Novel Synthesis, Properties, and Oxidizing Ability of 11,13-Disubstituted 3,8-Methanocycloundeca[8,9-b]pyrimido[5,4-d]-furan-12(11H),14(13H)-dionylium Tetrafluoroborates

Article abstract of DOI:10.1021/jo035213w

Synthesis of novel 4,9-methanoundecafulvene [5-(4,9-methanocycloundeca-2′,4′,6′,8′, 10′-pentaenylidene)pyrimidine-2(1H),4(3H),6(SH)-trione] derivatives 10a-c was accomplished. Their structural characteristics were investigated on the basis of the ¹H and ¹³C NMR and UV-vis spectra. Upon treatment with DDQ, 10a-c underwent oxidative cyclization to give novel 11,13-disubstituted 3,8-methanocycloundeca[8,9-b]pyrimido[5,4-d]furan-12(11H),14(13H)-dionylium tetrafluoroborates 11a-c·BF₄^- in good yields. The spectroscopic properties of 11a-c·BF₄^- were studied, and the structural characterization of 11b·BF₄ ^- was performed by the X-ray crystal analysis. Cations 11a-c were very stable, and their pK_R+ values were determined spectrophotometrically to be 8.3-8.9. The electrochemical reduction of 11a-c exhibited low reduction potentials at -0.43 to -0.45 (V vs Ag/AgNO₃) upon cyclic voltammetry (CV). In a search for reactivity, reactions of 11a·BF₄^- with some nucleophiles, hydride and diethylamine, were carried out to clarify that the methano-bridge controls the nucleophilic attacks to occur with endo-selectivity. The photoinduced oxidation reactions of 11a·BF₄^- toward some amines under aerobic conditions were carried out to give the corresponding carbonyl compounds in more than 100% yield.

Full text of DOI:10.1021/jo035213w

Novel Syn th esis, P r op er ties, a n d Oxid izin g Ability of
11,13-Disu bstitu ted 3,8-Meth a n ocyclou n d eca [8,9-b]p yr im id o[5,4-d ]-
fu r a n -12(11H),14(13H)-d ion yliu m Tetr a flu or obor a tes
Shin-ichi Naya, Yohei Yamaguchi, and Makoto Nitta*
Department of Chemistry, School of Science and Engineering, Waseda University,
Shinjuku-ku, Tokyo 169-8555, J apan
nitta@waseda.jp
Received August 19, 2003
Synthesis of novel 4,9-methanoundecafulvene [5-(4,9-methanocycloundeca-2′,4′,6′,8′,10′-pentae-
nylidene)pyrimidine-2(1H),4(3H),6(5H)-trione] derivatives 10a -c was accomplished. Their struc-
1
tural characteristics were investigated on the basis of the H and 13C NMR and UV-vis spectra.
Upon treatment with DDQ, 10a -c underwent oxidative cyclization to give novel 11,13-disubstituted
3,8-methanocycloundeca[8,9-b]pyrimido[5,4-d]furan-12(11H),14(13H)-dionylium tetrafluoroborates
-
-
11a -c‚BF 4 in good yields. The spectroscopic properties of 11a -c‚BF 4 were studied, and the
structural characterization of 11b‚BF 4- was performed by the X-ray crystal analysis. Cations 11a -c
were very stable, and their pKR+ values were determined spectrophotometrically to be 8.3-8.9.
The electrochemical reduction of 11a -c exhibited low reduction potentials at -0.43 to -0.45 (V vs
-
Ag/AgNO3) upon cyclic voltammetry (CV). In a search for reactivity, reactions of 11a ‚BF 4 with
some nucleophiles, hydride and diethylamine, were carried out to clarify that the methano-bridge
controls the nucleophilic attacks to occur with endo-selectivity. The photoinduced oxidation reactions
of 11a ‚BF 4- toward some amines under aerobic conditions were carried out to give the corresponding
carbonyl compounds in more than 100% yield.
In tr od u ction
d]furan-8,10(9H)-dione, which are the structural isomers
of 5-deazaflavin and 5-deaza-10-oxaflavin, respectively,
and their ability in oxidizing some alcohols to the
corresponding carbonyl compounds.5,6 In this relation, we
have recently reported the oxidative cyclization of novel
heptafulvenes 1a -c by using DDQ to afford cyclohepta-
[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetraflu-
Flavins are known to play an important role as
cofactors in a wide variety of biological redox reactions.1
The flavin-redox systems have been investigated exten-
sively through synthetic model systems and theoretical
calculations.2 Among these, 5-deazaflavins3 and 5-deaza-
10-oxaflavins4 (2H-chromeno[2,3-d]pyrimidine-2,4(3H)-
diones) have been studied extensively in the hope of
providing mechanistic insight into flavin-catalyzed reac-
tions. On the basis of the above observations, we have
previously studied convenient preparations of 6-substi-
tuted 9-methylcyclohepta[b]pyrimido[5,4-d]pyrrole-8(6H),-
10(9H)-diones and 9-methylcyclohepta[b]pyrimido[5,4-
oroborates 2a -c‚BF 4 (Figure 1).7 Furthermore, alter-
-
- 8
native synthesis, properties, and reactivity of 2a ‚BF 4
and its sulfur and nitrogen analogues 3a -c‚BF 4- 8-10 as
well as their novel photoinduced autorecycling oxidizing
reactions toward some alcohols were investigated. Thus,
structural modifications of the uracil-annulated hetero-
-
-
azulenes such as 2a ‚BF 4 and 3a -c‚BF 4 are a very
interesting project from the viewpoint of exploration of
novel functions. Much of the motivation for studying the
properties of organic molecules stems from manipulation
of the primary chemical structure. Strategies for raising
or lowering the HOMO and LUMO levels include conju-
gation length control, as well as the introduction of an
electron-withdrawing or -donating group to the parent
molecular skeleton. Based on this concept, we have now
embarked on investigation of the synthesis of 11, 13-
* To whom correspondence should be addressed. Tel:+81-(0)3-5286-
3236. Fax: +81-(0)3-3208-2735.
(1) Muller, F. In Chemistry and Biochemistry of Flavoenzymes;
Muller, F., Ed.; CRC Press: Boca Raton, 1991; Vol. 1, p 1 and
references therein.
(2) (a) Chiu, C. C.; Pan, K.; J ordan, F. J . Am. Chem. Soc. 1995, 117,
7027. (b) Kim, J .; Hoegy, S. E.; Mariano, P. S. J Am. Chem. Soc. 1995,
117, 100. (c) Murahashi, S.; Ono, S.; Imada, Y. Angew. Chem., Int. Ed.
2002, 41, 2366. (d) Bergstad, K.; J onsson, S.; Ba¨chvall, J . J . Am. Chem.
Soc. 1999, 121, 10424. (e) Van Houten, K. A.; Kim, J .; Bogdan, M. A.;
Ferri, D. C.; Mariano, P. S. J . Am. Chem. Soc. 1998, 120, 5864. (f)
Zheng, Y.; Ornstein, R. L. J . Am. Chem. Soc. 1996, 118, 9402. (g)
Breinlinger, E. C.; Keenan, C. J .; Rotello, V. M. J . Am. Chem. Soc.
1998, 120, 8606. (h) Hasford, J . J .; Rizzo, C. J . J . Am. Chem. Soc.,
1998, 120, 2251. (i) Antony, J .; Medvedev, D. M.; Stuchebrukhov, A.
A. J . Am. Chem. Soc. 2000, 122, 1057.
(5) Nitta, M.; Tajima, Y. Synthesis 2000, 651.
(6) Takayasu, T.; Mizuta, Y.; Nitta, M. Heterocycles 2001, 54, 601.
(7) Naya, S.; Nitta, M. Tetrahedron 2003, 59, 3709.
(8) Naya, S.; Miyama, H.; Yasu, K.; Takayasu, T.; Nitta, M.
Tetrahedron 2003, 59, 1811.
(9) Naya, S.; Miyama, H.; Yasu, K.; Takayasu, T.; Nitta, M.
Tetrahedron 2003, 59, 4929.
(3) Yoneda, F.; Kokel, B. In Chemistry and Biochemistry of Fla-
voenzymes; Muller, F., Ed.; CRC Press: Boca Raton, 1991; Vol. 1, p
121 and references therein.
(4) Yoneda, F.; Hirayama, R.; Yamashita, M. Chem. Lett. 1980, 1157.
(10) Naya, S.; Nitta, M. Tetrahedron 2003, 59, 7291.
10.1021/jo035213w CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/29/2003
9284
J . Org. Chem. 2003, 68, 9284-9291

Synthesis of 4,9-Methanoundecafulvene Derivatives
SCHEME 1 a
F IGURE 1.
a
Reagents and conditions: (i) Ac2O, 120 °C, 1.5 h; (ii) (a) DDQ,
CH2Cl2, rt, 1 h, (b) 42% aq HBF4, Ac2O, 0 °C, 1 h.
disubstituted 3,8-methanocycloundeca[8,9-b]pyrimido-
[5,4-d]furan-12(11H),14(13H)-dionylium tetrafluorobo-
rates 11a -c‚BF 4 (Scheme 1) for the first time, which
-
appeared.18 From this viewpoint, the preparation and
are vinylogous compounds of 2a -c, to involve 1,6-
methano[11]annulenylium ion 4 instead of tropylium ion
5. The 1,6-methano[11]annulenylium ion 4, which is an
aromatic 10π-electron analogue of the tropylium ion 5,
has high thermodynamic stability (pKR+ ) 6.2)11 as
compared with tropylium ion 5 (pKR+ ) 3.9).12 Thus,
-
resolution of chiral 11a -c‚BF 4 are also interesting in
relation to their functions. Thus, we studied the two-step
synthesis, properties, and oxidizing ability of novel
11,13-disubstituted 3,8-methanocycloundeca[8,9-b]py-
rimido[5,4-d]furan-12(11H),14(13H)-dionylium tetrafluo-
roborates 11a -c‚BF 4-, which were characterized on the
basis of the spectral data and pKR+ as well as redox
potentials. The detailed structural characterization of
11b was also carried out by X-ray crystal analysis. The
photoinduced autorecycling oxidation of alcohols and
amines to give the corresponding carbonyl compounds is
studied as well.
-
cations 11a -c‚BF 4 are expected to have extended
π-conjugation and higher thermodynamic stability. As for
methano-bridged aromatic compounds having 14π-elec-
tron (azulene and 1-heteraazulene vinylogues), Prinzbach
et al.13 and our research group14 have reported on a series
of 5,10-methanocyclopentacycloundecene ring system 6a
and 4,9-methanocyclopentacycloundecene ring system 7a
as vinylogous compounds of azulene, respectively. We
have also reported the synthesis and spectroscopic prop-
erties of 6,11- and 4,9-methanocycloundeca[b]pyrrole ring
systems 6b and 7b as 1-azaaulene vinylogues.15 Regard-
ing these compounds, the chemical shifts of the methano-
bridge protons and carbon can conveniently evaluate
their aromaticity.16,17 In addition, since the methano-
bridge can become a planar-chiral source, the synthesis
and resolution of chiral methano-bridge compounds have
Resu lts a n d Discu ssion
Syn th esis. The synthetic strategy of 11a -c is, at first,
to obtain 4,9-methanoundecafulvene [5-(4,9-methanocy-
cloundeca-2′,4′,6′,8′,10′-pentaenylidene)pirimidine-2(1H),4-
(3H),6(5H)-trione] derivatives 10a -c followed by oxida-
tive cyclization.7 Condensation reactions of 4,9-
methano[11]annulenone 819 with disubstituted barbituric
acids 9a -c in Ac2O under reflux afforded 10a -c as
orange or reddish needles in moderate to good yields
(Scheme 1). Compounds 10a -c have a Cs symmetry, and
thus, 10a -c are apparently achiral compounds. Oxida-
tion reactions of 10a -c with DDQ in CH2Cl2 at room
temperature and subsequent anion-exchange reaction by
using 42% aq HBF4 in Ac2O afforded possible racemic
compounds, 11a -c‚BF 4- in good yields. The results are
summarized in Table 1. Apparently, the oxidative cy-
clization at the C-2′ position of 10a -c gives (R)-11a -c‚
BF 4-, while the cyclization at the C-11′ position affords
(11) (a) Grimme, W.; Hoffmann, H.; Vogel, E. Angew. Chem., Int.
Ed. Engl. 1965, 4, 354. (b) Vogel, E.; Feldmann, R.; Du¨wel, H.
Tetrahedron Lett. 1970, 1, 1941.
(12) Okamoto, K.; Takeuchi, K.; Komatsu, K.; Kubota, Y.; Ohara,
R.; Arima, M.; Takahashi, K.; Waki Y.; Shirai, S. Tetrahedron 1983,
39, 4011 and references therein.
(13) (a) Prinzbach, H.; Knothe, L. Pure Appl. Chem. 1986, 58, 25.
(b) Beck, A.; Hunkler, D.; Prinzbach, H. Tetrahedron Lett. 1984, 25,
1785.
(14) (a) Takayasu, T.; Nitta, M. J . Chem. Soc., Perkin Trans. 1 1997,
3255. (b) Nitta, M.; Akaogi, A.; Tomioka, H. Heterocycles 2003, 60, 365.
(15) Kanomata, N.; Kamae, K.; Iino, Y.; Nitta, M. J . Org. Chem.
1992, 57, 5313.
(16) (a) Vogel, E.; Roth, H. D. Angew. Chem. 1964, 76, 145. (b)
Cremer, D.; Dick, B. Angew. Chem., Int. Ed. Engl. 1982, 21, 865 and
references therein.
(17) (a) Masamune, S.; Brooks, D. W. Tetrahedron Lett. 1977, 8,
3239. (b) Scott, L. T.; Brunsvold, W. R.; Kirms, M. A.; Erden, I., J .
Am. Chem. Soc. 1981, 103, 5216. (c) Scott, L. T.; Sumpter, C. A.; Oda,
M.; Erden, I. Tetrahedron Lett. 1989, 30, 305.
(18) (a) Chen, L.; Ding, K.; Tian, W. Chem. Commun. 2003, 838. (b)
Marshall, J . A.; Conrow, R. E. J . Am. Chem. Soc. 1980, 102, 4274. (c)
Marshall, J . A.; Conrow, R. E. J . Am. Chem. Soc. 1983, 105, 5679. (d)
Meyer, A.; Schlo¨gl, K.; Lerch, U.; Vogel, E. Chem. Ber. 1988, 121, 917.
(e) Schlo¨gl, K.; Widhalm, M. Chem. Ber. 1982, 115, 3042.
(19) Vogel, E. 23rd Int. Congr. Pure Appl. Chem. 1971, 1, 275.
J . Org. Chem, Vol. 68, No. 24, 2003 9285

Naya et al.
SCHEME 2
F IGURE 2. UV-vis spectra of 10a -c and 1a in CH3CN.
TABLE 1. Resu lts for th e P r ep a r a tion of 10a -c a n d
-
Ca tion s 11a -c‚BF 4
condensation
product yield (%)
oxidative cyclization
annulenylium ion 4 (pKR+, 6.2)11 is more stable than
tropylium ion 7 (pKR+, 3.9).12 This feature is similar to
the cases of dicyano-substituted heptafulvene and 4,9-
methanoundecafulvene; the C-12 carbon signal of 12,12-
dicyano-4,9-methanoundecafulvene (δc 83.6)20 is shifted
to lower field as compared with the C-8 carbon signal of
8,8-dicyanoheptafulvene (δc 70.1).21 Moreover, the proton
signals of the H-2′ and the H-11′ of 10a -c appearing at
δ 7.11-7.30 are remarkably shifted to higher field as
compared with those of 1a -c (δ 9.14-9.38).7 These
features are ascribed to the bending structure of the 11-
membered ring of compounds 10a -c, which do not allow
the H-2′ and the H-11′ to locate in the deshielding region
of the carbonyl groups in the barbitric acid moiety. The
redox property of 10a -c was determined by cyclic vol-
tammetry (CV) in acetonitrile. The oxidation and reduc-
tion waves of 10a -c were irreversible under the condi-
tions of CV measurements, and thus, the peak potentials
are summarized in Table 2. The redox processes of 10a -c
are depicted in Scheme 2. At the first reduction potentials
(E1red) of 10a -c, radical anions 12a -c would be gener-
ated. On the other hand, radical cations 13a -c seem to
be generated at the first oxidation potentials (E1ox). While
the values (E1red) of 10a -c are more positive in the order
10a < 10b ) 10c, the values (E1ox) of 10a -c are more
positive in the order 10a > 10c > 10b. The first reduction
potentials (E1red) of 10a -c are more positive than those
of 1a -c,7 while the first oxidation potentials (E1ox) of
10a -c are more negative than those of 1a -c.7 These
features suggest that compounds 10a -c have the elon-
gated π-conjugation as vinylogous compounds of 1a -c.
After the first cycle of CV measurements of 10a -c, other
reduction waves were recorded at -0.43, -0.39, and
-0.46 V, respectively. These waves are suggested to be
the reduction waves of 11a -c, which are generated by
oxidative cyclization reactions of 10a -c under CV mea-
surement. This feature is similar to the behavior of
compounds 1a -c.7 Thus, DDQ-prompted oxidative cy-
clization of 10a -c affording cations 11a -c would proceed
via a pathway similar to that of compounds 1a -c. The
run
compd
product
yield (%)
-
1
2
3
9a
9b
9c
10a
10b
10c
81
53
66
11a ‚BF 4-
11b‚BF 4-
11c‚BF 4
100
100
95
TABLE 2. 1H a n d 13C NMR Sp ectr a l Da ta a n d Red ox
P oten tia ls of Meth a n ou n d eca fu lven es 10a -c a n d
Refer en ce Com p ou n d s 1a -c
NMR
redox potentiala
compd
1H/δ
13C/δd
E1red
E1ox
10a
10b
10c
1a e
1be
1ce
7.12b
7.11b
7.30b
9.20c
9.14c
9.38c
110.5
109.4
108.0
102.7
103.1
101.9
-1.19
-1.14
-1.14
-1.23
-1.25
-1.18
+0.78
+0.90
+0.84
+1.07
+1.07
+1.10
V vs Ag/AgNO3; cathodic peak potential. H-2′, 11′. c H-2′, 7′.
a
b
C-5 of the barbitric acid moiety. e Reference 7.
d
(S)-11a -c‚BF 4-. Thus, the oxidative cyclization results
in the formation of racemic compounds (R),(S)-11a -c‚
BF 4-. Unfortunately, attempted optical resolution by
means of recrystallization and conversion to diastereo-
mers by adding chiral amine is unsuccessful at the
present stage (vide infra). Thus, racemic compounds
-
-
(R),(S)-11a -c‚BF 4 are represented as 11a -c‚BF 4
.
-
Compounds 10a -c and 11a -c‚BF 4 were fully charac-
1
terized on the basis of the H NMR, 13C NMR, IR. UV-
vis, and mass spectral data, as well as elemental analy-
ses.
P r op er ties. The UV-vis spectra of 10a -c in CH3CN
are shown in Figure 2, together with those of reference
compounds 1a .7 The spectra of 10a -c resemble each
other, and the longest wavelength absorption maxima
appear at 488 nm (10a ), 488 nm (10b), and 493 nm (10c).
These values are longer by ca. 50 nm than those of 1a ,
suggesting the elongated π-conjugation of compounds
10a -c as compared with those of 1a -c.7 In the 13C NMR
spectra, signals of the carbon atom (C-5) of the barbituric
acid moiety in 10a -c appear at low field (δC 108.0-
110.5), suggesting the low electron density as compared
with those of 1a -c (δC 101.7-103.1) (Table 2).7 Thus,
contribution of the zwitterionic canonical structure B of
10a -c seems to be less important as compared with the
canonical structure A; however, the 1,6-methano[11]-
(20) Knothe, L.; Prinzbach, H. Liebigs. Ann. Chem. 1977, 687.
(21) (a) Ikeda, Y.; Yin, B. Z.; Kato, N.; Mori, A.; Takeshita, H. Bull.
Chem. Soc. J pn. 1996, 69, 1319. (b) Ikeda, Y.; Yin, B. Z.; Kato, N.;
Mori, A.; Takeshita, H.; Chem. Lett. 1992, 1453. (c) Ikeda, Y.; Yin, B.
Z.; Kato, N.; Mori, A.; Takeshita, H. Heterocycles 1993, 36, 1725.
9286 J . Org. Chem., Vol. 68, No. 24, 2003

Synthesis of 4,9-Methanoundecafulvene Derivatives
F IGURE 3. UV-vis spectra of 11a -c and 2a in CH3CN.
radical cations 13a -c, which are generated by one-
electron oxidation of 10a -c, undergo cyclization reaction
to give intermediates 14a -c, the hydrogen abstraction
of which give cations 11a -c. Subsequent anion exchange
F IGURE 4. Chemical shifts and coupling constants of 10a ,
11b‚BF 4-, 15, and 16.
reaction with aq HBF4 solution results in the formation
-
of cations 11a -c‚BF 4
.
-
In a similar fashion, cations 11a -c‚BF 4 were fully
1
characterized on the basis of the H and 13C NMR, IR,
UV-vis, and mass spectral data, as well as elemental
analyses. Mass spectra of these compounds exhibited the
correct M+ - BF4- ion peaks, which are indicative of the
cationic structures of these compounds. The characteristic
-
absorption bands for the counterion of BF4 were ob-
served at 1084 cm-1 in the IR spectra of these compounds,
respectively. The UV-vis spectra of 11a -c‚BF 4- in CH3-
CN are shown in Figure 3, together with those of
-
2a ‚BF 4-. The spectra of 11a -c‚BF 4 resemble each
other, and the longest wavelength absorption maxima
show a red-shift by ca. 80 nm as compared with those of
2a ‚BF 4-, suggesting the elongated π-conjugation of 11a -
1
c‚BF 4-. The H NMR spectra of cations 11a -c‚BF 4- are
noteworthy, since the chemical shifts of bridged-annulene
systems are quite useful in determining such structural
properties as diatropicity and bond alternation. Unam-
-
F IGURE 5. ORTEP drawing of 11b‚BF 4 with thermal
1
ellipsoid plot (50% probability). Selected bond lengths (Å): O1-
C10, 1.403(3); O1-C14, 1.332(3); C1-C2, 1.361(4); C1-C11,
1.421(3); C2-C3, 1.392(4); C3-C4, 1.402(4); C4-C5, 1.383(4);
C5-C6, 1.409(4); C6-C7, 1.388(4); C7-C8, 1.386(4); C8-C9,
1.388(4); C9-C10, 1.364(4); C10-C11, 1.435(3); C11-C13,
1.417(4); C13-C14, 1.363(3); C14-N1, 1.322(3).
biguous proton assignment was made by analyzing H
NMR, H-H COSY, and NOE spectra. Since a similar
tendency is observed in a series of 11a -c‚BF 4-, the
chemical shifts of the bridge protons and selected cou-
pling constants of the peripheral protons of 11a ‚BF 4- are
shown in Figure 4 together with those of the reference
compounds 10a , 15,22 and 16.15 The large geminal
coupling constant of the methylene protons (J E,Z ) 11.5
Hz) supports the absence of a norcaradiene structure for
11a ‚BF 4-. The bridge protons of 11a ‚BF 4- appear at very
high field (δ -0.42 and δ -1.20), and the peripheral
protons appear in the aromatic region (δ 8.39-9.78),
suggesting a large diatropic ring current.23 The differ-
Destro and Simonetta have reported that 1,6-methano-
[11]annulenylium ion 4 is described as a delocalized 10π-
electron system rather than a homotropylium ion.24
A
-
single crystal of 11b‚BF 4 was obtained by recrystalli-
zation from CH3CN/Et2O, and thus, X-ray structure
analysis was carried out and the ORTEP drawing of
-
11b‚BF 4 is shown in Figure 5. Although solvent mol-
-
ecules (CH3CN) exist in the single crystal, the solvent
molecule and counteranion are omitted for clarity in the
ORTEP drawing. The single crystal is a racemic mixture,
and not conglomerate, and thus, optical resolution through
recrystallization seems to be difficult (vide supra). The
compound 11b‚BF 4- has a nearly planar structure, and
selected bond lengths of 11b obtained by X-ray analysis
are also summarized in Figure 5. The bond lengths of
C4-C5 and C6-C7 are shorter than those of C3-C4,
ences in vicinal coupling constants in 11a ‚BF 4 are
smaller than those found in 10a and 15, suggesting that
-
the bond alternation of 11a ‚BF 4 is smaller than those
of 10a and 15. While the vicinal coupling constants in
-
11a ‚BF 4 and 16 are similar, the chemical shifts of the
-
bridge protons of 11a ‚BF 4 are even higher than those
of 16.
(22) Nitta, M.; Naya, S. J . Chem. Res., Synop. 1998, 522-523; J .
Chem. Res., Miniprint 1998, 2363-2380.
(23) Vogel, E. Chem. Soc. Spec. Publ. 1967, 21, 113.
(24) Destro, R.; Simonetta, M. Acta Crystallogr. 1979, B35, 1846.
J . Org. Chem, Vol. 68, No. 24, 2003 9287

Naya et al.
SCHEME 3 a
a
F IGURE 6.
Reagents and conditions: (i) NaBH4, CH3CN, rt, 1 h; (ii) (a)
DDQ, CH2Cl2, rt, 1 h, (b) 42% aq HBF4, Ac2O, 0 °C, 1 h.
TABLE 3. p KR+ Va lu es a n d Red u ction P oten tia ls of
Ca tion s 11a -ca a n d Refer en ce Com p ou n d s 2a -c, 4, a n d 5
determined to be 8.3-8.9, which are much larger than
those of 2a , 4, 5. In addition, the pKR+ values are larger
in the order 11b < 11a < 11c. The pKR+ value difference
between 11a and 2a is ca. 2.7 pH unit, which is similar
to the difference between 1,6-methano[11]annulenylium
ion 4 (pKR+ 6.2)11 and tropylium ion 5 (pKR+ 3.9).12
The reduction potentials of 11a -c were determined by
cyclic voltammentry (CV) in CH3CN. The reduction waves
of 11a -c were irreversible under the conditions of the
CV measurements; the peak potentials are summarized
in Table 3, together with those of the reference com-
pounds 2a -c7 and 5.12 The values (E1red) are more
negative in the order 11a > 11b > 11c. The E1red of
11a -c are more positive by 0.15, 0.17, and 0.13 V than
those of 2a -c, respectively. The irreversible nature is
probably due to the formation of a radical species and
its dimerization, as reported to be a typical property of
uracil-annulated heteroazulenylium ions.7-10 The pKR+
values of 11a -c are larger than that of 2a ; however, the
first reduction potentials (E1red) of 11a -c are more
positive than that of 2a . This feature is rationalized by
the elongated π-conjugation of 11a -c as compared with
2a -c.
reduction potentialb
compd
pKR+
E1red
E2red
11a
11b
11c
2a c
2bd
2cd
4e
8.7
8.3
8.9
ca. 6.0
-0.43
-0.44
-0.45
-0.58
-0.61
-0.58
-1.09
6.2
3.9
5f
-0.51
a
-
b
Salts 11a -c‚BF 4 are used for the measurement. V vs Ag/
d
AgNO3; cathodic peak potential. c Reference 8. Reference 7.
e Reference 11. Reference 12.
f
C5-C6, and C7-C8. This feature is similar to those of
1,6-methano[11]annulenylium ion 4,24 and the large
transannular distance between C3 and C8 (11b: 2.32 Å;
cf. 2.30 Å for 4) also indicates that the homoconjugation
is not important in this cation. The bond lengths of C1-
C2 and C9-C10 are much shorter than those of C11-
C1, C2-C3, and C8-C9, suggesting the canonical struc-
tures 11b-B and 11b-C are important for 11b (Figure
6). In addition, since the bond length of O1-C14 is
shorter than that of O1-C10, a contribution of 11b-D
seems to be less important. MO calculation of 11a was
carried out by the 6-31G* basis set of the MP2 levels.25
The bond length alternation obtained by MO calculation
for 11a is similar to that obtained by X-ray analysis for
11b.
The affinity of the carbocation toward hydroxide ions
expressed by the pKR+ value is the most common criterion
of carbocation stability.26 The pKR+ values of cations
11a -c were determined spectrophotometrically in buffer
solutions prepared in 50% aqueous CH3CN and are
summarized in Table 3, along with those of reference
compounds 2a ,8 4,11 5.12 The pKR+ values of 11a -c were
-
Rea ctivity of 11a . The reaction site of 2a ‚BF 4 and
-
3a -c‚BF 4 with some nucleophiles depends on the
nucleophiles and heteroatom on the cations.8-10 Thus, the
-
reactions of 11a ‚BF 4 with some nucleophiles were
carried out. Reduction of 11a ‚BF 4- with NaBH4 in CH3-
CN afforded compound 17a in good yield (Scheme 3).
Compound 17a is oxidized by DDQ to regenerate
11a ‚BF 4- in good yield. Previously, we have reported that
-
-
the reaction of 2a ‚BF 4 or 3a -c‚BF 4 with NaBH4
proceeded at the 1-, 3-, and 5-positions to afford a mixture
of three regio-isomers.8-10 On the other hand, the reaction
-
of 11a ‚BF 4 proceeded at only the 9-position to afford
single isomer 17a , and other regioisomers, such as 18a ,
are not obtained. There is a well-known tendency for
double bond fixation in the methano[11]annulene system
to favor a cycloheptatriene system in the compound 17a
predominantly over a 1,6-dimethylenecyclohepta-2,4-
diene system in compound 18a .27 Thus, the reaction of
11a ‚BF 4- with NaBH4 would occur at the 9-position. To
clarify the selectivity of endo-addition or exo-addition, a
(25) Gaussian 98, revision A.11: Frisch, M. J .; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J . R.;
Zakrzewski, V. G.; Montgomery, J . A., J r.; Stratmann, R. E.; Burant,
J . C.; Dapprich, S.; Millam, J . M.; Daniels, A. D.; Kudin, K. N.; Strain,
M. C.; Farkas, O.; Tomasi, J .; Barone, V.; Cossi, M.; Cammi, R.;
Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J .;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.; Cioslowski, J .; Ortiz,
J . V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J .; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P.
M. W.; J ohnson, B. G.; Chen, W.; Wong, M. W.; Andres, J . L.; Head-
Gordon, M.; Replogle, E. S.; Pople, J . A. Gaussian, Inc.: Pittsburgh,
PA, 2001.
-
reaction of 11a ‚BF 4 with NaBD4 was also carried out
to give only endo-adduct 17a -D selectively. This result
shows that the methano[11]annulene system is useful for
(27) (a) Paquette, L. A.; Berk, H. C.; Ley, S. V. J . Org. Chem. 1975,
40, 902 and references therein. (b) Reisdorff, J .; Vogel, E. Angew.
Chem., Int. Ed. Engl. 1972, 11, 218.
(26) Freedman, H. H. Carbonium Ions; Olah, G. A., Schleyer, P.,
Eds.; Wiley-Interscience: New York, 1973.
9288 J . Org. Chem., Vol. 68, No. 24, 2003

Synthesis of 4,9-Methanoundecafulvene Derivatives
F IGURE 7. Chemical shifts (bold) and coupling constants of
17a .
SCHEME 4 a
F IGURE 8. Chemical shifts (bold) and coupling constants of
20a and 21a .
-
of Et2NH2‚BF 4 and/or Et2NH (cf. Supporting Informa-
tion). The 1H NMR spectrum of 20a is unequivocally
assigned by using the H-H COSY and NOE spectra, and
the chemical shifts and coupling constants are shown in
Figure 8. In the NOE spectrum of 20a , correlation
between the H-9 and the H-z is suggested, and thus, the
endo-orientation of the diethylamino group is assessed.
This feature shows also that the methano[11]annulene
system is useful for the chiral auxiliary. While the
9-adduct 20a was stable at low temperature (-65 to -35
°C for 1 h), isomerization reaction was observed at higher
temperature (>-35 °C) to give 21a (Scheme 4). The
rearrangement would proceed via 9a-adduct 22a . Al-
though compound 21a is stable in dilute solution, it
decomposes during concentration in vacuo. Thus, the
optical resolution based on the generation of diastereo-
mers by addition reaction of 11a ‚BF 4- with chiral amines
a
Reagents and conditions: (i) Et2NH, CD3CN, -65 °C, 30 s;
(ii) warm to rt, 1h; (iii) 42% aq HBF4, Ac2O, 0 °C, 1 h.
1
seems to be difficult. Satisfactory H and 13C NMR and
HRMS spectra were obtained for 21a . Based on the study
of 1H and 13C NMR, we have reported that compound 19a
has a larger contribution of the canonical structure 19a -
B.8 On the contrary, large coupling constant between the
H-10′ and the H-11′ as well as small coupling constants
between the H-5′ and the H-6′ and between the H-7′ and
the H-8′ suggest that compound 21a has a larger con-
tribution of the canonical structure 21a -A probably due
to the stability of the cycloheptatriene-moiety.27
the chiral auxiliary. Preferential endo-selectivity of nu-
cleophiles toward 11a is probably due to the bridge
methylene, which experiences appreciable steric hin-
drance with the incorporating nucleophiles. The struc-
tural assignment of 17a and 17a -D was based on the
1
NMR, IR and mass spectral data. The H NMR spectra
are assigned by using H-H COSY and NOE spectra, and
the chemical shifts and coupling constants of 17a are
shown in Figure 7.
-
The reaction of 2a ‚BF 4 with diethylamine gives 5a-
Au tor ecyclin g Oxid a tion of Alcoh ols a n d Am in es.
Compounds 2a ‚BF 4- and 3a -c‚BF 4- undergo autorecy-
cling oxidation toward some alcohols under photoirradia-
tion.8-10 In this context and in a search for oxidizing
ability of 11a ‚BF 4-, we examined the oxidation of some
adduct, which undergoes ring-opening reaction to give
-
19a (Scheme 4).8 Thus, the reaction of 11a ‚BF 4 with
diethylamine was monitored by NMR spectroscopy in
-
CD3CN. Initially, diethylamine addition of 11a ‚BF 4
-
occurred at the 9-position under low temperature (-65
°C) to afford 20a (Scheme 4). The structural assignment
alcohols and amines by using 11a ‚BF 4 under aerobic
and photoirradiation conditions (RPR-100, 350 nm lamps).
Although alcohols and cyclohexylamine were not oxidized
1
of 20a was based on the H and 13C NMR, although the
13C NMR spectrum of 20a shows more than eight signals
by 11a ‚BF 4-, we found that compound 11a ‚BF 4 has
-
of sp3-carbon, which are contaminated with some signals
oxidizing ability toward benzylamine and 1-phenylethy-
J . Org. Chem, Vol. 68, No. 24, 2003 9289

Naya et al.
TABLE 4. Au tor ecyclin g Oxid a tion of Som e Alcoh ols
dene)pyrimidine-2(1H),4(3H),6(5H)-trione] derivatives
10a -c was accomplished. On the basis of the UV-vis
a n d Som e Am in es by 11a ‚BF 4 u n d er P h otoir r a d ia tion a
-
1
and 13C and H NMR spectra, the structural character-
entry additive alcohol or amine carbonyl compdb yieldc/%
istics and electrochemical properties were investigated.
Upon oxidative cyclization with DDQ, 10a -c were con-
verted to 11,13-disubstituted 3,8-methanocycloundeca-
[8,9-b]pyrimido[5,4-d]furan-12(11H),14(13H)-dionylium
tetrafluoroborates 11a -c‚BF 4- in good yields. The spec-
troscopic properties were studied, and structural char-
-
1
2
3
4
5
6
11a ‚BF 4- PhCH2NH2
PhCHO
PhCOMe
3007
2267
11a ‚BF 4- PhCH(NH2)Me
11a ‚BF 4- cyclohexylamine cyclohexanone
0e
11a ‚BF 4- PhCH2OHd
PhCHO
PhCOMe
PhCOMe
0e
0e
1860
11a ‚BF 4
2a ‚BF 4
PhCH(OH)Me
PhCH(OH)Me
-f
a
-
CH3CN solution was irradiated by RPR-100 350 nm lamps
acterization of 11b‚BF 4 based on the X-ray crystal
under aerobic conditions. b Isolated as 2,4-dinitrophenylhydrazone.
c Based on 11a ‚BF 4- used; the yield, called “blank", is subtracted
from the total yield of carbonyl compound in the presence of
analysis was also performed. Due to the elongated
π-conjugation, the pKR+ values of 11a -c are remarkably
larger than that of 2a , while the first reduction potentials
(E1red) of 11a -c are more positive than that of 2a . In
-
d
11a ‚BF 4
.
In the presence of K2CO3 (1 mmol). e The “blank”
-
yield was higher than the yield in the presence of 11a ‚BF 4
.
f Reference 8.
-
the reactions of 11a ‚BF 4 with some nucleophiles, hy-
dride
and
diethyl-
amine, the methano-bridge is proposed for a useful chiral
auxiliary. Although some alcohols and cyclohexylamine
were not oxidized by 11a ‚BF 4-, the photoinduced oxida-
lamine to give benzaldehyde and acetophenone. The
results are summarized in Table 4. Direct irradiation of
the alcohols and amines in the absence of 11a ‚BF 4
-
-
tion reactions of 11a ‚BF 4 toward some amines under
(named “blank”) gives the corresponding carbonyl com-
pounds in low to modest yields. Thus, the yields are
calculated by subtraction of the “blank” yield from the
yield of the carbonyl compound in the presence of
aerobic conditions were carried out to give the corre-
sponding carbonyl compounds in more than 100% yields.
On the basis of the present study, uracil-annulated
-
-
2a ‚BF 4 and 11a ‚BF 4-. More than 100% yields are
methano-bridged aromatic compounds, such as 11a ‚BF 4
,
obtained [based on compounds 11a ‚BF 4-] (Table 4,
entries 1 and 2), and thus, autorecycling oxidation clearly
proceeds. In a search for the mechanistic aspect of the
photoinduced oxidation reaction, the fluorescence spectra
of 2a ‚BF 4- and 3a ‚BF 4- are studied.8,9 The fluorescence
of 2a and 3a are 491 and 500 nm, and the storks-shifts
are 94 and 86 nm, respectively.8,9 The quantum yields
(Φ) for 2a and 3a are 0.087 and 0.054, respectively, as
determined by using quinine bisulfate as standard.28 By
addition of 1-phenylethanol to the solutions of 2a or 3a ,
quenching of the fluorescence was observed, suggesting
interaction of the singlet excited state of 2a or 3a with
the alcohol. On the other hand, the fluorescence of 11a
appeared at 531 nm, and the storks-shift was 38 nm. The
fluorescence of 11a is very weak, and the quantum yield
(Φ) of 11a was determined to be 0.00394 by using quinine
bisulfate as standard.28 In addition, by addition of 1-phe-
nylethanol (500 equiv) to the solution of 11a (under
similar conditions for oxidation reaction), no quenching
of the fluorescence was observed. These features suggest
very small interaction of the singlet excited state of 11a
with alcohol, and thus, alcohols would not be oxidized
by 11a ‚BF 4-. On the other hand, the present autorecy-
cling oxidation of amines would proceed via addition
products, similar to the lumiflavinium ions.29 However,
attempted detection of the intermediate such as an
addition product or reduced compound 17a in the oxida-
tion reaction was unsuccessful at the present stage. Thus,
further investigations are required to clarify the present
autorecycling oxidation reaction.
are expected to provide chiral redox systems. Further
studies concerning this aspect will be continued.
Exp er im en ta l Section
General experimental conditions and spectroscopic instru-
mentation used are described in the Supporting Information.
Gen er a l P r oced u r e for th e P r ep a r a tion of 10a -c. A
solution of each of barbituric acid 9a (624 mg, 4 mmol), 9b
(736 mg, 4 mmol), and 9c (1120 mg, 4 mmol) and 4,9-methano-
[11]annulenone 8 (170 mg, 1 mmol) in Ac2O (2 mL) was heated
at 120 °C for 1.5 h. After the reaction was completed, the
reaction mixture was concentrated in vacuo. The resulting
residue was purified through column chromatography on Al2O3
by using hexane-AcOEt (2:1) as the eluent to give the products
10a (248 mg, 81%), 10b (177 mg, 53%), or 10c (286 mg, 66%).
Gen er a l P r oced u r e for th e P r ep a r a tion of Sa lts 11a -
c‚BF 4-. To a stirred solution of each 10a (64 mg, 0.2 mmol),
10b (68 mg, 0.2 mmol), and 10c (86 mg, 0.2 mmol) in CH2Cl2
(2 mL) was added DDQ (91 mg, 0.4 mmol), and the mixture
was stirred at rt for 1 h until the reaction was completed. After
evaporation of the CH2Cl2, the residue was dissolved in Ac2O
(5 mL) and 42% HBF 4 (1 mL) at 0 °C, and the mixture was
stirred for 1 h. To the mixture was added Et2O (100 mL), and
the precipitates were collected by filtration to give 11a ‚BF 4
(86 mg, 100%), 11b‚BF 4 (80 mg, 94%), or 11c‚BF 4 (110 mg,
100%).
X-r a y Str u ctu r e Deter m in a tion of 11b‚BF 4-: reddish
prisms, C22H22BF4N3O3, M ) 463.24, monoclinic, space group
P21/ c, a ) 8.844(6) Å, b ) 15.79(1) Å, c ) 15.418(6) Å, â )
98.12(2)°, V ) 2131.6(2) Å3, Z ) 4, Dc ) 1.443 g cm-3, crystal
dimensions 0.80 × 0.40 × 0.20 mm. Data were measured on a
Rigaku RAXIS-RAPID radiation diffractomater with graphite-
monochromated Mo KR radiation. A total 19 281 reflections
were collected, using the ω - 2θ scan technique to a maximum
2θ value of 55.0°. The structure was solved by direct methods
and refined by a full-matrix least-squares method using SIR92
structure analysis software,31 with 320 variables and 3217
observed reflections [I > 3.00σ(I)]. The non-hydrogen atoms
were refined anisotropically. The weighting scheme w )
[3.0000σc2(Fo) + 0.0010Fo2 + 0.5000]-1 gave satisfactory agree-
Su m m a r y
The synthesis of novel 4,9-methanoundecafulvene
[5-(4,9-methanocycloundeca-2′,4′,6′,8′,10′-pentaenyli-
(28) Melhuish, W. H. J . Phys. Chem. 1961, 65, 229.
(29) (a) Hoegy, S. E.; Mariano, P. S. Tetrahedron 1997, 53, 5027.
(b) Kim, J .; Hoegy, S. E.; Mariano, P. S. J . Am. Chem. Soc. 1995, 117,
100. (c) Kim, J .; Bogdan, M. A.; Mariano, P. S. J . Am. Chem. Soc. 1993,
115, 10591.
(30) Ridi, M.; Aldo, G. Gazz. Chim. Ital. 1952, 82, 13.
(31) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J . Appl. Crystallogr. 1994,
27, 435.
9290 J . Org. Chem., Vol. 68, No. 24, 2003

Synthesis of 4,9-Methanoundecafulvene Derivatives
ment analysis. The final R and Rw values were 0.0530 and
1H NMR Mon itor in g of th e Rea ction of 11a ‚BF 4- w ith
-
0.0680. The maximum peak and minimum peak in the final
Et2NH. To a solution of compound 11a ‚BF 4 (0.01 mmol) in
3
difference map were 0.40 and -0.37 e-/ Å .
CD3CN (0.5 mL) was added at -65 °C Et2NH (7.3 mg, 0.1
mmol) in an NMR tube. The NMR measurement was carried
out immediately (after ca. 30 s) at -65 °C, and complete
conversion of 11a ‚BF 4- to 20a was observed. After the reaction
mixture was warmed to -35 °C and kept at -35 °C for 1 h, no
change was observed. Then, the reaction mixture was warmed
to rt during 1 h, and complete conversion of 20a to 21a was
observed.
Deter m in a tion of p KR+ Va lu e of Ca tion s 11a -c. Buffer
solutions of slightly different acidities were prepared by mixing
aqueous solutions of KH2PO4 (0.1 M) and NaOH (0.1 M) (for
pH 6.0-8.0), Na2B4O7 (0.025 M) and HCl (0.1 M) (for pH 8.2-
9.0), and Na2B4O7 (0.025 M) and NaOH (0.1 M) (for 9.2-10.8)
in various portions. For the preparation of sample solutions,
1 mL portions of the stock solution, prepared by dissolving
-
3-5 mg of compounds 11a -c‚BF 4 in CH3CN (20 mL), were
Rea ction of 21a w ith HBF 4. To a solution of 21a (0.05
mmol) and Et2NH in CH3CN, which was prepared by the
reaction of 11a ‚BF 4- (20 mg, 0.05 mmol) with Et2NH (7.3 mg,
0.1 mmol) in CH3CN (20 mL), was added a mixture of Ac2O (5
mL) and 42% aq HBF 4 (1 mL) at 0 °C. The mixture was stirred
for 1 h. To the mixture was added Et2O (50 mL), and the
precipitate was collected by filtration to give 11a ‚BF 4- (20 mg,
100%).
diluted to 10 mL with the buffer solution (8 mL) and CH3CN
(1 mL). The UV-vis spectrum was recorded for each cation
11a -c in 20 different buffer solutions. Immediately after
recording the spectrum, the pH of each solution was deter-
mined on a pH meter calibrated with standard buffers. The
observed absorbance at the specific absorption wavelengths
(488 nm for 11a ; 490 nm for 11b; 487 nm for 11c) of each
cation 11a -c was plotted against pH to give a classical
titration curve, whose midpoint was taken as the pKR+ value.
Cyclic Volta m m etr y of 10a -c a n d 11a -c. The reduction
potentials of 10a -c and 11a -c were determined by means of
CV-27 voltammetry controller (BAS Co). A three-electrode cell
was used, consisting of Pt working and counter electrodes and
a reference Ag/AgNO3 electrode. Nitrogen was bubbled through
an acetonitrile solution (4 mL) of each compound (0.5 mmol
L-1) and Bu4NClO4 (0.1 mol L-1) to deaerate it. The measure-
Gen er a l P r oced u r e of Au tor ecyclin g Oxid a tion of
Som e Alcoh ols a n d a m in Es by 11a ‚BF 4-. A CH3CN (16 mL)
-
solution of compound 11a ‚BF 4 (2.0 mg, 0.005 mmol) and an
alcohol or an amine (2.5 mmol, 500 equiv.) in a Pyrex tube
was irradiated by RPR-100, 350 nm lamps under aerobic
condition for 16 h. The reaction mixture was concentrated in
vacuo, and the residue was dissolved in Et2O and filtered. The
filtrate was treated with saturated 2,4-dinitrophenylhydrazine
in 6% HCl to give 2,4-dinytrophenylhydrazone. The results are
summarized in Table 4.
ments were made at
a
scan rate of 0.1 V s-1 and the
voltammograms were recorded on a WX-1000-UM-019 (Graph-
tec Co) X-Y recorder. Immediately after the measurements,
ferrocene (0.1 mmol) (E1/2 ) +0.083) was added as the internal
standard, and the observed peak potentials were corrected
with reference to this standard. The compounds exhibited no
reversible reduction wave: each of the reduction potentials
was measured through independent scan, and they are sum-
marized in Tables 2 and 3.
Ack n ow led gm en t. Financial support from a Wase-
da University Grant for Special Research Project and
21COE “Practical Nano-Chemistry” from MEXT, J apan,
is gratefully acknowledged. We thank the Materials
Characterization Central Laboratory, Waseda Univer-
sity, for technical assistance with the spectral data,
elemental analyses, and X-ray analysis.
-
Rea ction of 11a ‚BF 4 w ith Na BH4 [a n d Na BD4]. A
solution of 11a ‚BF 4- (0.5 mmol) and NaBH4 (19 mg, 0.5 mmol)
[or NaBD4 (20 mg, 0.5 mmol)] in CH3CN (10 mL) was stirred
at rt for 1 h. To the mixture was added saturated aqueous
NH4Cl solution, and the mixture was extracted with CH2Cl2.
The extract was dried over Na2SO4 and concentrated in vacuo
to give 17a (154 mg, 100%) [or 17a -D (155 mg, 100%)].
Oxid a tion of 17a . To a stirred solution of 17a (0.5 mmol)
in CH2Cl2 (5 mL) was added DDQ (176 mg, 0.75 mmol), and
the mixture was stirred at rt for 1 h. After evaporation of the
CH2Cl2, the residue was dissolved in a mixture of Ac2O (5 mL)
and 42% HBF4 (1 mL) at 0 °C, and the mixture was stirred
for another 1 h. To the mixture was added Et2O (50 mL), and
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of 10a -c, 11a -c‚BF 4-, 17a , 17a -D, 20a , and 21a .
HMQC and HMBC spectra of 10b and 11a ‚BF 4-. H-H COSY
spectra of 10c, 11a ‚BF 4-, 20a , and 21a . NOE spectra of
11a ‚BF 4-, 17a -D, and 20a . Analytical and spectroscopic data
of 10a -c, 11a -c‚BF 4-, 17a , 17a -D, 20a , and 21a . MO
calculation data of 11a . This material is available free of
charge via the Internet at http://pubs.acs.org.
-
the precipitates were collected by filtration to give 11a ‚BF 4
(179 mg, 91%).
J O035213W
J . Org. Chem, Vol. 68, No. 24, 2003 9291