C-Branched pyrrolidines from 2-C-acetylmethyl-glycosylazides. Reduction of imines formed by monosaccharide ring opening

Article abstract of DOI:10.1016/j.carres.2006.07.005

Reduction of the protected 2-C-acetylmethyl-β-glucopyranosyl azide derivative 1 produced the corresponding β-glucosylamine 3. Rather than forming a conformationally strained 1,2-trans-fused bicyclic imine, we propose that the β-glycosylamine underwent ano

Full text of DOI:10.1016/j.carres.2006.07.005

Carbohydrate Research 341 (2006) 2434–2438
Note
C-Branched pyrrolidines from
2-C-acetylmethyl-glycosylazides. Reduction of
imines formed by monosaccharide ring opening
Huawu Shao,a Dean T. Williams,a Shih-Hsiung Wub and Wei Zoua,*
aInstitute for Biological Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario, Canada K1A 0R6
bInstitute of Biological Chemistry, Academy Sinica, Taipei, Taiwan, ROC
Received 1 June 2006; received in revised form 4 July 2006; accepted 12 July 2006
Available online 1 August 2006
Abstract—Reduction of the protected 2-C-acetylmethyl-b-glucopyranosyl azide derivative 1 produced the corresponding b-glucosyl-
amine 3. Rather than forming a conformationally strained 1,2-trans-fused bicyclic imine, we propose that the b-glycosylamine
underwent anomerization to an acyclic imine (4) followed by an intramolecular ring closure by the 5-hydroxy group. The resultant
2-C-acetylmethyl-a-glucopyranosylamine 5, which possesses the 1,2-cis-configuration was immediately converted to a bicyclic imine
(2) in excellent yield. Attempts to selectively reduce the C@N double bond of 2 using sodium borohydride and cyanoborohydride
failed to produce bicyclic amine 6. Instead, compound 6 underwent another ring-opening elimination and further reduction to pro-
duce a C-branched pyrrolidine (8) in good yield. Catalytic hydrogenation of 1 and 2 also provided the C-branched pyrrolidine (10).
Crown Copyright ꢀ 2006 Published by Elsevier Ltd. All rights reserved.
Keywords: C-Branched pyrrolidines; Glycosylamine; Glycosylimine; Reductive amination; Epimerization
Heterocyclic structures are ubiquitous features of
natural products and these compounds play a prominent
role in drug development. For example, one class of
nitrogen heterocycles, azasugars, are potent glycosidase
inhibitors due to their ability to mimic the oxacarbe-
nium ion transition state in enzymatic catalysis.1 How-
ever, prior to the cleavage of the glycosidic bond that
generates the oxacarbenium ion, the initial step in
hydrolysis is likely the protonation of the glycosidic oxy-
gen.2 Therefore, it is conceivable that compounds mim-
icking a glycosidic oxonium ion may also inhibit
enzymatic glycoside hydrolysis. In fact, acarbose, a drug
used for the control of diabetes, contains a pseudo-
glycosylamine at the nonreducing end. This compound
inhibits a-glycosidase (a-amylase) activity by imitating
the transition state conformation and by the interaction
of the glycosylaminium ion with the catalytic carboxyl-
ate residue.3 Consequently, glycosylamines and pseu-
do-glycosylamines have been synthesized and tested
against various glycosidases.2,4
We have been working on the synthesis of aza-C-gly-
cosides and thio-C-glycosides as inhibitors of glycopro-
cessing enzymes,5 and also have 2-C-acetylmethyl-b-
glycosyl azides in hand.6 The presence of both carbonyl
and azido (amino) groups in the same molecule
prompted us to investigate a reductive amination reac-
tion sequence to yield a bicyclic glycosylamine as a
potential glycosidase inhibitor. Unfortunately, although
we were able to obtain a stable bicyclic glycosylimine,
the corresponding bicyclic glycosylamine was not
obtained because the intermediate underwent a ring-
opening elimination that was converted under reductive
conditions to a C-branched monocyclic pyrrolidine.
The 2-C-branched glycosyl azide (1, Scheme 1) was
previously prepared from a 1,2 cyclopropanated sugar,
and its b-anomeric configuration was unambiguously
assigned based on NMR spectroscopic analysis.6 Treat-
ment of compound 1 with triphenylphosphine provided
*
Corresponding author. Tel.: +1 613 990 0821; fax: +1 613 952
9092; e-mail: wei.zou@nrc-cnrc.gc.ca
0008-6215/$ - see front matter Crown Copyright ꢀ 2006 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2006.07.005

H. Shao et al. / Carbohydrate Research 341 (2006) 2434–2438
2435
BnO
BnO
BnO
BnO
BnO
BnO
O
Ph P/THF-H O
O
3
2
N
3
85%
N
O
1
2
BnO
BnO
BnO
BnO
BnO
BnO
BnO
OH
O
O
BnO
BnO
O
NH
NH
2
NH
2
O
O
3
4
5
Scheme 1. Formation of 1,2-cis-fused bicyclic imine.
a bicyclic glycosylimine (2) in 85% yield. NMR analysis
indicated that the stereochemistry at C-1 was inverted.
The newly formed bicyclic imine was fused to the pyran-
oside with 1,2-cis-configuration as evidenced by the
observation of a strong NOE between H-1 and H-2
and by the lack of a NOE between H-1 and H-3/H-5.
This outcome was not a complete surprise because there
is evidence in the literature to support this transforma-
tion. It has been documented that mutarotation of gly-
cosylamines may occur under basic, neutral, and acidic
conditions.7 Also, the equilibrium between a glycosyl-
amine and its open-chain imine has provided the basis
for the use of the Grignard reaction as a key step in
the synthesis of aza-C-glycosides.8 We propose that be-
cause b-glycosylamine 3, obtained from reduction of
azide 1, was unable to effectively react with the ketone
group to form a conformationally strained 1,2-trans
fused bicyclic imine, a ring-opening elimination to an
acyclic imine (4) occurred. This species further equili-
brated by way of a ring-closing reaction, to the a-
anomer 5 as illustrated in Scheme 1. The 1,2-cis
configuration in 5 thus allowed the reaction between
the amino and ketone groups to occur to form the bi-
cyclic glycosylimine 2.
Natural glycosylamine derivatives are often N-substi-
tuted with electron-withdrawing groups, such as N-acyl
in N-glycoproteins and adenine, cytosine, guanine, and
thymine in nucleosides. However, stable N,O-acetals
are also found as subunits in natural products such as
norzoanthamine9 and azaspiracid-1.10 Being aware of
the possible rearrangements associated with glycosyl-
amine derivatives, we examined the reduction of com-
pound 2 under different conditions, including treatment
with sodium cyanoborohydride, sodium borohydride,
and catalytic hydrogenation, which we hoped would lead
to the formation of the bicyclic glycosylamine 6 (Scheme
2). Compounds of this kind may be potential glucos-
aminidase inhibitors due to their structural similarity to
the oxazolinium intermediate that forms during enzy-
matic hydrolysis of 2-acetamido-glycosides.11
Treatment of 2 with sodium cyanoborohydride or
sodium borohydride did not yield the desired bicyclic
glycosylamine. Instead, the major product obtained
was a C-branched pyrrolidine 8 (Scheme 2), which was iso-
lated as a mixture of two diastereomers that were insep-
arable by silica gel chromatography. Thus, we assigned
their stereochemistry and estimated the R/S (or cis/
trans) ratio (ꢀ4:1) by NMR spectroscopy. We believe
NaCNBH , 75%
3
or NaBH , 76%
4
Ac
or H /Pd-C/EtOAc/Et N, 62%
H
2
3
*
N
Ac O/Py
2
BnO
*
2
N
BnO
87%
BnO
3'
2'
1'
BnO
8 (cis/trans 4:1)
9
AcO OBn
4'
OH OBn
BnO
BnO
O
BnO
BnO
OH
BnO
BnO
NH
*
N
*
6
7
Scheme 2. A sugar-ring opening reductive amination.

2436
H. Shao et al. / Carbohydrate Research 341 (2006) 2434–2438
that the bicyclic glycosylamine 6 was formed by initial
reduction of imine. Unfortunately, this unstable com-
pound appears to have rearranged in a fashion similar
to the conversion of 3 to 4, via a ring-opening elimina-
tion to give a monocyclic imine intermediate (7) that
was immediately reduced to pyrrolidine 8. The diastereo-
selectivity at the 2-position (marked by an asterisk in
Scheme 2) was apparently determined in the initial
reduction of the C@N bond (from 2 to 6). As expected,
the R-configuration (2,4-cis) is favored due to the
hydride addition from the less hindered Si-face.12
We also attempted to obtain an N-acetylated deriva-
tive of 6 by subjecting 2 to catalytic hydrogenation in
ethyl acetate under basic conditions. We previously ob-
served that reduction of azido groups under such condi-
tions often led to in situ N-acetylation. Unfortunately,
after 48 h we obtained only 8 as the major product
together with a small amount of polar by-products,
presumably from partial de-O-benzylation. To further
confirm the proposed structure, 8 was converted to the
N,O-diacetate 9 by treatment of 8 with acetic anhydride
and pyridine (Scheme 2).
Based on the proposed mechanism illustrated in
Schemes 1 and 2, we were able to simplify the synthesis
of the C-branched pyrrolidine by subjecting 1 or 2 to
catalytic hydrogenation thus affording 10 (Scheme 3).
To facilitate the purification of the pyrrolidine, com-
pound 10 was further acetylated to give 11.
In summary, we describe here a procedure for the syn-
thesis of C-branched pyrrolidines from 2-C-branched
glycosylamines under various reductive conditions.
The transformation is achieved either by a direct reduc-
tion of the 2-C-branched glycosylazide or by way of an
indirect stepwise approach through a stable, isolable gly-
cosylimine. The key conversion involves a ring-opening
elimination followed by imine reduction to the mono-
cyclic pyrrolidine.
parts per million downfield from the signal of internal
1
Me4Si and were assigned on the basis of 2D H COSY
1
and H–13C chemical-shift correlated experiments. All
chemicals were purchased from Aldrich Chemical Co.
and used without further purification.
1.2. 3,4,6-Tri-O-benzyl-[1-N,2-C-(isopropylimino)]-1,2-
di-deoxy-a-glucopyranosylimine (2)
A solution of 1 (50 mg, 0.097 mmol) and triphenylphos-
phine (40 mg, 0.152 mmol) in THF (10 mL) with a drop
of H2O was stirred at rt for 24 h. The solvent was evap-
orated to a residue, which was purified by chromato-
graphy (1:1, hexane/EtOAc!100:1, EtOAc/CH3OH)
to give compound 2 (38.8 mg, 85%) as a syrup: [a]D
1
+33.6 (c 1.0, CHCl3); H NMR (CDCl3): dH 1.91 (d,
3H, –CH3, J = 1.6 Hz), 2.20 (d, 1H, –CH2–, J =
16.0 Hz), 2.37–2.48 (m, 2H, H-2, –CH2–), 3.16 (dd,
1H, H-3, J = 8.4, 8.4 Hz), 3.59 (ddd, 1H, H-5, J = 9.6,
2.8, 2.8 Hz), 3.74 (dd, 1H, H-4, J = 9.6, 8.4 Hz), 3.76
(dd, 1H, H-6, J = 10.8, 4.0 Hz), 3.85 (dd, 1H, H-60,
J = 10.8, 2.8 Hz), 4.55 (d, 1H, CH2Ph, J = 12.0 Hz),
4.62 (d, 1H, CH2Ph, J = 11.6 Hz), 4.63 (d, 1H, CH2Ph,
J = 10.8 Hz), 4.69 (d, 1H, CH2Ph, J = 12.0 Hz), 4.76 (d,
1H, CH2Ph, J = 10.8 Hz), 4.84 (d, 1H, CH2Ph,
J = 11.6 Hz), 5.43 (m, 1H, H-1), 7.18–7.37 (m, 15H,
3 · Ph); 13C NMR (CDCl3): dC 20.8 (CH3), 43.4
(–CH2–), 44.2 (C-2), 69.0 (C-6), 73.0 (C-5), 73.9
(–CH2Ph), 74.6 (2 · –CH2Ph), 78.3 (C-4), 81.8 (C-3),
99.2 (C-1), 127.8, 127.9, 128.1, 128.2, 128.5, 128.6 (2),
128.7, 138.2, 138.4, 138.7 (3 · Ph), 176.5 (–C@N–);
FABMS: m/z calcd for C30H34O4N [M+H]+: 472.2488.
Found: 472.2483.
1.3. 2R/S-Methyl-4S-(10R,20S,40-tri-benzyloxy-30R-
hydroxy-butyl)-pyrrolidine (8)
Method A: To a solution of compound 2 (60 mg,
0.127 mmol) in CH3OH (6 mL) under N2 at rt was
added NaBH4 (15 mg, 0.39 mmol) in portions. The mix-
ture was stirred for 16 h, and was diluted with EtOAc
and washed with H2O and HOAc until neutral and then
concentrated to give a crude product. Purification by
chromatography (10:1, i-PrOH/1 M aq NH4OAc) affor-
ded compound 8 (46 mg, 76%) as a syrup. FABMS: m/z
calcd for C30H38O4N [M+H]+: 476.2801. Found:
476.2771. Compound 8-cis: 1H NMR (CD3OD): dH
1.33 (d, 3H, –CH3, J = 6.0 Hz), 1.50 (ddd, 1H, H-3a,
J = 11.6, 11.2, 11.2 Hz), 2.28 (m, 1H, H-3b), 2.75 (m,
1H, H-4), 2.95 (dd, 1H, H-5a, J = 9.6, 9.6 Hz), 3.12
(dd, 1H, H-5b, J = 9.6, 9.6 Hz), 3.51 (m, 1H, H-2),
3.59–3.71 (m, 4H, H-30, H-40a, H-40b, H-10), 3.93 (m,
1H, H-20), 4.51–4.63 (m, 5H, 2.5 · CH2Ph), 4.75 (d,
1H, CH2Ph, J = 11.6 Hz), 7.23–7.34 (m, 15H, 3 · Ph);
13C NMR (CD3OD): dC 16.2 (CH3), 35.5 (C-3), 40.6
(C-4), 46.3 (C-5), 56.0 (C-2), 70.7 (C-20), 71.3 (C-40),
1. Experimental
1.1. General methods
1H and 13C NMR spectra were recorded in CDCl3 or
CD3OD at 400 and 100 MHz, respectively, with a Var-
ian instrument at 293 K. Chemical shifts are given in
Ac
N
H
N
1
Ac O/Py
2
*
*
H /Pd-C
2
AcO
HO
or
AcO
HO
2
10
AcO OAc
OH
HO
81-86% two steps
11 (cis/trans 3:1)
Scheme 3. C-Branched pyrrolidine formation by hydrogenation.

H. Shao et al. / Carbohydrate Research 341 (2006) 2434–2438
2437
73.3 (–CH2Ph), 73.8 (–CH2Ph), 74.2 (–CH2Ph), 79.6 (C-
10), 80.1 (C-30), 127.6, 127.7, 128.0, 128.2, 128.3, 138.3,
138.4 (3 · Ph). Compound 8-trans: 1H NMR (CD3OD):
dH 1.26 (d, 3H, –CH3, J = 6.4 Hz), 1.70 (m, 1H, H-3a),
2.15 (m, 1H, H-3b), 2.73–2.78 (m, 2H, H-4, H-5a), 3.18
(m, 1H, H-5b), 3.53–3.70 (m, 5H, H-2, H-30, H-40a,
H-40b, H-10), 3.92 (m, 1H, H-20), 7.23–7.34 (m, 15H,
3 · Ph); 13C NMR (CD3OD): dC 16.5 (CH3), 33.4 (C-
3), 39.0 (C-4), 46.3 (C-5), 55.6 (C-2), 70.8 (C-20), 71.3
(C-40), 73.3 (–CH2Ph), 73.9 (–CH2Ph), 74.3 (–CH2Ph),
79.4 (C-10), 80.2 (C-30), 127.6, 127.7, 128.0, 128.2,
128.3, 138.3, 138.4 (3 · Ph).
3.05 (m, 1H, H-5a), 3.52 (m, 1H, H-10), 3.74–3.95 (m,
4H, H-20, H-40a, H-40b, H-2), 4.02 (dd, 1H, H-5b,
J = 11.6, 7.2 Hz), 4.46 (d, 1H, CH2Ph, J = 10.8 Hz),
4.51 (s, 2H, CH2Ph), 4.62–4.69 (m, 2H, CH2Ph), 4.72
(d, 1H, CH2Ph, J = 10.4 Hz), 5.21 (m, 1H, H-8), 7.23–
7.31 (m, 15H, 3 · Ph); 13C NMR (CDCl3): dC 20.8
(CH3), 21.4 (COCH3), 21.8 (COCH3), 36.2 (C-3), 39.4
(C-4), 47.5 (C-5), 53.6 (C-2), 68.2 (C-40), 73.3 (C-30),
73.4 (–CH2Ph), 74.9 (–CH2Ph), 75.0 (–CH2Ph), 79.6
(C-10), 80.9 (C-20), 127.9, 128.0, 128.1, 128.2, 128.3,
128.6, 128.7, 138.0, 138.2, 138.3 (3 · Ph), 169.4, 170.7
(2 · COCH3).
Method B: To a solution of 2 (30 mg, 0.064 mmol) in
CH3OH (3 mL) under N2 at rt was added NaCNBH3
(10 mg, 0.16 mmol) in portions. The mixture was stirred
for 16 h and then worked up as described above and the
product was purified by column chromatography (10:1,
i-PrOH/1 M aq NH4OAc) gave 8 (22.8 mg, 75%) as a
syrup.
Method C: A solution of 2 (28 mg, 0.06 mmol) in
EtOAc (3 mL) and Et3N (0.2 mL) containing Pd/C
(10%, 28 mg) was stirred under H2 at atmospheric pres-
sure and rt for 18 h. Work-up as described above and
purification by chromatography (10:1, i-PrOH/1 M aq
NH4OAc) afforded 8 (17.5 mg, 62%) as a syrup.
1.5. N-Acetyl-2R/S-methyl-4S-(10R,20S,30R,4-tetra-acet-
yloxy-butyl)-pyrrolidine (11)
A solution of compound 1 (108 mg, 0.21 mmol) in
CH3OH (8 mL) containing Pd/C (10%, 60 mg) was stir-
red in the presence of H2 at atmospheric pressure and rt
for 18 h. The catalyst was filtered, and the solvent was
evaporated to give crude 10, which was dissolved in
Ac2O (1.8 mL) and pyridine (2.7 mL). The mixture
was stirred at rt for 8 h and poured into ice H2O and ex-
tracted with EtOAc. The organic phase was washed with
H2O and aq NaHCO3 until neutral, and then dried and
concentrated to a crude product, which was passed
through a silica gel column (1:1!6:1, EtOAc/hexane)
to afford compound 11 (73 mg, 84%) as a syrup.
FABMS: m/z calcd for C19H30O9N [M+H]+: 416.1921.
Found: 416.1906. Compound 11-cis: 1H NMR (CDCl3):
dH 1.28 (d, 3H, –CH3, J = 6.0 Hz), 1.40 (ddd, 1H, H-3,
J = 11.6, 10.8, 10.8 Hz), 2.10–2.28 (m, 2H, H-3, H-4),
3.15 (dd, 1H, H-5a, J = 10.4, 10.4 Hz), 3.70 (dd, 1H,
H-5b, J = 10.4, 7.6 Hz), 3.98 (m, 1H, H-2), 4.13 (dd,
1H, H-40a, J = 12.4, 4.8 Hz), 4.22 (dd, 1H, H-40b,
J = 12.4, 2.4 Hz), 5.05 (m, 1H, H-3), 5.21–5.28 (m,
2H, H-10, H-20); 13C NMR (CDCl3): dC 20.6 (CH3),
20.9 (2), 21.1 (2), 23.6 (5 · –COCH3), 36.8 (C-3), 39.4
(C-4), 50.0 (C-5), 53.2 (C-2), 62.0 (C-40), 68.3 (C-30),
69.9 (C-20), 71.4 (C-10), 169.3, 170.1, 170.2, 170.6,
170.8 (5 · –CO–). Compound 11-trans: 1H NMR
(CDCl3): dH 1.27 (d, 3H, –CH3, J = 6.0 Hz), 1.53 (m,
1H, H-3), 2.22–2.28 (m, 2H, H-3, H-4), 2.92 (m, 1H,
H-5a), 3.88 (m, 1H, H-2), 4.10–4.18 (m, 3H, H-5b,
H-40a, H-40b), 5.04 (m, 1H, H-30), 5.16–5.26 (m, 2H,
H-10, H-20) ppm; 13C NMR (CDCl3): dC 20.6 (CH3),
20.9 (2), 21.1, 21.9, 22.7 (5 · –COCH3), 37.5 (C-3),
38.1 (C-4), 47.6 (C-5), 53.5 (C-2), 61.8 (C-40), 68.5
(C-30), 70.0 (C-20), 71.5 (C-10), 169.4, 169.7, 170.2,
170.7, 170.9 (5 · –CO–) ppm.
1.4. N-Acetyl-2R/S-methyl-4S-(30R-acetyloxy-10R,20S,4-
tri-benzyloxy-butyl)-pyrrolidine (9)
A solution of 8 (30 mg, 0.063 mmol) in Ac2O (1.2 mL)
and pyridine (1.8 mL) was stirred at rt for 16 h. The
mixture was diluted with ice H2O and extracted with
EtOAc. The organic phase was dried and concentrated.
Purification by column chromatography (1:1!6:1,
EtOAc/hexane) afforded compound 9 (30.6 mg, 87%)
as a syrup. FABMS: m/z calcd for C34H42O6N
[M+H]+: 560.3012. Found: 560.2997. Compound 9-cis:
1H NMR (CDCl3): dH 1.25 (d, 3H, –CH3, J = 6.4 Hz),
1.47 (m, 1H, H-3a), 1.87 (s, 3H, –COCH3), 2.02 (s,
3H, –COCH3), 2.27–2.36 (m, 2H, H-3b, H-4), 3.02 (m,
1H, H-5a), 3.15 (dd, 1H, H-5b, J = 10.0, 7.6 Hz), 3.51
(m, 1H, H-10), 3.72–3.78 (m, 2H, H-20, H-40a), 3.86
(dd, 1H, H-40b, J = 10.4, 4.4 Hz), 3.94 (m, 1H, H-2),
4.43 (d, 1H, CH2Ph, J = 10.4 Hz), 4.52 (s, 2H, CH2Ph),
4.61 (d, 1H, CH2Ph, J = 11.2 Hz), 4.68 (d, 1H, CH2Ph,
J = 11.2 Hz), 4.73 (d, 1H, CH2Ph, J = 10.4 Hz), 5.21
(m, 1H, H-30), 7.23–7.31 (m, 15H, 3 · Ph); 13C
NMR (CDCl3): dC 21.4 (CH3), 21.5 (COCH3), 23.6
(COCH3), 35.6 (C-3), 40.1 (C-4), 50.0 (C-5), 53.1
(C-2), 68.3 (C-40), 73.0 (C-30), 73.6 (–CH2Ph), 74.6
(–CH2Ph), 74.7 (–CH2Ph), 79.6 (C-10), 80.2 (C-20), 127.9,
128.0, 128.1, 128.2, 128.3, 128.6, 128.7, 138.0, 138.2,
138.3 (3 · Ph), 168.9, 170.5 (2 · COCH3). Compound
9-trans: 1H NMR (CDCl3): dH 1.24 (d, 3H, –CH3,
J = 6.0 Hz), 1.62 (m, 1H, H-3a), 2.00 (s, 3H, –COCH3),
2.06 (s, 3H, –COCH3), 2.31–2.42 (m, 2H, H-4, H-3b),
Acknowledgments
This contribution is NRCC publication No. 42509. We
thank Jacek Stupak and Ken Chan for MS analysis.

2438
H. Shao et al. / Carbohydrate Research 341 (2006) 2434–2438
T.; Kameda, Y.; Asano, N.; Matsui, K. J. Med. Chem.
Supplementary data
1986, 29, 1038–1046.
5. (a) Yi, T.; Wu, S. H.; Zou, W. Carbohydr. Res. 2005, 340,
235–244; (b) Yi, T.; Wu, A.; Wu, S.-H.; Zou, W.
Tetrahedron 2005, 61, 11716–11722; (c) Zou, W.; Lacroix,
E.; Wang, Z. R.; Wu, S. H. Tetrahedron Lett. 2003, 44,
4431–4433.
NMR spectra (1H, 13C, HSQC, NOESY) for com-
pounds 2, 8, 9, and 11. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.carres.2006.07.005.
6. (a) Shao, H.; Ekthawatchai, S.; Chen, C.-S.; Wu, S.-H.;
Zou, W. J. Org. Chem. 2005, 70, 4726–4734; (b) Shao, H.;
Ekthawatchai, S.; Wu, S.-H.; Zou, W. Org. Lett. 2004, 6,
3497–3499.
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