Total synthesis of fomitellic acid B

Article abstract of DOI:10.1016/j.tetlet.2009.09.088

Herein, we describe the first total synthesis of fomitellic acid B (2), a potent inhibitor of DNA polymerases α and β. There are two key features in this synthesis: (i) the AB ring system, with all requisite chiral centers, is stereoselectively constructe

Full text of DOI:10.1016/j.tetlet.2009.09.088

Tetrahedron Letters 50 (2009) 6764–6768
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Tetrahedron Letters
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Total synthesis of fomitellic acid B
*
Makoto Yamaoka, Atsuo Nakazaki, Susumu Kobayashi
Faculty of Pharmaceutical Sciences, Tokyo University of Science (RIKADAI), 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein, we describe the first total synthesis of fomitellic acid B (2), a potent inhibitor of DNA polymerases
Received 2 September 2009
Revised 16 September 2009
Accepted 17 September 2009
Available online 22 September 2009
a
and b. There are two key features in this synthesis: (i) the AB ring system, with all requisite chiral
centers, is stereoselectively constructed by means of titanium(III)-mediated radical cascade cyclization
of epoxypolyene; and (ii) an enone moiety in the B-ring is generated by isomerization of an olefin
followed by allylic oxidation.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Fomitellic acid B
Titanium(III)-mediated radical cascade
cyclization
Total synthesis
Triterpenoid
Fomitellic acids, originally isolated by Sakaguchi and co-work-
can be stereoselectively prepared from the known dione 95 derived
from the (ꢀ)-Wieland–Miescher ketone.
ers1 from the mycelium of a basidiomycete, Fomitella fraxinea, are
potent inhibitors of calf DNA polymerase
a
, rat DNA polymerase
Therefore, our initial task was the preparation of vinyl iodide 86
as shown in Scheme 2. The synthesis commenced with the known
dione 95 which was available in two steps from the (ꢀ)-Wieland–
Miescher ketone. Selective ketalization of the carbonyl function of
the six-membered ring in dione 9, followed by the Wittig reaction
afforded the olefin 145c,e in 46% yield (80% br sm) for the two steps.
After hydroboration–oxidation of 14, the resultant alcohol was
subjected to a Dess–Martin periodinane7 oxidation to give the cor-
responding ketone as a diastereomeric mixture. Epimerization pro-
ceeded successfully by treatment of the mixture with a base to give
the desired ketone 15 as a single diastereomer in 88% overall yield
from 14.5c The ketone 15 was converted to the olefin by the Wittig
reaction. Stereoselective hydroboration of the olefin with 9–BBN,8
b, and human DNA topoisomerases I and II (Fig. 1).2 Structurally,
fomitellic acids belong to the lanostane-type triterpenoids, which
are characterized by a highly oxygenated steroidal AB ring moiety
with five contiguous stereogenic centers. Among several efficient
methodologies for constructing a trans-decaline ring system, we
were particularly interested in a cascade cyclization of epoxypoly-
ene, considering the presence of the hydroxy group at C3. Recently,
we have achieved the stereoselective synthesis of the fully func-
tionalized AB ring moiety (5) as a model study using a tita-
nium(III)-mediated radical cascade cyclization of epoxypolyene.3
In this Letter, we describe the first asymmetric total synthesis of
fomitellic acid B (2).
Our retrosynthetic analysis is shown in Scheme 1. Fomitellic
acid B (2) can be derived from the key tetracyclic intermediate 6
containing all requisite stereogenic centers. As a key step, we rea-
soned that the tetracyclic skeleton could be stereoselectively con-
structed by means of titanium(III)-mediated radical cascade
cyclization4 of epoxypolyene 7. The cyclization precursor 7 was di-
vided into two fragments, the vinyl iodide part 8 and the aldehyde
part 10. We planned to couple these two parts by 1,2-addition of
the vinyl lithium species derived from 8 to the aldehyde 10. We
have already reported the enantioselective synthesis of the alde-
hyde 10 using stereoselective vinylogous Mukaiyama aldol reac-
tion and Sharpless asymmetric epoxidation.3 The vinyl iodide 8
R3
11
OH
A
HO
1
D
C
R1
R2
A
B
B
3
HO
HO2C
O
O
7
H
H
R1
R2
OH OH
OH
OH OMe
R3
HO2C
Fomitellic Acid A (1) :
B (2) :
H
H
H
AB Ring Moiety (5)
H
C (3) :
D (4) : OH
H
H
* Corresponding author. Fax: +81 4 7121 3671.
E-mail address: kobayash@rs.noda.tus.ac.jp (S. Kobayashi).
Figure 1. Structures of fomitellic acids.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.088

M. Yamaoka et al. / Tetrahedron Letters 50 (2009) 6764–6768
6765
OTBS
OTBS
TBSO
TBSO
HO
HO
OAc
HO
HO2C
O
H
O
H
OBn
O
OBn
6
Fomitellic Acid B
7
O
OTBS
O
O
I
(−)-WMK
8
9
+
TBSO
TBSO
OTBDPS
+
CHO
CHO
N
O
OTBDPS
O
TBSO
O
BnO
OH
10
11
12
13
Scheme 1. Synthetic strategy for fomitellic acid B (2).
1) 1,2-ethanediol
.
1) BH3 THF, THF, 0 oC to rt
O
p-TsOH, CH(OMe)3
toluene, 70 oC
O
2) NaOH, H2O2, THF, 0 oC to rt
ref. 5
2) Ph3P+EtBr−, NaH, NaI
3) DMP, CH2Cl2, rt
4) EtONa, EtOH, rt
88% (4 steps)
O
O
O
DMSO, 60 o
46% (80% brsm, 2 steps)
C
O
(−)-WMK
9
14
O
1) Ph3P+MeBr−, n-BuLi, THF, rt
2) 9-BBN, THF, 0 oC
CO2Et
1) TEMPO, KBr, NaOCl
NaHCO3 aq., CH2Cl2, rt
OH
3) NaOH, H2O2, THF, 0 oC to rt
CO2Et
3) H2, Pd/C, EtOH, rt
Ph3P
, CH Cl , rt
2)
2
2
O
O
O
O
O
O
15
16
17
86% (3 steps)
1) TBSCl, imidazole
CH2Cl2, 0 oC to rt
OH
OTBS
1) LAH, THF, 0 oC
2) H2NNH2, n-BuOH, 100 oC
2) p-TsOH
3) I2, Et3N, Et2O, 0 o
C
acetone/H2O, reflux
O
I
18
8
85% (from 16)
90% (3 steps)
Scheme 2. Preparation of vinyl iodide 8.
followed by the oxidative workup provided the desired alcohol 16
as a single diastereomer in 86% yield for the three steps. Oxidation
of the primary alcohol with TEMPO, two-carbon elongation by the
Wittig reaction, and hydrogenation of the resulting unsaturated
olefin afforded ethyl ester 17. Reduction of the ethyl ester with
LiAlH4, followed by an acid-catalyzed hydrolysis of the ketal gave
ketoalcohol 18 in 85% overall yield from 16. After protection of
the hydroxyl group in 18 as the TBS ether, the ketone was trans-
formed into the vinyl iodide 8 according to Barton’s procedure9
in 90% yield for the three steps.
Next, the coupling of vinyl iodide 8 and epoxyaldehyde 10 was
examined (Scheme 3). Iodine–lithium exchange of vinyl iodide 8
with t-BuLi in Et2O at ꢀ78 °C, followed by the addition of epoxyalde-
hyde 10, gave a 1:1 diastereomeric mixture of alcohol 19 in 58%
yield. Subsequent acetylation of the secondary alcohol provided
epoxypolyenes 7a and 7b in 48% and 51% yield, respectively (separa-
ble by silica gel column chromatography). Stereochemistry at the C7
position was determined using modified Mosher’s method.10,11
With epoxypolyene in hand, we then examined a titanium(III)-
mediated radical cascade cyclization (Table 1). Thus, epoxypolyene
7a was treated with 3 equiv of Cp2TiCl,12 prepared from Cp2TiCl2
and Zn, in THF at 60 °C to afford the desired tetracyclic product 6
as a single diastereomer in 30% yield, along with the monocycliza-
tion product 20a13 in 33% yield (entry 1). However, a remarkable
increase in yield was observed by carrying out the reaction at
100 °C in toluene–THF (2:1) (entry 3). By contrast, epimer 7b did
not undergo cascade cyclization to give the monocyclization prod-
uct 20b (entry 4). Nevertheless, in the model study, both epimers
underwent a cascade cyclization.3 These differences will be dis-
cussed later by considering the corresponding transition states.
Therefore, 7b was converted into 7a in 60% yield for the four steps
(i.e., deacetylation, oxidation, Luche reduction, and acetylation).11

6766
M. Yamaoka et al. / Tetrahedron Letters 50 (2009) 6764–6768
OTBS
TBSO
O
TBSO
t-BuLi, 8
Ac2O, Et3N, DMAP
CHO
7
Et2O
CH2Cl2
rt
OH
−78 o
C
O
OBn
OBn
19
58%
10
(diastereomeric mixture)
OTBS
OTBS
TBSO
TBSO
+
7
7
OAc
OAc
O
O
OBn
OBn
7a (48%)
7b (51%)
Scheme 3. Preparation of epoxypolyenes 7a and 7b.
Table 1
Titanium(III)-mediated radical cascade cyclization
OTBS
OTBS
OTBS
TBSO
TBSO
TBSO
Cp2TiCl (3.0 equiv)
+
H
*
*
HO
HO
OAc
20a
OAc
H
H
O
7a (7S)
7b (7R)
6
OBn
OBn
OBn
20b
Entry
Substrate
Solvent
THF
Toluene–THF = 2:1
Toluene–THF = 2:1
THF
Temperature (°C)
Yield (%)
6
20a or 20b
1
2
3
4
7a
7a
7a
7b
60
60
100
60
30
33
26
3
43
58
ND
34
OTBS
OTBS
TBSO
TBSO
BzCl, DMAP
H
H
CH2Cl2, rt
85%
BzO
HO
H
H
OBn
OBn
6
21
Me
H
OTBS
H
Me
13
Me
TBSO
1
9
14
10
BzO
3
Me
5
7
H
H
H
H
OBn
Figure 2. Key NOEs observed in 21.

M. Yamaoka et al. / Tetrahedron Letters 50 (2009) 6764–6768
6767
7a
Me
Me
Me
Me
Me
TBSO
Me
TBSO
O
OAc
Me
O
Me
OAc
O
ClCp2Ti
O
ClCp2Ti
Bn
H
Bn
H
TS1
9β-H epimer
TS2
7b
model system
Me
Me
Me
Me
Me
TBSO
TBSO
O
OAc
O
H
O
Me
ClCp2Ti
Bn
O
ClCp2Ti
Bn
H
OAc
TS3
TS4
9α-H epimer
Figure 3. Possible transition states for the formation of the B-ring.
OTBS
OH
OH
TBSO
OH
OH
H
HCl
+
H
THF
rt
BzO
BzO
BzO
H
H
H
OBn
OBn
OBn
21
22 (57%)
23 (33%)
OAc
1) Ac2O, Et3N, DMAP, CH2Cl2, rt
2) H2, Pd/C, THF, rt
OAc
NHPI, NaClO2
OAc
H
OAc
3) DMP, CH2Cl2, rt
22
CH3CN/AcOEt/H2O
50 oC
4) NaClO2, NaH2PO4
BzO
HO2C
O
BzO
2-methyl-2-butene, t-BuOH/H2O, rt
H
HO2C
25
24
84% (4 steps)
40%
1) AcCl, MeOH, rt
OAc
H
OH
2) TEMPO, BAIB, CH2Cl2, rt
NaOH aq.
3) Ph3P+CHMe2I−, n-BuLi
THF, 0 oC to rt
MeOH/CH2Cl2
rt
BzO
HO2C
HO
O
O
H
HO2C
50% (3 steps)
26
94%
Fomitellic acid B
Scheme 4. Synthesis of fomitellic acid B (2).
NOE experiments of the corresponding benzoate 21 revealed that
the cyclization product 6 had the desired stereochemistry at all req-
uisite stereogenic centers (Fig. 2). Moreover, cyclization using 7a re-
sulted in the formation of the 9b-H epimer through a trans-fused
chair/boat-like transitionstateTS1(Fig. 3). Thisresultsuggestedthat
steric repulsion between C10 and C13 methyl groups preclude a
trans-fused chair/chair-like transition state TS2. By contrast, the
from 9b-H and the presence of the methyl groups at C13 and
C14. Therefore, we had to develop a different approach for the con-
struction of the enone moiety in the B-ring. To this end, the first to-
tal synthesis of fomitellic acid
B (2) was accomplished by
isomerization of the olefin and allylic oxidation as shown in
Scheme 4. Deprotection of the two TBS groups and isomerization
of the olefin were performed in one step by treatment of the ben-
zoate ester 21 with concentrated HCl to give the diol 22 in 57%
yield, along with its regioisomer 23 in 33% yield. After acetylation
of both the hydroxy groups in diol 22, the benzyl group was depro-
tected by hydrogenolysis, and the resultant primary alcohol was
oxidized to carboxylic acid 24 by a conventional two-step proce-
dure in 84% overall yield from 22. Allylic oxidation at the C7 posi-
tion was found to occur, following the treatment of 24 with NaClO2
and N-hydroxyphthalimide (NHPI) at 50 °C to give enone 25, albeit
in low yield.14 After selective cleavage of the primary acetyl group
model system involves stereoselective production of the 9a-H epi-
mer via a thermodynamically more favorable trans-fused chair/
chair-like transition state TS4. Furthermore, the fact that the cycliza-
tion of 7b terminated at the monocyclization stage can also be
understoodbyconsideringa stericrepulsionbetweentheC7acetoxy
group and the C14 methyl group (Fig. 3, TS3).
We were thus able to obtain the tetracyclic compound 6 with
C7–C8 double bond. However, the same approach for the synthesis
of 5 was unsuccessful due to the conformational change arising

6768
M. Yamaoka et al. / Tetrahedron Letters 50 (2009) 6764–6768
by the treatment with AcCl in MeOH,15 the resultant alcohol was
oxidized with TEMPO to give the aldehyde in 80% yield for the
two steps. The aldehyde was then subjected to the Wittig reaction
with n-BuLi and isopropyltriphenylphosphonium iodide in THF to
afford 26 in 63% yield. Finally, hydrolysis of the acyl-protecting
groups in 26 with NaOH completed the synthesis of fomitellic acid
B (2). The spectroscopic data (1H, 13C NMR and IR spectra, HRMS,
and optical rotation) for the synthetic material were identical with
those of natural fomitellic acid B.16
In summary, the first asymmetric total synthesis of fomitellic
acid B (2) was accomplished. The essential features of the present
synthesis are twofold; (i) stereoselective construction of the tetra-
cyclic skeleton by a titanium(III)-mediated radical cascade cycliza-
tion of epoxypolyene, and (ii) generation of the enone moiety in
the B-ring via isomerization of the olefin followed by allylic oxida-
tion. The synthesis of other fomitellic acids and the related struc-
ture–activity relationship study are currently underway.
5. (a) Reusch, W.; Grimm, K.; Karoglan, J. E.; Martin, J.; Subrahamanian, K. P.;
Toong, Y.-C.; Venkataramani, P. S.; Yordy, J. D.; Zoutendam, P. J. Am. Chem. Soc.
1977, 99, 1953–1958; (b) Reusch, W.; Grimm, K.; Karoglan, J. E.; Martin, J.;
Subrahamanian, K. P.; Venkataramani, P. S.; Yordy, J. D. J. Am. Chem. Soc. 1977,
99, 1958–1964; (c) Gibson, J. R.; Reusch, W. Tetrahedron 1983, 39, 55–59; (d)
Wang, W.-Y.; Reusch, W. Tetrahedron 1988, 44, 1007–1014; (e) Lemoine, S.;
Adam, P.; Albrecht, P.; Connan, J. Tetrahedron Lett. 1996, 37, 2837–2840.
6. Corey et al. reported the preparation of
a similar vinyl iodide using a
diastereoselective Simmons–Smith methylenation for the construction of the
quaternary methyl group at C14 in the total synthesis of 24,25-
dihydrolanosterol.: Corey, E. J.; Lee, J.; Liu, D. R. Tetrahedron Lett. 1994, 35,
9149–9152. Initial attempts using this approach for the preparation of 8 were
unsuccessful due to the predominant formation of the cis-fused isomer.11
7. Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155–4156.
8. Midland, M. M.; Kwon, Y. C. J. Am. Chem. Soc. 1983, 105, 3725–3727.
9. Barton, D. H. R.; Bashiardes, G.; Fourrey, J.-L. Tetrahedron 1988, 44, 147–162.
10. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113,
4092–4096.
11. See the Supplementary data.
12. For pioneer works on Cp2TiCl-mediated epoxide openings, see: (a) Nugent, W.
A.; RajanBabu, T. V. J. Am. Chem. Soc. 1988, 110, 8561–8562; (b) RajanBabu, T.
V.; Nugent, W. A. J. Am. Chem. Soc. 1994, 116, 986–997; For reviews, see: (c)
Gansäuer, A.; Bluhm, H. Chem. Rev. 2000, 100, 2771–2788; (d) Gansäuer, A.;
Narayan, S. Adv. Synth. Catal. 2002, 344, 465–475; (e) Gansäuer, A.; Lauterbach,
T.; Narayan, S. Angew. Chem., Int. Ed. 2003, 42, 5556–5573.
Acknowledgments
13. Cuerva et al. reported that the yield of monocyclization product, derived from
the premature trapping of intermediate radical species, increased by using
high concentrations of Cp2TiCl.17f Therefore, in order to prevent the
This research was supported in part by a Grant-in-Aid for Scien-
tific Research (B) (KAKENHI No. 18390010) from the Japan Society
for the Promotion of Science. We thank Professor Kengo Sakaguchi
and Professor Fumio Sugawara (Department of Applied Biological
Science, Tokyo University of Science) for kindly providing the spec-
tra (1H, 13C NMR, IR, and MS) of natural fomitellic acid B.
undesirable formation of monocyclization product 20a, employment of
a
catalytic amount of Cp2TiCl was also examined.17 Nonetheless, the best result
was obtained by using 3 equiv of Cp2TiCl.
14. Silvestre, S. M.; Salvador, J. A. R. Tetrahedron 2007, 63, 2439–2445.
15. Yeom, C.-E.; Lee, S. Y.; Kim, Y. J.; Kim, B. M. Synlett 2005, 1527–1530.
16. Data for the synthetic fomitellic acid B (2): Rf 0.37 (AcOEt/MeOH/H2O = 10/1/
0.5); ½a 2D3
ꢁ
+8.3 (c 0.18, MeOH); 1H NMR (400 MHz, CD3OD) d 5.13–5.05 (m, 1H),
4.04 (dd, J = 12.5, 4.4 Hz, 1H), 3.81 (dd, J = 11.2, 4.6 Hz, 1H), 2.91 (ddd, J = 21.7,
9.0, 9.0 Hz, 1H), 2.73 (dd, J = 16.6, 14.4 Hz, 1H), 2.57 (dd, J = 21.7, 6.3 Hz, 1H),
2.23 (dd, J = 14.4, 3.2 Hz, 1H), 2.10–1.76 (m, 8H), 1.75–1.60 (m, 1H), 1.66 (s,
3H), 1.60 (s, 3H), 1.52–1.23 (m, 5H), 1.17 (s, 3H), 1.17 (s, 3H), 1.12–1.00 (m,
1H), 0.95 (d, J = 6.1 Hz, 3H), 0.92 (s, 3H), 0.69 (s, 3H); 13C NMR (100 MHz,
CD3OD) d 200.6, 179.6, 169.3, 140.0, 131.8, 126.1, 73.1, 72.7, 54.3, 50.2, 49.2,
46.3, 45.7, 45.2, 38.6, 38.5, 37.4, 37.4, 33.5, 31.5, 29.8, 27.4, 25.9, 25.8, 25.5,
19.2, 17.7, 16.2, 14.4, 10.7; IR (KBr, cmꢀ1) 3400, 2927, 1710, 1682, 1645, 1375,
1269, 1036, 1014; HRMS (EI) calcd for C30H46O5: 486.3345, found: 486.3342.
17. For Cp2TiCl-catalyzed epoxide openings, see: (a) Gansäuer, A.; Bluhm, H. Chem.
Commun. 1998, 2143–2144; (b) Gansäuer, A.; Pierobon, M.; Bluhm, H. Angew.
Chem., Int. Ed. 1998, 37, 101–103; (c) Gansäuer, A.; Bluhm, H.; Pierobon, M. J.
Am. Chem. Soc. 1998, 120, 12849–12859; (d) Barrero, A. F.; Rosales, A.; Cuerva, J.
M.; Oltra, J. E. Org. Lett. 2003, 5, 1935–1938; (e) Fuse, S.; Hanochi, M.; Doi, T.;
Takahashi, T. Tetrahedron Lett. 2004, 45, 1961–1963; (f) Justicia, J.; Rosales, A.;
Buñuel, E.; Oller-López, J. L.; Valdivia, M.; Ha, A.; Oltra, J. E.; Barrero, A. F.;
Cárdenas, D. J.; Cuerva, J. M. Chem. Eur. J. 2004, 10, 1778–1788; (g) Justicia, J.;
Oltra, J. E.; Cuerva, J. M. J. Org. Chem. 2004, 69, 5803–5806; (h) Justicia, J.; Oller-
López, J. L.; Campaña, A. G.; Oltra, J. E.; Cuerva, J. M.; Buñuel, E.; Cárdenas, D. J. J.
Am. Chem. Soc. 2005, 127, 14911–14921; (i) Justicia, J.; Oltra, J. E.; Cuerva, J. M.
J. Org. Chem. 2005, 70, 8265–8272; (j) Gansäuer, A.; Justicia, J.; Rosales, A.;
Worgull, D.; Rinker, B.; Cuerva, J. M.; Oltra, J. E. Eur. J. Org. Chem. 2006, 4115–
4127; (k) Gansäuer, A.; Rosales, A.; Justicia, J. Synlett 2006, 927–929; (l)
Gansäuer, A.; Worgull, D.; Justicia, J. Synthesis 2006, 2151–2154; (m) Justicia, J.;
Álvarez de Cienfuegos, L.; Estévez, R. E.; Paradas, M.; Lasanta, A. M.; Oller, J. L.;
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Supplementary data
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tetlet.2009.09.088.
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