Total synthesis of leiocarpin C and (+)-goniodiol via an olefin cross-metathesis protocol

Article abstract of DOI:10.1002/hlca.201000365

A stereoselective total synthesis of leiocarpin C (2) and (+)-Goniodiol (1) by applying olefin cross-metathesis and substrate directed dihydroxylation as the key steps is reported (Schemea 3).

Full text of DOI:10.1002/hlca.201000365

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Helvetica Chimica Acta – Vol. 94 (2011)
Total Synthesis of Leiocarpin C and (þ)-Goniodiol via an Olefin Cross-
Metathesis Protocol
by Palakodety Radha Krishna* and Munagala Alivelu
D-211, Discovery Laboratory, Organic Chemistry Division-III, Indian Institute of Chemical Technology,
Hyderabad 500607, India
(phone: þ 91-40-27193158; fax: þ 91-40-27160387; e-mail: prkgenius@iict.res.in)
A stereoselective total synthesis of leiocarpin C (2) and (þ)-Goniodiol (1) by applying olefin cross-
metathesis and substrate directed dihydroxylation as the key steps is reported (Scheme 3).
Introduction. – Leiocarpin C (1) is a natural styryl lactone. Styryl lactones are
natural products that exhibit significant biological activities such as antitumor,
antifungal, and antibiotic properties [1]. Compound 1 was isolated from the stem bark
of the tropical plant Goniothalamus leiocarpus [2]. Recently, two syntheses of 1 were
reported [3]. (þ)-Goniodiol (2) was isolated from the stem bark of Goniothalamus
gigantus, and from the leaves and twigs of Goniothalamus sesquipedalis [4b], and it has
been shown as selectively cytotoxic against human lung carcinoma cells A-549 (ED50
0.12 mg/mlꢀ1) and p-388 murine leukemia cells (IC50 4.56 mg mlꢀ1) [4]. Several syntheses
of 2 have been already reported [3][5]. Impressed by the bio-profile of these
compounds, coupled with our interest in the synthesis of such pyrone containing natural
products [6], we embarked on their total synthesis. Here, we report the total synthesis
of 1 and 2 by a novel methodology involving olefin cross-metathesis and dihydrox-
ylation as the key steps to access the basic premise of the skeleton and thence its
elaboration to the targets.
Retrosynthetically (Scheme 1), 2 could be obtained from 1 according to a reported
procedure [3b]. The latter could be obtained by the desilylation/concomitant
lactonization of acyclic ester 3, which, in turn, could be prepared by the substrate-
controlled dihydroxylation of the cross-metathesis product 4. Formation of 4 could be
realized from the cross-metathesis reaction of two olefinic fragments 5 and 6. While,
fragment 5 could be prepared from the known compound 7 [7a] by protection of the
secondary alcohol, oxidation of the primary alcohol, and esterification of the
ꢀ 2011 Verlag Helvetica Chimica Acta AG, Zꢁrich

Helvetica Chimica Acta – Vol. 94 (2011)
Scheme 1. Retrosynthethic Analysis
1103
corresponding acid as the key steps, the other fragment 6 could be prepared by the
(t-Bu)Me2Si (TBS) etherification of the known (S)-1-phenylprop-2-en-1-ol ((S)-8)
that has been earlier prepared by us [7b].
Results and Discussion. – The synthesis of 1 commenced by accessing the two
fragments 5 and 6 independently. First, olefinic ester 5 was synthesized as depicted in
Scheme 2. The secondary OH group of diol 7 was protected as its TBS ether. To
differentiate between the primary and secondary alcohols, a selective deprotection of
the bis-TBS ether in the primary position was employed: the bis-TBS ether 9 was
formed, followed by the selective removal of the primary silyl group to afford 10
(74%). The primary alcohol 10 was oxidized to the corresponding aldehyde under
Swern oxidation conditions, which was further oxidized [8] to the acid 11 (86% over
two steps). The latter was subsequently esterified to afford 5 (85%) using CH2N2 in
Et2O.
Scheme 2. Synthesis of Fragment 5
a) (t-Bu)Me2SiCl (TBSCl), 1H-imidazole, CH2Cl2, 08, 2 h; 95%. b) TsOH, MeOH, ꢀ 108, 0.5 h; 74%. c)
1. DMSO, (COCl)2, Et3N, CH2Cl2, ꢀ 788, 1 h; 2. NaClO2, NaH2PO4, 2-methylbut-2-ene, H2O, 08, 3 h;
86% (overe two steps). d) CH2N2, Et2O, 08, 5 min; 85%.

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Helvetica Chimica Acta – Vol. 94 (2011)
The known methanol derivative 8 was protected (Scheme 3) as its silyl ether 6.
Based on our earlier results, this protection was warranted to enable a facile cross-
metathesis reaction.
Scheme 3. Synthesis of Fragment 6
a) TBSCl, 1H-imidazole, CH2Cl2, 08, 0.5 h, 80%.
As outlined in Scheme 4, the crucial olefin cross-metathesis reaction [9] between
olefinic ester 5 and 6 (1:1.5 ratio) using Grubbs-II catalyst, A, resulted in product 4
(60%) as a single stereoisomer and the homo-dimer 12 (5%) of 5. Compound 12 was
effectively converted to the desired olefin 4 (60%) by using a second cross-metathesis
reaction with 6 under the same conditions. Subsequently, 4 was dihydroxylated [10] to
give the desired diol 3 as the major isomer (75%) after column chromatography. The
minor diastereoisomer was obtained in 18% yield. Diol 3 was treated with Amberlyst 15
resin in MeCN to furnish 1 (93%). The physical and spectroscopic data of 1 were
identical to the reported ones of natural leiocarpin C [a]2D5 ¼ ꢀ63.1 (c ¼ 0.30, CHCl3)
[2][3]. Finally, 1 was converted to 2 according to the known procedure [3b]. The
spectroscopic data of 2 were in agreement with those reported in [3][5].
Scheme 4
a) Grubbs-II (A, 10 mol-%), CH2Cl2, r.t., 24 h, 60%. b) OsO4, N-methylmorpholine N-oxide (NMO),
acetone/H2O 4 :1, 24 h, 75%. c) Amberlyst 15, MeCN, r.t., 2 h, 93%. d) 6, Grubbs-II (A, 10 mol-%),
CH2Cl2, r.t., 24 h, 60%.
In summary, a stereoselective synthesis of 1 was achieved in ten steps and in 24.5%
overall yield by cross-metathesis, followed by dihydroxylation as the key steps to access
the advanced intermediate 3 that was endowed with all stereogenic centers and
functionalities; and 3 was easily converted to the target compound 1. Further, the
synthesis of 2 was also accomplished.

Helvetica Chimica Acta – Vol. 94 (2011)
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Experimental Part
General. Org. solns. were dried over anh. Na2SO4 and concentrated below 408 in vacuo. Column
chromatography (CC): silica gel (SiO2; Acmeꢂs, 60 – 120 mesh). Optical rotations : JASCO DIP 300
digital polarimeter at 258. IR Spectra: Perkin-Elmer IR-683 spectrophotometer with NaCl optics. 1H- and
13C-NMR: Bruker Avance-300 MHz, and Inova 500 MHz; 7 – 10 mm solns. in CDCl3; TMS as internal
standard; J values in Hz. MS: Finnigan Mat 1210 double-focusing mass spectrometers operating at a
direct inlet system.
(5R)-2,2,3,3,9,9,10,10-Octamethyl-5-(prop-2-en-1-yl)-4,8-dioxa-3,9-disilaundecane (9). To a stirred
soln. of diol 7 [7a] (0.30 g, 2.5 mmol) in CH2Cl2 (3.5 ml), 1H-imidazole (0.7 g, 10.3 mmol) was added at
08, and the mixture was stirred for 5 min, then TBS-Cl (0.85 g, 5.6 mmol) was added, and the stirring was
continued for 2 h at r.t. The mixture was diluted with CH2Cl2 (4 ml), and the org. layer was washed with
H2O (5 ml), followed by brine (5 ml). The combined org. layers were dried (Na2SO4), evaporated in
vacuo, and purified by CC (SiO2; 1% AcOEt/hexane) to afford 9 (0.84 g, 95%). Colorless oily liquid.
1
[a]2D5 ¼ ꢀ60.4 (c ¼ 0.8, CHCl3). IR (neat): 3454, 2954, 2857, 1742, 1638, 1253, 1088, 834, 776. H-NMR
(300 MHz, CDCl3): 6.03 – 5.86 (m, 1 H); 5.16 (dd, J ¼ 1.5, 12.1, 2 H); 4.03 (quint., J ¼ 6.0, 1 H); 3.81 (t,
J ¼ 6.7, 2 H); 2.47 – 2.27 (m, 2 H); 1.83 – 1.71 (m, 2 H); 1.05 (s, 18 H); 0.21 (s, 6 H); 0.19 (s, 6 H).
13C-NMR (75 MHz, CDCl3): 135.0; 116.9; 68.7; 59.6; 42.2; 39.7; 26.1 (6 C); 18.1 (2 C); ꢀ 4.1 (2 C); ꢀ 5.1
(2 C). ESI-MS: 345 (71, [M þ H]þ), 367 (100, [M þ Na]þ).
(3R)-3-{[(tert-Butyl)(dimethyl)silyl]oxy}hex-5-en-1-ol (10). To a stirred soln. of 9 (0.82 g,
2.38 mmol) in dry MeOH (10 ml), TsOH (0.04 g, 0.17 mmol) was added at ꢀ 108, and the mixture was
stirred for 0.5 h. The reaction was quenched with 1.5 ml of Et3N, the solvent was evaporated, the residue
was dissolved in AcOEt (2 ꢁ 5 ml), and washed with H2O (6 ml), and brine (6 ml). The combined org.
layers were dried (Na2SO4), concentrated in vacuo, and purified by CC (SiO2; 10% AcOEt/hexane) to
afford 10 (0.40 g, 74%). Syrupy liquid. [a]2D5 ¼ ꢀ64.4 (c ¼ 0.85, CHCl3). IR (neat): 3380, 2932, 2858, 1740,
1640, 1253, 1068, 835, 774. 1H-NMR (300 MHz, CDCl3): 5.82 – 5.67 (m, 1 H); 5.06 (d, J ¼ 4.1, 1 H); 5.01
(s, 1 H); 3.99 – 3.91 (m, 1 H); 3.83 – 3.72 (m, 1 H); 3.70 – 3.61 (m, 1 H); 2.28 (t, J ¼ 6.2, 2 H); 1.82 – 1.72
(m, 1 H); 1.67 – 1.56 (m, 1 H); 0.89 (m, 9 H); 0.09 (s, 3 H); 0.08 (s, 3 H). 13C-NMR (75 MHz, CDCl3):
134.4; 117.3; 71.1; 60.0; 41.6; 37.6; 25.7; 17.9; ꢀ 4.4; ꢀ 4.8. ESI-MS: 231 (23, [M þ H]þ), 253 (100, [M þ
Na]þ). HR-MS: 253.1601 (C12H26NaO2Siþ; calc. 253.1599).
(3R)-3-{[(tert-Butyl)(dimethyl)silyl]oxy}hex-5-enoic Acid (11). To a soln. of oxalyl chloride
(0.28 ml, 2.47 mmol) in dry CH2Cl2 (3 ml) at ꢀ 788, dry DMSO (0.27 ml, 4.95 mmol) was added
dropwise and stirred for 10 min. A soln. of 10 (0.38 g, 1.65 mmol) in dry CH2Cl2 (2 ml) was added, and the
mixture was stirred for 1 h at ꢀ 788. The reaction was quenched with Et3N (1.28 ml, 9.9 mmol), and the
mixture was diluted with CH2Cl2 (5 ml), washed with H2O (4 ml) and brine (4 ml), dried (Na2SO4), and
evaporated to furnish the corresponding aldehyde. To a cooled (08) soln. of the above aldehyde (0.38 g,
1.65 mmol) in t-BuOH (2 ml), 2-methylbut-2-ene (1 ml), followed by a soln. of NaClO2 (0.3 g, 3.3 mmol)
and NaH2PO4 (0.51 g, 3.3 mmol) in H2O (1 ml), was added, and the mixture was stirred at r.t. for 3 h. The
solvent was evaporated, the residue was dissolved in AcOEt (5 ml) and washed with H2O (5 ml) and
brine (5 ml), and dried (Na2SO4). Evaporation of the solvent under reduced pressure and purification of
the residue by CC (SiO2; 12% AcOEt/hexane) gave 11 (0.34 g, 86% over two steps). Yellow syrup.
1
[a]2D5 ¼ ꢀ48.2 (c ¼ 0.7, CHCl3). IR (neat): 3380, 2932, 2858, 1740, 1640, 1253, 1068, 835, 774. H-NMR
(300 MHz, CDCl3): 5.84 – 5.67 (m, 1 H); 5.09 (s, 1 H); 5.05 (d, J ¼ 3.5, 1 H); 4.18 (quint., J ¼ 5.6, 1 H);
2.51 – 2.37 (m, 2 H); 2.27 (pseudo-t, J ¼ 6.2, 2 H); 0.86 (s, 9 H); 0.07 (s, 3 H); 0.04 (s, 3 H). 13C-NMR
(75 MHz, CDCl3): 177.3; 133.7; 118.0; 68.8; 41.9; 41.7; 25.7; 17.9; ꢀ 4.5; ꢀ 4.9. ESI-MS: 245 (26, [M þ
H]þ), 267 (100, [M þ Na]þ). HR-MS: 267.1405 (C12H24NaO3Siþ; calc. 267.1392).
Methyl (3R)-3-{[(tert-Butyl)(dimethyl)silyl]oxy}hex-5-enoate (5). To a stirred soln. of 11 (0.32 g,
1.31 mmol) in Et2O (5 ml), CH2N2 in Et2O was added, until yellow color sustained at 08, and the mixture
was stirred for 5 min. Evaporation of the solvent under reduced pressure and purification of the residue
by CC (SiO2; 2% AcOEt/hexane) afforded 5 (0.28 g, 85%). Yellow oil. [a]2D5 ¼ ꢀ68.4 (c ¼ 0.55, CHCl3).
1
IR (neat): 3078, 2932, 2858, 1711, 1644, 1433, 1254, 1090, 834, 776. H-NMR (300 MHz, CDCl3): 5.84 –
5.68 (m, 1 H); 5.07 (s, 1 H); 5.03 – 4.99 (m, 1 H); 4.17 (quint., J ¼ 6.0, 1 H); 3.64 (s, 3 H); 2.40 – 2.38 (m,
2 H); 2.27 – 2.23 (m, 2 H); 0.85 (s, 9 H); 0.06 (s, 3 H); 0.02 (s, 3 H). 13C-NMR (75 MHz, CDCl3): 177.3;

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Helvetica Chimica Acta – Vol. 94 (2011)
133.7; 118.0; 68.8; 61.5; 41.9; 41.7; 25.6; 17.9; ꢀ 4.5; ꢀ 5.01. ESI-MS: 259 (5, [M þ H]þ), 281 (70, [M þ
Na]þ). HR-MS: 281.1561 (C13H26NaO3Siþ; calc. 281.1548).
(tert-Butyl)(dimethyl){[(1S)-1-phenylprop-2-en-1-yl]oxy}silane (6). To a stirred soln. of 8 [7b]
(0.25 g, 1.86 mmol) in CH2Cl2 (3.5 ml), 1H-imidazole (0.25 g, 3.67 mmol) was added at 08, and the
mixture was stirred for 5 min; then, TBS-Cl (0.30 g, 2.04 mmol) was added, and the mixture was stirred
for 0.5 h at r.t. The mixture was diluted with CH2Cl2 (4 ml), and the org. layer was washed with H2O
(5 ml), followed by brine (5 ml). The combined org. layers were dried (Na2SO4), evaporated in vacuo,
and purified by CC (SiO2; 1% AcOEt/hexane) to afford 6 (0.37 g, 80%). Colorless oil. [a]2D5 ¼ ꢀ46.0 (c ¼
0.25, CHCl3). IR (neat): 3450, 2930, 2860, 1710, 1676, 1390, 1219, 830. 1H-NMR (300 MHz, CDCl3): 7.38
(d, J ¼ 7.3, 1 H); 7.31 – 7.17 (m, 4 H); 5.95 – 5.84 (m, 1 H); 5.27 (td, J ¼ 1.3, 16.9, 1 H); 5.15 (d, J ¼ 5.6,
1 H); 5.04 (td, J ¼ 1.3, 10.1, 1 H); 0.93 (s, 9 H); 0.14 (s, 3 H); 0.08 (s, 3 H). 13C-NMR (75 MHz, CDCl3):
141.6; 128.1; 126.9; 125.9; 125.5; 113.3; 75.8; 25.8; 18.3; ꢀ 4.6; ꢀ 4.8. ESI-MS: 271 (70, [M þ Na]þ).
Methyl (3R,5E,7S)-3,7-Bis{[(tert-butyl)(dimethyl)silyl]oxy}-7-phenylhept-5-enoate (4). To a soln. of
5 (0.24 g, 0.93 mmol) and 6 (0.34 g, 1.37 mmol) in CH2Cl2 (5 ml), 10 mol-% Grubbsꢂ catalyst II (A;
0.078 g, 0.09 mmol) was added and stirred at r.t. for 24 h under N2. Most of the solvent was then distilled
off, and the concentrated soln. was left to be stirred at r.t. for 2 h under air bubbling in order to
decompose the catalyst. The mixture was evaporated to dryness to give a brown residue, which was
purified by CC (SiO2; 1% AcOEt/hexane) to give 4 (0.26, 60%) as a yellow color liquid, and dimethyl
(3R,5E,8R)-3,8-bis{(tert-butyl)(dimethyl)silyl]oxy}dec-5-enedioate (12) (0.022, 5%) as colorless syrup.
Data of 4: [a]2D5 ¼ ꢀ40.8 (c ¼ 0.75, CHCl3). IR (neat): 3452, 2931, 2857, 1742, 1634, 1253, 1087, 835,
775. 1H-NMR (300 MHz, CDCl3): 7.31 – 7.16 (m, 5 H); 5.72 – 5.55 (m, 1 H); 5.54 – 5.35 (m, 1 H); 5.11 (d,
J ¼ 5.2, 1 H); 4.24 – 4.12 (m, 1 H); 3.64 (s, 3 H); 2.47 – 2.34 (m, 2 H); 2.28 – 2.17 (m, 2 H); 0.93 (s, 9 H);
0.86 (s, 9 H); 0.087 (s, 3 H); 0.08 (s, 3 H); 0.06 (s, 3 H); 0.05 (s, 3 H). 13C-NMR (75 MHz, CDCl3): 172.1;
137.0; 128.1; 126.8; 125.8; 124.7; 117.7; 75.3; 69.0; 51.4; 41.8; 40.2; 25.6; 25.8;17.9; 17.5; ꢀ 4.2; ꢀ 4.5; ꢀ 4.7;
ꢀ 5.0. ESI-MS: 501 (100, [M þ Na]þ). HR-MS: 501.2821 (C26H46NaO4Siþ2 ; calc. 501.2832).
Data of 12: [a]2D5 ¼ ꢀ53.1 (c ¼ 0.5, CHCl3). IR (neat): 3078, 2932, 2858, 1711, 1644, 1433, 1254, 1090,
1
834, 776. H-NMR (300 MHz, CDCl3): 5.72 – 5.57 (m, 2 H); 4.31 (quint., J ¼ 5.8, 2 H); 3.82 (s, 6 H);
2.58 – 2.53 (m, 4 H); 2.39 (t, J ¼ 5.2, 4 H); 1.04 (s, 18 H); 0.23 (s, 6 H); 0.20 (s, 6 H). 13C-NMR (75 MHz,
CDCl3): 172.3; 129.2; 69.8; 51.8; 42.4; 40.9; 25.8; 18.2; ꢀ 4.0; ꢀ 5.1. ESI-MS: 489 (28, [M þ H]þ), 511
(100, [M þ Na]þ). HR-MS: 511.2892 (C24H48NaO6Siþ2 ; calc. 511.2887).
Conversion of 12 to 4. To a soln. of 12 (0.010 g, 0.02 mmol) and 6 (0.007 g, 0.02 mmol) in CH2Cl2
(1 ml), 10 mol-% A (0.004 g, 0.047 mmol) was added, and the mixture was stirred at r.t. for 24 h under
N2. Most of the solvent was then distilled off, and the concentrated soln. was left to be stirred at r.t. for 2 h
under air bubbling in order to decompose the catalyst. The mixture was evaporated to dryness to give a
brown residue, which was purified by CC (SiO2; 1% AcOEt/hexane) to give 4 (0.005, 60%).
Methyl (3R,5R,6S,7R)-3,7-Bis{[(tert-butyl)(dimethyl)silyl]oxy}-5,6-dihydroxy-7-phenylheptanoate
(3). To a stirred soln. of 4 (0.24 g, 0.50 mmol) in acetone/H2O 4 :1 (2 ml), NMO (0.11 ml, 1 mmol)
and 2 ml of 1m toluene soln. of OsO4 were added at r.t., and the mixture was stirred for 24 h. The reaction
was quenched with Na2SO3 (0.2 g), the mixture was filtered and washed with AcOEt, and the resulting
filtrate was washed with H2O (4 ml), followed by brine (4 ml). The combined org. layers were dried
(Na2SO4), evaporated in vacuo, and purified by CC (SiO2; 5% AcOEt/hexane) to afford 3 (0.19 g, 75%).
Syrupy liquid. [a]D25 ¼ ꢀ34.1 (c ¼ 0.2, CHCl3). IR (neat): 3451, 2856, 1739, 1639, 1254, 836, 777, 700.
1H-NMR (500 MHz, CDCl3): 7.57 – 7.38 (m, 5 H); 5.10 (d, J ¼ 4.5, 1 H); 4.54 – 4.39 (m, 1 H); 4.22 (d, J ¼
10.5, 1 H); 3.83 (s, 3 H); 3.54 – 3.43 (m, 1 H); 3.38 (br. s, OH); 2.70 – 2.54 (m, 3 H); 2.09 – 1.87 (m, 1 H);
1.12 (s, 9 H); 0.97 (s, 9 H); 0.30 (s, 3 H); 0.21 (s, 3 H); 0.02 (s, 3 H); 0.09 (s, 3 H). 13C-NMR (75 MHz,
CDCl3): 171.3; 141.0; 128.4; 127.6; 126.2; 78.7; 66.6; 65.5; 64.7; 51.4; 42.7; 41.1; 25.9; 25.8; 18.2; 17.9; ꢀ 4.4;
ꢀ 4.6; ꢀ 4.77; ꢀ 5.02. ESI-MS: 513 (42, [M þ H]þ), 535 (100, [M þ Na]þ). HR-MS: 535.2891
(C26H48NaO6Siþ2 ; calc. 535.2887).
(4R,6R)-6-[(1R,2R)-1,2-Dihydroxy-2-phenylethyl]-4-hydroxytetrahydro-2H-pyran-2-one (1). To a
stirred soln. of 3 (0.16 g, 0.31 mmol) in MeCN (2 ml), Amberlyst 15 (0.005 g) was added at r.t. and stirred
for 2 h. The mixture was filtered and washed with AcOEt, and the resulting filtrate was evaporated in
vacuo and purified by CC (SiO2; 35% AcOEt/hexane) to afford 1 (0.073 g, 93%). Colorless needles.
[a]2D5 ¼ ꢀ63.1 (c ¼ 0.3, CHCl3). IR (neat): 3423, 2920, 2850, 1720, 1656, 1460, 1219, 606. 1H-NMR

Helvetica Chimica Acta – Vol. 94 (2011)
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(500 MHz, CDCl3): 7.47 – 7.25 (m, 5 H); 4.91 (d, J ¼ 7.8, 1 H); 4.73 (dd, J ¼ 4.4, 9.8, 1 H); 4.28 (quint., J ¼
6.8, 1 H); 3.7 (d, J ¼ 8.8, 1 H); 2.86 (dd, J ¼ 5.4, 17.2, 1 H); 2.55 (dd, J ¼ 7.3, 17.2, 1 H); 2.20 – 2.14 (m,
1 H); 2.08 – 1.99 (m, 1 H). 13C-NMR (75 MHz, CDCl3): 172.1; 141.3; 128.5; 127.8; 127.2; 78.0; 68.9; 67.0;
66.8; 41.1; 40.1. ESI-MS: 275 (100, [M þ Na]þ). HR-MS: 275.0907 (C13H16NaOþ5 ; calc. 275.0895).
One of the authors, M. A., thanks the CSIR, New Delhi, for financial support in the form of
fellowship.
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Received September 27, 2010