Aryl C-glycosylation of phenols with glycosyl trifluoroacetimidates

Article abstract of DOI:10.1016/j.carres.2006.08.022

Aryl C-glycosylation of a variety of phenols with glycosyl trifluoroacetimidates in the presence of TMSOTf was examined, leading to the corresponding ortho-hydroxyaryl C-glycosides in variable yields.

Full text of DOI:10.1016/j.carres.2006.08.022

Carbohydrate Research 341 (2006) 2717–2722
Note
Aryl C-glycosylation of phenols with glycosyl trifluoroacetimidates
Yuwen Li, Guo Wei and Biao Yu*
State Key Laboratory of Bio-organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, China
Received 27 June 2006; received in revised form 21 August 2006; accepted 25 August 2006
Available online 15 September 2006
Abstract—Aryl C-glycosylation of a variety of phenols with glycosyl trifluoroacetimidates in the presence of TMSOTf was exam-
ined, leading to the corresponding ortho-hydroxyaryl C-glycosides in variable yields.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: C-Glycosylation; Aryl C-glycosides; Glycosyl trifluoroacetimidates; Phenols
The Friedel–Crafts reaction between glycosyl donors
and electron-rich aromatic compounds is the earliest
and the most straightforward approach,1 which is also
employed by Nature,2 for the synthesis of aryl C-glycos-
ides.3 When a phenol is used in the reaction, the corre-
sponding O-glycoside might be produced first, which
then undergoes an O ! C rearrangement to give the
ortho-hydroxyaryl C-glycoside in a regioselective man-
ner.3,4 In this thermodynamically controlled C-glycosyl-
ation reaction, matching of the reactivities of the
glycosyl donor and the aromatic acceptor, which deter-
mines the reaction conditions, has proven to be critical
for producing the desired C-glycoside. Otherwise, com-
peting side reactions of the two components and the
resulting coupling products will lead to complex mix-
tures. Among the various glycosyl donors that have
been applied in the aryl C-glycosylation, glycosyl tri-
chloroacetimidates are one of the most successful types
and require mild activation conditions using TMSOTf
or BF3ÆOEt2 as promoters.5 Recently, glycosyl trifluoro-
acetimidates have been developed as valuable alterna-
tives to the glycosyl trichloroacetimidates.6 The
glycosyl trifluoroacetimidates are relatively more self
stable, but can behave as efficiently as the corresponding
glycosyl trichloroacetimidates during glycosylation.
Here, we report the C-glycosylation of a variety of
phenols with perbenzylated gluco-, galacto-, and
mannopyranosyl trifluoroacetimidates.
We first tried the coupling of 2,3,4,6-tetra-O-benzyl-
glucopyranosyl trifluoroacetimidate (1a)6 with p-methyl-
phenol (2a) and 7-hydroxycoumarin (2b). Even under
forcing conditions (1.2 equiv of TMSOTf at rt), only
the corresponding a-O-glycosides (3aa and 3ab) were
isolated as the major products (Table 1, entries 1 and
2); the desired C-glycosides were not detected. The more
electron-rich mono- and dimethoxyphenols 2c–f were
then employed as acceptors for the C-glycosylation with
trifluoroacetimidate 1a. Under a variety of conditions
with TMSOTf or BF3ÆEtO2 as a promoter, the reactions
led to complex mixtures. Thus, only the major products
(1.2 equiv of TMSOTf, 0 °C ! rt) were isolated, which
were characterized and shown to be the expected
ortho-hydroxyaryl-b-C-glycosides (3ac–af). The yields
were quite low (21–37%) except for 3af (71%). When
naphthen-2-ol (2g), which bears an exceedingly nucleo-
philic 1-carbon, was used as an acceptor, the C-glyco-
side 3ag was obtained in a good 69% yield. These
results are in accordance with those of the correspond-
ing coupling reactions using 2,3,4,6-tetra-O-benzyl-
glucopyranosyl trichloroacetimidate (0.05–0.1 equiv of
TMSOTf, ꢀ30 °C ! rt)5c and 2,3,4,6-tetra-O-benzyl-
glucopyranosyl diphenylphosphate (1.2 equiv of
TMSOTf, 0 °C ! rt)7 as donors. However, when a
catalytic amount of TMSOTf (0.1 equiv) was used
to promote the coupling of 1a and 2g, only the
*
Corresponding author. Tel.: +21 54925131; fax: +21 64166128;
e-mail: byu@mail.sioc.ac.cn
0008-6215/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2006.08.022

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Y. Li et al. / Carbohydrate Research 341 (2006) 2717–2722
Table 1. Coupling of the glycosyl N-phenyltrifluoroacetimidates (1a–c) with phenol acceptors (2a–h)a
Entry
1
Donor
Acceptor
Product
Yield (%)b
47c
OBn
O
1a
2a
Me
BnO
BnO
3aa
BnO
O
OBn
O
BnO
BnO
2
3
4
5
2b
2c
2d
2e
41
27
21
37
5c
3ab
BnO
O
O
O
OBn
O
OMe
BnO
BnO
5c
3ac
OBn
OH
OMe
OBn
O
OMe
OMe
BnO
BnO
5c
5c
3ad
3ae
OBn
MeO
OH
OH
BnO
O
BnO
BnO
OBn
OMe
OMe
BnO
O
BnO
BnO
5c
6
2f
71
3af
OBn
OH
BnO
O
BnO
BnO
7
7
2g
2h
2c
2d
2e
69
3ag
OBn
HO
HO
BnO
8
56
O
BnO
BnO
3ah
OBn
OH
OMe
OBn
BnO
OBn
59c
45c
67c
O
9
1b
3bc
OBn
OH
OMe
OBn
BnO
OBn
O
OMe
10
11
3bd
OBn
OH
OBn
OBn
BnO
MeO
OMe
O
3be
OBn
OH
d
12
13
2f
—
—
OBn
OBn
O
2g
76c
69c
3bg
BnO
OBn
OH
OH
OBn
HO
OBn
BnO
14
2h
O
3bh
OBn

Y. Li et al. / Carbohydrate Research 341 (2006) 2717–2722
2719
Table 1 (continued)
Entry
15
Donor
Acceptor
Product
—
Yield (%)b
d
1c
2c
—
OMe
OMe
O
O
BnO
BnO
48e
76e
3cd
16
17
2d
2e
HO
MeO
BnO
OBn
BnO
BnO
3ce
OMe
HO
BnO
OBn
d
18
19
2f
—
—
HO
O
BnO
BnO
2g
96
7
3cg
OBn
BnO
OH
O
BnO
BnO
20
2h
74e
3ch
HO
BnO
OBn
a
˚
Reaction conditions: 1 (1.2 equiv), 2 (1.0 equiv), TMSOTf (1.2 equiv), CH2Cl2, 4 A MS, 0 °C ! rt.
b Isolated yields (based on phenol 2).
c The anomeric configuration was confirmed by 1H NMR analysis of the corresponding debenzylation product.
d It was not possible to purify the desired C-glycoside.
e The anomeric configuration of the mannopyranosides 3cd, 3ce, and 3ch was assumed to be a, as that of the known compound 3cg.
corresponding a-O-glycoside7 was obtained (ꢁ60%).
The last phenol employed as an acceptor is 4,6-di-tert-
butyl-1,3-dihydroxybenzene (2h) in an attempt to obtain
the corresponding 2-(b-D-glucopyranosyl)-1,3-dihydr-
oxybenzene derivative 3ah. Compound 3ah, if its 4,6-
di-tert-butyl group could be taken off, would be a key
intermediate for the synthesis of puerarin and related
isoflavone-C-glycosides.8 Gratifyingly, coupling of the
resorcinol derivative 2h with trifluoroacetimidate 1a
afforded the desired 2-C-b-D-glucopyranoside 3ah in a
good 56% yield (Fig. 1).
However, deprotection of the 4,6-di-tert-butyl groups
in 3ah under a variety of acidic conditions (in the pres-
ence of CF3COOH, H3PO4, AlCl3, K-10, or Zn(OAc)2)9
led to complex mixtures; the desired compound 4 was
obtained in not more than 20% yield (CF3COOH, rt)
OBn
OBn
OBn
O
BnO
BnO
BnO
OBn
O
O
BnO
BnO
O
CF3
O
CF3
NPh
O
CF3
NPh
BnO
OBn
1a
OBn
NPh
1c
1b
OMe
OMe
Me
OMe
O
O
HO
HO
HO
HO
2a
OMe
2b
2c
2d
OMe
OH
HO
MeO
HO
HO
HO
OMe
2g
2f
2h
2e
Figure 1. Glycosyl trifluoroacetimidate donors (1a–c) and phenol acceptors (2a–h).

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Y. Li et al. / Carbohydrate Research 341 (2006) 2717–2722
BnO HO
HO
O
HO
HO
HO
O
BnO
BnO
b
a
3ah
OH
5
OBn
HO
4
HO
Scheme 1. Reagents and conditions: (a) CF3COOH, rt, 19%; (b) H2, Pd/C, CH3OH/EtOAc, rt, 64%.
(Scheme 1). The removal of the benzyl groups in 4 by
hydrogenolysis gave 2-(b-D-glucopyranosyl)-1,3-dihydr-
oxybenzene 5. The analytical data of 5 are in full
agreement with those for the natural product isolated
from Pterocarpes marsupium,10 which further confirmed
the assigned structure for the C-glycosylation product
3ah.
The resulting mixture was filtered through Celite. The
filtrates were concentrated to give a brown syrup, which
was subjected to silica gel column chromatography
(15:1, petroleum ether–EtOAc) to afford 3cg (319 mg,
96%) as a yellow oil.
1.2.1. p-Methylphenyl 2,3,4,6-tetra-O-benzyl-a-D-gluco-
24
1
The C-glycosylation of the electron-rich phenols 2c–h
with 2,3,4,6-tetra-O-benzyl-galacto- and mannopyranos-
yl trifluoroacetimidates (1b and 1c)6 was also examined
with the promotion of TMSOTf (1.2 equiv, 0 °C ! rt).
All the reactions led to a number of spots on the TLC;
only the major products were purified and characterized
(Table 1). In general, the C-mannopyranosides were
obtained in the highest yields, while the correspon-
ding C-glucopyranosides gave the lowest yields (cf.,
3ad/3bd/3cd; 3ae/3be/3ce; 3ag/3bg/3cg; and 3ah/3bh/
3ch).
pyranoside (3aa). ½aꢂD +72.8 (c 1.0, CHCl3); H NMR
(300 MHz, CDCl3): d 7.36–6.94 (m, 23H), 5.51 (br s,
1H), 5.04 (d, J = 11.1 Hz, 1H), 4.88 (d, J = 10.8 Hz,
2H), 4.77–4.42 (m, 5H), 4.20 (t, J = 9.6 Hz, 1H), 3.92
(d, J = 9.3 Hz, 1H), 3.82 (d, J = 9.9 Hz, 1H), 3.75 (br
s, 2H), 3.62 (d, J = 10.5 Hz, 1H), 2.32 (s, 3H); 13C
NMR (CDCl3, 75 MHz): d 138.8, 138.3, 138.2, 137.9,
130.7, 128.3, 128.0, 127.9, 127.7, 127.6, 126.8, 122.0,
114.6, 96.1, 81.9, 80.1, 75.7, 75.1, 73.4, 72.9, 70.9, 68.4,
29.7. HR-MALDI MS (m/z): calcd for C41H42O6Na
[M+Na]+: 653.2873. Found: 653.2897.
In summary, the treatment of the electron-rich phe-
nols with glycosyl trifluoroacetimidates in the presence
of TMSOTf led to the corresponding ortho-hydroxyaryl
C-glycosides as the major products. The regioselectivity
of the C-glycosylation was fully determined by the phe-
nolic acceptor and the stereoselectivity was controlled
by the glycosyl donors to give the relatively more stable
C-b-gluco- and galactopyranosides and the C-a-manno-
pyranosides. It was found that C-glycosylation with
mannopyranosyl donors gave the highest yields, while
the glucopyranosyl donors gave the lowest yields.
1.2.2. 4,6-Di-tert-butyl-2-(2,3,4,6-tetra-O-benzyl-b-D-glu-
24
copyranosyl)-1,3-dihydroxybenzene (3ah). ½aꢂD +18.1
1
(c 1.1, CHCl3); H NMR (300 MHz, CDCl3): d 7.37–
6.97 (m, 21H), 4.98 (d, J = 11.1 Hz, 2H), 4.91 (d,
J = 11.7 Hz, 1H), 4.85 (d, J = 11.8 Hz, 1H), 4.64 (dd,
J = 11.8, 6.9 Hz, 2H), 4.55 (d, J = 12.0 Hz, 1H), 4.48
(d, J = 12.0 Hz, 1H), 4.29 (d, J = 10.8 Hz, 1H), 3.99–
3.88 (m, 1H), 3.82 (d, J = 9.0 Hz, 1H), 3.78–3.66 (m,
3H), 3.57 (d, J = 9.6 Hz, 1H), 1.26 (s, 18H); 13C NMR
(75 MHz, CDCl3): d 138.4, 137.9, 137.8, 128.4, 128.3,
128.0, 127.8, 127.6, 127.4, 124.7, 112.6, 86.5, 81.5,
78.7, 75.7, 75.4, 75.2, 73.4, 67.5, 34.6; HR-MALDI
MS (m/z): calcd for C48H56O7Na [M+Na]+: 767.3918.
Found: 767.3929.
1. Experimental
1.1. General methods
See Ref. 11.
1.2.3. 2-(2,3,4,6-Tetra-O-benzyl-b-D-galactopyranosyl)-5-
24
methoxyphenol (3bc). ½aꢂD ꢀ8.94 (c 1.0 CHCl3); 1H
NMR (300 MHz, CDCl3): d 7.58 (s, 1H), 7.35–7.24 (m,
17H), 7.06 (d, J = 6.9 Hz, 3H), 6.49 (s, 1H), 6.44 (d,
J = 6.9 Hz, 2H), 5.07 (d, J = 8.7 Hz, 1H), 4.76 (s, 2H),
4.67 (d, J = 11.4 Hz, 1H), 4.49–4.41 (m, 3H), 4.30 (d,
J = 9.3 Hz, 1H), 4.17 (t, J = 9.3 Hz, 1H), 4.07 (s, 1H),
3.79 (s, 3H), 3.66 (d, J = 6.6 Hz, 2H), 3.57 (d,
J = 6.3 Hz, 2H); 13C NMR (75 MHz, CDCl3): d
161.1, 156.9, 138.4, 137.7, 130.2, 128.5, 128.4, 128.3,
128.2, 127.9, 127.7, 127.4, 116.2, 105.6, 102.6, 83.6,
81.9, 75.5, 74.4, 73.7, 72.6, 68.5, 55.3; HR-MALDI MS
(m/z): calcd C41H42O7Na [M+Na]+: 669.2823. Found:
669.2831.
1.2. Typical procedure for the coupling of glycosyl
N-phenyltrifluoroacetimidates with phenols
A mixture of the glycosyl trifluoroacetimidate 1c
(426 mg, 0.6 mmol), naphthen-2-ol 2g (72 mg, 0.5
˚
mmol), and 4 A MS (ꢁ200 mg) in anhydrous CH2Cl2
(5 mL) was stirred at 0 °C for 30 min under a N2 atmo-
sphere. Then, TMSOTf (0.12 mL, 0.6 mmol) was added.
After stirring at 0 °C for 1 h, the mixture was warmed to
rt within ꢁ3 h. The reaction was quenched by the addi-
tion of Et3N (0.5 mL) and stirring continued for 30 min.

Y. Li et al. / Carbohydrate Research 341 (2006) 2717–2722
2721
1.2.4. 6-(2,3,4,6-Tetra-O-benzyl-b-D-galactopyranosyl)-
75 MHz): d 138.6, 137.9, 137.7, 136.8, 128.5, 128.2,
127.9, 127.5, 127.4, 124.6, 112.9, 84.4, 78.9, 77.6, 75.9,
75.8, 74.2, 73.5, 72.8, 72.1, 68.1, 34.5; HR-MALDI
MS (m/z): calcd for C48H56O8Na [M+Na]+: 767.3918.
Found: 767.3923.
27
3,4-dimethoxyphenol (3bd). ½aꢂD +3.4 (c 1.0, CHCl3);
1H NMR (300 MHz, CDCl3): d 7.39–7.24 (m, 18H),
7.06 (br s, 2H), 6.65 (s, 1H), 6.54 (s, 1H), 5.09 (d, J =
11.7 Hz, 1H), 4.79 (br s, 2H), 4.68 (d, J = 12.0 Hz, 1H),
4.57 (d, J = 9.9 Hz, 1H), 4.47 (d, J = 12.0 Hz, 1H), 4.41
(d, J = 12.3 Hz, 1H), 4.23–4.18 (m, 2H), 4.07 (br s, 1H),
3.92 (d, J = 10.5 Hz, 1H), 3.88 (s, 3H), 3.74 (s, 3H),
3.68 (d, J = 6.6 Hz, 2H), 3.59 (d, J = 5.1 Hz, 2H); 13C
NMR (75 MHz, CDCl3): d 149.9, 138.5, 138.4, 137.7,
128.5, 128.3, 128.2, 127.9, 127.8, 127.7, 127.6, 127.5,
127.4, 114.3, 112.8, 101.8, 83.6, 81.9, 78.8, 75.6, 74.5,
73.7, 73.6, 72.6, 68.5, 56.3, 55.9; HR-MALDI MS (m/
z): calcd C42H44O8Na [M+Na]+: 699.2928. Found:
699.2940.
1.2.8. 6-(2,3,4,6-Tetra-O-benzyl-a-D-mannopyranosyl)-
27
3,4-dimethoxyphenol (3cd). ½aꢂD +10.6 (c 1.2, CHCl3);
1H NMR (300 MHz, CDCl3): d 7.93 (br s, 1H), 7.35–
7.13 (m, 20H), 6.49 (s, 1H), 6.33 (s, 1H), 4.91 (d,
J = 10.8 Hz, 1H), 4.69 (s, 2H), 4.67–4.46 (m, 6H), 4.17
(t, J = 9.6 Hz, 1H), 3.96 (s, 1H), 3.86 (s, 3H), 3.76–
3.72 (m, 6H), 3.54 (d, J = 9.3 Hz, 1H); 13C NMR
(CDCl3, 75 MHz): d 128.4, 128.3, 128.2, 128.1, 127.9,
127.7, 127.4, 112.6, 111.5, 101.9, 84.2, 82.3, 79.6, 77.8,
75.3, 74.6, 74.4, 73.5, 72.3, 68.8, 56.8, 55.9; HR-MALDI
MS (m/z): calcd for C42H44O8Na [M+Na]+: 699.2928.
Found: 699.2913.
1.2.5.
2-(Tetra-O-benzyl-b-D-galactopyranosyl)-3,5-di-
27
methoxyphenol (3be). ½aꢂD +24.5 (c 1.1, CHCl3); 1H
NMR (300 MHz, CDCl3): d 7.67 (s, 1H), 7.28–7.13
(m, 18H), 6.97 (br s, 2H), 6.04 (s, 1H), 5.97 (s, 1H),
5.05 (d, J = 11.7 Hz, 1H), 4.99 (d, J = 11.7 Hz, 1H),
4.90 (d, J = 6.3 Hz, 1H), 4.69 (t, J = 11.4 Hz, 2H),
4.58 (d, J = 11.7 Hz, 1H), 4.44 (d, J = 10.2 Hz, 1H),
4.36 (d, J = 12.3 Hz, 1H), 4.19 (t, J = 9.3 Hz, 1H),
3.99 (s, 1H), 3.95 (d, J = 11.2 Hz, 1H), 3.69 (s, 3H),
3.61 (s, 3H), 3.61–3.43 (m, 4H); 13C NMR (75 MHz,
CDCl3): d 161.4, 158.4, 158.2, 138.6, 138.2, 137.7,
128.4, 128.3, 128.1, 127.9, 127.8, 127.6, 127.5, 127.4,
105.5, 94.5, 90.0, 83.9, 78.9, 75.5, 74.4, 74.1, 73.8, 73.5,
72.8, 68.4, 55.5, 55.3; HR-MALDIMS (m/z): calcd
C42H44O8Na [M+ Na]+: 699.2928. Found: 699.2949.
1.2.9. 2-(2,3,4,6-Tetra-O-benzyl-a-D-mannopyranosyl)-
27
3,5-dimethoxyphenol (3ce). ½aꢂD +20.2 (c 1.2, CHCl3);
1H NMR (300 MHz, CDCl3): d 8.66 (s, 1H), 7.32–7.14
(m, 20H), 6.11 (s, 1H), 5.93 (s, 1H), 4.91 (s, 1H), 4.90
(d, J = 10.8 Hz, 1H), 4.67–4.42 (m, 7H), 4.15 (t,
J = 9.6 Hz, 1H), 3.93 (br s, 1H), 3.77 (s, 3H), 3.74 (t,
J = 9.0 Hz, 3H), 3.66 (s, 3H), 3.53 (d, J = 9.0 Hz, 1H);
13C NMR (CDCl3, 75 MHz): d 160.9, 158.9, 157.1,
138.5, 138.3, 128.3, 128.2, 128.0, 127.9, 127.7, 127.6,
127.5, 127.2, 103.2, 94.5, 90.3, 84.1, 79.6, 76.8, 76.2,
75.2, 74.5, 74.4, 73.4, 72.1, 68.8, 55.4, 55.3; HR-MALDI
MS (m/z): calcd for C42H44O8Na [M+Na]+: 699.2928.
Found: 699.2919.
1.2.6. 1-(2,3,4,6-Tetra-O-benzyl-b-D-galactopyranosyl)-
24
naphthen-2-ol (3bg). ½aꢂD 50.1 (c 1.4, CHCl3); 1H
1.2.10. 4,6-Di-tert-butyl-2-(2,3,4,6-tetra-O-benzyl-a-D-
27
NMR (300 MHz, CDCl3): d 8.19 (s, 1H), 7.94 (d,
J = 8.4 Hz, 1H), 7.69 (t, J = 8.4 Hz, 2H), 7.35–7.08
(m, 18H), 7.01 (t, J = 7.8 Hz, 1H), 6.90 (t, J = 7.2 Hz,
2H), 6.32 (d, J = 7.8 Hz, 2H), 5.28 (d, J = 9.6 Hz,
1H), 5.05 (d, J = 11.7 Hz, 1H), 4.72 (s, 2H), 4.65
(d, J = 11.4 Hz, 1H), 4.39–4.29 (m, 3H), 4.11 (d,
J = 9.6 Hz, 1H), 4.06 (s, 1H), 3.73 (t, J = 6.6 Hz, 2H),
3.52 (d, J = 6.3 Hz, 2H), 3.45 (d, J = 9.9 Hz, 1H); 13C
NMR (CDCl3, 75 MHz): d 154.8, 138.5, 137.7, 137.2,
132.7, 130.3, 128.6, 127.9, 127.8, 127.6, 127.5, 127.4,
126.5, 122.9, 122.8, 119.5, 115.5, 100.3, 83.9, 79.1,
75.6, 74.5, 73.8, 73.6, 73.3, 73.1, 72.9, 72.8, 72.2, 68.5;
HR-MALDI MS (m/z): calcd for C42H44O8Na
[M+Na]+: 689.2874. Found: 689.2889.
mannopyranosyl)-1,3-dihydroxybenzene
(3ch). ½aꢂD
1
ꢀ1.1 (c 1.1, CHCl3); H NMR (300 MHz, CDCl3): d
7.25–6.90 (m, 21H), 5.37 (d, J = 11.8 Hz, 1H), 4.52–
4.36 (m, 5H), 4.29 (d, J = 9.3 Hz, 2H), 4.14–4.02 (m,
3H), 3.89–3.71 (m, 4H), 1.31 (s, 18H); 13C NMR
(CDCl3, 75 MHz): d 138.0, 137.7, 136.6, 128.7, 128.4,
128.1, 127.9, 127.6, 124.3, 112.9, 75.9, 75.6, 73.3, 73.2,
72.5, 71.5, 67.2, 34.6; HR-MALDI MS (m/z): calcd for
C48H56O8Na [M+Na]+: 767.3907. Found: 767.3907.
Acknowledgements
This work was supported by the Chinese Academy of
Sciences (KGCX2-SW-213-05).
1.2.7.
galactopyranosyl)-1,3-dihydroxybenzene
4,6-Di-tert-butyl-2-(2,3,4,6-tetra-O-benzyl-b-D-
27
(3bh). ½aꢂD
1
+4.6 (c 1.2, CHCl3); H NMR (300 MHz, CDCl3): d
7.36–7.01 (m, 21H), 5.06 (d, J = 12.0 Hz, 1H), 4.88–
4.64 (m, 5H), 4.49 (br s, 1H), 4.33 (d, J = 10.5 Hz,
2H), 4.12 (br s, 1H), 3.72 (t, J = 8.1 Hz, 2H), 3.60 (d,
J = 5.7 Hz, 2H), 1.36 (s, 18H);13C NMR (CDCl3,
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