Gram-scale quaternarization of the anomeric position of carbohydrates: Dramatic effects of molecular sieves on rhodium(II)-mediated decomposition of diazo sugars

Article abstract of DOI:10.1055/s-0032-1317013

The optimization of rhodium(II) carbene mediated quaternarization of the anomeric position of carbohydrates is reported. Preparation of ketopyranosides in good and reliable yields requires reverse addition of the substrate to a highly diluted suspension o

Full text of DOI:10.1055/s-0032-1317013

PRACTICAL SYNTHETIC PROCEDURES
▌3731
practical synthetic procedures
Gram-Scale Quaternarization of the Anomeric Position of Carbohydrates:
Dramatic Effects of Molecular Sieves on Rhodium(II)-Mediated Decomposi-
tion of Diazo Sugars
Large-Scale Preparation of Ketopyranosides
Mélissa Boultadakis-Arapinis, Camille Lescot, Laurent Micouin, Thomas Lecourt*
Laboratoire de Chimie Thérapeutique (UMR CNRS 8638), Université Paris Descartes, Sorbonne Paris Cité,
Faculté des Sciences Pharmaceutiques et Biologiques, 4 avenue de l’Observatoire, 75006 Paris, France
Fax +33(1)43291403; E-mail: Thomas.Lecourt@parisdescartes.fr
PSP
Received: 21.05.2012; Accepted after revision: 10.07.2012
No239
Abstract: The optimization of rhodium(II) carbene mediated quaternarization of the anomeric position of carbohydrates is reported. Prep-
aration of ketopyranosides in good and reliable yields requires reverse addition of the substrate to a highly diluted suspension of the catalyst
in refluxing 1,2-dichloroethane, as well as addition of a carefully controlled amount of molecular sieves, and vigorous stirring. Following
these optimized reaction conditions, functionalization of the anomeric position of carbohydrates can finally be performed on a preparative
scale.
Key words: rhodium, carbenes, molecular sieves, insertion, carbohydrates
Ph
O
Ph
O
Ph
O
O
O
O
O
O
O
O
O
O
OMe
OMe
N2
AcO
AcO
OMe
AcO
Rh2(OAc)4
Rh2(OAc)4 (1 mol%)
1,2-DCE, reflux
O
O
O
via:
O
O
4 Å MS
vigorous stirring
O
N2
Rh2(OAc)4
O
O
O
Ph
O
O
AcO
Ph
O
O
AcO
( >1000 rpm)
Ph
O
O
AcO
O
O
O
OMe
OMe
up to 1 gram scale
OMe
up to 65%
Scheme 1 Carbene-mediated functionalization of the anomeric C–H bond of carbohydrates
bromoacetate at position 2 was converted into a diazo ac-
etate. This carbene precursor then induced quaternariza-
Introduction
Since the pioneering work of Teyssié and co-workers in tion of the anomeric position in a late stage of the
early 1970s,1 rhodium(II) carboxylates and carboxami- synthetic process (Scheme 2). This preliminary study re-
dates have been widely used to catalyze carbene-mediated vealed that slow addition of the substrate to a highly dilut-
transformations.2 Thus, smooth decomposition of diazo ed suspension of the catalyst in refluxing 1,2-
compounds by rhodium(II) salts delivers metal carbenes dichloroethane was critical to prevent carbene dimeriza-
with finely tuned chemical properties arising from the tion.
unique structure and coordinating properties of the cata-
lyst.3,4 Among carbene-mediated transformations, stereo-
O
O
1) stereoselective
glycosylation
specific formation of a new C–C bond from an unreactive
C–H bond is of particular interest because it opens new
ways for planning the synthesis of complex molecules.5 In
this context, we recently reported that insertion of Rh(II)
carbenes into the anomeric C–H bond of carbohydrates
provides an efficient access toward both α- and β-ketopy-
ranosides.6 Thus, after stereoselective formation of a gly-
Br
P6O
P4O
P3O
O
P6O
P4O
P3O
O
O
O
2) diazo transfer
3) C–H functionalization
SPh
O
R
Scheme 2 Preparation of ketopyranosides by functionalization of the
anomeric C–H bond
cosidic linkage by anchimeric assistance,
a key
However, adaptation of this procedure to the large-scale
preparation of ketopyranosides might be tricky, and we
therefore undertook studies on new model compounds in
order to identify reaction conditions enabling quater-
SYNTHESIS 2012, 44, 3731–3734
Advanced online publication: 29.08.2012
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0032-1317013; Art ID: SS-2012-T0456-PSP
© Georg Thieme Verlag Stuttgart · New York

3732
M. Boultadakis-Arapinis et al.
PRACTICAL SYNTHETIC PROCEDURES
narization of carbohydrates on a preparative scale More careful distillation of the solvent (twice over P2O5)
(Scheme 1).
did not improve the yield of γ-lactone 2 (entry 4), but ad-
dition of 4 Å molecular sieves9 finally fully prevented this
side reaction, even when simply using commercially
available anhydrous 1,2-dichloroethane (entry 5). Howev-
er, while increasing the scale to one mmol under these
Scope and Limitations
Carbene precursor 1 was first prepared after adaptation of newly optimized conditions, 2 could only be obtained in
a bromoacetylation/diazo transfer sequence recently re- moderate yield. Careful analysis of the crude product
ported by Fukuyama.7,8 Decomposition of 1 under showed some recovered starting material,10 together with
Rh2(OAc)4 catalysis was then performed on a 0.1 mmol alcohol 5 (Figure 1) that might result from degradation of
scale. Thus, at a sub-millimolar concentration (Table 1, a transient oxonium ylide.11
entry 1) the targeted γ-lactone 2 was obtained only in low
Suspecting that partial conversion of the substrate and for-
yield together with large amounts of dimer 3. As expect-
mation of 5 might have been induced by the additive, we
ed, decreasing catalyst loading and concentration dimin-
then moved back to 3-O-pivalate compound 6, a substrate
that was already quaternarized in good yield on a 0.5
ished this intermolecular side reaction (entry 2).
Nevertheless, further dilution delivered 2 in poor yield be-
cause of competitive formation of 4 (Figure 1) by inser-
tion of the reactive moiety into water (entry 3).
mmol scale in the absence of molecular sieves (Table 2,
entry 1).6 The influence of the amount of additive was
evaluated first, a parameter that we did not carefully con-
trol until then, on the outcome of the transformation.
Thus, decomposition of 6 with 0.5 mol% of catalyst was
complete with up to 10 grams of molecular sieves per mil-
ligram of catalyst, but dramatically decreased at higher ra-
tio (Table 2, entries 2–6). Moreover, additional stirring of
the substrate over molecular sieves before its reverse ad-
dition to a suspension of the catalyst and drying agent re-
sulted in moderate to low conversion (entries 7 and 8),
whereas using powder instead of mesh (entries 9 and 10)
had no visible effect.
O
OH
OH
O
O
Ph
O
Ph
O
O
O
O
AcO
AcO
OMe
OMe
4
5
Figure 1 By-products arising from the decomposition of 1
Table 1 Catalytic Decomposition of α-Mannoside 1
O
Table 2 Influence of Molecular Sieves on the Decomposition of 6
O
Ph
O
O
O
O
O
AcO
O
N2
O
O
Rh2(OAc)4 (0.5 mol%)
4 Å MS
OMe
Ph
O
N2
2
Ph
O
O
O
O
O
O
Rh2(OAc)4
1,2-DCE, reflux
Ph
O
PivO
PivO
O
1,2-DCE, reflux
O
+
OMe
O
OMe
AcO
OMe
6
7
OAc
O
OMe
O
Entry 4 Å MS
MS/Rh2(OAc)4 Conv. (%)a Yield (%)
O
Ph
1
O
O
16
2
no
–
100
100
100
100
70
77b
15–70a
62a
Ph
O
O
O
O
AcO
3
8–12 mesh
8–12 mesh
8–12 mesh
8–12 mesh
8–12 mesh
8–12 mesh
8–12 mesh
powder
1 g/mg
3
OMe
3
Entry Rh2(OAc)4
Rate
4 Å MS Yield (%) Yield (%)
4
10
20
40
3
45a
(mol%, concn) (μmol/h)
of 2
of 3
43b
64b
30b
45b
90f
5
53a
1a
2a
3c
4d
5e
5, 0.8 mM
2, 0.16 mM
2, 0.08 mM
2, 0.08 mM
2, 0.08 mM
40
40
12
12
12
no
no
no
no
yes
42b
11b
0
6
50
21a
7c
8c
9
80
63a
20
1
40
15a
0
100
100
65a
0
59a
a 1,2-Dichloroethane freshly distilled over P2O5.
b Evaluated by 1H NMR of the crude product.
c Additional 55% of 4 from insertion into water.
d 1,2-dichloroethane freshly distilled twice over P2O5.
e Commercially available anhydrous 1,2-dichloroethane.
f After purification by chromatography.
10
powder
3
a Evaluated by 1H NMR of the crude product.
b After purification by chromatography.
c Substrate dried over 4 Å MS before the reaction.
Synthesis 2012, 44, 3731–3734
© Georg Thieme Verlag Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Large-Scale Preparation of Ketopyranosides
3733
(230–400 mesh, E. Merck). Anhyd 1,2-dichloroethane and CH2Cl2
were purchased from Sigma-Aldrich. THF was distilled over sodi-
um/benzophenone. All reactions were conducted under an argon at-
mosphere in flame-dried glassware. Rh2(OAc)4, was purchased
from Sigma-Aldrich, and Deloxan® was purchased from Strem
Chemicals.
However, even if full conversion of diazo sugar 6 could
always be ensured at low molecular sieves/catalyst ratio,
the yield of γ-lactone 7 was dramatically nonreproducible
(Table 2, entry 2) because of the erratic formation of a by-
product related to 5. Suspecting that this side reaction
might be due to the high heterogeneity of the reaction
mixture, we became very careful at controlling the stir-
ring, and were very pleased to see that 7 could be obtained
in reproducible yield when the reaction mixture was
placed under vigorous stirring (>1000 rpm).
Methyl 3-O-Acetyl-4,6-O-benzylidene-2-O-diazoacetyl-α-D-
mannopyranoside (1)
To a solution of methyl 3-O-acetyl-4,6-O-benzylidene-α-D-manno-
pyranoside (1.06 g, 2.380 mmol) and pyridine (0.77 mL, 9.496
mmol) in anhyd CH2Cl2 (30 mL) at 0 °C was added bromoacetyl
bromide (0.41 mL, 4.748 mmol). After stirring for 2 h at 0 °C, the
reaction mixture was quenched with MeOH (3 mL), while TLC (cy-
clohexane–EtOAc, 1:1) showed complete consumption of the start-
Having a better understanding of the effects of molecular
sieves on this quaternarization process, the decomposition
of diazo sugar 1 was finally conducted on a one mmol ing material, and aq 1 M HCl (20 mL) was added. After extractions
with CH2Cl2 (3 × 20 mL), the combined organic layers were dried
(MgSO4), filtered, and concentrated under vacuum. To a solution of
the residue and N,N′-ditosylhydrazine (1.62 g, 4.760 mmol) in dis-
tilled THF (35 mL) at 0 °C was added DBU (1.78 mL, 11.900
scale in 52% yield. Moreover, the carbene precursor 8 in
the β-gluco series was also prepared and functionalization
of its axial anomeric C–H bond on a gram-scale in 65%
yield was performed (Scheme 3).
mmol). After TLC analysis (cyclohexane–EtOAc, 3:1) showed
complete consumption of the starting material, sat. aq NaHCO3 (30
mL) and EtOAc (30 mL) were added. The organic layer was sepa-
rated, dried (MgSO4), filtered, and concentrated under vacuum. The
residue was purified by silica gel chromatography (cyclohexane–
EtOAc, 3:1) to give the diazo sugar 1 as a bright yellow foam (660
mg, 71%); [α]D20 –25.2 (c = 1.0, CHCl3).
Ph
O
Ph
O
O
O
O
O
Rh2(OAc)4 (1 mol%)
4 Å MS
O
O
OMe
N2
OMe
AcO
AcO
1,2-DCE, reflux
stirring >1000 rpm
O
O
IR (film): 2116, 1746, 1698, 1457, 1384, 1370, 1283, 1229, 1175,
1133, 1097, 1077, 1029 cm–1.
8
gram scale
10 µmol/h
9
65%
1H NMR (300 MHz, CDCl3): δ = 7.48–7.28 (m, 5 Harom), 5.58 (s, 1
H, H-7), 5.46–5.31 (m, 2 H, H-2, H-3), 4.91 (br s, 1 H, H-8), 4.72
(s, 1 H, H-1), 4.30 (dd, J = 10.0, 4.0 Hz, 1 H, H-6eq), 4.06–3.91 (m,
2 H, H-4, H-5), 3.90–3.79 (m, 1 H, H-6ax), 3.42 (s, 3 H, OCH3), 2.05
(s, 3 H, CH3C=O).
Scheme 3 Gram-scale decomposition of 8
13C NMR (75 MHz, CDCl3): δ = 169.9 (C=O), 165.9 (C=O), 137.0
(Carom), 129.1 (CHarom), 128.2 (CHarom), 126.1 (CHarom), 101.9 (C-7),
99.5 (C-1), 76.0 (C-4), 70.2 (C-2), 68.7 (C-6), 68.3 (C-3), 63.6 (C-
5), 55.2 (OCH3), 46.5 (C-8), 20.8 (CH3C=O).
Conclusion
In summary, we have shown that molecular sieves have a
dramatic effect on carbene-mediated functionalization of
the anomeric C–H bond of carbohydrates. Thus, the
amount of additive must be carefully controlled, and the
reaction mixture must be placed under vigorous stirring in
order to prevent partial conversion of the substrate and
loss of the reactive moiety. After optimization of the reac-
tion conditions, decomposition of diazo sugars was finally
performed on a gram-scale in the presence of 1 mol% of
catalyst using commercially available anhydrous solvent.
Functionalization of the anomeric C–H bond of carbohy-
drates on preparative scale now opens the way to the de-
sign and preparation of new chemical tools for
glycobiology.12
MS (ESI): m/z = 415 (MNa+).
HRMS (ESI): m/z calcd for C18H20N2O8 + Na: 415.1117; found:
415.1115.
(2R,4aR,5aS,8aS,9S,9aR)-5a-Methoxy-7-oxo-2-phenyloctahy-
drofuro[2′,3′:5,6]pyrano[3,2-d][1,3]dioxin-9-yl Acetate (2)
A suspension of Rh2(OAc)4 (5.7 mg, 12.970 μmol) in commercially
available anhyd 1,2-dichloroethane (200 mL) was stirred for 1 h at
r.t. with activated 4 Å molecular sieves (8 to 12 mesh, 6 g). A solu-
tion of 1 (510 mg, 1.297 mmol) in the same solvent (10 mL) was
then added dropwise via a syringe pump (10 μmol/h) to the reflux-
ing reaction mixture under vigorous stirring (>1000 rpm). After the
end of the addition, the reaction mixture was stirred for 3 h in the
presence of Deloxan®, filtered over Celite, and concentrated under
reduced pressure. The residue was purified by silica gel chromatog-
raphy (cyclohexane–EtOAc, 3:1) to give the γ-lactone 2 (245 mg,
52%) as a colorless oil; [α]D20 –78.4 (c = 1.0, CHCl3).
Optical rotations were measured at 20 °C with a Perkin-Elmer Mod-
el 341 polarimeter, in a 10 cm, 1 mL cell. IR spectra were recorded
with a Nicolet 510 FT-IR spectrometer. Mass spectrometry spectra
were recorded on a Waters ZQ 2000 spectrometer. High-resolution
mass spectra were recorded on a Bruker MicrO-Tof-Q 2 spectrom-
IR (film): 1801, 1741, 1457, 1372, 1306, 1285, 1221, 1183, 1159,
1127, 1096, 1075, 1034, 1004 cm–1.
1H NMR (300 MHz, CDCl3): δ = 7.53–7.29 (m, 5 Harom), 5.55 (s, 1
H, H-7), 5.33 (dd, J = 9.9, 3.8 Hz, 1 H, H-3), 4.78 (d, J = 3.8 Hz, 1
H, H-2), 4.30 (dd, J = 10.0, 4.6 Hz, 1 H, H-6eq), 4.02 (t, J = 9.9 Hz,
1 H, H-4), 3.95–3.74 (m, 2 H, H-5, H-6ax), 3.38 (s, 3 H, OCH3), 2.82
(d, J = 16.4 Hz, 1 H, H-8), 2.70 (d, J = 16.4 Hz, 1 H, H-8′), 2.13 (s,
3 H, CH3C=O).
13C NMR (75 MHz, CDCl3): δ = 171.3 (C=O), 170.3 (C=O), 136.7
(Carom), 129.2 (CHarom), 128.2 (CHarom), 126.1 (CHarom), 103.5 (C-1),
101.8 (C-7), 78.6 (C-2), 74.5 (C-4), 68.4 (C-3), 68.1 (C-6), 64.4 (C-
5), 51.8 (OCH3), 40.1 (C-8), 20.8 (CH3C=O).
1
eter at CRMPO (Rennes, France). H NMR spectra were recorded
at 300 MHz with a Bruker Avance 300 spectrometer. 13C NMR
spectra were recorded at 75 MHz with a Bruker Avance 300 spec-
trometer with adoption of 77.00 ppm for the central line of CDCl3.
The relaxation delay (D1) was increased to 60 seconds for 13C NMR
spectra of diazo sugars. Reactions were monitored by TLC on a pre-
coated silica gel 60 F254 plate (layer thickness 0.2 mm; E. Merck,
Darmstadt, Germany) and detection by charring phosphomolybdic
acid. Flash column chromatography was performed on silica gel 60
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 3731–3734

3734
M. Boultadakis-Arapinis et al.
PRACTICAL SYNTHETIC PROCEDURES
MS (ESI): m/z = 387 (MNa+).
13C NMR (75 MHz, CDCl3): δ = 170.8 (C=O), 169.5 (C=O), 136.5
(Carom), 129.3 (CHarom), 128.3 (CHarom), 126.1 (CHarom), 103.7 (C-1),
101.7 (C-7), 81.9 (C-2), 75.8 (C-4), 72.0 (C-3), 68.8 (C-6), 66.3 (C-
5), 50.7 (OCH3), 36.6 (C-8), 20.8 (CH3C=O).
HRMS (ESI): m/z calcd for C18H20O8 + Na: 387.1056; found:
387.1058.
MS (ESI): m/z = 387 (MNa+).
Methyl 3-O-Acetyl-4,6-O-benzylidene-2-O-diazoacetyl-β-D-glu-
copyranoside (8)
HRMS (ESI): m/z calcd for C18H20O8 + Na: 387.1056; found:
To a solution of methyl 3-O-acetyl-4,6-O-benzylidene-β-D-gluco-
pyranoside (4.09 g, 12.611 mmol) and pyridine (2.56 mL, 31.528
mmol) in anhyd CH2Cl2 (150 mL) at 0 °C was added bromoacetyl
bromide (2.2 mL, 25.222 mmol). After stirring for 2 h at 0 °C, the
reaction mixture was quenched with MeOH (1 mL), while TLC (cy-
clohexane–EtOAc, 1:1) showed complete consumption of the start-
ing material, and a solution of aq 1 M HCl (50 mL) was added. After
extractions with CH2Cl2 (3 × 70 mL), the combined organic layers
were dried (MgSO4), filtered, and concentrated under vacuum. To a
solution of the residue and N,N′-ditosylhydrazine (8.6 g, 25.200
mmol) in distilled THF (350 mL) at 0 °C was added DBU (9.43 mL,
63.0 mmol). After TLC analysis (cyclohexane–EtOAc, 3:1) showed
complete consumption of the starting material, sat. aq NaHCO3 (50
mL) and EtOAc (50 mL) were added. The organic layer was sepa-
rated, dried (MgSO4), filtered, and concentrated under vacuum. The
residue was purified by silica gel chromatography (cyclohexane–
EtOAc, 3:1) to give the diazo sugar 8 as a bright yellow foam (4.43
g, 90%); [α]D20 –53.3 (c = 1.0, CHCl3).
387.1057.
Acknowledgment
Financial support of this work by CNRS (Research Fellowship to
T.L.) and University Paris Descartes (MESR grant to M.B.A.) is
gratefully acknowledged.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis. Included
are 1H and 13C NMR spectra for compounds 1, 2, 4, 8, and 9.SunpIgfpi
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References
(1) Paulissen, R.; Reimlinger, H.; Hayez, E.; Hubert, A. J.;
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(3) For catalyst-induced chemoselective transformations, see:
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128, 2524.
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(10) Padwa, A.; Dean, D. C.; Zhi, L. J. Am. Chem. Soc. 1992,
114, 593.
IR (film): 2114, 1744, 1699, 1453, 1380, 1227, 1174, 1155, 1089,
1062, 1041, 1030, 1010 cm–1.
1H NMR (300 MHz, CDCl3): δ = 7.49–7.29 (m, 5 Harom), 5.52 (s, 1
H, H-7), 5.34 (t, J = 9.4 Hz, 1 H, H-3), 5.06 (t, J = 9.4 Hz, 1 H, H-
2), 4.80 (br s, 1 H, H-8), 4.51 (d, J = 9.4 Hz, 1 H, H-1), 4.39 (dd,
J = 10.4, 4.8 Hz, 1 H, H-6eq), 3.82 (t, J = 10.4 Hz, 1 H, H-6ax), 3.71
(t, J = 9.4 Hz, 1 H, H-4), 3.59–3.47 (m, 4 H, OCH3, H-5), 2.08 (s, 3
H, CH3C=O).
13C NMR (75 MHz, CDCl3): δ = 170.1 (C=O), 136.7 (Carom), 129.1
(CHarom), 128.2 (CHarom), 126.1 (CHarom), 102.3 (C-1), 101.4 (C-7),
78.3 (C-4), 72.4 (C-2), 71.6 (C-3), 68.5 (C-6), 66.3 (C-5), 57.3
(OCH3), 46.4 (C-8), 20.8 (CH3C=O).
MS (ESI): m/z = 415 (MNa+).
HRMS (ESI): m/z calcd for C18H20N2O8 + Na: 415.1117; found:
415.1112.
(2R,4aR,5aR,8aR,9S,9aR)-5a-Methoxy-7-oxo-2-phenyloctahy-
drofuro[2′,3′:5,6]pyrano[3,2-d][1,3]dioxin-9-yl Acetate (9)
A suspension of Rh2(OAc)4 (11 mg, 25 μmol) in commercially
available anhyd 1,2-dichloroethane (300 mL) was stirred for 1 h at
r.t. with activated 4 Å molecular sieves (8 to 12 mesh, 11 g). A so-
lution of 8 (1.010 g, 2.58 mmol) in the same solvent (20 mL) was
then added dropwise via a syringe pump (10 μmol/h) to the reflux-
ing reaction mixture under vigorous stirring (>1000 rpm). After the
end of the addition, the reaction mixture was stirred for 3 h in the
presence of Deloxan®, filtered over Celite, and concentrated under
reduced pressure. The residue was purified by silica gel chromatog-
raphy to give the γ-lactone 9 (615 mg, 65%) as a colorless oil;
[α]D20 +6.1 (c = 1.0, CHCl3).
(11) Kumaraswamy, G.; Sadaiah, K.; Ramakrishna, D. S.; Police,
N.; Sridhar, B.; Bharatam, J. Chem. Commun. 2008, 5324.
(12) For recent reviews on the modification of carbohydrate
scaffolds, see: (a) Doores, K. J.; Gamblin, D. P.; Davis, B. G.
Chem.–Eur. J. 2006, 12, 656. (b) Koester, D. C.;
Holkenbrink, A.; Werz, D. B. Synthesis 2010, 3217.
(c) Awan, S. I.; Werz, D. B. Bioorg. Med. Chem. 2012, 20,
1846.
IR (film): 1797, 1755, 1371, 1226, 1178, 1081, 1036 cm–1.
1H NMR (300 MHz, CDCl3): δ = 7.49–7.33 (m, 5 Harom), 5.54 (s, 1
H, H-7), 5.30 (dd, J = 10.2, 5.8 Hz, 1 H, H-3), 4.45 (d, J = 5.8 Hz,
1 H, H-2), 4.37 (d, J = 5.8 Hz, 1 H, H-6), 3.95 (td, J = 10.2, 1.7 Hz,
1 H, H-4), 3.83–3.71 (m, 2 H, H-5, H-6), 3.41 (s, 3 H, OCH3), 2.91
(d, J = 17.1 Hz, 1 H, H-8), 2.69 (d, J = 17.1 Hz, 1 H, H-8′), 2.13 (s,
3 H, CH3C=O).
Synthesis 2012, 44, 3731–3734
© Georg Thieme Verlag Stuttgart · New York