Formal total synthesis of (±)-trichodiene via Claisen rearrangement and Robinson annulation

Article abstract of DOI:10.1515/znb-2008-0312

A facile formal total synthesis of (±)-trichodiene was achieved by employing Claisen rearrangement and Robinson annulation as synthetic key strategies.

Full text of DOI:10.1515/znb-2008-0312

Formal Total Synthesis of (± )-Trichodiene via Claisen Rearrangement
and Robinson Annulation
Jae-Chul Jung , Young-Jo Jung , and Oee-Sook Park
a
b
c
a
Department of Neuroscience and Medical Research Institute, College of Medicine,
Ewha Womans University, Ewha Global Challenge BK21, Seoul 158-710, South Korea
Union Chemical Corporation, Cheongju 361-290, Chungbuk, South Korea
Department of Chemistry, Institute for Basic Sciences, College of Natural Sciences,
Chungbuk National University, Cheongju 361-763, Chungbuk, South Korea
b
c
Reprint requests to Prof. Oee-Sook Park. Fax: +82-43-267-227. E-mail: ospark@cbnu.ac.kr
Z. Naturforsch. 2008, 63b, 293 – 297; received October 23, 2007
A facile formal total synthesis of (± )-trichodiene was achieved by employing Claisen rearrange-
ment and Robinson annulation as synthetic key strategies.
Key words: (± )-Trichodiene, Claisen Rearrangement, Robinson Annulation
Introduction
(
± )-Trichodiene (1) is a volatile sesquiterpene
which was isolated from the extract of the mycelium of
the fungus Trichothecium roseum Link [1]. The struc-
ture of (± )-trichodiene (1) was determined by No-
zoe and Machida in 1970 via degradation reactions
and by spectroscopic methods [2]. (± )-Trichodienehas Fig. 1. Structures of (± )-trichodiene (1) and (–)-trichodermin
(
2).
been shown to be the biogenetic precursor of the tri-
chothecane family of sesquiterpenoids as characterized
by the cytotoxic fungal metabolite (–)-trichodermin
are the Claisen rearrangement of vinyl ether 4 and the
Robinson annulation of aldehyde 10a in order to con-
struct quaternary centers of enone 3 which has been
converted into (± )-trichodiene (1).
(
2, Fig. 1) [3]. The trichothecane family of sesquiter-
penoids has stimulated significant interest due to
its wide range of intriguing biological activities [4]
such as antifungal [5], antibacterial [6], cytotoxic [7],
and phytotoxic effects [8], and the inhibition of en-
zymes [9]. Several elegant synthetic approaches have
Results and Discussion
Our retrosynthetic analysis of (± )-trichodiene (1) is
been reported [10] in the literature; most of these meth- shown in Scheme 1. (± )-Trichodiene (1) has been syn-
ods are racemic syntheses, and only a few approaches thesized from enone 3 by using a Wittig reaction and
are applicable to the stereocontrolled synthesis of (+)- metal reduction [14]. We envisaged that the enone 3
trichodiene or (–)-trichodiene. The Lange group [11] would be accessible from the vinyl ether 4 via a Claisen
has performed a synthesis of (± )-trichodiene using a rearrangement [15] and a Robinson annulation of alde-
free radical fragmentation approach. Meyer et al. [12] hyde 10a [16] (Scheme 2).
reported the asymmetric synthesis of (–)-trichodiene
Cyclohexane was treated with acetyl chloride in
based on the generation of vicinal stereogenic quater- the presence of aluminum chloride in chloroform to
nary centers via a thio-Claisen rearrangement. Gilbert give 1-acetyl-2-methylcyclopentene (6) [17]. Com-
et al. [13] described an enantioselective synthesis of pound 6 was oxidized with freshly prepared potas-
(
–)-trichodiene through Ireland-Claisen rearrangement sium hypochlorite(KOCl) solution (calcium hypochlo-
of a ketene silyl acetal with excellent stereoselectivity. rite was treated with 0.5 equiv. of potassium hydrox-
Herein, we wish to report a formal total synthesis of ide and 0.75 equiv. of potassium carbonate in H O) to
± )-trichodiene (1). Key synthetic steps of our strategy obtain 2-methyl-1-cyclopentenecarboxylic acid (7) in
2
(
0
932–0776 / 08 / 0300–0293 $ 06.00 ꢀc 2008 Verlag der Zeitschrift f u¨ r Naturforschung, T u¨ bingen · http://znaturforsch.com
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294
J.-C. Jung et al. · Formal Total Synthesis of (± )-Trichodiene
Scheme 1. Retrosynthetic analysis
of (± )-trichodiene (1).
Scheme 2. Synthesis of (± )-
trichodiene (1): (a) AlCl , AcCl,
3
CHCl , reflux, 2 h; then r. t., 48 h;
3
then KOH, MeOH, r. t., 16
h
(
32 %); (b) KOCl [Ca(OCl) , KOH,
2
◦
K CO ], H O, 0 C, 16 h (80 %);
2
3
2
(
(
c) LAH, diethyl ether, r. t., 1 h
78 %); (d) Hg(OAc) , ethyl vinyl
2
ether, reflux, 28 h (72 %); (e) neat,
◦
2
35 – 240 C, 1 h (83 %); or Ph P,
3
Et AlCl, 1,2-dichloroethane, r. t.,
2
1
5
(
h (51 %); (f) KH, CH I, THF,
3
◦
C, 30 min; then r. t., 6 h (70 %);
g) KH, 3-buten-2-one, r. t., 1 h;
◦
then 80 – 90 C, 1 h (60 %).
8
0 % yield [18]. Subsequent reduction of compound 7 forded 10a and 10b (1 : 1, 70 % combined yield). The
was accomplished by treatment with lithium aluminum desired component 10a was subjected to Robinson an-
hydride (LAH) in dry THF to give 2-methyl-1-cyclo- nulation with 3-buten-2-one at 80– 90 C for 1 h in a
◦
pentenylmethanol (8) in good yield [19]. Alcohol 8 steam bath to afford a 1.2 : 1 mixture of compound 3.
was treated with freshly distilled ethyl vinyl ether This mixture could be separated by flash chromatog-
in the presence of mercury acetate, [Hg(OAc) ], to raphy [20] with 5 % EtOAc in hexane by using twice
2
yield 1-methyl-2-vinyloxymethylcyclopentene (4) in the amount of silica gel specified in [20] (1.2 : 1, 60 %
7
2 % yield, which was subjected to Claisen rearrange- combined yield). Most diagnostic for the assignment
◦
ment at 235 – 240 C for 1 h in a silicon oil bath to af- of the relative stereochemistry to the isomers were the
ford 1-methyl-1-formylmethyl-2-methylenecyclopent- singlets δ = 1.18 and 1.22 that are ascribable to the
ane (9) in 83 % yield (including recovered starting ma- methyl group at C-4 of the cyclohexene ring: The res-
terial).
onance at higher field is assigned to 3a by analogy to
1
Vinyl ether 4 was also treated with triphenylphos- literature H NMR data of 3a [14].
phine and diethyl aluminum chloride in 1,2-dichloro-
ethane to give compound 9 in 5l % yield. Although
the latter reaction condition was more convenient for
handling, the thermal Claisen rearrangement method
Conclusion
In conclusion, we have developed a concise formal
afforded a clean product and superior yield. Methyla- total synthesis of (± )-trichodiene (1), starting from 1-
tion of compound 9 was performed with KH (30 wt % methyl-2-vinyloxymethylcyclopentene (4). The ad-
dispersion in mineral oil) and CH I in dry THF to vantage of this synthesis over previously reported
3
provide a 1 : 1 mixture of aldehyde 10. Separation of methods, that also employ a [3,3]-sigmatropic rear-
the resulting mixture by preparative TLC (0.5 mm, rangement, is that the starting materials can be pre-
2
2
0 × 20 cm , silica gel, 2 % EtOAc in hexane) af- pared easily and from inexpensive reagents.
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J.-C. Jung et al. · Formal Total Synthesis of (± )-Trichodiene
295
Experimental Section
and compound 6 (1.0 g, 8.1 mmol) was added dropwise
under argon atmosphere. The reaction mixture was stirred
Reactions requiring anhydrous conditions were performed
with the usual precautions for rigorous exclusion of air and
moisture. Tetrahydrofuran was distilled from sodium ben-
zophenone ketyl prior to use. Thin layer chromatography
◦
at 0 C for 16 h and allowed to warm to r. t. The mixture
was quenched by addition of sodium hydrogen sulfite (1.0 g)
and extracted with diethyl ether (20 mL). The aqueous phase
was acidified by using an 18 % aqueous HCl solution and
extracted with dichloromethane (3×10 mL). The combined
(
TLC) was performed on precoated silica gel G and GP
uniplates from Analtech and visualized with 254 nm UV
light. Flash chromatography was carried out on silica gel 60
organic phases were dried over anhydrous MgSO , filtered,
4
and concentrated under reduced pressure to give 7 (0.81 g,
(
6
Scientific Adsorbents Incorporated (SAI), particle size 32 –
3 µm, pore size 60 A). H and C NMR spectra were
−1
8
–
2
0 %). – IR (neat, NaCl): ν = 3460, 2960, 1461, 1280 cm
.
˚
1
13
1
H NMR (CDCl , 400 MHz): δ = 10.9 (br s, 1H), 2.81 –
.30 (m, 4H), 2.1 (s, 3H), 2.05 – 1.62 (m, 2H). – HRMS
3
recorded on a Bruker DPX 400 instrument at 400 MHz
and 100 MHz, respectively. Chemical shifts are reported in
parts per million (ppm) downfield from tetramethylsilane,
and J values are given in Hz. Infrared (IR) spectra were ob-
tained on an ATI Mattson FT/IR spectrometer. Mass spectra
were recorded with a Waters Micromass ZQ LC-Mass sys-
tem and high-resolution mass spectra (HRMS) were mea-
(
[
(+)-ESI): m/z = 127.0773 (calcd. 127.0759 for C H O ,
M+H] ).
7 11 2
+
2-Methyl-1-cyclopentenylmethanol (8)
To a stirred suspension of LAH (1.70 g, 44.8 mmol) in an-
sured with a Bruker BioApex FTMS system by direct in- hydrous diethyl ether (100 mL) was added dropwise acid 7
◦
jection using an electrospray interface (ESI). When neces- (3.80 g, 30.0 mmol) at −45 C, and the mixture was warmed
sary, chemicals were purified according to the reported pro- to r. t. and stirred for 1 h. After the reaction was completed,
cedures [21].
the mixture was quenched with a 3 % aqueous HCl solution
20 mL), and the organic phase was separated and washed
with a saturated aqueous NH Cl solution (30 mL) and brine
(
1-Methyl-2-vinyloxymethylcyclopentene (4)
4
(
40 mL). The organic layer was separated, dried over an-
To a stirred solution of mercury acetate (3.20 g,
0.0 mmol) in freshly distilled ethyl vinyl ether (75 mL) was
added alcohol 8 (1.70 g, 15.1 mmol) at r. t., and the mix-
ture was refluxed for 28 h. The reaction mixture was cooled
to r. t. and poured to a 5 % aqueous KOH solution (15 mL).
The organic phase was separated and washed with a saturated
hydrous MgSO and concentrated under reduced pressure.
The residue was purified by flash column chromatography
4
1
(
silica gel, 10 % ethyl acetate in hexanes) to give alcohol 8
(2.60 g, 78 %) as a colorless oil. – IR (neat, NaCl): ν = 3320,
−
1
1
2
940, 1450, l082 cm . – H NMR (CDCl , 400 MHz): δ =
3
4
(
1
.15 (s, 2H), 2.42 – 2.20 (m, 4H), 1.95 – 1.81 (m, 2H), 1.67
s, 3H), 1.21 (s, 1H). – C NMR (CDCl , 100 MHz): δ =
36.0, 134.5, 59.2, 38.4, 33.9, 21.8, 13.3.
aqueous NH Cl solution (20 mL). The organic layer was sep-
4
13
3
arated, dried over anhydrous MgSO and concentrated under
4
reduced pressure. The residue was purified by flash column
chromatography (silica gel, 5 % ethyl acetate in hexanes) to
give 4 (1.50 g, 72 %) as an oil. – IR (neat, NaCl): ν = 2976,
1
-Methyl-1-formylmethyl-2-methylenecyclopentane (9)
−
1
1
1
6
1
465, 1100, 845 cm . – H NMR (CDCl , 400 MHz): δ =
3
Method A: Vinyl ether 4 (1.38 g, 10.0 mmol) was heated
.46 (dd, J = 15.6, 7.2 Hz, 1H), 4.81 (dd, J = 15.6, 2.0 Hz,
H), 4.52 (dd, J = 7.2, 2.0 Hz), 4.25 (s, 2H), 2.41 – 2.30
◦
at 235 – 240 C for 1 h on a silicon oil bath. The mixture
was cooled to r. t. and diluted with diethyl ether (55 mL) and
then treated with a 10 % aqueous sodium bicarbonate solu-
tion (30 mL). The organic phase was separated and washed
(
(
m, 4H), 2.12 – 1.91 (m, 2H), 1.70 (br s, 3H). – 13C NMR
CDCl , 100 MHz): δ = 155.1, 140.4, 131.7, 86.5, 65.2, 38.1,
3
3
1.2, 22.4, 17.1. – HRMS ((+)-ESI): m/z = 161.0925 (calcd.
with a saturated aqueous NH Cl solution (25 mL) and brine
4
+
161.0942 for C H ONa, [M+Na] ). – C H O (138.1):
9
14
9 14 1
(25 mL). The organic layer was separated, dried over an-
calcd. C 78.21, H 10.21; found C 78.15, H 10.12.
hydrous MgSO and concentrated under reduced pressure.
4
The residue was purified by flash column chromatography
2
-Methyl-1-cyclopentenecarboxylic acid (7)
(silica gel, 7 % ethyl acetate in hexanes) to give aldehyde 9
To a stirred solution of potassium carbonate (2.5 g, (1.15 g, 83 %) as a colorless oil. – IR (neat, NaCl): ν = 2960,
−
7.7 mmol) in H O (7 mL) was added potassium hydrox- 1720, 1460, 1122 cm . – H NMR (CDCl , 400 MHz): δ =
2 3
1
1
1
ide (0.7 g, 12.5 mmol), followed by a suspension of calcium 9.70 (s, lH), 4.88 – 4.75 (m, 2H), 2.41 – 2.30 (m, 4H), l.70 –
13
hypochlorite (3.5 g, 24.0 mmol) in H O (14 mL) at r. t. The 1.61 (m, 4H), 1.12 (s, 3H). – C NMR (CDCl , 100 MHz):
2
3
mixture was stirred at r. t. for 1 h. The potassium hypochlo- δ = l98.8, 170.1, 105.5, 55.6, 53.1, 52.3, 37.2, 25.1, 24.7.
rite solution (containing approximately 14.0 mmol) was then – HRMS ((+)-ESI): m/z = 139.1132 (calcd. 139.1123 for
+
filtered to remove the calcium salts, and the filtered cake was C H O , [M+H] ). – C H O (138.1): calcd. C 78.21,
9
15
1
9 14 1
◦
rinsed with H O (5 mL). The residue was cooled to 0 C, H 10.21; found C 78.11, H 10.15.
2
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296
J.-C. Jung et al. · Formal Total Synthesis of (± )-Trichodiene
−1
1
Method B: To a stirred solution of triphenylphosphine 1082 cm . – H NMR (CDCl , 400 MHz): δ = 9.60 (s, lH),
3
(
1.20 g, 4.6 mmol) in 1,2-dichloroethane (5 mL) was added 4.85 (d, J = 7.0 Hz, 2H), 2.49 – 2.31 (m, 1H), 2.29 – 2.18 (m,
diethyl aluminum chloride (4 mL, 4.0 mmol, 1.0 M sol. 2H), l.78 – 1.53 (m, 4H), 1.10 (s, 3H), 1.07 (s, 3H). – HRMS
in hexane), and the mixture was stirred at r. t. for 15 min. ((+)-ESI): m/z = 153.1291 (calcd. 153.1279 for C H O ,
1
0 17 1
+
A solution of vinyl ether 4 (0.28 g, 2.0 mmol) in 1,2- [M+H] ). – C H16O1 (152.1): calcd. C 78.90, H 10.59;
10
dichloroethane (5 mL) was added dropwise to the reaction found C 78.75, H 10.68.
mixture, and the resulting reaction mixture was stirred at
r. t. for 1 h. The mixture was diluted with dichloromethane 4-Methyl-4-[1-methyl-2-methylenecyclopent-1-yl]-2-
(
20 mL) and treated with a 5 % aqueous HCl solution cyclohexenone (3)
(
15 mL). The organic phase was separated and washed with
To a stirred suspension of potassium hydride (0.20 g,
.0 mmol, 30 wt.-% dispersion in mineral oil) in dry
a saturated aqueous NH Cl solution (15 mL) and brine
(
drous MgSO and concentrated under reduced pressure. The
residue was purified by flash column chromatography (silica
gel, 7 % ethyl acetate in hexanes) to give aldehyde 9 (0.14 g,
5
4
5
15 mL). The organic layer was separated, dried over anhy-
THF (10 mL) was added dropwise aldehyde 10a (0.68 g,
4
◦
4
.5 mmol) in dry THF (5 mL) at 5 C, and the mixture was
stirred at r. t. for 30 min. A solution of 3-buten-2-one (0.35 g,
.0 mmol) in dry THF (7 mL) was added to the mixture at r. t.,
5
1 %) as a colorless oil.
and the resulting mixture was stirred at r. t. for 1 h and then
◦
heated on the steam bath (80 – 90 C) for 1 h. The mixture
2
-[1-Methyl-2-methylenecyclopentyl]propanal (10)
was cooled to r. t. and treated with a 5 % aqueous HCl so-
A suspension of KH (0.44 g, 11.0 mmol, 30 wt.-% lution (5 mL). The mixture was concentrated under reduced
dispersion in mineral oil) in dry THF (15 mL) was added to pressure, and the residue was extracted with dichloromethane
a solution of 9 (1.38 g, 10.0 mmol) in dry THF (5 mL), and (20 mL). The organic phase was separated, washed with a
the mixture was stirred at r. t. for 30 min. After hydrogen saturated aqueous NH Cl solution (10 mL), dried over an-
4
evolution has ceased, a solution of CH I (1.56 g, 11.0 mmol) hydrous MgSO and concentrated under reduced pressure.
3
4
◦
in THF (5 mL) was added dropwise at 5 C, and the resulting The residue was purified by flash chromatography (silica
◦
mixture was stirred at 5 C for 30 min. The reaction mixture gel, 5 % EtOAc in hexane) to give 3a (0.30 g, 33 %) and
was warmed to r. t. and stirred at same temperature for 6 h. its epimer 3b (0.25 g, 27 %) as pale yellow oils. 3a: IR
−1
1
The mixture was quenched with cold water (10 mL) and (neat, NaCl): ν = 1670, 1610 cm . – H NMR (CDCl3,
extracted with diethyl ether (2×20 mL). The organic phase 400 MHz): δ = 6.98 (dd, J = 10.5, 2.0 Hz, 1H), 5.93 (dd,
was separated and washed with a saturated aqueous NH Cl J = 10.5, 0.7 Hz, 1H), 5.05 (d, J = 2.8 Hz, 1H), 4.88 (d,
4
solution (20 mL) and brine (20 mL). The organic layer was J = 2.8 Hz, 1H), 2.53 (ddd, J = 17.2, 14.2, 5.5 Hz, 1H),
separated, dried over anhydrous MgSO4 and concentrated 2.43 – 2.32 (m, 2H), 2.29 – 2.22 (m, 1H), 2.11 (dt, J = 13.8,
under reduced pressure. The residue was purified by flash 5.0 Hz, 1H) 1.85 – 1.64 (m, 3H), 1.53 – 1.37 (m, 2H), 1.18 (s,
column chromatography (silica gel, 5 % ethyl acetate in 3H), 1.12 (s, 3H). – HRMS ((+)-ESI): m/z = 205.1889 (calcd.
+
hexane) to give the products. Separation of the resulting 205.1592 for C H O , [M+H] ). 3b: IR (neat, NaCl): ν =
1
4 21 1
−
1
1
mixture by preparative TLC (0.5 mm, 20 × 20 cm, silica 1670, 1610 cm . – H NMR (CDCl , 400 MHz): δ = 6.92
3
gel, 2 % EtOAc in hexane) afforded 10a (0.54 g, 35 %) and (dd, J = 10.2, 2.2 Hz, 1H), 5.95 (dd, J = 10.2, 0.7 Hz, 1H),
its diastereomer 10b (0.54 g, 35 %) as pale yellow liquids. 5.05 (d, J = 2.8 Hz, 1H), 4.88 (d, J = 2.8 Hz, 1H), 2.53
−1
1
0a: IR (neat, NaCl): ν =1725, 1452, 1120, 1065 cm
.
(ddd, J = 17.0, 14.2, 5.2 Hz, 1H), 2.48 – 2.35 (m, 2H), 2.30 –
1
–
H NMR (CDCl , 400 MHz): δ = 9.70 (s, lH), 4.78 (br 2.25 (m, 1H), 2.11 (dt, J = 13.8, 5.0 Hz, 1H) 1.90 – 1.72 (m,
3
s, 1H), 5.05 (br s, 1H), 2.54 (m, 1H), 2.05 – 1.98 (m, 2H), 3H), 1.55 – 1.40 (m, 2H), 1.22 (s, 3H), 1.12 (s, 3H). – HRMS
l.78 – 1.42 (m, 4H), 1.16 (s, 3H), 1.08 (s, 3H). – 13C NMR ((+)-ESI): m/z = 205.1877 (calcd. 205.1592 for C H O ,
14 21 1
+
(
4
1
CDCl , 100 MHz): δ = 201.1, 170.0, 102.1, 55.6, 49.8, [M+H] ).
3
3.9, 36.4, 25.1, 21.9, 10.2. – HRMS ((+)-ESI): m/z =
+
Acknowledgement
This work was supported by a Chungbuk National Uni-
H 10.53. 10b: IR (neat, NaCl): ν = 1725, 1452, 1122, versity Grant in 2006.
53.1298 (calcd. 153.1279 for C10H17O , [M+H] ). –
1
C H O (152.1): calcd. C 78.90, H 10.59; found C 78.61,
16
10 1
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