Synthesis and muscarinic M2 subtype antagonistic activity of unnatural ent-himbacine and an enantiomeric pair of (2′S,6′R)-Diepihimbacine

Article abstract of DOI:10.1016/S0960-894X(02)00566-8

The title three compounds ent-1, 6, and ent-6 were synthesized by coupling the chiral sulfone 4 or ent-4 with the chiral piperidinaldehyde 5 or ent-5, which were readily prepared following the synthetic routes previously established by the novel total syn

Full text of DOI:10.1016/S0960-894X(02)00566-8

Bioorganic & Medicinal ChemistryLetters 12 (2002) 2871–2873
Synthesis and Muscarinic M2 Subtype Antagonistic Activity of
Unnatural ent-Himbacine and an Enantiomeric Pair of
(20S,60R)-Diepihimbacine
Masanori Takadoia,* and Shiro Terashimab
aDiscovery Research Laboratories, Kyorin Pharmaceutical Company Ltd., 2399-1 Nogi, Nogi-machi, Tochigi 329-0114, Japan
bSagami Chemical Research Center, 2743-1 Hayakawa, Ayase, Kanagawa 252-1193, Japan
Received 13 June 2002; accepted 15 July2002
Abstract—The title three compounds ent-1, 6, and ent-6 were synthesized by coupling the chiral sulfone 4 or ent-4 with the chiral
piperidinaldehyde 5 or ent-5, which were readilyprepared following the synthetic routes previouslyestablished bythe novel total
synthesis of natural himbacine 1. Their muscarinic M2 subtype binding affinity was evaluated in comparison to that of 1, disclosing
that the stereochemistryof both the tricyclic moietyand the piperidine part of 1 plays crucial roles in its potent activity.
# 2002 Elsevier Science Ltd. All rights reserved.
Himbacine 1 is a piperidine alkaloid isolated from the
bark of Galbulimima baccata, which is a species of the
magnolia family.1 It has been reported that 1 behaves as
a potent antagonist of the muscarinic receptor of the M2
subtype with a 10–20-fold selectivity toward the M1
receptor.2 Accordingly, it is anticipated that 1 is one of
the potential candidates or promising lead compounds
for the treatment of Alzheimer’s disease, because its
potent M2 subtype antagonistic activity can shut down
the negative feedback mechanism of presynaptic recep-
tors, therebyincreasing the acetylcholine levels in the
synapses.3,4
because various structural types of furan, chiral furan-
2(5H)-one, and aldehyde derivatives can be employed as
starting materials or reagents. Therefore, we next
focused on elucidating the relationships between the
stereochemistryof 1 and its antagonistic activity. We
wish to report here on the synthesis of unnatural ent-him-
bacine ent-1 and an enantiomeric pair of (20S,60R)-diepi-
himbacine 6 and ent-6, and their binding affinityagainst
the muscarinic receptor of the M2 subtype (Fig. 1).8 The
latter two congeners 6 and ent-6 were designed with the
aim of exploring whether the stereochemistryof the tri-
cyclic moiety or that of the lower piperidine part is
crucial for the antagonistic activity.
Recently, we have reported a novel total synthesis of 1
employing a highly stereoselective intermolecular Diels–
Alder reaction.5 The outline of our synthetic route to 1
is the construction of the tricyclic lactone moiety by
means of the intermolecular Diels–Alder reaction of the
furan derivative 2 and the chiral furan-2(5H)-one 3, and
the subsequent connection of the chiral sulfone 4 with
the chiral piperidinaldehyde 5, according to the Hart’s
procedure featuring the Julia–Lythgoe olefination reac-
tion (Scheme 1).6 Our methodologyis considered to be
more convergent and flexible than other methods so far
reported,7 especiallyfor synthesizing the congeners of 1,
The synthesis of ent-19 was achieved uneventfully,
starting with 2 and ent-3, and followed bythe connec-
tion of ent-48 and ent-58 according to the same synthetic
*Corresponding author. Tel.: +81-280-56-2201; fax: +81-280-57-
1293; e-mail: masanori.takadoi@mb2.kyorin-pharm.co.jp
Scheme 1.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00566-8

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M. Takadoi, S. Terashima / Bioorg. Med. Chem. Lett. 12 (2002) 2871–2873
Table 1. In vitro muscarinic M1 and M2 receptors binding affinityof
novel himbacine congeners
EntryCompd
ꢀlogKi
M2 (brainstem)
M1 (cortex)
1
2
3
4
1
7.2
6.0
6.5
6.5
8.0
6.1
6.7
6.7
ent-1
6
ent-6
Figure 1.
veryweak binding affinitywith the same level against
the muscarinic M1 and M2 subtype. Accordingly, it
appears evident that the stereochemistryof both the
tricyclic moiety and the piperidine part of 1 plays
important roles for its strong muscarinic M2 binding
affinity.
In conclusion, we have succeeded in synthesizing the
novel himbacine congeners ent-1, 6, and ent-6 by
employing the synthetic Scheme which was previously
established byour total synthesis of natural 1. From the
results of the muscarinic M2 subtype binding affinity
assayof these congeners, it was discovered that the ste-
reochemistryof both the tricyclic moietyand the piper-
idine part of 1 is crucial for its potent antagonistic
activity.
Scheme 2. (a) (i) nBuLi, DME, ꢀ78 ꢁC, 2 h; (ii) benzoyl chloride,
ꢀ78 ꢁC, 0.5 h; (iii) 3-(dimethylamino)propylamine, 0 ꢁC, 0.5 h; (b) 5%
Na-Hg, Na2HPO4, MeOH, rt, 2.5 h, 33% (four steps) (recoveryof 4:
51%); (c) Jones reagent, acetone, rt, 1 h, 68%; (d) trifluoroacetic acid,
CH2Cl2, rt, 1.5 h, 91%; (e) 37% HCHO aq, NaBH3CN, CH3CN, rt,
0.5 h, 73%.
Acknowledgements
We are grateful to Dr. H. Ohkubo and Dr. T. Ishizaki,
Kyorin Pharmaceutical Co. Ltd., for their many valu-
able suggestions and encouragement. We would also
like to thank Dr. Y. Fukuda, Kyorin Pharmaceutical
Co. Ltd., for his discussions with us and for his helpful
suggestions. The in vitro binding activityassaywas
performed byDr. T. Kawai, Koyrin Pharmaceutical
Co. Ltd., to whom our thanks are also due.
Scheme employed in the preparation of 1. Next, in order
to produce 6, the Julia–Lythgoe coupling reaction of 4
and ent-5 was examined. However, being different from
the cases for the synthesis of 1 and ent-1, the starting
material 4 was fullyrecovered when the reaction was
quenched byadding water. It seems likelythat the retro-
aldol type reaction might occur during quenching with
water, probablydue to the decreased stabilityof the in
situ formed lithium aldolate or the aldol type product.
After much experimentation, it was finallyfound that
quenching the reaction of 4 and ent-5 bythe addition of
excess benzoyl chloride successfully gave the desired
benzoates 7 as a diastereomeric mixture, after removal
of the excess benzoyl chloride with 3-(dimethylamino)-
propylamine (Scheme 2). Without separation, the reac-
tion mixture was directlysubjected to reductive
elimination, giving rise to (E)-olefin 8 in a 33% yield
from 4, along with a 51% recoveryof 4. According to
the same synthetic procedure as for 1, 8 was converted
to the target compound 69 in a three step sequence
involving oxidation of the hemiacetal moiety, deprotec-
tion of the N-Boc group, and reductive N-methylation.
The enantiomer of 6, ent-6,9 was also synthesized
employing ent-4 and 5 in the same manner as described
for the synthesis of 6.
References and Notes
1. (a) Brown, R. F. C.; Drummond, R.; Fogerty, A. C.;
Hughes, G. K.; Pinhey, J. T.; Ritchie, E.; Taylor, W. C. Aust.
J. Chem. 1956, 9, 283. (b) Pinhey, J. T.; Ritchie, E.; Taylor,
W. C. Aust. J. Chem. 1961, 14, 106.
2. (a) Miller, J. H.; Aagaard, P. J.; Gibson, V. A.; McKinney,
M. J. Pharmacol. Exp. Ther. 1992, 263, 663. (b) McKinney,
M.; Miller, J. H.; Aagaard, P. J. J. Pharmacol. Exp. Ther.
1993, 264, 74. (c) Doods, H. N.; Quirion, R.; Mihm, G.; Engel,
W.; Rudolf, K.; Entzeroth, M.; Schiavi, G. B.; Ladinsky, H.;
Bechtel, W. D.; Ensinger, H. A.; Mendla, K. D.; Eberlein, W.
Life Sci. 1993, 52, 497.
3. (a) Perry, E. K. Br. Med. Bull. 1986, 42, 63. (b) Review for
muscarinic receptors: Broadley, K. J.; Kelly, D. R. Molecules
2001, 6, 142.
4. (a) McKinney, M.; Coyle, J. T. Mayo Clinic Proc. 1991, 66,
1225. (b) Iversen, L. L. Trends Pharmacol. Sci. Suppl. 1986, 44.
5. Takadoi, M.; Katoh, T.; Ishiwata, A.; Terashima, S. Tet-
rahedron Lett. 1999, 40, 3399.
With the synthetic targets ent-1, 6, and ent-6 in hand,
their muscarinic M1 and M2 subtype binding affinity
10
assaywas evaluated. The results are shown in Table 1
along with that of 1. Being different from 1, all the tes-
ted compounds ent-1, 6, and ent-6 were found to show
6. (a) Julia, M.; Paris, J.-M. Tetrahedron Lett. 1973, 49, 4833.
(b) Kocienski, P. J.; Lythgoe, B.; Waterhouse, I. J. Chem.
Soc., Perkin Trans. 1 1980, 1045.

M. Takadoi, S. Terashima / Bioorg. Med. Chem. Lett. 12 (2002) 2871–2873
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7. (a) Hart, D. J.; Wu, W.-L.; Kozikowski, A. P. J. Am.
Chem. Soc. 1995, 117, 9369. (b) Hart, D. J.; Li, J.; Wu, W.-L.;
Kozikowski, A. P. J. Org. Chem. 1997, 62, 5023. (c) Chack-
alamannil, S.; Davies, R. J.; Asberom, T.; Doller, D.;
Leone, D. J. Am. Chem. Soc. 1996, 118, 9812. (d) Chack-
alamannil, S.; Davies, R. J.; Wang, Y.; Asberom, T.; Doller,
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64, 1932.
8. The starting materials, 4, ent-4, 5, and ent-5 were synthe-
sized from the opticallypure starting materials, respectively,
according to our reported procedures.5 The specific rotations
of 4, ent-4, 5, and ent-5 are as follows: 4, [a]2D0 +104ꢁ (c 0.35,
CHCl3) [lit.7b [a]2D0 +100ꢁ (c 0.35, CHCl3)]; ent-4, [a]2D2
ꢀ104ꢁ (c 0.35, CHCl3); 5, [a]D26 +120ꢁ (c 1.03, CHCl3)
[lit.7b [a]2D0 +122ꢁ (c 0.96, CHCl3)]; ent-5, [a]2D2 ꢀ128ꢁ (c
0.82, CHCl3).
0.40, CHCl3). All compounds were characterized by 1H and
13C NMR, IR, and Mass spectroscopic methods.
10. Binding assay: the receptor binding analysis for the mus-
carinic receptor of the M1 and M2 subtypes was performed
using homogenates of the cerebral cortex and brainstem of a
rat, respectively. The radioligands used were [3H]-pirenzepine
for the cerebral cortex and [3H]-quinuclidinyl benzilate (QNB)
for the brainstem, respectively. The homogenates were incu-
bated in a 50 mM Tris–buffer (pH 7.4) at 25 ꢁC for 90 min,
and rapidlyfiltrated on Whatman GF-B filters. The radio-
activities were counted using a liquid scintillation counter. Non-
specific binding was defined in the presence of 2 mM atropine. The
test compounds were dissolved in DMSO and diluted with buffer
to final concentrations. The competition binding experiments
were performed in the presence of less than 0.1% DMSO, which
did not affect the specific binding. The equilibrium dissociation
constants (Ki) were calculated using the Cheng–Prusoff equation,
Ki=IC50/(1+L/Kd), where L and Kd were the concentration and
the dissociation constant of radioligand, respectively. The Kd
values were determined bya Scatchard analysis.
9. The physical data on ent-1, 6, and ent-6 are as follows: ent-
1, mp 128–130 ꢁC, [a]D23 ꢀ59ꢁ (c 0.29, CHCl3) [1,5 mp 127–
128 ꢁC, [a]2D4 +55ꢁ (c 0.21, CHCl3)]; 6, mp 136–138 ꢁC, [a]D24
+6.6ꢁ (c 0.10, CHCl3); ent-6, mp 135–137 ꢁC, [a]2D1 ꢀ7.0ꢁ (c