Formal total synthesis of (+)-gephyrotoxin

Article abstract of DOI:10.1021/jo801150e

(Chemical Equation Presented) An efficient formal total synthesis of (+)-gephyrotoxin is described. The key step of our strategy relies on the diastereoselective reduction of a chiral pyrrolidine β-enamino ester obtained by condensation of (S)-phenylglycinol on a protected 8-hydroxy-3,6-dioxooctanoate.

Full text of DOI:10.1021/jo801150e

described a shorter access to Kishi’s intermediate 2 (10 steps)
but included a poorly diastereoselective step.
Formal Total Synthesis of (+)-Gephyrotoxin
Marco Santarem, Corinne Vanucci-Bacque´,* and
Ge´rard Lhommet*
UPMC UniV Paris 06, CNRS, UMR 7611, Laboratoire de
Chimie Organique, Equipe Chimie des He´te´rocycles, 4 Place
Jussieu, F-75005 Paris, France
corinne.bacque@upmc.fr; gerard.lhommet@upmc.fr
In his strategy,6 Kishi synthesized derivative 2 via the
enantiopure cis-2,5-disubstituted pyrrolidine 3 that was obtained
in 15 steps from L-pyroglutamic acid. We have recently reported
the preparation of enantiopure cis-2,5-disubstituted pyrrolidine
ꢀ-amino esters8 by the diastereoselective reduction of chiral
ꢀ-enamino esters, which in turn were obtained by condensation
of a chiral amine on an ω-oxo ꢀ-ketoester.9 We envisioned that
we could apply our methodology to access to pyrrolidine 3.
We report herein the results of this work that afforded an
efficient formal synthesis of (+)-gephyrotoxin (1).
ReceiVed May 27, 2008
Retrosynthetically, we anticipated that compound 3 could
stem from the diastereocontrolled reduction of the appropriate
chiral bicyclic pyrrolidine ꢀ-enamino ester 4. The latter could
be obtained by condensation of (S)-2-phenylglycinol on ad-
equately protected 8-hydroxy-3,6-dioxooctanoate 5 (Scheme 1).
An efficient formal total synthesis of (+)-gephyrotoxin is
described. The key step of our strategy relies on the
diastereoselective reduction of a chiral pyrrolidine ꢀ-enamino
ester obtained by condensation of (S)-phenylglycinol on a
protected 8-hydroxy-3,6-dioxooctanoate.
SCHEME 1
Gephyrotoxin is a naturally occurring alkaloid first isolated
and characterized by Daly and co-workers in 1977 from the
skin secretion of tropical frogs Dendrobates histrionicus.1 This
compound is relatively nontoxic and originally exhibited weak
activity as a muscarinic antagonist.2 More recent studies have
shown it to display an interesting array of neurological activi-
ties.3 As a result of these biological activities and the extreme
scarcity of this natural product, the synthesis of gephyrotoxin
and analogues has attracted much interest from several
laboratories.4,5 Among the different approaches, only two
enantioselective syntheses of (+)-gephyrotoxin (1) have been
reported both involving a common enantiopure tricyclic inter-
mediate 2. The first and only total synthesis has been reported
by Kishi and co-workers,6 who prepared intermediate derivative
2 in 18 steps from L-pyroglutamic acid. More recently, a formal
synthesis of this alkaloid performed by Hsung and co-workers7
The most obvious access to our starting compound 5 involves
the alkylation of the Weiler dianion of methyl acetoacetate 610
by suitably functionalized derivatives. However, this approach
turned out to lead to some disappointing results when we first
attempted the condensation of the dianion of 6 with different
compounds such as benzyloxy epoxide 7,11 variously protected
1,3-butanediol iodides 8 (R′ ) Ac,12 TBS, SiEt3), and the cyclic
sulfate ester 913 (Scheme 2). In the case of success, these
reactions would have led to the expected compounds 5 (R)
Bn) after an oxidation step or a deprotection-oxidation
sequence. Unfortunately, in all cases, only starting materials
were surprisingly recovered.14
Funk and co-workers15 reported recently the condensation
of various ꢀ-keto ester dianions on 6-bromomethyl-4H-1,3-
(1) Daly, J. W.; Witkop, B.; Tokuyama, T.; Nishikawa, T.; Karle, I. L. HelV.
Chem. Acta 1977, 60, 1128–1140.
(2) Mensah-Dwumah, M.; Daly, J. W. Toxicon 1978, 16, 189–194.
(3) Daly, J. W.; Spande, T. F. In Alkaloids: Chemical and Biological
PerspectiVes; Pelletier S. W. Ed.; John Wiley and Sons: New York, 1986; Vol.
4, pp 95-122.
(8) Calvet-Vitale, S.; Vanucci-Bacque´, C.; Fargeau-Bellassoued, M.-C.;
Lhommet, G. Tetrahedron 2005, 61, 7774–7782.
(9) David, O.; Calvet, S.; Chau, F.; Vanucci-Bacque´, C.; Fargeau-Bellassoued,
M.-C.; Lhommet, G. J. Org. Chem. 2004, 69, 2888–2891.
(10) Huckin, S. N.; Weiler, L. J. Am. Chem. Soc. 1974, 96, 1082–1087.
(11) Lygo, B. Tetrahedron 1988, 44, 6889–6896.
(4) For previous total syntheses of racemic gephyrotoxin, see: (a) Fujimoto,
R.; Kishi, Y.; Blount, J. F. J. Am. Chem. Soc. 1980, 102, 7154–7156. (b) Hart,
D. J.; Kanai, K. J. Am. Chem. Soc. 1983, 105, 1255–1263. (c) Overman, L. E.;
Lesuisse, D.; Hashimoto, M. J. Am. Chem. Soc. 1983, 105, 5373–5379.
(5) For previous formal syntheses of racemic gephyrotoxin, see: (a) Ito, Y.;
Nakajo, E.; Nakatsuka, M.; Saegusa, T. Tetrahedron Lett. 1983, 24, 2881–2884.
(b) Pearson, W. H.; Fang, W.-k. J. Org. Chem. 2000, 65, 7158–7174.
(6) Fujimoto, R.; Kishi, Y. Tetrahedron Lett. 1981, 42, 4197–4198.
(7) Wei, L.-L.; Hsung, R. P.; Sklenicka, H. M.; Gerasyuto, A. I. Angew.
Chem., Int. Ed. 2001, 40, 1516–1518.
(12) Kwon, D. W.; Kim, Y. H.; Lee, K. J. Org. Chem. 2002, 67, 9488–
9491.
(13) Bennett, C. E.; Figueroa, R.; Hart, D. J.; Yang, D. Heterocycles 2006,
70, 119–128.
(14) This lack of reactivity in particular with epoxide 7 was unexpected as
the ring opening of this compound with the dianion of methyl 2-methylacetoac-
etate has been previously reported.11
(15) Greshock, T. J.; Funk, R. L. J. Am. Chem. Soc. 2002, 124, 754–755.
6466 J. Org. Chem. 2008, 73, 6466–6469
10.1021/jo801150e CCC: $40.75 2008 American Chemical Society
Published on Web 07/19/2008

SCHEME 2
SCHEME 4
SCHEME 5
SCHEME 3
compound 5c with (S)-2-phenylglycinol under the latter reaction
conditions provided the expected oxazolidine 4c (R ) THP) in
62% isolated yield along with 25% recovery of 5c. At this stage,
compound 4c appeared as an inseparable 1:1 mixture of
diastereoisomers according to the NMR spectra. As we assumed
that the condensation was diastereocontrolled at the C-7a center,9
we assigned the epimeric carbon to be localized at C-2 of the
THP group. This assumption was verified by the ultimate
removal of the THP group as shown below. Considering the
latter result, we chose to carry on the synthesis of our target
compound from bicyclic ꢀ-enamino ester 4c.
The key step of our strategy to prepare enantiopure compound
3 relied on the diastereoselective reduction of 4c to give a cis-
2,5-disubstituted pyrrolidine. As we have shown recently,8
reduction using in situ generated sodium triacetoxyborohydride
in acetic acid was considered an adequate method to obtain the
target compound. Indeed, under these reaction conditions, the
epimeric mixture of 4c was diastereoselectively reduced to
pyrrolidine 12 as two epimers in the same equimolar ratio
(Scheme 5). In order to confirm the diastereoselective outcome
of this reduction step, compound 12 was submitted to depro-
tection of the alcohol moiety in the presence of p-TsOH to give
amino alcohol 13 as a single diastereoisomer according to GC
and NMR (de > 95%). On the basis of previously obtained
results,8 the absolute configuration of 13 (and thus that of 12)
was assigned as to be (2S,5R). This stereochemistry was
ultimately determined by the synthesis of compound 2. Subse-
quent hydroxymethylbenzyl group removal of compound 12 (H2,
Pd(OH)2/C) followed by in situ protection in the presence of
Boc2O gave rise to derivative 14 in 92% isolated overall yield
from compound 4c (two steps). Treatment with lithium alumi-
num hydride at 0 °C led to the chemoselective reduction of the
ester moiety to give alcohol 15 in 75% isolated yield. The latter
was subsequently O-benzylated (NaH, BnBr, DMF) to give 16
dioxin 10 as an equivalent of bromomethyl vinyl ketone. As
for us, we envisioned compound 10 as a protected ꢀ-hydroxy
ketone moiety. To our delight, reaction of the dianion of 6 with
compound 10 afforded the expected ꢀ-keto ester 11 in 79%
isolated yield (Scheme 3). While mild thermal retrocyclization
of the 1,3-dioxin would give the corresponding vinyl ketone
ꢀ-keto ester,15 we found that acidic hydrolysis of the 1,3-dioxin
moiety in the presence of p-TsOH and water afforded, as
anticipated, ꢀ-hydroxy keto derivative 5a (R ) H) in 37%
isolated yield (Scheme 3). At this stage, we envisaged to
improve the isolated yield by in situ protection of the alcohol
function. When the above deprotection step was performed in
the presence of TBSCl, a mixture of starting material, alcohol
5a, and unidentifiable products was obtained. On the other hand,
reaction in the presence of acetic anhydride gave rise to acetate
5b in 52% yield. The best result was obtained in the presence
of dihydropyran providing tetrahydropyranyl ether 5c in 83%
yield (Scheme 3).
With the required protected ω-oxo ꢀ-keto esters 5b and 5c
in hand, we turned our attention to their diastereoselective
condensation with (S)-2-phenylglycinol to give the correspond-
ing bicyclic chiral pyrrolidine ꢀ-enamino ester 49 (Scheme 4).
The first attempt to react 5b in the presence of p-TsOH in
benzene as previously described for analogous compounds9 gave
rise to the expected compound 4b (R ) Ac) as a single isomer
in a disappointing 10% yield. A better result was obtained using
16
catalytic amount of Zn(ClO4)2.6H2O and MgSO4 to give
oxazolidine 4b in a still modest 34% yield. Its absolute
configuration at C-7a was assigned as (S) based on previously
obtained results for analogous compounds.9 Condensation of
(16) Noe¨l, R.; Vanucci-Bacque´, C.; Fargeau-Bellassoued, M.-C.; Lhommet,
G. J. Org. Chem. 2005, 70, 9044–9047.
J. Org. Chem. Vol. 73, No. 16, 2008 6467

1
SCHEME 6
1613 cm-1; H NMR (250 MHz, CDCl3) δ 1.53-1.82 (m, 6H),
2.00-2.15 (m, 3H), 2.46-2.57 (m, 1H), 2.97-3.12 (m, 1H),
3.46-3.56 (m, 2 H), 3.59 (s, 3H), 3.74-3.99 (m, 4H), 4.55-4.73
(m, 4H), 7.21-7.38 (m, 5 H); 13C NMR (62.5 MHz, CDCl3) δ
19.5, 19.7, 25.4, 30.7, 32.7, 34.3, 34.5, 35.9, 50.4, 62.3, 62.6, 63.1,
63.4, 63.5, 74.9, 85.1, 98.9, 99.2, 105.4, 105.5, 125.4, 127.7, 129.0,
139.4, 166.6, 166.7, 169.0; HRMS (ESI) m/z calcd for
C22H29NaNO5 (MNa+) 410.1937, found 410.1933.
(2S,5R)-[1-(2-Hydroxy-1(S)-phenylethyl)-5-[2-(tetrahydropyran-
2-yloxy)ethyl]pyrrolidin-2-yl]acetic Acid Methyl Ester (12). A
solution of NaBH(OAc)3 was prepared by portionwise addition of
NaBH4 (0.2 g, 5.3 mmol) to glacial acetic acid (3 mL, 53 mmol)
at 0 °C. After the hydrogen evolution ceased (30 min), a solution
of 4c (0.41 g, 1.05 mmol) in acetonitrile (10 mL) was added. After
being stirred for 48 h at rt, the resulting mixture was quenched
with saturated aqueous NaHCO3 until pH ) 9 and extracted with
CH2Cl2 (3 × 15 mL). The organic layer was washed with water
and brine, dried with Na2SO4, and evaporated under vacuum to
give crude 12 (0.42 g, 1:1 mixture of isomers) as a yellow oil which
was used in the next step without further purification. An analytical
sample was obtained by silica gel column chromatography (ethyl
in 95% isolated yield. Finally, simultaneous tetrahydropyranyl
and Boc deprotections (HCl, MeOH) afforded the target
pyrrolidine 3 in 90% isolated yield. Overall, this compound was
obtained in eight steps from compound 10 in 31% overall yield
to afford a new formal total synthesis of (+)-gephyrotoxin.
However, as compound 3 was not completely described in
the literature,6 we converted it into tricyclic derivative 2, whose
spectroscopic and physical data are known,4a,5a,6 in order to
secure its absolute configuration (Scheme 6). This was ac-
complished following Kishi’s route4a by initial condensation
with cyclohexane-1,3-dione (p-TsOH, PhCH3) to give enamine
17 in 91% isolated yield. Access to tricyclic derivative 18 was
carried out using a slightly modified procedure. The expected
compound 18 was obtained in 96% overall yield by intramo-
lecular cyclization in refluxing acetonitrile in the presence of
NaI using the nonisolated bromide intermediate, which was in
turn directly obtained by reacting alcohol 17 with PBr3 in
CH2Cl2.17 Final hydrogenolysis of 18 (H2, Pd/C, HClO4, MeOH)
gave rise to Kishi’s intermediate 2 in 86% isolated yield. The
spectroscopic data, the physical properties and the optical
1
acetate-cyclohexane, 6:4): IR (neat) 3445, 1731 cm-1; H NMR
(250 MHz, CDCl3) δ 1.18-1.26 (m, 1H), 1.42-1.78 (m, 11H),
2.06-2.11 (m, 1H), 2.33-2.37 (m, 1H), 2.45-2.51 (m, 1H),
3.20-3.28 (m, 1H), 3.43-3.56 (m, 3H), 3.55-3.80 (m, 1H), 3.72
(s, 3H), 3.82-3.94 (m, 4H), 4.58-4.63 (m, 1 H), 7.25-7.35 (m,
5H); 13C NMR (62.5 MHz, CDCl3) δ 19.7, 19.8, 25.6, 30.2, 30.3,
30.7, 30.8, 30.9, 36.4, 43.2, 51.7, 54.3, 54.4, 60.8, 61.0, 62.1, 62.5,
62.6, 64.9, 65.0, 65.1, 65.4, 99.0, 99.3, 128.0, 128.5, 129.0, 137.0,
172.8; HRMS (ESI) m/z calcd for C22H34NO5 (MH+) 392.2431,
found 392.2428.
(2S,5R)-2-Methoxycarbonylmethyl-5-[2-(tetrahydropyran-2-
yloxy)ethyl]pyrrolidine-1-carboxylic Acid tert-Butyl Ester (14). A
solution of crude 12 (0.28 g, 0.71 mmol) in methyl acetate (25
mL) was subjected to hydrogenation (1 atm) in the presence of
Pd(OH)2/C (84 mg) and Boc2O (0.17 g, 0.79 mmol) at rt for 12 h.
The reaction mixture was filtered through Celite and the solvent
removed under vacuum. Purification by silica gel column chroma-
tography (ethyl acetate-cyclohexane, 3:7) afforded 14 (1:1 mixture
of isomers) as a colorless oil (0.24 g, 92% over two steps from
rotation of this compound [[R]20 +537 (c 2.0, EtOH)]
D
corresponded to those reported in the literature6 for the (1S,
3aS)-2 diastereomer [[R]20D +538 (c 1.4, EtOH)], thus validating
our configurational assignments.
1
4c): IR (neat)1737, 1691 cm-1; H NMR (250 MHz, CDCl3) δ
1.39-1.43 (m, 11H), 1.50-1.73 (m, 8H), 1.93-2.11 (m, 3H), 2.28
(dd, J ) 15.0, 10.0 Hz, 1H), 3.35-3.49 (m, 2H), 3.63 (s, 3H),
3.75-3.91 (m, 3H), 4.03-4.09 (m, 1H), 4.48-4.53 (m, 1H); 13C
NMR (62.5 MHz, CDCl3) δ 19.6, 25.5, 27.0, 29.6, 29.9, 30.8, 35.7,
35.9, 37.4, 40.3, 51.6, 54.1, 55.2, 56.5, 56.7, 62.3, 65.1, 65.4, 79.6,
99.0, 154.6, 172.1; HRMS (ESI) m/z calcd for C19H33NO6Na
(MNa+) 394.220, found 394.2200.
In conclusion, we have completed an efficient total formal
synthesis of (+)-gephyrotoxin. Our strategy was based on the
obtention of an enantiopure cis-2,5-disubstituted pyrrolidine by
the diastereoselective reduction of a chiral phenylglycinol-
derived oxazolopiperidine enamino ester. Contrary to the other
reported enantioselective syntheses which both involved one
poorly diastereoselective step, our approach allowed the simul-
taneous creation of the two stereogenic centers of the target
compound in a totally stereocontrolled manner and therefore
compares very favorably to previous approaches.
(2S,5R)-2-(2-Hydroxyethyl)-5-[2-(tetrahydropyran-2-yloxy)eth-
yl]pyrrolidine-1-carboxylic Acid tert-Butyl Ester (15). To a solution
of 14 (1.3 g, 3.66 mmol) in THF (35 mL) at 0 °C was added LiAlH4
(0.34 g, 9.10 mmol). The mixture was stirred at room temperature
for 2 h. The reaction mixture was quenched with saturated aqueous
Na2SO4. Then the reaction mixture was filtered through Celite and
the solvent removed under vacuum. Purification by silica gel column
chromatography (ethyl acetate-cyclohexane, 6:4) afforded alcohol
15 (1:1 mixture of isomers) as a colorless oil (0.9 g, 75%): IR (neat)
Experimental Section
(3S,7aS)-[3-Phenyl-7a-[2-(tetrahydro-pyran-2-yloxy)ethyl]tet-
rahydropyrrolo[2,1-b]oxazol-5-ylidene]acetic Acid Methyl Ester
(4c). To a solution of ꢀ-keto ester 5c (3 g, 10.4 mmol) in CH2Cl2
(100 mL) was added (S)-phenylglycinol (1.44 g, 31.4 mol) followed
by ZnClO4 ·6H2O (0.39 g, 1 mmol) and MgSO4 (0.38 g, 3.1 mmol).
The mixture was stirred at rt for 24 h. The resulting mixture was
filtered and concentrated. Purification by silica gel column chro-
matography (ethyl acetate-cyclohexane, 3:7) afforded the isolation
of 4c (1:1 mixture of isomers) as a yellow oil (2.5 g, 62%) along
with some recovered starting material (0.75 g, 25%): IR (neat) 1695,
1
3441, 1689, 1666 cm-1; H NMR (250 MHz, CDCl3) δ 1.45 (s,
9H), 1.40-1.85 (m, 11H), 1.90-2.30 (m, 3H), 3.34-3.59 (m, 4H),
3.71-3.84 (m, 3H), 4.10-4.25 (m, 1H), 4.40-4.50 (m, 1H),
4.52-4.57 (m, 1H); 13C NMR (62.5 MHz, CDCl3) δ 19.6, 25.5,
28.5, 30.5, 30.8, 36.9, 38.8, 54.8, 56.4, 56.7, 59.1, 62.3, 64.9, 65.1,
80.2, 98.7, 99.0, 156.6; HRMS (ESI) m/z calcd for C18H33NO5Na
(MNa+) 366.2250, found 366.2247.
(2S,5R)-2-(2-Benzyloxyethyl)-5-[2-(tetrahydropyran-2-yloxy)eth-
yl]pyrrolidine-1-carboxylic Acid tert-Butyl Ester (16). To a solution
of 15 (0.16 g, 0.46 mmol) in DMF (15 mL) was added at 0 °C
sodium hydride (74 mg, 1.86 mmol). The mixture was stirred at 0
°C for 1 h. Then, benzyl bromide (62 µL, 0.51 mmol) and
(17) Kishi prepared compound 18 by cyclisation in DMF of the nonisolated
bromide intermediate, which was obtained after mesylation of alcohol 17.6
6468 J. Org. Chem. Vol. 73, No. 16, 2008

tetrabutylammonium iodide (17 mg, 0.046 mmol) were added. After
being stirred for 36 h at rt, the mixture was diluted with ethyl acetate
(10 mL), washed successively with saturated aqueous NH4Cl (10
mL), water (10 mL), and saturated aqueous NaCl (10 mL), dried
with Na2SO4, and concentrated under vacuum. Purification by silica
gel column chromatography (ethyl acetate-cyclohexane, 3:7)
provided 16 (1:1 mixture of isomers) as a colorless oil (0.19 g,
NaOH aqueous solution (2 mL) and saturated aqueous NaCl (5
mL). The combined extracts were dried (Na2SO4) and concentrated
under vacuum. Purification by silica gel column chromatography
(methanol-7 N ammonia in MeOH, 9:1) provided 3 as a colorless
oil (0.12 g, 90%): IR (neat) 3319 cm-1; 1H NMR (250 MHz, CDCl3)
δ 1.45-1.62 (m, 4H), 1.74-1.90 (m, 4H), 3.22-3.35 (m, 1H),
3.36-3.40 (m, 1H), 3.54 (t, J ) 6.2 Hz, 2H), 3.66-3.74 (m, 1H),
3.78-3.85 (m, 1H), 4.44-4.55 (m, 4H), 7.28-7.34 (m, 5H); 13C
NMR (62.5 MHz, CDCl3) δ 30.2, 31.2, 36.4, 36.5, 56.4, 57.8, 61.1,
68.3, 72.8, 127.5, 127.6, 128.3, 138.4; HRMS (ESI) m/z calcd for
1
95%): IR (neat) 1683 cm-1; H NMR (250 MHz, CDCl3) δ 1.45
(s, 9H), 1.50-1.80 (m, 10H), 1.85-2.00 (m, 2H), 2.05-2.15 (m,
2H), 3.38-3.55 (m, 4H), 3.76-3.88 (m, 4H), 4.49-4.50 (m, 2 H),
4.55-4.60 (m, 1H), 7.26-7.34 (m, 5H); 13C NMR (62.5 MHz,
CDCl3) δ 19.7, 25.6, 28.6, 30.2, 30.8, 30.7, 36.1, 56.3, 56.5, 62.2,
62.3, 65.2, 65.5, 68.3, 73.0, 79.2, 98.7, 99.0, 127.6, 127.7, 128.4,
138.6, 154.9; HRMS (ESI) m/z calcd for C25H39NO5Na (MNa+)
456.2720, found 456.2719.
(2R,5S)-2-[5-(2-Benzyloxyethyl)pyrrolidin-2-yl]ethanol (3). To a
solution of 16 (0.23 g, 0.53 mmol) in methanol (6 mL) at 0 °C
was added dropwise acetyl chloride (1.6 mL, 1.6 mmol). The
mixture was stirred at rt for 2 h. The mixture was quenched with
water. Methanol was partially evaporated. The crude mixture was
diluted with CH2Cl2 (10 mL) and washed successively with a 2 M
C15H24NO2 (MH+) 250.1801, found 250.1797; [R]20 +8 (c 2.00,
D
CHCl3).
Supporting Information Available: Experimental proce-
dures and characterization data for compounds 11, 5a-c, 4b,
1
13, 17, 18, and 2. Copies of H and 13C NMR spectra of all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
JO801150E
J. Org. Chem. Vol. 73, No. 16, 2008 6469