Indium-mediated alkynylation in C-glycoside synthesis

Article abstract of DOI:10.1055/s-2006-926353

The indium-mediated alkynylation of perbenzylated or peracetylated formylglucose is a pathway for the synthesis of various C-glycosides. Hydroxylated, ketonic, mono- or difluorinated, and methylated C-glycosides were thus obtained. Georg Thieme Verlag Stu

Full text of DOI:10.1055/s-2006-926353

PAPER
979
Indium-Mediated Alkynylation in C-Glycoside Synthesis
Indium
M
u
ediated Alky
l
nylatio
i
n
in C-
G
e
lycoside
S
n
ynthesis Picard, Nadège Lubin-Germain,* Jacques Uziel, Jacques Augé*
Laboratoire de Synthèse Organique Sélective et Chimie Organométallique, UMR 8123 CNRS UCP ESCOM, Université de Cergy-Pontoise,
5 mail Gay-Lussac, 95031 Cergy-Pontoise Cedex, France
Fax +33(134)257071; E-mail: nadege.lubin-germain@chim.u-cergy.fr; E-mail: jacques.auge@chim.u-cergy.fr
Received 28 August 2005
ene iodide 3 in dichloromethane in the presence of indium
Abstract: The indium-mediated alkynylation of perbenzylated or
(2.4 equiv). It led to the corresponding propargylic alco-
peracetylated formylglucose is a pathway for the synthesis of vari-
hols 4 and 5 in 87% and 71% yields, respectively; both
were obtained as mixtures of diastereomers in a ratio of
65:35 (Scheme 2). In the first case, the diastereomers
were separated by chromatography on silica gel; the major
product was less polar than the minor product. Even
though the diastereoselectivity was low, it was in the same
sense as in the case of Mg, Zn, or Ce reagents described
by Genêt and co-workers.7
ous C-glycosides. Hydroxylated, ketonic, mono- or difluorinated,
and methylated C-glycosides were thus obtained.
Key words: indium, alkynylation, C-glycoside, fluorination
The synthesis of non-natural glycosides, in which the gly-
cosidic oxygen is replaced by a methylene group, has re-
ceived great interest during the last decades. Numerous
methods have been described for the preparation of C-gly-
cosides.1 Moreover, the replacement of the anomeric oxy-
gen bond by a difluoromethylene group is promising for
the synthesis of new glycoconjugates, as this unit is con-
sidered bioisosteric and isoplanar to oxygen.2
In order to obtain the corresponding propargylic ketone 6,
we first applied the conditions allowing the in situ oxida-
tion of the alcohol (excess of aldehyde in dichloroethane).
Unfortunately, the expected ketone was not obtained,
which is not surprising as we showed that the indium-me-
diated oxidation did not always proceed in the case of eno-
lisable aldehydes.3b On the other hand, ketone 6 was easily
We recently described the indium-mediated alkynylation
of carbonyl derivatives.3 In the case of aldehyde alkynyla-
tion, this reaction allows the preparation of propargylic al-
cohols or ketones according to the experimental
conditions depicted in Scheme 1. In refluxing dichlo-
romethane in the presence of two equivalents of alkynyl
iodide the propargylic alcohol was obtained, whereas in
refluxing dichloroethane and with the aldehyde in excess
(2.3 equiv) the reaction led to the propargylic ketone via
an in situ Oppenauer-type oxidation.
OR
O
O
RO
RO
H
OR
1 R = Bn
2 R = Ac
Ph
I
(3)
In
CH2Cl2, reflux, 8 h
71–87%
OH
O
In (2.4 equiv)
+
R2
I
R1
R1
H
CH2Cl2
reflux
R2
OR
OR
2 equiv
OH
OH
O
O
RO
RO
RO
RO
+
O
O
OR
OR
In (1.5 equiv)
ClCH2CH2Cl
R2
I
Ph
Ph
+
R1
H
R1
R2
4a R = Bn
5a R = Ac
4b R = Bn
5b R = Ac
2.3 equiv
reflux
IBX
Scheme 1 Indium-mediated aldehyde alkynylation
DMSO, r.t., 8 h
97%
Continuing our work in C-glycoside synthesis,4 we ap-
plied this reaction to 2,3,4,6-tetra-O-benzyl-1-formylglu-
copyranose (1) and 2,3,4,6-tetra-O-acetyl-1-formyl-
glucopyranose (2) prepared according to literature proce-
dures.5,6 The alkynylation was accomplished by mixing
the aldehyde 1 or 2 with two equivalents of phenylacetyl-
OBn
O
O
BnO
BnO
OBn
Ph
6
Scheme 2 Indium-mediated alkynylation of 2,3,4,6-tetra-O-benzyl-
1-formylglucopyranose (1) and 2,3,4,6-tetra-O-acetyl-1-formylglu-
copyranose (2)
SYNTHESIS 2006, No. 6, pp 0979–0982
Advanced online publication: 27.02.2006
DOI: 10.1055/s-2006-926353; Art ID: P13405SS
x
x
.
x
x
.2
0
0
6
© Georg Thieme Verlag Stuttgart · New York

980
J. Picard et al.
PAPER
obtained in 97% yield from the mixture of propargylic al- synthetic pathway starting from propargylic alcohol 5
cohols 4a and 4b by an oxidation with 2-iodoxybenzoic bearing acetate protecting groups on the sugar moiety. In
acid (IBX) (Scheme 2).
this case, after palladium-catalyzed hydrogenation, the
alcohol 9 was obtained and led to the desired dehydroxy-
lated product by successive treatment with thio-
carbonyldiimidazole and tributyltin hydride (Scheme 5).
These two compounds 4 and 6 can be fluorinated in order
to lead to the corresponding mono- and gem-difluorinated
derivatives. We put the fluorination of propargylic alcohol
4a into practice by treatment with commercial diethylami- We have shown herein the utility of the indium-mediated
nosulfur trifluoride (DAST) at room temperature in alkynylation reaction for the synthesis of a variety of C-
dichloromethane. The expected fluorinated derivative 7 glycosides starting from aldehydic sugar moieties. We are
was thus obtained in 96% yield as a mixture of diaste- presently investigating the application of this pathway for
reomers in a ratio of 62:38. Compound 4b led to 7 in 90% the synthesis of C-glycosylated aminoacids using an acet-
yield and in a similar diastereomeric ratio (68:32). Both ylenic iodide of an aminoacid derivative.
reactions afforded the same diastereomer as main product
(Scheme 3).
IR spectra were recorded on a Bruker Tensor 27 spectrophotometer.
1H, 13C, and 19F NMR spectra were recorded on a Bruker Avance
In the case of propargylic ketone 6 the difluorination was
performed in neat DAST at 55 °C leading to 8 in 42%
yield (Scheme 4). This last compound is interesting be-
cause the difluoromethylene group is isosteric to oxygen.
250 DPX (250 MHz) spectrometer. Indium was purchased from Al-
drich and was activated by stirring in vacuo for 30 min. CH2Cl2 was
dried over anhyd P2O5. 2-Iodoxybenzoic acid (IBX) was prepared
according to a literature procedure.8 Optical rotations were deter-
mined at 25 °C in CHCl3, 589 nm, on a JASPO DIP 370 instrument.
In order to access a C-glycoside with a methylene group
in b-anomeric position, we first tried the Barton–McCom-
bie deoxygenation of alcohol 4.
Iodophenylacetylene (3)
A mixture of I2 (1.218 g, 4.8 mmol) and morpholine (1.14 mL, 13.1
mmol) in benzene (5 mL) was stirred for 30 min at r.t. until the for-
mation of an orange solution. Phenylacetylene (980 mg, 4.35 mmol)
diluted in benzene (8 mL) was then added and the medium was
stirred at 45 °C for 24 h. After filtration and washing with Et2O (20
mL), the organic phase was washed with aq NH4Cl soln, NaHCO3
soln, and H2O. After drying with MgSO4 and filtration, the solvent
was removed under reduced pressure. The crude product was puri-
Unfortunately no reduction of the xanthate intermediate
or the thiocarbonyldiimidazoyl derivative occurred by
treatment with Bu3SnH/AIBN. This was certainly due to
the presence of the conjugated C–C triple bond. As the re-
duction of this triple bond by treatment with tosyl-
hydrazide did not succeed, we decided to realize the
OBn
OBn
OH
O
F
O
DAST
CH2Cl2, r.t.
BnO
BnO
BnO
BnO
OBn
OBn
Ph
Ph
dr: 62/38
dr: 68/32
4a
4b
96%
90%
7
Scheme 3 Propargylic alcohol fluorination by DAST
OBn
OBn
O
O
F
F
O
DAST
BnO
BnO
BnO
BnO
OBn
neat, 55 °C, 6 h
OBn
Ph
Ph
42%
8
6
Scheme 4 Propargylic ketone fluorination by DAST
OAc
OAc
OAc
O
OH
O
O
OH
i
ii, iii
AcO
AcO
AcO
AcO
AcO
OAc
AcO
OAc
Ph
OAc
Ph
Ph
9
10
5
Scheme 5 Reagents and conditions: (i) H2, 10% Pd/C, dioxane–EtOH–H2O (2:2:1), 3 h, 95%; (ii) In2C=S (10 equiv), DMAP (15 equiv), THF,
70 °C, 5 h; (iii) Bu3SnH (10 equiv), AIBN (1 equiv), toluene, 85 °C, 3 h, 54%.
Synthesis 2006, No. 6, 979–982 © Thieme Stuttgart · New York

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Indium Mediated Alkynylation in C-Glycoside Synthesis
981
fied by flash chromatography on silica gel (PE–EtOAc, 9:1) to give
3 (2.01 g, 92%) as a yellow oil.
1H NMR (250 MHz, CDCl3): d = 7.25 (m, 3 H), 7.42 (m, 3 H).
13C NMR (62 MHz, CDCl3): d = 6.6, 94.1, 123.2, 128.1, 128.7,
132.2.
the solvent was removed under reduced pressure. The crude oil was
purified by flash chromatography on silica gel (PE–EtOAc, 85:15)
to give 6 (232 mg, 97%); [a]D25 –5.8 (c 6.9 CHCl3).
IR (neat): 3030, 2917, 2203, 1673, 1496 cm–1.
1H NMR (250 MHz, CDCl3): d = 3.55–3.80 (m, 6 H, H-2 to H-6, H-
6¢), 4.03 (d, J = 9.3 Hz, 1 H, H-1), 4.56–4.92 (m, 8 H, PhCH2),
7.20–7.49 (m, 25 H, Ar)
13C NMR (62 MHz, CDCl3): d = 68.7 (C-6), 73.5,75.1, 74.8,
(PhCH2), 78.1, 79.4, 79.5, 83.7 (C-2 to C-5), 86.4 (C-1), 87.0
(C≡C), 94.5 (C≡C), 119.7 (Ar-C), 127.6–128.5 (Ar-CH), 133.4 (Ar-
CH), 137.5–138.3 (Ar-C), 183.33 (CO).
1-(2,3,4,6-Tetra-O-benzyl-D-gluco-b-C-pyranosyl)-3-phenyl-
prop-2-yn-1-ol (4)
To activated In (276 mg, 2.40 mmol) was added a solution of io-
dophenylacetylene (3) (456 mg, 2.00 mmol) in anhyd CH2Cl2 (2.5
mL). Aldehyde 1 (553 mg, 1.00 mmol) in CH2Cl2 (10 mL) was in-
troduced to the medium, which was refluxed overnight. The mixture
was treated with a sat. NaHCO3 soln (10 mL) and extracted with
CH2Cl2 (10 × 10 mL). The combined organic phases were dried
over MgSO4 and the solvent was removed under reduced pressure.
The crude product was purified by flash chromatography on silica
gel (PE–EtOAc, 85:15) and the two diastereomers 4a (374 mg,
57%) and 4b (198 mg, 30%) were thus separated.
HRMS (CI): m/z [M + NH4]+ calcd for C43H44NO6: 670.3169;
found: 670.3170.
3-(2,3,4,6-Tetra-O-benzyl-D-gluco-b-C-pyranosyl)-3-fluoro-1-
phenylpropyne (7)
To a DAST soln (54 mg, 0.33 mmol) in anhyd CH2Cl2 at –78 °C
was added dropwise the alcohol 4a or 4b (154 mg, 0.235 mmol) in
2 mL of CH2Cl2. After stirring for 1 h, the temperature was allowed
to reach 20 °C and stirring was continued for one additional hour.
The mixture was treated with MeOH (2 mL) and a spatula of
NaHCO3 was added. The crude product was concentrated in vacuo
and then purified by flash chromatography on silica gel (PE–
EtOAc, 9:1) to give 7 as a mixture of diastereomers (149 mg, 96%
starting from 4a; 139 mg, 90% starting from 4b).
4a
IR (neat): 3357, 3030, 2864, 1490 cm–1.
1H NMR (250 MHz, CDCl3): d = 3.00 (s, 1 H, OH), 3.49–3.86 (m,
7 H, H-1 to H-6, H-6¢), 4.58–4.99 (m, 9 H, PhCH2, CHOH), 7.18–
7.44 (m, 25 H, Ar).
13C NMR (62 MHz, CDCl3): d = : 62.1 (CHOH), 68.6 (C-6), 73.3,
75, 75.3, 75.5 (PhCH2), 78.1, 78.3, 78.9, 80.5 (C-2 to C-5), 84.9
(C≡C), 86.8 (C-1), 88.4 (C≡C), 122.7 (Ar-C), 126.9–128.4 (Ar-
CH), 131.7 (Ar-CH), 137.8–138.4 (Ar-C).
1H NMR (250 MHz, CDCl3): d = 3.43–3.76 (m, 7 H, H-1 to H-6, H-
6¢), 4.53–4.85 (m, 8 H, PhCH2), 5.55 (d, J = 48 Hz, 1 H, CHF),
7.11–7.35 (m, 25 H, Ar).
13C NMR (62 MHz, CDCl3; major diastereomer): d = 68.4 (C-6),
73.4, 75.1, 75.2, 75.8 (PhCH2), 78.1, 78.4, 79.5, 79.9 (C-2 to C-5),
82.3 (C≡C), 83.20 (d, J = 123 Hz, CHF), 86.84 (C-1), 90 (C≡C),
121.5 (Ar-C), 127.4–129.0 (Ar-CH), 132.0 (Ar-CH), 137.7–138.4
(Ar-C).
13C NMR (62 MHz, CDCl3; minor diastereomer): d = 68.9 (C-6),
73.4, 75.1, 75.2, 75.6 (PhCH2), 77.8, 78.5, 79.5, 80.1 (C-2 to C-5),
80.4 (d, J = 123 Hz, CHF), 82.5 (C≡C), 86.84 (C-1), 89.8 (C≡C),
121.5 (Ar-C), 127.4–129.0 (Ar-CH), 132.0 (Ar-CH), 137.7–138.4
(Ar-C).
4b
IR: 3431, 2862, 1490 cm–1.
1H NMR (250 MHz, CDCl3): d = 3.00 (s, 1 H, OH), 3.51–3.82 (m,
7 H, H-1 to H-6, H-6¢), 4.60–4.94 (m, 9 H, PhCH2, CHOH), 7.22–
7.41 (m, 25 H, Ar).
13C NMR (62 MHz, CDCl3): d = 63.2 (CHOH), 68.6 (C-6), 73.3,
75.1, 75.4, 75.7 (PhCH2), 78.3, 79.2, 79.8, 80.7 (C-2 to C-5), 86.4
(C≡C), 86.5 (C≡C), 86.8 (C-1), 122.4 (Ar-C), 127.6–128.4 (Ar-
CH), 131.7 (Ar-CH), 138.0–138.3 (Ar-C).
19F NMR (235 MHz, CDCl3; major diastereomer): d = –185.5 (dd,
J = 47, 11.8 Hz).
19F NMR (235 MHz, CDCl3; minor diastereomer): d = –189.5 (dd,
J = 47, 23.6 Hz).
HRMS (CI): m/z [M + NH4]+ calcd for C43H45NO5F: 674.3282;
found: 674.3270.
1-(2,3,4,6-Tetra-O-acetyl-D-gluco-b-C-pyranosyl)-3-phenyl-
prop-2-yn-1-ol (5)
According to the same procedure as for 4, 2 (389 mg, 1.08 mmol)
was transformed into 5 (355 mg, 71%) and obtained as a mixture of
diastereomers.
1H NMR (250 MHz, CDCl3; 5a + 5b): d = 1.99–2.09 (m, 12 H,
OAc), 2.77 (s, 1 H, OH), 3.61–3.83 (m, 2 H, H-1, H-5), 4.10–4.37
(m, 2 H, H-6, H-6¢), 4.58 (s, 1 H, CHOH), 5.10–5.30 (m, 3 H, H-2
to H-4), 7.23–7.47 (m, 5 H, Ar).
3-(2,3,4,6-Tetra-O-benzyl-D-gluco-b-C-pyranosyl)-3,3-difluoro-
1-phenylpropyne (8)
13C NMR (62 MHz, CDCl3; 5a): d = 20.6 (CH3COO), 61.7
(CHOH), 62.8 (C-6), 68.4, 68.9, 74.0 (C-2 to C-4), 75.9 (C-5) 79.3
(C-1), 85.0 (C≡C), 85.8 (C≡C), 122.1, 128.3, 128.8 (Ar-CH), 131.6
(Ar-C), 169.4, 169.8, 170.3, 170.7 (CO).
13C NMR (62 MHz, CDCl3; 5b): d = 20.6 (CH3COO), 61.7
(CHOH), 62.1 (C-6), 68.3, 68.7, 74.1 (C-2 to C-4), 75.9 (C-5) 79.4
(C-1), 85.0 (C≡C), 86 (C≡C), 122.1, 128.3, 128.8 (Ar-CH), 131.8
(Ar-C), 169.4, 169.8, 170.3, 170.7 (CO).
Compound 6 (183 mg, 0.280 mmol) and DAST (361 mg, 2.24
mmol) were heated at 55 °C for 6 h. The mixture was treated with
MeOH (2 mL) and a spatula of NaHCO3 was added. The crude
product was concentrated under reduced pressure and then purified
by flash chromatography on silica gel (PE–EtOAc, 9:1) to give 8 as
a yellow oil (80 mg, 42%); [a]D25 –0.15 (c 5.4 CHCl3).
IR (neat): 3030, 2918, 2242, 1493 cm–1.
1H NMR (250 MHz, CDCl3): d = 3.48–3.77 (m, 7 H, H-1 to H-6, H-
6¢), 4.47–4.83 (m, 8 H, PhCH2), 7.10–7.38 (m, 25 H, Ar).
13C NMR (62 MHz, CDCl3): d = 68.39 (C-6), 77.7, 78.8, 79.2, 79.5
(C-2 to C-5), 80.1 (C≡C), 86.5 (C-1), 88.6 (C≡C), 113.3 (dd,
J = 239 Hz, CF2), 119.9 (Ar-C), 127.4–128.4 (Ar-CH), 132.2 (Ar-
CH), 137.7–138.3 (Ar-C).
19F NMR (235 MHz, CDCl3): d = –88.66 (dd, J = 285, 7 Hz, 1 F), –
91.44 (dd, J = 284, 7 Hz, 1 F).
1-(2,3,4,6-Tetra-O-benzyl-D-gluco-b-C-pyranosyl)-3-phenyl-
prop-2-yn-1-one (6)
To the mixture of 4a and 4b (240 mg, 0.366 mmol) in anhyd DMSO
(6.5 mL) was added in one portion IBX (154 mg, 0.550 mmol). Af-
ter stirring overnight, H2O (10 mL) was added and the mixture was
extracted with Et2O (3 × 10 mL). The combined organic phases
were washed with brine and then dried over MgSO4. After filtration,
Synthesis 2006, No. 6, 979–982 © Thieme Stuttgart · New York

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J. Picard et al.
PAPER
HRMS (CI): m/z [M + NH4]+ calcd for C43H44NO6F2: 692.3188;
found: 692.3190.
1H NMR (250 MHz, CDCl3): d = 1.46 (m, 4 H, CHCH2CH2), 1.91
(m, 12 H, OAc), 2.53 (t, J = 7.6 Hz, 2 H, PhCH2), 3.33 (m, 1 H, H-
1), 3.52 (m, 1 H, H-1), 4.00 (dd, J = 12.0, 2.0 Hz, 1 H, H-6), 4.17
(dd, J = 12.3, 5.0 Hz, 1 H, H-6¢), 4.80 (t, J = 9.6 Hz, 1 H, H-2), 4.96
(t, J = 9.7 Hz, 1 H, H-3), 5.08 (t, J = 9.2 Hz, 1 H, H-4), 7.07–7.23
(m, 5 H, Ar-CH).
13C NMR (62 MHz, CDCl3): d = 20.7 (CH3COO), 26.6
(CHCH2CH2), 30.5 (CHCH2), 35.3 (PhCH2), 62.3 (C-6), 68.6 (C-3),
71.8 (C-2), 74.3 (C-4), 75.6 (C-5), 77.5 (C-1), 125.7, 128.2, 128.3
(Ar-CH), 141.9 (Ar-C), 169.4, 169.6, 170.3, 170.6 (CO).
1-(2,3,4,6-Tetra-O-acetyl-D-gluco-b-C-pyranosyl)-3-phenylpro-
pan-1-ol (9)
To a solution of propargylic alcohol 5 (355 mg, 0.770 mmol) in di-
oxane–EtOH–H2O, (2:2:1) (6 mL) was added 10% palladium on
carbon (81 mg). The medium was shaken for 3 h under 1 atm of H2,
then filtered through celite and the solvents were removed under re-
duced pressure. Compound 9 (340 mg, 95%) was obtained as mix-
ture of diastereomers and used without purification.
1H NMR (250 MHz, CDCl3; 9a+9b): d = 1.91–2.00 (m, 14 H, OAc,
CH2CHOH), 2.45 (s, 1 H, OH), 2.58 (m, 1 H, PhCHH), 2.75 (m, 1
H, PhCHH), 3.21 (d, J = 8.4 Hz, 1 H, H-1), 3.39 (s, 1 H, CHOH),
3.50–3.59 (m, 1 H, H-5), 3.97–4.22 (m, 2 H, H-6, H-6¢), 4.92–5.16
(m, 3 H, H-2 to H-4), 7.10–7.23 (m, 5 H, Ar).
13C NMR (62 MHz, CDCl3; 9a): d = 21.0 (CH3COO), 32.5
(CH2CHOH), 35.1 (PhCH2), 62.7 (C-6), 68.0 (CHOH), 69.0, 69.5,
74.5 (C-2 to C-4), 76.2 (C-5) 79.6 (C-1), 125.5, 128.0, 128.2 (Ar-
CH), 141.4 (Ar-C), 169.1, 169.9, 170.2, 170.7 (CO).
13C NMR (62 MHz, CDCl3; 9b): d = 21.1 (CH3COO), 32.1
(CH2CHOH), 33.0 (PhCH2), 62.5 (C-6), 70.31 (CHOH), 68.6, 68.8,
74.9 (C-2 to C-4), 76.2 (C-5) 80.7 (C-1), 125.5, 128.0, 128.2 (Ar-
CH), 141.4 (Ar-C), 169.1, 169.9, 170.2, 170.7 (CO).
HRMS (CI): m/z [M + H]+ calcd for C23H31O9: 451.1968; found:
451.1971.
Acknowledgment
The authors thank Dr. Stéphanie Norsikian for a gift of aldehyde 2
and the French Ministry of Research for a grant (JP).
References
(1) (a) Postema, M. H. D. C-Glycoside Synthesis; CRC Press:
Boca Raton, 1995. (b) Levy, D. E.; Tang, C. The Chemistry
of C-Glycosides; Pergamon Press: Oxford, 1995.
(2) Blackburn, G. M.; Kent, D. E.; Kolkmann, F. J. Chem. Soc.,
Perkin Trans. 1 1984, 119.
(3) (a) Augé, J.; Lubin-Germain, N.; Seghrouchni, L.
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(4) Boucard, V.; Larrieu, K.; Lubin-Germain, N.; Uziel, J.;
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(5) Lasterra-Sanchez, M. E.; Michelet, V.; Besnier, I.; Genêt, J.
P. Synlett 1994, 705.
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1-(2,3,4,6-Tetra-O-acetyl-D-gluco-b-C-pyranosyl)-3-phenylpro-
pane (10)
To a solution of 9 (99 mg, 0.214 mmol) in 2 mL of THF under Ar
were added thiocarbonyldiimidazole (382 mg, 2.14 mmol) and
DMAP (392 mg, 3.21 mmol) and the mixture was heated at 70 °C
for 5 h. The medium was then concentrated under reduced pressure
and filtered through celite in order to remove the excess thiocar-
bonyldiimidazole and DMAP. The residue was then diluted with
anhyd toluene and Bu3SnH (577 mL, 2.14 mmol) and AIBN (35 mg,
0.21 mmol) were successively added under Ar. This mixture was
heated at 85 °C for 3 h. After concentration in vacuo, the crude
product was purified by flash chromatography on silica gel (cyclo-
hexane–EtOAc, 1:1) to give 10 (52 mg, 54%) as a colorless oil;
[a]D25 –9.9 (c 3.5 CHCl3).
Synthesis 2006, No. 6, 979–982 © Thieme Stuttgart · New York