A Morita-Baylis-Hillman adduct allows the diastereoselective synthesis of styryl lactones

Article abstract of DOI:10.1016/j.tetlet.2011.09.044

We disclosed herein a diastereoselective approach for the total syntheses of (±)-Leiocarpin A and (±)-Goniodiol. These biologically active styryl lactones were obtained from a common intermediate, prepared in five steps and 40% overall yield, using a simp

Full text of DOI:10.1016/j.tetlet.2011.09.044

Tetrahedron Letters 52 (2011) 6180–6184
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A Morita–Baylis–Hillman adduct allows the diastereoselective synthesis
of styryl lactones
⇑
Paulo H.S. Paioti, Fernando Coelho
Laboratório de Síntese de Produtos Naturais e Fármacos, Universidade Estadual de Campinas – UNICAMP, Caixa Postal 6154, 13083-970 Campinas, São Paulo, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
We disclosed herein a diastereoselective approach for the total syntheses of ( )-Leiocarpin A and
( )-Goniodiol. These biologically active styryl lactones were obtained from a common intermediate,
prepared in five steps and 40% overall yield, using a simple synthetic sequence starting from a
Morita–Baylis–Hillman adduct. The total syntheses of these styryl lactones were accomplished in nine
steps. This is the first report on the total synthesis of this class of natural products starting from
Morita–Baylis–Hillman adduct.
Received 20 August 2011
Revised 8 September 2011
Accepted 11 September 2011
Available online 16 September 2011
Keywords:
Leiocarpin A
Ó 2011 Elsevier Ltd. All rights reserved.
Goniodiol
Lactones
Morita–Baylis–Hillman
Ring closing metathesis
The plants of the Goniothalamus genus (Annonaceae family)
provide multi-functionalized molecules known as styryl lactones.
These molecules are associated with a wide variety of biological ef-
fects, such as anti-tumor,1 anti-parasitic,2 abortifacient3, and as in-
sect repellents.4 Styryl lactones also display great structural
variations usually classified into six main groups (Fig. 1).1a
The styrylpyrone skeleton is always present and the biochemi-
cal pathway for its synthesis is related to shikimic acid and
acetylcoenzyme A.5 The unique structural patterns exhibited by
these molecules associated with their biological effects have
attracted great attention.6
Leiocarpin A (1) presents a pyranopyrone skeleton and was
isolated from the ethanolic extract of stem barks of the plant
Goniothalamus leiocapus (Fig. 2).7 Leiocarpin A showed excellent
biological profile against some types of cancer cells.8 Structurally,
Leiocarpin A is a bicyclo[3.3.1]heptane having substituents on C7
and C8. In addition, Leiocarpin A has four stereogenic centers, three
of them adjacent. Despite its promising biological profile, few ef-
forts have been reported so far in developing a synthetic route to
Leiocarpin A (Fig. 2).9
tone presents three adjacent stereogenic centers with relative ste-
reochemistry 1,2-anti-1,3-anti.
Contrary to Leiocarpin A, many routes to the total synthesis of
Goniodiol have been reported.12
Morita–Baylis–Hillman (MBH) is an organocatalyzed reaction
which provides functionalized small molecules showing great po-
tential as the starting material for organic synthesis.13,14 This
MBH reaction also presents high atom economy and may be per-
formed with low environmental impacts.15
In a research program focused on finding relevant synthetic
applications for MBH adducts associated with the need to obtain
styryl lactones for biological assays led us to investigate the diaste-
reoselective synthesis of ( )-Leiocarpin A using a suitably function-
alized aldehyde as the key precursor, which was easily prepared
from a MBH adduct. Additionally, we also describe herein a diaste-
reoselective synthesis of ( )-Goniodiol.
Leiocarpin A can be synthesized according to the retrosynthetic
analysis depicted in Scheme 1.
Leiocarpin A can be prepared according to Scheme 1 from al-
kene 3 using a classical sequence.9b,16 Ring closing metathesis
Goniodiol (2,Fig. 2) is a hydroxylated styrylpyrone isolated from
leaves and twigs of Goniothalamus sesquipedalis3a and also from
stem barks of Goniothalamus giganteus.10 Goniodiol also showed
biological profile as anti-tumor and selective cytotoxicity against
A549 lung carcinoma cells3a and P-388 leukemia cells.11 This lac-
(RCM) of 3 gives, therefore, an a,b-unsaturated lactone, which after
removal of the protecting group in basic medium cyclizes directly
to Leiocarpin A. Alkene 3 can be promptly prepared from aldehyde
4 by stereoselective allylation followed by the acylation of the
resulting allyl alcohol with an adequate acylating agent. Depending
on the stereoselectivity achieved in the allylation step both natural
products can be prepared using the same synthetic sequence. Fi-
nally, the key aldehyde 4 can be prepared using an original and dia-
stereoselective sequence from a MBH adduct.
⇑
Corresponding author. Tel.: +55 19 3521 3085; fax: +55 19 3521 3023.
E-mail address: coelho@iqm.unicamp.br (F. Coelho).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.044

P. H. S. Paioti, F. Coelho / Tetrahedron Letters 52 (2011) 6180–6184
6181
O
O
O
O
O
O
O
O
Styrilpyrone
furanopyrone
furanofurone
O
O
O
O
O
O
O
butenolide
pyranopyrone
heptolide
Figure 1. Main styryl lactones skeletons found in plants of Goniothalamus genus.
after 15 min an ozonide, which was treated under reductive condi-
tions with dimethylsulfide at the same temperature to give the
corresponding carbonyl compound. The solvents were removed
and the residue was dissolved in DCM and treated with NaBH4 at
À72 °C to give anti-dihydroxylated ester 6, as the only product,
and high diastereoisomeric purity (695% de).17 Ester 6 was ob-
tained in 70% yield and was sufficiently pure to be used in the next
step without purification (Scheme 2). The diastereoselectivity
achieved in the reduction step could be rationalized by assuming
the Cram-chelate model for 1,2-induction in acyloins.18
O
OH
O
O
5
4
6
O
Ph
3
Ph
O
9
1
O
H
OH
7
O
O
8
2
O
HO
Leiocarpin A (1)
Goniodiol (2)
Figure 2. Structures of Leiocarpin A and Goniodiol.
Following Scheme 1, ester 6 was treated with TBSCl in the pres-
ence of imidazole and DMF to provide the di-silylated ester 7, in al-
most quantitative yield, even after chromatographic filtration. The
silylated product was then reduced with DIBAl-H at À72 °C to give
We began our synthetic trial by preparing adduct 5. The reac-
tion between benzaldehyde and methyl acrylate gave the required
adduct in 85% yield. Ozonolysis in methanol at À72 °C furnished
O
OR
O
OR
O
OH
O
O
O
Ph
R
O
H
O
OR
3 ,R= TBS
OR
4, R= TBS
O
Leiocarpin A (1)
MBH adduct
5, R= Phenyl
Scheme 1. Retrosynthetic analysis for the synthesis of Leiocarpin A.
O
OH
O
OH
O
a
O
b
O
OH
Benzaldehyde
5
6
, < 95% d.e.
c
TBS
TBS
O
O
O
R
OTBS
40% overall yield
Only a purification
step is needed
O
OTBS
d,e
7
8, R = CH2OH
4, R = CHO
Scheme 2. Reagents and conditions: (a) methyl acrylate, rt, 96 h, 85%; (b) (i) O3, MeOH, À72 °C, then (CH3)2S, 1 h; (ii) DCM, NaBH4, À72 °C, 12 h, 70%; (c) TBSCl, Imidazole, rt,
12 h, 98%; (d) DIBAl-H, DCM, À72 °C, 1 h, 79%; (e) IBX, DMSO, rt, 1 h, 88%.

6182
P. H. S. Paioti, F. Coelho / Tetrahedron Letters 52 (2011) 6180–6184
Table 1
Allylation reaction with aldehyde 4
Entry
Nucleophile
L. A.c
Solvent
Temperature (°C)
Time (min)
%,d dre
1
2
3
allylMgBra
allylMgBra
Allyl(Sn)nBu3
—
BF3OEt
TiCl4
THF
THF
CH2Cl2
0
À22
À72
10
20
60
82 (1.2:1)
87 (2.5:1)
79 (5:1)
b
a
b
c
1.3 equiv were used.
1.5 equiv were used.
1.0 equiv was used.
d
e
Yields refer to isolated and purified products. No chromatographic separation was possible at this stage.
Diastereoisomeric ratio was determined by measuring the signals attributed to carbinolic hydrogen in NMR spectra.
O
TBS
TBS
TBS
O
OH
O
OH
TBS
O
O
TBS
O
O
O
OH
a
+
a
OTBS
OTBS
OTBS
OTBS
OTBS
4
5 : 1
10
9
(1,2-anti; 1,3-anti)
(1,2-anti; 1,3-syn)
3
9
(1,2-anti; 1,3-syn) 5:1
Scheme 3. Reagents and conditions: (a) allylSn(nBu)3, TiCl4, CH2Cl2, À72 °C, 1 h,
79% (diastereoisomeric ratio 5: 1, 9:10).
b
O
TBS
the triol di-silylated 8 in 79%. Finally, 8 was reacted with IBX in
DMSO to give the required aldehyde 4 in 88% yield.19 To increase
the synthetic efficiency and to avoid the two-step transformation,
we tried to reduce the ethyl ester directly to aldehyde 4 with DI-
BAL-H at À72 °C, but all attempts failed and only a mixture of alde-
hyde and alcohol was obtained.
O
O
O
Ph
c
O
H
O
O
OTBS
11, <95% d.e.(by NMR)
(+/-)-Leiocarpin A (1)
The key aldehyde 4 was thus prepared with a high diastereose-
lectivity in five steps from a cheap and abundant commercially
available aldehyde in 40% overall yield. The whole sequence is very
easy to be performed. Analogs of this aldehyde having different
protecting groups have already been used in the total synthesis
of styryl lactones, but more complicated procedures have been
used.
Scheme 4. Reagents and conditions: (a) acryloyl chloride, NEt3, CH2Cl2, 0 °C, 83%;
(b) Grubbs I, CH2Cl2, reflux, 18 h, 64%; (c) TBAF, THF, 0 °C to rt, 12 h, 50%.
was thus synthesized in nine steps in 8.5% overall yield. This is
the first approach to this natural product starting from a Morita–
Baylis–Hillman adduct.
Allylation is a very relevant chemical transformation able to
transfer three carbon atoms at once thus creating new stereogenic
centers and a homoallyl alcohol that can be used for further chem-
ical transformations.20
The spectral data of our synthetic Leiocarpin A were compared
with those described both for natural and synthetic products and
found fully compatible (see Supplementary data for details, page
S18).8,9
Initially aldehyde 4 was treated with phenylmagnesium bro-
mide in THF at 0 °C to give after 5 min a mixture of diastereoiso-
meric compounds 9 and 10 in a poor ratio. We tested, therefore,
some alternatives (Table 1).
Allylation using allyl tributylstanane in the presence of a Lewis
acid at À72 °C gave the best diastereoselectivity (entry 3). Unfortu-
nately, diastereoisomers are not separable by usual chromato-
graphic techniques, but after purification we observed net
diastereoisomeric enrichment in the reaction performed with allyl
stannane (see Supplementary data for details).
By comparing our data with the reported data for allyl com-
pounds 9 and 10, it was possible to assume that the major isomer
obtained was that having a 1,2-anti-1,3-syn stereochemical rela-
tionship (9, Scheme 3).9b The attained diastereoselectivity can be
rationalized assuming the Felkin–Ahn model.18,21
The diastereoisomeric enriched mixture was used in the next
step. So, allylic alcohol 9 (>90% de, after purification) was reacted
with acryloyl chloride in the presence of Et3N in DCM at 0 °C to
provide alkene 3 in 83% yield. Ring closing metathesis of this olefin
In the final steps of the total synthesis of Leiocarpin, we ob-
served that diastereoisomeric lactones could be easily separable
by silica gel column chromatography. We, therefore, decided to
take advantage of this separation and accomplish also the total
synthesis of ( )-Goniodiol.
The diastereoisomeric mixture of allylic alcohols 9 and 10 ob-
tained when allylation was conducted without Lewis acid (see en-
try 1, Table 1) was used. Acylation with acryloyl chloride followed
by ring closing metathesis using Grubbs I catalyst provided a mix-
ture of diastereoisomeric lactones 11 (1,2-anti; 1,3-syn) and 13
(1,2-anti; 1,3-anti). (Scheme 5). The diastereoisomeric lactones
were easily separated by silica gel chromatography providing 11
and 13, as pure diastereoisomers.
An acetonitrile solution of the pure a,b-unsaturated lactone 13
was then treated with HF/pyridine at room temperature for 24 h to
give ( )-Goniodiol (2) in 74% yield. Lactone 11 was also treated
with TBAF in THF to give ( )-Leiocarpin A in 50% yield.
The spectral data of ( )-Goniodiol synthesized in this work were
compared with those described for the natural and synthetic prod-
ucts and also found to be fully compatible (see Supplementary data
for details, pages S29–30).23
In summary, we have described the diastereoselective total syn-
theses of ( )-Leiocarpin A and ( )-Goniodiol using a simple alterna-
tive approach. Both the molecules were prepared from the unique
aldehyde 4, which in turn can be easily synthesized via MBH reac-
tion using a facile and scalable synthetic sequence.
was performed using a first generation Grubb’s catalyst to give a,b-
unsaturated cyclohexenone 11 in 64% yield (Scheme 4).22 After
chromatographic purification the diastereoisomeric purity of 11
was superior to 95% (determined by NMR).
To finish the synthetic sequence the unsaturated lactone 11 was
cyclized by treatment with TBAF in THF to ( )-Leiocarpin A (1) in
50% yield and high diastereoisomeric purity. ( )-Leiocarpin A (1)

P. H. S. Paioti, F. Coelho / Tetrahedron Letters 52 (2011) 6180–6184
6183
TBS
TBS
O
R
O
R
R1
R1
OTBS
3 : 12,
a
OTBS
1.5:1
9
10
, 1.2: 1
:
, R= OAcryloyl, R1 = H
, R= OH, R1 = H
3
9
, R = H; R1 = OAcryloyl
, R = H; R1 = OH
10
12
b,c
O
O
TBS
TBS
O
O
O
O
OTBS
OTBS
11
13
d
e
O
O
O
Ph
OH
O
H
O
O
OH
(+/-)-Goniodiol (2)
(+/-)-Leiocarpin A (1)
Scheme 5. Reagents and conditions: (a) acryloyl chloride, NEt3, CH2Cl2, 0 °C, 83%; (b) Grubbs I, CH2Cl2, reflux, 18 h, 64%; (c) chromatographic separation; (d) see Scheme 3; (e)
HF, pyridine, CH3CN, rt, 24 h, 74%.
the synthesis of styryl lactones see: (c) Prasad, K. R.; Dhaware, M. G. Synlett
2007, 1112; (d) Sabitha, G.; Sudhakar, K.; Yadav, J. S. Synthesis 2007, 385; (e)
Prasad, K. R.; Gholap, S. L. Tetrahedron Lett. 2007, 48, 4679.
This sequence can also allow the synthesis of these natural
products in their enantiomeric forms simply starting with a chiral
Morita–Baylis–Hillman adduct.
7. Mu, Q.; Tang, W.; Li, C.; Lu, Y.; Sun, H.; Zheng, H.; Hao, X.; Zheng, Q.; Wu, N.;
Lou, L.; Xu, B. Heterocycles 1999, 51, 2969.
8. Mu, Q.; Li, C. M.; He, Y. N.; Sun, H. D.; Zheng, H. L.; Lu, Y.; Zheng, Q. T.; Jiang, W.
Chin. Chem. Lett. 1999, 10, 35.
Acknowledgments
9. At the present only two approaches for the total synthesis of Leiocarpin A have
been described. (a) Chen, J.; Lin, G. Q.; Liu, H. Q. Tetrahedron Lett. 2004, 45,
8111; (b) Nagaiah, K.; Sreenu, D.; Purnima, K. V.; Rao, R. S.; Yadav, J. S. Synthesis
2009, 1386.
10. Fang, X.-P.; Anderson, J. E.; Chang, C.-J.; McLaughlin, J. L. J. Nat. Prod. 1991, 54,
1034.
Authors thank Fapesp for financial support and fellowship
(P.H.S.P). F.C. thanks CNPq research fellowship and financial sup-
port. Authors thank Professor Carol Collins for the english revision
of this manuscript.
11. Tsubuki, M.; Kanai, T.; Honda, T. J. Chem. Soc. Chem. Commun. 1992, 1640.
12. For some ouststanding examples concerning the total synthesis of Goniodiol,
see: (a) Surivet, J.-P.; Vatele, J.-M. Tetrahedron 1999, 55, 13011; (b) Tsukubi, M.;
Kanai, K.; Nagase, H.; Honda, T. Tetrahedron 1999, 55, 2493; (c) Ramachandran,
P. V.; Chandra, J. S.; Reddy, M. V. R. J. Org. Chem. 2002, 67, 7547; (d) Tate, E. W.;
Dixon, D. J.; Ley, S. V. Org. Biomol. Chem. 2006, 4, 1698; (e) Yoshida, T.;
Yamauchi, S.; Tago, R.; Maruyama, M.; Akyiama, K.; Sugahara, T.; Kishida, T.;
Koba, Y. Biosci. Biotechnol. Biochem. 2008, 72, 2342; (f) Yadav, J. S.; Premalatha,
K.; Harshavardhan, S. J.; Reddy, B. V. S. Tetrahedron Lett. 2008, 49, 6765; (g)
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Supplementary data
Supplementary data (Complete experimental synthetic descrip-
tions and characterizations of all the compounds.) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.09.044.
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