Syntheses and structures of isomeric [6.6]- and [8.8]cyclophanes with 1,4-dioxabut-2-yne and 1,6-dioxahexa-2,4-diyne bridges

Article abstract of DOI:10.1021/jo015574g

All three isomers (ortho, meta, and para) of [8.8] cyclophane bearing 1,6-dioxahexa-2,4-diyne bridges have been synthesized and structually characterized by single-crystal X-ray crystallography to determine the conformation of the cyclophanes and their cavity dimensions. The three isomeric [6.6]cyclophanes bearing 1,4-dioxabut-2-yne bridges have also been synthesized from but-2-yne-1,4-diol ditosylate and the isomeric dihydroxybenzenes. The [6.6]orthocyclophane has been structurally characterized by single-crystal X-ray crystallography. The energy-minimized structures from the semiempirical AM1 calculations of these cyclophanes compare very well with the structures obtained by X-ray crystallography.

Full text of DOI:10.1021/jo015574g

J . Org. Chem. 2001, 66, 4299-4303
4299
Syn th eses a n d Str u ctu r es of Isom er ic [6.6]- a n d [8.8]Cyclop h a n es
w ith 1,4-Dioxa bu t-2-yn e a n d 1,6-Dioxa h exa -2,4-d iyn e Br id ges†
Manivannan Srinivasan,‡ Sethuraman Sankararaman,*,‡ Henning Hopf,§ Ina Dix,§ and
Peter G. J ones|
Department of Chemistry, Indian Institute of Technology, Madras 600 036, India,
Institute of Organic Chemistry, Hagenring 30, University of Braunschweig, D-38106 Braunschweig,
Germany, and Institute of Inorganic and Analytical Chemistry, Hagenring 30,
University of Braunschweig, D-38106 Braunschweig, Germany
sanka@acer.iitm.ernet.in
Received February 14, 2001
All three isomers (ortho, meta, and para) of [8.8]cyclophane bearing 1,6-dioxahexa-2,4-diyne bridges
have been synthesized and structually characterized by single-crystal X-ray crystallography to
determine the conformation of the cyclophanes and their cavity dimensions. The three isomeric
[6.6]cyclophanes bearing 1,4-dioxabut-2-yne bridges have also been synthesized from but-2-yne-
1,4-diol ditosylate and the isomeric dihydroxybenzenes. The [6.6]orthocyclophane has been
structurally characterized by single-crystal X-ray crystallography. The energy-minimized structures
from the semiempirical AM1 calculations of these cyclophanes compare very well with the structures
obtained by X-ray crystallography.
In tr od u ction
synthesis of cyclophanes has been rare. Recently, Gokel
has reported the synthesis and selective ion complexing
ability of [6.6]paracyclophane 1c.5 Our interest in the
area of cyclophanes bearing acetylenic bridging units6 is
to study the structure, conformation and cavity dimen-
sions of these cyclophanes for their use as donors to form
electron donor-acceptor complexes with acceptors of
suitable dimensions. Herein we report the syntheses and
X-ray crystal structures of isomeric [8.8]cyclophanes and
also the syntheses of isomeric [6.6]cyclophanes and the
X-ray crystal structure of the ortho isomer 1a (Scheme
1). The energy-minimized structures based on semiem-
pirical AM1 calculations7 are also reported and compared
with the structures from the X-ray crystallography.
The design and synthesis of cyclophanes possessing
rigidly defined cavities and shape-persistent structures
of molecular dimensions is of interest as molecular hosts
in the areas of host-guest and electron donor-acceptor
complexes.1 The dimensions of the cavity depend on the
spacer group and its connectivity to the arene units.
Acetylenic units as bridges impart rigidity to the cyclo-
phanes, and the number of acetylenic units in the bridge
and its connectivity to the arene units control the size of
the cavity possessed by the cyclophane. Two such readily
available units are 1,4-dioxabut-2-yne and 1,6-dioxahexa-
2,4-diyne. Whitlock has used the latter unit for the
construction of several [8.8]paracyclophanes, which are
substituted derivatives of 2c.2 However, the parent
cyclophane, namely 2c, has not been reported so far.
Breslow has reported a triply bridged [8.8.8]cyclophane
consisting of the 1,6-dioxahexa-2,4-diyne units bridging
two triphenylmethyl moieties.3 Similarly, Vo¨gtle has also
reported the synthesis of an [8.8.8]cyclophane connecting
two triphenylmethyl cations as the arene units.4 The use
of 1,4-dioxabut-2-yne moiety as the bridging unit in the
Resu lts a n d Discu ssion :
Syn th eses of [8.8]Cyclop h a n es. The isomeric [8.8]-
cyclophanes (2a -c) (Scheme 1) were synthesized from
the corresponding benzenediols (3a -c) by a two-step
procedure (Scheme 2).
Propargylation of the benzenediols proceeded cleanly
to yield the corresponding isomeric bispropargyl ethers
(4a -c) in good yields. When the bispropargyl ethers were
subjected to Glaser-Eglington coupling, the cyclophanes
were obtained in rather poor yields. In an attempt to
improve the overall yield, an alternative route involving
the stepwise construction of the bridges was attempted
† Dedicated to Prof. H. W. Whitlock, J r., on the occasion of his 65th
birthday.
‡ Department of Chemistry, Indian Institute of Technology.
§ Institute of Organic Chemistry, University of Braunschweig.
| Institute of Inorganic and Analytical Chemistry, University of
Braunschweig.
(1) (a) Vo¨gtle, F. Cyclophane Chemistry; J ohn Wiley and Sons:
Chichester, 1993. (b) Diederich, F. Cyclophanes; Royal Society of
Chemistry: Cambridge, 1991. (c) Cram, D. J .; Cram, J . M. Container
Molecules and Their Guests; Royal Society of Chemistry: Cambridge,
1994. (d) Morrison, D. L.; Ho¨ger, S. Chem. Commun. 1996, 2313-2314.
(e) Tobe, Y.; Nagano, A.; Kawabata, K.; Sonoda, M.; Naemura, K. Org.
Lett. 2000, 2, 3265-3268 and refs 2-4 cited therein.
(2) (a) J arvi, E. T.; Whitlock, H. W., J r. J . Am. Chem. Soc. 1980,
102, 657-662. (b) Whitlock, B. J .; Whitlock, H. W., J r. J . Am. Chem.
Soc. 1983, 105, 838-844. (c) Whitlock, B. J .; Whitlock, H. W., J r. J .
Am. Chem. Soc. 1985, 107, 1325-1329. (d) Brown, A. B.; Whitlock, H.
W., J r. J . Am. Chem. Soc, 1989, 111, 3640-3651.
(3) O’Krongly, D.; Denmeade, S. R.; Chiang, M. Y.; Breslow, R. J .
Am. Chem. Soc. 1985, 107, 5544-5545.
(4) (a) Berscheid, R.; Vo¨gtle, F. Synthesis 1992, 58-62. (b) Berscheid,
R.; Nieger, M.; Vo¨gtle, F. Chem. Ber. 1992, 125, 2539-2552.
(5) (a) Behm, R.; Gloeckner, C.; Grayson, M. A.; Gross, M. L.; Gokel,
G. W. Chem. Commun. 2000, 2377-2378. For recent reports on related
cyclophanes, see: (b) Suffert, J .; Raeppel, S.; Raeppel, F.; Didier, B.
Synlett 2000, 874-876. (c) Yamaguchi, Y.; Kobayashi, S.; Wakamiya,
T.; Matsubara, Y.; Yoshida, Z. J . Am. Chem. Soc. 2000, 122, 7404-
7405.
(6) Srinivasan, M.; Sankararaman, S.; Dix. I.; J ones, P. G. Org. Lett.
2000, 2, 3849-3851.
(7) Calculations were performed using PC Spartan Pro Version 1.1,
Wavefunction, Inc., Irvine, CA.
10.1021/jo015574g CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/19/2001

4300 J . Org. Chem., Vol. 66, No. 12, 2001
Srinivasan et al.
Sch em e 1
Sch em e 4
sponding acetates (7a -c) and the coupling of the acetates
was carried out to give the singly bridged precursor
acetates (8a -c).
In spite of carrying out the coupling reaction under a
variety of literature reported conditions8 and also under
high dilution conditions, the yields of the cyclophanes
could not be improved. Thus, in Schemes 2 and 3 the
bottleneck has been the Glaser-Eglington coupling step.
In spite of the low yields, the cyclophanes were isolated
and purified by column chromatography and repeated
crystallization and were thoroughly characterized by
spectroscopic, analytical, and single-crystal X-ray crystal-
lographic data.
Syn th eses of [6.6]Cyclop h a n es. The isomeric [6.6]-
cyclophanes (1a -c) were synthesized starting from the
corresponding benzenediols (3a -c) by a single step using
buta-2-yne-1,4-diol ditosylate (Scheme 4). Although the
desired cyclophanes were formed in poor yields, they were
isolated and purified by column chromatography and
repeated crystallization. The cyclophanes were fully
characterized by spectroscopic and analytical data. The
ortho isomer has also been characterized by single-crystal
X-ray crystallographic data.
Sch em e 2a
a
Key: (a) propargyl bromide, K2CO3, acetone, reflux, 22 h, 80-
89%; (b) Cu(OAc)2‚H2O, CH3CN, pyridine, 60 °C, 2 h, 8-11%.
Cr ysta l Str u ctu r es of th e Cyclop h a n es, Th eir
Con for m a tion s, a n d Th eir Ca vity Dim en sion s. The
crystal structures of all the three isomers of the [8.8]-
cyclophanes (2a -c) are shown in Figure 1. The cavities
in these cyclophanes can be approximated to be a
rectangular box, and the cavity dimensions mentioned
herein are the center-to-center distance between bridges
and the distance between the two arene units. In the case
of [8.8]orthocyclophane (2a ), there are two independent
molecules in the unit cell stacked alternatively along the
crystallographic b axis. Each of the independent mol-
ecules possesses a center of symmetry. The cavity dimen-
sions are 4.1 × 10. 2 Å. The structure of the meta isomer
2b corresponds to an elongated anti-chair conformation,
and the structure possesses an inversion center. The
cavity dimensions are 4.4 × 10.4 Å. The distance between
the intraannular aromatic hydrogens is 6.705 Å. The
structure of the para isomer 2c in the crystal corresponds
to an anti-chair conformation with the two arene units
parallel to each other and forming an angle of 55° with
the plane connecting the sp-carbons of the two bridges.
The cavity dimensions are 7.0 × 7.9 Å. The center-to-
center distance between the aromatic units is 7.87 Å. The
molecular structure of [6.6]orthocyclophane (1a ) in the
crystal is very similar to that of 2a , possessing a center
of symmetry (Figure 2). Of the four structures presented
here, only in the ortho-cyclophane 1a are the molecules
stacked on top of each other in a crystal lattice forming
Sch em e 3a
a
Key: (a) propargyl bromide, K2CO3, acetone, reflux, 22 h; (b)
Cu(OAc)2‚H2O, CH3CN, py, 60 °C, 2 h; (c) Ac2O, py, CH2Cl2, rt, 5
h, 70-75%; (d) Na2CO3, MeOH, rt, 1 day, 60-70%.
(Scheme 3). The benzenediols were first converted into
the monopropargyl ethers (5a -c) followed by the Glaser-
Eglington coupling to give the singly bridged precursors
(6a -c). Further propargylation followed by coupling of
the terminal acetylene units in 9a -c yielded the isomeric
cyclophanes (2a -c), again in poor yields. A slight im-
provement in the yield was observed when the monopro-
pargyl ethers (5a -c) were first converted to the corre-
(8) For a recent review: (a) Siemsen, P.; Livingston, R. C.; Diederich,
F. Angew. Chem., Int. Ed. 2000, 39, 2632-2657. (b) J arvi, E. T.;
Whitlock J r. H. W. J . Am. Chem. Soc. 1988, 104, 7196-7204. (c)
Whitlock, B. J .; J arvi, E. T.; Whitlock, J r. H. W. J . Org. Chem. 1981,
46, 1832-1835. (d) Dietrich-Buchecker, C. O.; Hemmert, C.; Khemiss,
A.; Sauvage, J . P. J . Am. Chem. Soc. 1990, 112, 8002-8008. (e)
Campbell, I. D.; Eglinton, G. Organic Syntheses; J ohn Wiley: New
York, 1973; Collect. Vol. V, pp 517-520. (f) Collins, S. K.; Yap, G. P.
A.; Fallis, A. G. Angew. Chem., Int. Ed. 2000, 39, 385-388.

Syntheses and Structures of Isomeric [6.6]- and [8.8]Cyclophanes
J . Org. Chem., Vol. 66, No. 12, 2001 4301
Ta ble 1. Com p a r ison of th e Ca vity Dim en sion s (in Å) of
th e Cyclop h a n esa
cyclophane
exptl
calcd
1a
1b
1c
2a
2b
2c
4.0 × 7.9
4.0 × 7.6
4.9 × 8.0 (3.8)
5.5 × 7.0
5.5 × 7.0
4.1 × 10.2
4.4 × 10.4 (6.7)
7.0 × 7.9
4.1 × 9.7
4.8 × 10.2 (6.30)
7.7 × 7.9
a
The dimensions are the center-to-center distance between
bridges and the distance between the two arene units, respectively,
in Å. The numbers in the parentheses are the distance between
the intraannular hydrogens of the metacyclophanes. The experi-
mental data for 1c is from ref 5a.
F igu r e 3. Calculated energy-minimized structure of 1b.
case of 2c reported herein. Although the structure of the
meta isomer 2b in the solid state is anti, in solution it
could exist as a mixture of syn and anti conformers. To
1
investigate this possibility, the H NMR spectrum of 2b
was recorded in the temperature range of -95 to +135
°C. Under these conditions, the NMR spectrum remained
unchanged, and no coalescence (or decoalescence) of the
NMR signals could be observed. We conclude that the
cavity dimensions of 2b are large enough for a rapid syn-
anti interconversion in solution even at -95 °C.
F igu r e 1. Structures of cyclophanes 2a -c (top to bottom,
respectively) in the crystal.
Ca lcu la tion of En er gy-Min im ized Con for m a tion
of th e Cyclop h a n es.7 The energy minimization calcula-
tions were carried out by semiempirical AM1 methods.
Although the energy-minimized conformation/geometry
highly depends on the local geometry from the starting
model, we find that the energy-minimized structures in
the case of 2a -c and 1a are in good agreeement with
the X-ray structures in terms of the conformation cyclo-
phanes and their cavity dimensions. For example, cyclo-
phanes 1c and 2c that exist in an elongated chair
conformation in the solid state (crystal) show the same
conformation in the calculated energy-minimized struc-
ture as well. Similarly, cyclophane 2b, which exists in
an anti chair conformation in the solid state (crystal),
shows an identical conformation in the calculated energy-
minimized structure. The cavity dimensions from the
calculations and the X-ray structures are compared in
Table 1. Cyclophane 1b is the only member of this series
for which the X-ray crystal structure is not known. The
calculated energy-minimized structure of 1b (Figure 3)
clearly shows an anti chair conformation similar to the
larger cyclophane 2b. From the AM1 calculations, we
have estimated the differences in the heat of formation
between the syn and anti conformations of 1b and 2b to
be approximately 1.27 and 0.33 kcal/mol, respectively.
F igu r e 2. Structure of 1a in the crystal (top) and packing
diagram of 1a in the crystal (bottom).
a columnar structure along the crystallographic b axis
(Figure 2). The cavity dimensions of 1a are 4.0 × 7.5 Å.
Recently, Gokel5 has reported the X-ray crystal structure
of the [6.6]paracyclophane 1c, and in the solid state the
molecule exists in the chair conformation just as in the
F or m a tion of Ch a r ge-Tr a n sfer (CT) Com p lexes.
The cyclophanes 1a -c and 2a -c are good electron donors

4302 J . Org. Chem., Vol. 66, No. 12, 2001
Srinivasan et al.
Exp er im en ta l Section
Ta ble 2. Absor p tion Ma xim u m (in n m ) of th e
Ch a r ge-Tr a n sfer Com p lexes in CH2Cl2
Gen er a l P r oced u r e for th e Gla ser -Eglin gton Cou -
p lin g of th e Bis-P r op a r gyl Eth er s 4a -c. To a suspension
of cupric acetate monohydrate (6.78 g, 34.0 mmol) in a mixture
of acetonitrile (240 mL) and pyridine (60 mL) at 60 °C was
added a solution of the bis propargyl ether (4a -c) (3.0 g, 15.8
mmol) in acetonitrile (20 mL). The color of the solution changed
from deep blue to green. The mixture was stirred for 2 h,
during which time the reaction was monitored by TLC. The
reaction mixture was cooled to room temperature, and water
was added (500 mL). The mixture was extracted with CH2Cl2
(3 × 100 mL). The combined organic phase was washed with
4 N HCl (2 × 100 mL), saturated NaHCO3 (200 mL), water (2
× 200 mL), and saturated brine (200 mL). The organic layer
was dried over anhydrous Na2SO4 and filtered, and the solvent
was removed. The crude product was chromatographed on
silica gel and eluted with a hexane/ethyl acetate (7:3 v/v)
mixture to afford the cyclophanes 2a -c as colorless solids.
Gen er a l P r oced u r e for th e Syn th esis of [6.6]Cyclo-
p h a n es (1a -c). To a solution of the benzenediol (3a -c) (0.5
g, 4.54 mmol) in dry acetone (30 mL) was added anhydrous
K2CO3 (3.14 g, 22.7 mmol), and the mixture was refluxed for
0.5 h. To the mixture was added dropwise a solution of
2-butyne-1,4-diol ditosylate (1.8 g, 4.54 mmol) in dry acetone
(10 mL) over a period of 0.5 h. The resulting mixture was
stirred and refluxed for an additional 20 h. The mixture was
cooled and filtered, and the filtrate was evaporated. The
residue was dissolved in CH2Cl2 (100 mL) and washed with
water (2 × 100 mL) and saturated brine (100 mL). The organic
layer was dried over Na2SO4, and removal of the solvent
yielded a white solid. The crude product was chromatographed
on silica gel and eluted with a mixture of hexane/ethyl acetate
(4:1 v/v) to afford the cyclophanes 1a -c as colorless solids.
Sp ectr oscop ic Ch a r a cter iza tion of th e Cyclop h a n es.
1,6,13,18-Tetraoxa[6.6]orthocyclophane-3,15-diyne (1a ): yield
0.084 g (0.26 mmol) (11.6% from 0.5 g, 4.5 mmol of 3a );
colorless crystalline solid; mp 174-176 °C; IR (KBr) 1600, 1497
acceptor
donor
TCNE
TCNQa
DDQ
1a
1b
1c
2b
4a
4b
4c
420, 520
420, 520
400, 560
400, 500
410, 520
420, 500
380, 550
460-700
460-700
560
440-700
460-700
460-700
540
560
560
650
540
560
570
630
a
Except for 1c and 4c no distinct absorption maximum was
observed. The range given is the tail portion of the CT band in
the visible region.
due to the electron-rich dialkoxyarene units. The forma-
tion of CT complexes of the cyclophanes was studied with
TCNE, TCNQ, and DDQ as the acceptors in dichlo-
romethane using electronic spectroscopy.9 The bisprop-
argyloxybenzenes 4a -c were also studied for comparison.
Formation of purple CT complexes was observed with
TCNE with all the substrates studied. The formation of
the CT complexes was accompanied by the observation
of a new CT band in the visible region of the electronic
spectra of the solutions containing the donor and TCNE.
The CT complexes of TCNE showed multiple CT bands
with two absorption maxima, which is a very character-
istic feature of the TCNE complexes with arene donors.10
The CT complexes of TCNQ were dark yellow in color,
and they did not display any characteristic absorption
maximum in the visible region of the electronic spectra,
rather a broad tail absorption in the region of 460-700
nm was observed in all the cases. The CT complexes of
DDQ were greenish yellow in color, and they showed a
broad featureless CT band in the electronic spectra. The
absorption spectral data of the CT complexes are given
in Table 2.
1
cm-1; H NMR (CDCl3, 400 MHz) δ 6.82-6.72 (AA′BB′ mul-
tiplet, 8H), 4.70 (s, 8H); 13C NMR(CDCl3, 100 MHz) δ 147.89
(s), 122.79 (d), 117.40 (d), 82.09 (s), 57.79 (t); MS (EI, 70 eV)
321(18), 320 (M+, 92), 160 (100); HRMS calcd for C20H16O4
320.10486, found 320.1042. Anal. Calcd: C, 74.97; H, 5.03.
Found: C, 74.67; H, 5.03.
The CT spectra of the cyclophanes were nearly identi-
cal to the CT spectra of the model substrates 4a -c. From
the data it is evident that the dialkoxyarene unit is
responsible for the formation of the CT complexes with
various acceptors. The determination of the formation
constant of the CT complexes using the Benesi-Hilde-
brand method11 was attempted. The magnitude of the
1,6,13,18-Tetraoxa[6.6]metacyclophane-3,15-diyne (1b): yield
0.053 g (0.17 mmol) (7.3% from 0.5 g, 4.5 mmol of 3b); colorless
crystalline solid; mp 157-158 °C; IR (KBr): 1616, 1491, 1146
1
cm-1; H NMR (CDCl3, 400 MHz) δ 7.14 (t, 2H, J ) 8.2 Hz),
6.54 (dd, 4H, J ) 8.2, 2.4 Hz), 6.20 (t, 2H, J ) 2.4 Hz), 4.69 (s,
8H); 13C NMR(CDCl3, 100 MHz) δ 158.59 (s), 130.01 (d), 108.52
(d). 102.25 (d), 82.54 (s), 56.05 (t); MS (EI, 70 eV) 321 (12),
320 (M+, 64), 160 (100); HRMS calcd for C20H16O4 320.10486,
found 320.1042. Anal. Calcd: C, 74.97; H, 5.03. Found: C,
75.44; H, 5.00.
association constants was in the range of 0.3-10 M-1
.
Con clu sion s
The synthesis of the three isomers of [6.6]- and [8.8]-
cyclophanes bearing 1,4-dioxabut-2-yne and 1,6-dioxa-
hexa-2,4-diyne bridges, respectively, has been reported.
The three isomers of the [8.8]cyclophane and the ortho
isomer of the [6.6]cyclophane have been structurally
characterized by single-crystal X-ray diffraction. The
energy-minimized structures obtained by semiempirical
AM1 calculations are in good agreement with the crystal
structures of these cyclophanes. The cyclophanes form
colored CT complexes with electron acceptors such as
TCNE, TCNQ, and DDQ due to the electron-rich di-
alkoxyarene units.
1,6,13,18-Tetraoxa[6.6]paracyclophane-3,15-diyne (1c): yield
0.048 g (0.15 mmol) (6.6% from 0.5 g, 4.5 mmol of 3c); colorless
crystalline solid; mp 190-192 °C; 1H NMR (CDCl3, 400 MHz)
δ 6.84 (s, 8H), 4.67 (s, 8H); 13C NMR (CDCl3, 100 MHz) δ
152.10 (s), 116.26 (d), 82.65 (s), 56.61 (t); MS (EI, 70 eV) 321
(10), 320 (M+, 56), 52 (100); HRMS calcd for C20H16O4 320.10486,
found 320.1048.
1,8,15,22-Tetraoxa[8.8]orthocyclophane-3,5,17,19-tetrayne
(2a ): yield 0.39 g (1.0 mmol) (7.9% from 5.0 g, 27.0 mmol of
4a ); colorless solid; mp 170-172 °C; IR (KBr) 1587, 1468, 1235,
1
1116, 1011 cm-1; H NMR (CDCl3, 400 MHz) δ 6.98 (AA′BB′
multiplet, 8H), 4.82 (s, 8H); 13C NMR(CDCl3, 100 MHz) δ
148.13 (s), 123.35 (d), 117.68 (d), 74.21 (s), 71.25 (s), 58.53 (t);
MS (EI, 70 eV) 368 (M+, 40), 76 (100); HRMS calcd for C24H16O4
368.1049, found 368.1040.
(9) Foster, R. Organic Charge-Transfer Complexes; Academic
Press: New York, 1969; Chapter 3.
1,8,15,22-Tetraoxa[8.8]metacyclophane-3,5,17,19-tetrayne
(2b): yield 0.28 g (0.76 mmol) (7.1% from 4.0 g, 21.5 mmol of
4b); colorless solid; mp 192-194 °C; IR (KBr) 1590, 1484, 1020
cm-1; 1H NMR (CDCl3, 400 MHz) δ_7.18 (t, J ) 8.30 Hz, 2H),
6.66 (t, J ) 1.96 Hz, 2H), 6.58 (dd, J ) 8.30 and 2.45 Hz, 4H),
4.78 (s, 8H); 13C NMR(CDCl3, 100 MHz) δ 158.28 (s), 130.06
(10) (a) Merrifield, R. E.; Phillips, W. D. J . Am. Chem. Soc. 1958,
80, 2778-2782. (b) Voigt, E. M.; Reid, C. J . Am. Chem. Soc. 1964, 86,
3930-3934. (c) Briegleb, G.; Czekalla, J .; Reuss, G. Z. Phys. Chem.
Frankf. Ausg. 1961, 30, 316-320.
(11) Benesi, H. A.; Hildebrand, J . H. J . Am. Chem. Soc. 1949, 71,
2703-2707.

Syntheses and Structures of Isomeric [6.6]- and [8.8]Cyclophanes
J . Org. Chem., Vol. 66, No. 12, 2001 4303
(d), 109.62 (d), 101.49 (d), 74.57 (s), 71.14 (s), 56.02 (t); MS
(EI, 70 eV) 368 (M+, 25), 202 (100); HRMS calcd for C24H16O4
368.1049, found 368.1018.
1,8,15,22-Tetraoxa[8.8]paracyclophane-3,5,17,19-tetrayne
(2c): yield 0.54 g (1.4 mmol) (10.8% from 5.0 g, 27.0 mmol of
4c); colorless solid; mp 165-166 °C; 1H NMR (CDCl3, 400 MHz)
δ 6.90 (s, 8H), 4.72 (s, 8H); 13C NMR(CDCl3, 100 MHz) δ 152.23
(s), 116.12 (d), 74.79 (s), 71.08 (s), 56.85 (t); MS (EI, 70 eV)
368 (M+, 100), 292 (25), 183 (20); HRMS calcd for C24H16O4
368.1049, found 368. 1044.
support from CSIR, New Delhi, India, and Volkswagen
Stiftung, Hannover, Germany, and a graduate fellow-
ship from IIT, Madras, for M.S. are gratefully acknowl-
edged.
Su p p or tin g In for m a tion Ava ila ble: Crystal structure
data for compounds 1a and 2a -c. General experimental
procedures for the syntheses of 4a -c, 5a -c, 6a -c, and 7a -
c. Spectroscopic and analytical data for compounds 5b,c, 7a ,
6a -c, 8a -c, and 9b-c. This material is available free of
charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. We thank Dr. L. Ernst for the
measurement of variable-temperature NMR. Financial
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