Asymmetric synthesis of (-)-6-epi-centrolobine

Article abstract of DOI:10.1055/s-2008-1067218

A stereoselective total synthesis of (-)-6-epi-centrolobine, an unnatural analogue of (-)-centrolobine, starting from readily available tri-O-acetyl-D-glucal has been described for the first time. The key steps involved in this synthetic approach are stereoselective C-glycosidation, dehydroxylation and Wittig reaction. The target molecule was achieved in nine steps with 49% overall yield. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1067218

PAPER
2939
Asymmetric Synthesis of (–)-6-epi-Centrolobine
A
symmetric Syn
h
a
6
-epi-Centr
d
olobine a Raji Reddy,* Pasupulety Phani Madhavi, Srivari Chandrasekhar
Organic Division – I, Indian Institute of Chemical Technology, Hyderabad 500007, India
Fax +91(40)27160512; E-mail: rajireddy@iict.res.in
Received 14 May 2008; revised 28 May 2008
by a hydrogenation reaction. We postulated that this pre-
cursor 4 could be accessed by a palladium-catalyzed ste-
reoselective C-glycosidation of tri-O-acetyl-D-glucal 5
followed by dehydroxylation as key reactions
Abstract: A stereoselective total synthesis of (–)-6-epi-centro-
lobine, an unnatural analogue of (–)-centrolobine, starting from
readily available tri-O-acetyl-D-glucal has been described for the
first time. The key steps involved in this synthetic approach are ste-
reoselective C-glycosidation, dehydroxylation and Wittig reaction. (Scheme 1).
The target molecule was achieved in nine steps with 49% overall
yield.
OMe
Key words: centrolobine, tri-O-acetyl-D-glucal, C-glycosidation,
dehydroxylation, Wittig reaction
PPh3+Br–
O
2
HO
+
BnO
3
4
(–)-Centrolobine (1) is a natural product obtained from the
Amazon forest, isolated from the heartwood of Centrolo-
bium robustum and from the stem of Brosinium potabile.1
Centrolobine is known to exhibit anti-leishmanial activity
against Leishmania amazonesis prosmatigotes.2 The ab-
solute configuration of this natural product was deter-
mined in 2002 by Colobert et al. via asymmetric total
synthesis.3 Due to the biological importance and relatively
simple structure of (–)-centrolobine, it makes a perfect
target for synthetic organic chemists and its enantioselec-
tive synthesis has been achieved by various groups,4 in-
cluding ours.5 After the synthesis of (–)-centrolobine (1),
we turned our attention to the synthesis of its analogues,
unnatural derivatives, which can be used to study struc-
ture–activity relationships (SARs), with the anticipation
of obtaining better biological activity. Towards this aim,
herein we report the asymmetric synthesis of (–)-6-epi-
centrolobine (2; Figure 1), a non-natural analogue.
O
AcO
AcO
OAc
5
Scheme 1 Retrosynthetic approach to (–)-6-epi-centrolobine
Thus, C-aryl pseudoglycal 7 was prepared by the stereo-
selective C-glycosidation of tri-O-acetyl-D-glucal (5)
with 4-methoxyphenylboronic acid (6) in the presence of
Pd(OAc)2 in 78% yield (Scheme 2).6 Next, deacetylation
of compound 7 was carried out using potassium carbonate
to afford the diol 8 in 96% yield. The primary hydroxyl
group of diol 8 was selectively protected as its tert-bu-
tyldimethylsilyl (TBDMS) ether 9 using imidazole and
TBDMSCl in 91% yield. Dehydroxylation of the second-
ary hydroxyl group of compound 9 was achieved via tosy-
lation of the hydroxyl group using p-tolunesulfonyl
chloride/pyridine followed by the treatment with lithium
aluminum hydride to give 10 (88% yield, two steps). At
this stage, the TBDMS protection in 10 was removed by
tetrabutylamonium fluoride (TBAF) to reveal the primary
alcohol 4 in 97% yield. The alcohol 4 was oxidized using
Dess–Martin periodinane into the aldehyde, which was
then subjected to a Wittig olefination with 4-benzyloxy-
benzyl triphenylphosphonium bromide (3), which was
prepared using a known procedure.7,4b In the presence of
n-butyllithium in tetrahydrofuran, compound 11 was ob-
tained as a mixture of E/Z isomers (3:2) in 87% yield (two
steps). Finally, one-pot deprotection of the benzyl ether
and reduction of two double-bonds in compound 11 (mix-
ture of E/Z, 3:2) using hydrogenation in the presence of
PtO2 (98% yield), completed the total synthesis of (–)-6-
epi-centrolobine (2). This unnatural analogue of (–)-cen-
From a retrosynthetic perspective, we envisioned 2, 6-
trans-disubstituted dihydropyran 4 as a precursor, which
would lead to 2 through a Wittig reaction with 3 followed
HO
OMe
S
O
R
6
2
(–)-centrolobine 1
HO
OMe
S
S
O
2
6
(–)-6-epi-centrolobine 2
Figure 1 Structures of (–)-centrolobine and its 6-epimer
1
trolobine was fully characterized by IR, HRMS, H and
SYNTHESIS 2008, No. 18, pp 2939–2942
x
x.
x
x
.
2
0
0
8
Advanced online publication: 06.08.2008
DOI: 10.1055/s-2008-1067218; Art ID: Z11108SS
13C NMR spectral data.
© Georg Thieme Verlag Stuttgart · New York

2940
C. R. Reddy et al.
PAPER
B(OH)2
OMe
OMe
O
AcO
AcO
Pd(OAc)2, MeCN
r.t., 16 h, 78%
K2CO3, MeOH
r.t., 2 h, 96%
O
O
+
AcO
AcO
HO
HO
OAc
OMe
7
8
5
6
OMe
OMe
i) py, TsCl,
CH2Cl2, 0 °C, 2 h
imidazole, TBDMSCl
r.t., 1.5 h, 91%
TBAF in THF (1 M)
THF, r.t., 1 h, 97%
O
O
TBSO
TBSO
ii) LAH, THF,
r.t., 3 h
88% (two steps)
HO
9
10
i) Dess–Martin periodinane,
CH2Cl2, 0 °C, 30 min
BnO
OMe
OMe
PtO2, H2 (balloon)
O
2
O
ii) 3, n-BuLi, THF,
0 °C to r.t., 2 h
87% (two steps)
HO
2
6
EtOAc–MeOH (3:1),
r.t., 8 h, 98%
4
11
Scheme 2
13C NMR (75 MHz, CDCl3): d = 170.7, 170.3, 159.5, 131.6, 130.7,
129.3 (2 × C), 124.9, 113.7 (2 × C), 73.7, 68.8, 65.0, 62.8, 55.1,
20.9, 20.7.
HRMS (ESI): m/z [M + Na]+ calcd for C17H20O6Na: 343.1157;
found: 343.1159.
The synthesis of other analogues of (–)-centrolobine in-
cluding its aza-analogue using different synthetic ap-
proaches is in progress. Upon completion of the synthesis,
all these analogues will be tested for anti-leishmanial ac-
tivity by comparison with the natural centrolobine and the
results will be published later.
(2S,5S,6R)-6-(Hydroxymethyl)-2-(4-methoxyphenyl)-5,6-dihy-
dro-2H-pyran-5-ol (8)
In conclusion, we have successfully achieved the asym-
metric total synthesis of (–)-6-epi-centrolobine from com-
mercially available tri-O-acetyl-D-glual in nine steps with
49% overall yield.
To a solution of 7 (0.35 g, 1.09 mmol) in MeOH (10 mL), was added
K2CO3 (0.45 g, 3.28 mmol) at 27 °C and the reaction was stirred for
2 h. Upon completion (reaction monitored by TLC), the solvent was
removed in vacuo, H2O (10 mL) was added and the residue was ex-
tracted with EtOAc (3 × 10 mL). The combined organic layer was
washed with brine (10 mL), dried over Na2SO4 and evaporated un-
der reduced pressure. The residue obtained was purified by silica
gel column chromatography (EtOAc–hexanes, 3:2) to afford 8.
All solvents and reagents were purified by standard techniques.
Crude products were purified by column chromatography on silica
gel (60–120 mesh). IR spectra were recorded on a Perkin–Elmer
683 spectrometer. Optical rotations were obtained on Jasco Dip 360
digital polarimeter. 1H and 13C NMR spectra were recorded in
CDCl3 on a Varian Gemini 200 and Brucker Avance 300. Chemical
shifts are reported in parts per million (ppm) with respect to internal
TMS. Coupling constants (J) are quoted in Hz. Mass spectra were
recorded on CEC-21-11013 or Fannigan Mat 1210 double focusing
mass spectrometers operating with a direct inlet system or LC/MSD
Trap SL (Agilent Technologies).
Yield: 0.46 g (96%); white solid; mp 85–86 °C; [a]D25 –88.4 (c 1.3,
CHCl3).
IR (neat): 3305, 3038, 2879, 1610, 1511, 1242, 1174, 1067, 1024,
823 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.27 (d, J = 8.5 Hz, 2 H), 6.84 (d,
J = 8.5 Hz, 2 H), 6.0 (s, 2 H), 5.17 (s, 1 H), 4.28–4.16 (m, 1 H), 3.79
(s, 3 H), 3.68 (t, J = 4.6 Hz, 2 H), 3.37–3.32 (m, 1 H).
13C NMR (75 MHz, CDCl3): d = 159.4, 131.3, 130.2, 129.7 (2 × C),
(2S,5S,6R)-6-(Acetoxymethyl)-2-(4-methoxyphenyl)-5,6-dihy-
dro-2H-pyran-5-yl Acetate (7)
128.8, 113.8 (2 × C), 73.7, 72.4, 63.5, 62.4, 55.3.
HRMS (ESI): m/z [M + Na]+ calcd for C13H16O4Na: 259.0946;
found: 259.0944.
To a mixture of the tri-O-acetyl-D-glucal (5; 0.5 g, 1.83 mmol) and
4-methoxyphenylboronic acid (6; 0.56 g, 3.67 mmol) in MeCN (5
mL) was added Pd(OAc)2 (0.046 g, 0.18 mmol). The resulting sus-
pension was stirred at r.t. for 16 h. After this time, the reaction mix-
ture was diluted with CH2Cl2 (15 mL) and filtered through a pad of
silica gel. The filtrate was concentrated and subjected to column
chromatography (EtOAc–hexanes, 1:4) to afford 7.
(2S,5S,6R)-6-[(tert-Butyldimethylsilyloxy)methyl]-2-(4-meth-
oxyphenyl)-5,6-dihydro-2H-pyran-5-ol (9)
To a solution of diol 8 (0.5 g, 2.11 mmol) in anhydrous CH2Cl2 (10
mL) at 0 °C was added imidazole (0.29 g, 4.2 mmol) followed by
TBDMSCl (0.32 g, 2.11 mmol) in anhydrous CH2Cl2 (5 mL) and
the reaction was allowed to warm to r.t. After stirring for 1.5 h, the
reaction mixture was diluted with H2O (10 mL), the organic layer
was separated and the aqueous layer was extracted with CH2Cl2
(3 × 10mL). The combined organic layer was washed with brine (15
mL), dried over Na2SO4 and concentrated to dryness. Purification of
the residue by silica gel column chromatography (EtOAc–hexanes,
7%) gave 9.
Yield: 0.67 g (78%); white solid; mp 39–41 °C; [a]D25 –6.2 (c 1.1,
CHCl3).
IR (neat): 3008, 2929, 1739, 1606, 1510, 1370, 1259, 1033, 817
cm–1.
1H NMR (300 MHz, CDCl3): d = 7.29 (d, J = 8.3 Hz, 2 H), 6.85 (d,
J = 8.3 Hz, 2 H), 6.15–6.1 (m, 1 H), 5.98–5.92 (m, 1 H), 5.28–5.21
(m, 2 H), 4.24–4.17 (m, 1 H), 4.05–3.99 (m, 1 H), 3.8 (s, 3 H),
3.76–3.7 (m, 1 H), 2.08 (s, 3 H), 2.05 (s, 3 H).
Yield: 0.67 g (91%); viscous liquid; [a]D25 –58.0 (c 1, CHCl3).
Synthesis 2008, No. 18, 2939–2942 © Thieme Stuttgart · New York

PAPER
Asymmetric Synthesis of (–)-6-epi-Centrolobine
2941
IR (neat): 3443, 2926, 2855, 1609, 1510, 1249, 1081, 778 cm–1.
13C NMR (75 MHz, CDCl3): d = 159.4, 132.8, 129.6 (2 × C), 127.6,
125.4, 113.8 (2 × C), 73.8, 67.7, 65.5, 55.4, 26.5.
1H NMR (300 MHz, CDCl3): d = 7.22 (d, J = 9.0 Hz, 2 H), 6.76 (d,
J = 9.0 Hz, 2 H), 5.9 (s, 2 H), 5.06 (s, 1 H), 4.1 (d, J = 6.7 Hz, 1 H),
3.71 (s, 3 H), 3.71–3.67 (m, 1 H), 3.64–3.57 (m, 1 H), 3.33–3.25
(m, 1 H), 2.93 (br s, 1 H), 0.82 (s, 9 H), 0.01 (s, 6 H).
HRMS (ESI): m/z [M + Na]+ calcd for C13H16O3Na: 243.0997;
found: 243.1000.
(2S,6S)-6-[4-(Benzyloxy)styryl]-2-(4-methoxyphenyl)-5,6-dihy-
dro-2H-pyran (11)
13C NMR (75 MHz, CDCl3): d = 159.6, 131.6, 129.8 (2 × C), 129.5,
128.7, 113.8 (2 × C), 73.7, 70.9, 67.4, 66.0, 55.4, 26.0 (3 × C), 18.3,
–5.3, –5.4.
HRMS (ESI): m/z [M + Na]+ calcd for C19H30O4NaSi: 373.1811;
found: 373.1823.
To a solution of alcohol 4 (0.1 g, 0.45 mmol) under a nitrogen atmo-
sphere was added anhydrous CH2Cl2 (5 mL) and Dess–Martin peri-
odinane (0.19 g, 0.45 mmol) at 0 °C and the reaction was stirred at
the same temperature for 30 min. The reaction mixture was
quenched with aq sat. Na2S2O3 (10 mL), the organic layer was sep-
arated and the aqueous layer was extracted with CH2Cl2 (3 × 10
mL). The combined organic layers were washed with brine (15
mL), dried over Na2SO4 and evaporated in vacuo to obtain the crude
product, which was used for the next step without purification.
(2S,6S)-6-[(tert-Butyldimethylsilyloxy)methyl]-2-(4-methoxy-
phenyl)-5,6-dihydro-2H-pyran (10)
To a solution of compound 9 (0.25 g, 0.71 mmol) in CH2Cl2 (5 mL)
was added pyridine (0.18 mL, 2.2 mmol), PTSA (0.17 g, 0.9 mmol)
and a catalytic amount of DMAP at 0 °C, and the reaction was
stirred for 2 h at the same temperature. After the reaction was com-
plete (monitored by TLC), the reaction mixture was diluted with
CH2Cl2 (5 mL), washed with aq sat. CuSO4 (2 × 5 mL), brine (5
mL), dried over Na2SO4 and concentrated to dryness. The residue
was passed through a silica gel pad (EtOAc–hexanes, 1:9), to obtain
the corresponding tosylated compound (~0.36 g), which was imme-
diately used in the next step.
To a solution of phosphonium salt 3 (1.24 g, 2.29 mmol) in THF (5
mL) at 0 °C was added n-BuLi (1.07 mL, 1.72 mmol, 1.6 M in hex-
anes). The solution turned orange-red and was stirred for 20 min at
0 °C. To this, a solution of the above prepared aldehyde (0.25 g,
1.15 mmol) in THF (5 mL) was added and the reaction mixture was
stirred for 30 min at 0 °C. After the reaction was complete (moni-
tored by TLC), the mixture was quenched with aq sat. NH4Cl (10
mL) and extracted with EtOAc (3 × 10 mL). The combined organic
layers were washed with brine (15 mL), dried over Na2SO4 and con-
centrated to dryness. Purification of the residue by silica gel column
chromatography (EtOAc–hexanes, 3%) gave 11.
To a suspension of LAH (0.077 g, 2.01 mmol) in anhydrous THF (5
mL) at 0 °C was added the above obtained tosylated compound
(0.36 g, 0.67 mmol) in THF (5 mL) and the reaction was stirred at
r.t. for 3 h. After the reaction was complete (monitored by TLC), the
mixture was quenched with moistened Na2SO4 (50 mg) at 0 °C and
stirring was continued for 3 h. The white colored salts were filtered
on a sintered-glass funnel and the residue was rinsed with H2O (20
mL), EtOAc (3 × 10 mL). The organic layer was separated and
washed with brine (15 mL), dried over Na2SO4 and concentrated on
rotary evaporator. The residue obtained was purified by silica gel
column chromatography (EtOAc–hexanes, 1:4) to yield 10.
Yield: 0.198 g (88%); viscous oil; [a]D25 –132.5 (c 1.4, CHCl3).
IR (neat): 2928, 2856, 1604, 1512, 1252, 1106, 838 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.25 (d, J = 8.3 Hz, 2 H), 6.78 (d,
J = 8.3 Hz, 2 H), 6.0–5.9 (m, 2 H), 5.14 (s, 1 H), 3.77 (s, 3 H), 3.76–
3.67 (m, 1 H), 3.64–3.47 (m, 2 H), 2.26–2.18 (m, 1 H), 2.04–1.99
(m, 1 H), 0.85 (s, 9 H), 0.01 (s, 6 H).
Yield: 0.39 g (87.3%); a 2:3 mixture of E/Z isomers.
IR (neat): 3448, 2922, 2852, 1605, 1509, 1244, 1174, 1032, 838,
739 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.42–7.21 (m, 9 H), 6.92–6.76 (m,
4 H), 6.51 (d, J = 9.0 Hz, 1 H), 6.43 (d, J = 11.3 Hz, 1 H), 6.11–5.86
(m, 2 H), 5.24 (s, 1 H), 5.04 (s, 1 H), 4.95 (s, 1 H), 4.51–4.41 (m,
1 H), 3.81 (s, 3 H), 2.23–2.14 (m, 2 H).
13C NMR (75 MHz, CDCl3): d = 159.6, 159.3, 158.5, 157.9, 137.1,
133.4, 133.1, 132.9, 130.5, 130.4, 130.2, 130.0, 129.5, 128.7, 128.1,
127.9, 127.8, 127.6, 125.5, 125.3, 115.0, 114, 4, 113.9, 113.8, 74.0,
73.7, 70.2, 69.9, 68.4, 64.1, 55.5, 31.5, 30.9.
HRMS (ESI): m/z [M + Na]+ calcd for C27H26O3Na: 421.1779;
found: 421.1795.
13C NMR (75 MHz, CDCl3): d = 159.1, 133.3, 129.4 (2 × C), 127.9,
125.4, 113.6 (2 × C), 73.6, 68.2, 66.3, 55.3, 27.5, 26.0 (3 × C), 18.4,
–5.2 (2 × C).
(2S,6S)-6-[4-(Hydroxy)phenethyl]-2-(4-methoxyphenyl)-tetra-
hydro-2H-pyran (2)
To a solution of compound 11 (0.03 g, 0.07 mmol) in a mixture of
EtOAc–MeOH (3:1, 3 mL) was added a catalytic amount of PtO2
and the mixture was kept under a H2 atmosphere (balloon) for 8 h at
27 °C. After completion of the reaction (monitored by TLC), the re-
action mixture was filtered through a pad of celite and concentrated
to dryness. Purification of the residue by silica gel column chroma-
tography (EtOAc–hexanes, 1:5) afforded the desired product 2.
HRMS (ESI): m/z [M + Na]+ calcd for C19H30O4NaSi: 357.1861;
found: 357.1874.
(2S,6S)-6-(Hydroxymethyl)-2-(4-methoxyphenyl)-5,6-dihydro-
2H-pyran (4)
To a solution of tert-butyldimethylsilyl ether 10 (0.5 g, 1.49 mmol)
in THF (7 mL) was added TBAF (0.52 mL, 1.79 mmol, 1M in THF)
at r.t., and the mixture was stirred for 1 h. The reaction mixture was
quenched with aq sat. NH4Cl (10 mL), diluted with H2O (10 mL)
and the mixture was extracted with EtOAc (3 × 10 mL). The com-
bined organic layers were washed with brine (15 mL), dried over
Na2SO4 and evaporated in vacuo to give the crude product, which
was purified by silica gel column chromatography (EtOAc-hex-
anes, 1:3) to obtain 4.
Yield: 0.33 g (97.5%); white solid; mp 41–43 °C; [a]D25 –149.9 (c
1.1, CHCl3).
IR (neat): 3420, 2924, 1610, 1511, 1245,1032, 827 cm–1.
Yield: 0.22 g (98%); [a]D25 –1.4 (c 0.4, CHCl3).
IR (neat): 3441, 2923, 2853, 1613, 1512, 1244, 1033, 760 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.29 (d, J = 8.7 Hz, 2 H), 7.02 (d,
J = 8.4 Hz, 2 H), 6.86 (d, J = 9.0 Hz, 2 H), 6.69 (d, J = 8.4 Hz, 2 H),
4.97 (br s, 1 H), 4.78 (t, J = 5.4 Hz, 1 H), 3.78 (s, 3 H), 3.76–3.72
(m, 1 H), 2.76–2.66 (m, 1 H), 2.57–2.47 (m, 1 H), 1.88–1.82 (m,
2 H), 1.75–1.57 (m, 6 H).
13C NMR (75 MHz, CDCl3): d = 158.8, 153.8, 134.7, 134.5, 129.6
(2 × C), 128.0 (2 × C), 115.3 (2 × C), 113.9 (2 × C), 72.0, 71.4, 55.5,
35.4, 31.5, 30.3, 30.1, 19.2.
1H NMR (300 MHz, CDCl3): d = 7.26 (d, J = 8.3 Hz, 2 H), 6.83 (d,
J = 9.0 Hz, 2 H), 6.08–5.91 (m, 2 H), 5.19 (s, 1 H), 3.79 (s, 3 H),
3.76–3.68 (m, 1 H), 3.64–3.45 (m, 2 H), 1.96–1.86 (m, 2 H).
HRMS (ESI): m/z [M + Na]+ calcd for C20H24O3Na: 335.1623;
found: 335.1634.
Synthesis 2008, No. 18, 2939–2942 © Thieme Stuttgart · New York

2942
C. R. Reddy et al.
PAPER
Bull. Korean Chem. Soc. 2004, 25, 1609. (e) Boulad, L.;
Acknowledgment
BouzzBouz, S.; Cossy, J.; Franck, X.; Figadère, B.
Tetrahedron Lett. 2004, 45, 6603. (f) Clarke, P. A.; Martin,
W. H. C. Tetrahedron Lett. 2004, 45, 9061. (g) Chan, K. P.;
Loh, T. P. Org. Lett. 2005, 61, 4491. (h) Clarke, P. A.;
Martin, W. H. C. Tetrahedron 2005, 61, 5433. (i) Jennings,
M. P.; Clemens, R. T. Tetrahedron Lett. 2005, 46, 2021.
(j) Bohrsch, V.; Blechert, S. Chem. Commun. 2006, 1968.
(k) Lee, C. H. A.; Loh, T. P. Tetrahedron Lett. 2006, 47,
1641. (l) Prasad, K. R.; Anbarasan, P. Tetrahedron 2007, 63,
1089. (m) Washio, T.; Yamaguchi, R.; Abe, T.; Nambu, H.;
Anada, M.; Hashimolo, S. Tetrahedron 2007, 63, 12037.
(n) Dziedzic, M.; Furman, B. Tetrahedron Lett. 2008, 49,
678.
PPM thanks the CSIR, New Delhi for financial assistance. CRR is
thankful to Dr. J. S. Yadav, Director, Indian Institute of Chemical
Technology, Hyderabad for his encouragement.
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Synthesis 2008, No. 18, 2939–2942 © Thieme Stuttgart · New York