Total synthesis of the G2/M DNA damage checkpoint inhibitor psilostachyin C

Article abstract of DOI:10.1021/jo2001275

A concise total synthesis of the G2/M DNA damage checkpoint inhibitor psilostachyin C is reported using a 1,4-addition-aldol condensation-ring-closing metathesis (RCM) strategy. Initial biological studies indicate that psilostachyin C could enhance the sensitivity of the HeLa cell toward camptothecin (CPT) treatment via the activation of the caspase-3 mediated apoptosis pathway.

Full text of DOI:10.1021/jo2001275

NOTE
pubs.acs.org/joc
Total Synthesis of the G2/M DNA Damage Checkpoint Inhibitor
Psilostachyin C
Chao Li,† Shanghui Tu,‡ Shuguang Wen,‡ Shan Li,‡ Junbiao Chang,§ Feng Shao,‡ and Xiaoguang Lei*,†,‡
†School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, China
‡National Institute of Biological Sciences (NIBS), Beijing 102206, China
§Zhengzhou University, Zhengzhou 450001, China
S Supporting Information
b
ABSTRACT: A concise total synthesis of the G2/M DNA damage
checkpoint inhibitor psilostachyin C is reported using a 1,4-addition-
aldol condensation-ring-closing metathesis (RCM) strategy. Initial
biological studies indicate that psilostachyin C could enhance the
sensitivity of the HeLa cell toward camptothecin (CPT) treatment via
the activation of the caspase-3 mediated apoptosis pathway.
NA damage activates cell cycle checkpoints, which arrest
cell cycle progression in order to allow for DNA repair.1 In
Initially, we planned to develop a one-pot protocol for the
3-component tandem 1,4-addition-aldol condensation; however,
the results were not encouraging.9 Therefore, we shifted our
focus to the stepwise sequence. The synthesis commenced with
D
cancer cells, the G1/S checkpoint is normally inactive due to
its dependence on p53, which is mutated in the variety of
cancers.2 Therefore, the mainly p53-independent G2/M
checkpoint is likely to play a vital role in tumor cell sensitivity
toward many conventional anticancer treatments (including:
ionizing radiation, hyperthermia, DNA alkylating agents, and
DNA topoisomerase inhibitors, etc.). Small-molecule inhibi-
tors of G2/M DNA damage checkpoint may thus find
therapeutical applications in combination with other anti-
cancer agents.3 Recently, the novel naturally occurring
inhibitor of the G2/M checkpoint, psilostachyin C (1), was
identified by Roberge and co-workers.4 This molecule falls
into a general class of sesquiterpene lactone natural products,
which was originally isolated from Ambrosia psilostachya DC.5
This family of natural products has attracted substantial
attention from the scientific community for its interesting
biological activity as well as complex molecular architecture.6
Herein, we report a concise strategy for the total synthesis of
psilostachyin C.
Our retrosynthetic analysis for psilostachyin C is depicted in
Figure 1. According to the previously reported method, psilos-
tachyin C should be prepared from the structurally related
natural product damsin 2 through direct BaeyerꢀVilliger
oxidation.5,6a Damsin 2 may be derived from a highly functiona-
lized 5,7-bicyclic intermediate 3. The construction of the seven-
membered ring should be achieved by ring-closing metathesis
(RCM), which has been proved to be a powerful method for the
synthesis of a midsized ring.7 We envisioned the key intermediate
4 could be rapidly and efficiently prepared from three readily
available starting materials through an intermolecular tandem
1,4-addition-aldol condensation.8
the commercially available 2-methylcyclopent-2-enone
5
(Scheme 1). Treatment of 5 with freshly prepared isopropenyl
magnesium bromide and anhydrous cuprous iodide generated
the 1,4-addition intermediate which was effectively trapped with
TMSCl to afford trimethylsilyl enol ether 8.10 Mukaiyama-aldol
condensation between compound 8 and pent-4-enal 7 using
BF3 Et2O as an optimal Lewis acid afforded alcohol 4 in 69%
3
yield as a mixture of 4:1 diastereomers.11 The diastereoselectivity
is not important for our synthesis, since we will oxidize the
hydroxyl group in the latter step. Ring-closing metathesis (RCM)
of compound 4 using Grubbs second-generation catalyst cleanly
provided the desired seven-membered ring intermediate 3 in
92% yield.12 Hydrogenation of the double bond using Adam’s
catalyst afforded compound 9 in excellent yield and diastereo-
selectivity (95%, >20:1).
It is not surprising that protection of the sterically hindered
ketone moiety on compound 9 is difficult. After screening a
number of conditions, we found the desired ketal 10 could be
smoothly formed using a combination of 2-methoxy-5,5-dimethyl-
1,3-dioxane and 2,2-dimethylpropane-1,3-diol in the presence of
a catalytic amount of p-TsOH (Scheme 2).13 Oxidation of
alcohol 10 by Dess-Martin periodinane afforded ketone 11,
which was subjected to LDA and R-bromoethylacetate to gen-
erate ester 12 (92%).6b Hydrolysis and epimerization of 12 under
basic conditions afforded acid 13. The relative stereochemistry of
13 was unambiguously confirmed by the X-ray crystal structure
Received: February 5, 2011
Published: March 21, 2011
r
2011 American Chemical Society
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Scheme 2. Synthesis of Tricyclic Compound 14
Figure 1. Retrosynthetic analysis for psilostachyin C.
Scheme 1. Synthesis of 5,7-Bicyclic Ketone 9
further development of an asymmetric strategy by which to gain
access to optically active natural products is necessary.
It was previously reported that psilostachyin C could block the
G2/M checkpoint upon DNA damage but did not show poten-
tiation of the inhibition effect against cell proliferation in
combination with γ irradiation.4 The biological activity of the
structurally related natural product damsin has not been studied
to date. While evaluating the biological activity of our synthetic
psilostachyin C, we failed to confirm the reported checkpoint
inhibitor activity, at least in commonly used HeLa cells. As DNA
damage agent is known to induce apoptosis as anticancer agents,
we then tested whether psilostachyin C and damsin could
sensitize DNA damage and facilitate cell apoptosis. Camptothe-
cin (CPT), a classical DNA damage agent, can trigger basic
caspase-3 activation in the damaged HeLa cells.22 The in vivo
substrate of caspase-3, poly(ADP-ribose) polymerase (PARP),
can be partially cleaved upon CPT treatment. Herein, caspase-3
and PARP are both intrinsic apoptosis markers.23 As shown in
Figure 2, when HeLa cells were treated with CPT and damsin or
psilostachyin C together for 4 h, more caspase-3 was activated
than the basic level in DNA damaged cells. In combination with
CPT treatment, damsin or psilostachyin C could promote the
DNA damaged HeLa cells to apoptosis in a dose dependent
manner, whereas the damsin or psilostachyin C alone did not
activate caspase-3. Interestingly we found that at the concentra-
tion of 50 μM, damsin showed higher potency than psilostachyin
C. These results suggest that damsin or psilostachyin C could
sensitize cancer cell toward camptothecin (CPT) treatment by
the activation of apoptosis. Therefore, damsin or psilostachyin C
could serve as a very useful small molecule chemical probe for the
identification of its cellular target(s).
analysis (see the Supporting Information). Treatment of acid 13
with NaOAc in Ac2O smoothly afforded the desired lactone 14
which was directly used in the next step.14
With the lactone 14 in hand, we evaluated a series of
conditions for the installation of exo-methylene moiety. After
considerable experimentation, we identified a highly efficient
two-step protocol to achieve this goal (Scheme 3). Selective
hydrogenation of 14 cleanly generated the cis-fused tricyclic
lactone 15. Treatment of γ-butyrolactone 15 with NaH and
paraformaldehyde under 100 °C in THF in a sealed tube
smoothly afforded the desired R-alkylidene-γ-butyrolactone
16.15 Removal of dimethylketal under mild acidic conditions
furnished damsin 2 in 98% yield. Final transformation using
direct BaeyerꢀVilliger oxidation of damsin to generate psilos-
tachyin C turned out to be a great challenge. Attempted condi-
tions including m-CPBA,16 CH3CO3H (15%),6b PhSeO2H-
H2O2 (36%),17 TFAA-H2O2,18 H2O2-NaOH,19 and Ph3COOLi20
led to either poor reactivity or excessive side reactions. After
extensive screening of reaction conditions, we were pleased to
identify an effective condition using bis(trimethylsilyl)-peroxide
(TMSO)2 and TMSOTf,21 which allowed us to accomplish the
total synthesis of racemic psilostachyin C in 60% yield. Synthetic
1 was confirmed to be identical to natural psilostachyin C by 1H
and 13C NMR data and high-resolution mass spectrometry. The
relative stereochemistry of psilostachyin C was further unam-
biguously confirmed by the X-ray crystal structure. Since natural
damsin and psilostachyin C are chiral nonracemic substrates,5
In conclusion, the concise total synthesis of the G2/M DNA
damage checkpoint inhibitor psilostachyin C has been achieved
in only 13 steps with 14% overall yield. The synthesis relies on a
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NOTE
Scheme 3. Total Synthesis of Damsin and Psilostachyin C
2-methylcyclopent-2-enone (3.94 mL, 40.0 mmol) and cuprous iodide
(760 mg, 4.0 mmol) in anhydrous THF (20 mL). Stirring was continued
for 3 h at ꢀ40 °C, and an equimolar mixture of trimethylsilylchloride
(10.2 mL, 80.0 mmol) and triethylamine (free from triethylamine hydro-
chloride) (11.1 mL, 80.0 mmol) was added at ꢀ15 °C. The resulting mixture
was allowed to warm to room temperature and stirred for 3 h. Then the
reaction was quenched by addition of water and extracted with pentane. The
combined organic phase was washed with brine, dried with Na2SO4, and
concentrated in vacuo. The enol ether was purified by flash chromatography
(pentane) to provide 8as a colorless oil (7.0 g, 85% yield). Physical properties
were identical to those of a previous report.10
Diene 4. Boron trifluoride etherate (2.0 mL, 15.5 mmol, 1.0 equiv)
was added dropwise to a 0.1 M solution of 8 (3.6 g, 17.1 mmol, 1.1 equiv)
and pent-4-enal (1.3 g, 15.5 mmol, 1.0 equiv) in CH2Cl2 at ꢀ78 °C. The
reaction was stirred for 3 h, quenched at ꢀ78 °C by addition of an
equivalent volume of saturated aqueous NaHCO3, and then warmed to
ambient temperature. The mixture was diluted with CH2Cl2 and washed
with saturated aqueous NaHCO3. The aqueous phase was extracted
once with CH2Cl2. The combined organic layers were dried over
anhydrous Na2SO4, concentrated in vacuo. The residue was purified
by silica gel chromatography (PE/ EtOAc = 10:1) to afford diene 4
(2.3 g, 69%, 4:1 mixture) as a colorless oil. 1H NMR (400 MHz, CDCl3,
major isomer) 5.88ꢀ5.78 (m, 1H), 5.08ꢀ4.96 (m, 2H), 4.95 (s, 1H),
4.83 (s, 1H), 3.68ꢀ3.64 (m, 1H), 3.00 (dd, 1H, J1 = 8.1 Hz, J2 = 6.4 Hz),
2.43ꢀ2.19 (m, 3H), 2.16ꢀ2.08 (m, 2H), 2.04ꢀ1.96 (m, 1H),
1.94ꢀ1.83 (m, 1H), 1.81 (s, 3H), 1.80ꢀ1.72 (m, 1H), 1.63ꢀ1.54 (m,
1), 0.94 (s, 3H); 13C NMR (100 MHz, CDCl3, major isomer) δ 222.7,
144.9, 138.3, 115.1, 114.0, 75.1, 55.9, 48.8, 37.9, 31.3, 31.0, 24.1, 23.3,
14.3; IR (neat) νmax 3461, 3077, 2963, 2918, 2850, 1728, 1641,
1451 cmꢀ1; HRMS (ESI) [M þ Naþ] calculated for C14H22NaO2,
245.1512; found, 245.1506.
Alcohol 3. A solution of diene 4 (2.3 g, 10.4 mmol) in anhydrous
CH2Cl2 (2100 mL) was treated with Grubbs’ II catalyst (90 mg, 0.104
mmol, 1 mol %). The mixture was stirred at reflux for 1 h and then
cooled to room temperature. The mixture was stirred open to air
overnight and concentrated in vacuo. Purification by flash chromatog-
raphy (PE/EtOAc = 5:1) afforded alcohol 3 (1.83 g, 92%) as a colorless
oil. 1H NMR (400 MHz, CDCl3) 5.70ꢀ5.67 (m, 1H), 4.37 (s, 1H), 3.69
(dd, 1H, J1 = 11.2 Hz, J2 = 7.2 Hz), 2.78 (t, 1H, J = 8.8 Hz), 2.53ꢀ2.46
(m, 1H), 2.31ꢀ2.21 (m, 1H), 2.09ꢀ1.90 (m, 3H), 1.75, s, 3H),
1.71ꢀ1.64 (m, 1H), 1.36ꢀ1.23 (m, 1H), 0.89(s, 3H); 13C NMR (100
MHz, CDCl3) δ 227.1, 136.5, 126.4, 77.3, 51.7, 44.6, 37.0, 27.5, 22.8,
22.3, 22.2, 8.5; IR (neat) νmax 3499, 2962, 2935, 2854, 1718, 1446, 1286,
1070, 1050 cmꢀ1; HRMS (ESI) [M þ Naþ] calculated for
C12H18NaO2, 217.1199; found, 217.1192.
Ketone 9. A solution of 1.41 g (7.3 mmol) of alcohol 3 and 100 mg
(5 wt %) of platinum oxide in 100 mL of absolute methanol was stirred
at ꢀ15 °C and under atmospheric hydrogen gas. After 2 h, the uptake of
hydrogen ceased and the reaction mixture was filtered and concentrated
Figure 2. Biological evaluation of damsin and psilostachyin C.
1
1,4-addition-aldol condensation-RCM strategy to efficiently construct
the key 5,7-bicyclic skeleton. Initial biological studies indicate that
both damsin and psilostachyin C could enhance the sensitivity of
HeLa cell toward camptothecin (CPT) treatment via the activation of
caspase-3 mediated apoptosis pathway. Further studies toward the
asymmetric synthesis and biological evaluation of damsin and
psilostachyin C derivatives to identify suitable small molecule chemi-
cal probes for further understanding of the G2/M checkpoint path-
way are in progress and will be reported in due course.
to afford 1.35 g (95%) of the product 9 as a colorless oil. H NMR
(400 MHz, CDCl3) 4.55 (s, 1H), 3.55 (dd, 1H, J1 = 11.2 Hz, J2 = 4.8 Hz),
2.50ꢀ2.43 (m, 1H), 2.31ꢀ2.21 (m, 1H), 2.16ꢀ1.92 (m, 5H),
1.89ꢀ1.81 (m, 1H), 1.70ꢀ1.63 (m, 1H), 1.52ꢀ1.43 (m, 1H),
1.37ꢀ1.19 (m, 1H), 1.06 (s, 3H), 1.00 (d, 3H), J = 6.4 Hz); 13C
NMR (100 MHz, CDCl3) δ 228.4, 76.7, 54.4, 43.6, 37.6, 36.3, 34.8, 32.9,
23.7, 20.9, 18.8, 11.4; IR (neat) νmax 3490, 2959, 2929, 2860, 1719, 1467,
1411, 1057 cmꢀ1; HRMS (ESI) [M þ Naþ] calculated for C12H20-
NaO2, 219.1356; found, 219.1349.
Ketal 10. Ketone 9 (1.18 g, 6.02 mmol) was combined with 2,2-
dimethylpropane-1,3-diol (7.51 g, 72.24 mmol, 12 equiv) and crude
2-methoxy-5,5-dimethyl-1,3-dioxane (2.64 g, 18.06 mmol, 3 equiv) in
12 mL of anhydrous THF containing p-TsOH (58 mg, 0.33 mmol, 0.05
equiv) at 23 °C. The reaction was stirred at room temperature overnight
and quenched with half-saturated aqueous NaHCO3. Then the aqueous
’ EXPERIMENTAL SECTION
(2-Methyl-3-(prop-1-en-2-yl)cyclopent-1-enyloxy)trimethyl-
ilane 8. To a solution of isopropenyl magnesium bromide (60.0 mmol,
60 mL) in anhydrous THF was added at ꢀ60 °C a mixture of
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NOTE
phase was extracted with EtOAc, dried over anhydrous Na2SO4, filtered,
and concentrated to give the crude product. Purification by the silica gel
chromatography (PEꢀEtOAc = 10:1) provided 1.6 g (94%) of ketal 10
as a white solid. Mp 70ꢀ71 °C; 1H NMR (400 MHz, CDCl3) 4.14 (dd,
1H, J1 = 11.2 Hz, J2 = 4.8 Hz), 3.75ꢀ3.73 (m, 2H), 3.57ꢀ3.39 (m, 3H),
2.38ꢀ2.32 (m, 1H), 2.16ꢀ2.09 (m, 1H), 2.00ꢀ1.67(m, 5H), 1.63ꢀ1.52
(m, 2H), 1.47ꢀ1.40 (m, 1H), 1.36ꢀ1.22 (m, 2H), 1.21 (s, 3H), 1.01 (s,
3H), 0.97 (d, 3H, J = 5.1 Hz), 0.74 (s, 3H); 13C NMR (100 MHz,
CDCl3) δ 111.1, 74.7, 72.9, 70.6, 54.7, 40.3, 36.7, 35.0, 32.7, 30.0, 24.9,
23.5, 23.0, 22.3, 21.0, 19.0, 11.8; IR (neat) νmax 3536, 2953, 2927, 2865,
1728, 1470, 1301, 1260, 1124 cmꢀ1; HRMS (ESI) [M þ Naþ]
calculated for C17H30NaO3, 305.2087; found, 305.2084.
Ketone 11. A solution of ketal 10 (1.47 g, 5.18 mmol) in 10 mL of
anhydrous CH2Cl2 was added to a solution of Dess-Martin periodinane
(3.3 g, 7.8 mmol, 1.5 equiv) in 10 mL of anhydrous CH2Cl2 with stirring.
After 2 h, the reaction mixture was diluted with ether and the resulting
suspension was added to 50 mL of 1.3 M NaOH aq. After the mixture
was stirred for 10 min, the ether layer was washed with 50 mL of 1.3 M
NaOH, 50 mL of water, and brine. The organic layer was then dried over
anhydrous Na2SO4 and concentrated in vacuo. Purification by flash
chromatography (PEꢀEtOAc = 10:1) provided 1.35 g (92%) of ketone
11 as a white solid. Mp 108ꢀ109 °C; 1H NMR (400 MHz, CDCl3) 3.57
(d, 1H, J = 11.2 Hz), 3.47ꢀ3.38 (m, 2H), 3.28ꢀ3.35 (m, 1H),
3.08ꢀ3.00 (m, 2H), 2.48ꢀ2.32 (m, 2H), 2.05ꢀ2.02 (m, 1H),
1.85ꢀ1.62 (m, 6H), 1.44ꢀ1.37 (m, 1H), 1.26 (s, 3H), 1.03 (s, 3H),
0.96 (d, 3H, J = 7.6 Hz), 0.68 (s, 3H); 13C NMR (100 MHz, CDCl3) δ
215.6, 110.6, 72.2, 70.5, 61.3, 43.1, 43.0, 35.0, 34.0, 30.2, 27.1, 23.9, 22.4,
21.9, 20.6, 16.9, 14.3; IR (thin film) νmax 2957, 2916, 2861, 1703, 1473,
1308, 1175, 1133, 1070 cmꢀ1; HRMS (ESI) [M þ Naþ] calculated for
C17H28NaO3, 303.1931; found, 303.1927.
11.2 Hz, 3.44ꢀ3.36 (m, 2H), 3.27ꢀ3.23 (m, 1H), 3.19ꢀ3.13 (m, 1H),
2.70 (dd, 1H, J1 = 16 Hz, J2 = 6 Hz), 2.41ꢀ2.18 (m, 2H), 2.06ꢀ1.91 (m,
2H), 1.86ꢀ1.66 (m, 4H), 1.46ꢀ1.32 (m, 2H), 1.12 (s, 3H), 0.96ꢀ0.94
(m, 6H), 0.66 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 214.9, 178.2,
110.7, 72.2, 70.4, 61.4, 45.8, 42.3, 36.9, 34.2, 33.1, 30.1, 29.0, 27.4, 23.8,
22.1, 21.9, 17.3, 13.8; IR (neat) νmax 2958, 2866, 1739, 1711, 1474, 1306,
1124, 1067 cmꢀ1; HRMS (ESI) [M þ Naþ] calculated for C19H30-
NaO5, 361.1986; found, 361.1983.
Lactone 15. A mixture of 75 mg (0.22 mmol) of acid 13 and 273 mg
(3.3 mmol) of sodium acetate in 5 mL of acetic anhydride was heated at
150 °C for 1 h. The resulting mixture was cooled to 0 °C, and 2 mL of
EtOAc was added followed by filtration. The filtrate was concentrated in
vacuo to provide 53 mg (75%) of butenolide 14 as a colorless oil. The
crude product 14 was directly used for the next step. To the solution of
14 (53 mg, 0.17 mmol) in 4.0 mL of anhydrous EtOAc was added 8 mg
of PtO2. The reaction mixture was placed under atmospheric hydrogen
gas and stirred at 0 °C for 12 h. The resulting mixture was filtered
through Celite, and the filtrate was then concentrated in vacuo and
purified by flash column chromatography (PEꢀEtOAc = 10:1) to give
48 mg (93%) as a white solid. Mp 136ꢀ137 °C; 1H NMR (400 MHz,
CDCl3) 5.17 (d, 1H, J = 8.4 Hz), 3.67 (d, 1H, J = 11.6 Hz), 3.42 (m, 3H),
2.80 (m, 1H), 2.67 (dd, 1H, J1 = 17.2 Hz, J2 = 9.2 Hz), 2.43ꢀ2.34 (m,
1H), 2.24 (dd, 1H, J1 = 17.6 Hz, J2 = 8.4 Hz), 2.16ꢀ2.09 (m, 1H),
2.04ꢀ1.99 (m, 1H), 1.89ꢀ1.53 (m, 6H), 1.25 (s, 3H), 1.08 (s, 3H), 1.06
(d, 3H, J = 7.6 Hz), 0.70 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 177.3,
109.7, 83.3, 72.1, 71.0, 54.7, 44.9, 40.3, 37.6, 34.3, 33.7, 30.2, 25.3, 24.9,
24.2, 23.0, 22.1, 15.2, 13.6; IR (neat) νmax 2950, 2862, 1772, 1471, 1130,
1112 cmꢀ1; HRMS (ESI) [M þ Naþ] calculated for C19H30NaO4,
345.2036; found, 345.2030.
r-Methylene Lactone 16. To a solution of lactone 15 (44 mg,
0.14 mmol) in 2 mL of anhydrous THF in a sealed tube was added dry
paraformaldehyde (130 mg, 4.3 mmol) and NaH (19 mg, 60 wt %, 0.46
mmol). The resulting mixture was stirred for 15 min at 100 °C before the
resulting brown solution was cooled to 0 °C. The reaction mixture was
diluted with EtOAc, washed by water. The aqueous layer was extracted
twice with EtOAc, and the combined organic layers were dried over
Na2SO4, filtered, and concentrated in vacuo. The residue was purified by
flash chromatography (EA/PE = 1:9) to give the product 16 (40 mg,
81%) as a white solid. Mp 141ꢀ142 °C; 1H NMR (400 MHz, CDCl3)
6.16 (d, 1H, J = 3.6 Hz), 5.42 (d, 1H, J = 3.6 Hz), 5.24 (d, 1H, J = 9.2 Hz),
3.67 (d, 1H, J = 13.2 Hz), 3.43ꢀ3.40 (m, 3H), 3.35ꢀ3.28 (m, 1H),
2.44ꢀ2.38 (m, 1H), 2.16ꢀ2.09 (m, 1H), 2.05ꢀ2.00 (m, 1H),
1.95ꢀ1.90 (m, 2H), 1.86ꢀ1.81 (m, 2H), 1.65ꢀ1.57 (m, 3H), 1.28 (s,
3H), 1.03 (d, 3H, J = 8.0 Hz), 0.93 (s, 3H), 0.71 (s, 3H); 13C NMR (100
MHz, CDCl3) δ 170.8, 141.7, 118.6, 109.6, 81.5, 72.2, 71.1, 55.1, 45.1,
43.7, 33.6, 33.3, 30.3, 25.2, 24.4, 24.2, 23.0, 22.2, 14.8, 12.5; IR (neat)
νmax 2949, 2866, 1758, 1474, 1152, 1112 cmꢀ1; HRMS (ESI) [M þ
Naþ] calculated for C20H30NaO4, 357.2036; found, 357.2031.
Damsin 2. A solution of 16 (37 mg, 0.111 mmol) and HCl (0.3 mL,
0.2 M) in THF (1.5 mL) was stirred at room temperature for 20 h and
poured into a saturated aqueous solution of NaHCO3 (20 mL). The
reaction mixture was extracted with EtOAc, washed with brine, dried
over anhydrous Na2SO4, and concentrated in vacuo. The crude product
was purified by flash chromatography (PEꢀEtOAc = 3:2) to give 27 mg
(98%) of damsin 2 as a white solid. Mp 107ꢀ108 °C; 1H NMR (400
MHz, CDCl3) 6.26 (d, 1H, J = 3.2 Hz), 5.54 (d, 1H, J = 3.2 Hz), 4.53 (d,
1H, J = 8.8 Hz), 3.30ꢀ3.28 (m, 1H), 2.50ꢀ2.43 (m, 1H), 2.29ꢀ2.20 (m,
2H), 2.10ꢀ1.96 (m, 3H), 1.90ꢀ1.80 (m, 3H), 1.78ꢀ1.69 (m, 1H), 1.09
(s, 3H), 1.08 (d, 3H, J = 12.0 Hz); 13C NMR (100 MHz, CDCl3) δ
218.7, 170.1, 139.6, 120.8, 81.7, 54.9, 46.1, 44.4, 36.0, 34.3, 33.4, 25.7,
23.9, 15.8, 13.8; IR (neat) νmax 2923, 2872, 1755, 1738, 1270, 1160,
1014 cmꢀ1; HRMS (ESI) [M þ Naþ] calculated for C15H20NaO3,
271.1305; found, 271.1300.
Keto Ester 12. To a stirred solution of diisopropylamine (233 μL,
1.65 mmol, 1.1 equiv) in 2 mL of anhydrous THF at ꢀ78 °C was added a
solution of n-butyllithium (630 μL 2.5 M, 1.58 mmol, 1.05 equiv) in
hexane. Then the reaction mixture was warmed to 0 °C. After stirring at
0 °C for 30 min, the reaction mixture was cooled to ꢀ78 °C, and a
solution of ketone 11 (423 mg, 1.5 mmol) in 3 mL of anhydrous THF
was added. After 1 h, the reaction mixture was warmed to ꢀ20 °C slowly.
Then the reaction mixture was cooled down to ꢀ78 °C again and a
solution of ethyl bromoacetate (183 μL, 1.65 mmol, 1.1 equiv) and
HMPA (287 μL, 1.65 mmol, 1.1 equiv) in 1 mL of anhydrous THF was
added. The reaction mixture was warmed to room temperature over 2 h.
Then the reaction was quenched with saturated NH4Cl solution and
extracted with EtOAc. The organic layer was washed with water and
brine subsequently, dried over anhydrous Na2SO4, concentrated in
vacuo, and purified by flash column chromatography (PEꢀEtOAc =
5:1) to give keto ester 12 (498 mg, 92% yield) as a white solid. Mp
84ꢀ85 °C; 1H NMR (400 MHz, CDCl3) 4.12 (q, 2H, J = 7.2 Hz), 3.57
(d, 1H, J = 11.2 Hz), 3.48ꢀ3.38 (m, 2H), 3.32ꢀ3.28 (m, 1H),
3.16ꢀ3.10 (m, 1H), 3.06ꢀ3.00 (m, 1H), 2.61 (d, 2H, J = 8.0 Hz),
2.41ꢀ2.34 (m, 1H), 2.03ꢀ1.78 (m, 4H), 1.68ꢀ1.43 (m, 3H),
1.29ꢀ1.17 (m, 4H), 1.11 (s, 3H), 1.03 (s, 3H), 1.00 (d, 3H, J = 7.2
Hz), 0.68 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 215.4, 172.4, 110.1,
72.0, 70.4, 62.7, 60.3, 52.1, 39.9, 38.8, 34.5, 32.3, 30.2, 26.6, 26.3, 22.6,
22.3, 21.8, 16.9, 16.3, 14.2; IR (neat) νmax 2952, 2858, 1734, 1689, 1472,
1180, 1134, 1124 cmꢀ1; HRMS (ESI) [M þ Naþ] calculated for
C21H34NaO5, 389.2299; found, 389.2289.
Acid 13. The solution of 200 mg (0.55 mmol) of keto ester 12 and
150 mg of potassium hydroxide in 3 mL of methanol was heated at reflux
for 2 h. The solution was cooled, poured into water, and washed with
ether. The aqueous layer was carefully acidified with 2 M HCl, and the
product was isolated with ether. After concentration in vacuo, the crude
product was purified by flash column chromatography (PEꢀEtOAc =
3:1) to give the acid 13 (185 mg, 100%) as a white solid. Mp
148ꢀ149 °C; 1H NMR (400 MHz, CDCl3) 3.72 (m, 1H), 3.54, d, J =
3569
dx.doi.org/10.1021/jo2001275 |J. Org. Chem. 2011, 76, 3566–3570

The Journal of Organic Chemistry
NOTE
Psilotachyin C 1. To a solution of the TMSOTf (1 mg, 0.0044
mmol, 0.1 equiv) in 0.5 mL of anhydrous CH2Cl2 was slowly added
bis(trimethylsilyl)peroxide (TMSO)2 (40 mg, 0.22 mmol, 5 equiv)
at ꢀ78 °C, followed by a solution of damsin 2 (11 mg, 0.044 mmol) in
0.2 mL of anhydrous CH2Cl2. The reaction mixture was warmed
to ꢀ50 °C and stirred at this temperature for 24 h, before quenched
by an ice cooled, saturated NaHCO3 aqueous solution (5 mL). The
mixture was extracted with EtOAc, and the combined organic layers
were washed with brine, dried over Na2SO4, and concentrated in vacuo.
The residue was purified by reverse phase HPLC [(50% waterꢀ50%
methanol f 0% waterꢀ100% methanol (10 min), (15 mL/min)] to
give 7 mg (60%) of psilotachyin C 1 as a white solid. Mp 223ꢀ225 °C;
1H NMR (400 MHz, CDCl3) 6.26 (d, 1H, J = 3.2 Hz), 5.51 (d, 1H, J =
3.2 Hz), 4.65 (d, 1H, J = 9.6 Hz), 3.42ꢀ3.39 (m, 1H), 2.72ꢀ2.65 (m, 1H),
2.52ꢀ2.42 (m, 1H), 2.21ꢀ2.18 (m, 1H), 2.17ꢀ2.02 (m, 2H), 1.97ꢀ1.92
(m, 1H), 1.87ꢀ1.67 (m, 4H), 1.31 (s, 3H), 1.02 (d, 3H, J = 15.2 Hz); 13C
NMR (100 MHz, CDCl3) δ 169.6, 168.9, 138.4, 120.4, 89.8, 86.2, 77.3,
43.2, 41.4, 35.3, 31.7, 30.9, 24.0, 22.5, 18.9, 14.4; IR (thin film) νmax 2918,
2882, 1761, 1729, 1259, 1235, 1145, 984 cmꢀ1; HRMS (ESI) [M þ Naþ]
calculated for C15H20NaO4, 287.1254; found, 287.1255.
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Corresponding Author
*E-mail: leixiaoguan@nibs.ac.cn
’ ACKNOWLEDGMENT
We thank Ms. Mingyan Zhao (NIBS) for NMR and LCꢀMS
analysis and Dr. Jiang Zhou (Peking University) for HRMS
analysis. Financial support from the National High Technology
Projects 863 (2008AA022317) and NSFC (20802050, 21072-
150) is gratefully acknowledged.
3570
dx.doi.org/10.1021/jo2001275 |J. Org. Chem. 2011, 76, 3566–3570