Total synthesis of taiwaniadducts B, C, and D

Article abstract of DOI:10.1021/ja503972p

The first total syntheses of taiwaniadducts B, C, and D have been accomplished. Two diterpenoid segments were prepared with high enantiopurity, both through Ir-catalyzed asymmetric polyene cyclization. A sterically demanding intermolecular Diels-Alder reaction promoted by Er(fod)₃ assembled the scaffold of taiwaniadducts B and C. A carbonyl-ene cyclization forged the cage motif of taiwaniadduct D at a late stage, providing over 200 mg of this compound.

Full text of DOI:10.1021/ja503972p

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Communication
Total Synthesis of Taiwaniadducts B, C, and D
Jun Deng, Shupeng Zhou, Wenhao Zhang, Jian Li, Ruofan Li, and Ang Li
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja503972p • Publication Date (Web): 27 May 2014
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Journal of the American Chemical Society
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Total Synthesis of Taiwaniadducts B, C, and D
Jun Deng,† Shupeng Zhou,† Wenhao Zhang, Jian Li, Ruofan Li, and Ang Li*
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State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chi-
nese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
Supporting Information Placeholder
of 3 from the Diels–Alder reaction.5,6 Herein, we report the
ABSTRACT: The first total synthesis of taiwaniadducts B, C,
total synthesis of taiwaniadducts B, C, and D based on the
and D have been accomplished. Two diterpenoid segments
above biosynthetic hypothesis.
were prepared with high enantiopurity, both through Ir-
Scheme 1. Retrosynthetic Analysis
catalyzed asymmetric polyene cyclization. A sterically de-
manding intermolecular Diels–Alder reaction promoted by
Er(fod)3 assembled the scaffold of taiwaniadducts B and C. A
carbonyl-ene cyclization forged the cage motif of taiwani-
adduct D at a late stage, providing over 200 mg of this com-
pound.
OH
OH
Me
Me
O
O
Me
Me
Me
Me
carbonyl-ene
Me
Me
H
Me
H
O
O
Me
CO2H
Me
CO2H
6
H
H
H
14'
CHO
Me Me
Me Me
HO
1
2
: taiwaniadduct D
Me
: taiwaniadduct B
Taiwaniaquinoids are a class of terpenoids with impressive
biological activities isolated from the endangered species
Taiwania cryptomerioides,1 which have attracted remarkable
attentions from a synthetic perspective.1,2 A few members of
this family, namely taiwaniadducts A–J,3 possess a character-
istic Diels–Alder cycloadduct scaffold. From a biosynthetic
perspective, taiwaniadduct D (1, Figure 1), the most complex
molecule among them, could be derived from taiwaniadduct
B (2) through a carbonyl-ene cyclization, and 2 may arise
from a intermolecular Diels–Alder reaction between natural-
ly occurring taiwaniaquinone A or F (3 or 4)3 and trans-ozic
acid (5).4 Taiwaniadduct C (6) is presumably the regioisomer
Diels Alder
OMe
O
O
Me
Me
Me
Me
+
H
CHO
Me Me
Me CO2Me
7
4
: taiwaniaquinone F
12
OMe Me
Me
MeO
Me
O
Me
Me
4
OMe
H
H
10
8
Ir-catalyzed asymmetric
polyene cyclization
OMe Me
OH
Me
MeO
Me
Me
OH
Me
OMe
OH
11
9
I
OMe Me
OTBS
Me
MeO
Me
Me
+
+
OMe
R2B
R2B
OTBS
12
13
14
We first undertook a retrosynthetic analysis of taiwani-
adduct D (1), as illustrated in Scheme 1. The initial discon-
nection takes place at the C6−C14′ bond to provide
Figure 1. Taiwaniadducts B, C, and D, taiwaniaquinones A
and F, and trans-ozic acid.
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Scheme 2. Synthesis of the Dienophile and Diene through Ir-Catalyzed Asymmetric Polyene Cyclization
1
2
3
4
5
6
7
8
OMe Me
OMe Me
Me
OMe
OMe Me
Me
OMe
e) K2OsO2(OH)4
NaIO4, 2,6-lutidine
f) t-BuOK, MeI
d) [Ir(cod)]2 (4%)
a) 9-BBN
b) aq. NaOH,
Pd(dppf)Cl2 (10%)
OMe Me
Me
OMe
MeO
Me
MeO
Me
MeO
Me
16
12
,
(R)- (16%),
Me
OMe
MeO
Zn(OTf)2 (20%)
4
c) TBAF
69%, >99%
ee
89% (2 steps)
H
HO
H
73% (3 steps)
Me
CHO
17
15
9
8
O
g) N2H4, 160 oC;
OTBS
P
O
N
I
72%
KOH, 180 o
C
9
Me
12
16
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
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40
41
42
43
44
45
46
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51
52
53
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57
58
59
60
OMe Me
Me
OMe
OMe
OMe
O
OMe
O
O
MeO
Me
Me
Me
Me
Me
Me
Me
MeO
Me
h) K2CO3
i) CAN
5 steps
ref. 2h
Me
Me
or
7
7
O
or i) CAN
OMe
H
H
H
CHO
CHO
20
CHO
H
Me Me
4
Me Me
Me Me
Me Me
19
18
: taiwaniaquinone F
82%
76% (2 steps)
n) [Ir(cod)]2 (4%),
OH
Me
16
(R)- (16%),
j) TBSCl, Im
k) Sia2BH
Me
H
Me
Me
O
O
OH
9
p) O3
96%
Zn(OTf)2 (20%)
Me
Me
o) BF3 OEt2
12
,
l) aq. NaOH,
Pd(dppf)Cl2 (10%)
m) TBAF
Me
H
HO
O
79% (2 steps)
>99% ee, 10:1 dr
21
11
10
22
56% (4 steps)
q) t-BuOK,
BOMCl;
aq. HCl
70%
Me
OMe
N
O
12
r) N2H4, 160 oC;
t) MeNHOMe HCl
i-PrMgCl, 78 to 0 o
s) RuCl3 3H2O,
NaIO4, pH 7 buffer
O
OH
KOH, 180 o
C
C
Me
Me
Me
H
Me
O
O
O
Me
Me
Me
Me
4
79%
81%
Me
67%
H
H
H
Me
HO
Me
BzO
Me
OHC
BnO
BnO
Ph3P
28
27
26
25
23
CHO
u) SOCl2, py, 78 o
v) DIBAL-H, 78 o
C
58%
(2 steps)
Me
Me
Me
C
y) AZADO (5%),
NaClO2, pH 4.6 buffer
z) TMSCHN2,
MeOH
w) , 120 oC
x) Ph3P=CH2, 0 o
Me
H
CHO
28
O
Me
Me
C
Me
54% (2 steps)
67% (2
steps)
BOMO
H
H
H
Me
HO
Me
HO
Me CO2Me
30
29
7
24
26
ORTEP of
taiwaniadduct B (2) as a carbonyl-ene substrate, which could
be further disassembled through a retro Diels–Alder fashion
to give taiwaniaquinone F (4) and trans-ozic acid methyl
ester (7). It should be noted that the both fragments need to
be prepared in the enantioenriched forms, to avoid the dia-
stereomerically mismatched Diels–Alder reactions. Having
noticed the inherent similarity between 3 and 7, we consider
the Ir-catalyzed asymmetric polyene cyclization7 elegantly
developed by Carreira and co-workers as a general tool to
construct the carbon skeleton of the both compounds. Based
on the ring-contraction strategy reported by us previously,2h
2 could be directly simplified to an optically active 6,6,6-
tricycle 8, which is further traced back to the substrate for
the polyene cyclization, allylic alcohol 9. In parallel, the syn-
thesis of 7 may exploit an enantioenriched 6,6,5-tricycle 10;
modifications at C4 and C12 would install the carboxylate
and diene functionalities, respectively. This intermediate is
anticipated to arise from dienyl alcohol 11, through a mecha-
nistically similar Ir-mediated cyclization, despite few prece-
dents of heteroatom trapping.7,8 The linear precursors 9 and
11 could be prepared by cross-coupling of known iodide 127a
with alkylboranes 13 and 14, respectively.
The synthesis commenced with the enantioselective con-
structions of the two segments 4 and 7 (Scheme 2). Hydrobo-
ration of styrene 15 with 9-BBN gave a 13-type alkylborane,
which underwent Suzuki–Miyaura coupling [cat. Pd(dppf)Cl2,
aq. NaOH]2h,7a with 12 and subsequent desilylation to afford
alcohol 9 in 73% overall yield, setting the stage for the Ir-
catalyzed asymmetric polyene cyclization. Thus, 9 was sub-
jected to Carreira′s catalyst system {[Ir(cod)Cl]2, (R)-16,
Zn(OTf)2}7,8 to yield the desired tricycle 8 (69%) with an ex-
cellent level of stereoselectivity (> 99% ee, a single detectable
diastereomer). Notably, this reaction was successfully carried
out on 15 gram scale with the consistent efficiency and selec-
tivity. Olefin cleavage (OsO4, NaIO4) followed by C-
methylation (KOt-Bu, MeI) of the resulting aldehyde fur-
nished compound 17 in 89% yield as a single diastereomer at
C4, which was converted to tricycle 18 (72% yield) under
Wolff–Kishner–Huang conditions (N2H4•H2O, KOH, 180 °C).9
This compound entered a 5-step sequence involving Wolff
ring contraction developed by us previously to reach 6,5,6-
trycylic aldehyde 19; treatment with K2CO3/EtOH provided
its epimer at C7.2h The both aldehydes were oxidized by CAN
to generate essentially enantiopure taiwaniaquinone F (4)
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Journal of the American Chemical Society
and its C7-epimer 20, respectively, which served as the
dienophiles for the devised Diels–Alder reaction.
ly, a variety of conventional conditions, such as thermal,
acidic, neat, and high-pressure conditions,15 failed to effect
the cycloaddition. The instability of both taiwaniaquinone F
(4) and its alternative 7-epi-taiwaniaquinone F (20) under
forcing conditions was found to be a severe problem. The
undesired reaction of 20 was depicted in Scheme 3; exposure
to Sc(OTf)3 gave a naturally occurring taiwaniaquinoid,
namely taiwaniaquinol D (31),1,2i,3 presumably through a dou-
ble tautomerization process with the intermediacy of 32. 7-
epi-Taiwaniaquinone A (33) underwent a similar sequence
followed by spontaneous aerobic oxidation to arrive at an-
other natural product taiwaniaquinone D (34).1,2i,3 Distinct
from its 7-epimer, taiwaniaquinone F (4) underwent a much
slower but more complicated decomposition. These observa-
tions suggest a plausible biosynthetic model of 31 and 32, and
imply the fate of 7-epi-taiwaniaquinones A and F which have
not been isolated as natural products.
1
2
3
4
5
6
7
8
The synthesis of the diene fragment 7 also took advantage
of the Ir chemistry (Scheme 2). Silylation of known dienyl
alcohol 2110 followed by selective hydroboration of the mono-
substituted C═C bond with Sia2BH afforded a 14-type alkyl-
borane,7b which was subjected to a similar Suzuki–Miyaura
coupling/desilylation sequence to furnish diol 11. This com-
pound turned out to be a suitable substrate for the expected
heteroatom-terminating Ir-catalyzed cyclization; 6,6,5-
tricycle 10 was obtained in 59% yield and > 99% ee under the
standard conditions. Notably, a high level of diastereoselec-
tivity at C9 (ca. 10:1) was also achieved, making this chemis-
try applicable to the synthesis of the framework of a wide
range of terpenoids. Mono-cyclization products (an olefin
mixture)7a were isolated in 25% yield, which were readily
converted to 10 (80% yield) by exposure to BF3•OEt2. Thus,
the overall yield of 10 from 11 reached 79%. These transfor-
mations were easily amplified to 5 gram scale. With 10 in
hand, we introduced the carboxylate and diene functionali-
ties to its scaffold. Similar to the sequence used for the
dienophile synthesis, double bond cleavage by ozonolysis
gave aldehyde 22 (96% yield), and subsequent alkylation
with BOMCl generated compound 23 in 70% yield as a single
diastereomer at C4. The by-product 24 resulted from O-
alkylation was hydrolyzed during acid work up, leading to 26%
of recovered 22. Aldehyde 23 underwent Wolff–Kishner–
Huang reduction to afford compound 25 (67% yield). Treat-
ment of 25 with RuCl3/NaIO4 resulted in the C12−H oxida-
tion,11 providing lactone 26 in 81% yield, the structure of
which was confirmed by X-ray crystallographic analysis
(Scheme 2). Under these conditions, the benzyl was also oxi-
dized to a benzoyl for convenient removal. Lactone opening
(MeNHOMe•HCl, i-PrMgCl)12 formed amide 27 (79% yield)
with the primary hydroxyl released. Dehydration (SOCl2,
py)12,13 and DIBAL-H reduction, followed by two Wittig ole-
finations (with reagent 28 and methylenetriphenylphospho-
rane, respectively) gave 1,3-diene 29 with the intermediacy of
aldehyde 30. Finally, oxidation (AZADO, NaClO2)14 followed
by esterification (TMSCHN2, MeOH) rendered trans-ozic
acid methyl ester (7) in an essentially enantiopure form.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
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40
41
42
43
44
45
46
47
48
49
50
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53
54
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57
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59
60
Scheme 4. Intermolecular Diels–Alder Reaction and
Completion of the Total Synthesis of Taiwaniadducts
B, C, and D
Scheme 3. Unexpected Reactions of 7-epi-
Taiwaniaquinones A and F under Acidic Conditions
At this point, we directed our attention to the dienophiles
more stable against acidic and thermal conditions (Scheme
4). Alcohol 35, which is readily available from aldehyde 19
and secured from the tautomerization observed before, was
With the both diene and dienophile in hand, we investi-
gated the intermolecular Diels–Alder reaction. Unfortunate-
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(1) A review of taiwaniaquinoids: Majetich, G.; Shimkus, J. M. J. Nat.
Prod. 2010, 73, 284.
considered as such an alternative. This compound was pre-
1
2
3
4
5
6
7
8
pared on 4 gram scale and used for extensive examinations of
Diels−Alder conditions. To our delight, Er(fod)3 turned out
to be a effective promoter,16 despite few precedents of utiliz-
ing it in Diels−Alder reactions to our knowledge. Neat condi-
tions and elevated temperature were also required. 35 (1.0
eq.) and 7 (1.2 eq) reacted under these optimized conditions
to afford cycloadduct 36 (52% yield) and its regioisomer 37
(21% yield), and no other positional or diastereomeric iso-
mers were detected. The site selectivity toward the C8-olefin
over the C12-olefin may be attributable to the bulky isopropyl
and the electron-donating methoxyl that make the latter
olefin a worse dienophile. The facial selectivity may arise
from the steric effect of the axial C20 methyl group (see the
single crystal structure in ref. 2h for information). The homo-
dimeric cycloaddition of 7 was not observed either, presuma-
bly due to its poor dienophilicity.17 The both cycloadducts
were subjected to a three-step sequence of oxidation (DMP)
and demethylations (LiI and then t-BuOK/DMSO),18,19 to
furnish taiwaniadducts B and C (2 and 6) via the intermedia-
cy of 38 and 39, respectively. Treatment of 2 with Et2AlCl
realized the final carbonyl-ene cyclization to render taiwani-
adduct D (1) in 91% yield; over 200 mg of 1 were prepared.
The structure of the bis-methylated derivative of 1 (com-
pound 40) was verified by X-ray crystallographic analysis
(Scheme 4). The synthetic samples display identical spectral
and physical properties with those of authentic samples
(supporting information).
(2) For the syntheses of taiwaniaquinoids not included in Ref. 1: (a)
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Scopelliti, R.; Gademann, K. Synthesis 2010, 2223. (c) Jana, C. K.;
Scopelliti, R.; Gademann, K. Chem. Eur. J. 2010, 16, 7692. (d) Alvarez-
Manzaneda, E.; Chahboun, R.; Alvarez, E.; Tapia, R.; Alvarez-
Manzaneda, R. Chem. Commun. 2010, 9244. (e) Liao, X.; Stanley, L.
M.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 2088. (f) Tapia, R.;
Guardia, J. J.; Alvarez, E.; Haidour, A.; Ramos, J. M.; Alvarez-
Manzaneda, R.; Chahboun, R.; Alvarez-Manzaneda, E. J. Org. Chem.
2012, 77, 573. (g) Thommen, C.; Jana, C. K.; Neuburger, M.; Gade-
mann, K. Org. Lett. 2013, 15, 1390. (h) Deng, J.; Li, R. F.; Luo, Y. J.; Li,
J.; Zhou, S. P.; Li, Y. J.; Hu, J. Y.; Li, A. Org. Lett. 2013, 15, 2022. (i)
Ozeki, M.; Satake, M.; Toizume, T.; Fukutome, S.; Arimitsu, K.; Ho-
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9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(3) (a) Lin, W. H.; Fang, J. M.; Cheng, Y. S. Phytochemistry 1995, 40,
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In summary, we have accomplished the first total synthesis
of taiwaniadducts B, C, and D (2, 6, and 1). Ir-catalyzed
asymmetric polyene cyclization was exploited to construct
the scaffolds of the both diene and dienenophile. Er(fod)3
promoted intermolecular Diels−Alder and Et2AlCl mediated
carbonyl-ene reactions forged the core of 1. The chemistry
may find further applications in terpenoid synthesis.
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures and compound characterization
(cif, pdf). This material is available free of charge via the In-
ternet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
ali@sioc.ac.cn
Author Contributions
†These authors contributed equally.
ACKNOWLEDGMENT
This paper is dedicated to Prof. Li-Xin Dai on the occasion of
his 90th birthday. We thank Profs. Yunheng Shen and Lihong
Hu for helpful discussions. Financial support was provided
by Ministry of Science & Technology (2013CB836900) and
National Natural Science Foundation of China (21290180,
21172235, and 21222202).
REFERENCES
(19) Chang, F. C.; Wood, N. F. Tetrahdron Lett. 1964, 40, 2969.
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