Stereoselective formal total synthesis of novel antibiotic (-)-centrolobine

Article abstract of DOI:10.3987/COM-08-S(S)1

A concise and stereoselective formal total synthesis of (-)-centrolobine is achieved utilizing Mioskowski's Lewis acid mediated epoxide opening followed by ring-closing metathesis as the key reaction.

Full text of DOI:10.3987/COM-08-S(S)1

HETEROCYCLES, Vol. 80, No. 1, 2010
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HETEROCYCLES, Vol. 80, No. 1, 2010, pp. 219 - 227. © The Japan Institute of Heterocyclic Chemistry
Received, 2nd December, 2008, Accepted, 16th January, 2009, Published online, 19th January, 2009
DOI: 10.3987/COM-08-S(S)1
STEREOSELECTIVE FORMAL TOTAL SYNTHESIS OF NOVEL
ANTIBIOTIC (−)-CENTROLOBINE
Debendra K. Mohapatra,*† Rita Pal, Hasibur Rahaman, and
Mukund K. Gurjar
Division of Organic Chemistry, National Chemical Laboratory,
Pune-411 008, India
Abstract – A concise and stereoselective formal total synthesis of
(−)-centrolobine is achieved utilizing Mioskowski’s Lewis acid mediated epoxide
opening followed by ring-closing metathesis as the key reaction.
INTRODUCTION
Syntheses of natural products and analogues thereof embodying substituted tetrahydropyrans are of
substantial interest to organic chemistry and chemical biology.1,2 Several natural products possessing
2,6-disubstituted tetrahydropyran, tetrahydrofurans, and oxepanes such as ()-centrolobine,3
de-O-methylcentrolobine, calyxins,4 diospongins,5 annonaceae acetogenins,6 ionophores,7 etc. exhibit a
wide range of biological activities. Owing to the challenges posed by the substitution pattern as well as
the widespread occurrence of substituted tetrahydropyran moieties in natural products with interesting
biological activities,8 has inspired the development of numerous creative synthetic approaches to this
important structural subunit.
O
MeO
OH
1
Figure 1. Structure of ()-centrolobine 1.
()-Centrolobine, 6-[-(p-hydroxyphenyl)ethyl]-2-(p-methoxyphenyl)tetrahydropyran, is a crystalline
substance (Figure 1) isolated from the heartwood of Centrolobine robustum and from the stem of Brosinium
_____________________________________________________________________________________________________
This paper is dedicated to Prof. Akira Suzuki on the occasion of his 80th birthday.
† Current Address: Division of Organic Chemistry, IICT, Hyderabad-500007, India

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HETEROCYCLES, Vol. 80, No. 1, 2010
potabile in the amazon rain forest in 1962. Although the basic structure of ()-centrolobine was
elucidated in 1964,1 its absolute configuration was only established in 2002 by an enantioselective total
syntheis.9 Afterward several research groups have accomplished the total synthesis of ()-centrolobine. A
variety of approaches starting with optically active building blocks, obtained by well-established
asymmetric reactions or the chiral pool method, have been devised to provide access to the
cis-2,6-disubstituted tetrahydropyran rings. These include the Prins and related cyclizations10a,h,k,o
reductive etherifications,9,10b one-pot cross metathesis–hydrogenation –lactonization procedure,10c radical
cyclization,10e nucleophilic addition-stereoselective reduction protocol,10f intramolecular oxy-Michael
reaction,10g diastereoselective ring rearrangement metathesis–isomerization sequence,10l FeCl3-mediated
cyclization of 1,5-diol,10m hetero-Diels-Alder reaction.10n
In continuation of our research on the synthesis of biologically active natural products containing
tetrahydrofuran and tetrahydropyran rings,11 we herein disclose a new strategy towards the synthesis of
tetrahydropyran derivatives using Mioskowski’s Lewis acid12 catalyzed regioselective epoxide opening
followed by ring-closing metathesis as the key steps. This novel strategy was applied for the synthesis of
()-centrolobine. Retrosynthetic analysis of ()-centrolobine (1), as depicted in Scheme 1, revealed two
fragments 6 and 7.
Ref. 9
O
O
MeO
MeO
MeO
MeO
2
OH
OBn
1
O
OBn
O
4
O
OH
3
OH
O
+
OBn
OBn
O
MeO
OH
MeO
7
5
6
Scheme 1. Retrosynthetic analysis.
RESULTS AND DISCUSSION
As summarized in Scheme 1, one of the stereogenic centers of ()-centrolobine was established in a
reagent-controlled fashion, whereas the other capitalized on a stereocontrolled asymmetric allylation. For
synthesizing fragment 6, we followed Keck allylation on 4-tosyloxybenzaldehyde.13 Detosylation14

HETEROCYCLES, Vol. 80, No. 1, 2010
221
followed by selective methylation10a of phenolic hydroxy group accomplished the fragment 6 in 83%
yield over two steps.
OH
CHO
Ref. 10g
TsO
TsO
8
9
a, b
OH
MeO
6
Scheme 2. Reagents and conditions: (a) Mg, MeOH, rt, 3 h, 94%; (b) MeI, K2CO3, acetone, rt, 12 h,
88%.
The other fragment 7 was synthesized starting from cis-2-butene-1,4-diol. Accordingly, the alcohol 10
was mono protected as its benzyl ether with NaH and benzyl bromide followed by Sharpless asymmetric
epoxidation with L-(+)-DET, TBHP afforded the epoxy alcohol 11.15 IBX oxidation of the primary
hydroxyl group to the corresponding aldehyde and Wittig reaction using methyltriphenylphosphonium
bromide11 afforded vinyl substituted epoxide 7 in an overall yield of 86% in two steps.
O
Ref. 16
HO
OH
OBn
HO
11
10
a, b
O
OBn
7
Scheme 3. Reagents and conditions: (a) IBX, DMSO, THF, 0 oC - rt, 4 h; (ii) Ph3P+CH3Br−, NaHMDS,
THF, 0 oC - rt, 12 h, 86% over two steps.
After having enantiomerically pure homoallylic alcohol 6 and the vinyl epoxide derivative 7 in hand, we
proceeded further for the Mioskowski’s Lewis acid mediated epoxide opening reaction. Accordingly, the
.
two segments were treated with BF3 OEt2 in dichloromethane to afford the diene 5 in 67% yield. Barely
detectable amounts of isomeric diene could be detected in the product by HPLC analysis or by NMR
spectroscopy. Now, the stage is set to effect the ring-closing metathesis reaction. On exposure of 5 to

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HETEROCYCLES, Vol. 80, No. 1, 2010
Grubbs second generation catalyst16 furnished the pyran ring in 73% yield. The product was assigned by
1H, 13C NMR, mass and elemental analysis.
OH
O
a
O
OBn
OH
+
OBn
OH
OH
MeO
b
MeO
MeO
5
7
6
c
O
O
OBn
OH
MeO
MeO
12
4
d
Ref. 9
O
O
O
MeO
OH
3
1
Scheme 4. Reagents and conditions: (a) BF3-OEt2, CH2Cl2, rt, 45 min, 67%; (b) Grubbs second
generation catalyst, toluene, 70 oC, 73%; (c) Pd/C, HCl EtOH:EtOAc:H2O (5:1:1), 1 h, 75%; (d) NaIO4
(impregnated over silica gel, CH2Cl2, rt, 30 min, 92%.
Double bond reduction as well as benzyl ether cleavage was accomplished in one pot by exposure of 4 to
Pd/C catalyst under hydrogen atmosphere10g to afford the diol 12 in 75% yield. Compound 12 was then
treated with NaIO4 impregnated over silica gel in dichloromethane to afford the aldehyde 3 in 92% yield.
The analytical data were exactly matching with the reported values. Though this constitutes a formal
synthesis of ()-centrolobine, we were interested to complete its synthesis and compare the data with the
natural product. The Wittig reaction between the 4-benzyloxybenzyltriphenylphosphonium bromide9b and
the aldehyde 3 was carried out in the presence of n-BuLi to obtain 2 followed by simultaneous reduction
of the double bond and deprotection of the benzyl ether by catalytic hydrogenation gave ()-centrolobine
1
13
25
(1) in 78% yield over two steps. H and C NMR spectra, IR, melting point, and optical rotation ([]D
–91.5 (c 1.2, CHCl3)) of 1 were in good agreement with the natural product.10
CONCLUSION
In summary, a new synthetic approach has been designed for the stereoselective formal synthesis of
()-centrolobine following Lewis acid mediated epoxide opening followed by ring-closing metathesis

HETEROCYCLES, Vol. 80, No. 1, 2010
223
reaction for the first time. Total syntheses of other pyran ring containing natural products are in progress
in our laboratory following the above approach and will be reported in due course.
EXPERIMENTAL
General
All chemicals used in this study were purchased from Aldrich, Fluka or Lancaster and used as received.
All the moisture-sensitive reactions were performed in an inert atmosphere of either N2 or Ar using dry
solvents. The elemental analyses were recorded on Elmentar-Vario-EL (Heraeus Company Ltd.
Germany). The NMR spectra were obtained on a Bruker 200, 400, or 500 Fourier transform spectrometer.
Optical rotations were measured with a JASCO DIP 370 digital polarimeter. All reactions are monitored
by Thin Layer Chromatography (TLC) carried out on 0.25 mm E- Merck silica gel plates (60F-254) with
UV, I2 or anisaldehyde in ethanol as development reagents.
(2R,3S)-2-(Benzyloxymethyl)-3-vinyloxirane (7)
To a solution of IBX (6.5 g, 23.19 mmol) in DMSO (14 mL), was added pyridine (5 mL) followed by
epoxy alcohol 11 (3.0 g, 15.46 mmol) in dry THF (20 mL) at rt and stirred for 4 h. After completion of
the reaction (monitored by TLC), water (H2O) (30 mL) was added, and diluted with Et2O (50 mL). The
solid was filtered through a pad of Celite and organic layer was separated. The aqueous layer was
extracted with Et2O (2 x 50 mL). The combined organic layer was washed with brine, dried over Na2SO4,
concentrated to give crude aldehyde (2.9 g, crude product) which was used immediately for the next
reaction.
To a suspension of methyltriphenylphosphonium bromide salt (16.56 g, 46.39 mmol) in THF (50 mL) at
0 oC, was added NaHMDS (31.0 mL, 1M solution in toluene) drop-wise. The reaction mixture was stirred
for 1 h and the crude aldehyde (2.9 g in 20 mL THF) was added slowly. After 2 h, the reaction was
quenched with saturated solution of NH4Cl and extracted with EtOAc. The organic layers were washed
with brine, dried over Na2SO4 and concentrated under reduced pressure. Purification of the crude product
by silica gel (60-120 mesh) column chromatography afforded vinyl epoxide 7 (2.5 g, 86% over two steps)
as a yellow color liquid. [α]D25 −1.95 (c 1.65, CHCl3); IR (CHCl3): 3030, 2989, 2920, 2860, 1639, 1496,
1
1453, 1387, 1148, 1096, 1028, 929, 739.cm-1; H NMR (200 MHz, CDCl3): δ 3.24 (dt, J = 4.4, 5.9 Hz,
1H), 3.38 (dd, J = 4.4, 6.7 Hz, 1H), 3.45-3.62 (m, 2H), 4.44 (d, J = 11.9 Hz, 1H), 4.53 (d, J = 11.9 Hz,
1H), 5.22-5.44 (m, 2H), 5.51-5.68 (m, 1H), 7.17-7.27 (m, 5H) ppm; 13C NMR (50 MHz, CDCl3): δ 56.0,
56.7, 67.8, 73.2, 120.8, 127.7, 128.38, 131.9, 137.8 ppm; ESI-MS m/z 213.2 [M+Na]+; Anal. Calcd for
C12H14O2: C, 75.76; H, 7.42. Found: C, 75.59; H, 7.35.
(2S,3R)-1-(Benzyloxy)-3-((S)-1-(4-methoxyphenyl) but-3-enyloxy)pent-4-en-2-ol (5)

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To a mixture of epoxide 7 (250 mg 1.31 mmol) and homoallyl alcohol 6 (350 mg, 1.97 mmol) in CH2Cl2
(20 mL) was added drop-wise a 19% solution of BF3·Et2O (0.4 mL, 0.065 mmol) in freshly dried CH2Cl2 at
rt. After 45 minutes of stirring at rt, the solution was diluted with CH2Cl2 and washed with saturated
aqueous NaHCO3 solution and brine. The organic layer was dried over Na2SO4, filtered and concentrated
under reduced pressure. The crude product was purified by flash column chromatography (light
petroleum/EtOAc: 9/1) to afford diene 5 (0.32 g, 67%) as a colorless liquid. [α]D25 +3.8 (c 2.6, CHCl3); IR
(CHCl3): 3436, 3009, 2906, 2862, 1639, 1611, 1512, 1496, 1454, 1302, 1247, 1175, 1035, 923, 832, 755,
698 cm-1; 1H NMR (200 MHz, CDCl3): δ 2.24-2.58 (m, 2H), 3.30-3.65 (m, 3H), 3.71 (s, 3H), 3.84-3.96 (m,
1H), 4.24-4.48 (m, 3H), 4.88-5.25 (m, 4H), 5.40-5.76 (m, 2H), 6.72-6.80 (m, 2H), 7.07-7.25 (m, 7H) ppm;
13C NMR (50 MHz, CDCl3): δ 43.7, 55.1, 70.8, 72.9, 73.2, 77.4, 77.6, 113.6, 116.8, 118.0, 127.0, 127.6,
128.2, 133.0, 134.6, 134.8, 135.1, 159.1 ppm; ESI-MS m/z391.48 [M + Na]+; Anal. Calcd for C23H28O4: C,
74.97; H, 7.66. Found: C, 74.79; H, 7.49.
(S)-2-(Benzyloxy)-1-((2R,6S)-6-(4-methoxyphenyl)-5,6-dihydro-2H-pyran-2-yl)ethanol (4). A mixture
of compound 5 (0.31 g, 0.84 mmol) and Grubbs’ II catalyst (0.021 g, 0.025 mmol) in degassed toluene (60
o
mL) was heated at 70 C for 12 h. After completion of the reaction (monitored by TLC), solvent was
evaporated under reduced pressure and the crude purified on silica gel column chromatography by eluting
with light petroleum: EtOAc (19:1) to afford 4 (0.21 g, 73%) as a colorless viscous liquid. [α]D25 +4.3 (c 1.4,
CHCl3); IR (CHCl3): 3401, 2922, 2851, 1612, 1514, 1454, 1385, 1303, 1247, 1174, 1085, 829, 770 cm-1; 1H
NMR (200 MHz, CDCl3): δ 2.19-2.31 (m, 2H), 2.58 (bs, 1H), 3.53-3.72 (m, 2H), 3.78 (s, 3H), 3.78-3.90 (m,
1H), 4.48-4.61 (m, 4H), 5.74-5.80 (m, 1H), 5.94-6.04 (m, 1H), 6.85 (d, J = 8.7 Hz, 2H), 7.22-7.33 (m, 7H)
13
ppm; C NMR (50 MHz, CDCl3): δ 32.9, 55.2, 70.9, 72.3, 73.4, 75.2, 75.7, 113.7, 126.8, 126.8, 127.0,
127.7, 128.4, 134.5, 138.1, 159.0 ppm; ESI-MS m/z 363.4 (M + Na)+; Anal. Calcd for C21H24O4: C, 74.09;
H, 7.11. Found: C, 73.97; H, 7.00.
(S)-1-((2R,6S)-6-(4-Methoxyphenyl)tetrahydro-2H-pyran-2-yl)ethane-1,2-diol (12). A solution of 4
(0.34 g, 1.0 mmol) in EtOH:EtOAc:water (25:5:5) was hydrogenated in the presence of 10% Pd/C (20 mg)
under acidic condition (conc. HCl) at rt. After 1 h, the reaction mixture was filtered through a pad of Celite,
concentrated and the residue purified on silica gel column chromatography using EtOAc: light petroleum
(1:4) to afford 12 (0.19 g, 75%) as a colorless liquid. [α]D25 +4.8 (c 1.25, CHCl3); IR (CHCl3): 3392, 2932,
1613, 1514, 1384, 1246, 1175, 1035, 830, 771 cm-1; 1H NMR (400 MHz, CDCl3): δ 1.43-1.48 (m, 2H), 1.56
(m, 1H), 1.61 (m, 1H), 1.70-1.73 (m, 1H), 1.89-1.93 (m, 1H), 2.55 (bs, 2H), 3.52-3.57 (m, 2H), 3.59 (dd, J
= 4.5, 11.7 Hz, 1H), 3.67 (dd, J = 11.7, 3.4 Hz, 1H), 3.73 (s, 3H), 4.28 (dd, J = 11.4, 2.1 Hz, 1H), 6.80 (d, J
= 8.7 Hz, 2H), 7.18 (d, J = 8.7 Hz, 2H) ppm; 13C NMR (50 MHz, CDCl3): δ 23.3, 26.8, 33.2, 55.2, 63.6, 74.2,
79.1, 79.7, 113.6, 127.1, 134.9, 158.9 ppm; ESI-MS m/z 275.1 [M + Na]+; Anal. Calcd for C14H20O4: C,

HETEROCYCLES, Vol. 80, No. 1, 2010
225
66.65; H, 7.99. Found: C, 66.54; H, 7.80.
(2R,6S)-2-(4-(Benzyloxy)styryl)-6-(4-methoxyphenyl)tetrahydro-2H-pyran (2). Compound 12 (0.08 g,
0.32 mmol) in CH2Cl2 (5 mL) was stirred with sodium metaperiodate impregnated over silica gel (0.64 g,
2.0 g/ mmol) for 0.5 h. Silica was separated by filtration, organic layer concentrated under reduced pressure
and the crude aldehyde 3 used for the next reaction without further purification.
To a stirred solution of p-benzyoxybenzyltriphenylphosphonium bromide (0.41 g, 0.95 mmol) in THF (5
mL) under nitrogen was added (0.63 mL, of 1.6 M solution in hexane) of n-butyllithium. After stirring for
0.5 h, crude aldehyde 3 was added with THF (3 mL) and the reaction mixture stirred for an additional 1 h.
The reaction mixture was quenched with saturated aqueous NH4Cl and diluted with Et2O. The organic layer
was separated and washed with water, brine and dried over Na2SO4. Solvent was evaporated under reduced
pressure and the crude product purified by silica gel column chromatography eluting with EtOAc: light
petroleum (3:97) to afford product 2 (0.07 g, 55%) as a colorless liquid. [α]D25 +15.0 (c 1.2, CHCl3); IR
(CHCl3): 3368, 2929, 2853, 1607, 1511, 1454, 1382, 1247, 1174, 1074, 1029, 834, 769 cm-1; 1H NMR (200
MHz, CDCl3): δ 1.51-2.24 (m, 6H), 3.71 (s, 3H), 4.03-4.13 (m, 0.27H), 4.27-4.35 (m, 1.63H), 4.97 (s,
0.55H), 4.99 (s, 1.42H), 5.58 (dd, J = 8.7, 11.6 Hz, 0.68H), 6.06 (dd, J = 5.9, 16.0 Hz, 0.27H), 6.41 (d, J =
11.8 Hz, 0.8H), 6.63 (d, J = 16.0 Hz, 0.17H), 6.76-7.37 (m, 13H) ppm; 13C NMR (100 MHz, CDCl3): δ 23.9,
24.1, 31.6, 31.8, 32.9, 33.4, 55.2, 69.9, 74.9, 78.8, 79.0, 79.6, 113.6, 114.5, 114.8, 127.2, 127.3, 127.4,
127.6, 127.9, 128.0, 128.5, 128.6, 129.0, 129.2, 129.9, 130.1, 130.8, 131.5, 135.4, 135.6, 137.0, 158.0,
158.3, 159.0 ppm; ESI-MS m/z 423.5 [M + Na]+; Anal. Calcd for C27H28O3: C; 80.97, H; 7.05. Found: C;
80.78, H; 6.89.
4-(2-((2R,6S)-6-(4-Methoxyphenyl)tetrahydro-2H-pyran-2-yl)ethyl)phenol (1). Compound 2 was
hydrogenated by the same procedure as used for the transformation of 4 to 12. The crude product was
purified by silica gel column chromatography eluting with EtOAc: light petroleum (1:10) to obtain
25
25
()-centrolobine (1) (0.02 g, 89%). [α]D −91.5 (c 1.2, CHCl3), lit.,3 [α]D −92.2 (c 1.0, CHCl3); IR
1
(CHCl3): 3420, 2930, 1650, 1490, 1220, 1110 cm-1; H NMR (200 MHz, CDCl3): δ 1.42-1.58 (m, 4H),
1.61-1.69 (m, 2H), 1.78-1.89 (m, 2H), 2.54-2.69 (m, 2H), 3.34-3.39 (m, 1H), 3.73 (s, 3H), 4.22 (dd, J = 1.9,
11.1 Hz, 1H), 6.67 (d, J = 8.4 Hz, 2H), 6.81 (d, J = 8.6 Hz, 2H), 6.98 (d, J = 8.4 Hz, 2H), 7.24 (d, J = 8.6 Hz,
2H) ppm; 13C NMR (50 MHz, CDCl3): δ 24.0, 30.7, 31.3, 33.3, 38.3, 55.3, 77.1, 79.1, 113.6, 115.1, 127.1,
129.6, 134.7, 135.9, 153.5, 158.7 ppm; ESI-MS m/z 344.4 [M + Na]+; Anal. Calcd for C20H24O3: C; 76.89,
H; 7.74. Found: C; 76.66, H; 7.67.
ACKNOWLEDGEMENT
RP and HR thank CSIR, New Delhi for financial support in the form of research fellowship. We thank Dr.

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Ganesh Pandey, HOD, Organic Chemistry Division, for his constant support and encouragement. We also
thank Dr. P. R. Rajmohanan for the NMR data.
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