Self-Assembly of Interlocked Supramolecular Dendrimers

Article abstract of DOI:10.1021/jo035798u

Interlocked supramolecular dendrimers were spontaneously self-assembled from molecular components, metallocycles, and dumbbells bearing benzyl ether repeating units. Here, the metallocycles were in situ self-assembled from L-shaped ligands with dendritic

Full text of DOI:10.1021/jo035798u

2a -c, of which the last two components possess dendritic
branches of benzyl ether repeating units (Scheme 1).
Rotaxanes can be divided into two distinct molecules,
a macrocycle and a dumbbell, having complementary
binding sites inside. As reported previously,7 the rotax-
anes can be self-assembled in solution when metallocycles
with reversible coordinative bonds are used. The metal-
locycles can be also prepared using various self-as-
sembling motifs, that is, combinations of transition
metals and ligands.8 With this in mind, we prepared
bispyridyl ligands 1a -c and dumbbells 2a -c for syn-
thesis of interlocked supramolecular dendrimers. Syn-
theses of 1a -c and 2a -c are outlined in Schemes 2 and
3, respectively. The benzyl ether dendritic branches were
prepared following a literature procedure9 and attached
to the positions remote from the internal hydrogen-
bonding sites, the 4-position of the pyridine in 1a -c and
the ends of 2a -c, to minimize possible steric crowding
upon their assembly.
Self-Assem bly of In ter lock ed
Su p r a m olecu la r Den d r im er s
Kyu-Sung J eong* and Eun-J in Park
Center for Bioactive Molecular Hybrids and Department of
Chemistry, Yonsei University, Seoul 120-749, Korea
ksjeong@yonsei.ac.kr
Received December 9, 2003
Abstr a ct: Interlocked supramolecular dendrimers were
spontaneously self-assembled from molecular components,
metallocycles, and dumbbells bearing benzyl ether repeating
units. Here, the metallocycles were in situ self-assembled
from L-shaped ligands with dendritic branches, 2,3-di-
methyl-2-butene and osmium tetraoxide. The supramolecu-
lar dendrimers were stabilized by hydrogen-bonding inter-
actions between the pyridine-2,6-dicarboxamide unit in the
metallocycle and the adipamide unit in the dumbbell.
Here, the interlocked dendrimers were assembled in
two different ways: direct self-assembly from the ligand,
olefin, OsO4, and the dumbbell in one pot and stepwise
assembly, self-assembly of the metallocycle first, followed
by the rotaxane.
Self-assembly processes have been proven a powerful
tool for construction of supramolecular dendrimers with
structural diversity over the past decade.1 Owing to the
directionality and strength, hydrogen bonds are particu-
larly useful for the self-assembly of smaller molecular
components to form the supramolecular dendrimers in a
programmed way. Since the pioneering work reported by
Zimmerman,2 several examples of the self-assembled
supramolecular dendrimers mediated by hydrogen-bond-
ing interactions have been reported to date.3-5 Here, we
report on the preparation and kinetic and thermodynamic
stabilities of rotaxane-type dendrimers5,6 that were spon-
taneously assembled by combination of osmylation, co-
ordination, and hydrogen bonding between four different
components, 2,3-dimethyl-2-butene, osmium tetraoxide,
bispyridyl ligands 1a -c, and dumbbell-shaped molecules
The direct self-assembly was investigated using 1H
NMR spectroscopy (Figure 1a-d). When a ligand 1a and
2,3-dimethylbut-2-ene were added to a CDCl3 solution of
a dumbbell 2a , the 1H NMR spectral change of each
compound was negligible (Figure 1c), implying that there
is no appreciable aggregation between these components.
However, two sets of new signals spontaneously appeared
when osmium tetraoxide was added to the above solution
(Figure 1d). One originates from the metallocycle 3a , self-
(5) For pseudorotaxane-based supramolecular dendrimers, see: (a)
Yamaguchi, N.; Hamilton, L. M.; Gibson, H. W. Angew. Chem., Int.
Ed. 1998, 37, 3275-3279. (b) Gibson, H. W.; Hamilton, L., Yamaguchi,
N. Polym. Adv. Technol. 2000, 11, 791-797. (c) Gibson, H. W.;
Yamaguchi, N.; Hamilton, L.; J ones, J . W. Am. Chem. Soc. 2002, 124,
4653-4665. (d) Elizarov, A. M.; Chang, T.; Chiu, S.-H.; Stoddart, J . F.
Org. Lett. 2002, 4, 3565-3568. (e) Dykes, G. M.; Smith, D. K.; Seeley,
G. J . Angew. Chem., Int. Ed. 2002, 41, 3254-3257. (f) J ones, J . W.;
Bryant, W. S.; Bosman, A. W.; J anssen, R. A. J .; Meijer, E. W.; Gibson,
H. W. J . Org. Chem. 2003, 68, 2385-2389. Also see: (g) Kenda, B.;
Diederich, F. Angew. Chem., Int. Ed. 1998, 37, 3154-3158. (h) Lee, J .
W.; Ko, Y. H.; Park, S.-H.; Yamaguchi, K.; Kim, K. Angew. Chem., Int.
Ed. 2001, 40, 746-749.
(6) For rotaxanes bearing dendritic branches, see: (a) Amabilino,
D. B.; Ashton, P. R.; Balzani V.; Brown, C. L.; Credi, A.; Fre´chet, J .
M. J .; Leon, J . W.; Raymo, F. M.; Spencer, N.; Stoddart, J . F.; Venturi,
M. J . Am. Chem. Soc. 1996, 118, 12012-12020. (b) Hu¨bner, G. M.;
Nachtsheim, G.; Li, Q. Y.; Seel, C.; Vo¨gtle, F. Angew. Chem., Int. Ed.
2000, 39, 1269-1272. (c) Reuter, C.; Pawlitzki, G.; Wo¨rsdo¨rfer, U.;
Plevoets, M.; Mohry, A.; Kubota, T.; Okamoto, Y.; Vo¨gtle, F. Eur. J .
Org. Chem. 2000, 3059-3067. (d) Elizarov, A. M.; Chiu, S.-H.; Glink,
P. T., Stoddart, J . F. Org. Lett. 2002, 4, 679-682.
* To whom correspondence should be addressed. Fax: (+) 82-2-364-
7050. Tel: (+) 82-2-2123-2643.
(1) For reviews, see: (a) Zeng, F.; Zimmerman, S. C. Chem. Rev.
1997, 97, 1681-1712. (b) Balzani, V.; Campagna, S.; Denti, G.; J uris,
A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26-34. (c)
Narayanan, V. V.; Newkome, G. R. Top. Curr. Chem. 1998, 197, 19-
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10.1021/jo035798u CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/03/2004
2618
J . Org. Chem. 2004, 69, 2618-2621

SCHEME 1. Syn th esis of Rota xa n e-Typ e Den d r im er s
SCHEME 2. Syn th esis of Bisp yr id yl Liga n d s 1a -ca
SCHEME 3. Syn th esis of Du m bbells 2a -ca
a
Reaction conditions: (i) Den-Br a -c, NaH, THF, reflux, 80-
86%; (ii) CBr4, PPh3, THF, rt, 68-72%; (iii) phthalimide, K2CO3,
DMF, 60-70 °C, then NH2NH2, EtOH, reflux, 75-85%; (iv) adipoyl
chloride, DIPEA, CH2Cl2, rt, 50-82%.
shifted (∆δ ) 1.5 ppm) relative to those of the free 3a ,
indicative of hydrogen-bond formation. Second, the sig-
nals for the methylene units -(CH2)4- of the adipamide
stations appear at 0.9 and 0.3 ppm, while those in the
free 2a resonance at 2.1 and 1.3 ppm. These large upfield
shifts are clear evidence for insertion of the adipamide
part into the cavity surrounded by four pyridyl rings.
These observations are consistent with those seen in the
metallocycle-based rotaxanes studied previously in our
laboratory.7a-d
On the other hand, the dendrimer 4a can be prepared
by the stepwise assembly using the preassembled met-
allocycle 3a and the dumbbell 2a . When two components
were mixed in component in CDCl3 at 24 °C, the complex
4a was immediately formed by hydrogen-bond inter-
a
Reaction conditions: (i) Den-Br a -c, K2CO3, acetone, 18-
1
actions (Figure 1f). The H NMR spectrum (Figure 1f) is
identical to that (Figure 1d) obtained from the direct self-
crown-6, reflux, 75-93%; (ii) LiOH, THF, H2O, reflux, 90-95%;
(iii) (COCl)2, DMF, CH2Cl2, rt, argon, then 4-amino-3,5-lutidine,
DIPEA, CH2Cl2, rt, argon, 50-83%.
(10) (a) J eong, K.-S.; Cho, Y. L.; Song, J . U.; Chang, H.-Y.; Choi,
M.-G. J . Am. Chem. Soc. 1998, 120, 10982-10983. (b) J eong, K. S.;
Cho, Y. L.; Chang, S.-Y.; Park, T.-Y.; Song, J . U. J . Org. Chem. 1999,
64, 9459-9466. (c) Cho, Y. L.; Uh, H.; Chang, S.-Y.; Chang, H.-Y.; Choi,
M.-G.; Shin, I.; J eong, K.-S. J . Am. Chem. Soc. 2001, 123, 1258-1259.
(11) For hydrogen bond-based rotaxanes derived from covalent
tetralactams, see: (a) Schalley, C. A.; Beizai, K.; Vo¨gtle, F. Acc. Chem.
Res. 2001, 34, 465-476. (b) Gatti, F. G.; Leigh, D. A.; Nepogodiev, S.
A.; Slawin, A. M. Z. Teat, S. J .; Wong, J . K. Y. J . Am. Chem. Soc. 2001,
123, 5983-5989. (c) Altieri, A.; Gatti, F. G.; Kay, E. R.; Leigh, D. A.;
Martel, D.; Paolucci, F.; Slawin, A. M. Z.; Wong, J . K. Y. J . Am. Chem.
Soc. 2003, 1253, 8644-8654 and references are therein.
assembled in situ from osmium tetraoxide, 2,3-dimeth-
ylbut-2-ene, and the ligand 1a . This was confirmed by
comparison of the 1H NMR spectrum of an authentic
sample prepared independently (Figure 1e).10 The other
corresponds to the interlocked dendrimer 4a stabilized
by hydrogen-bonding interactions11 between 2a and 3a
as shown in Scheme 1. Evidence is as follows. First, the
NH signal in the metallocycle is considerably downfield
J . Org. Chem, Vol. 69, No. 7, 2004 2619

EXSY experiments.12 The dendrimers 4a -c all are
reversibly converted into their components, a metallo-
cycle and a dumbbell, at ambient temperature. The
activation free energies of the interconversion were
determined to be 15.9-16.8 kcal/mol (Table 1), based on
diagonal and cross-peaks for the NH signals of the
complex and the free metallocycle. The activation ener-
gies are slightly increased in the dendrimers with higher
generations, probably due to an increased steric hin-
drance on the assembly and disassembly process. Overall,
the magnitudes are comparable to that (15.5 kcal/mol)
of the interlocked complexes reported previously.7a-c
In conclusion, we have demonstrated the self-assembly
of interlocked dendrimers from smaller components by
combination of osmylation, coordination, and hydrogen
bonds. Considering that various processes for self-as-
sembly of metallocycles are well-known, this approach
can be applied to the preparation of supramolecular
functional dendrimers that contain transition metals
with diverse electrochemical or photochemical properties.
Exp er im en ta l Section
Substituted benzyl bromides with different generations of
benzyl ether dendritic branches were prepared according to
literature procedures.9 Ligands 1a -c and dumbbells 2a -c were
prepared as shown in Schemes 2 and 3, and experimental details
have been described in the Supporting Information.
1a : mp 118-120 °C; 1H NMR (500 MHz, CDCl3) δ 9.79 (s,
2H, NH), 8.25 (s, 4H), 8.05 (s, 2H), 7.42-7.32 (m, 10H), 6.66 (s,
2H), 6.61 (s, 1H), 5.21 (s, 2H, ArCH2O), 5.05 (s, 4H, PhCH2O),
2.21 (s, 12H, Py-CH3); 13C NMR (126 MHz, CDCl3) δ 168.2, 161.7,
160.7, 150.8, 149.5, 142.6, 137.4, 136.9, 130.9, 129.0, 128.5, 127.9,
113.0, 106.7, 102.5, 71.0, 70.6, 15.9; IR (KBr) 3400-3200, 1686
cm-1; MS (MALDI-TOF) m/z 694.3 [M + H]+. Anal. Calcd for
F IGURE 1. 1H NMR spectra (500 MHz, CDCl3, 24 °C) of (a)
ligand 1a , (b) dumbbell 2a , (c) 1a + 2a + 2,3-dimethylbut-2-
ene, (d) 1a + 2a + 2,3-dimethylbut-2-ene + OsO4, (e) metal-
locycle 3a , (f) 3a + 2a . b: complex 4a . O: Free 2a and 3a .
TABLE 1. Associa tion Con sta n ts (Ka , M-1) a n d
Activa tion En er gy (∆Gq, k ca l m ol-1) of th e In ter lock ed
Den d r im er s 4a , 4b, a n d 4c a t 24 ( 1 °C in CDCl3
C
42H39N5O5: C, 72.71; H, 5.67; N, 10.09. Found: C, 72.69; H,
5.60; N, 10.10.
1b: mp 112-114 °C; 1H NMR (500 MHz, CDCl3) δ 9.57 (s,
2H, NH), 8.32 (s, 4H), 8.09 (s, 2H), 7.41-7.31 (m, 20H), 6.70 (br
s, 4H), 6.67 (br s, 2H), 6.60 (br s, 3H), 5.25 (s, 2H, ArCH2O),
5.05 (s, 8H, PhCH2O), 5.00 (s, 4H, ArCH2O), 2.26 (s, 12H, Py-
CH3); 13C NMR (126 MHz, CDCl3) δ 167.9, 161.0, 160.2 (two
overlapped signals), 150.4, 149.1, 142.0, 138.9, 136.9, 136.7,
130.1, 128.5, 128.0, 127.5, 112.6, 106.3 (two overlapped signals),
102.1, 101.6, 70.7, 70.1 (two overlapped signals), 15.5; IR (KBr)
3400-3200, 1686 cm-1; MS (MALDI-TOF) m/z 1118.5 [M + H]+.
Anal. Calcd for C70H63N5O9: C, 75.18; H, 5.68; N, 6.26. Found:
C, 75.32; H, 5.65; N, 6.09.
dendrimer
(molar mass)
association constant
(Ka, M-1
activation energy
)
(∆Gq, kcal/mol)
4a (3047)
4b (4776)
4c (8172)
2800 ( 350
1200 ( 150
600 ( 50
15.9
16.2
16.8
assembly. The relative intensities of the signals corre-
sponding to the complex and its components depend on
concentrations of the components; the signals for the
complex increase at higher concentrations, but they
decrease at lower concentrations. The binding affinity
between two components, 2a and 3a , was measured to
be 2800 ( 350 M-1, based on 1H NMR integrations of
five different stock solutions containing 1-5 mM of each
component (see the Supporting Information).
The other interlocked dendrimers 4b and 4c with
higher generations of dendritic branches were also self-
assembled by two different processes from the corre-
sponding components, respectively, and the spectral
behaviors are exactly the same as those described above
in the self-assembly of 4a (see the Supporting Informa-
tion). However, the binding affinities decrease as the
generation of the branches increases, as summarized in
Table 1. The reduction in the affinities is attributed to
the increased steric crowding between dendritic branches
in both the metallocycle and the dumbbell.
1
1c: mp 96-98 °C; H NMR (500 MHz, CDCl3) δ 9.49 (s, 2H,
NH), 8.30 (s, 4H), 8.05 (s, 2H), 7.39-7.28 (m, 40H), 6.66-6.54
(m, 21H), 5.18 (s, 2H, ArCH2O), 5.00 (s, 16H, PhCH2O), 4.96 (s,
12H, ArCH2O) 2.25 (s, 12H, Py-CH3); 13C NMR (126 MHz,
CDCl3) δ 167.9, 160.9, 160.1 (two overlapped signals), 150.3,
149.2, 141.8, 139.1, 138.9, 136.7, 129.9, 128.5, 127.9, 127.5, 112.6,
106.3 (two overlapped signals), 101.5 (three overlapped signals),
70.1 (three overlapped signals), 15.5; IR (KBr) 3400-3200, 1686
cm-1; MS (MALDI-TOF) m/z 1967.0 [M + H]+. Anal. Calcd for
C126H111N5O17: C, 76.93; H, 5.69; N, 3.56. Found: C, 76.92; H,
5.60; N, 3.30.
2a : mp 82-84 °C; 1H NMR (500 MHz, CDCl3) δ 7.42-7.30
(m, 20H), 6.60 (s, 4H), 6.54 (s, 2H), 6.11 (s, 2H, NH), 5.02 (s,
8H, PhCH2O), 4.50 (s, 4H, ArCH2O), 3.64-3.62 (m, 16H), 3.52
(t, J ) 8.7 Hz, 4H), 3.41 (t, J ) 8.7 Hz, 4H), 2.14 (br s, 4H), 1.34
(br s, 4H); 13C NMR (126 MHz, CDCl3) δ 172.7, 160.0, 140.6,
136.8, 128.5, 127.9, 127.5, 106.5, 101.2, 73.0, 70.5, 70.1, 70.0,
69.7, 69.3 (5 peaks of 6 expected between 69.3 and 70.5), 39.1,
(12) (a) Perrin, C. L.; Dwyer, T. J . Chem. Rev. 1990, 90, 935-967.
(b) Pons, M.; Millet, O. Prog. Nucl. Magn. Reson. Spectrosc. 2001, 38,
267-324.
The kinetic stabilities of the self-assembled, supra-
molecular dendrimers 4a -c were revealed with 2D-
2620 J . Org. Chem., Vol. 69, No. 7, 2004

36.0, 25.0; IR (KBr) 3334, 1669 cm-1; MS (MALDI-TOF) m/z
1013.4 [M + H]+. Anal. Calcd for C60H72N2O12: C, 71.12; H, 7.16;
N, 2.76. Found: C, 71.13; H, 7.06; N, 2.66.
NMR (126 MHz, CDCl3) δ 168.4, 160.3, 160.0, 149.7, 148.0, 145.7,
136.6, 136.4, 131.0, 128.6, 128.1, 127.5, 112.9, 106.2, 102.1, 90.1,
70.8, 70.2, 24.5, 16.2; IR (KBr) 3283, 1694, 829 cm-1. Anal. Calcd
for C96H102N10O18Os2: C, 55.85; H, 4.98; N, 6.79. Found: C,
55.89; H, 4.79; N, 6.83.
2b: colorless oil; 1H NMR (500 MHz, CDCl3) δ 7.42-7.32 (m,
40H), 6.68 (br s, 8H), 6.59(s, 4H), 6.52 (br s, 6H), 6.07 (s, 2H,
NH), 5.03 (s, 16H, PhCH2O), 4.96 (s, 8H, ArCH2O), 4.50 (s, 4H,
ArCH2O), 3.64-3.61 (m, 16H), 3.51 (m, 4H), 3.40 (m, 4H), 2.12
(br s, 4H), 1.61 (br s, 4H); 13C NMR (126 MHz, CDCl3) δ 172.6,
160.1, 159.9, 140.6, 139.2, 136.7, 128.5, 127.9, 127.5, 106.5, 106.3,
101.4, 101.2, 73.0, 70.5, 70.0, 69.9, 69.8, 69.3 (5 peaks of 7
expected between 69.3 and 70.5), 39.0, 36.0, 25.0; IR (film) 3354,
1664 cm-1; MS (MALDI-TOF) m/z 1899.8 [M + K]+. Anal. Calcd
for C116H120N2O20: C, 74.82; H, 6.50; N, 1.50. Found: C, 74.89;
H, 6.59; N, 1.49.
3b: 1H NMR (500 MHz, CDCl3) δ 9.13 (s, 4H, NH), 8.64 (s,
8H), 8.05 (s, 4H), 7.40-7.31 (m, 40H), 6.67 (br s, 8H), 6.61 (br s,
4H), 6.61 (br s, 6H), 5.23 (s, 4H, ArCH2O), 5.03 (s, 16H,
PhCH2O), 4.99 (s, 8H, ArCH2O), 2.23 (s, 24H, Py-CH3), 1.50 (s,
24H, osmate-CH3); 13C NMR (126 MHz, CDCl3) δ 168.4, 161.0,
160.2 (two overlapped signals), 149.8, 148.0, 145.7, 139.0, 136.7,
131.0, 128.5, 128.0, 127.5, 112.9, 106.3 (two overlapped signals),
102.2, 101.7, 90.1, 70.1 (three overlapped signals), 24.5, 16.2;
IR (KBr) 3304, 1695, 829 cm-1. Anal. Calcd for C152H150N10O26
-
2c: colorless oil; 1H NMR (500 MHz, CDCl3) δ 7.37-7.26 (m,
80H), 6.65 (br s, 24H), 6.57-6.51 (m, 18H), 6.02 (s, 2H, NH),
5.06 (s, 32H, PhCH2O), 4.99 (s, 24H, ArCH2O), 4.46 (s, 4H,
ArCH2O), 3.57 (m, 16H), 3.47-3.45 (m, 4H), 3.39-3.35 (m, 4H),
2.10 (br s, 4H), 1.57 (br s, 4H); 13C NMR (126 MHz, CDCl3) δ
173.1, 160.1 (two overlapped signals), 160.0, 140.7, 139.2 (two
overlapped signals), 136.7, 128.5, 128.0, 127.5, 106.4 (three
overlapped signals), 101.6 (two overlapped signals), 101.2, 73.0,
70.5, 70.1, 69.9, 69.6, 69.4 (5 peaks of 8 expected between
Os2: C, 62.66; H, 5.19; N, 4.81. Found: C, 62.68; H, 5.13; N,
4.74.
3c: 1H NMR (500 MHz, CDCl3) δ 9.09 (s, 4H, NH), 8.65 (s,
8H), 8.01 (s, 4H), 7.53-7.29 (m, 80H), 6.65-6.53 (m, 42H), 5.17
(s, 4H, ArCH2O), 4.98-4.95 (m, 56 H, ArCH2O and PhCH2O),
2.19 (s, 24H, Py-CH3), 1.50 (s, 24H, osmate-CH3); 13C NMR (126
MHz, CDCl3): δ 168.3, 160.1 (four overlapped signals), 149.6,
147.9, 145.7, 139.1, 138.9, 136.7 (two overlapped signals), 131.0,
128.5, 127.9, 127.5, 112.8, 106.4 (two overlapped signals), 106.2,
102.1, 101.5, 90.1, 70.0 (four overlapped signals), 24.5, 16.1; IR
(KBr) 3314, 1694, 829 cm-1. Anal. Calcd for C264H246N10O42Os2:
C, 68.76; H, 5.38; N, 3.04. Found: C, 68.96; H, 5.42; N, 2.92.
69.4 and 70.5), 39.3, 35.7, 24.9; IR (film) 3339, 1670 cm-1
;
MS (MALDI-TOF) m/z 3583.0 [M + K]+. Anal. Calcd for
C228H216N2O36: C, 76.92; H, 6.12; N, 0.79. Found: C, 76.92; H,
6.26; N, 0.63.
Ack n ow led gm en t. This work was financially sup-
ported by Korea Institute of Science & Technology
Evaluation and Planning (M10213030001-02B1503-
00310) and KOSEF-CBMH.
Gen er a l P r oced u r e for P r ep a r a t ion of Met a llocycles
3a -c. These compounds were all prepared by the same method,
and synthesis of 3a will be here described as a representative.
Osmium tetraoxide (0.50 M in CH2Cl2, 360 µL, 0.18 mmol) was
added to a CH2Cl2 solution of ligand 1a (84 mg, 0.12 mmol) and
2,3-dimethylbut-2-ene (15 mg, 0.18 mmol). After the solution was
stirred for 40 min at room temperature, the brown precipitate
was collected, washed with diethyl ether, and dried under
vacuum to give 3a (110 mg, 92%) as a gray solid, which was
slowly decomposed on heating: 1H NMR (500 MHz, CDCl3) δ
9.14 (s, 4H, NH), 8.64 (s, 8H), 8.05 (s, 4H), 7.41-7.33 (m, 20H),
6.64 (s, 4H), 6.61 (s, 2H), 5.24 (s, 4H, ArCH2O), 5.05 (s, 8H,
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures of 1a -c and 2a -c, physical properties and spectro-
scopic data of their intermediates, 1H NMR studies on self-
assembly of 4b and 4c, and determination of binding affinities
and 2D-EXSY experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
PhCH2O), 2.24 (s, 24H, Py-CH3), 1.49 (s, 24H, osmate-CH3); 13
C
J O035798U
J . Org. Chem, Vol. 69, No. 7, 2004 2621