Total synthesis of pyrrolizidines 223H', 239K', 265H', and 267H' found in madagascan frogs (Mantella) and their affinities for nicotinic acetylcholine receptor

Article abstract of DOI:10.1016/S0960-894X(00)00221-3

The total synthesis of pyrrolizidines 223H', 239K', 265H' and 267H' has been achieved starting from 1,5-hexadiene via a common synthetic intermediate 5. The affinity of 1-4 for nicotinic acetylcholine receptor was evaluated. (C) 2000 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0960-894X(00)00221-3

Bioorganic & Medicinal Chemistry Letters 10 (2000) 1293±1295
Total Synthesis of Pyrrolizidines 223H0, 239K0, 265H0, and
267H0 Found in Madagascan Frogs (Mantella) and Their
Anities for Nicotinic Acetylcholine Receptor
Hiroki Takahata, a,* Seiki Takahashi, a Nehad Azer, b Amira T. Eldefrawi b
and Mohyee E. Eldefrawi b
aToyama Medical and Pharmaceutical University, Sugitani 2630, Toyama 930-0194, Japan
bDepartment of Pharmacology and Experimental Therapeutics, School of Medicine, University of Maryland at
Baltimore, 655 West Baltimore Street, Bressler Building, Room 4-029, Baltimore, MA 21201, USA
Received 13 March 2000; accepted 5 April 2000
AbstractÐThe total synthesis of pyrrolizidines 223H0, 239K0, 265H0, and 267H0 has been achieved starting from 1,5-hexadiene via a
common synthetic intermediate 5. The anity of 1±4 for nicotinic acetylcholine receptor was evaluated. # 2000 Elsevier Science
Ltd. All rights reserved.
Amphibian skin has provided a wide range of biologi-
cally active alkaloids. During the past 30 years, over 400
alkaloids of over 20 structural classes have been detec-
ted.1 Recently, four 3,5-disubstituted pyrrolizidines
223H0, 239K0, 265H0, and 267H0 were isolated in Mada-
gascan frogs (Mantella)2. Among them, it is known the
relative structure of pyrrolizididine 223H0 is identical
with that of xenovenine found in ant (Solenopsis xenove-
neum).3 So far, the syntheses of the xenovenine has been
reported several times in chiral forms,4 whereas the
syntheses of the other three have never been reported in
both racemic and chiral forms. Among the poison-dart
frog alkaloids, 3,5-disubstituted, 5,8-disubstituted, and
5-substituted indolizidines5 appear to represent an aty-
pical and potent class of noncompetitive blockers for
muscle-type and ganglionic nicotinic receptor-channels.
However, the similar biological activity for homologues
such as 3,5-disubstituted pyrrolizidines has not been
investigated. Accordingly, we were stimulated into the
development of a comprehensive synthetic program for
these alkaloids. In this letter we communicate an asym-
metric synthesis of 3,5-disubstituted pyrrolizidines 223H0,
239K0, 265H0, and 267H0 from trans-2,5-disubstituted
pyrrolidine as a common synthetic intermediate available
from 1,5-hexadiene together with their binding test for
the nicotinic acetylcholine receptor (nAChR) (Chart 1).
A retrosynthetic plan is shown in Scheme 1. Namely,
the stereoselective construction of 3,5-disubstituted
pyrrolizidines could be achieved by the reductive annu-
lation of a ketopyrrolidines II via transient iminium
ions I.4d The pyrrolidines would be transformed from
the common intermediate, trans-2-hydroxymethyl-5-(6-
heptenyl)pyrrolidine (5), prepared by the kinetically
controlled intramolecular amidomercuration4d of a
chiral 4-pentenylcarbamate 6, which would be obtained
from achiral 1,5-hexadiene using the Sharpless asym-
metric dihydroxylation (AD)6 as chiral source.
The synthesis of the pyrrolidine 5 was carried out as
shown in Scheme 2. The mono (DHQD)2-PYR7 ligand-
induced AD reaction of 1,5-hexadiene (7) gave the dihy-
dorxyole®n 8 in 64% yield (80% ee).8 The ole®n 8 was
successively subjected to epoxidation,9 and the regiose-
lective cleavage of the resulting epoxide ring with 6-hex-
enylmagnesium bromide in combination with a cuprous
iodide to give the alcohol 9 in 53% overall yield from 8.
The secondary alcohol was transformed by a four-step
sequence (1. mesylation; 2. azidation; 3. reduction; 4.
carbamation) into the desired N-benzyloxycarbonyl
group (N-Cbz) 6. The unsaturated carbamate underwent
the cyclization mediated by mercuric acetate in THF
followed by treatment with aqueous NaBr to a€ord the
organomercurial, which was oxidatively demercuration
to provide the trans diastereomer 5 in 63% yield with-
out concomitant formation of the cis isomer and the
azocine.
*Corresponding author. Tel.:+81-76-4347531, ext. 2631; fax:+81-
764-344656; e-mail: takahata@ms.toyama-mpu.ac.jp
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00221-3

1294
H. Takahata et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1293±1295
Chart 1.
Scheme 3. (a) (1) (COCl)2/DMSO/Et3N; (2) CH3COCH2PO(OCH3)2/
iPr2EtN/LiCl or n-C3H7COCH2PO(OCH3)2/iPr2EtN/LiCl; (b) H2/cat.
Pd(OH)2; (c) O2/cat. PdCl2/CuCl/DMF/H2O.
Next, we turned our attention to the synthesis of pyr-
rolizidines 239K0 2 and 267H0 4. The installation of
hydroxyl group of the C-3 appendage was performed by
the AD reaction. Since the AD reaction of 10 gave a
mixture of the diol and the tetraol, the reduction of
conjugate ole®n was carried out with Red-Al, in the
presence of CuBr10 to provide the monoole®n 15 in 75%
yield. The (DHQD)2-PYR7 ligand-induced AD reaction
of 15 a€orded the diol 16 in 94% yield, of which selec-
tive monotosylation was performed with a two-step
sequence (1. Bu2SnO 2. p-TsCl) to give the tosylate 17 in
95% yield. Exposure of 17 to hydrogen in the presence
of Pd(OH)2 as a catalyst yield the pyrrolizidine 18,
which, without puri®cation, was reduced with Sup0er-
Hydride1, to the desired pyrrolizidine 235H0 ( )-(6 S-
hydroxy)-211 in 58% yield12 from 17. Similarly, ( )-
(60R)-211 was obtained from 15 via the (DHQ)2-PYR7
ligand-induced AD reaction of 1512 (Scheme 4).
Scheme 1.
Scheme 2. (a) AD-mix-b [(DHQD)2-PYR ligand]; (b) (1) (CH3O)3
CCH3/PPTS; (2) CH3COBr/cat. Et3N; (3) K2CO3/CH3OH/Et2O; (c)
6-hexenyllmagnesium bromide/CuI; (d) (1) MsCl/pyridine; (2) NaN3:
(3) LiAiH4; (4) CbzCl/K2CO3; (e) (1) Hg(OAc)2/THF; (2) NaBr; (3)
NaBH4/O2/DMF.
With the common synthetic intermediate 5 in hand, the
elaboration of ring appendage (hydroxymethyl) was
carried out. The Swern oxidation followed by the Hor-
ner±Emmons reaction using Masamune-Roush's condi-
tion gave the a,b-unsaturated ketons 10 and 11 in 70
and 72% yields, respectively. Exposure of 10 to hydro-
Finally, two pyrrolizidines 267H0 (-)-(60S)-and(-)-(60R)-
411,12 were prepared from 11 shown in Scheme 5.
gen in the presence of Pd(OH)2 as a catalyst in methanol
26
provided the desired ( )-pyrrolizidine 223H0 1 ‰ꢀŠ
d
11.6 (CHCl3), in 59% yield
20
d
10.9 (CHCl3), lit.4d ‰aŠ
With six chiral pyrrolizidines 1±4 in hand, our attention
was directed toward their biologically activity. The
interaction of the six pyrrolizidines 1±4 with binding
sites on carbamylcholine-activated nicotinic acetylcho-
line receptor (nAChR) channel complex from Torpedo
californica electric organ was investigated using radi-
olabeled probe, [3H]-thienyl-cyclohexylpiperidine ([3H]-
along with its C-5 epimer 12 (16%). The spectral data of
1 were in agreement with those reported.4 The Wacker
oxidation of the terminal ole®n on 11 followed by
exposure of the resulting ketone to hydrogen in an
above condition gave (-)-pyrrolizidines 265H0 3 in 55%
yield together with its C-5 epimer 14 (11%) (Scheme 3).
Scheme 4. (a) Red-A1/CuBr; (b) AD-mix-b [DHQD)2-PYR ligand; (c) (1) Bu2SnO; (2) TsCl; (d) H2/cat. Pd(OH)2; (e) Super-Hydride1; (f) AD-mix-
a [(DHQ)2-PYR ligand].

H. Takahata et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1293±1295
1295
Acknowledgements
We are grateful to Professor Kinzo Matsumoto,
Toyama Medical and Pharmaceutical University, for
many valuable discussions.
Scheme 5. (a) Red-A1/CuBr; (b) AD-mix-b [DHQD)2-PYR ligand;
(c) (1) Bu2SnO; (2) TsCl; (d) H2/cat. Pd(OH)2; (e) Super-Hydride1; (f)
AD-mix-a [(DHQ)2-PYR ligand].
References and Notes
1. (a) Daly, J. W. J. Nat. Prod. 1998, 61, 162. (b) Daly, J. W.
In The Alkaloids; Cordell, G. A., Ed.; Academic Press: San
Diego, 1998, Vol. 50, pp 141±169. (c) Daly, J. W.; Garra€o, H.
M.; Spande, T. F. In The Alkaloids; Cordell, G. A., Ed.; Aca-
demic Press: San Diego, 1993, Vol. 43, pp 185±288.
2. Garra€o, H. M.; Caceres, J.; Daly, J. W.; Spande, T. F. J.
Nat. Prod. 1993, 56, 1016±1038.
Table 1. Evaluation of the anities of 1±4 for the nAChR of Torpedo
californica14
Compounds
1
(60S)-2 (60R)-2
3.3 8.3
3
(60S)-4 (60R)-4 20
21
Ki, mM
0.05
0.83
3.1 3.1 0.42 0.37
3. Jones, T. H.; Blum, M. S.; Fales, H. M.; Thompson, C. R.
J. Org. Chem. 1980, 45, 4778.
4. (a) Arredondo, V. C.; Tian, S.; McDonard, F. E.; Marks, T.
J. J. Am. Chem. Soc. 1999, 121, 3633. (b) Dhimane, H.;
Vanucci-Bacque, C.; Hamon, L.; Lhommet, G. Eur. J. Org.
Chem. 1998, 1955. (c) Oppolzer, C. G.; Bochet, E.; Meri®eld,
E. Tetrahedron Lett. 1994, 35, 7015. (d) Takahata, H.; Ban-
doh, H.; Momose, T. J. Org. Chem. 1992, 57, 4401.
5. (a) Aronstam, R. S.; Daly, J. W.; Spande, T. F.; Nar-
ayanan, T. K.; Albuquerque, E. X. Neurochem. Res. 1986, 11,
1227. (b) Daly, J. W.; Nishizawa, Y.; Padgett, W. L.;
Tokuyama, T.; Smith, A. L.; Holmes, A. B.; Kibayashi, C.;
Aronstam, R. S. Neurochem. Res. 1991, 16, 1213. (c) Taka-
hata, H.; Kubota, M.; Ihara, K; Okamoto, N.; Momose, T.;
Azer, N.; Eldefrawi, A. T.; Eldefrawi, M. E. Tetrahedron
Asymmetry 1998, 9, 3289.
Chart 2.
Chart 3.
6. Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B.
Chem. Rev. 1994, 94, 2483.
7. Crispino, G. A.; Jeong, K.-S.; Kolb, H. C.; Wang, Z.-M.;
Xu, D.; Sharpless, K. B. J. Org. Chem. 1993, 58, 3785.
8. Takahata, H.; Takahashi, S.; Kouno, S.; Momose, T. J.
Org. Chem. 1998, 63, 2224.
TCP).13 The Ki values for inhibition of [3H]-TCP by 1±
4, compared to those of 3,5-disubstituted indolizidines
20 and 21,5a are shown in Table 1. Interestingly, anity
of 1 was increased one order compared with those of the
corresponding indolizidines. Since the introduction of a
hydroxyl moiety in a side chain such as 2 and 4 remark-
ably decreases anity, the structure±activity relation-
ships suggest an important contribution of hydrophobic
interactions. As a result, stereocon®guration of hydroxyl
had little e€ect on ion channel interactions (Chart 2).
9. Kolb, H. C.; Sharpless, K. B. Tetrahedron 1992, 48, 10515.
10. Sommelhack, M. F.; Sta€er, R. D.; Yamashita, A. J. Org.
Chem. 1977, 42, 3180.
11. The diastereomeric excess (de) of (60S)-2 and (60R)-2 was
1
estimated to be 71 and 64% by H NMR observation using
bis-(+)-MTPA ester 22 and 23, respectively. Similarly, the des
of (60S)- and (60R)-4 were estimated to be 73 and 68%,
respectively, see Chart 3.
12. Although C-5 epimer of 4 may exist in a reaction mixture,
we could not isolate that.
13. Katz, E. J.; Cortes, V. I.; Eldefrawi, M. E.; Eldefrawi, A.
T. Toxicol. Appl. Pharmacol. 1997, 146, 227.
14. Biological test method is referenced in 5c.
In summary, the total synthesis of six 3,5-disubstituted
pyrrolizidines 1, (60S)-2, (60R)-2, 3, (60S)-4, and (60R)-4
has been asymmetrically achieved starting from a sym-
metrical 1,5-hexadiene. Their anities for nAChR
channel of T. californica were for the ®rst time evaluated.