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Convenient New Synthesis of [7]Circulene.

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tion of its molecular model, has a chiral, saddle-shaped structure (yellow prisms, m.p. 225-227"C, 24% yield)['21and was
isolated by chromatography on silica gel.
The mechanism for this striking transformation presumably
involves the initial formation of the ethano-bridged hexahelicene 9 by pyrolysis, followed by dehydrogenation at both ends
of the hexahelicene.['3] MM3 calculations['4] predict a strain
energy about 32.2 kcalmol-' for 10, which should make it more
than 12 kcalmo1-1 more stable than 9 (44.3 kcalmol- l). The
500 MHz 1H NMR spectrum of 10 in CDC1, shows a single
peak for the two enantiotopic methylene groups at room temperature (6 = 2,70), but sets of peaks for the aliphatic hydrogens centered at 6 = 2.98 and 3.75 at - 50 " c (A" = 149.5 Hz)
characteristic of an AABB' pattern. The coalescence temperature for those two signals was found to be - 10 "c,from which
we calculate the barrier for ring inversion in 10 to be
AG * = 12.2 kcalmol- 1 at this temperature.
Finally, dehydrogenation of 10 with 5 YOPdjC in 1-methylnaphthalene at 280 "C for 3 h led to [7]circulene 11 (m.p. 295296"C, 85% yield) with a twisted saddle-shaped geometry.[']
The electrochemical properties of 11 were examined
by cyclic voltammetry. The voltammograms exhibited
one irreversible oxidation peak and two reduction
waves, the first of which was reversible, the second
irreversible, as in the case of coronene itself (Table 1).
A comparison of the oxidation and redox potentials
of 11 with those of ~ o r o n e n e [ ' ~indicate
that it is
easier to remove or add electrons to 11. This observation reveals that structures such as 12 and 13 make a
large contribution to the electronic structure of the
radical cation and anion, respectively, of the non-alternated benzenoid hydrocarbon 1l.[l6]
Convenient New Synthesis of [7]Circulene**
Koji Yamamoto,* Hiroyuki Sonobe,
Hiroshi Matsubara, Masaaki Sato,
Susumu Okamoto, and Kazuo Kitaura*
Recent advances in carbon cluster chemistry (fullerenes) have
renewed the interest in nonplanar polycychc aromatic cOmpounds with a circular arrangement of benzene rings, known as
circulenes.[" w e previously reported the preparation Of
[7]circulene 11 (pleiadann~lene)'~~
whose twisted, saddleshaped structure resulting from a circular arrangement of seven
benzene rings can be considered as a subunit of the inner surface
of helix-shaped nanotube~.[~]
w e report here a convenient new
synthesis of 11 via the 2,1 Sethano-bridged hexahelicene intermediate 9, and the electrochemical properties of 11.
The Wittig condensation of 2,7-naphthalenedicarboxaldehyde 1 with triphenyl-para-xylylphosphoniumbromide in
sodium methoxide/DMF gave a mixture of stereoisomers 2.
*@; - @
\ /
\ / \
2 R=C H = C H + 2 H 3
3 R=CHs
4 R = CHpBr
/ \
\ / \
*" 1 @
* '@
This mixture was dissolved in benzene and irradiated with a
high-pressure mercury lampL6]for 2 h to give 2,15-dimethylhexahelicene 3 (m.p. 212-214"C, 59% yield from l).['] Bromination of 3 with N-bromosuccinimide afforded the dibromide 4
(m.p. 196-198 "C, 40%),[*]which was oxidized with O-natrio2-aci-nitropropane in ethanollbenzene to give the dialdehyde 5
(m.p. 202-204 "C, 72 %) .[91
Although, reductive coupling of 5 to give the bridged hexahelicene 6 by using low-valent titanium["] was unsuccessful, invariably giving a polymer as the product, intramolecular cyclization of 4 with sodium sulfide nonahydrate in refluxing
benzene/ethanol afforded the hexahelicene sulfide 7 (m.p. 252254 "C, 48 YOyield)" whose oxidation with hydrogen peroxide
gave the sulfone 8 (m.p. > 300 "C) in quantitative yield. Flash
vacuum pyrolysis (550°C, 0.01 Torr) of 8 gives the dihydro[7]circulene 10 directly which, as expected from the examina[*] Prof. Dr. K. Yamamoto, Prof. Dr. K. Kitaura, H. Sonobe, Dr. H. Matsubara,
Dr. M. Sato
Department of Chemistry, Faculty of Integrated Arts and Sciences
University of Osaka Prefecture, Sakai, Osaka 593 (Japan)
Fax: Int. code +(722) 55-2981
S. Okamoto
Department of Chemistry. Minatogawa College
Yotsutsuji. Sanda, Hyogo 669-13 (Japan)
This work was supported by a Grant-in-Aid for Scientific Research on Priority
Areas "Carbon Cluster" (No. 05233105) from the Ministry of Education,
Science and Culture. Japan.
Anxeu.. Chem. I n r . E d Engl. 1996, 35, N o . I
For a better
initio MO calculations
of the radical
formed with the GAUSSIAN 92 program.[171The
calculated HOMO energy of [7]circulene is -7.1 eV
Table 1. Results of cyclic voltammetric studies [a].
E,, [b, dl
Erca[c, fl
[C, el
- 2.41
[a] Tetrabutylammonium perchlorate (0.2 moldm-3) as a supporting electrolyte,
scan rate 0.3 Vs-', at 25°C. [b] Potential in Volts vs. Ag/Agf with a glassy carbon
working electrode in dichloromethane. (c] Potential in Volts vs. SCE with a hanging
mercury drop working electrode in DMF, [d] Irreversible oxidation. [el Reduction
peak potential of a quasi-reversible redox reaction. [fl Irreversible reduction.
at the 3-21G SCF level, whereas that of coronene is -7.3 eV.['81
Thus the calculations predict that the ionization potential is
smaller for [7]circulene than for coronene, which is consistent
with the observed redox potentials. The geometry optimizations
of the radical cation and anion of 11 were carried out at the
STO-3G U H F level. The doublet spin state was assumed for
both ions. Both of the
are saddle-shaped
C , symmetry, and are
They are also very similar to the structure of
the neutral molecule.
Verlagsgesellsrhufi mbH, 0-69451 Weinhcim, 1996
\ I
\ /
0570-0833/96/350I-O069 $ 10.00+,2510
/ /
The major differences are that the out-of-plane distortion is a
little greater and that the bonds a and b are about 0.05 8, longer
in the radical ions. This suggests some contribution of the localized Kekulean forms 12 and 13 or of other unidentified localized
forms to the resonance hybrids of cation and anion. Details of
the calculations will be reported elsewhere.
We are currently investigating the potentially fascinating
chemistry of 11 including the energy barrier for the inversion of
the twisted saddle structure in mono-substituted derivatives and
the synthesis of metal complexes.
Received: April 10. 1995
Revised version: October 18, 1995 [Z 7878 I€]
German version: Aiigew. Chwi. 1996, 108. 69-70
Keywords: arenes . circulenes . polycycles
[l] J. H. Dopper, H.Wynberg, J. Org. Clirm. 1975, 40, 1957.~1966.
[2] K. Yamamoto, T. Harada, Y. Okamoto, H . Chikamatsu, M. Nakazaki, Y Kai,
T. Takano, M. Tanaka, S. Harada, N. Kasai, J. A m . Ciiein. SUC.1988, 110.
3578-3584; K. Yamamoto, Y. Saitoh, D. Iwaki, T. Ooka. Angeu.. Cheiii 1991,
103, 1202-1203; Angew. Chem. I n l . Ed. Engl. 1991. 3fJ, 1173-1174: K. ydmamoto, Pure Appl. Chem. 1993, 65, 157-163.
[3] J. S. Siege], T. J. Seiders, Chmi. Br. 1995, 313-316.
[4] S. Iijima, P. M. Ajayan. T. Ichihashi. PIILT.Rev. Le/t. 1992.69, 3100-3104; V.
Ivanov. J. B. Nagy, P. Lambin, A. Lucas, X. B. Zhang. X. F. Zhang, D. Bernaerts. G. van Tendeloo. S. Amelinckx, J. van Landuyt, Cliem. Pliys. Lett. 1994,
223, 329--335.
[5] Compound 1 was prepared by lithiation of 2.7-dibromonaphthalene followed
by formylation with DMF.
[6] Halos, ET-500, Eikosha Co.. Osaka (Japan).
[7] J. H. Borkent, W. H. Laarhoven, Tetruhedrun 1978, 34, 2565-2567.
[8] Satisfactory analytical and spectroscopic data have been obtained for all new
[9] K. Yamamoto. T. Ikeda. T. Kitsuki. Y Okamoto, H. Chikamatsu. M. Nakaraki, J. Cheiii. SOC.Perktn Trans. 1 1990, 271 -276.
[lo] J. E. McMurry, K. L. Kess. J. Org. C h m . 1977. 42. 2655-2656: A. L. Baumstark, C. J. Closkey, K . E. Witt, ;bid. 1978, 43, 3609.~3611
[Ill 7: MS (El, 75 eV): i ~ / 386
z ( M ’ ) ; ‘H NMR (270 MHz, CDCI,, 25‘C. TMS):
6 = 2.55 (d, J =12.82 Hz, 2H. CH,). 3.74(d. J=12.82 Hz. 2 H , C H , ) , 7.40(s,
2H. ArH), 7.93 -8.13 (m, 12H. ArH).
1121 10: MS: m / z : 352 ( M ’ ) ; ‘H N M R (500 MHz. CDCI,, 2 5 - C , TMS): 6 = 2.70
(s, 4 H. CH,). 7.50 (d, J =7.63 Hz, 2 H , ArH). 7.59 (d. J = 8.55 Hz. 2 H, ArH).
7.62 (d, J = 7 . 6 3 Hz. 2H. ArH). 7.68 (d. J = 8.24 Hz, 2H. ArH). 7.69 (d.
J = 8.55 Hz, 2H. ArH). 7.80 (d, .I = 8.24 Hz. ZH, ArH): UViVis (cyclohexane): i m a X ( i ; ) = 251 (46080), 273 (30590). 383sh (28430). 296sh (21 180),
312 (20400). 333 (9800) nm.
[I31 J. H. Dopper, D. Oudman. H. Wynberg, J. Org. CAem. 1975. 40. 3398-3401.
[I41 N. L. Allinger, Y. H. Yuh. J.-H. Lii, J. Am. Chem. Suc. 1989. I l l , 8551 -8566;
J.C. Tai, L. Yang. N . L . Allinger. &id. 1993, 115. 11906-11917.
1151 B. S. Jensen, V. D. Parker, J. Ani. CI11,ni. SOC.
1975, Y7, 5211-5217.
[16] J. Janata, J. Gendell. C:Y. Ling, W. Barth. L. Backer, H. B. Mark. R. D.
Lawton, J. Am. C h ~ m Sor.
1967. 89, 3056-3058.
[17] M. J. Frisch, G. W. Trucks. M. Head-Gordon. P. M. W. Gill. M. W. Wong, J. B.
F o r e m a n , B. G. Johnson, H. B. Schlegel. M. A. Robb. E. S. Replogle, R.
Gomperts, J. L. Andres, K. Raghavachari. J. S. Binkley, C. Gonzalez. R. L.
Martin, D. J. Fox, D. J. DeFrees. D. J. Baker, J. J. Stewart, J. A. Pople. GAUSSIAN 92, Gaussian Inc., Pittsburgh. PA. USA, 1992.
[18] The STO-3G SCF optimized geometries were used. The geometries were taken
from M. Shen, I. S. Igiiatgev. Y. Xie. H. F. Schaefer 111, J. Pliys. Cliein. 1993,
97, 3212 -3216 for [7]circulene and from J. M. Schulman, R. C. Peck. R. L.
Disch. J. Am. Chem. SOC.1989. I l l , 5675-5680 for coronene.
2-Nucleos-5’-0-yl-4H-1,3,2-benzodioxaphosphinin-2-oxides-A New Concept for Lipophilic,
Potential Prodrugs of Biologically Active
Nucleoside Monophosphates””
Chris Meier*
Nucleoside analogues such as 3’-azido-2’,3’-dideoxythymidine
(AZT), 2’,3’-dideoxy-2’,3’-didehydrothymidine(d4T), 2’,3‘dideoxyinosine (ddI), and 2’,3’-dideoxycytidine (ddC) are used
in antiviral chemotherapy against AIDS.[‘] One of the major
problems in the use of this type of compounds is the intracellular
bioavailability of their 5’-phosphorylated derivatives. One concept to overcome this limitation is the intracellular delivery of
the 5’-monophosphates from prodrugs (“kinase-bypass”) .r21 In
view of the high antiviral activity of nucleoside analogues, the
development of nucleotide prodrugs is a challenging
As part of our ongoing programf4]to develop new prodrug
systems, the synthesis of the title compounds 3a-e and 4a-e of
the nucleoside analogues 2,3’-dideoxythymidine (ddT) 1 and
d4T 2, respectively, bearing different donor and acceptor substituents on the benzene ring is reported. Furthermore the partition coefficients of 3 and 4 in I-octanol/phosphate buffer and
their hydrolysis behavior in phosphate buffer (pH 7.5) at 37 ’C
will be discussed.[4”The approach reported here differs from other prodrug concepts in that it leads to nucleotides after a coupledcleavage of the
phenyl- and the benzylester bond in the phosphotriesters 3 and
4, respectively. The difference in hydrolytic stability of phenyl*I has already been reported :
and benzylpho~photriesters[~~
whereas acceptor-substituted phenylphosphotriesters are rapidly hydrolyzed under alkaline conditions, a fast hydrolysis of
benzylphosphotriesters was only observed in the case of their
donor-substituted derivative^.[^] Regardless of the type of phosphotriester, the resulting phosphodiesters are very stable against
nonenzymatic hydrolysis.[’0] The herein reported prodrug concept of the title compounds 3 and 4 uses precisely this difference
in stability of the phenyl and the benzyl phosphate esters for the
design of a three-parted prodrug for the selective liberation of
phosphorylated nucleosides. The hydrolysis should proceed as
follows: in the initial step the phenolic phosphate ester bond is
cleaved selectively, since the resulting negative charge can be
delocalized over the phenolic oxygen atom in the aromatic ring.
The acceptor substituent in the ortho position prevents the
cleavage of the benzylester bond. As a consequence of this cleavage, together with the “umpolung” of the substituent in the
ortho position to the methylene group, the second step, the
cleavage of the 2-hydroxybenzyl group is induced. The presence
of the electron-donating substituent allows cleavage of the benzylic phosphate ester bond, releasing the nucleotides 7 or 8
and the 2-hydroxybenzyl alcohols 9[3d. (tandem-reaction;
Scheme 1 ) .
Furthermore, according to this concept it should be possible
to control the initial activation step by varying the substituents
in the para position to the phenolic phosphate ester bond. For
this reason, different para-substituted salicyl alcohols 9a, b, d, e
were used as precursors in the synthesis of 3 and 4. In addition,
the different substitution in the para position allows one to
[*] Dr. C. Meier
Institut f i r Organische Chemie der Universitit
Marie-Curie-Strasse 11. D-60439 Frankfurt am Main (Germany)
Fax: Int. code + (69)7982-9148
Q VCH ~ r / u g g r s i ~ / l . ~ c hmbH,
u f i 0-69453 Wcmheini. 1996
This work was supported by the Deutsche Forschungsgemeinschaft, the AdolfMesser-Stiftung. and the Fonds der Chemischen Industrie. I thank Prof. Dr.
J. W. Engels for his generous support and encouragement.
0570-0833/96i3501-0070$ 10.00 + .2S/O
Aiigew. C/ieiw. Int. Ed. Eiigl.
1996, 35, N u . 1
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synthesis, convenient, circulene, new
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