Mechanism of 2-O→3-O silyl migration in cyclomaltohexaose (α-cyclodextrin)

Article abstract of DOI:10.1016/S0040-4039(03)01149-3

To gain insight into the mechanism of the well-known 2-O→3-O silyl migration of 2-O-silylated cyclomaltooligosaccharides (cyclodextrins) under basic conditions, we have undertaken studies of the reaction of 2^A-O-TBDMSi-6^A-deoxy-6^A-S-phenyl-α- cyclodextrin with MeI and NaH. Under these conditions, the TBDMSi group on the C-2 oxygen was found to migrate onto the C-3 oxygen in the glucose residue in which the C-2 oxygen is located, and not onto the C-3 oxygen in the adjacent glucose residue.

Full text of DOI:10.1016/S0040-4039(03)01149-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4843–4848
Mechanism of 2-O“3-O silyl migration in cyclomaltohexaose
(a-cyclodextrin)
Katsunori Teranishi* and Fumiko Ueno
Faculty of Bioresources, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan
Received 12 March 2003; revised 28 April 2003; accepted 2 May 2003
Abstract—To gain insight into the mechanism of the well-known 2-O“3-O silyl migration of 2-O-silylated cyclomaltooligosaccha-
rides (cyclodextrins) under basic conditions, we have undertaken studies of the reaction of 2A-O-TBDMSi-6A-deoxy-6A-S-phenyl-
a-cyclodextrin with MeI and NaH. Under these conditions, the TBDMSi group on the C-2 oxygen was found to migrate onto the
C-3 oxygen in the glucose residue in which the C-2 oxygen is located, and not onto the C-3 oxygen in the adjacent glucose residue.
© 2003 Elsevier Science Ltd. All rights reserved.
Cyclomaltooligosaccharides (cyclodextrins) are cyclic
oligosaccharides, typically consisting of six, seven, or
Figure 1, Stoddart et al.5a proposed intra-glucosidic
2-O“3-O silyl migration mechanism (Route A),
whereas Konig et al.5c suggested possibility of inter-
glucosidic 2-O“3-O silyl migration (Route B). In the
cases where partially 2-O-silylated cyclodextrins are
employed for the 2-O“3-O silyl migration, it is
extremely important to predict to which C-3 oxygens
eight a-1,4-linked
D
-(+)-glucopyranose units (a-, b-,
and g-cyclodextrins, respectively), forming cavities that
are hydrophobic and optically active. Each glucopyran-
ose unit possesses a primary hydroxyl group at the C-6
position and secondary hydroxyl groups at the C-2 and
C-3 positions.1 Because of the commercial availability
of these cyclodextrins, they are widely utilized in the
formation of inclusion complexes with various guest
molecules, as transporters of hydrophobic molecules, or
as building blocks for molecular mimics of enzymes.2 In
these cyclodextrins, the secondary hydroxyl faces,
which are larger than the primary hydroxyl faces, serve
as the preferential locus for the molecular inclusion of
large molecules.3 Accordingly, cyclodextrin derivatives
that have modified secondary hydroxyl faces often pos-
sess significantly different properties than those that
have their primary hydroxyl faces modified,4 and conse-
quently, functionalization of the secondary hydroxyl
faces have been further investigated in order to enhance
the utility of cyclodextrins. During these investigations,
2-O“3-O silyl migration of 2-O-silyl-cyclodextrins
under basic conditions was observed,5 which was subse-
quently employed as a method for the silyl protection
of C-3 hydroxyl groups.5a,d,6 Although the understand-
ing of silyl migration is important towards the prepara-
tion of various 3-O-silyl-cyclodextrins, the mechanism
of the migration has remained unclear. As shown in
Keywords: cyclodextrin; silyl migration.
* Corresponding author. Tel.: +81-59-231-9615; fax: +81-59-231-9615;
e-mail: teranisi@bio.mi-u.ac.jp
Figure 1. Proposed mechanisms for intra-glucosidic (route A)
and inter-glucosidic (route B) 2-O“3-O silyl migration.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01149-3

4844
K. Teranishi, F. Ueno / Tetrahedron Letters 44 (2003) 4843–4848
the silyl groups will migrate. Herein we report on our
studies on mono-2-O-silyl-a-cyclodextrin, which pro-
vided valuable clues to describe the mechanisms of
2-O“3-O silyl migration.
regioisomers—the desired 2A-O-TBDMSi-6A-O-(p-
toluenesulfonyl)-a-cyclodextrin (2), three regioisomers
(3–5), and a mixture of two regioisomers (6 and 7) were
readily separated by open reverse-phase column chro-
matography (15×150 mm, Fuji Silisia Chromatorex-
ODS DM1020T gel, 10–50% aqueous MeOH), followed
by preparative HPLC (20×250 mm, Cosmosil 5C18-MS
column, 50–60% aqueous MeOH). The regiochemistries
of the TBDMSi and p-toluenesulfonyl groups were
Recently, as part of our studies on the regioselective
chemical modification of the C-2 hydroxyl groups of
cyclodextrins,7 the regioselective mono-silylation of the
C-2 hydroxyl group of a-cyclodextrin was successfully
1
accomplished
by
using
tert-butyldimethylsilyl
determined by H NMR spectroscopy (Fig. 2).
(TBDMSi) imidazole in the presence of molecular
sieves in N,N-dimethylformamide (DMF).8 The
regioselective silylation in affording 2-O-silylated
cyclodextrins has proven to be valuable since the free
C-6 hydroxyl groups can be subsequently modified
without a deprotection step, since 6-O-silyl protecting
groups are typically employed for 2-O-silylations.5a,d,9
Consequently, this improved methodology allowed the
introduction of a substituent as a ‘marker’ on the C-6
hydroxyl group of mono-2-O-TBDMSi-a-cyclodextrin
(1)8 for the mechanistic studies of the 2-O“3-O silyl
migration of 2-O-silyl cyclodextrins.
1
The H NMR spectrum of 1 (5% D2O in DMSO-d6;
60°C; ref., DMSO, l 2.49 ppm) exhibited a signal
corresponding to one H-2 proton of the silylated glu-
cose residue at l 3.50 ppm, which was downfield from
those of the five remaining H-2 protons (l 3.26–3.33
ppm).8a In contrast, the 1H NMR spectrum (similar
conditions as above) of mono-6-O-(p-toluenesulfonyl)-
a-cyclodextrin10 showed a signal for the H-2 proton of
the sulfonylated glucose residue at l 3.20 ppm, which
was upfield from those of the five remaining H-2 pro-
tons (l 3.25-3.31 ppm).11 The H NMR properties of 1
1
and mono-6-O-(p-toluenesulfonyl)-a-cyclodextrin sug-
gested that when one C-2 hydroxyl group and one C-6
hydroxyl group on the same glucose residue of a-
cyclodextrin are simultaneously silylated and p-toluene-
Initially, as shown in Scheme 1, 2A-O-TBDMSi-6A-O-
(p-toluenesulfonyl)-a-cyclodextrin (2) was prepared in a
5.4% yield by reaction of 1 (1.0 g, 0.921 mmol) with
p-toluenesulfonyl chloride (0.263 g, 1.38 mmol) in pyri-
dine at 0°C for 2 h; these conditions are typically
employed for the sulfonylation of the C-6 hydroxyl
groups of cyclodextrins.10 As expected, this sulfonyl
reaction resulted in the formation of six 6-O-sulfonyl
1
sulfonylated, respectively, the H NMR signal of the
H-2 proton of the sulfonylated glucose residue should
be affected by a combination of the downfield-shift
effect by the silyl group and the upfield-shift effect by
the sulfonyl group, and should be present at around l
Scheme 1.

K. Teranishi, F. Ueno / Tetrahedron Letters 44 (2003) 4843–4848
4845
1
Figure 2. Partial H NMR spectra of 2–7 (5%D2O in DMSO-d6; ref., DMSO, l 2.49 ppm). The assigned signals are numbered
according to the usual convention shown in Scheme 1, and the symbols * and ** refer to the 2-O-silylated glucose residue and
the 6-O-sulfonylated glucose residue, respectively. In case of 2, the symbol * refers to the 2-O-silylated and 6-O-sulfonylated
1
1
1
1
glucose residue. [a] H NMR of 2 at 80°C;12 [b] H NMR of 3 at 60°C; [c] H NMR of 4 at 60°C; [d] H NMR of 5 at 60°C;
1
[e] H NMR of a mixture of 6 and 7 at 60°C.
3.50 ppm as the result of only the downfield-shift effect
by the silyl group, whereas the H-2 proton of the
sulfonylated glucose residue should be expected at
around l 3.20 ppm as the result of only the upfield-shift
effect by the sulfonyl group.
3.40 ppm. The five remaining H-2 protons should be at
around l 3.25–3.33 ppm. On the other hand, if the
2-O-silyl group and the 6-O-sulfonyl group are located
on different glucose residues, the H-2 proton of the
silylated glucose residue should be expected at around l

4846
K. Teranishi, F. Ueno / Tetrahedron Letters 44 (2003) 4843–4848
1
THF and DMF (10:1)15 at 0–20°C, as shown in Scheme
1. A ratio of the resulting crude products, methylated
3A-O-TBDMSi-6A-deoxy-6A-S-phenyl-a-cyclodextrin
(9)17 and methylated 2A-O-TBDMSi-6A-deoxy-6A-S-
phenyl-a-cyclodextrin (10)18 was determined by HPLC
The H NMR spectra of 2–7, as shown in Figure 2,
include partial spectral assignments of 2–7 using H–1H
1
COSY and HOHAHA experiments. As shown in Fig-
ure 2(a),12 the H NMR spectrum of 213 exhibited an
1
appreciable downfield-shift of one H-2 proton (l 3.38
ppm). HOHAHA experiments showed a correlation
between this proton and the H-6 protons that are
located at the sulfonylated C-6 position, indicating that
the structure of 2 is 2A-O-TBDMSi-6A-O-(p-toluenesul-
fonyl)-a-cyclodextrin (the glucose residues are desig-
nated using letters A through F, clockwise, as viewed
1
and H NMR as 4.5:1 and 4.3:1, respectively, and then
these products were isolated using preparative HPLC
(20×250 mm, Cosmosil 5C18-MS column, 80% aqueous
EtOH), in 56 and 7.8% yields, respectively. The struc-
1
tures of 9 and 10 were confirmed by H and 13C NMR
1
spectroscopy. As shown in Figure 3(a), the H NMR
1
from the primary hydroxyl face). Furthermore, the H
NMR characteristics of the H-2 proton of 2 were in
agreement to the expected H NMR chemical shift of
spectrum of 9, which were assigned using 1H–1H
COSY, HOHAHA 13C–1H COSY, and DEPT experi-
ments, showed a significant downfield-shift of the H-3
proton (l 4.13 ppm) of the 6-phenylthiolated glucose
residue relative to the H-3 protons (l 3.52–3.63 ppm) of
other glucose residues. Furthermore, the H-2 proton (l
3.12 ppm) of the 6-phenylthiolated glucose residue
showed a slight upfield-shift against the remaining H-2
protons (l 3.13–3.20 ppm), indicating that the TBDMSi
group was not located on the C-2 oxygen of the 6-
phenylthiolated glucose residue. If the silyl group was
located on the C-2 oxygen, the 1H NMR spectrum
should show a downfield shift of the H-2 proton of the
2-O-silylated glucose residue against the remaining H-2
1
the H-2 proton that was described in the above section.
1
The H NMR spectra of 3–7, as shown in Figure 2(b)
and (e), respectively, showed characteristic signals for
the H-2 proton at around l 3.20 ppm and for the H-2
protons at around l 3.50 ppm. The results of the
HOHAHA experiments indicated that the former pro-
tons corresponded to the 6-O-sulfonylated glucose
residues, and therefore, the latter protons can be
assigned as the H-2 protons of the 2-O-silylated glucose
1
residues, as expected from the H NMR chemical shifts
of the H-2 proton as described in the above section.
The results of these studies indicated that 3–7 do not
possess TBDMSi and sulfonyl groups in the same glu-
cose residue of the cyclodextrin molecules. Detailed
regiochemistries of 4–7 were not obtained using NMR
spectroscopic studies. However, the assignment of 3 as
2A-O-TBDMSi-6F-O-(p-toluenesulfonyl)-a-cyclodextrin
was accomplished using 2D ROESY NMR technique,
which indicated a cross-peak between the H-4 proton of
the 6-O-sulfonylated glucose residue and the H-1 pro-
ton of the 2-O-silylated glucose residue.
1
proton. These H NMR characteristics of the H-2 and
H-3 protons of the 6-phenylthiolated glucose residue of
9 indicated that the TBDMSi group on the C-2 oxygen
of the 6-phenylthiolated glucose residue of 8 migrated
onto the C-3 oxygen of the same glucose residue. As
1
shown in Figure 3(c), the H NMR spectrum of 10,
which was assigned using 1H-1H COSY, HOHAHA
13C–1H COSY, and DEPT experiments, exhibited i)
appreciable downfield-shift of the H-2 proton (l 3.57
ppm) of the 6-phenylthiolated glucose residue relative
to the remaining H-2 protons (l 3.12–3.20 ppm) and ii)
an insignificant shift of the H-3 proton (approximately
3.50 ppm) of the 6-phenylthiolated glucose residue rela-
tive to the remaining H-3 protons (l 3.50–3.65 ppm).
These results indicated that the TBDMSi group was
located on the C-2 oxygen of the 6-phenylthiolated
glucose residues of 10. The 13C NMR spectra of 9 and
10, as shown in Figure 3(b) and (d), respectively, which
was assigned using 13C–1H COSY and DEPT experi-
ments, showed significant upfield-shifts of one C-3 car-
bon of 9 and one C-2 carbon of 10 relative to the other
carbons of 9 and 10, thus supporting that the TBDMSi
group was located on the C-3 oxygen of 9 or on the C-2
oxygen of 10. These results undoubtedly demonstrated
that the TBDMSi group on the C-2 oxygen of 8
rearranged onto the C-3 oxygen via intra-glucosidic
silyl migration (Fig. 1, Route A), and not via inter-glu-
cosidic silyl migration (Fig. 1, Route B), which affords
3A -O-TBDMSi-6B -deoxy-6B -S-phenyl-a-cyclodextrin
(11) (Scheme 1). In the case of the present methylation
of 8 under the basic condition, HPLC, NMR, and TLC
techniques showed that only 9 and 10 were afforded as
the methylated products, without other methylated O-
TBDMSi-6-S-phenyl-a-cyclodextrins, thus indicating
that the silyl migration mechanism for 8 must occur
through intra-glucosidic silyl migration.
Kaneda et al. have reported the successful methylation
of mono-6-O-(p-toluenesulfonyl)-cyclodextrins under
basic conditions using MeI and NaH in DMF without
the cleavage of the p-toluenesulfonyl group.14 In this
present study, 2 was treated with MeI and NaH in a
mixture of THF and DMF (5:1) at 0–20°C, resulting in
the cleavage of the p-toluenesulfonyl group as the
‘marker’, which can be attributed to the high propor-
tion of THF in the reaction solvent.15 Since the p-tolu-
enesulfonyl group cannot function as the ‘marker’
under basic conditions, 2 was consequently derivatized
as 2A-O-TBDMSi-6A-deoxy-6A-S-phenyl-a-cyclodextrin
(8)16 in a 91% yield by treatment with NaSPh (2.0
equiv.) in DMSO under argon gas (Scheme 1). The
structure of 8 was determined by 1H NMR, which
exhibited a significant downfield-shift of one H-2 pro-
ton (l 3.55 ppm) against the remaining H-2 protons (l
3.26–3.40 ppm), indicating that the TBDMSi group did
not migrate to the C-3 position. Additionally, treatment
of compound 8 with NaH without MeI in a mixture of
THF and DMF (10:1) afforded only 8, indicating that
the 2-O-TBDMSi group is unable to migrate in the
basic condition without MeI and supporting that the
TBDMSi group of 2 did not migrate to other hydroxyl
groups in the reaction with NaSPh that is more basic-
less than NaH. Subsequently, the silyl migration of 8
was investigated using MeI and NaH in the mixture of

K. Teranishi, F. Ueno / Tetrahedron Letters 44 (2003) 4843–4848
4847
1
Figure 3. Partial H and 13C NMR spectra of 9 and 10 (CDCl3; 20°C; ref., TMS, l 0.00 ppm). The assigned signals are numbered
1
according to the usual convention shown in Scheme 1, and the symbol * refers to the silylated glucose residue. (a) H NMR of
1
9; (b) 13C NMR of 9; (c) H NMR of 10; (d) 13C NMR of 10.
In conclusion, we have demonstrated that the silyl
migration mechanism of 2A-O-TBDMSi-6A-deoxy-6A-
S-phenyl-a-cyclodextrin occurs via intra-glucosidic silyl
migration. This insight can be useful for the prepara-
tion of partial-3-O-TBDMSi-a-cyclodextrins from par-
tial-2-O-TBDMSi-a-cyclodextrins by means of silyl
migration, because the regiochemistry between the
TBDMSi groups in the partial-3-O-TBDMSi-a-
cyclodextrins can be expected to remain unchanged
from that of partial-2-O-TBDMSi-a-cyclodextrins.
Stoddart et al.5a reported that the efficiencies of the silyl
migration reactions of per(2,6-di-O-TBDMSi)-a-, b-,
and g-cyclodextrins with MeI and NaH were different,
suggesting that there may be a possibility that the type
of cyclodextrin influences the migration route, such as
intra- and/or inter-glucosidic migrations. Investigation
on the silyl migration mechanism of 2-O-TBDMSi-b-
and 2-O-TBDMSi-g-cyclodextrins is currently in
progress.
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Acknowledgements
We are grateful for Grant-in-Aid for Scientific Research
on Priority Areas (A) (No. 13024241) and Grant-in-Aid
for Scientific Research (C) (No. 14560082) from the
Japanese Ministry of Education, Science, and Culture.
The NMR instrument used in this research was
installed at the Cooperative Research Center of Mie
University.

4848
K. Teranishi, F. Ueno / Tetrahedron Letters 44 (2003) 4843–4848
6. Chiu, S-H.; Myles, D. C.; Garrell, R. L.; Stoddart, J. F.
DMSO-d6; 80°; ref., DMSO, l 39.50 ppm): l −3.44
(SiCH3), 17.52 (SiC(CH3)3), 20.93 (PhCH3), 25.65
(SiC(CH3)3), 59.91–60.09 (C-6), 69.18 (C-5*), 69.44 (C-
6*), 71.54–73.11 (C-2, C-3, C-5, C-2*, C-3*), 81.61–82.04
(C-4, C-4*), 101.39–101.98 (C-1, C-1*), 127.38 (ArC),
129.80 (ArC), 132.75 (ArC), and 144.71 (ArC). The
symbol * refers to the silylated glucose residue. MALDI-
TOF-MS m/z 1263.3 for [M+Na]+. Anal. Found: C,
47.05; H, 6.88%, Calcd for C49H80O32SSi: C, 47.41; H,
6.50%.
J. Org. Chem. 2000, 65, 2792–2796.
7. (a) Teranishi, K.; Watanabe, K.; Hisamatsu, M.;
Yamada, T. J. Carbohydr. Chem. 1998, 17, 489–494; (b)
Teranishi, K.; Tanabe, S.; Hisamatsu, M.; Yamada, T.
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8. (a) Teranishi, K.; Ueno, F. Tetrahedron Lett. 2002, 43,
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15. During our studies on the silyl migration, it was shown
that the higher the proportion of DMF in the mixture of
THF and DMF as the reaction solvent, the lower the
efficiency of the silyl migration. Consequently, to attain
efficient migration, the proportion of DMF had to be as
low as possible, and thus the mixture of THF and DMF
was reluctantly used due to the insolubility of 8 for THF.
16. Data for 8. Colorless powder. Mp. 240°C (dec.). IR
(KBr) w 3398 and 2929 cm−1 1H NMR (5% D2O in
.
DMSO-d6; 50°C; ref., DMSO, l 2.49 ppm): l 0.09 (3H, s,
SiCH3), 0.12 (3H, s, SiCH3), 0.88 (9H, s, SiC(CH3)3), 3.13
(1H, dd, J=7.9, and 13.5 Hz, CH2SPh), 3.26–3.84 (m),
4.77–4.81 (5H, m, H-1 protons of glucose residues), 4.84
(1H, d, J=3.7 Hz), 7.12 (1H, t, J=7.3 Hz, ArH), 7.20
(2H, t, J=7.3 Hz, ArH), and 7.30 (2H, d, J=7.3 Hz,
ArH). MALDI-TOF-MS m/z 1201.1 for [M+Na]+. Anal.
Found: C, 48.52; H, 6.84%, Calcd for C48H78O29SSi: C,
48.89; H, 6.67%.
11. 1H NMR chemical shifts (5%D2O in DMSO-d6; 60°C;
ref., DMSO, l 2.49 ppm) of mono-6-O-(p-toluenesul-
fonyl)-a-cyclodextrin: l 2.39 (3H, s, CH3), 3.20 (1H, dd,
J=3.1 and 9.8 Hz, H-2 proton of sulfonylated glucose
residue), 3.25 (1H, t, J=9.2 Hz, H-4 proton of sulfonyl-
ated glucose residue), 3.25–3.31 (5H, m, H-2 protons of
unsulfonylated glucose residues), 3.34–3.41 (5H, m, H-4
protons of unsulfonylated glucose residues), 3.47–3.76
(m), 3.82 (1H, H-5 proton of sulfonylated glucose
residue), 4.26–4.32 (2H, m, H-6 protons of sulfonylated
glucose residue), 4.66 (1H, d, J=3.1 Hz, H-1 proton of
unsulfonylated glucose residue), 4.72 (1H, d, J=3.7 Hz,
H-1 proton of sulfonylated glucose residue), 4.77–4.81
(4H, m, H-1 proton of no-sulfonylated glucose residue),
7.42 (2H, d, J=7.9 Hz, ArH), and 7.72 (2H, d, J=7.9
Hz, ArH).
17. Data for 9. Colorless powder. Mp. 103–106°C. IR (KBr)
w 3449 and 2929 cm−1 1H NMR (CDCl3; 20°C; ref.,
.
TMS, l 0.00 ppm) omitted in Figure 3(a): l 0.15 (3H, s,
SiCH3), 0.21 (3H, s, SiCH3), 0.94 (9H, s, SiC(CH3)3), 7.16
(1H, t, J=7.3 Hz, ArH), 7.27 (2H, t, J=7.3 Hz, ArH),
and 7.40 (2H, d, J=7.3 Hz, ArH). 13C NMR (CDCl3;
20°C; ref., TMS, l 0.00 ppm) omitted in Figure 3(b): l
−3.68 (SiCH3), −3.08 (SiCH3), 18.27 (SiC(CH3)3), 26.17
(SiC(CH3)3), 125.79 (ArC), 128.79 (ArC), 129.08 (ArC),
and 137.11 (ArC). MALDI-TOF-MS m/z 1425.4 for
[M+Na]+. Anal. Found: C, 54.51; H, 8.13%, Calcd for
C64H110O29SSi: C, 54.76; H, 7.90%.
12. The 1H NMR chemical shifts of 2 measured at 60°C
(5%D2O in DMSO-d6) were not significantly different
from those measured at 80°C. However, since the signals
observed in the spectrum at 60°C were slightly broader,
the spectrum at 80°C is presented herein.
18. Data for 10. Colorless powder. Mp. 108–112°C. IR (KBr)
w 3449 and 2930 cm−1 1H NMR (CDCl3; 20°C; ref.,
.
TMS, l 0.00 ppm) omitted in Figure 3(c): l 0.15 (6H, s,
SiCH3), 0.97 (9H, s, SiC(CH3)3), 7.19 (1H, t, J=7.3 Hz,
ArH), 7.28 (2H, t, J=7.3 Hz, ArH), and 7.35 (2H, d,
J=7.3 Hz, ArH). 13C NMR (CDCl3; 20°C; ref., TMS, l
0.00 ppm) omitted in Figure 3(d): l −4.65 (SiCH3), 18.32
(SiC(CH3)3), 25.90 (SiC(CH3)3), 125.84 (ArC), 128.82
(ArC), 128.82 (ArC), and 137.66 (ArC). MALDI-TOF-
MS m/z 1425.3 for [M+Na]+. Anal. Found: C, 54.48; H,
8.17%, Calcd for C64H110O29SSi: C, 54.76; H, 7.90%.
13. Data for 2. Colorless powder. Mp. 142°C (dec.). IR
(KBr) w 3389 and 2928 cm−1 1H NMR (5% D2O in
.
DMSO-d6; 80°C; ref., DMSO, l 2.49 ppm) omitted in
Fig. 2(a): l 0.08 (3H, s, SiCH3), 0.10 (3H, s, SiCH3), 0.87
(9H, s, SiC(CH3)3), 7.41 (2H, d, J=8.6 Hz, ArH), and
7.72 (2H, d, J=8.6 Hz, ArH). 13C NMR (5% D2O in