An efficient preparation, the structure and the properties of 1-isopropyl-4-(3-guaiazulenylmethylium)benzene hexafluorophosphate and tetrafluoroborate

Article abstract of DOI:10.1016/S0040-4039(02)00207-1

Reactions of guaiazulene (1) with 4-isopropylbenzaldehyde in acetic acid in the presence of hexafluorophosphoric acid (and tetrafluoroboronic acid) at 25°C for 2 h under aerobic conditions quantitatively give the new title monocarbocation compounds, 1-isopropyl-4-(3-guaiazulenylmethylium)benzene hexafluorophosphate (4) and the tetrafluoroborate (5), which upon reaction with sodium methoxide dissolved in methanol in acetonitrile at 0°C for 20 min, respectively, afford as high as 92% isolated yield of 1-isopropyl-4-(3-guaiazulenylmethoxymethyl)benzene (8). The crystal structure of 5, as the first example of carbocations stabilized by azulenyl groups, and the characteristic and chemical properties of 4 and 5 are reported.

Full text of DOI:10.1016/S0040-4039(02)00207-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2073–2078
An efficient preparation, the structure and the properties of
1-isopropyl-4-(3-guaiazulenylmethylium)benzene
hexafluorophosphate and tetrafluoroborate
Shin-ichi Takekuma,* Mariko Tanizawa, Masato Sasaki, Takuya Matsumoto and Hideko Takekuma
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka-shi,
Osaka 577-8502, Japan
Received 12 December 2001; accepted 25 January 2002
Abstract—Reactions of guaiazulene (1) with 4-isopropylbenzaldehyde in acetic acid in the presence of hexafluorophosphoric acid
(and tetrafluoroboronic acid) at 25°C for 2 h under aerobic conditions quantitatively give the new title monocarbocation
compounds, 1-isopropyl-4-(3-guaiazulenylmethylium)benzene hexafluorophosphate (4) and the tetrafluoroborate (5), which upon
reaction with sodium methoxide dissolved in methanol in acetonitrile at 0°C for 20 min, respectively, afford as high as 92%
isolated yield of 1-isopropyl-4-(3-guaiazulenylmethoxymethyl)benzene (8). The crystal structure of 5, as the first example of
carbocations stabilized by azulenyl groups, and the characteristic and chemical properties of 4 and 5 are reported. © 2002 Elsevier
Science Ltd. All rights reserved.
Although the chemistry of carbocations stabilized by
azulenyl groups has been extensively studied and the
chemical structures and the characteristic properties of
a number of these compounds have been well docu-
mented, nothing has so far been achieved towards
their accurate X-ray crystallographic analyses. In the
previous papers, we reported an efficient preparation
and the characteristic and chemical properties of two
dicarbocations stabilized by two guaiazulenyl groups,
1,2- and 1,4-bis(3-guaiazulenylmethylium)benzene bis-
hexafluorophosphate (21 and 31,2); however, their accu-
rate X-ray crystallographic analyses have not yet been
achieved because of difficulty in obtaining single crys-
tals suitable for this purpose. In a series of basic
studies on the above chemistry, our interest has been
focused on the X-ray crystallographic analyses of car-
bocations stabilized by azulenyl groups, establishing
the crystal (molecular) structures and their packing
structures, and on the characteristic and chemical
properties of those molecules in which the single crys-
tals are formed. We now wish to report our detailed
studies on a one-pot synthesis, the chemical structure
and the characteristic and chemical properties of the
title new monocarbocation compound, 1-isopropyl-4-
(3-guaiazulenylmethylium)benzene
hexafluorophos-
phate (4) with a view to comparison and discussion
with those of 2 and 3, and further, on the crystal
structure of 1-isopropyl-4-(3-guaiazulenylmethylium)-
benzene tetrafluoroborate (5), which formed a p-
stacking structure in the single crystal, as the first
example for carbocations stabilized by azulenyl
groups.
-
PF6
-
PF6
-
H
+
PF6
4
1
+
+ H
H
H
1
+
-
PF6
2
2
3
Keywords: azulenes; carbonium ions; X-ray crystal structures.
* Corresponding author. Tel.: +00 81 6 6730 5880 (ext. 4020); fax: +00 81 6 6727 4301; e-mail: takekuma@apch.kindai.ac.jp
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00207-1

2074
S. Takekuma et al. / Tetrahedron Letters 43 (2002) 2073–2078
Compound 4 was prepared by the following procedure:
To a solution of guaiazulene (1) (50 mg, 0.25 mmol) in
acetic acid (0.4 mL) was added a solution of 4-iso-
propylbenzaldehyde (80 mL, 0.53 mmol) in acetic acid
(0.3 mL) containing hexafluorophosphoric acid (60%
aqueous solution, 0.18 mL). The mixture was stirred at
25°C for 2 h under aerobic conditions. After the reac-
tion, diethyl ether (3.0 mL) was slowly added and the
mixture was allowed to stand at 25°C for 24 h, afford-
ing very small single crystals of 4. The crystals thus
obtained were carefully washed with diethyl ether and
dried well in a vacuum desiccator to provide pure 4 as
stable crystals (113 mg, 95% yield).
Hz, H-5%), 8.61 (1H, d, J=2.0 Hz, H-8%), 8.79 (1H, brd
s, H-2%) and signals for the 4-1-isopropylphenyl group
at l 1.32 (6H, d, J=7.0 Hz, (CH
6 3)2CH-1), 3.07 (1H,
sept, J=7.0 Hz, Me2CH
6
-1), 7.53 (2H, ddd, J=8.0, 1.5,
1.0 Hz, H-2,6), 7.82 (2H, brd ddd, J=8.0, 1.5, 1.0 Hz,
1
H-3,5). Moreover, the H–1H COSY spectrum showed
cross peaks between the following signals (Me-1% and
H-2%; Me-1% and H-8%; Me-1% and HC+-a; Me-4% and
H-5%; Me-4% and H-2%; H-2% and H-3,5; H-2% and HC+-
a), indicating that each proton possesses a long-range
coupling (J value: <1.0 Hz each) between them. The
125 MHz 13C NMR (CD3CN) spectrum exhibited the
following 21 carbon signals assigned by the 1H–13C
1
COSY and H–13C COLOC techniques: l 171.3 (C-7%),
Compound 4 was reddish-orange needles, mp >118°C
[decomp., determined by thermal analysis (TGA and
DTA)]. The UV–vis [umax (CH3CN) nm (log m)] spec-
trum appeared at 223 (4.51), 250sh (4.36), 295 (4.27),
328 (4.18), 393 (4.30) and 471 (4.56) (Fig. 1). Similarly,
as in the cases of 21 and 3,1,2 no characteristic absorp-
tion bands for guaiazulene were observed, indicating
formation of a charge transfer (CT) complex between
161.2 (C-1%), 157.9 (C-4%), 155.5 (C-3a%), 153.8 (C-8a%),
151.0 (C-2%), 150.5 (C-5%), 145.9 (C-3%), 144.9 (C-6%),
141.7 (HC+-a), 139.9 (C-8%), 139.7 (C-4), 134.4 (C-3,5),
134.3 (C-1), 128.7 (C-2,6), 40.3 (Me2C
(Me2CH-1), 29.8 (Me-4%), 23.9 ((C
((C
6
H-7%), 35.1
6
6 H3)2CH-7%), 23.8
6
H3)2CH-1) and 13.8 (Me-1%). The chemical shifts (l
ppm) for the proton and carbon signals of the HC+-a
carbonium ion of 4 coincided with those of the HC+-1,4
dicarbonium ions (8.03 for 1H NMR; 141.0 for 13C
NMR) of 3.2 These spectroscopic data led to the struc-
ture, 1-isopropyl-4-(3-guaiazulenylmethylium)benzene
hexafluorophosphate, for 4.
the
1-isopropyl-4-(3-guaiazulenylmethylium)benzene
moiety with a delocalized p-electron system and the
−
counter anion (PF6 ). The absorption maximum
appeared at umax 471 nm, which showed a hyp-
sochromic shift (D 40 nm) and hypochromic effect in
comparison with that of 3 (umax 511 nm, log m=4.87)1
(Fig. 1). The IR (KBr) spectrum showed two specific
Although an X-ray crystallographic analysis of com-
pound 4 has not yet been achieved because of difficulty
in obtaining a single crystal suitable for this purpose,
the crystal structure of 1-isopropyl-4-(3-guaiazulenyl-
methylium)benzene tetrafluoroborate (5)3 has been
determined by means of X-ray diffraction, producing
accurate structural parameters.4,5 The ORTEP drawing
and the space-filling structure of 5 are shown in Fig.
2(b)6 together with the selected bond distances, reveal-
ing that the best plane of the 1-isopropylbenzene ring
twists by 29.2° from the best plane of the 3-guaiazu-
lenylmethylium substituent owing to the influence of
steric hindrance between the hydrogen atoms of the C-5
and C-2% positions. The 3-guaiazulenylmethylium sub-
stituent clearly indicates the bond alternation between
the single and double bonds in comparison with the
bond distances of the parent azulene.7 The counter
−
bands based on the counter anion (PF6 ) at wmax 783
and 448 cm−1, which showed larger low wavenumber
field shifts in comparison with those of 2 (wmax 837 and
556 cm−1)1 and 3 (wmax 841 and 559 cm−1).1 The
MALDI-TOF-MS (3,5-dimethoxy-4-hydroxycinnamic
acid matrix) spectrum showed only one ion peak at m/z
329 (100%, [M−PF6]+). The molecular formula C25H29
for the carbocation unit was determined by the exact
FAB-MS (3-nitrobenzyl alcohol matrix) spectrum
(found: m/z 329.2294. Calcd for C25H29: [M−PF6]+, m/z
1
329.2269.). The 500 MHz H NMR (CD3CN) spectrum
showed signals for the 3-guaiazulenylmethylium sub-
stituent at l 1.47 (6H, d, J=7.0 Hz, (CH
6 3)2CH-7%), 2.54
(3H, brd s, Me-1%), 3.38 (3H, s, Me-4%), 3.51 (1H, sept,
J=7.0 Hz, Me2CH6
-7%), 8.07 (1H, brd s, HC+-a), 8.43
−
anion (BF4 ) is located on the Ca carbonium ion (the
(1H, dd, J=11.0, 2.0 Hz, H-6%), 8.53 (1H, d, J=11.0
,
distance between the B-Ca atoms: 5.647 A). The aver-
age CꢀC bond distance for the seven-membered ring of
,
the 3-guaiazulenyl group (1.401 A) is slightly shorter
than the distance observed for that of the parent azu-
7
,
lene (1.412 A). Moreover, the distances for the five-
membered ring of the 3-guaiazulenyl group appreciably
vary between 1.347 and 1.470 A; in particular, the
C1%–C2% distance (1.347 A) is characteristically shorter
than the average CꢀC bond distance for the five-mem-
bered ring (1.436 A), which is slightly longer than the
distance observed for that of the parent azulene (1.427
,
,
,
7
,
,
A). The Ca–C3% distance (1.364 A) is also characteristi-
,
cally shorter than the C4–Ca distance (1.451 A), which
coincides with the bond distance between the
methylium carbon atom and the benzene ring for
8
Figure 1. The UV–vis spectra of compounds 31 and 4 in
CH3CN. Concentrations, 3: 0.020 g/L; 4: 0.020 g/L; length of
the cell, 1 cm each.
triphenylmethylium perchlorate (1.454 A). The average
,
,
CꢀC bond distance for the benzene ring (1.388 A)
coincides with the distance observed for that of

S. Takekuma et al. / Tetrahedron Letters 43 (2002) 2073–2078
2075
8
,
triphenylmethylium perchlorate (1.381 A). Along with
ring, forming a 3-guaiazulenium ion; and further (2)
from a result of the torsion angle between the 3-guai-
azulenylmethylium plane and the 1-isopropylbenzene
ring, formation of a conjugated p-electron system
between them is possible. Thus, the X-ray crystallo-
graphic analysis of 5 enabled us to presume the plausi-
ble crystal (molecular) structures of 4, 2 and 3; namely:
(1) the structural parameters for the 1-isopropyl-4-(3-
guaiazulenylmethylium)benzene unit of 4 are nearly the
same as those of 5; and (2) the two 3-guaiazulenyl-
methylium planes of 3 twist by ca. 30° from the benzene
ring, whereas in the case of 2, the two 3-guaiazulenyl-
methylium planes twist by >30° from the benzene ring
owing to the influence of steric hindrance between
them, the repulsion between the two 3-guaiazulenyl-
methylium ions and so on. In addition to the X-ray
the crystal structure of 5, the two different (top and
side) views for the packing (molecular) structure of 5
revealed that this molecule formed a p-stacking struc-
ture in the single crystal and that the inter-plane dis-
tances between the overlapping molecules (i.e.
,
3-guaiazulenylmethylium planes) were 3.56–3.64 A
(Fig. 2c, d). From a comparative study with the bond
distances of the parent azulene,7 the azulenium ions (i.e.
the average CꢀC bond distance for the seven-membered
9
,
rings of the azulenium ions: 1.38 A) and triphenyl-
methylium perchlorate,8 it can be inferred that: (1)
although the positive charge of 5 in the single crystal is
mainly localized at the Ca carbon atom, forming a
3-guaiazulenylmethylium ion, the positive charge
apparently is slightly transferred to the seven-membered
Figure 2. (a) Structures of 1-isopropyl-4-(3-guaiazulenylmethylium)benzene hexafluorophosphate and tetrafluoroborate (4 and 5).
(b) The ORTEP drawing with the numbering scheme (30% probability thermal ellipsoids) and the space-filling structure of 5. The
,
selected bond distances (A) of 5 are as follows: C1–C2; 1.381(9), C2–C3; 1.376(8), C3–C4; 1.408(8), C4–C5; 1.393(8), C5–C6;
1.376(9), C6–C1; 1.388(9), C4–Ca; 1.451(8), Ca–C3%; 1.364(8), C1%–C2%; 1.347(8), C2%–C3%; 1.456(8), C3%–C3a%; 1.470(8), C3a%–C4%;
1.391(8), C4%–C5%; 1.411(8), C5%–C6%; 1.383(8), C6%–C7%; 1.400(7), C7%–C8%; 1.380(8), C8%–C8a%; 1.373(8), C8a%–C1%; 1.440(8) and
C8a%–C3a%; 1.466(8). (c and d) The two different (top and side) views for the packing (molecular) structure of 5; hydrogen atoms
are omitted for reasons of clarity.

2076
S. Takekuma et al. / Tetrahedron Letters 43 (2002) 2073–2078
crystallographic analysis of 5, the accurate parameters
for the crystal structure of 5 were transferred to a
WinMOPAC program10 and then the atomic-charges of
5 were calculated. As a result, it was suggested that a
carbocation was mainly localized at the Ca carbon
atom (the calculation value: +0.243).
electron oxidation at the potentials of +0.57sh (Epa) and
+0.65 (Epa) by CV. The two oxidation and two reduc-
tion potentials [+0.65 (Epa) and +0.57sh (Epa) V; −1.85
(E1/2) and −1.99 (Epc) V] observed by CV are based on
the redox potentials of the two 3-guaiazulenyl groups of
7, which are supported by the corresponding redox
potentials of 1,12 2,1 31,13 and 6.14
We have been interested further in a comparative study
of the electrochemical properties of the previously
obtained dicarbonium hexafluorophosphates 2,1 31 and
the title monocarbonium hexafluorophosphate 4. The
electrochemical behavior of 4 was, therefore, measured
by means of CV and DPV (potential/V versus SCE) in
0.1 M [n-Bu4N]PF6, CH3CN (Fig. 3).11 Three reduction
potentials observed by DPV were positioned at the Ep
values of −0.22, −1.80 and −1.94sh V, while five redox
potentials determined by CV were located at the values
In addition to the above investigations, we studied the
chemical properties of 4 and 5 in which the single
crystals were formed, and have found that the reactions
of 4 and 5 with sodium methoxide dissolved in
methanol in acetonitrile at 0°C for 20 min, respectively,
afforded as high as 92% isolated yield of 1-isopropyl-4-
(3-guaiazulenylmethoxymethyl)benzene (8),15 indicating
that the nucleophilic substitution reactions readily
occur at the Ca carbon atoms of 4 and 5 (Scheme 1).
Thus, similarly as the above computation for the
atomic-charges of the crystal structure 5, the present
work also bears out that the carbonium ions of 4 and 5
in acetonitrile are mainly localized at their Ca carbon
atoms. Moreover, when dissolved in a mixed solvent of
acetonitrile and water and allowed to stand at room
temperature for 2 weeks under aerobic conditions, 8
was gradually converted into 1-isopropyl-4-[bis(3-guai-
azulenyl)methyl]benzene (9) and the starting 4-iso-
propylbenzaldehyde, quantitatively (Scheme 1).4,16
Application of the nucleophilic substitution reactions
by using types of the title carbocation compounds as
the starting materials is currently under intensive
investigation.
of +0.65 (Epa), +0.57sh (Epa), −0.29 (Epc), −1.85 (E1/2
)
and −1.99 (Epc) V. From a comparative study with the
redox potentials of 1,12 31,13 (Fig. 3) and 1-isopropyl-4-
(3-guaiazulenylmethyl)benzene (6)14 obtained by the
reduction of 4 with NaBH4, it can be inferred that: (1)
4 undergoes one-electron reduction at a potential of
−0.29 (Epc) V by CV (corresponding to −0.22 V by
DPV), generating the corresponding 3-(4-1-isopropyl-
phenyl)guaiazulenylmethyl radical species, which is
rapidly converted into the radical homo-coupling
product 7; (2) the dimer 7 yielded is stepwise reduced to
the di-anion at the potentials of −1.85 (E1/2) and −1.99
(Epc) V by CV (corresponding to −1.80 and −1.94sh V
by DPV); and further; (3) 7 stepwise undergoes two-
Figure 3. Cyclic (a) and differential pulse (b) voltammograms of compounds 313 (6.0 mg, 7.6 mmol) and 4 (6.0 mg, 12.6 mmol) in
0.1 M [n-Bu4N]PF6, CH3CN (10 mL) at a glassy carbon (ID: 3 mm) and platinum wire served as the working and auxiliary
electrodes; scan rates 100 mV s−1 at 25°C under argon, respectively.
Scheme 1.

S. Takekuma et al. / Tetrahedron Letters 43 (2002) 2073–2078
2077
Acknowledgements
affinity) and the dipole moment were calculated to be
−9.693 and −3.555 eV (i.e. the gap between them: 6.138
eV) and 24.94649 D, respectively.
This work was partially supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education,
Science, Sports and Culture, Japan.
11. For comparative purposes, the oxidation potential using
ferrocene as a standard material showed +0.42 (Ep) V by
DPV and +0.40 (E1/2) V by CV under the same electro-
chemical conditions as 4.
12. 1: DPV (Ep), +0.59 and −1.77 V; CV, +0.69 (Epa) and
−1.83 (E1/2) V.
References
13. 3: DPV (Ep), +0.04, −0.25, −1.63 and −1.87 V; CV, +0.64
(Epa), −0.02 (Epc), −0.29 (Epc), −1.67 (Epc), −1.89 (Epc),
−2.07sh (Epc) and −1.84 (Epa) V.
1. Takekuma, S.; Takata, S.; Sasaki, M.; Takekuma, H.
Tetrahedron Lett. 2001, 42, 5921–5924.
2. Takekuma, S.; Sasaki, M.; Takekuma, H.; Yamamoto,
14. Compound 6 was prepared by the following procedure:
To a solution of NaBH4 (5 mg, 132 mmol) in ethanol (1
mL) was added a solution of 4 (30 mg, 63 mmol) in
acetonitrile (2 mL). The mixture was stirred at 25°C for
20 min under aerobic conditions and then evaporated in
vacuo. The residue thus obtained was dissolved in hexane
and filtered. The hexane filtrate was evaporated in vacuo,
giving pure 6 as a blue paste (20 mg, 96% yield). Com-
pound 6: Rf=0.51 on silica gel TLC (hexane/AcOEt=96/
4, vol/vol); UV–vis umax (hexane) nm (log m), 213 (4.30),
247 (4.36), 291 (4.66), 306 (4.21), 353 (3.80), 371 (3.76),
623 (2.62), 678sh (2.52) and 754sh (2.05); exact EI-MS,
found: m/z 330.2359; calcd for C25H30: M+, m/z
H. Chem. Lett. 1999, 999–1000.
3. The same procedure as the preparation method of com-
pound 4 was employed: reaction of guaiazulene (1) (50
mg, 0.25 mmol) with 4-isopropylbenzaldehyde (80 mL,
0.53 mmol) in acetic acid (0.7 mL) containing tetra-
fluoroboronic acid (42% aqueous solution, 0.18 mL) at
25°C for 2 h under aerobic conditions afforded 5 as
stable single crystals (60 mg, 58% isolated yield). Com-
pound 5: Reddish-orange needles, mp 160.8°C [deter-
mined by thermal analysis (TGA and DTA)]; UV–vis
umax (CH3CN) nm (log m), 226 (4.41), 250sh (4.25), 295
(4.16), 326 (4.08), 392 (4.21) and 472 (4.47); IR wmax
(KBr) cm−1, 1080, 1049 and 521 (BF4−); found: H, 6.89;
C, 72.09%; calcd for C25H29BF4: H, 7.02; C, 72.13%. The
1H and 13C NMR spectral data of 5 coincided with those
of 4.
1
330.2348; H NMR (CD3CN), l 1.14 (6H, d, J=7.0 Hz,
(CH
(3H, s, Me-1%), 2.77 (3H, s, Me-4%), 2.80 (1H, sept, J=7.0
Hz, Me2CH-1), 3.00 (1H, sept, J=7.0 Hz, Me2CH-7%),
6 3)2CH-1), 1.28 (6H, d, J=7.0 Hz, (CH6 3)2CH-7%), 2.54
6
6
4. Details will be reported elsewhere.
4.50 (2H, s, CH2-4), 6.78 (1H, d, J=11.0 Hz, H-5%), 6.88
(2H, brd ddd, J=8.0, 1.5, 1.0 Hz, H-2,6), 7.07 (2H, ddd,
J=8.0, 1.5, 1.0 Hz, H-3,5), 7.27 (1H, dd, J=11.0, 2.0 Hz,
H-6%), 7.36 (1H, brd s, H-2%) and 8.07 (1H, d, J=2.0 Hz,
H-8%); DPV (Ep), +0.63 and −1.82 V; CV, +0.63 (Epa) and
−1.84 (E1/2) V.
5. Crystallographic data for compound 5: C25H29BF4
(FW=416.31), reddish-orange needle (the crystal size,
0.20×0.25×0.08 mm), monoclinic, C2/c (c15), a=
,
33.348(7), b=9.856(4), c=14.272(3) A, i=98.61(3)°, V=
3
4638(2) A , Z=8, Dcalcd=1.192 g/cm3, v(Mo-Ka)=0.89
,
cm−1, measured reflections=3610, observed reflections=
3376, R1=0.066, wR2=0.092. A total 3610 reflections
with 2qmax=50.0° were collected on a Rigaku AFC-5R
automated four-circle diffractometer with graphite
15. (a) Compound 8 was prepared by the following proce-
dure: To a solution of sodium methoxide (5 mg, 93 mmol)
in methanol (1 mL) was added a solution of 4 (30 mg, 63
mmol) in acetonitrile (2 mL). The mixture was stirred at
0°C for 20 min under aerobic conditions and then evapo-
rated in vacuo. The residue thus obtained was dissolved
in hexane and filtered to remove the yielded NaPF6. The
hexane filtrate was evaporated in vacuo, giving pure 8 as
a blue paste (21 mg, 92% yield). Compound 8: Rf=0.28
on silica gel TLC (hexane/AcOEt=96/4, vol/vol); UV–vis
umax (hexane) nm (log m), 218 (4.27), 247 (4.38), 285sh
(4.59), 290sh (4.64), 293 (4.65), 306sh (4.25), 340sh (3.59),
354 (3.77), 371 (3.76), 610 (2.59), 665sh (2.49) and 738sh
(1.99); exact FAB-MS (3-nitrobenzyl alcohol matrix),
found: m/z 360.2422; calcd for C26H32O: M+, m/z
,
monochromated Mo-Ka radiation (u=0.71069 A, rotat-
ing anode: 50 kV, 180 mA) at 296 K. The structure was
solved by direct methods (SIR97) and expanded using
Fourier techniques (DIRDIF94). The non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
included but not refined. The final cycle of full-matrix
least-squares refinement was based on F2. All calculations
were performed using the teXsan crystallographic soft-
ware package.
6. The ORTEP drawing and the space-filling structure of 5
in Fig. 2(b) were drawn using the following softwares,
Ortep-3 for Windows Ver. 1.06 and POV-Ray for Win-
dows Ver. 3.1g.
7. Zeller, K.-P. Methoden der organishen Chemie, 4th ed.;
Georg Thieme Verlag: Stuttgart, 1985; Vol. V/2c, p. 127.
8. Gomes de Mesquita, A. H.; MacGillavry, C. H.; Eriks,
K. Acta Crystallogr. 1965, 18, 437–443.
9. Oda, M.; Fukuta, A.; Kajioka, T.; Uchiyama, T.;
Kainuma, H.; Miyatake, R.; Kuroda, S. Tetrahedron
2000, 56, 9917–9925.
10. This computer program (Ver. 3.0) was developed by
Fujitsu Ltd., Japan. A keyword (1SCF) was used. More-
over, the p-HOMO (corresponding to the ionization
potential), p-LUMO (corresponding to the electron
1
360.2453; H NMR (CD3CN), l 1.21 (6H, d, J=7.0 Hz,
(CH
(3H, s, Me-1%), 2.89 (1H, sept, J=7.0 Hz, Me2CH
(3H, s, Me-4%), 3.07 (1H, sept, J=7.0 Hz, Me2CH
6
3)2CH-1), 1.33 (6H, d, J=7.0 Hz, (CH
6
3)2CH-7%), 2.56
-1), 2.95
-7%),
6
6
3.33 (3H, s, -OCH3), 6.22 (1H, s, CH-4), 6.97 (1H, d,
J=11.0 Hz, H-5%), 7.20 (4H, s, H-2,3,5,6), 7.38 (1H, dd,
J=11.0, 2.0 Hz, H-6%), 7.49 (1H, brd s, H-2%) and 8.16
(1H, d, J=2.0 Hz, H-8%); 13C NMR (CD3CN), l 148.7
(C-3%), 146.0 (C-4%), 142.0 (C-8a%), 141.3 (C-3a%), 139.5
(C-7%), 139.3 (C-2%), 135.7 (C-6%), 134.6 (C-8%), 134.1 (C-4),
128.9 (C-1), 128.6 (C-3,5), 128.0 (C-5%), 127.1 (C-2,6),
125.4 (C-1%), 81.4 (CH-4), 57.0 (-OCH3), 38.3 (Me2C
6 H-
7%), 34.5 (Me2CH-1), 27.8 (Me-4%), 24.7 ((CH3)2CH-7%),
6
6

2078
S. Takekuma et al. / Tetrahedron Letters 43 (2002) 2073–2078
6 H3)2CH-1) and 13.1 (Me-1%); (b) similarly, the
EI-MS, found: m/z 526.3620; calcd for C40H46: M+, m/z
24.3 ((C
1
reaction of compound 5 with sodium methoxide provided
the same results as 4.
526.3600; H NMR (CD3CN), l 1.24 (6H, d, J=7.0 Hz,
(CH3)2CH-1), 1.34 (12H, d, J=7.0 Hz, (CH3)2CH-7%,7¦),
2.51 (6H, s, Me-1%,1¦), 2.88 (1H, sept, J=7.0 Hz, Me2CH
1), 2.89 (6H, s, Me-4%,4¦), 3.02 (2H, sept, J=7.0 Hz,
Me2CH-7%,7¦), 6.76 (2H, d, J=11.0 Hz, H-5%,5¦), 6.85
6
6
16. Compound 8 (30 mg, 83 mmol) was dissolved in a mixed
solvent (6 mL) of acetonitrile and water (70:30, vol/vol)
and allowed to stand at room temperature for 2 weeks
under aerobic conditions, providing only crystals of 1-iso-
propyl-4-[bis(3-guaiazulenyl)methyl]benzene (9) (18 mg,
34 mmol, 41% yield) in the solution. Another product of
4-isopropylbenzaldehyde was identified by TLC and
HPLC analyses of the solution compared with those of
the authentic sample. Compound 9: Dark-blue prisms,
mp 201°C [determined by thermal analysis (TGA and
DTA)]; Rf=0.44 on silica gel TLC (hexane/AcOEt=96/4,
vol/vol); UV–vis umax (hexane) nm (log m), 219 (4.48), 248
(4.61), 284sh (4.81), 291 (4.81), 310 (4.62), 344sh (3.98),
358 (4.09), 375 (4.13), 580sh (2.82), 628 (2.94), 664sh
(2.87), 686sh (2.84), 736sh (2.47) and 768sh (2.34); exact
6
-
6
(2H, ddd, J=8.0, 1.5, 1.0 Hz, H-2,6), 7.02 (2H, s, H-
2%,2¦), 7.09 (2H, ddd, J=8.0, 1.5, 1.0 Hz, H-3,5), 7.24
(2H, dd, J=11.0, 2.0 Hz, H-6%,6¦), 7.28 (1H, s, CH-4) and
8.08 (2H, d, J=2.0 Hz, H-8%,8¦); 13C NMR (CD3CN), l
146.43, 146.39, 145.4, 141.6 (C-2%,2¦), 139.4, 138.1, 135.1
(C-6%,6¦), 134.0 (C-8%,8¦), 132.7, 132.2, 129.7 (C-2,6),
126.8 (C-5%,5¦), 126.5 (C-3,5), 124.4, 47.5 (CH-4), 38.0
(Me2C
6
H-7%,7¦), 34.0 (Me2C
6 H-1), 27.3 (Me-4%,4¦), 24.7
((CH3)2CH-7%,7¦), 24.2 ((C
6
6
H3)2CH-1) and 13.1 (Me-1%,1¦);
found: H, 8.64; C, 91.14%; calcd for C40H46: H, 8.80; C,
91.20%.