A ring contraction strategy toward a diastereoselective total synthesis of (+)-bakkenolide A

Article abstract of DOI:10.1021/jo100108b

A diastereoselective route to (+)-bakkenolide A is presented from the readily available optically active Wieland-Miescher ketone. This novel synthesis of this sesquiterpene lactone features the following as key stereoselective transformations: (i) the ring contraction reaction of a octalone mediated by thallium(III) nitrate (TTN); (ii) a hydrogenation to create the cis-fused junction; and (iii) the formation of the C7 quaternary center through an enolate intermediate. Furthermore, during this work, the absolute configuration of a trinorsesquiterpene isolated from Senecio Humillimus was assigned.

Full text of DOI:10.1021/jo100108b

pubs.acs.org/joc
A Ring Contraction Strategy toward a Diastereoselective
Total Synthesis of (þ)-Bakkenolide A
Vania M. T. Carneiro, Helena M. C. Ferraz, Tiago O. Vieira, Eloisa E. Ishikawa, and
Luiz F. Silva, Jr.*
†
^
´
~
Instituto de Quımica, Universidade de Sao Paulo, Av. Prof. Lineu Prestes, 748, CP 26077,
CEP 05513-970 Sa~o Paulo SP, Brazil. †Deceased.
luizfsjr@iq.usp.br
Received January 22, 2010
A diastereoselective route to (þ)-bakkenolide A is presented from the readily available optically active
Wieland-Miescher ketone. This novel synthesis of this sesquiterpene lactone features the following as
key stereoselective transformations: (i) the ring contraction reaction of a octalone mediated by
thallium(III) nitrate (TTN); (ii) a hydrogenation to create the cis-fused junction; and (iii) the formation
of the C7 quaternary center through an enolate intermediate. Furthermore, during this work, the
absolute configuration of a trinorsesquiterpene isolated from Senecio Humillimus was assigned.
Introduction
The most famous member of the bakkanes class of natural
products is (þ)-bakkenolide A, (þ)-1 (Figure 1), which was
first isolated in the late 600s.1 This sesquiterpene lactone
displays cytotoxic activity against several cells lines, as well
as insect antifeedant activity.2 Several routes have been
developed for the synthesis of bakkanes,3 including for
bakkenolide A.4 Among them, the comprehensive work
of Greene and his group must be highlighted.5 However,
FIGURE 1. Bakkane skeleton and structure of (þ)-bakkenolide A.
in their 11-step route for (þ)-bakkenolide A,4d,h the C7
quaternary carbon was not formed diastereoselectively
and, eventually, (þ)-1 was obtained together with its C-7
epimer in a 3:1 mixture, respectively. Indeed, this is a key
issue in the synthesis of bakkanes and low diastereoselective
(1) (a) Naya, K.; Takagi, I.; Hayashi, M.; Nakamura, S.; Kobayashi, M.;
Katsumura, S. Chem. Ind. 1968, 318. (b) Abe, N.; Onoda, R.; Shirahata, K.;
Kato, T.; Woods, M. C.; Kitahara, Y. Tetrahedron Lett. 1968, 369.
(2) (a) Jamieson, G. R.; Reid, E. H.; Turner, B. P.; Jamieson, A. T.
Phytochemistry 1976, 15, 1713. (b) Kano, K.; Hayashi, K.; Mitsuhashi, H.
Chem. Pharm. Bull. 1982, 30, 1198. (c) Nawrot, J.; Bloszyk, E.; Harmatha, J.;
(4) Syntheses of bakkenolide A: (a) Evans, D. A.; Sims, C. L. Tetrahedron
Lett. 1973, 4691. (b) Evans, D. A.; Sims, C. L.; Andrews, G. C. J. Am. Chem.
ꢀ
Novotny, L. J. Appl. Entomol. 1984, 98, 394. (d) Nawrot, J.; Harmatha, J.;
ꢀ
Novotny, L. Biochem. Syst. Ecol. 1984, 12, 99. (e) Nawrot, J.; Bloszyk, E.;
Harmatha, J.; Novotny, L.; Drozdz, B. Acta Entomol. Bohemoslov. 1986, 83,
ꢀ
Soc. 1977, 99, 5453. (c) Greene, A. E.; Depres, J.-P.; Coelho, F.; Brocksom,
ꢀ
ꢀ
T. J. J. Org. Chem. 1985, 50, 3943. (d) Greene, A. E.; Coelho, F.; Depres,
327. (f) Kreckova, J.; Krecek, J.; Harmatha, J. Pr. Nauk. Inst. Chem. Org. Fiz.
Politech. Wroclaw. 1988, 33, 105. (g) Rosinski, R.; Bloszyk, E.; Harmatha, J.;
Knapik, A. Pr. Nauk. Inst. Chem. Org. Fiz. Politech. Wroclaw. 1988, 33, 91.
(h) Nawrot, J.; Koul, O.; Isman, M. B.; Harmatha, J. J. Appl. Entomol. 1991,
112, 194. (i) Harmatha, J.; Nawrot, J. Biochem. Syst. Ecol. 1984, 12, 95. (j) Li,
E.-W.; Gao, K.; Jia, Z.-J. Pharmazie 2004, 59, 646.
(3) For a review, see: (a) Silva, L. F., Jr. Synthesis 2001, 671. For an
account, see: (b) Brocksom, T. J.; Brocksom, U.; Constantino, M. G. Quim.
Nova 2008, 31, 937. For some papers concerning the synthesis of bakkanes,
see: (c) Jiang, C.-H.; Bhattacharyya, A.; Sha, C.-K. Org. Lett. 2007, 9, 3241.
(d) Srikrishna, A.; Reddy, T. J. Arkivoc 2001, 9. (e) Srikrishna, A.; Nagaraju,
S.; Venkateswarlu, S.; Hiremath, U. S.; Reddy, T. J.; Venugopalan, P.
J. Chem. Soc., Perkin Trans. 1 1999, 2069.
J.-P.; Brocksom, T. J. Tetrahedron Lett. 1988, 29, 5661. (e) Srikrishna, A.;
Reddy, T. J.; Nagaraju, S.; Sattigeri, J. A. Tetrahedron Lett. 1994, 35, 7841.
(f) Back, T. G.; Payne, J. E. Org. Lett. 1999, 1, 663. (g) Back, T. G.; Nava-
Salgado, V. O.; Payne, J. E. J. Org. Chem. 2001, 66, 4361. (h) Brocksom, T. J.;
ꢀ
Coelho, F.; Depres, J.-P.; Greene, A. E.; de Lima, M. E. F.; Hamelin, O.;
Hartmann, B.; Kanazawa, A. M.; Wang, Y. J. Am. Chem. Soc. 2002, 124,
15313. (i) Reddy, D. S.; Kozmin, S. A. J. Org. Chem. 2004, 69, 4860.
(j) Reddy, D. S. Org. Lett. 2004, 6, 3345. (k) Constantino, M. G.; de Oliveira,
K. T.; Polo, E. C.; da Silva, G. V. J.; Brocksom, T. J. J. Org. Chem. 2006, 71,
9880. (l) Kato, K.; Motodate, S.; Takaishi, S.; Kusakabe, T.; Akita, H.
Tetrahedron. 2008, 64, 4627. (m) Kusakabe, T.; Kato, K.; Motodate, S.;
Takaishi, S.; Akita, H. Chem. Pharm. Bull. 2008, 56, 1436. (n) Maity, S.;
Ghosh, S. Tetrahedron 2009, 65, 9202.
DOI: 10.1021/jo100108b
r
Published on Web 04/08/2010
J. Org. Chem. 2010, 75, 2877–2882 2877
2010 American Chemical Society

Carneiro et al.
JOCArticle
SCHEME 1. Ring Contraction Route for (þ)-Bakkenolide A
SCHEME 2. Preparation of the Octalone (þ)-4
in the creation of the carbon that originates the spiro lactone
moiety can also be noted in racemic syntheses of bakkenolide
A.4c,f,g,k Another approach to obtain (þ)-1 was recently
developed by Kato and co-workers.4l,m In their 13-step
synthesis from 2-methyl-1,3-cyclohexanedione, the required
optically active intermediate was obtained in 22% yield and
in 95% of enantiomeric excess, through a kinetic resolution.
In this scenario, we herein described a new and diastereose-
lective route to (þ)-bakkenolide A.
with thallium(III) has not been investigated in these papers.
The preparation of (þ)-4 has been described from the readily
available Wieland-Miescher ketone (þ)-5 (Scheme 1). Several
protocols have been developed for the preparation of (þ)-5 in
optically pure form.8 Furthermore, ketone (þ)-5 and its
enantiomer are commercially available.
Results and Discussion
The starting Wieland-Mischer ketone (þ)-5 was obtained
in large amounts and in high enantiomeric excess from methyl
vinyl ketone and 2-methylcyclohexane-1,3-dione through
Robinson annulation catalyzed by 5 mol % of S-proline.8a,b
The octalone (þ)-4 was prepared from (þ)-5, as described by
Paquette and co-workers,9 although their route was modified
in some steps (Scheme 2). The main alteration was the use of
the iodine(III) reagent PhI(OCOCF3)2 (PIFA) instead of
TTN to perform the deprotection of the thioketal group of
8.10 Additionally, thereduction of the mesylate was performed
with LiAlH4 in the place of Super-Hydride (LiBHEt3). Other
small modifications are described in the Supporting Informa-
tion. Racemic 4 was also prepared for testing the key ring
contraction step. Two different routes were used.11 The proto-
col of Zoretic and co-workers gave (()-4 in two steps,
although the product was obtained together with its epimer
in 9:1 ratio.11a An adaptation of routes described in the
literature11b,c gave (()-4 as a single diastereomer, but in
several steps (see the Supporting Information).
We envisioned that (þ)-1 could be synthesized from the
keto-ester 2 by R-oxidation of the carbonyl group followed by
lactonization. The keto-ester 2 would be obtained from the
ketone 3 through the acylation of an enolate intermediate.
Finally, the ketone 3 would be obtained by the oxidative
rearrangement of the optically active octalone 4, followed by
diastereoselective hydrogenation and functional group trans-
formations. Thallium(III) and iodine(III) reagents could a
priori be used in such a rearrangement.6 There are no pre-
cedents for the ring contraction of octalones using I(III).
Regarding Tl(III), ring contraction product was obtained
using thallium(III) nitrate (TTN) in a mixture of trimethyl-
orthoformate (TMOF) and MeOH.7a On the other hand,
dehydrogenation is observed after treatment with thallium(III)
acetate (TTA) in acetic acid.7b The oxidation of octalone 4
ꢀ
(5) (a) Coelho, F.; Depres, J.-P.; Brocksom, T. J.; Greene, A. E. Tetra-
hedron Lett. 1989, 30, 565. (b) Hartmann, B.; Kanazawa, A. M.; Depres,
ꢀ
J.-P.; Greene, A. E. Tetrahedron Lett. 1991, 32, 767. (c) Hartmann, B.;
ꢀ
Depres, J.-P.; Greene, A. E.; de Lima, M. E. F. Tetrahedron Lett. 1993, 34,
1487. (d) Hamelin, O.; Depres, J.-P.; Greene, A. E.; Tinant, B.; Declercq,
ꢀ
J.-P. J. Am. Chem. Soc. 1996, 118, 9992. (e) Hamelin, O.; Depres, J.-P.;
Heidenhain, S.; Greene, A. E. Nat. Prod. Lett. 1997, 10, 99. (f) Hamelin, O.;
With the octalone 4 in hand, we start to investigate the
rearrangement step using iodine(III) or thallium(III). The
expected products of this ring contraction (9/10) bear a
double bond and, consequently, are also prone to react with
electrophilic species, such as thallium(III) and iodine(III).
This constitutes one of the challenges of this transformation.
Several conditions were tested with (þ)- and/or (()-4 and we
summarize the main results in the following paragraphs.
The reaction of 4 with iodine(III) was tested with [hydroxy-
(tosyloxy)iodo]benzene (HTIB) (Koser’s reagent), with dia-
cetoxyiodobenzene (DIB), and with PIFA (Table 1). A mix-
ture of several compounds was obtained under all conditions,
except in the cases shown in entries 1 and 7, where no reaction
was observed. The best yield (40%) for the ring contraction
products (9/10) was obtained by using DIB in TMOF:MeOH
ꢀ
ꢀ
Wang, Y.; Depres, J.-P.; Greene, A. E. Angew. Chem., Int. Ed. 2000, 39, 4314.
(g) Hartmann, B.; Kanazawa, A. M.; Depres, J.-P.; Greene, A. E. Tetra-
ꢀ
hedron Lett. 1993, 34, 3875. (h) See also refs 4c, 4d, and 4h.
(6) For reviews concerning ring contraction reactions, see: (a) Redmore,
D.; Gutsche, C. D. In Advances in Alicyclic Chemistry; Hart, H., Karabastos,
G. J., Eds.; Academic Press: New York , 1971; Vol. 3, p 1. (b) Silva, L. F., Jr.
Tetrahedron 2002, 58, 9137. For a review concerning Tl(III)-mediated ring
contractions, see: (c) Ferraz, H. M. C.; Silva, L. F., Jr. Quim. Nova 2000, 23, 216.
See also: (d) Silva, L. F., Jr.; Carneiro, V. M. T. Synthesis 2010, 1059. For a
review concerning I(III)-mediated ring contractions, see: (e) Silva, L. F., Jr.
Molecules 2006, 11, 421.
(7) (a) Mincione, E.; Bovicelli, P.; Gil, J. B.; Forcellese, M. L. Gazz. Chim.
~
Ital. 1985, 115, 37. (b) Banerjee, A. K.; Carrasco, M. C.; Pena-Matheud,
C. A. Recl. Trav. Chim. Pays-Bas 1989, 108, 94.
(8) The procedure of the following papers was used in this work:
(a) Barrack, S. A.; Okamura, W. H. J. Org. Chem. 1986, 51, 3201.
€
(b) Buchschacher, P.; Furst, A.; Gutzwiller, J. Org. Synth. 1985, 63, 37.
For other selected recents papers, see: (c) Bui, T.; Barbas, C. F., III
Tetrahedron Lett. 2000, 41, 6951. (d) Kriis, K.; Kanger, T.; Laars, M.;
Kailas, T.; Muurisepp, A.-M.; Pehk, T.; Lopp, M. Synlett 2006, 1699.
(e) Akahara, Y.; Inage, N.; Nagamine., T.; Inomata, K.; Endo, Y. Hetero-
(9) Paquette, L. A.; Wang, T.-Z.; Phillippo, C. M. G.; Wang, S. J. Am.
Chem. Soc. 1994, 116, 3367.
(10) Stork, G.; Zhao, K. Tetrahedron Lett. 1989, 30, 287.
(11) (a) Zoretic, P. A.; Golen, J. A.; Saltzman, M. D. J. Org. Chem. 1981,
46, 3554. (b) Piers, E.; Britton, R. W.; de Waal, W. Can. J. Chem. 1969, 47,
ꢀ
cycles 2007, 74, 637. (f) Arriba, A. L. F.; Simon, L.; Raposo, C.; Alcazar, V.;
Moran, J. R. Tetrahedron 2009, 65, 4841. (g) Almasi, D.; Alonso, D. A.;
ꢀ
ꢀ
ꢀ
Balaguer, A.-N.; Najera, C. Adv. Synth. Catal. 2009, 351, 1123. (h) Akahane,
Y.; Inomata, K.; Endo, Y. Heterocycles 2009, 77, 1065.
ꢀ
4307. (c) Cuesta, X.; Gonzalez, A.; Bonjoch, J. Tetrahedron: Asymmetry
1999, 10, 3365.
2878 J. Org. Chem. Vol. 75, No. 9, 2010

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TABLE 1. Oxidation of Octalone 4 with Iodine(III)
product
(isolated yield)
entry
conditions
1
2
3
4
5
6
7
8
9
HTIB, MeCN, rt, 24 ha
no reaction
9/10 (GC: 6%)
9/10 (10%)
HTIB, TMOF, 0 °C, 1 mina
DIB, TMOF, 10% HClO4, rt, 20 hb
PIFA, TMOF, 10% HClO4, rt, 30 minc
DIB, TMOF, p-TsOH, rt, 1 mind
DIB, TMOF, p-TsOH, 0 °C to rt, 2 hd
DIB, TMOF:MeOH (7:3), rt, 4 daysc,e
DIB, TMOF:MeOH (7:3), HNO3, rt, 17.5 hd,e
DIB, TMOF:MeOH (7:3), p-TsOH, rt, 1 hd,e
FIGURE 2. NOE cross-peaks of ring contraction product 9.
9/10 (GC: 8%)
complex mixture
9/10 (28%)
SCHEME 3. Mechanism for the Ring Contraction of Octalones
no reaction
9/10 (40%)
9/10 (36%)
a1 equiv of HTIB. b1.2 equiv of DIB. c2 equiv of I(III) reagent. d2 equiv
of DIB and 1 equiv of acid. Total volume of solvent: 20 mL/1 mmol,
whereas in other conditions 7 mL/1 mmol was used.
e
TABLE 2. Oxidation of Octalone 4 with 1.1 equiv of TTN
product
(isolated yield)
entry
conditions
rearrangement toward other intermolecular oxidations and,
thus, increase the yield of 9. Indeed, by using 20 mL of solvent
for each mmol of substrate, the ring contraction product 9 was
isolated in 59% yield (entry 8), whereas in 10 mL the yield was
52% (entry 7). Increasing the amount further to 40 mL did
not lead to a better result (entry 9). Then, the condition with
20 mL/1 mmol was applied to (þ)-4, giving an analogous yield,
as expected (entry 10).
The ring contraction mediated by thallium(III) led to 9, as
a single diastereomer. The relative configuration of C7 in this
compound was assessed by means of 2D COSY, 2D HET-
COR and 2D NOESY. Figure 2 shows the most important
NOE cross-peaks.
The suggested mechanism can explain the observed diaster-
eoselectivity.13 The first step would be the attack of Tl(III) to
the enol ether 11 giving the thallonium ion 12. The trans-diaxial
ring-opening of this onium ion by the methanol led to the
organothallium intermediate 13. The exit of thallium(I) would
occur by the intramolecular attack of the methoxyl oxygen,
giving the oxonium-like ion 14, on which the rearrangement
would take place delivering the ring contraction product 9
(Scheme 3). The ring contraction mediated by iodine(III)
would occur by an analogous mechanism. However, the acidic
medium would promote the epimerization and a mixture of
diastereomers was isolated (Table 1).
1
2
3
4
5
6
7
8
9
TMOF:MeOH (4:3), 0 °C, 30 min
TMOF, 0 °C, 15 min
MeCN:MeOH:TMOF (1:1:1), rt, 10 min
TMOF:TFEc (1:1), 0 °C, 5 min
(þ)-9 (48%)a
(þ)-9 (36-40%)b
(þ)-9 (38%)b
(()-9 (15%)b
TMOF:MeOH (4:3), 0 °C, 15 min 7 mL/1 mmol (þ)-9 (36-38%)
TMOF:MeOH (7:3), 0 °C, 15 min 10 mL/1 mmol (()-9 (51%)
TMOF:MeOH (7:3), rt, 5 min 10 mL/1 mmol
TMOF:MeOH (7:3), rt, 10 min 20 mL/1 mmol
TMOF:MeOH (7:3), rt, 30 min 40 mL/1 mmol
(()-9 (52%)
(()-9 (59%)
(()-9 (58%)
(þ)-9 (59%)
10 TMOF:MeOH (7:3), rt, 10 min 20 mL/1 mmol
aTotal volume of solvent: 12 mL/1 mmol; 1.2 equiv of TTN. bTotal
volume of solvent: 7 mL/1 mmol. cTFE: 2,2,2-trifluoroethanol.
(7:3), in the presence of HNO3, with a total volume of solvent
of 20 mL for each mmol of substrate (entry 8).
The reaction of 4 with thallium(III) was also studied.
On the basis of previous work7 and on our experience,6b-d
we focus the attempts on TTN. By using the conditions
described by Mincione et al.,7a the desired hydrindene 9 was
obtained in 48% yield (Table 2, entry 1). Several other
conditions were then tried with use of TTN, but the yield
of 9 did not improved (entries 2-5).12 By increasing the
amount of solvent from 7 mL/1 mmol to 10 mL and the
proportion of TMOF to MeOH, the racemic product 9 was
isolated in 51% yield (entry 6). Reduction on reaction time
was achieved by increasing the temperature, without signifi-
cantly altering the yield (entries 6 and 7). Finally, the effect
of dilution was investigated for (()-4 (entries 7-9). A
more diluted condition could favor the intramolecular
The ring contraction step was tested by using enol ether 16
as the substrate, which was prepared from (þ)-(S,S)-4.14
(13) (a) McKillop, A.; Hunt, J. D.; Taylor, E. C. J. Org. Chem. 1972, 37,
3381. (b) Ferraz, H. M. C.; Silva, L. F., Jr. Tetrahedron Lett. 1997, 38, 1899.
(c) Ferraz, H. M. C.; Silva, L. F., Jr. J. Org. Chem. 1998, 63, 1716. (d) Ferraz,
H. M. C.; Silva, L. F., Jr. J. Braz. Chem. Soc. 2001, 12, 548.
(14) (a) Govindan, S. V.; Fuchs, P. L. J. Org. Chem. 1988, 53, 2593.
(b) Tanabe, M.; Crowe, D. F. J. Chem. Soc., Chem. Commun. 1973, 16, 564.
(12) No reaction was observed with the following conditions: MeOH:
THF (2:1), H2O cat., rt, 4 h; MeCN, rt, 3 h; MeCN:MeOH (1:1), rt, 1 h;
CH2Cl2, rt. Employing HClO4 as solvent, a complex mixture of products was
formed.
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SCHEME 4. Synthesis of the Trinorsesquiterpene 17
SCHEME 5. Synthesis of the cis-Hydrindane 3
SCHEME 6. Creation of the C7 Quaternary Center
TABLE 3. Oxidation of Octalones 18 and 21 with I(III) or with Tl(III)
Table 2) are better than those reported by Mincione et al.7a
(entry 1, Table 2). On the other hand, for the octalones 18 and
21, the protocol of Mincione is more efficient (compare entries
3 and 5, and 4 and 6 of Table 3).
After a detailed study of the ring contraction step, the total
synthesis was continued. The unsaturated ester 9 was hydro-
genated giving the required cis-hydrindane (þ)-24. The for-
mation of the corresponding compound with a trans junction
was not observed in the crude product. The chemical shift of
the methyl group atC3a canbe used to assignthe ringjunction
as cis or trans in hydrindanes, as 24. For the trans diaster-
eomer, the chemical shift would be around 0.7 ppm, whereas
for the corresponding cis it would be at ca. 1.0 ppm.13c,d The
high selectivity can be explained by the delivery of the hydro-
gen to the less hindered convex face, where the CO2Me group
may have an influence. The ester 24 was then hydrolyzed and
the carboxylic acid treated with MeLi,16 leading to the
corresponding ketone 3, in 86% yield from 9 (Scheme 5).
The basic conditions led to a partial epimerization of C7,
which is not important at this stage, because the next step
would involve the formation of the enolate of the ketone 3.
Exposing the cis-hydrindane 3 to HMDS and TMSI led to
the thermodynamic enol ether 25, which was treated with
MeLi delivering the enolate 26. This enolate reacted with
CNCO2Me only by the convex face giving the keto-ester 2, as
a single diastereomer (Scheme 6).4j,17
a2 equiv of DIB and 1 equiv of p-TsOH. b1.2 equiv of Tl(NO3)3
3
3H2O. c1.1 equiv of Tl(NO3)3 3H2O.
3
When 16 was treated with TTN, the hydrindene (þ)-9 was
isolated as the minor component. The major product was the
dienone (þ)-(S,S)-17 (Scheme 4). The dienone 17 is a trinor-
sesquiterpene that was isolated from Senecio Humillimus, as
the levorotatory enantiomer [R]24 -12.4 (c 1.0, CHCl3).15
D
On the basis of the absolute configuration of related natural
products and on biosynthetic considerations, Bohlmann
suggested the absolute configuration of 17 as (S,S).15 How-
ever, in conclusion, on the basis of our results the absolute
configuration of (-)-17 is (R,R).
The oxidation of other octalones, namely 18 and 21, was
also investigated with iodine(III) and thallium(III) under
several conditions. Representative results are herein discussed.
The reaction of enone 18 with DIB led to the ring contraction
products 19 and 20 as a 2.3:1 mixture, respectively, in 22%
yield (Table 3, entry 1). Under similar conditions, the octalone
21 gave 22 and 23, together with other unidentified compounds
in a total yield of 27% (entry 2). The conditions optimized by
us for the thallium(III)-mediated ring contraction of 4 (entry 8,
After the diastereoselective creation of the C7 quaternary
center, the formation of the spiro lactone moiety would be
required to accomplish the total synthesis of 1. Among many
options, we decided to perform the R-oxidation of (þ)-2
using hypervalent iodine.18 Indeed, treatment of 2 with DIB
gave the R-hydroxyketone 27.18c Subsequent TFA-catalyzed
lactonization furnished the keto-lactone 28, in 61% yield
(16) Adcock, W.; Abeywickrema, A. N. J. Org. Chem. 1982, 47, 2951.
(17) (a) Mori, K.; Matsushima, Y. Synthesis 1995, 845. (b) Ferraz,
H. M. C.; Vieira, T. O.; Silva, L. F., Jr. Synthesis 2006, 2748.
(18) (a) Moriarty, R. M.; Prakash, O. Org. React. 1999, 54, 273.
(b) Moriarty, R. M. J. Org. Chem. 2005, 70, 2893. (c) Moriarty, R. M.; Berglund,
B. A.; Penmasta, R. Tetrahedron Lett. 1992, 33, 6065. (d) See also, ref 6e.
(15) Bohlmann, F.; Kramp, W.; Robinson, H.; King, R. M. Phytochemistry
1981, 20, 1739.
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SCHEME 7. Formation of the Spiro Lactone Moiety
3.5 h, when the mixture was filtered with Et2O (30 mL) as eluent.
The organic phase was dried over anhyd MgSO4. The solvent was
removed under reduced pressure to give (þ)-24 (0.166 g) as a
colorless oil, which was used in the next step without purification.
Rf 0.70 (10% AcOEt/hexanes); IR (film) 2958, 2927, 2865, 1737,
1199, 1172 cm-1; 1H NMR (500 MHz, CDCl3) δ 0.79 (3H, d, J =
7.0 Hz), 0.88 (3H, s), 1.06-1.16 (1H, m), 1.37-1.60 (7H, m),
1.67-1.73 (1H, m), 1.79-1.85 (1H, m), 2.05-2.13 (2H, m), 2.85
(1H, dddd, J = 5.3, 8.8, 9.5, and 11.4 Hz), 3.67 (3H, s); 13C NMR
(125 MHz, CDCl3) δ 16.4, 19.9, 21.1, 24.2, 30.7, 32.8, 33.7, 40.6,
41.6, 43.0, 47.3, 51.6, 177.7; LRMS m/z (%) 210 (Mþ•, 11%), 178
(59), 151 (48), 109 (45), 95 (65), 81 (100), 41 (62); HRMS m/z
C13H22O2 (M þ H)þ calcd 211.1693, found 211.1689; [R]25
D
from 2.19 Finally, Wittig olefination of 28 led to (þ)-bakke-
nolide A (þ)-(1), in 62% yield (Scheme 7). The analytical
data of our sample of 1 are equivalent to those previously
reported.4i,20
þ25.9 (c 0.85, CHCl3).
1-((3aR,4S,7aR)-Octahydro-3a,4-dimethyl-1H-inden-2-yl)-
ethanone, 3. To a stirred solution of the ester (þ)-24 (0.166 g) in
MeOH (1.6 mL) was added dropwise a 10% aqueous solution
of KOH (1.6 mL). The mixture was stirred for 5 h at rt and
the reaction was quenched with a 10% aqueous solution of HCl
(until pH <3). H2O (20 mL) was added. The organic layer was
extracted with EtOAc (3 ꢀ 25 mL), washed with brine (20 mL),
and dried over anhyd MgSO4. The solvent was removed under
reduced pressure. The carboxylic acid was obtained as a colorless
oil (0.155 g) and was used in the next step without purification.
(2S,3aR,4S,7aR)-octahydro-3a,4-dimethyl-1H-indene-2-carboxylic
acid: IR (film) 2957, 2920, 2855, 1696 cm-1; 1H NMR (300 MHz,
CDCl3) δ 0.78 (3H, d, J = 6.9 Hz), 0.89 (3H, s), 1.03-1.28 (2H, m),
1.37-1.60 (7H, m), 1.67-1.76 (1H, m), 1.81-1.90 (1H, m), 2.05-
2.19 (2H, m), 2.90 (1H, dddd, J = 5.2, 8.9, 9.5, and 11.5 Hz);
13C NMR (75 MHz, CDCl3) δ 16.4, 19.9, 21.1, 24.2, 30.7, 32.6,
33.7, 40.6, 41.5, 43.1, 47.3, 182.7; LRMS m/z (%) 196 (Mþ•, 14%),
178 (29), 151 (30), 109 (37), 95 (64), 81 (100); HRMS m/z C12H20O2
(M þ Na)þ calcd 219.1356, found 219.1358; [R]25D þ18.7 (c 0.42,
CHCl3).
Conclusion
In summary, a novel total synthesis of (þ)-bakkenolide A
(þ)-1 was accomplished in 15 steps and in 6% yield from the
readily available Wieland-Miescher ketone (þ)-(5), which
can be prepared in large amounts by an organocatalytic reac-
tion. This stereoselective synthesis features the following as key
steps: (i) the ring contraction reaction of the octalone 4; (ii) a
stereoselective hydrogenation to create the required cis-fused
junction; and (iii) the diastereoselective formation of the C7
quaternary center through the enolate 26. Moreover, the abso-
lute configuration of the trinorsesquiterpene (-)-17, which was
isolated from Senecio Humillimus, was assigned as (R,R).
Experimental Section
To a stirred solution of the carboxylic acid (0.155 g) in anhyd
Et2O (4 mL) was added dropwise MeLi (0.60 mL, 3 M in
diethoxymethane, 1.8 mmol) at 0 °C under inert atmosphere.
The mixture was refluxed for 2.5 h and cooled to 0 °C. H2O
(6 mL) was added. After stirring for 10 min, the organic layer
was extracted with Et2O (3 ꢀ 25 mL) and dried over anhyd
MgSO4. The solvent was concentrated by reduced pressure. The
crude product was purified by flash chromatography (hexanes:
Et2O, 9:1 as eluent), giving 3 (86%, 0.140 g, 0.722 mmol) as a 7:1
mixture of diastereomers, as a colorless oil. Rf 0.50 (10% Et2O/
(2R,7S,7aR)-Methyl 1,2,4,5,6,7,7a-Heptahydro-7,7a-dimethyl-
2H-indene-2-carboxylate, (þ)-9. ToastirredsolutionofTl(NO3)3
3
3H2O (0.488 g, 1.1 mmol) in TMOF (6 mL) and in MeOH (6 mL),
stirred for 10 min, was added the octalone (þ)-4 (0.179 g, 1.0 mmol)
in TMOF (8 mL) at rt. After 2 min an abundant precipitation was
observed and the reaction mixture was stirred for 10 min at rt. The
resulting suspension was filtered through a silica gel pad (ca. 10 cm),
using CH2Cl2 (100 mL) as eluent. The filtrate was washed with a
saturated solution of NaHCO3 (30 mL) and brine (30 mL) and
dried over anhyd MgSO4. The solvent was removed under reduced
pressure. The crude product was purified by flash chromatography
(hexanes:Et2O, 9:1 as eluent) immediately after concentration of the
solvent, to give (þ)-9 (59%, 0.123 g, 0.591 mmol) as a light yellow
oil. Rf 0.70 (10% AcOEt/hexanes); IR (film) 2956, 2931, 2859, 1740,
1436, 1201, 1172 cm-1; 1H NMR (300 MHz, CDCl3) δ 0.85 (3H, d,
J = 6.6 Hz), 0.91 (3H, s), 1.19-1.48 (4H, m), 1.71-1.79 (1H, m),
1.87 (1H, dd, J = 12.8 and 9.0 Hz), 1.90-2.09 (1H, m), 2.04 (1H,
dd, J = 12.8 and 8.1 Hz), 2.26-2.33 (1H, m), 3.54 (1H, ddt, J =
8.67, 4.01, and 1.78 Hz), 3.69 (3H, s), 5.17 (t, J = 1.9, 1H); 13C
NMR (75 MHz, CDCl3) δ 16.5, 16.9, 25.8, 26.7, 30.3, 42.9,
44.0, 47.5, 49.6, 51.7, 117.0, 153.7, 176.1; LRMS m/z (%) 208
(Mþ•, 19), 149 (100), 148 (51), 133 (29), 93 (66), 91 (40); HRMS m/z
C13H20O2 (M þ H)þ calcd 209.1536, found 209.1535; [R]25D þ56.2
(c 1.0, CHCl3).
hexanes);IR(film) 2958, 2925, 2873, 1711, 1462, 1358, 1174 cm-1
;
1H NMR (300 MHz, CDCl3) δ (major diastereomer) 0.77 (3H, d,
J = 6.6 Hz), 0.89 (3H, s), 1.00-1.14 (1H, m), 1.24-1.58 (7H, m),
1.69-1.81 (2H, m), 1.97-2.11 (2H, m), 2.15 (3H, s), 2.89-3.06
(1H, m), (minor diastereomer) 0.80 (3H, d, J = 6.6 Hz), 2.14 (3H,
s); 13C NMR(75MHz, CDCl3) δ (major diastereomer) 16.4, 19.9,
21.1, 24.3, 28.9, 30.7, 31.5, 33.5, 39.9, 43.0, 47.3, 49.5, 211.1,
(minor diastereomer) 16.7, 19.0, 20.9, 23.9, 28.7, 29.6, 30.4, 32.6,
41.4, 44.3, 45.7, 48.2, 211.1; LRMS m/z (%) (major diastereomer)
194 (Mþ•, 0.3%), 176 (3), 109 (8), 95 (7), 81 (10), 67 (8), 55 (9),
43 (100), (minor diastereomer) 194 (Mþ•, 2%), 124 (23), 109 (34),
95 (19), 81 (22), 71 (17), 67 (15), 55 (14), 43 (100); HRMS m/z
C13H22O (M þ H)þ calcd 195.1743, found 195.1741.
(2R,3aR,4S,7aR)-Methyl 2-Acetyloctahydro-3a,4-dimethyl-
1H-indene-2-carboxylate, (þ)-2. To a solution of ketone 3(0.089 g,
0.46 mmol) in CH2Cl2 (4.5 mL) was added HMDS (292 μL,
0.222 mg, 1.38 mmol) and TMSI (131 μL, 0.184 g, 0.92 mmol)
at -40 °C under inert atmosphere. The light yellow solution
was stirred for 10 min at -40 °C and for 30 min at 0 °C. The
reaction mixture was poured into a mixture of pentane (25 mL)
and a saturated solution of NaHCO3 (10 mL). The organic
phase was separated and the aqueous phase was extracted with
pentane (2 ꢀ 10 mL). The combined organic phase was dried
over anhyd Na2SO4 and the solvent was concentrated by
(2S,3aR,4S,7aR)-Methyl Octahydro-3a,4-dimethyl-1H-indene-
2-carboxylate, (þ)-24. An autoclave charged with (þ)-9 (0.175 g,
0.841 mmol), 10% (w/w) Pd/C (0.009 g, 5% w/w), and anhyd
MeOH (2.7 mL) was purged 3 times with H2. The autoclave was
charged with 2.5 atm of H2. The reaction mixture was stirred for
(19) Ferraboschi, P.; Casati, S.; Grisenti, P.; Santaniello, E. Tetrahedron
1994, 50, 3251.
(20) Naya, K.; Hayashi, M.; Takagi, I.; Nakamura, S.; Kobayashi, M.
Bull. Chem. Soc. Jpn. 1972, 45, 3673.
J. Org. Chem. Vol. 75, No. 9, 2010 2881

Carneiro et al.
JOCArticle
reduced pressure. The residue was filtered through a small
column of silica gel (pentane as eluent) to furnish silyl-enol
ether, after evaporation of pentane. The intermediate was
dissolved in THF (4.5 mL) under inert atmosphere of N2.
MeLi (0.460 mL, 3 M in diethoxymethane, 1.38 mmol) was
added dropwise at -40 °C. After being stirred for 10 min at
-40 °C and for 10 min at 0 °C, methylcyanoformate (120 μL,
128 mg, 1.51 mmol) was added dropwise. The reaction mixture
was stirred for 15 min at -40 °C and for 45 min at 0 °C. The
reaction was quenched with H2O (10 mL) and extracted with
Et2O (25 þ 2 ꢀ 13 mL). The combined organic layer was
washed with a saturated aqueous solution of NaHCO3 (6 mL)
then brine (6 mL), and dried over anhyd Na2SO4. After solvent
evaporation, the crude product was purified by flash chromato-
graphy (hexanes:Et2O, 9:1 as eluent) to afford (þ)-2(64%, 0.074 g,
0.29 mmol) as a colorless oil. Rf 0.55 (10% Et2O/hexanes); IR
NaHCO3 (3 mL), and dried over anhyd MgSO4. The solvent was
removed under reduced pressure, giving (þ)-28 (61%, 0.030 g,
0.13 mmol) as a white solid, which was used in the next step
without purification. Mp 96.0-97.1 °C; IR (KBr) 2959, 2920,
2853, 1794, 1750, 1052 cm-1; 1H NMR (500 MHz, CDCl3) δ 0.81
(3 H, d, J = 6.7 Hz), 0.99 (3H, s), 1.06-1.20 (1H, m), 1.46-1.59
(6H, m), 1.73-1.85 (2H, m), 1.90-2.01 (1H, m), 2.11 (1H, d, J =
13.6 Hz), 2.27-2.30 (1H, m), 4.62 (1H, d, J = 20.2 Hz), 4.69 (1H,
d, J = 20.2 Hz); 13C NMR (75 MHz, CDCl3) δ 16.4, 19.0, 20.7,
23.3, 30.6, 32.3, 39.6, 45.2, 46.8, 47.0, 51.5, 72.4, 179.2, 210.6;
LRMS m/z (%) 236 (Mþ•, 1.0%), 124 (18), 123 (100), 109 (30), 81
(19), 41 (35); [R]25D þ43.1 (c 1.0, CHCl3).
(þ)-Bakkenolide A, (þ)-1. To a stirred solution of Ph3PCH3Br
(0.130 g, 0.36 mmol) in anhyd THF (3 mL) under a strong flow
of N2 was added dropwise NaHMDS (330 μL, 1 M in hexanes,
0.33 mmol) at 0 °C. The mixture was stirred for 45 min at 0 °C
resulting in a yellow solution. A solution of (þ)-28 (0.026 g,
0.11 mmol) in anhyd THF (0.5 mL) was added dropwise. The
resulting light yellow solution was stirred for 1 h at rt. The
reaction was quenched with H2O (10 mL) and extracted with
Et2O (3 ꢀ 10 mL). The organic phase was washed with brine
(5 mL) and dried over anhyd MgSO4. After solvent evaporation,
thecrudeproductwaspurifiedbyflashchromatography(hexanes:
AcOEt, 9:1 as eluent), affording (þ)-bakkenolide A (þ)-1 (62%,
0.016 g, 0.068 mmol) as a white solid. Mp 80.3-80.6 °C (lit.20 mp
1
(film) 2958, 2927, 1745, 1715, 1251, 1235, 1152 cm-1; H NMR
(500 MHz, CDCl3) δ 0.77 (3 H, d, J = 6.5 Hz), 0.91 (3H, s),
1.02-1.10 (1H, m), 1.15-1.21 (1H, m), 1.32-1.37 (1H, m),
1.42-1.46 (2H, m), 1.53-1.56 (2H, m), 1.75 (1H, d, J = 14.0
Hz), 1.87-1.93(1H, m), 2.11-2.16 (1H, m), 2.13(3H, s), 2.30(1H,
t, J = 13.3 Hz), 2.52 (1H, d, J = 14.0 Hz), 3.72 (3H, s); 13C NMR
(125 MHz, CDCl3) δ 16.3, 19.4, 20.9, 23.5, 26.5, 30.5, 32.9,
35.3, 43.5, 44.0, 46.2, 52.6, 64.4, 174.6, 203.7; LRMS m/z (%)
252 (Mþ•, 0.55%), 210 (75), 192 (9), 129 (26), 109 (35), 100 (20),
43 (100); HRMS m/z C15H24O3 (M þ Na)þ calcd 275.1618, found
275.1628; [R]25D þ60.8 (c 1.0, CHCl3).
80.5-80.6 °C); IR (KBr) 2959, 2923, 2853, 1777, 1655, 1021 cm-1
;
1H NMR(500 MHz, CDCl3) δ0.85 (3 H, d, J =6.8Hz), 0.99 (3H,
s), 1.12-1.21 (1H, m), 1.41-1.64 (6H, m), 1.94-1.99 (3H, m),
2.09 (1H, t, J = 13.1 Hz), 2.24-2.30 (1H, m), 4.77 (2H, tq, J =
12.8 and 2.1 Hz), 5.11 (1H, dt, J = 2.3 and 0.7 Hz), 5.03 (1H, dt,
J = 2.0 and 0.7 Hz); 13C NMR (125 MHz, CDCl3) δ 16.3, 19.2,
21.0, 23.3, 30.9, 33.9, 42.3, 44.0, 46.2, 48.5, 49.8, 70.4, 105.8, 150.4,
182.6; LRMS m/z (%) 234 (Mþ•, 1.8%), 124 (58), 122 (34), 109
(87), 111 (52), 95 (20), 41 (100); HRMS m/z C15H22O2 (M þ Na)þ
(20R,3a0R,40S,7a0R)-3a0,40-Dimethyloctahydro-2H-spiro[furan-
3,20-indene]-2,4(5H)-dione, (þ)-28. A solution of (þ)-2 (0.052 g,
0.21 mmol) in anhyd methanol(1.6 mL) was addeddropwisewith
stirring to a cooled solution (0 °C) of KOH (0.070 g, 1.25 mmol)
in anhyd MeOH (1 mL). After stirring for 15 min, DIB (0.133 g,
0.413 mmol) was added in small portions. The reaction mixture
was stirred at 0 °C for 45 min and then a 5% aqueous solution of
H2SO4 (1.3 mL) was added. The mixture was stirred for 35 min at
0 °C, then H2O (3 mL) was added. The mixture was extracted
with CH2Cl2 (20 þ 2 ꢀ 6 mL). The combined organic layer was
washed with brine (5 mL) and dried over anhyd MgSO4. The
solvent was removed under reduced pressure. The crude product
was purified by flash chromatography (hexanes:Et2O, 9:1 as
eluent), affording 27 (0.036 g) as a colorless oil. 27 was dissolved
in CH2Cl2 (1.2 mL) and acidified with TFA (12 μL). The reaction
mixture was stirred overnight at rt. The resulting solution was
dilutedwith CH2Cl2 (10 mL), washed with a saturatedsolution of
calcd 257.1512, found 257.1509; [R]25 þ16.9 (c 0.95, MeOH)
D
{lit.20 [R]20D þ17 (c 1.0, MeOH)}.
Acknowledgment. The authors wish to thank FAPESP,
CNPq, and CAPES for financial support.
Supporting Information Available: Spectroscopic data and
experimental procedures. This material is available free of
charge via the Internet at http://pubs.acs.org.
2882 J. Org. Chem. Vol. 75, No. 9, 2010