Stereoselective total synthesis of furofuran lignans through dianion aldol condensation

Article abstract of DOI:10.1016/j.tetlet.2006.06.127

Highly stereoselective total synthesis of (+)-eudesmin, (+)-yangambin, (-)-eudesmin, and (-)-yangambin is described. This method is useful to generate the core skeleton of furofuran rings utilizing modification of Evans asymmetric aldol condensation.

Full text of DOI:10.1016/j.tetlet.2006.06.127

Tetrahedron Letters 47 (2006) 6433–6437
Stereoselective total synthesis of furofuran lignans through
dianion aldol condensation
Jae-Chul Jung,a Ju-Cheun Kim,a Hyung-In Moonb and Oee-Sook Parkc,*
aDepartment of Medicinal Chemistry, School of Pharmacy, University of Mississippi, PO Box 1848, University, MS 38677-1848, USA
bNational Center of Natural Product Research, School of Pharmacy, University of Mississippi, PO Box 1848,
University, MS 38677-1848, USA
cDepartment of Chemistry, Institute for Basic Sciences, College of Natural Sciences, Chungbuk National University,
Cheongju 361-763, Chungbuk, South Korea
Received 12 May 2006; revised 22 June 2006; accepted 23 June 2006
Available online 24 July 2006
Abstract—Highly stereoselective total synthesis of (+)-eudesmin, (+)-yangambin, (À)-eudesmin, and (À)-yangambin is described.
This method is useful to generate the core skeleton of furofuran rings utilizing modification of Evans asymmetric aldol condensation.
ꢀ 2006 Elsevier Ltd. All rights reserved.
R3
R3
1. Introduction
R2
R2
O
O
O
O
Furofuran lignans have stimulated significant interest
due to their wide range of intriguing biological
activities1 such as antitumor,2 antimitomic,3 antiviral,4
antioxidant,5 antihypertensive,6 inhibition of platelet-
activating factor (PAF),7 and Ca2+ channels,8 cAMP
phosphodiesterase inhibitory,9 sodium selective diuretic
properties,10 and microsomal monooxygenases inhibi-
tory effects for insects.11 Several elegant synthetic ap-
proaches have been reported12 in the literature, most
of these methods are substrate specific providing an en-
try to one particular stereochemical series and only a
few approaches are applicable to the stereocontrolled
synthesis of exo–exo furofuran ( )-sesamin, ( )-eudes-
min, ( )-yangambin, and ( )-epiasarinin (Fig. 1).12f,g
2
4
6
1
R1
R1
H
H
H
H
R1
R2
R1
R2
8
R3
R3
1a (+)-Sesamin
2a (+)-Eudesmin R1=H, R2=R3=-OCH3 2b (-)-Eudesmin
3a (+)-Yangambin R1=R2=R3=-OCH3 3b (-)-Yangambin
R1=H, R2=R3=-OCH2O- 1b (-)-Sesamin
Figure 1. Structures of furofuran lignans.
been interested in the asymmetric total synthesis of
lignans of the furofuran series.
We recently reported13 an efficient asymmetric synthesis
of (+)-sesamin and (À)-sesamin. Key steps include
highly stereoselective aldol condensation of piperonal
with the dianion of chiral auxiliary, followed by intra-
molecular ring cyclization of aldol product in high yield.
In connection with our synthetic approaches, we have
Herein, we described the efficient stereoselective total
synthesis of (+)-eudesmin, (+)-yangambin, (À)-
eudesmin, and (À)-yangambin by modification of asym-
metric aldol condensation and intramolecular ring
cyclization.
2. Results and discussion
Keywords: Furofuran lignans; (+)-Eudesmin; (+)-Yangambin;
(À)-Eudesmin; (À)-Yangambin; Stereoselective aldol condensation;
Intramolecular ring cyclization.
The general features of our initial approach to the gen-
eration of the 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane
skeleton of furofuran lignans are summarized in
*
Corresponding author. Tel.: +82 43 261 2283; fax: +82 43 267
2279; e-mail: ospark@cbnu.ac.kr
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.06.127

6434
J.-C. Jung et al. / Tetrahedron Letters 47 (2006) 6433–6437
of the furofuran lignans shown in Scheme 1. A diaste-
O
O
Ar
O
Ar
reoselective double aldol condensation of chelated metal
complex 6 and aldehyde 7, followed by intramolecular
ring cyclization and double-reduction to complete the
synthesis of furofuran lignans 4.
H
H
H
H
Ar
Ar
O
O
O
4
5
The synthesis of (+)-eudesmin (2a) and (+)-yangambin
(3a) was accomplished as shown in Scheme 2. Thus, suc-
cinic acid (8) was treated with thionyl chloride to give
succinyl chloride, which was reacted with freshly
prepared (S)-(À)-4-benzyl-2-oxazolidinone and (S)-(À)-
2,10-camphorsultam as chiral auxiliaries [(S)-(À)-4-benz-
yl-2-oxazolidinone was prepared14 from (S)-phenylalaniol,
diethyl carbonate, and sodium bicarbonate in 80% yield;
(S)-(À)-2,10-camphorsultam was obtained from (+)-
camphor-10-sulfonic acid with thionyl chloride, ammo-
nium hydroxide, and sodium borohydride in a 65%
three steps yield15] in the presence of n-BuLi (1.6 M soln,
in hexane) at 0 ꢁC to afford the required 1,4-bis[4-(S)-
benzyl-2-oxazolidin-3-yl]butane-1,4-dione (9a) and (S)-
(À)-sultam (12) in 85% and 88% yields, respectively.
M
N
O
O
Aux
N
Ar-CHO
+
Aux
7
O
O
M
6
M = B, Li, Ti, Sn
Aux = Chiral auxiliary
Scheme 1. Retrosynthetic analysis of eudesmin and yangambin.
Scheme 1. The chelated metal complex 6 (derived from
9a or 12 with enolating agents) would be served as a
key intermediate according to the retrosynthetic analysis
The stereoselective aldol condensation of aldehydes
(3,4-dimethoxybenzaldehyde, 2,3,4-trimethoxybenzalde-
hyde) and chiral auxiliaries 9a, 12 into the dichloro-
Bn
O
HO
Ar
O
O
O
O
a
b
OH
H
N
O
HO
R
O
N
R
O
H
O
O
Bn
Ar
OH
9a
8
10a Ar=3,4-di-OMe-C6H3, R=(-)-Benzoxazolidinyl
11a Ar=2,3,4-tri-OMe-C6H2, R=(-)-Benzoxazolidinyl
c
d
O
O
N
HO
Ar
O
S
O
O
Ar
O
H
e
d
R
N
H
H
R
S
H
O
Ar
O
O
O
O
O
Ar
OH
12
15a Ar=3,4-di-OMe-C6H3
16a Ar=2,3,4-tri-OMe-C6H2
13a Ar=3,4-di-OMe-C6H3, R=(-)-sltam
14a Ar=2,3,4-tri-OMe-C6H2, R=(-)-sltam
O
O
O
Ar
HO
Ar
f
g
H
H
H
H
Ar
Ar
OH
O
2a (+)-Eudesmin
3a (+)-Yangambin
17a Ar=3,4-di-OMe-C6H3
18a Ar=2,3,4-tri-OMe-C6H2
Scheme 2. Reagents and conditions: (a) SOCl2, CH2Cl2, reflux, 1 h; then (S)-(À)-4-benzyl-2-oxazolidinone, n-BuLi/THF, 0 ꢁC, 2 h, 85%;
(b) Bu2BOTf, DIPEA/CH2Cl2, À78 ꢁC, 3,4-dimethoxybenzaldehyde, 20 min; (c) SOCl2, CH2Cl2, reflux, 1 h; then (S)-(À)-2,10-camphorsultam,
n-BuLi/THF, 0 ꢁC, 2 h, 88%; (d) KH2PO4 (aq), H2O2 (28%)/MeOH, 25 ꢁC, 8 h, 84% for 15a, 81% for 16a; (e) Bu2BOTf, DIPEA/CH2Cl2, À78 ꢁC,
2,3,4-trimethoxybenzaldehyde, 20 min; (f) DIBAL-H/THF, À25 ꢁC, 1 h, 42% for 17a, 45% for 18a; (g) Et3SiH, BF3ÆEt2O/CH2Cl2, À78 ꢁC, 1 h, 55%
for 2a, 58% for 3a.

J.-C. Jung et al. / Tetrahedron Letters 47 (2006) 6433–6437
6435
methane was performed with dibutylboron triflate at
À78 ꢁC. The resultant dihydroxyl compounds 13a and
14a were unstable and we were not able to isolate them
in sufficient purity or yield. Compounds 13a and 14a
were subjected to intramolecular ring cyclization using
KH2PO4 and H2O2 (28%) in MeOH to yield dilactone
15a and 16a in 84% and 81% two step yields, with
18:1, 20:1 diasteromeric selectivity, respectively. Conver-
sion of 15a and 16a was achieved by reduction using DI-
BAL–H in THF, followed dehydroxylation of dilactols
17a and 18a to generate (+)-eudesmin (2a) and (+)-yan-
gambin (3a) (Scheme 2).
original report,13 oxazolidinine 9a was used as an enol-
izable substrate resulting in a 19:1 diastereomeric ratio
of dilactones as (+)-, (À)-sesamin precursors in good
yield. Having pure enolizable substrate 9a and 12 in
hand, we tested several enolating agents16 such as Et2-
BOTf/CH2Cl2, Bu2BOTf/CH2Cl2, n-BuLi, ZnCl2/THF,
LDA/THF, TiCl4/CH2Cl2, TiCl(OiPr)3/CH2Cl2, and
SnOTf/CH2Cl2 under standard conditions.17 The
best results were obtained with Bu2BOTf/CH2Cl2 at
À78 ꢁC to 0 ꢁC for 1 h, regarding diastereoselectivity
and yield (Table 1). Although stereoselectivity was en-
hanced, the reaction failed to go to completion. Aldol
reactions with lithium, titanium, and tin failed to en-
hance the stereoselectivity. Also, the enhancement in
diastereoselectivity was not sufficient to compensate
for the low yields in both aldol condensation and intra-
molecular ring cyclization. We have found that our ori-
ginal reaction conditions using the chelated boron
complex of chiral auxiliaries 9a and 12, followed by
intramolecular ring cyclization with KH2PO4 and
The key step in our synthesis involved the stereoselective
aldol coupling of enolizable substrates 9a and 12 with
aldehydes 7. The primary goal in a reaction of this type
is to obtain maximum diastereoselectivity. Therefore, we
sought to determine the conditions necessary to achieve
this goal by varying the enolizable substrate, and the
reaction conditions for the aldol condensation. In our
Table 1. Stereoselective aldol condensation and intramolecular ring cyclization of 10a and 11a to form dilactones 15a and 16a
Entry
Substrate
Enolization conditions
Temp (ꢁC)
Time (h)
Selectivity; yield (%)
15a
16a
1
2
3
4
5
6
7
Et2BOTf, DIPEA/CH2Cl2
Bu2BOTf, DIPEA/CH2Cl2
n-BuLi, ZnCl2/THF
LDA/THF
TiCl4/CH2Cl2
TiCl(OiPr)3/CH2Cl2
Sn(OTf)2/CH2Cl2
À78 to 0
À78 to 0
À40 to 0
À78
1
1
1
0.5
2
15:1a (48)b
18:1 (61)
20:1 (45)
5:1 (34)
10:1 (38)
10:1 (30)
5:1 (22)
10:1 (33)
9:1 (28)
25:1 (35)
10:1 (37)
9a or 12
c
À78 to 0
À78 to 0
À78 to 0
—
c
2
1
—
c
—
a Stereoselectivity was determined by HPLC analysis.
b Yield was isolated yield.
c No isolation.
Bn
HO
H
Ar
O
O
O
O
R
N
O
a
b
OH
R
O
N
HO
H
O
O
O
O
Ar
OH
Bn
9b
8
10b Ar=3,4-di-OMe-C6H3,R=R-(-)-Benzoxazolidinyl
11b Ar=2,3,4-di-OMe-C6H2,R=R-(-)-Benzoxazolidinyl
c
O
O
O
O
HO
Ar
O
Ar
Ar
e
d
H
H
H
H
H
H
Ar
OH
Ar
O
Ar
O
O
2b (-)-Eudesmin
3b (-)-Yangambin
15b Ar=3,4-di-OMe-C6H3
16b Ar=2,3,4-di-OMe-C6H2
17b Ar=3,4-di-OMe-C6H3
18b Ar=2,3,4-di-OMe-C6H2
Scheme 3. Reagents and conditions: (a) SOCl2, CH2Cl2, reflux, 1 h; then (S)-(À)-4-benzyl-2-oxazolidinone, n-BuLi/THF, 0 ꢁC, 2 h, 86%; (b)
Bu2BOTf, DIPEA/CH2Cl2, À78 ꢁC, 3,4-dimethoxybenzaldehyde or 2,3,4-trimethoxybenzaldehyde, 20 min; (c) KH2PO4 (aq), H2O2 (28%)/MeOH,
25 ꢁC, 8 h, 80% for 15b, 83% for 16b; (d) DIBAL–H/THF, À25 ꢁC, 1 h, 42% for 17b, 45% for 18b; (e) Et3SiH, BF3ÆEt2O/CH2Cl2, À78 ꢁC, 1 h, 51%
for 2b, 50% for 3b.

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J.-C. Jung et al. / Tetrahedron Letters 47 (2006) 6433–6437
9. Nikaido, T.; Ohmoto, T.; Kinoshita, T.; Sankawa, U.;
Nishibe, S.; Hisada, S. Chem. Pharm. Bull. 1981, 29, 3586–
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H2O2 (28%) provided acceptable selectivity and yield for
preparation of the furofuran lignans 4.
10. Plante, G. E.; Prevost, C.; Chainey, A.; Braquet, P.; Sirois,
P. Am. J. Physiol. 1987, 253, R375–R378.
Likewise, treatment of oxazolidinone 9b with aldehydes
(3,4-dimethoxybenzaldehyde, 2,3,4-trimethoxybenzalde-
hyde) in the presence of Bu2BOTf in dry dichlorometh-
ane gave diastereomeric boron complex mixtures, which
were subjected to intramolecular ring cyclization with
KH2PO4 and H2O2 (28%) in MeOH to afford dilactones
15b and 16b in 80% and 83% yields, with 19:1 diastero-
meric selectivity, respectively. Reduction of dilactones
15b and 16b with DIBAL-H in THF gave dilactols 17b
and 18b in good yields, which were treated with Et3SiH
and BF3ÆEt2O in dichloromethane to give (À)-eudesmin
(2b) and (À)-yangambin (3b) (Scheme 3). The properties
of 2b and 3b were identical to the reported spectral and
physical data for the these compounds.12f,l
11. Bernard, C. B.; Arnason, J. T.; Philogene, B. J. R.; Lam,
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metry 2002, 13, 1845–1847; (s) Aldous, D. J.; Dalencon, A.
J.; Steel, P. G. J. Org. Chem. 2003, 68, 9159–9161; (t)
Kasahara, H.; Miyazawa, M.; Kameoka, H. Phytochem-
istry 1997, 44, 1479–1482; (u) Banerjee, B.; Roy, S. C.
Synthesis 2005, 2913–2919.
13. Kim, J. C.; Kim, K. H.; Jung, J. C.; Park, O. S.
Tetrahedron: Asymmetry 2006, 17, 3–6.
14. Gage, J. R.; Evans, D. A. Org. Synth. 1990, 68, 77–82.
15. Jung, J. C.; Kache, R.; Vines, K. K.; Zheng, Y.; Bijoy, P.;
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16. (a) Inoue, T.; Liu, J. F.; Buske, D. C.; Abiko, A. J. Org.
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17. General procedure for the preparation of dilactones 15a and
16a: To a stirred solution of substrates 9a and 12
(2.2 mmol) in a specified solvent (12 mL) was added
dropwise a specified enolizable reagent (4.8 mmol) and
DIPEA (8.8 mmol, for boron aldol reagents) at À78 ꢁC,
and the reaction mixture was stirred at that temperature
for 10 min. A solution of aldehydes (3,4-dimethoxybenz-
aldehyde, 2,3,4-trimethoxybenzaldehyde; 5.5 mmol) in
solvent (THF or CH2Cl2; 2 mL) was added dropwise at
À78 ꢁC, and the resulting mixture was warmed to À40 or
0 ꢁC for an appropriate reaction time (Table 1). The
reaction mixture was quenched by the slow addition of
satd aqueous NH4Cl solution (5 mL). The mixture was
warmed to room temperature, and diluted with dichloro-
methane (10 mL). The organic phase was separated, and
the aqueous layer was extracted with dichloromethane
(8 mL). The combined organic phases were dried over
anhydrous MgSO4, filtered, and concentrated under
reduced pressure to give the aldol products 10a and 11a,
In conclusion, we have developed a method for the effi-
cient synthesis of (+)-, (À)-eudesmin and (+)-, (À)-yan-
gambin. This method is useful to generate the core
skeleton of furofuran rings by modification of asymmet-
ric aldol condensation.
Acknowledgement
This work was supported by KOSEF (Grant No. RO5-
2004-000-10742-0).
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J.-C. Jung et al. / Tetrahedron Letters 47 (2006) 6433–6437
6437
which were treated with KH2PO4 (10 mL), H2O2 (10 mL,
28%), MeOH (15 mL), and the resulting mixture was
vigorously stirred at room temperature for 8 h. The
mixture was extracted with dichloromethane (3 · 30 mL).
The combined organic phase was dried over anhydrous
MgSO4, filtered, and concentrated under reduced pressure
to give dilactones, which were purified by flash column
chromatography (silica gel, 25% ethyl acetate in hexanes)
to afford pure dilactones 15a and 16a. Compound 15a: Mp
203–204 ꢁC (lit.,12h,18 199 ꢁC); IR (neat, NaCl) 3045, 2987,
CDCl3) d 175.0, 149.6, 130.4, 116.7, 111.4, 107.9, 81.8,
56.1, 56.0, 48.4; HRMS calcd for C22H23O8: 415.1393
[M+H]+, found: 415.1398; 16a: mp 170–171 ꢁC; IR (neat,
1
NaCl) 3030, 2975, 1778, 1600, 1520, 1261, 1045 cmÀ1; H
NMR12i (400 MHz, CDCl3) d 6.51 (s, 4H), 5.87 (s, 2H),
3.87 (s, 12H), 3.81 (s, 6H), 3.57 (s, 2H); 13C NMR
(100 MHz, CDCl3) d 174.9, 153.9, 138.3, 133.6, 101.3,
81.6, 60.9, 56.3, 48.5; MS (ESI) (m/z) 475 [M+H]+, 307,
154 (base peak); HRMS calcd for C24H27O10: 475.1604
[M+H]+, found: 475.1610.
1
1780, 1490, 1242, 1045, 1038 cmÀ1; H NMR (400 MHz,
18. Anjaneyulu, A. S. R.; Rao, A. M.; Rao, V. K.; Row, L.
R.; Pelter, A.; Ward, R. S. Tetrahedron 1977, 33, 133–
143.
CDCl3) d 6.86 (s, 4H), 6.79 (s, 2H), 5.89 (s, 2H), 3.90 (s,
6H), 3.87 (s, 6H), 3.57 (s, 2H); 13C NMR (100 MHz,