Gas-phase generation of highly reactive hexatriyne-bridged [6 n]paracyclophynes

Article abstract of DOI:10.1021/jo050833d

[6_n]Paracyclophenediynes 2a-d (n = 3-6) having [4.3.2]-propellatriene units were prepared as precursors of the corresponding [6_n]paracyclophynes. Laser irradiation of 2a-d produced the negative ions of [6_n]paracyclophynes

Full text of DOI:10.1021/jo050833d

homologue, a butadiyne-bridged [46]paracyclophyne de-
rivative, was synthesized by Ohkita and Tsuji by using
photochemical valence isomerization of the corresponding
Dewar benzene valence isomer.3 Although the individual
triple bond is not much deformed, the compound is air-
sensitive and decomposes gradually within several days.
Gas-Phase Generation of Highly Reactive
Hexatriyne-Bridged [6n]Paracyclophynes
Rui Umeda, Takayoshi Morinaka, Motohiro Sonoda, and
Yoshito Tobe*
Division of Frontier Materials Science, Graduate School of
Engineering Science, Osaka University, and CREST (JST),
1-3 Machikaneyama, Toyonaka 560-8531, Japan
tobe@chem.es.osaka-u.ac.jp
Received April 25, 2005
In view of the known reactivity of polyynes,4 it is
expected that the elongation of the sp-carbon chains of
the cyclophynes would reduce their kinetic stabilities.5
Indeed, attempts to prepare octatetrayne-bridged [82]para-
cyclophyne by oxidative decomplexation of the corre-
sponding cobalt complex precursor was not successful.6
Similarly, the longer homologue, [122]paracyclophyne
(C36H8), also eluded characterization, while its anion was
detected in the laser-desorption time-of-flight (LD-TOF)
mass spectrometry of the corresponding precursor having
[4.3.2]propellatriene units.7 It should be pointed out,
however, that the corresponding chloro derivative (C36Cl8)
lost all chlorine atoms to form a C36 carbon cluster,
although its structure has not been clarified yet. This
suggests that highly reactive [mn]paracyclophynes with
long sp-carbon chains may serve as precursors of small
carbon clusters with a specific carbon number. From the
above points of view, we became interested in hexatriyne-
bridged [6n]paracyclophynes (1a-d; n ) 3-6).
[6n]Paracyclophenediynes 2a-d (n ) 3-6) having [4.3.2]-
propellatriene units were prepared as precursors of the
corresponding [6n]paracyclophynes. Laser irradiation of
2a-d produced the negative ions of [6n]paracyclophynes
1a-d (n ) 3-6) by extrusion of the indan fragments, which
were detected by time-of-flight mass spectrometry.
The [2n]- and [4n]paracyclophynes possessing phenyl-
ene ethynylenes or phenylene butadiynylenes as repeat-
ing units adopt rigid and belt-shaped structures with
well-defined cavities.1 Because of the deformation of the
benzene rings and triple bonds, the electronic properties
inside/outside the cavities are different. In this respect,
considerable interest has been shown in the unique
complexation behavior of these molecules. The first series
of this class of compounds, [2n]paracyclophynes (n )
6-9), were prepared by Kawase and Oda.2 They demon-
strated that [26]paracyclophyne having a cavity with a
diameter of 1.31 nm formed 1:1 inclusion complexes with
fullerenes (C60, C70, and a methano[60]fullerene deriva-
tives) in solution as well as in the solid state.2c,d Moreover,
the formation of double-inclusion complexes composed of
two paracyclophyne molecules with different ring size
and a C60 molecule was reported.2e The next higher
(3) Ohkita, M.; Ando, K.; Tsuji, T. Chem. Commun. 2001, 2570-
2571.
(4) For reviews, see: (a) Huntsman, W. D. In The Chemistry of the
Carbon-Carbon Triple Bond; Patai, S., Ed.; John Wiley & Sons:
Chichester, 1978; Part 2, pp 553-620. (b) Tobe, Y.; Wakabayashi, T.
In Acetylene Chemistry: Chemistry, Biology and Material Science;
Diederich, F., Stang, P. J., Tykwinski, R. R., Eds.; Wiley-VCH:
Weinheim, 2005; pp 387-426.
(5) (a) Tobe, Y.; Ohki, I.; Sonoda, M.; Niino, H.; Sato, T.; Waka-
bayashi, T. J. Am. Chem. Soc. 2003, 125, 5614-5615. (b) Hisaki, I.;
Eda, T.; Sonoda, M.; Tobe, Y. Chem. Lett. 2004, 33, 620-621. (c) Hisaki,
I.; Eda, T.; Sonoda, M.; Niino, H.; Sato, T.; Wakabayashi, T.; Tobe, Y.
J. Org. Chem. 2005, 70, 1853-1864.
(1) For recent reviews, see: (a) Tobe, Y.; Sonoda, M. In Modern
Cyclophane Chemistry; Gleiter, R., Hopf, H., Eds.; Wiley-VCH: Wein-
heim, 2004; pp 1-40. (b) Jones, C. S.; O’Connor, M. J.; Haley, M. M.
In Acetylene Chemistry: Chemistry, Biology and Material Science;
Diederich, F., Stang, P. J., Tykwinski, R. R., Eds.; Wiley-VCH:
Weinheim, 2005; pp 303-385.
(2) (a) Kawase, T.; Darabi, H. R.; Oda, M. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2664-2667. (b) Kawase, T.; Ueda, N.; Tanaka, K.;
Seirai, Y.; Oda, M. Tetrahedron Lett. 2001, 42, 5509-5511. (c) Kawase,
T.; Tanaka, K.; Fujiwara, N.; Darabi, H. R.; Oda, M. Angew. Chem.,
Int. Ed. 2003, 42, 1624-1628. (d) Kawase, T.; Tanaka, K.; Seirai, Y.;
Shiono, N.; Oda, M. Angew. Chem., Int. Ed. 2003, 42, 5597-5600. (e)
Kawase, T.; Tanaka, K.; Shiono, N.; Seirai, Y.; Oda, M. Angew. Chem.,
Int. Ed. 2004, 43, 1722-1724.
(6) Haley, M. M.; Langsdorf, B. L. Chem. Commun. 1997, 1121-
1122.
(7) Tobe, Y.; Furukawa, R.; Sonoda, M.; Wakabayashi, T. Angew.
Chem., Int. Ed. 2001, 40, 4072-4074.
10.1021/jo050833d CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/29/2005
J. Org. Chem. 2005, 70, 6133-6136
6133

Because of the expected deformations,8 these com-
pounds may not be accessible by the conventional cou-
pling reactions to form a triyne unit.9 We designed,
therefore, [6n]paracyclophynes having [4.3.2]propella-
triene units 2a-d (n ) 3-6) as precursors of 1a-d on
the basis of our previous work on the generation of
reactive polyynes based on [2+2] cycloreversion.5,10 In this
paper, we report the synthesis of precursors 2a-d,
photolysis of 2a in solution, and the generation of the
anions of 1a-d from 2a-d by laser irradiation.
SCHEME 1a
a Reagents and conditions: (a) K2CO3, rt; (b) p-N3Et2C6H4I,
Pd(PPh3)4, CuI, Et3N, rt, 85%; (c) CH3I, 100 °C, 94%; (d) Bun4NF,
THF, rt; (e) Pd(PPh3)4, CuI, Et3N, rt.
SCHEME 2a
a Reagents and conditions: (a) Bun4NF, AcOH, THF, rt; (b) 3b,
Pd(PPh3)4, CuI, Et3N, reflux, 87% for 5a, 88% for 6a; (c) CH3I,
100 °C, 90% for 5b, 96% for 6b, 96% for 8b; (d) Pd(PPh3)4, CuI,
Et3N, 50 to 70 °C, 80% for 2a, 9% for 2c; (e) 5b, Pd(PPh3)4, CuI,
Et3N, 50 °C, 89%; (f) 3b, Pd(PPh3)4, CuI, Et3N, 50 °C, 77%.
For the preparation of cyclophenediynes 2a-d, we
designed 3a as a key building block having both masked
iodobenzene and protected terminal acetylene moieties.
This compound 3a was prepared from the known di-
ethynylpropellane 410 by desilylation and subsequent
palladium-catalyzed coupling with 1-(4-iodophenyl)-3,3-
diethyltriazene (Scheme 1). For the preparation of 2a-
d, we first examined the one-shot cross coupling reaction
of 3b. Triazenyl to iodo transformation11 of 3a gave 3b.
After removal of the protecting group of 3b with TBAF,
palladium-catalyzed coupling under high dilute condi-
tions gave cyclic dehydrotrimer 2a12 as a major product
(6%) together with cyclic dehydrotetramer 2b12 (trace),
cyclic dehydropentamer 2c,12 and cyclic dehydrohexamer
2d12 (2%; combined yield of 2c and 2d) (Scheme 1).
Because of the low yield of the products and the
difficulty in their separation, we turned to stepwise, size-
selective synthesis of 2a-d. The synthesis of cyclic
dehydrotrimer 2a is shown in Scheme 2. After desilyl-
ation of 3a, palladium-catalyzed coupling with 3b gave
dehydrodimer 5a.12 Linear dehydrotrimer 6a12 was pre-
pared by desilylation of 5a followed by palladium-
catalyzed coupling with 3b. The triazenyl to iodo trans-
formation of 6a yielded 6b.12 Finally, desilylation of 6b
and subsequent palladium-catalyzed coupling under high
dilute conditions afforded cyclic dehydrotrimer 2a in good
yield (80%). Similarly, cyclic dehydropentamer 2c was
also prepared selectively in a stepwise manner as shown
SCHEME 3a
a Reagents and conditions: (a) Bun4NF, THF, rt; (b) Pd(PPh3)4,
CuI, Et3N, rt, 54% for 2b, 7% for 2d.
in Scheme 2. Removal of the protecting group of 5a
followed by coupling with 5b,12 prepared from 5a by
treatment with CH3I, gave linear dehydrotetramer 7.12
Linear dehydropentamer 8a12 was obtained by desilyl-
ation of 7 followed by coupling with 3b. Finally, triazenyl
to iodo transformation to give 8b,12 desilylation, and
subsequent palladium-catalyzed coupling under dilute
conditions afforded cyclic dehydropentamer 2c albeit in
low yield (9%).
To obtain even-numbered cyclic dehydrooligomers such
as 2b and 2d, palladium-catalyzed coupling of the de-
hydrodimer unit was examined (Scheme 3). Desilylation
of 5b with TBAF followed by palladium-catalyzed cou-
pling under dilute conditions yielded cyclic dehydro-
tetramer 2b and cyclic dehydrohexamer 2d in 54% and
7% yields, respectively.
The 1H NMR chemical shift of the aromatic protons in
cyclic dehydrotrimer 2a (δ 7.35 ppm) exhibits slight
upfield shifts as compared to those of larger cyclic
dehydrooligomers 2b-d (δ 7.43-7.45 ppm) because of the
anisotropic shielding effect of the neighboring aromatic
rings, which are located closer than in 2b-d. Except for
this, the spectral properties of 2a-d are very similar to
each other.
(8) AM1 calculations predict that the triple bonds of 1a-d are
moderately deformed from linearity; the calculated bond angles of the
triyne unit, C(sp2)-C(1)tC(2), C(1)tC(2)-C(3), and C(2)-C(3)tC(4),
are as follows: 1a, 166.1°, 167.1°, and 167.0°; 1b, 169.6°, 170.3°, and
170.2°; 1c, 171.6°, 172.3°, and 172.2°; 1d, 173.0°, 173.5°, and 173.5°.
(9) For example, see: (a) Bell, M. L.; Chiechi, R. C.; Johnson, C. A.;
Kimball, D. B.; Matzger, A. J.; Wan, W. B.; Weakley, T. J. R.; Haley,
M. M. Tetrahedron 2001, 57, 3507-3520. (b) Ye, F.; Orita, A.; Yaruva,
J.; Hamada, T.; Otera, J. Chem. Lett. 2004, 33, 528-529.
(10) Tobe, Y.; Fujii, T.; Matsumoto, H.; Tsumuraya, K.; Noguchi,
D.; Nakagawa, N.; Sonoda, M.; Naemura, K.; Achiba, Y.; Wakabayashi,
T. J. Am. Chem. Soc. 2000, 122, 1762-1775.
(11) Moore, J. S.; Weinstein, E. J.; Wu, Z. Tetrahedron Lett. 1991,
32, 2465-2466.
(12) Compounds 2a-d, 5a,b, 6a,b, 7, 8a,b, and 11a,b are mixtures
of diastereomers. The structures of only one of them are drawn for
clarity.
To generate [63]paracyclophyne 1a in solution, a de-
gassed solution of 2a in THF-d8 was irradiated with a
1
low-pressure mercury lamp. However, whereas the H
NMR spectrum exhibited signals due to indan, we were
not able to detect signals due to 1a or intermediates such
as 9 leading to 1a, indicating that these are too reactive
in solution. Photolysis was then carried out in furan to
intercept reactive intermediates under similar conditions,
yielding a 1:3 mixture of two different furan adducts
6134 J. Org. Chem., Vol. 70, No. 15, 2005

11a12 and 11b12 (total yield of 26%) together with
recovered 2a (27%) (Scheme 4). Although 11a and 11b
could not be separated, their structures were elucidated
on the basis of the NMR spectra of the mixture. In
particular, the 13C NMR spectrum exhibited an sp carbon
signal (C5) at 80 ppm, characteristic of the central carbon
of a deformed butadiyne unit3 and the quaternary sp2
carbon signal (C1) at considerable low field (164 ppm),
which we assigned to the one in the oxanorbornadiene
framework adjacent to the benzene ring.13 These signals
are consistent with the unsymmetrical [4+2] adduct 11b.
SCHEME 4
bonds of the hexatriyne moiety of 9 (166.1° vs 166.5° (av)
by AM1 calculations).
In view of the low efficiency of the photolysis of 2a in
solution, we examined the generation of ions of 1a-d (n
) 3-6) by laser irradiation in the gas phase.14 As shown
in Figure 1a-d, the negative mode LD-TOF mass spectra
of 2a-d exhibited the peaks due to the paracyclophynes
1a-d formed by the loss of the aromatic indan frag-
ments.15 Although peaks of the parent anions 2a-d were
not observed, those due to the anions designated as
1a(indan)-, 1b(indan)-, 1c(indan)-, and 1d(indan)-,
in which one of the propellane units is still intact, were
also observed together with those due to the loss of C2H4
unit from their propellatriene moieties. However, peaks
due to carbon cluster anions such as C36-, C48-, C60-, and
C72-, that would be derived from cyclophynes 1a-d by
the loss of hydrogen atoms, were not detected, in contrast
This means that the intermediate 9 reacted at the both
triple bonds of the hexatriyne moiety. In contrast, we
observed that related decapentayne-bridged compound
10 reacted with furan regioselectively at the central triple
bond of the decapentayne moiety.10 The difference be-
tween the regioselectivity can be explained in terms of
strain of the triple bonds. The calculated bond angle of
the central triple bond of the decapentayne moiety of 10
is most severely deformed (159.5° vs 162.0° (av) and
168.2° (av) for the other two triple bonds by AM1
calculations), in contrast to the similarly deformed triple
to the three-dimensional cyclophyne anion C60H6-, which
- 16
lost all hydrogen atoms to form C60
.
In conclusion, we synthesized [6n]paracyclophene-
diynes 2a-d as precursors of [6n]paracyclophynes 1a-d
(n ) 3-6), respectively. While we were unable to spec-
FIGURE 1. Laser-desorption time-of-flight mass spectra of cyclic dehydrooligomers: (a) 2a [C36H12(indan)3], (b) 2b [C48H16-
(indan)4], (c) 2c [C60H20(indan)5], and (d) 2d [C72H24(indan)6]. Inset: Expansion of the peaks of the paracyclophyne anions 1a-d.
J. Org. Chem, Vol. 70, No. 15, 2005 6135

into the reaction mixture. The mixture was extracted with Et2O,
and the extract was washed with brine, and dried over anhy-
drous MgSO4. The solvent was removed by evaporation to give
deprotected product, which was used in the next step without
purification. The above deprotected product was dissolved in
Et3N (30 mL), which had been degassed thoroughly by bubbling
argon for 1 h, and the solution was added dropwise into a
solution of Pd(PPh3)4 (150 mg, 130 µmol) and CuI (25.4 mg, 133
µmol) in degassed Et3N (30 mL) over 90 min at room tempera-
ture under an argon atmosphere. After being stirred for 2 h,
the reaction mixture was concentrated in vacuo. The residue was
subjected to chromatography on silica gel (4:1:1, hexanes:CHCl3:
AcOEt). The products were separated by preparative HPLC to
give fractions containing dehydrotetramer 2b and dehydro-
hexamer 2d, respectively. The fraction of 2b was washed with
hexanes and CHCl3 to afford 2b (40 mg, 54%) as a bright yellow
solid. The fraction of 2d was further purified by chromatography
on silica gel (95:5, hexanes:AcOEt) to give 2d (5.3 mg, 7%) as a
bright yellow solid. 2b: dec > 205 °C; 1H NMR (300 MHz, CDCl3)
δ 7.45 (s, 16H), 5.91 (brs, 16H), 2.09-2.03 (m, 8H), 1.70-1.49
(m, 8H), 1.36-1.27 (m, 8H); 13C NMR (75 MHz, CDCl3) δ 132.8
(s), 131.7 (d), 128.5 (d), 122.9 (s), 121.8 (d), 95.1 (s), 84.3 (s), 56.3
(s), 33.2 (t), 19.0 (t); IR (KBr) 2175, 835, 756, 698 cm-1; MS
(LDMS), see Figure 1b. 2d: mp 138-140 °C; 1H NMR (300 MHz,
CDCl3) δ 7.43 (s, 24H), 5.91 (brs, 24H), 2.09-2.02 (m, 12H),
1.70-1.49 (m, 12H), 1.36-1.26 (m, 12H); 13C NMR (75 MHz,
CDCl3) δ 132.4 (s), 131.7 (d), 128.6 (d), 122.9 (s), 121.8 (d), 94.9
(s), 84.1 (s), 56.4 (s), 33.1 (t), 19.0 (t); IR (KBr) 2179, 835, 756,
705 cm-1; MS (LDMS), see Figure 1d.
Photolysis of 2a in Furan. A solution of cyclic dehydro-
trimer 2a (30.3 mg, 37.9 µmol) in furan (30 mL) was charged to
a quartz tube, and then the reaction mixture was degassed by
freeze-thaw cycles. The solution was irradiated in a water bath
with a 60 W low-pressure mercury lamp for 50 h. Similarly,
irradiation of a solution of 2a (18.0 mg, 22.5 µmol) in furan (30
mL) was conducted as described above for 50 h. Both reaction
mixtures were combined, the solvent and indan were removed
by evaporation, and the products were subjected to chromatog-
raphy on silica gel (9:1, hexanes:AcOEt). Purification by pre-
parative HPLC afforded a 1:3 mixture of two different furan
adducts 11a and 11b (12.6 mg, 26%) as an orange solid together
with recovered 2a (13.0 mg, 27%). 11a and 11b: dec > 126 °C,
1H NMR (270 MHz, CDCl3) δ 7.64-7.58 (m, 2H for 11b), 7.45-
7.26 (m, 14H), 7.21-7.18 (m, 2.6H), 5.95 (brs, 11.7H), 5.61 (brs,
0.6H for 11a), 5.57 (brs, 1H for 11b), 2.36-2.05 (m, 5.3H), 1.66-
1.50 (m, 5.3H), 1.37-1.25 (m, 5.3H); 13C NMR (67.8 MHz, CDCl3)
δ 163.6 (s), 143.8 (t), 143.4 (s), 143.1 (t), 141.4 (t), 136.6 (s), 135.4
(s), 134.8 (s), 133.11 (s), 133.08 (s), 132.8 (s), 132.2 (d), 132.1
(d), 131.8 (d), 131.7 (d), 131.6 (d), 131.5 (d), 131.3 (d), 131.2 (d),
129.7 (s), 128.6 (t), 128.5 (t), 128.3 (t), 125.1 (t), 125.0 (t), 123.9
(s), 123.6 (s), 123.2 (s), 122.93 (s), 122.91 (s), 121.83 (d), 121.78
(s), 98.5 (s), 97.6 (s), 97.4 (s), 96.6 (s), 96.4 (s), 95.5 (s), 95.17 (s),
95.15 (s), 90.4 (s), 90.3 (s), 89.64 (s), 89.59 (s), 88.1 (s), 87.0 (s),
86.6 (s), 86.3 (s), 85.9 (s), 85.4 (s), 84.72 (s), 84.66 (s), 84.6 (s),
84.4 (s), 79.9 (s), 56.34 (s), 56.26 (s), 56.24 (s), 56.19 (s), 56.1 (s),
33.4 (t), 19.14 (t), 19.10 (t), 19.0 (t); IR (KBr) 2175, 1272, 1031,
882, 834, 755, 710, 696 cm-1; MS (APCI) m/z 748 (M+).
troscopically characterize [63]paracyclophyne 1a by ir-
radiation of 2a in solution, laser irradiation of 2a-d
induced expulsion of the aromatic fragment, indan, giving
[6n]paracyclophyne anions 1a-d (n ) 3-6), respectively,
which were detected by TOF mass spectrometry. We are
planning to synthesize the chloro derivatives of 2a-d,
which would generate the corresponding carbon clusters
in a size-selective manner by further elimination of
chlorine atoms.7
Experimental Section
Cyclic Dehydrotrimer 2a. A solution of tetra-n-butyl-
ammonium fluoride (TBAF) (1 M in THF, 780 µL, 780 µmol) and
acetic acid (70 µL, 74 mg, 1.2 mmol) was added dropwise into a
solution of 6b (274 mg, 253 µmol) in THF (10 mL) at room
temperature. After the mixture was stirred for 2 h, water was
added into the mixture. The mixture was extracted with Et2O,
and the extract was washed with saturated NaHCO3 solution
and brine, and dried over anhydrous MgSO4. The solvent was
removed by evaporation to give deprotected product, which was
used in the next step without purification. The above deprotected
product was dissolved in Et3N (180 mL), which had been
degassed thoroughly by bubbling argon for 1 h, and the solution
was added dropwise into a solution of Pd(PPh3)4 (292 mg, 253
µmol) and CuI (57.9 mg, 304 µmol) in degassed Et3N (40 mL) at
50 °C over 1 h under an argon atmosphere. After being stirred
for 1 h at 70 °C, the reaction mixture was cooled to room
temperature and concentrated in vacuo. The residue was diluted
with CHCl3, and water was added. After extraction with CHCl3,
the organic layer was washed with water, 1 N HCl, and brine,
and dried over anhydrous MgSO4. After removal of the solvent,
the residue was subjected to chromatography on silica gel (99:
1, hexanes:AcOEt). The product was further purified by pre-
parative HPLC to give 2a (161 mg, 80% for two steps) as a yellow
solid: dec > 188 °C; 1H NMR (300 MHz, CDCl3) δ 7.35 (s, 12H),
6.00-5.92 (m, 12H), 2.10-1.96 (m, 6H), 1.72-1.49 (m, 6H),
1.39-1.27 (m, 6H); 13C NMR (75 MHz, CDCl3) δ 134.8 (s), 131.7
(d), 128.54 (d), 128.52 (d), 123.0 (s), 121.9 (d), 96.4 (s), 85.3 (s),
56.1 (s), 33.3 (t), 19.0 (t); IR (KBr) 2161, 861, 833, 755, 704 cm-1
MS (LDMS), see Figure 1a.
;
Cyclic Dehydropentamer 2c. Deprotection of 8b (136 mg,
84.2 µmol) and subsequent Pd-catalyzed coupling were carried
out as described above to afford 2c (7.4 mg, 9% for two steps) as
a yellow solid: dec > 210 °C; 1H NMR (300 MHz, CDCl3) δ 7.45
(s, 20H), 5.91 (brs, 20H), 2.09-2.03 (m, 10H), 1.70-1.49 (m,
10H), 1.36-1.27 (m, 10H); 13C NMR (67.8 MHz, CDCl3) δ 132.4
(s), 131.7 (d), 128.6 (d), 122.9 (s), 121.9 (d), 94.9 (s), 84.2 (s), 56.4
(s), 33.1 (t), 19.0 (t); IR (KBr) 2178, 835, 756, 709 cm-1; MS
(LDMS), see Figure 1c.
Cyclic Dehydrotetramer 2b and Dehydrohexamer 2d.
A solution of TBAF (1 M in THF, 220 µL, 220 µmol) was added
dropwise into a solution of 5b (113 mg, 138 µmol) in THF (13
mL). After the mixture was stirred for 2 h, water was added
(13) de Meijere, A.; Heinze, J.; Meerholz, K.; Reiser, O.; Ko¨nig, B.
Angew. Chem., Int. Ed. Engl. 1990, 29, 1418-1419.
Acknowledgment. Support to R.U. from the 21st
Century COE Program “Integrated Eco-Chemistry” is
gratefully acknowledged.
(14) LD-TOF mass spectra were recorded by irradiation with a
nitrogen laser (337 nm) of the solid samples followed by detection of
the negative ions in the reflectron mode.
Supporting Information Available: Experimental pro-
cedures and characterization of new compounds. Copies of 1H
and 13C NMR spectra for new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) Although we measured positive mode LD-TOF mass spectra of
2a-d, their spectra did not show clearly peaks for 1a-d.
(16) (a) Tobe, Y.; Nakagawa, N.; Naemura, K.; Wakabayashi, T.;
Shida, T.; Achiba, Y. J. Am. Chem. Soc. 1998, 120, 4544-4545. (b)
Tobe, Y.; Nakagawa, N.; Kishi, J.; Sonoda, M.; Naemura, K.; Waka-
bayashi, T.; Shida, T.; Achiba, Y. Tetrahedron 2001, 57, 3629-3636.
JO050833D
6136 J. Org. Chem., Vol. 70, No. 15, 2005