R1
R1
R2
Ph
RuII catalyst
105 °C
OH
Triptycene, a molecule in which three benzene rings are
annelated with barrelene, is widely used as a rigid propeller-
shaped segment for the development of three-dimensional
functional materials.[1] For example, triptycene derivatives have
been used in molecular machines,[2] ligands,[3] functional
polymers,[4] and domain-boundary-free organic thin films.[5] In
1942, Bartlett achieved the first synthesis of triptycene, while this
synthesis was not practical due to long reaction steps.[6a] In 1956,
Wittig and Ludwig reported the practical one-step synthesis of
triptycene by the Diels-Alder reaction of anthracene with
benzyne.[6b] After this report, a number of efficient methods (e.g.,
the Friedel-Crafts reaction,[6c] the [2+2+2] cycloaddition,[6d,e] and
the [2+2] cycloaddition[6f,g] approaches) have been reported for
the synthesis of structurally diverse triptycene derivatives.
Ph
Br
HO
Br
moerate ee (58% ee)
"three benzenes"
b) Concept of this work (enantioselective [2+2+2] cycloaddition)
R
R
chiral
R
RhI+ catalyst
enantioselective
[2+2+2]
cycloaddition
R
3 / high ee
1
2a
(R ≠ H)
Ar
Diels-Alder
reaction
dienophile
Ar
Besides, since triptycene has two sp3 carbon atoms,
molecular asymmetry is manifested by introducing substituents
into the aromatic rings, and chiral triptycene derivatives have
been used in chiral ligands[7] and circularly polarized
luminescence materials.[8] However, only one example of the
enantioselective synthesis of chiral triptycene derivatives has
been reported so far,[9] although several examples of optical
resolution of racemic chiral triptycene derivatives by using chiral
reagents[10] or chiral HPLC columns[11] have been reported. In
2015, Shibata reported the enantioselective synthesis of chiral
triptycenes, consisting of three substituted benzene rings, by the
sparteine-mediated enantioselective alkynylation of 1,5-
Ar
R
aromatization
R
H
H
R
R
5
4 / single diastereomer
"three distinct aromatic rings"
"large distortion"
Scheme 1. Enantioselective synthesis of chiral triptycenes.
Recently, our research group reported the highly
enantioselective synthesis of chiral multicyclic cyclohexadienes
by the cationic rhodium(I)/difluorphos complex-catalyzed [2+2+2]
cycloaddition of biphenyl-linked internal 1,7-diynes with
indene.[12,13] From this successful enantioselective catalysis, we
came up with the following approach for the enantioselective
synthesis of
rhodium(I)-catalyzed enantioselective [2+2+2] cycloaddition[14,15]
of biphenyl-linked internal 1,7-diyne with 1,2-
dihydronaphthalene (2a) instead of indene would also afford
chiral multicyclic cyclohexadiene 3 with high yield and ee value.
The subsequent Diels-Alder reaction with dienophile would
proceed selectively on the convex face to give chiral
dihydrobarrelene 4 as a single diastereomer. The subsequent
a chiral triptycene. We anticipated that the
[a]
Y. Aida, Dr. Y. Shibata, Prof. Dr. K. Tanaka
Department of Chemical Science and Engineering, Tokyo Institute
of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
E-mail: ktanaka@apc.titech.ac.jp
1
http://www.apc.titech.ac.jp/~ktanaka/
Supporting information for this article is given via a link at the end
of the document.
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oxidative aromatization would afford π-extended chiral triptycene
5, consisting of three distinct aromatic rings[16] (Scheme 1b).
However, the large distortion would exist in the targeted chiral
triptycene 5 due to the presence of three large steric repulsion
between the bridgehead substituents (R) and the neighboring
aromatic rings.[6g] Because of this large distortion, there are
concerns about the stability of 5. In this Communication, we
have established that the appropriate introduction of substituents
into the biphenyl-linked 1,7-diyne 1 and 1,2-dihydronaphthalene
(2a) realizes this concept.
naphthoquinone followed by reductive aromatization using
LiAlH4 and the Burgess reagent[17] proceeded to give the desired
triptycene precursor (+)-4aa in 30% yield with complete
diastereoselectivity. However, unfortunately, the treatment of
(+)-4aa with DDQ did not afford the desired chiral triptycene (+)-
5aa, instead, an unidentified complex mixture of products was
generated.[18]
Possible reaction pathways for the treatment of 4 with DDQ
are shown in Scheme 3. The oxidation of 4 with DDQ would
generate tertiary carbocation A. However, strain-relieving
carbocation rearrangement[19] might proceed to give tertiary
carbocation B, which leads to a complex mixture of products.
We anticipated that the introduction of an electron-donating
group at the 6-position (R2) of 1,2-dihydronaphthalene (2a)
facilitates the oxidation reaction and induces the generation of
regioisomeric secondary carbocation intermediate C, in which
the carbocation rearrangement dose not proceed. Deprotonation
from C would afford dihydronaphthalene D. Subsequent
Scheme 2 displays the enantioselective synthesis of chiral
triptycene 5. The enantioselective [2+2+2] cycloaddition of
biphenyl-linked
internal
1,7-diyne
1a
with
1,2-
dihydronaphthalene (2a) proceeded at 40 °C in the presence of
a cationic rhodium(I)/(R)-segphos complex (20 mol %) to give
the desired chiral cyclohexadiene (+)-3aa in good yield with high
ee value of 88% (entry 1). Although the Diels-Alder reaction of
(+)-3aa with benzyne did not proceed, that with 1,4-
R1
R2
1) 1,4-naphthoquinone
R1
Me
Me
PhCl, 130 °C
2) LiAlH4, Et2O/THF
0 °C to RT
5 mol %
[Rh(cod)2]BARF/
(R)-segphos
Me
Me
aromatization
R2
Me
Me
R1
R1
3) Burgess reagent
THF, RT
CH2Cl2, RT
O
R1
R2
1
R1
+
Me
Me
R1
R1
3
R2
O
PPh2
5
4 (single diastereomer)
PPh2
O
1) BBr3, CH2Cl2, 0 °C to RT
2) Tf2O, DIEA, CH2Cl2, –78 °C
3) Pd(OAc)2/dppf, HCO2NH4
Et3N, DMF, 60 °C
O
(R)-segphos
2 (5 equiv)
Me
F3C
Me
5ba from 5bb
F3C
X-ray crystal structure
(+)-4aa (R1 = R2 = H)
Entry
1[a]
1 (R1)
1a (H)
1a (H)
2 (R2)
3 / % yield
(% ee)
4 / % yield from 3 Aromatization
5 / % yield
(% ee)
5ba / % yield from
(% ee)
Conditions
5bb (% ee)
2a (H)
(–)-3aa / 69 (88)
(+)-4aa / 30 (87)
DDQ (5 equiv), PhCl
130 °C, 12 h
(+)-5aa / 0
–
2[a]
2b (OMe)
(±)-3ab / 63
(±)-4ab / 22
DDQ (5 equiv), PhCl
130 °C, 12 h
(±)-5ab / 0
–
3[a]
4
5[b]
1b (CF3)
1b (CF3)
1b (CF3)
2b (OMe)
2b (OMe)
2b (OMe)
(–)-3bb / 72 (78)
(–)-3bb / >99 (88)
(–)-3bb / 95 (88)
–
–
–
–
–
–
–
–
(+)-4bb / 43 (87)
DDQ (3.2 equiv), PhCl
40 °C, 72 h
(+)-5bb / 20 (87)
(–)-5bb / 19 (87)
(±)-5ba / 0
(+)-5ba / 62 (87)
6[b,c]
7[a]
1b (CF3)
1b (CF3)
2b (OMe)
2a (H)
(+)-3bb / 99 (87)
(±)-3ba / 79
(–)-4bb / 41 (87)
(±)-4ba / 18
DDQ (3.2 equiv), PhCl
40 °C, 72 h
(–)-5ba / 58 (87)
DDQ (5 equiv), PhCl
130 °C, 12 h
–
Scheme 2. Screening of substituents and reaction conditions for the enantioselective synthesis of chiral triptycenes. 1 (0.20 mmol) was used. BARF =
tetrakis[bis(3,5-trifluoromethyl)phenyl]borate. Burgess reagent = (methoxycarbonylsulfamoyl)triethylammonium hydroxide inner salt. DDQ = 2,3-dichloro-5,6-
dicyano-p-benzoquinone. Tf2O = trifluoromethanesulfonic anhydride. DIEA = N,N-diisopropylethylamine. DDQ = 2,3-dichloro-5,6-dicyano-p-benzoquinone. dppf =
1,1'-bis(diphenylphosphino)ferrocene. [a] [Rh(cod)2]BF4/(R)-segphos (20 mol %) was used at 40 °C. cod = 1,5-cyclooctadiene. [b] The preparative-scale reaction
using 1b (1.00 mmol). [c] (S)-segphos was used.
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COMMUNICATION
H
carbocation
rearrangement
Me
Me
complex mixture
Me
R1
R2 = H
H
Me
R1
R1
B
R1
A
Me
DDQ
R1
R2
Me
R1
R2 = OMe
4
DDQ
Me
Me
Me
R1
R1
R1
–H+
OMe
OMe
OMe
H
Me
Me
Me
R1
R1
R1
D
H
E
C
–H+
OMe
Me
R1
H+
carbocation
rearrangement
Me
R1
R1 = H
5
H
OMe
Me
R1 = CF3
H
Me
complex mixture
Me
Me
H
G
H
F
OMe
H
Scheme 3. Possible reaction pathways for treatment of 4 with DDQ.
oxidation with DDQ would afford tertiary carbocation E, which is
spontaneously aromatized through deprotonation to give the
desired triptycene 5. Thus, the oxidative aromatization of
racemic triptycene precursor (±)-4ab, prepared from 1a and 2a,
was conducted with DDQ, but we failed to obtain the
corresponding triptycene (±)-5ab. We anticipated that
protonation of the phenanthrene moiety of 5ab would occur to
generate carbocation F due to the large distortion. Subsequent
strain-relieving carbocation rearrangement would generate
carbocation G that leads to the decomposition of 5ab. We
expected that the use of 1,7-diyne 1b, bearing the electron-
withdrawing CF3 groups at the 5- and 5’-positions (R1), might
inhibit this protonation and stabilize triptycene 5.
derivatives (+)-5ba and (–)-5ba were synthesized from (+)-5bb
and (–)-5bb, respectively, by demethylation, triflation, and the
palladium(0)-catalyzed C-O bond cleavage[21] sequence
(Scheme 2). Both electron-deficient diyne 1b and electron-rich
alkene 2b are necessary to suppress the undesired carbocation
rearrangement. Thus, the oxidative aromatization of (±)-4ba,
derived from 1b and 1a, with DDQ was also unsuccessful (entry
7).[22]
A single crystal of (+)-4aa was obtained by recrystallization
from CH2Cl2/MeOH, and its crystal structure was unambiguously
determined by
a single crystal X-ray diffraction analysis
(Scheme 2, Figure S3).[23] Although single crystals of 5bb and
5ba were not obtained, the triptycene structures of 5bb and 5ba
were confirmed by 1H, 13C, and 2D-NMR spectroscopy, and
HRMS (high-resolution mass spectrometry).[18] To confirm the
existence of the large distortion, the theoretical optimization of
the structure of (9S,16R)-5ba was conducted, as shown in
Figure 1. Indeed, the large distortion exists in triptycene 5,
especially in the phenanthrene moiety.
Thus, the enantioselective [2+2+2] cycloaddition of 1,7-diyne
1b with 6-methoxy-1,2-dihydronaphthalene (2b) was conducted
under the same conditions for 1a and 2a. Although the desired
chiral cyclohexadiene (+)-3bb was obtained in good yield, the ee
value was lower than that of (+)-3aa (entry 3). Fortunately,
optimization of reaction conditions (Table S3) revealed that the
use of [Rh(cod)2]BARF instead of [Rh(cod)2]BF4 at room
temperature improved both the yield and the ee value to >99%
and 88%, respectively, even using the low catalyst loading of 5
(a)
(b)
mol
% (entry 4). The present enantioselective [2+2+2]
cycloaddition was scalable, and the preparative-scale reaction
using 1.00 mmol of 1b afforded (+)-3bb in 95% yield with 88%
ee (entry 5). The Diels-Alder reaction of (+)-3bb with 1,4-
naphthoquinone followed by reductive aromatization proceeded
to give triptycene precursor (+)-4bb in higher yield than that of
(+)-4aa. As expected, the oxidative aromatization of (+)-4bb with
DDQ proceeded under mild conditions[18] to give the desired
chiral triptycene (+)-5bb in 20% yield without racemization.[20]
The opposite enantiomer (–)-5bb was also prepared in a
preparative scale by the same sequence used for (+)-5bb
employing (S)-segphos as a ligand (entry 6). Finally, demethoxy
Figure 1. Optimized structures of (9S,16R)-5ba calculated at the wB97XD/6-
311G(d) level of theory, (a) side view, (b) diagonal view.
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The ECD (electronic circular dichroism) signals were
observed from (+)-5ba and (–)-5ba with opposite signs and
similar intensities, namely mirror images (Figure 2). Additionally,
the absolute configuration of (+)-5ba was determined to be
9S,16R by comparing the observed ECD spectrum of (+)-5ba
with the theoretical ECD spectrum of (9S,16R)-5ba (Figure 2).
(Japan). Y.A. thanks JSPS research fellowship for young
scientists (No. 17J08763). We thank Takasago International
Corporation for the gift of H8-binap, segphos, tol-segphos, xyl-
segphos, and dtbm-segphos, Umicore for generous support in
supplying the palladium and rhodium complexes, and Central
Glass Co., Ltd. for the gift of trifluoromethanesulfonic anhydride
and 1,1,1,3,3,3-hexafluoro-2-propanol.
This absolute configuration is consistent with
a plausible
enantioselection mechanism of the cationic rhodium(I)/(R)-
segphos complex-catalyzed [2+2+2] cycloaddition of 1b with 2b
(Figure S2).
Keywords: chiral triptycenes • cyclic alkenes • enantioselective
synthesis • rhodium • [2+2+2] cycloaddition
[1]
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Figure 2. Experimental ECD spectra of (+)-5ba (red plain line) and (–)-5ba
(blue plain line) in EtOH (1.0×10–5 M), and theoretical ECD spectrum of
(9S,16R)-5ba (black broken line) calculated by the TD-DFT method at the
wB97XD/6-311G(d) level with IEFPCM (ethanol).
[6]
In summary, we have achieved the highly enantioselective
synthesis of distorted π-extended chiral triptycenes, consisting
of three distinct aromatic rings, with high enantioselectivity (87%
ee) by the cationic rhodium(I)/segphos complex-catalyzed
enantioselective [2+2+2] cycloaddition of 2,2'-di(prop-1-yn-1-yl)-
[7]
[8]
5,5'-bis(trifluoromethyl)-1,1'-biphenyl
with
6-methoxy-1,2-
dihydronaphthalene followed by the diastereoselective Diels-
Alder reaction, and reductive and oxidative aromatization
reactions. Demethoxy derivatives were also synthesized by
demethylation, triflation, and the palladium(0)-catalyzed C-O
bond cleavage sequence. In this synthesis, the use of the
electron-deficient diyne and the electron-rich alkene is crucial to
a) Y. Wada, K. I. Shinohara, T. Ikai, Chem Commun 2019, 55, 11386;
b) T. Ikai, T. Yoshida, S. Awata, Y. Wada, K. Maeda, M. Mizuno, T. M.
Swager, ACS Macro Letters 2018, 7, 364; c) T. Ikai, Y. Wada, S. Awata,
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15, 8440.
[9]
T. Shibata, Y. Kamimura, Tetrahedron: Asymmetry 2015, 26, 41.
suppress
the
undesired
strain-relieving
carbocation
[10] a) N. Harada, Y. Tamai, Y. Takuma, H. Uda, J. Am. Chem. Soc. 1980,
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Chem. Soc. Jpn. 1962, 35, 853. See also refs 1b and 7b.
rearrangement in the oxidative aromatization step, leading to the
desired triptycene. Additionally, the introduction of the electron-
withdrawing CF3 groups to the triptycenes might stabilize the
distorted triptycene structure. We believe that the present
process would be a useful strategy for the enantioselective
synthesis of chiral triptycenes. Future works will include
functionalization of the thus obtained chiral triptycenes, leading
to chiral ligands.
[11] a) T. Ikai, T. Yoshida, K. I. Shinohara, T. Taniguchi, Y. Wada, T. M.
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2016, 18, 2672.
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[2+2+2] cycloaddition reactions of alkynes with cyclic 1,2-disubstituted
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83, 2617; b) T. Shibata, A. Kawachi, M. Ogawa, Y. Kuwata, K.
Tsuchikama, K. Endo, Tetrahedron 2007, 63, 12853; c) S. Ikeda, H.
Kondo, T. Arii, K. Odashima, Chem. Commun. 2002, 2422.
Acknowledgements
This research was supported partly by Grants-in-Aid for
Scientific Research (JP26102004 and JP19H00893) from JSPS
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10.1002/chem.201905519
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COMMUNICATION
[14] For reviews of the transition-metal-catalyzed [2+2+2] cycloaddition
involving alkenes, see: a) G. Dominguez, J. Perez-Castells, Chem. Eur.
J. 2016, 22, 6720; b) Y. Satoh, Y. Obora, Eur. J. Org. Chem. 2015,
2015, 5041; c) M. Amatore, C. Aubert, Eur. J. Org. Chem. 2015, 2015,
265; d) G. Domínguez, J. Pérez-Castells, [2+2+2] Cycloaddition, in
Comprehensive Organic Synthesis II, Vol. 5, 2nd ed. (Ed.: P. Knochel),
Elsevier, Amsterdam, 2014, p. 1537; e) N. Saito, D. Tanaka, M. Mori, Y.
Sato, Chem Rec 2011, 11, 186; f) T. Shibata, K. Tsuchikama, Org
Biomol Chem 2008, 6, 1317; g) P. R. Chopade, J. Louie, Adv. Synth.
Catal. 2006, 348, 2307.
[15] For selected recent reviews of the rhodium-catalyzed [2+2+2]
cycloaddition, see: a) Y. Shibata, K. Tanaka, Rhodium(I)-Catalyzed
[2+2+2] and [4+2] Cycloadditions, in Rhodium Catalysis in Organic
Synthesis: Methods and Reactions (Ed.: K. Tanaka), Wiley-VCH,
Weinheim, 2019, p. 183; b) K. Tanaka, Bull. Chem. Soc. Jpn. 2018, 91,
187; c) K. Tanaka, TCI Mail 2018, 179, 3; c) K. Tanaka, Y. Kimura, K.
Murayama, Bull. Chem. Soc. Jpn. 2015, 88, 375; d) K. Tanaka,
Rhodium-Mediated [2+2+2] Cycloaddition, in Transition-Metal-Mediated
Aromatic Ring Construction (Ed.: K. Tanaka), Wiley, Hoboken, USA,
2013, p. 127; e) K. Tanaka, Y. Shibata, Synthesis 2012, 44, 323-350.
[16] For an example of triptycene derivatives consisting of three distinct
aromatic rings, see: M. Sugihashi, R. Kawagita, T. Otsubo, Y. Sakata,
S. Misumi, Bull. Chem. Soc. Jpn. 1972, 45, 2836. See also refs 1b, 6g,
10b, and 10c.
[17] M. Hermann, D. Wassy, D. Kratzert, B. Esser, Chem. Eur. J. 2018, 24,
7374.
[18] See the Supporting Information for details.
[19] G. A. Olah, Acc. Chem. Res. 1976, 9, 41.
[20] The oxidative aromatization of dihydrobarrelene 4cb, bearing the
moderately electron-withdrawing chloro groups, with DDQ was also
examined. Although the desired triptycene 5cb was generated in a
trace amount, this compound decomposed even under weakly acidic
conditions, e.g., isolation on a silica gel chromatography and the NMR
measurement in CDCl3.
DDQ (3.2 equiv)
Me
OMe
Me
PhCl, 40 °C, 72 h
Cl
Cl
OMe
Me
4cb
Me
5cb / trace
Cl
Cl
[21] D. F. Fischer, R. Sarpong, J. Am. Chem. Soc. 2010, 132, 5926.
[22] The use of 6-methoxy-1,2-dihydronaphthalene (2b) is also crucial, and
the oxidative aromatization of 4bc, derived from 7-methoxy-1,2-
dihydronaphthalene (2c), with DDQ was unsuccessful.
Me
F3C
Me
F3C
OMe
4bc
[23] CCDC 1957627 [(+)-4aa] contains the supplementary crystallographic
data for this paper. This data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
This article is protected by copyright. All rights reserved.

10.1002/chem.201905519
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
F3
C
C
OMe
1) Diels-Alder
reaction with
1,4-naphtho-
quinone
Yukimasa Aida, Yu Shibata, and Ken
Tanaka*
F3
C
Me
Me
5 mol %
[Rh(cod)2]BARF/
(R)-segphos
Me
R
Me
2) reductive &
oxidative
aromatization
CH2Cl2, RT
+
F3
C
F3
OMe
Me
F3C
Page No. – Page No.
Me
>99%, 88% ee
F3C
R = OMe
R = H
demethoxylation
87% ee
Enantioselective Synthesis of
Distorted π-Extended Chiral
"three distinct aromatic rings"
"large distortion"
Triptycenes Consisting of Three
Distinct Aromatic Rings by Rhodium-
Catalyzed [2+2+2] Cycloaddition
The enantioselective synthesis of distorted π-extended chiral triptycenes, consisting
of three distinct aromatic rings, has been achieved with high ee value of 87% by the
rhodium(I)-catalyzed [2+2+2] cycloaddition followed by the Diels-Alder reaction and
aromatization. Demethoxy derivatives were also synthesized by the C-O bond
cleavage. In this synthesis, the use of an electron-deficient diyne and an electron-
rich alkene is crucial to suppress the undesired strain-relieving carbocation
rearrangement and stabilize the distorted triptycene structure.
This article is protected by copyright. All rights reserved.