Two-carbon bridge substituted cocaines: Enantioselective synthesis, attribution of the absolute configuration and biological activity of novel 6- and 7-methoxylated cocaines

Article abstract of DOI:10.1016/S0014-827X(99)00027-0

In an effort to learn more about the general structure-activity relationships of cocaine with the aim to elucidate those structural features that might confer antagonistic properties to such analogues, we describe herein our synthetic efforts to prepare two-carbon bridge functionalized (methoxylated and hydroxylated) analogues. Our approach makes use of a modification of the classical Willstatter synthesis of cocaine: Mannich type cyclization of acetonedicarboxylic acid monomethyl ester with methylamine hydrochloride and 2-methoxysuccindialdehyde in a citrate buffer solution afforded the 6- and 7-substituted 2-carbomethoxy-3-tropinones 3a,b and 4a,b in approximate yields of 64%. Reduction of the (±)-tropinone derivatives was performed with sodium amalgam in a sulfuric acid solution to afford a mixture of (±)-methoxyecgonine and (±)-methoxypseudoecgonine derivatives 5, 11 and 6, 7, 12, 13. Benzoylation of these alcohols yielded the desired cocaine and pseudococaine-like compounds 8, 14 and 9, 10, 15, 16. Additionally, we show that enzymatic hydrolysis of these cocaine analogues using pig liver esterase (PLE) affords a practical means for achieving their chemical resolution. The enantiomers of the methoxycocaine analogues were also prepared starting from chiral (±)- and(-)-6-methoxytropinone. All new analogues were examined for their ability to displace [³H]mazindol binding and to inhibit high-affinity uptake of [³H]dopamine into striatal nerve ending (synaptosomes). It appeared evident that methoxylation of the cocaine two-carbon bridge provides compounds of particular interest: the K(i) for the binding of the methoxypseudococaines is about two to four times smaller than the K(i) for inhibition of dopamine uptake, thus enabling these compounds capable of countering the effects of cocaine to some extent.

Full text of DOI:10.1016/S0014-827X(99)00027-0

www.elsevier.nl/locate/farmac
Il Farmaco 54 (1999) 275–287
Two-carbon bridge substituted cocaines: enantioselective synthesis,
attribution of the absolute configuration and biological activity of
novel 6- and 7-methoxylated cocaines
Daniele Simoni a,*, Marinella Roberti b, Vincenza Andrisano b, Monica Manferdini a,
Riccardo Rondanin a, Francesco Paolo Invidiata c
a Dipartimento di Scienze Farmaceutiche, Uni6ersita` di Ferrara, Via Fossato di Mortara 17-19, I-44100 Ferrara, Italy
b Dipartimento di Scienze Farmaceutiche, Uni6ersita` di Bologna, Bologna, Italy
c Istituto Farmacochimico, Uni6ersita` di Palermo, Palermo, Italy
Received 30 December 1998; accepted 1 March 1999
Abstract
In an effort to learn more about the general structure–activity relationships of cocaine with the aim to elucidate those structural
features that might confer antagonistic properties to such analogues, we describe herein our synthetic efforts to prepare
two-carbon bridge functionalized (methoxylated and hydroxylated) analogues. Our approach makes use of a modification of the
classical Willstatter synthesis of cocaine: Mannich type cyclization of acetonedicarboxylic acid monomethyl ester with methyl-
amine hydrochloride and 2-methoxysuccindialdehyde in a citrate buffer solution afforded the 6- and 7-substituted 2-car-
bomethoxy-3-tropinones 3a,b and 4a,b in approximate yields of 64%. Reduction of the (9)-tropinone derivatives was performed
with sodium amalgam in a sulfuric acid solution to afford a mixture of (9)-methoxyecgonine and (9)-methoxypseudoecgonine
derivatives 5, 11 and 6, 7, 12, 13. Benzoylation of these alcohols yielded the desired cocaine and pseudococaine-like compounds
8, 14 and 9, 10, 15, 16. Additionally, we show that enzymatic hydrolysis of these cocaine analogues using pig liver esterase (PLE)
affords a practical means for achieving their chemical resolution. The enantiomers of the methoxycocaine analogues were also
prepared starting from chiral (+)- and (−)-6-methoxytropinone. All new analogues were examined for their ability to displace
[3H]mazindol binding and to inhibit high-affinity uptake of [3H]dopamine into striatal nerve ending (synaptosomes). It appeared
evident that methoxylation of the cocaine two-carbon bridge provides compounds of particular interest: the Ki for the binding of
the methoxypseudococaines is about two to four times smaller than the Ki for inhibition of dopamine uptake, thus enabling these
compounds capable of countering the effects of cocaine to some extent. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Cocaine; Pseudococaine; Methoxycocaine derivatives; Antagonist properties; Enantioselective synthesis
1. Introduction
Cocaine inhibits reuptake of the neurotransmitters
norepinephrine and serotonin, as well as of dopamine.
It binds to the dopamine transporter labelled by mazin-
dol and to imipramine binding sites on serotoninergic
neurons. Cocaine also affects neurotransmission in his-
tamine, acetylcholine, and phenethylamine pathways.
None of these neurotransmitter actions are solely re-
sponsible for cocaine euphoria since each action is also
produced by other pharmacological agents that do not
produce euphoria, are not self-administered by animals,
and are not abused by humans [5–11]. Thus, although
the rewarding properties of cocaine clearly require acti-
vation of dopaminergic systems, the molecular mecha-
nism involved and whether activation is due only to
Cocaine abuse is one of the greatest concerns of
society today and, in the US alone, one to three million
cocaine abusers are estimated to be in need of treat-
ment, six times the number of heroin addicts [1–3]. It is
well documented that long-term cocaine abuse produces
neurophysiological alterations in specific systems, in the
central nervous system, that regulate the capacity to
experience pleasure. Clinical and preclinical data about
long-term cocaine exposure suggest cocaine causes a
neurophysiological addiction [4].
* Corresponding author.
0014-827X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0014-827X(99)00027-0

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D. Simoni et al. / Il Farmaco 54 (1999) 275–287
direct cocaine effects on dopaminergic neurons, or to
simultaneous collateral actions on other neurotransmit-
ters, is uncertain.
N-ethylmaleimide (NEM) alkylation of [3H]mazindol
binding sites: while cocaine, DA and amphetamine can
protect the most reactive site from NEM alkylation,
only cocaine can protect the less reactive site [18]. This
would appear to allow both classically competitive and
allosteric mechanisms for the inhibition of DA uptake
by cocaine. Furthermore, it has been demonstrated that
two aminoacidic residues, namely aspartate and serine,
are crucial for dopamine transporters and have differ-
ent importance for cocaine binding and dopamine up-
take [19].
Several laboratories have shown that cocaine inhibi-
tion of dopamine uptake into striatal synaptosomes is
consistent with a classic, fully competitive mechanism
[12–14]. However, these data are also consistent with
more complex models, including allosteric or partially
competitive models and several others involving steric
hindrance, distinct but overlapping binding sites, or
multiple binding sites in which at least one site is
required for both cocaine and dopamine binding
[15,16].
Present evidence has demonstrated that there are at
least two binding sites for (R)-cocaine associated with
the dopamine transporter, one with high and another
with low affinity [17].
Recently, it has been demonstrated that cocaine, DA
and amphetamine afford different protection against
Taken together, these results support the notion
that cocaine occupies a site that has both a distinct
domain as well as a domain that it shares with other
substrates such as DA and
D-amphetamine. Conse-
quently, this distinction may be sufficient to allow
appropriately designed drugs which bind to a region of
the cocaine binding domain without inhibiting do-
pamine uptake.
Table 1
Binding and dopamine uptake data for racemic compounds 8, 9, 14–16

D. Simoni et al. / Il Farmaco 54 (1999) 275–287
277
Table 2
Binding and dopamine uptake data for enantiomers 8–10 and 14–16
a Data of racemates previously reported [29].
b Less than 85% inhibition at highest concentration tested, 1,000 mM.
Based on this supposition, we hypothesized that
modifications of the basic skeleton of the tropane ring
of cocaine could afford compounds that allosterically
diminish cocaine binding, but with cocaine antagonist
properties. Support for a cocaine-based chemical ap-
proach can be found in the opiate area where useful
narcotic antagonists have been discovered through sim-
ple modifications of morphine [20]. Furthermore, we
were encouraged in this direction by the observation
that substitution at the nitrogen portion of cocaine
provides compounds endowed with cocaine antagonist
activity [21].
On the basis of a large body of SAR work available
principally from Carroll and co-workers [17,22–33], we
decided to explore the effect of substitution at the
two-carbon bridge of cocaine. In particular we consid-
ered of interest to study new cocaine derivatives bearing
a methoxy function at the two-carbon bridge. In con-
trast to 2,3,4-substituted tropanes, compounds with
substituents in the 6- or 7-positions are sterically con-

278
D. Simoni et al. / Il Farmaco 54 (1999) 275–287
Fig. 1.
strained due a fixed spatial relationship between the
tropane nitrogen and the above cited substituents. This
could offer a potential tool for the study of pharma-
cophore–receptor interactions. Moreover, in our opin-
ion, neglect of the two-carbon bridge substituted
cocaines is a consequence of synthetic difficulties, prin-
cipally in terms of the number of isomers which poten-
tially could be obtained when
introduced in such positions.
a substituent is
Preliminary results from our laboratory (see Tables 1
and 2; Fig. 1) demonstrated that methoxylation of the
cocaine two-carbon bridge provides compounds of par-
ticular interest: it appears that the Ki for the binding of
methoxypseudococaines is about two to four times
smaller than Ki for inhibition of dopamine uptake, thus
making these compounds capable of countering the
effects of cocaine to some extent [29].
Since it has been well documented by Carroll et al.
that stereochemistry plays an important role in modu-
lating the binding of cocaine analogues and that the
seven other stereoisomers of cocaine are less potent
than the original one [30,33], we have been developing
a methodology aimed at optical resolution of our
racemic mixtures.
Scheme 2.
In this lecture we describe the preparation of racemic
6- and 7-methoxylated cocaines 8 and 14 and the
pseudococaine-like derivatives 9, 10, 15 and 16, as well
as the 6- and 7-hydroxylated analogues 28a,b and
29a,b, together with new findings emerged during their
optical resolution. Biological results for racemates and
enantiomers of the methoxylated cocaines will also be
reported.
2. Synthesis of racemic methoxycocaines 8, 14 and
pseudomethoxycocaines 9, 10, 15 and 16
Since (−)-cocaine is the usual starting material, syn-
thetic flexibility is naturally restricted, causing serious
limitations in the preparation of cocaine analogues: the
logical consequence of this type of approach is the lack
of information about the biology of cocaine stereoiso-
mers. In order to overcome this limitation we reconsid-
ered the chemistry developed nearly seven decades ago
by Wilstatter et al. [34] (see retrosynthetic analysis in
Scheme 1) as a potential tool in synthesizing cocaine-
like derivatives as well as its stereoisomers: Mannich
type cyclization of acetonedicarboxylic acid mono-
methyl ester with methylamine hydrochloride and suc-
cindialdehyde in a citrate buffer solution affords the
2-carbomethoxy-3-tropinone. Reduction of the latter
tropinone intermediate may affords cocaine-, pseudo-
cocaine- and allopseudococaine-like derivatives.
Keeping this approach in mind, it was easy to go
about the introduction of different substituent on the
cocaine carbon skeleton ring starting from derivatives
Scheme 1.

D. Simoni et al. / Il Farmaco 54 (1999) 275–287
279
of acetonedicarboxylic acid monomethyl ester and suc-
cindialdehyde. Retrosynthetic analysis (Scheme 1) sug-
gestedthat6-or7-methoxy-substituted2-carbo-methoxy-
tropinone derivatives 3a,b and 4a,b incorporating the
methoxy function present in our desired cocaine
derivatives 8–10 and 14–16 could be suitable candi-
dates.
Accordingly, the desired two-carbon bridge methoxy-
cocaine derivatives 8–10 were prepared as reported in
Scheme 3, while the 2-carbomethoxytropinone deriva-
tives 3a,b and 4a,b were obtained starting from the
known methoxysuccindialdehyde (2) (Scheme 2). Fi-
nally, compounds 14–16 were prepared in the same
manner as described for 8–10 starting from 4a,b
(Scheme 4).
The monomethyl ester of acetonedicarboxylic acid,
prepared from the acetonedicarboxylic acid anhydride
and methanol at room temperature [35–37], was re-
acted with methylamine hydrochloride and 2-methoxy-
succindialdehyde (2) in a citrate buffer solution to give
the key tropinone derivatives 3a,b and 4a,b in 64% yield
(Scheme 2). The reaction produced a mixture (1:1 ratio)
of the (9)-6b-methoxy-2-carbomethoxy-3-tropinone
(3a) together with traces of the 6a isomer 3b and
the (9)-7b-methoxy-2-carbomethoxy-3-tropinone (4a).
Only a limited amount (6%) of the (9)-7a-methoxy-2-
carbomethoxy-3-tropinone (4b) was recovered. The
starting material 2-methoxysuccindialdehyde (2) was
obtained by exposure of
a
mixture of 2,3,5-
trimethoxytetrahydrofuran (1) to a 0.2 M aqueous sul-
furic acid solution. The 2,5-dimethoxy-2,5-dihydro-
furan was reacted with methanol [38] to afford the
desired 2,3,5-trimethoxytetrahydrofuran (1). Reduction
of the (9)-methoxytropinone derivatives 3a,b and 4a,b
was performed with sodium amalgam [35], in a pH 3–4
sulfuric acid solution, to give a mixture of the (9)-
methoxyecgonine derivatives 5, 11 and the (9)-
methoxypseudoecgonine derivatives 6, 7, 12, 13. All
compounds were purified by flash chromatography, the
alcohol (9)-7 was obtained as an inseparable mixture
with (9)-6 and used in the next step without any
further purification.
Benzoylation of alcohols 5–7 and 11–13 using ben-
zoyl chloride in the presence of triethylamine (TEA)
and a catalytic amount of 4-dimethylaminopyridine
(DMAP) produced the desired cocaine- and pseudoco-
caine-like derivatives 8–10 and 14–16 in 53–71% yield.
Structural assignments of the newly prepared benzoyl
1
derivatives were inferred by H and 13C NMR analyses
and by comparison with data of literature [39]. Posi-
tions of all protons in the tropane ring were assigned on
the basis of proton-decoupling experiments, starting
Scheme 3.

280
D. Simoni et al. / Il Farmaco 54 (1999) 275–287
Scheme 6.
ful. Moreover, further attempts (Scheme 5) involving
the baker’s yeast (B.Y.) reduction of the carbomethoxy-
tropinone analogues 3a,b and 4a,b, gave as unique
result the formation of the corresponding 6-methoxy-3-
tropinone. Although a large body of literature exists
concerning the use of the b-ketoester moiety in micro-
bial transformations aimed at the production of chiral
building blocks, the present finding was surprising and,
to the best of our knowledge, unprecedented.
Since the sodium-amalgam based reduction of the
2-carbomethoxy-3-tropinone is the only known access
to cocaine-like compounds, the enantioselective reduc-
tion of compounds 3a,b and 4a,b did not appear readily
feasible.
Scheme 4.
from the diagnostic H-3 proton that appears as a
multiplet centered at 5.80 ppm. Consequently, the H-6
proton, linked to the methoxy function, was easily
attributed in the 4.06 ppm region. The stereochemistry
was attributed on the basis of the coupling constants
between H-2 and H-3, H-5 and H-6 protons. A con-
stant of about 6 Hz between H-5 and H-6 was indica-
tive of an a-configuration of the methoxy function
whereas 10.6 Hz between H-2 and H-3 protons indi-
cated a pseudococaine-like structure. Finally, a con-
stant of about 6 Hz between H-2 and H-3 was
indicative of a cocaine-like configuration.
Recently, it was reported that cocaine is inactivated
in vivo through a methabolic pathway which involves
hydrolysis of the ester in position 3 [40,41]. This reac-
tion affords the formation of the ecgonine methyl ester.
Moreover, it has been well documented that (1S)-co-
caine is faster hydrolyzed, by baboon plasma butyryl-
cholinesterase, than the corresponding (1R)-isomer [42].
3. Pig liver esterase (PLE) resolution of racemic 6- and
7-methoxycocaines analogues 8–10 and 14–16
Initial attempts to resolve the racemic mixtures 8–10
and 14–16 by using the classical tartaric acid resolu-
tion, as described by Carroll et al. [30], were unsuccess-
Scheme 5.
Fig. 2.

D. Simoni et al. / Il Farmaco 54 (1999) 275–287
281
Fig. 3.
drolysis of the (−)-isomer takes place to afford (−)-
pseudoecgonine methyl ester (−)-20. In contrast, for
cocaine the (+)-isomer is hydrolyzed at a faster time
producing (+)-19. Attempts to effect the PLE-cata-
lyzed hydrolysis of (9)-allococaine (21) and (9)-al-
lopseudococaine (22) (Fig. 2) failed; the first analogue
was recovered unchanged under the reaction conditions
described, whereas the second compound was found to
undergo slow decomposition. The more rapid rate of
hydrolysis of (+)-cocaine by PLE is consistent with an
earlier report of the preferential hydrolysis of (1S)-co-
caine relative to its (1R)-counterpart which has been
observed in baboon plasma [43]. The preferred hydroly-
sis of the equatorial –OCOPh group can be rational-
ized by considering the active-site model first proposed
for PLE by the groups of Tamm [44] and Jones [45–
47]. In the case of six-membered ring substrates, it has
been shown that an equatorial, or pseudo-equatorial
ester orientation is preferred by PLE.
Scheme 7.
In view of these observations and of our own work in
which some of us have shown that PLE has a particular
affinity for the benzoic ester moiety [42], we were
attracted to effect an enzymatic resolution of our
racemic mixture.
With regard to the chemoselectivity of the hydrolysis,
i.e. the selective cleavage of the benzoate group rather
than the carbomethoxy group, we may advance the
hypothesis that together with a higher susceptibility of
the benzoate group to hydrolysis, the preferred en-
zyme–substrate complex must allow better alignment
of the –OCOPh group with the serine residue present in
the catalytic site. To this regard, it may be of interest to
The PLE-catalyzed hydrolysis of cocaine and pseudo-
cocaine (Scheme 6) was readily performed in aqueous
solution at 37°C, maintaining the pH at 7. The hydro-
lysis was continued until one-half equivalent of sodium
hydroxide had been consumed. The reaction time was
about 1 h for pseudococaine, whereas the hydrolysis of
the cocaine molecule required longer reaction times (7
h). In the case of (9)-pseudococaine, preferential hy-
Table 3
Products of PLE resolution
Starting
material
Products
Yield
(%)
[h]2D0
ee (%)
(9)-17
(9)-17
(9)-18
(9)-18
(9)-8
(−)-Cocaine
45
35
91
85
35
3
65
60
30
74
55
26
67
−13.1° (c 2.5, CHCl3); −16° (c 4, CHCl3) [26]
+41.5° (c 1.5, MeOH); +52.3° (c 1, MeOH) [13]
+43° (c 1, MeOH); +43° (c 1, MeOH) [27]
−22.2° (c 2, H2O); −22.5° (c 1, H2O) [27]
−16.4° (c 1, MeOH)
+24.3° (c 2.3, MeOH)
+16.1° (c 1, MeOH)
+35.9° (c 1, MeOH)
−18.0° (c 3, MeOH)
−37.0° (c 1, MeOH)
+46.1° (c 1, MeOH)
−14.2° (c 2.5, MeOH)
−46.7° (c 1, MeOH)
81.8
71
100
95
82
95
85
99
99
97
(+)-Ecgonine methyl ester hydrochloride
(+)-Pseudococaine hydrochloride
(−)-Pseudoecgonine methyl ester
(−)-6b-Methoxycocaine
(+)-6b-Methoxyecgonine methyl ester
(+)-6b-Methoxycocaine
(+)-7b-Methoxypseudococaine
(−)-7b-Methoxypseudoecgonine methyl ester
(−)-7b-Methoxypseudococaine
(+)-7a-Methoxypseudococaine
(−)-7-a-Methoxypseudoecgonine methyl ester
(−)-7-a-Methoxypseudococaine
(9)-8
(9)-15
(9)-15
(9)-16
(9)-16
97
95
97

282
D. Simoni et al. / Il Farmaco 54 (1999) 275–287
hydrolysis occurs preferentially for the (−)-isomer,
affording the corresponding (−)-pseudoecgonine
derivative (−)-6. This latter was, in turn, benzoylated
leading to the final (−)-pseudococaine-like analogue
(−)-9. On the contrary, the (+)-isomer was faster
hydrolyzed, with respect to the (−)-isomer, in the case
of the cocaine-like derivative (9)-8. This result is in
agreement with the above reported on the faster hydro-
lysis of (1S)-cocaine compared to (1R)-cocaine.
Therefore, application of the PLE-catalyzed hydroly-
sis reaction to the racemic mixture of cocaine and its
analogues provides efficient access to the optically pure
tropanes [48], all of which are obtained in good yields
and enantiomeric excess (Table 3).
4. Synthesis of two-carbon bridge hydroxylated
cocaines and pseudococaines 28a,b and 29a,b
To further explore the structure–activity relation-
ships of cocaine we needed access to 6- and 7-hydroxy-
lated cocaine analogues 28a,b, as well as to their
pseudo-stereoisomers 29a,b (Fig. 3). In spite of the fact
that tropane alkaloids bearing an oxygenated sub-
stituent at the two-carbon bridge are well represented in
nature, a search of the literature failed to reveal any
general methodologies for the synthesis of the above
mentioned 6- and 7-hydroxylated cocaine derivatives. It
appeared evident that ready access to the hydroxylated
analogues 28a,b and 29a,b could be achieved by simple
demethylation of the already-prepared methoxy-
cocaines.
Scheme 8.
note that the use of longer reaction times, and conse-
quently increased amounts of sodium hydroxide, led to
the isolation of some ecgonine.
Very interestingly, we found that PLE works effi-
ciently (Scheme 7) on our methoxycocaines affording a
good separation of the two isomers. In the case of the
pseudococaine (9)-9, the reaction was very fast and
Scheme 9.

D. Simoni et al. / Il Farmaco 54 (1999) 275–287
283
Scheme 10.
Unfortunately, all attempts to cleave the methyl ether
5. Enantioselective synthesis and attribution of the
absolute configuration of two-carbon bridge
methoxylated cocaines
group present in the starting methoxylated cocaines
were unsuccessful because the ester function underwent
cleavage more readily than the methyl group, thus
producing inseparable mixtures of compounds. Conse-
quently, we devised an alternative methodology starting
from the 6-hydroxytropinone (23) (Scheme 8), which in
turn was easily transformed into the b-ketoesters 25a,b,
two promising precursors of our desired hydroxyco-
caine derivatives 28a,b and 29a,b. Thus, the tropinone
derivative 23 was prepared as described in literature
and the hydroxy function was protected as its t-
butyldimethylsilyl ether. The ketone was deprotonated
with sodium hydride in the same manner as described
by Carroll et al. for tropinone [39]. The enolate was
reacted with dimethyl carbonate to afford in 85% yield
a mixture (1:1) of the corresponding carbomethoxylated
derivatives 25a,b. Careful flash chromatography al-
lowed the separation of the two isomers. Reduction of
the b-ketoesters (with NaHg or NaBH4) and subse-
quent benzoylation and desylilation allowed in good
yields the desired racemic hydroxycocaine derivatives
28a,b and hydroxypseudococaine derivatives 29a,b.
Therefore, this methodology provides ready access to
the 6- and 7-hydroxylated derivatives of cocaine and
pseudococaine [49].
A search of literature revealed that the absolute
stereochemistry of the majority of chiral natural
tropane alkaloids is known in only a few cases [50–52]:
the optical properties of the isolated compounds are
usually not known and most of the syntheses of these
natural products afford racemic mixtures. Therefore,
the development of new methodologies for the enan-
tioselective preparation of cocaine and its analogues
still represents an attractive goal. Accordingly, we have
devised a novel enantioselective approach for the
preparation of two-carbon bridge methoxylated co-
caines which in turn allows concomitant, facile attribu-
tion of their absolute configuration.
Retrosynthetic analysis suggested (Scheme 9) that
chiral (6R)- and (6S)-2-carbomethoxytropinone deriva-
tives (9)-3a, and the corresponding regioisomers (9)-
4a, possessing the known configuration at the carbon
atoms bearing the methoxy functionality, would consti-
tute appropriate starting materials for our purpose.
It was anticipated that carbomethoxylation of the
known chiral 6-methoxytropinones (9)-30 would al-
low a facile access to the desired 2-carbomethoxy

284
D. Simoni et al. / Il Farmaco 54 (1999) 275–287
derivatives (+)-3a, (+)-4a, (−)-3a, and (−)-4a.
Moreover, their stereoselective reduction could afford
the ecgonine methyl ester derivatives (+)-5, (+)-11
and, of course, the corresponding enantiomers (−)-5,
(−)-11, together with the pseudoecgonine derivatives
(−)-6, (+)-12, (+)-6 and (−)-12.
In conclusion, the use of the optical antipodes of the
6-b-methoxytropinone permits the synthesis of all the
enantiomers (see ee in Table 4) of the 6-b- and 7-b-
methoxycocaines as well as of the diastereoisomers 6-b-
and 7-b-methoxypseudococaines. It is noteworthy that
the synthetic methodology described above also al-
lowed the assignment of absolute configuration of the
new synthesized enantiomers.
With this approach in mind, it was easy to imagine
that absolute configuration of all the new synthesized
methoxycocaines could be unambiguously assigned.
Thus, the carbomethoxylation of (−)-30 (Scheme
10) afforded in 65% total yield the two regioisomers
(+)-3a and (+)-4a bearing the carbomethoxy function
at the 6- and 7-carbon atoms, respectively (see Scheme
11 for the carbomethoxylation of (+)-30). The b-ke-
toesters were reduced with sodium amalgam to generate
both the ecgonine-like derivatives (+)-5 and (+)-11
and the pseudoecgonine-like derivatives (−)-6 and
(+)-12 in 15–32 and 44–60% yields, respectively. Even
though this transformation was not stereoselective, the
two isomers were readily individualized by TLC and
careful flash chromatography allowed them to be sepa-
rated easily.
6. Biological results and discussion
Enantiomers (−)-8, (+)-9, (−)-9, (+)-15, (−)-15,
(+)-16 and (−)-16 and the racemic mixtures 10 and
14, were examined for their ability to displace
[3H]mazindol binding, a compound which has been
shown to label the cocaine binding sites on the do-
pamine transporter of rat striatal membranes. This
ligand binds with high affinity to a single sodium-de-
pendent site in striatal membranes, representing the
dopamine carrier. Additionally, the new compounds
were tested for their ability to inhibit high-affinity
uptake of [3H]dopamine into striatal nerve endings
(synaptosomes). All synthesized compounds were less
active as compared with the parent cocaine and pseudo-
cocaine. As a general rule, the (−)-isomers, belonging
to both cocaine- and pseudococaine-like series, were
more active than the racemic mixture in both, binding
Benzoylation of (+)-5, (+)-11, (−)-6 and (+)-12
provided easily (+)-8, (+)-14, (−)-9 and (+)-15.
Interestingly, the (+)-(6b)-methoxytropinone (+)-30,
afforded in the same manner as described above for the
methoxytropinone (−)-30, the new (6S)- and (7S)-
enantiomers (−)-8, (−)-14, (+)-9 and (−)-15.
Scheme 11.

D. Simoni et al. / Il Farmaco 54 (1999) 275–287
285
Table 4
Synthetic chiral methoxy derivatives of cocaine and pseudococaine
Starting material
Products
Yield (%)
[h]2D0
ee (%)
(+)-3a
(+)-5
(+)-4a
(+)-11
(+)-3a
(−)-6
(+)-4a
(+)-12
(−)-3a
(−)-5
(−)-4a
(−)-11
(−)-3a
(+)-6
(+)-6b-Methoxyecgonine methyl ester (+)-5
(+)-6b-Methoxycocaine (+)-8
(+)-7b-Methoxyecgonine methyl ester (+)-11
(+)-7b-Methoxycocaine (+)-14
(−)-6-b-Methoxypseudoecgonine methyl ester (−)-6
(−)-6b-Methoxypseudococaine (−)-9
(+)-7-b-Methoxypseudoecgonine methyl ester (+)-12
(+)-7b-Methoxypseudococaine (+)-15
(−)-6b-Methoxyecgonine methyl ester (−)-5
(−)-6b-Methoxycocaine (−)-8
(−)-7-b-Methoxyecgonine methyl ester (−)-11
(−)-7b-Methoxycocaine (−)-14
(+)-6b-Methoxypseudoecgonine methyl ester (+)-6
(+)-6b-Methoxypseudococaine (+)-9
(−)-7b-Methoxypseudoecgonine methyl ester (−)-12
(−)-7b-Methoxypseudococaine (−)-15
15
66
32
75
44
89
22
85
19
55
25
63
60
85
44
71
+24.5° (c 1.5, MeOH)
+18.3° (c 1.5, MeOH)
+50.5° (c 1, MeOH)
+14.0° (c 2, MeOH)
−7.45° (c 1, MeOH)
−30.3° (c 1.8, MeOH)
+39.5° (c 2, MeOH)
+37.2° (c 2, MeOH)
−24.3° (c 1.5, MeOH)
−18.0° (c 1, MeOH)
−50.1° (c 1, MeOH)
−14.0° (c 2, MeOH)
+6.7° (c 1, MeOH)
+29.3° (c 2, MeOH)
−38.7° (c 1.5, MeOH)
−37.0° (c 1.5, MeOH)
91
93
99
99
97
95
99
99
94
90
97
98
95
93
99
99
(−)-4a
(−)-12
dramatic, the results may still be significant. The data
provided for these pseudococaine-like derivatives show
that this difference does not relate solely to the a-
stereochemistry of the carbomethoxy group: the posi-
tion of the methoxy function in the two-carbon bridge
as well as its stereochemistry seems to play an impor-
tant role in modulating cocaine antagonist activity.
Therefore, these findings add further support to the
idea that the cocaine recognition site may be distinct
from the dopamine transporter: if cocaines were, in
fact, simply binding within the transporter channel,
then any analogue of cocaine would always inhibit
dopamine uptake.
and DA uptake inhibition, biological tests. The (−)-16
has the same degree of activity as compared to the
racemic mixture: this data being in contrast with the
expected pattern.
Important effects on activity were achieved by inver-
sion of the configuration: a decrease of activity was
observed for isomers (+)-15 and (+)-16 with respect
to the corresponding (−)-isomers. On the contrary,
compounds (9)-9 showed a different activity profile as
compared with the parent derivatives (9)-15 and (9)-
16: the two enantiomers were equally active and both
showed an increased activity with respect to the racemic
mixture.
Since differential binding and dopamine uptake Ki
values are obtained for compounds (+)-9, (+)-15,
(+)-16, then the mazindol/cocaine binding site may in
fact be located in a region which is distinct from the
transporter channel. These compounds may therefore
have the properties of partial agonists or antagonists,
and thus be capable of countering to some extent the
cocaine effects.
Finally, we believe of particular interest the biologi-
cal results related to the new racemic 6-methoxypseudo-
cocaine derivative 10: even though the introduction of a
methoxy function at 6a-position seriously compromises
binding affinity at the dopamine transporter (Ki=
151913), the dramatic decrease of the capability to
inhibit DA reuptake of [3H]dopamine (Ki\1000) into
striatal nerve endings makes this compound of particu-
lar interest. Based on these latter findings, it appears
reasonable to think that the introduction of a phenyl
group in the 3-position (‘Win’ type analogue) could
allow higher affinity at the dopamine transporter. Fur-
thermore, it would be considered of interest to prepare
additional compounds, related to 10, in order to see
whether further optimization of antagonist activity can
be achieved.
Very interestingly, the Ki for the binding of com-
pounds (+)-9, (+)-15 and (+)-16 is about 2.5–3
times smaller than their Ki for inhibition of dopamine
uptake. As compared with the values reported previ-
ously for racemic mixtures, the differences are higher
for derivatives (+)-9 and (+)-15, but, very surpris-
ingly, somewhat smaller for derivative (+)-16: this
occurrence, in contrast to the expected pattern, is
presently unexplained.
A difference between the binding affinities and do-
pamine reuptake inhibition is also retained for the
parent (−)-pseudococaine-like derivatives (−)-9 and
(−)-15 (Ki for binding is about 2.2–2.5 times smaller
than their Ki for inhibition of dopamine uptake); on the
contrary, the (−)-8 and (−)-16 isomers lose the dis-
crepancy in binding versus dopamine reuptake
inhibition.
Although the limited number of compounds does not
allow us to draw conclusions, it appears that (+)-iso-
mers could play an important role in the discovery of a
cocaine antagonist.
Moreover, notwithstanding that the differences in Ki
values for binding versus uptake inhibition are not

286
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stereoisomer derivatives were scarcely considered until
now, considerations emerging from our results provide
evidence that stereoisomers may play an important role
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are deserving of further research in this field.
Finally, about the PLE resolution of cocaine race-
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susceptibility to hydrolysis in plasma. To the best of
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