Himbacine analogs as muscarinic receptor antagonists - Effects of tether and heterocyclic variations

Article abstract of DOI:10.1016/j.bmcl.2004.05.047

A number of analogs of the natural product himbacine were synthesized employing variations at the heterocyclic unit and the tether that links the heterocyclic unit to the tricyclic motif. Several of these analogs had M ₂ affinity and M₁/M₂ selectivity comparable to those of himbacine. The structural and stereochemical requirements of the heterocyclic unit for muscarinic binding are discussed in the light of these data.

Full text of DOI:10.1016/j.bmcl.2004.05.047

Bioorganic & Medicinal Chemistry Letters 14 (2004) 3967–3970
Himbacine analogs as muscarinic receptor antagonists––effects
of tether and heterocyclic variations
*
ꢀ
Samuel Chackalamannil, Darıo Doller, Robert McQuade and Vilma Ruperto
Schering-Plough Research Institute, 2015 Galloping, Hill Rd, Kenilworth, NJ 07033, USA
Received 2 April 2004;accepted 21 May 2004
Available online 17 June 2004
Abstract—A number of analogs of the natural product himbacine were synthesized employing variations at the heterocyclic unit and
the tether that links the heterocyclic unit to the tricyclic motif. Several of these analogs had M2 affinity and M1/M2 selectivity
comparable to those of himbacine. The structural and stereochemical requirements of the heterocyclic unit for muscarinic binding
are discussed in the light of these data.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
himbacine have only yielded compounds with weaker
binding affinity.5 Similarly, ent-himbacine and a him-
bacine diastereomer possessing a piperidine ring with
absolute chirality opposite to that of natural himbacine
have been shown to be far less active suggesting the
importance of the absolute and relative stereochemistry
of the tricyclic and heterocyclic units for biological
activity.6 We have reported on the preliminary efforts of
varying the substituted piperidine moiety of
dihydrohimbacine using a parallel synthesis approach.7
However, there has been no report of replacing the
piperidine ring of himbacine itself with other hetero-
cycles and incorporating tether constraints. We wish to
Himbacine (1),1;2 a complex piperidine alkaloid isolated
from Galbulimima baccata, continues to attract consid-
erable interest as a potent muscarinic receptor antago-
nist.3 Blockade of pre-synaptic muscarinic M2 receptor
is a potentially promising anticholinergic approach for
the treatment of Alzheimer’s disease.4 The complex
structural features of himbacine have rendered a thor-
ough investigation of its structure–activity relationship
properties difficult. Therefore, structure–activity rela-
tionship information in this series has been slow forth-
coming. Attempts to simplify the tricyclic motif of
O
O
O
H
H
O
H
H
H
H
H
H
H
H
H
H
H
H
O
O
O
O
H
R
H
R
Me
Me
Me
Me
Me
R
H
H
H
H
R1
N
N
N
N
Me
Me
Me
Me
2 R = H, R1 = Me
Dihydrohimbacine
3 R, R1 = H
6 Isohimbacine
4 R = Me N-Methyl
himandravine
5 R = H Himandravine
1 R = Me Himbacine
Dihydrohimbeline
7 R = OH, R1 = Me
Hydroxyhimbacine
* Corresponding author. Tel.: +1-908-740-3474;fax: +1-908-740-7152;e-mail: samuel.chackalamannil@spcorp.com
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.05.047

3968
S. Chackalamannil et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3967–3970
HOOC
O
LnTi
PPh2
11
EDCI, DMAP,
CH2Cl2, TEMPO
(0.1%) 75%
HO
OTBS
CHO
1.
9
O
2. MeI 50%
10
Me
12
3. TFA-MeOH (1%, V/V)
8
O
O
O
H
O
N
H
O
H
H
O
H
H2, Ra-Ni,
PtO2
14
H
H
O
Me
MeI
O
H
H
O
1. Toluene, 190˚C
2. DBU
Me
90%
95%
H
Toluene, 135˚C
45%
H
Me
R
60%
N
N
13
I
16 R = Me
15
O
O
H
H
H
H
O
O
O
H
H
H
H
H
H
H
SOCl2,
O
Me
PhH, 50˚C
H2, PtO2, MeOH
90%
H
H
Y
Me
Me
R
75%
N
R
R
N
N
19 R = Me
20 R = Et
18 R = Me
17 R = Me, Y = OH
O
O
H
H
H
H
O
O
H
H
Me
Me
Me
Me
1. H2, Ra-Ni
MeI
16
R
93%
2. SOCl2, PhH
50˚C, 85%
N
N
I
23
21 R = Me
22 R = Et
Scheme 1.
report here the synthesis and antimuscarinic activities of
dimethyl pyrrolidinyl analogs of himbacine as well as
tether modified himbacine analogs.
12. Exo-selective intramolecular Diels–Alder reaction
was carried out under the previously reported conditions
followed by brief in situ treatment with DBU to give the
tricyclic alkene derivative 13. 1,3-Dipolar cycloaddi-
tion10 of commercially available nitrone 14 with the al-
kene 13 gave the isoxazolidine derivative 15 as the major
product.11 N-Methylation of the isoxazolidine 15 fol-
lowed by reduction using a 1:1 mixture of Raney nickel
and platinum oxide gave the fully saturated amino
alcohol 17. Dehydration of amino alcohol 17 using
thionyl chloride gave the exocyclic alkene 18 as a single
geometric isomer,12 which was catalytically reduced to
the nor-dihydrohimbacine 19 as an equimolar mixture
of diastereomers. Alternatively, Raney nickel mediated
reduction of isoxazolidinium salt 16 followed by thionyl
chloride induced dehydration gave the diene 21.
The syntheses of himbacine (1), dihydrohimbacine (2),
himandravine (5), isohimbacine (6), and hydroxy-
himbacine (7) have been reported earlier.2a;7 Dihydro-
himbeline (3) was generated from the previously
reported himbeline2a by catalytic reduction of the pen-
dent double bond. N-Methyl himandravine (4) was
generated from himandravine (5)2a by reductive N-
methylation as reported in the conversion of himbeline
to himbacine (1).
The synthesis of the dimethylpyrrolidine analogs of
himbacine was achieved using a [3+2] nitrone cycload-
dition to attach the pyrrolidinyl unit to the previously
reported tricyclic alkene 13 (Scheme 1). A novel syn-
thesis of tricyclic alkene 13 was achieved starting from
optically pure O-TBS-lactaldehyde (8).8 Condensation
of (diphenylphosphino)allyltitanium reagent 9 with
aldehyde 8 followed by treatment with methyl iodide
and subsequent O-deprotection gave the Z-diene 10,9
which was esterified with dienoic acid 11 to give the ester
2. Results and discussion
Binding assays were conducted on cloned human mus-
carinic receptors as described previously.7 As reported
before,7 dihydrohimbacine (2), which carries a saturated

S. Chackalamannil et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3967–3970
3969
two-carbon linker in place of the corresponding trans-
olefin of himbacine (1), is equipotent to himbacine with
comparable M1/M2 selectivity. Dihydrohimbeline (3),
which lacks the N-methyl group of dihydrohimbacine
(2) is fourfold less active toward the M2 receptor, sug-
gesting the importance of the N-methyl substituent on
piperidine for activity. N-methylhimandravine (4) is a
diastereomer of himbacine, being epimeric at the
piperidine carbon carrying the tether. This compound
was inactive against the M2 and M1 receptors, suggest-
ing that the stereochemistry of the substituted piperidine
moiety is important for activity. Isohimbacine (6)
maintained the affinity for the muscarinic receptors
suggesting that the piperidine moiety in isohimbacine
can adopt a binding conformation similar to that of
himbacine in spite of the presence of the exocyclic
double bond. The dimethylpyrrolidinyl series showed a
similar spectrum of activity to that of the himbacine
series. For example, compound 19, which is a direct
analog of dihydrohimbacine (2), showed a Ki value of
12.2 nM against the M2 receptor with a 10-fold M1/M2
selectivity. The corresponding N-ethyl analog 20 was
essentially inactive suggesting the steric limitations at
the binding site of the basic nitrogen. The exocyclic
olefin derivative 18, which is a close analog of iso-
himbacine (6), maintained a similar binding profile.
Presence of an extra double bond in the middle ring of
the tricyclic unit did not considerably alter the binding
profile as indicated by the activity of compound 21. The
isoxazolidine derivative 15 was inactive. The quaternary
ammonium salt 23 was also far less active than 21 sug-
gesting the importance of the basic nitrogen (Table 1).
reduced the muscarinic binding affinity. The absolute
and relative stereochemistry of the substituted hetero-
cyclic amine is important for muscarinic activity. Both
in the himbacine series and the pyrrolidinyl series, a
trans-disubstitution pattern adjacent to the nitrogen
seems to be important. In the dimethylpyrrolidinyl ser-
ies, one could envision a trans-relationship between one
of the gem-dimethyl groups and the two-carbon tether,
which is a plausible explanation for the retention of
affinity. Also important is the (R)-absolute configuration
at the tether-bearing piperidine a-carbon. N-Methyl-
himandravine (4), which has an (S)-configuration at the
tether-bearing a-carbon and a cis-methyl substituent,
was inactive. A diastereomer of himbacine with (S)-
configuration at the tether-bearing piperidine 10-carbon
and a trans-methyl substituent at the 60 position has
been reported to be far less active.6 While a two-carbon
tether is tolerated, steric and polar effects introduced by
the presence of a hydroxyl group or an isoxazolidine
group eliminate activity. The fact that isohimbacine 6
and its dimethylpyrrolidinyl analog 18 maintain the M2
activity and selectivity suggests that this double bond
geometry facilitates the binding conformation of the
molecule.
In summary, this study strongly suggests the following
requirements of himbacine derivatives for selective M2
binding. First, an N-methyl piperidinyl or pyrrolidinyl
subunit having an (R)-configuration at the tether-bear-
ing a-carbon and a trans-alkyl substituent at the a0-
carbon is necessary. Secondly, the two-carbon tether can
be saturated or it can incorporate a double bond with
specific geometry as in himbacine (1) or isohimbacine
(6), but substitution of the carbon tether with a hydroxyl
group or introduction of steric constraints as in the
isoxazolidine derivative 15 reduces the muscarinic
activity.
The following conclusions can be drawn from the above
observations. A basic tertiary amine such as the one
present in himbacine (1), and the dimethyl pyrrolidinyl
analog 19 is necessary for antimuscarinic activity.
Quaternary ammonium salt 23 was less active and
selective. The binding is sensitive to the steric environ-
ment of the basic nitrogen. N-Methyl substituent seems
to be optimal. Bulkier N-alkyl groups (e.g., 20 and 22)
Acknowledgements
Table 1. In vitro M2 and M1 inhibitory activity on cloned human
muscarinic receptorsa
The authors like to acknowledge Drs. William Greenlee,
Ashit Ganguly, Michael Czarniecki, and John Clader
for helpful discussions. We also like to thank Dr. P. Das
and B. Pramanik for mass spectral data, and Dr. M.
Puar for NMR analysis.
Compound number
Ki (nM)
M1
M2
1
4.5
4.3
48
32
68
2
3
16
>1400
>1400
4
>1400
>1400
5
References and notes
6
13.3
65.9
7
203
>1400
180
1567
>1400
790
1. (a) Pinhey, J. T.;Ritchie, E.;Taylor, W. C. Aust. J. Chem.
1961, 14, 106;(b) 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;(c) Ritchie, E.;
Taylor, W. C. In The Alkaloids;Manske, R. H. F., Ed.;
Academic: New York, 1967;Vol. 9, p 529.
2. (a) Chackalamannil, S.;Davies, R. J.;Wang, Y.;Asb-
erom, T.;Doller, D.;Wong, J.;Leone, D. J. Org. Chem.
1999, 64, 1932;(b) Takadoi, M.;Katoh, T.;Ishiwata, A.;
Terashima, S. Tetrahedron Lett. 1999, 40, 3399;(c) Hart,
D. J.;Li, J.;Wu, W.-L.;Kozikowski, A. P. J. Org. Chem.
15
16
17
18
19
20
21
22
23
>1400
43
>1400
418
122
12.2
449
68
811
177
>1400
571
640
446
a See Ref. 7 for assay conditions.

3970
S. Chackalamannil et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3967–3970
1997, 62, 5023;(d) Wong, L. S.-M.;Sharp, L. A.;Xavier, 901. In Figure 1 of this reference the M1 and M2 values
N. M. C.;Turner, P.;Sherburn, M. S. Org. Lett. 2002, 4,
1955;(e) Baldwin, J. E.;Chesworth, R.;Parker, J. S.;
Russell, A. T. Tetrahedron Lett. 1995, 36, 9551;(f)
Hofman, S.;De Baecke, G.;Kenda, B.;De Clercq, P. J.
Synthesis 1998, 479.
were inadvertently transposed.
8. Cainelly, G.;Giacomini, D.;Perciacante, F.;Trer
Tetrahedron: Asymmetry 1994, 5, 1913.
9. Ukai, J.;Ikeda, Y.;Ikeda, N.;Yamamoto, H. Tetrahedron
Lett. 1983, 24, 4029.
ꢁ
e, A.
3. (a) Takadoi, M.;Yamaguchi, K.;Terashima, S. Bioorg.
Med. Chem. 2003, 11, 1169;(b) Gao, L.-J.;Waelbroeck,
M.;Hofman, S.;Van Haver, D.;Milanesio, M.;Viterbo,
D.;De Clercq, P. J. Bioorg. Med. Chem. Lett. 2002, 12,
1909;(c) Takadoi, M.;Yamaguchi, K.;Terashima, S.
Bioorg. Med. Chem. Lett. 2002, 12, 3271.
10. Structural characterization of isoxazolidine 15 was carried
out by spectroscopic means (1H NMR, 13C NMR, and
FAB MS) and by comparison to the spectroscopic data for
the corresponding trans-6-methylpiperidine analog the
structure of which was confirmed by X-ray crystallo-
graphic analysis.2a
4. (a) Heardown, M. J. Expert Opin. Therap. Patents 2002,
12, 863;(b) Clader, J. W. Curr. Opin. Drug Discov. Dev.
1999, 2, 311;(c) Doods, H. N. Drugs Fut. 1995, 20, 157.
5. (a) Kozikowski, A. P.;Fauq, A. H.;Miller, J. H.;
McKinney, M. Bioorg. Med. Chem. Lett. 1992, 2, 797;
(b) Malaska, M. J.;Fauq, A. H.;Kozikowski, A. P.;
Aagaard, P. J.;McKinney, M. Bioorg. Med. Chem. Lett.
1993, 3, 1247;(c) Malaska, M. J.;Fauq, A. H.;Kozikow-
ski, A. P.;Aagaard, P. J.;McKinney, M. Bioorg. Med.
Chem. Lett. 1995, 5, 61.
11. (a) Torsell, K. B. G. In Nitrile Oxides, Nitrones, and
Nitronates in Organic Synthesis;Feuer, H., Ed.; Organic
Nitro Chemistry Series;VCH: New York, 1988;(b)
Padwa, A.;Schoffstall, A. M. In Advances in Cycloaddi-
tion;Curran, D. P., Ed.;Jai Press: Greenwich, Connect-
icut, 1990;Vol. 3, p 1;(c) Carruthers, W. In Cycloaddition
Reactions in Organic Synthesis;Baldwin, J. E., Magnus,
P., Eds.; Tetrahedron Organic Chemistry Series;Perg-
amon: Oxford, 1990;Vol. 8, p 269.
12. The geometry of the trisubstituted double bond was
deduced from NOE data. The allylic methylene protons of
18 showed strong NOE to the C-3 methyl group whereas
the C-12 vinyl proton displayed no NOE to the C-3 methyl
group.2a
6. Takadoi, M.;Terashima, S. Bioorg. Med. Chem. Lett.
2002, 12, 2871.
7. Doller, D.;Chackalamannil, S.;Czarniecki, M.;McQu-
ade, R.;Ruperto, V. Bioorg. Med. Chem. Lett. 1999, 9,