Synthesis of a hybrid analog of the acetylcholinesterase inhibitors huperzine A and huperzine B

Article abstract of DOI:10.1016/S0040-4039(01)01008-5

The synthesis of a new hybrid analog of the acetylcholinesterase (AChE) inhibitors huperzine A and B is reported. An intramolecular reductive dicarbonyl coupling was used as a key reaction for constructing the tetracyclic ring system.

Full text of DOI:10.1016/S0040-4039(01)01008-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 5359–5361
Synthesis of a hybrid analog of the acetylcholinesterase
inhibitors huperzine A and huperzine B
V. Rajendran,a Suo-Bao Rong,a Ashima Saxena,b Bhupendra P. Doctorb and Alan P. Kozikowskia,*
aDrug Discovery Program, Dept. of Neurology, Georgetown University Medical Center, 3900 Reservoir Road, Washington,
DC 20007-2197, USA
bDivision of Biochemistry, Walter Reed Army Institute of Research, Washington, DC 20307-5100, USA
Received 15 May 2001; revised 6 June 2001; accepted 7 June 2001
Abstract—The synthesis of a new hybrid analog of the acetylcholinesterase (AChE) inhibitors huperzine A and B is reported. An
intramolecular reductive dicarbonyl coupling was used as a key reaction for constructing the tetracyclic ring system. © 2001
Elsevier Science Ltd. All rights reserved.
Huperzine A (1, HA) is a potent, reversible inhibitor of
acetylcholinesterase (AChE), the key brain enzyme
responsible for the rapid degradation of the neurotrans-
mitter acetylcholine.1 Because numerous studies have
pointed to the existence of a cholinergic deficit in
patients suffering from neurodegenerative diseases of
the Alzheimer’s type, considerable credence has been
given to the use of AChE inhibitors in the amelioration
of the Alzheimer’s symptomatology.2 In this regard, the
long duration of action of huperzine A coupled with its
potency, selectivity, and lack of side effects encouraged
us and others to explore the structure–activity relation-
ships of this alkaloid.3 To date, we have been able to
identify several analogs of HA that have an activity
better than or comparable to that of HA.4 Huperzine B
(2, HB), a congener of HA, shows a lower AChE
inhibitory potency.5
HA.6 However, due to its reduced potency, few biolog-
ical studies have been reported on this alkaloid.7 In our
efforts to further explore the SAR of these naturally
occurring AChE inhibitors, we believed that it would
be of interest to examine an analog bearing structural
features of both alkaloids, i.e. a hybrid version of HA
and HB, namely structure 3 (Fig. 1).
A molecular modeling study of this hybrid analog in
complex with AChE revealed that in comparison to the
HA complex,8 the cation-p interaction between the
ammonium group and Trp 84 and Phe 330 is altered
only slightly by the presence of the ring methylene
groups. Additionally, the ring methylenes can engage in
hydrophobic interactions with the same residues. More-
over, the hybrid analog 3 is capable of restoring the
hydrogen bond interaction with the main chain car-
bonyl group of His 440 that is present in HA but
lacking in HB. From this preliminary modeling study, it
appeared that the hybrid structure 3 might show a
Interestingly, HB exhibits a higher therapeutic index
due to its longer duration of action in comparison with
Figure 1. Structures of huperzine A, huperzine B and the hybrid analog.
* Corresponding author. Tel.: 202-687-0686; fax: 202-687-5065; e-mail: kozikowa@giccs.georgetown
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01008-5

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V. Rajendran et al. / Tetrahedron Letters 42 (2001) 5359–5361
reasonably good AChE inhibitory potency, and it was
therefore deemed to be a reasonable candidate for
synthesis. In this paper, we describe a method for the
synthesis of hybrid analog 3 that is applicable to a
broader range of compounds.
by benzylation. Attempted hydrolysis of compound 9
with 6N NaOH in methanol resulted only in recovery
of starting material. On the other hand, the use of
LiI and NaCN in DMF at reflux for 4 h afforded 10
in 70% yield. Next, the carboxylic acid was subjected
to Curtius rearrangement, and the resulting intermedi-
ate isocyanate was reacted with 8N HCl at 100°C for
10 min to provide amino alcohol 11 with concomitant
O-debenzylation and double bond isomerization in
42% yield.10 Compound 11 was then subjected to a
Michael addition reaction employing methyl vinyl
ketone as an electrophile to afford the amino ketone
12 in 78% yield. Oxidation of compound 12 with
PDC in CH2Cl2 provided the diketone 13.11
Intramolecular reductive coupling of diketone 12
under McMurry reaction conditions12 using TiCl4/Zn
gave the desired cyclic olefin 14 in 62% yield.9 The
final hybrid structure 3 was obtained in 84% yield
from 14 by removal of the protecting group with
Me3SiI in CHCl3 at reflux.10
The pathway that was followed to assemble the
hybrid analog 3 is shown in Scheme 1. The interme-
diate 8 was obtained using our previously established
protocol.9 Accordingly, the commercially available
ketone 4 was treated with ethyl propiolate and
ammonia in methanol at 100°C in a pressure reactor
for 8 h to give the fused ring pyridone 5 in 65%
yield. After conversion of the pyridone to
methoxypyridine
by
O-methylation
employing
Ag2CO3/MeI, the ketal was deprotected with acetone/
HCl under reflux to obtain compound 6. Reaction of
ketone 6 with 2.2 equiv. of NaH and 4.0 equiv. of
dimethyl carbonate yielded the b-ketoester 7 in good
yield. The b-ketoester 7 was treated with 2-methylene-
1,3-propanediol diacetate, DBU, and
a catalytic
amount of Pd(OAc)2/PPh3 to provide the annulation
product 8 in 85% yield.
In conclusion, the present work provides a convenient
method for preparing structures embodying features
of both HA and HB. Preliminary biological studies
reveal that racemic 3 is able to inhibit AChE with a
potency that is comparable to that of HB. Details of
the enzymatic studies of 3 and related analogs will be
reported separately.
Attempted saponification of ester 8 to the corre-
sponding keto acid led only to isolation of the ring-
opened diacid. We therefore converted b-ketoester 8
to the protected hydroxy ester 9 in 78% yield by
reduction with NaBH4 in methanol at −78°C followed
Scheme 1. Reagents and conditions: (a) ethyl propiolate, NH3/MeOH (approx. 8N), 100°C, 8 h, 65%; (b) Ag2CO3, MeI, CHCl3,
reflux, 2.5 h, 95%; (c) acetone/3N HCl (1:1), reflux, 3 h, 90%; (d) 2.2 equiv. NaH, 4.0 equiv. (MeO)2CO, THF, reflux, 3.5 h, 87%;
(e) (CH3CO2CH2)2CꢀCH2, Pd(OAc)2/PPh3 (1:4.1), DBU, 1,4-dioxane, rt, 0.5 h, then reflux, 3.5 h, 85%; (f) NaBH4, MeOH, −78°C,
0.5 h; (g) BnBr, NaH, DMF, 0°C, 5 h, 78% (2 steps); (h) LiI, NaCN, DMF, reflux, 4 h, 70%; (i) (PhO)2P(O)N3, Et3N, toluene,
reflux, 3 h, then 8N HCl, reflux, 10 min, 42%; (j) CH2ꢀCHCOCH3, Et3N, CH2Cl2, rt, 15 h, 78%; (k) PDC, CH2Cl2, rt, 5 h, 71%;
(l) TiCl4, Zn, pyridine, THF, reflux, 20 h, 62%; (m) Me3SiI, CHCl3, reflux, 4 h, then MeOH, reflux, 15 h, 84%.

V. Rajendran et al. / Tetrahedron Letters 42 (2001) 5359–5361
5361
Acknowledgements
Huey, R.; Hart, W. E.; Belew, R. K.; Olson, A. J. J.
Comput. Chem. 1998, 19, 1639.
We are indebted to the Department of Defense
(DMAD17-00-P-09-62) for support of these studies.
9. Campiani, G.; Sun, L.-Q.; Kozikowski, A. P.; Aagaard,
P.; McKinney, M. J. Org. Chem. 1993, 58, 7660.
1
10. Spectral data for selected compounds: 11: H NMR (300
MHz, CDCl3): l 7.80 (d, 1H, J 9.0 Hz), 6.59 (d, 1H, J 9.0
Hz), 5.37–5.34 (m, 1H), 3.86 (s, 3H), 3.68 (s, 1H), 3.13
(dd, 1H, J 5.7, 17.7 Hz), 2.85–2.76 (m, 2H), 2.37 (d, 1H,
J 17.1 Hz), 1.81 (d, 1H, J 17.1 Hz), 1.58 (s, 3H); 13C
NMR (75 MHz, CDCl3): l 162.7, 152.0, 136.6, 133.2,
131.0, 120.9, 108.5, 73.6, 53.9, 53.3, 42.1, 38.4, 37.4, 22.8;
MS (EI) m/z 246 (M+, 48%), 231, 215, 190 (100%), 174,
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1
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