Synthesis, absolute configuration, and biological profile of the enantiomers of trans-[2-(2,6-dimethoxyphenoxy)ethyl][(3-p-tolyl-2,3-dihydro-1,4-benzodioxin-2- yl)methyl]amine (mephendioxan), a potent competitive α1A-adrenoreceptor antagonist

Article abstract of DOI:10.1021/jm960069a

The enantiomers of trans-[2-(2,6-dimethoxyphenoxy)ethyl][(3-p-tolyl-2,3-dihydro-1,4-benzodioxin-2- yl)methyl]amine (mephendioxan, 2) were synthesized from the chiral trans-3-p-tolyl- 2,3-dihydro-1,4-benzodioxin-2-carboxylic acids [(+)-3 and (-)-3] which in turn were obtained through the resolution of the racemic acid with (R)- and (S)-α-methylbenzylamine. Comparison of CD spectra of the enantiomers of 2 with that of (2S,3S)-3-methyl-2-phenyl-1,4-benzodioxane allowed the assignment of the 2S,3S configuration to the (-)-enantiomer of 2 and of the 2R,3R configuration to the other enantiomer. The binding profile of the enantiomers of 2 was assessed at α₁, α₂, D₂, and 5-HT_1A receptors, in comparison to WB 4101 (1), 5-methylurapidil, and (+)- niguldipine. In addition, the two enantiomers were investigated at native and cloned α₁-adrenoreceptor subtypes. (-)-2 was 10-30 times as potent as the (+)-enantiomer at α₁- adrenoreceptor subtypes in both functional and binding assays. It was 36-fold selective for the α_1A- versus α_1B-adrenoreceptor and 60- and 20-fold selective in binding to the α_1a- adrenoreceptor relative to α_1b and α_1d subtypes, respectively. Furthermore, the enantiomer (-)-2 displayed selectivities of 12000-, 2500-, and 250-fold in binding to α_1a-adrenoreceptors relative to α₂-adrenoreceptors and 5-HT_1A and D₂ receptors. These results indicate that (-)-2 may be a valuable tool in the characterization of α₁-adrenoreceptor subtypes.

Full text of DOI:10.1021/jm960069a

J . Med. Chem. 1996, 39, 2253-2258
2253
Syn th esis, Absolu te Con figu r a tion , a n d Biologica l P r ofile of th e En a n tiom er s of
tr a n s-[2-(2,6-Dim eth oxyp h en oxy)eth yl][(3-p-tolyl-2,3-d ih yd r o-1,4-
ben zod ioxin -2-yl)m eth yl]a m in e (Mep h en d ioxa n ), a P oten t Com p etitive
r1A-Ad r en or ecep tor An ta gon ist
Wilma Quaglia,† Maria Pigini,† Seyed K. Tayebati,† Alessandro Piergentili,† Mario Giannella,†
Amedeo Leonardi,‡ Carlo Taddei,‡ and Carlo Melchiorre*,§
Department of Chemical Sciences, University of Camerino, Via S. Agostino 1, 62032 Camerino, Italy, Research and
Development Division, Recordati S.p.A., Via Civitali 1, 20148 Milano, Italy, and Department of Pharmaceutical Sciences,
University of Bologna, Via Belmeloro 6, 40126 Bologna, Italy
Received J anuary 22, 1996X
The enantiomers of trans-[2-(2,6-dimethoxyphenoxy)ethyl][(3-p-tolyl-2,3-dihydro-1,4-benzo-
dioxin-2-yl)methyl]amine (mephendioxan, 2) were synthesized from the chiral trans-3-p-tolyl-
2,3-dihydro-1,4-benzodioxin-2-carboxylic acids [(+)-3 and (-)-3] which in turn were obtained
through the resolution of the racemic acid with (R)- and (S)-R-methylbenzylamine. Comparison
of CD spectra of the enantiomers of 2 with that of (2S,3S)-3-methyl-2-phenyl-1,4-benzodioxane
allowed the assignment of the 2S,3S configuration to the (-)-enantiomer of 2 and of the 2R,3R
configuration to the other enantiomer. The binding profile of the enantiomers of 2 was assessed
at R1, R2, D2, and 5-HT1A receptors, in comparison to WB 4101 (1), 5-methylurapidil, and (+)-
niguldipine. In addition, the two enantiomers were investigated at native and cloned
R1-adrenoreceptor subtypes. (-)-2 was 10-30 times as potent as the (+)-enantiomer at R1-
adrenoreceptor subtypes in both functional and binding assays. It was 36-fold selective for
the R1A- versus R1B-adrenoreceptor and 60- and 20-fold selective in binding to the R1a-
adrenoreceptor relative to R1b and R1d subtypes, respectively. Furthermore, the enantiomer
(-)-2 displayed selectivities of 12000-, 2500-, and 250-fold in binding to R1a-adrenoreceptors
relative to R2-adrenoreceptors and 5-HT1A and D2 receptors. These results indicate that (-)-2
may be a valuable tool in the characterization of R1-adrenoreceptor subtypes.
In tr od u ction
R1-Adrenoreceptors are not a homogeneous family as
clearly evidenced by pharmacological tools and molec-
ular biological techniques.1 Two native R1-adreno-
receptor subtypes, R1A and R1B, have been formerly
characterized by functional and binding assays. The R1A
subtype has high affinity for antagonists such as WB
Our research group has previously been involved in
4101, 5-methylurapidil, and (+)-niguldipine and is
designing new R1-adrenoreceptor antagonists structur-
ally related to WB 4101 {[2-(2,6-dimethoxyphenoxy)-
et h yl][(2,3-dih ydr o-1,4-ben zodioxin -2-yl)m et h yl]-
amine, 1} and in studying structure-affinity and
structure-selectivity relationships with the goal of
developing high-affinity, site-selective ligands for sub-
types of the R1-adrenoreceptor.8-11 Among a variety of
structural modifications performed on 1, we have dem-
onstrated that the insertion of a substituent at the
3-position having a trans relationship with the 2-side
chain markedly affects the affinity for R2-adrenorecep-
tors whereas that for R1-adrenoreceptors is only slightly
decreased.12 The overall result of that structural modi-
fication was a significant improvement in selectivity
toward rat vas deferens R1-adrenoreceptors compared
to 1. Thus, it is evident that a 3-substituent (trans) may
have a crucial role in the modulation of selectivity for
R1-adrenoreceptor subtypes.
insensitive to inactivation by chloroethylclonidine (CEC).
The R1B subtype displays lower affinity for the above
antagonists, but is preferentially inactivated by the
alkylating agent CEC.1 However, very recently it has
been demonstrated that the R1-adrenoreceptor mediat-
ing contraction upon activation in isolated rat aorta may
belong to the R1D subtype.2 Cloning studies have
confirmed the existence of at least three distinct R1-
adrenoreceptors, which are now designated as R1a, R1b,
and R1d subtypes.3-5 The recombinant R1a-adreno-
receptor (formerly designated as R1c)4 corresponds to the
native R1A-adrenoreceptor, the recombinant R1b to the
native R1B, and the R1d (formerly designated as R1a/d in
some publications) to the native R1D-adrenoreceptor
recently characterized in rat aorta. Thus, R1-adreno-
receptors are now classified as R1A (R1a), R1B (R1b), and
R1D (R1d), with upper and lower case subscripts being
used to designate native or recombinant receptor,
respectively.6,7
Among the analogues of 1 bearing a 3-substituent the
p-tolyl derivative mephendioxan (2) resulted in the most
potent and selective antagonist for the rat vas deferens
R1-adrenoreceptor subtype. Since the enantiomers of 1
have different affinity for R1-adrenoreceptors,13,14 we
† University of Camerino.
‡ Recordati S.p.A.
§ University of Bologna.
X Abstract published in Advance ACS Abstracts, April 15, 1996.
S0022-2623(96)00069-6 CCC: $12.00 © 1996 American Chemical Society

2254 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 11
Quaglia et al.
Sch em e 1a
F igu r e 1. CD spectra of enantiomers of 2 (oxalate salts: c
0.015, MeOH): (-)-2 (- - -) and (+)-2 (s).
was observed for (+)-2 and (-)-2 (as the free bases) at
δ 3.81 and 3.83 ppm, respectively.
Knowledge of the absolute configuration of (2S,3S)-
3-methyl-2-phenyl-1,4-benzodioxane15 (5) allows a de-
termination of the stereochemistry of the two enanti-
omers of 2. The CD spectra for the enantiomers (as
a
(a) (S)-(-)-R-methylbenzylamine, 95% EtOH, room tempera-
ture, 30 h; (b) (R)-(+)-R-methylbenzylamine, 95% EtOH, room
temperature, 30 h; (c) EtOCOCl, Et3N, CHCl3, 30 min, 0 °C, then
2-(2,6-dimethoxyphenoxy)ethylamine, room temperature, 12 h,
silica gel (petroleum ether-EtOAc, 75:25); (d) BH3‚Me2S, diglyme,
120 °C, 12 h, then MeOH, room temperature, 5 h, HCl, 120 °C, 4
h, silica gel (CHCl3-EtOAc, 95:5).
thought it of interest to investigate whether the enan-
tiomers of 2, that have an additional chiral center might
be able to discriminate among R1-adrenoreceptor sub-
types.
We report here the synthesis of the two enantiomers
of 2 together with their biological profile assessed by
functional and binding assays. The absolute configu-
ration of the enantiomers of 2 was assigned by correlat-
ing their CD spectra with that of (2S,3S)-3-methyl-2-
phenyl-1,4-benzodioxanes.15
oxalate salts) of mephendioxan (2) are shown in Figure
1. They show a short-wavelength maximum at ca. 229
nm, [θ] ≈ 20 000 and two nearly overlapping long-
wavelength maxima at ca. 275.5 and 281 nm, [θ] ≈ 3000,
of opposite sign from the short-wavelength transition.
The (-)-enantiomer of 2 showed negative Cotton effects
in the 275-281 nm region and a positive transition in
the 229 nm region. The (+)-enantiomer of 2 displayed
a CD spectrum which was the mirror image of that of
the other enantiomer, suggesting that the two enanti-
omers have similar optical purity (>98%). The (-)-
enantiomer of 2 and 515 have almost identical CD
spectra, which suggests that the (-)-enantiomer of 2 has
the same absolute configuration (2S,3S) of the reference
compound 5. It derives that (+)-2 has a 2R,3R config-
uration. Since the two enantiomers of 2 showed high
enantiomeric purity, it reasonably follows that, unless
there was an unlike complete inversion of configuration
in the course of reactions outlined in Scheme 1, (-)-3
and (+)-3 have a 2S,3R and 2R,3S configuration,
respectively, and, consequently, (+)-4 and (-)-4 have a
2S,3R and 2R,3S configuration, respectively.
Ch em istr y
1
All the compounds were characterized by H NMR,
IR, and elemental analyses. Efforts to resolve racemic
2 by fractional crystallization of the dibenzoyl l-tartrate
and d-tartrate salts were unsuccessful. Therefore, acid
(()-312 was resolved with (R)-(+)- and (S)-(-)-R-meth-
ylbenzylamine. The two enantiomeric salts were con-
verted into the free acids (-)-3 and (+)-3, followed by
amidation with 2-(2,6-dimethoxyphenoxy)ethylamine.16
Intermediate amides (+)-4 and (-)-4 were reduced with
borane-methyl sulfide complex to (+)-2 and (-)-2
(Scheme 1). Enantiomeric purity, determined by 1H
NMR spectroscopy in comparison to the spectrum of
racemic 2 and on addition of the chiral shift reagent (R)-
(+)-R-methoxy-R-(trifluoromethyl)phenylacetic acid [(+)-
MTPA],17 was found to be >98% (detection limit) for
both enantiomers. The spectrum of racemic 2 (as the
free base) in the presence of (+)-MTPA showed a double
singlet signal at δ 3.82 ppm for the methoxy protons of
the 2,6-methoxyphenoxy group whereas only a singlet
Biology
F u n ction a l Stu d ies. The pharmacological profile of
the enantiomers of 2 was evaluated at R1- and R2-
adrenoreceptors on isolated rat vas deferens tissues. To
allow comparison of the results, we used the same
techniques and statistical evaluation of the bioassays

Synthesis of Enantiomers of Mephendioxan
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 11 2255
as for other 1,4-benzodioxan-related compounds.12 WB
4101 (1) was used as the standard compound. R1-
Adrenoreceptor blocking activity was assessed by an-
tagonism of (-)-norepinephrine-induced contractions of
the epididymal portion, while R2-adrenoreceptor block-
ing activity was determined by antagonism of the
clonidine-induced depression of the twitch responses of
the field-stimulated prostatic portion of rat vas deferens.
The potencies of the drugs were expressed as pA2
values,18,19 or, in the case of compounds with low
potency, as -log KB values.20
Ta ble 1. R1- and R2-Adrenoreceptor pA2 Values in the Isolated
Rat Vas Deferensa
c
R1 pA2 against
norepinephrine
R2 -logKB
against clonidine
R1/R2
no.b
selectivity ratio
1
2
9.13 ( 0.05
8.67 ( 0.02
7.59 ( 0.10
8.92 ( 0.09
6.29 ( 0.07d
692
>4700
>390
<5e
<5e
<5e
(+)-2
(-)-2
>8300
a
pA2 and -log KB values, plus or minus standard error of
estimate, were calculated according to Arunlakshana and Schild18,19
and van Rossum,20 respectively. With the exception of 1 that was
b
a hydrochloride salt, all other compounds were oxalates. c The R1/
R2 selectivity ratio is the antilog of the difference between pA2 and
Bin d in g Exp er im en ts. Receptor subtype selectivity
of the enantiomers of 2 was further determined by
employing receptor binding assays. [3H]Prazosin was
used to label R1-adrenoreceptors binding sites of rat
d
-log KB values at R1- and R2-adrenoreceptors, respectively. Cal-
culated at only one concentration (10 µM). e Inactive up to 10 µM.
cerebral cortex homogenates as well as R1A- and R1B
-
enantiomers of 1.13 Thus a 2S,3S configuration as-
signed to (-)-2 is consistent with a 2S configuration for
the most active enantiomer of 1 as one would expect if
related compounds act on the same receptor site.
adrenoreceptor subtypes of CEC pretreated rat hippo-
campus and liver membranes. In addition, competition
assays were performed using [3H]prazosin, and mem-
branes were prepared from COS-7 cells expressing
bovine R1a-, hamster R1b-, or rat R1d-adrenoreceptor
subtypes. Finally, [3H]rauwolscine, [3H]-8-OH-DPAT,
and [3H]spiperone were used to label R2-adrenoreceptors
in rat cerebral cortex, 5-HT1A receptors in rat hippo-
campus, and D2 receptors in rat striatum, respectively.
Binding affinities were expressed as pKi values derived
using the Cheng-Prusoff equation.21 WB 4101 (1),
5-methylurapidil, and (+)-niguldipine were used as the
reference compounds.
An analysis of the binding affinities at native and
cloned R1-adrenoreceptor subtypes reveals that (-)-2 is
36-fold selective for R1A- versus R1B-adrenoreceptors.
Comparable results were obtained for 5-methylurapidil
and (+)-niguldipine that exhibited selectivities of 47-
and 60-fold, respectively. Compound (-)-2 bound to
cloned R1-adrenoreceptors with pKi values of 9.46 (R1a),
7.68 (R1b), and 8.18 (R1d) and exhibited selectivities of
60- and 20-fold in binding to the R1a-adrenoreceptors
relative to R1b- and R1d-adrenoreceptors, respectively. In
parallel experiments, 5-methylurapidil and (+)-nigul-
dipine displayed selectivities of 520- and 66-fold, re-
spectively, for the R1a-adrenoreceptor relative to the R1b
subtype, and selectivities of 87- and 210-fold, respec-
tively, for the R1a versus the R1d subtype. Clearly, the
enantiomer (-)-2 has an affinity profile at both native
and cloned R1-adrenoreceptor subtypes qualitatively
similar to that of 5-methylurapidil and (+)-niguldipine
which are classified as R1A-adrenoreceptor selective
antagonists. However, 5-methylurapidil and (+)-nigul-
dipine have also additional biological properties such
as high affinity for 5-HT1A receptors and potent calcium
channel blocking activity, respectively, which limit their
utility as pharmacological tools in receptor characteriza-
tion. Interestingly, (-)-mephendioxan [(-)-2] was also
12000-, 2500-, and 250-fold selective in binding to R1a-
adrenoreceptors relative to R2-adrenoreceptors and
5-HT1A and D2 receptors, respectively. On these bases,
(-)-2 can be considered as a lead R1A-selective antago-
nist having a similar profile but a structure very
different from the recently reported analogue of (+)-
niguldipine.24
Resu lts a n d Discu ssion
The characterization of R1-adrenoreceptor subtypes
has been difficult due to the lack of selective ligands.
Availability of R1-adrenoreceptor selective antagonists
is of paramount importance not only for receptor
characterization and classification but also for potential
therapeutic implications such as the treatment of benign
prostatic hyperplasia, cardiac arrhythmia and hyper-
tension.22 However, the design of receptor subtype
selective ligands is inherently difficult and remains a
formidable challenge to medicinal chemists due to the
high homology among receptors. For example, R-
adrenoreceptor subtypes share about 45% identity with
serotoninergic and dopaminergic receptors.23 As a
consequence, R1-adrenoreceptor antagonists which are
currently classified as subtype selective have also
significant affinity for other receptor systems. WB 4101
(1) is selective for R1A-adrenoreceptors versus R1B- and
R1d-adrenoreceptors but have also high affinity for R2-
adrenoreceptors and 5-HT1A receptors.
We have already demonstrated that the insertion of
a 3-substituent in 1 affording mephendioxan (2) and
related compounds decreases significantly the functional
affinity for R2-adrenoreceptors.12 This finding has been
confirmed by present investigation in both functional
and binding assays (Tables 1 and 2). Furthermore, in
comparison to 1, compound 2 has 100-fold lower affinity
for the 5-HT1A receptor.
In conclusion, the present investigation has demon-
strated that the insertion of a trans aryl substituent at
the 3-position of 1 affording 2 increases affinity and
selectivity for R1A-adrenoreceptors while significantly
decreasing the affinity for R2-adrenoreceptors and 5-HT1A
and D2 receptors in comparison to the prototype 1.
Furthermore, it has been shown that the affinity and
selectivity reside predominantly in the enantiomer (-)-
mephendioxan [(-)-2] which may represent not only a
valuable tool for the characterization of R1-adreno-
receptor subtypes but also a lead compound for the
development of selective R1A-adrenoreceptor antagonists
for the treatment of benign prostatic hypertrophy.22
The affinity profile of (+)-2 and (-)-2 is shown in
Tables 1 and 2. Clearly, (-)-mephendioxan [(-)-2] was
10-30-fold more potent than the other enantiomer
toward R1-adrenoreceptor subtypes in both functional
and binding assays. The observed stereoselectivity of
the enantiomers of 2 is similar to that reported for the

2256 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 11
Quaglia et al.
Ta ble 2. Affinity Constants (Ki) of the Enantiomers of Mephendioxan (2) for Native and Cloned R1-Adrenoreceptor Subtypes, Native
R2-Adrenoreceptor, and 5-HT1A and D2-Receptors in Comparison to Reference Compoundsa
Ki (nM), native receptors (rat)
Ki (nM), cloned receptors
hippocampus
+ 10 µM
CEC, R1A
hamster
smooth
muscle, R1b rat brain, R1d
cerebral
cortex, R1
cerebral
cortex, R2
hippocampus,
5-HT1A
bovine
striatum, D2 brain, R1a
no.
liver, R1B
1
8.72 ( 2.02 1.29 ( 0.39
25.64 ( 4.86 14.72 ( 1.16 7.26 ( 2.40
122.8( 25.7
102.9 ( 8.1 3501 ( 874 606.2 ( 52.1 82.34 ( 8.65 0.91 ( 0.21 51.38 ( 4.31 11.76 ( 2.43
(+)-2 121.2 ( 10.4 40.33 ( 4.11 774.3 ( 77.6 2142 ( 667 1262 ( 372 827.3 ( 5.2 8.34 ( 3.12 195.2 ( 36.4 191.1 ( 50.9
0.62 ( 0.16 57.34 ( 14.25 6.26 ( 1.18
(()-2 17.03 ( 2.45 5.32 ( 0.67
(-)-2 9.97 ( 0.92 1.76 ( 0.53
METb 28.25 ( 3.84 4.66 ( 1.82
NIGc 27.80 ( 7.40 6.38 ( 1.29
62.61 ( 2.38 4293 ( 1502 888.9 ( 379.0 86.48 ( 11.01 0.35 ( 0.18 21.08 ( 3.20 6.57 ( 0.87
220.2 ( 20.3 432.4 ( 65.1 1.20 ( 0.37
379.9 ( 41.0 1611 ( 235 > 10000
935.1 ( 115.6 2.02 ( 0.53 1058 ( 183
175.1 ( 21.3
294.9 ( 56.6 0.38 ( 0.06 25.01 ( 1.67 82.41 ( 11.95
a
Values are the mean ( SE of at least three separate experiments performed in triplicate. The pseudo-Hill coefficients (nH) were not
significantly different from unity (p > 0.05) with the exception of rat cerebral cortex, where a heterogeneous R1-adrenoreceptor population
is present. Equilibrium dissociation constants (Ki) were derived using the Cheng-Prusoff equation.21 Scatchard plots were linear or
almost linear in all preparation tested. The affinity estimates were derived from displacement of [3H]prazosin from R1-adrenoreceptors,
[3H]rauwolscine from R2-adrenoreceptors, [3H]spiperone from D2 receptors, and [3H]-8-hydroxy-2-(di-n-propylamino)tetraline from 5-HT1A
b
receptors. MET, 5-methylurapidil. c NIG, (+)-niguldipine.
[(+)-4]. Ethyl chlorocarbonate (0.34 g, 3.03 mmol) was added
dropwise to a stirred and cooled (0 °C) solution of (-)-3 (0.82
g, 3.03 mmol) and Et3N (0.31 g, 3.03 mmol) in chloroform (20
mL), followed after 30 min by the addition of a solution of
2-(2,6-dimethoxyphenoxy)ethylamine16 (0.59 g, 3.03 mmol) in
chloroform (10 mL). The resulting reaction mixture was
stirred overnight at room temperature and then washed with
2 N HCl, 2 N NaOH, and finally with water. Removal of dried
solvent gave a solid which was purified by column chroma-
tography. Eluting with petroleum ether-EtOAc (75:25) af-
forded (+)-4: 1.0 g (74% yield); mp 120-122 °C; [R]D +3.81°
(c 1, MeOH); 1H NMR (CDCl3) δ 2.32 (s, 3, CH3C6H4), 3.50 (m,
2, NCH2), 3.85 (s, 6, OCH3), 4.02 (t, 2, CH2O), 4.58 (d, J ) 6.7
Hz, 1, 2-H), 5.07 (d, J ) 6.7 Hz, 1, 3-H), 6.58-7.25 (m, 11,
ArH), 7.52 (br t, 1, NH exchangeable with D2O).
Exp er im en ta l Section
Ch em istr y. Melting points were taken in glass capillary
tubes on a Bu¨chi SMP-20 apparatus and are uncorrected. IR
1
and H NMR spectra were recorded on Perkin-Elmer 297 and
Varian VXR 300 instruments, respectively. Chemical shifts
are reported in parts per million (ppm) relative to tetrameth-
ylsilane (TMS), and spin multiplicities are given as s (singlet),
d (doublet), t (triplet), or m (multiplet). Although the IR
spectra data are not included (because of the lack of unusual
features), they were obtained for all compounds reported and
were consistent with the assigned structures. Optical activity
was measured at 20 °C with a Perkin-Elmer 241 polarimeter.
Circular dichroism (CD) spectra were measured at room
temperature with a J ASCO J 710 spectropolarimeter using a
0.1 cm path length cell. Data were handled with a mathematic
noise reduction routine (J ASCO software). The elemental
compositions of the compounds agreed to within (0.4% of the
calculated value. When the elemental analysis is not included,
crude compounds were used in the next step without further
purification. Chromatographic separations were performed on
silica gel columns (Kieselgel 40, 0.040-0.063 mm, Merck) by
flash chromatography. Petroleum ether refers to the fraction
with a boiling point of 40-60 °C. The term “dried” refers to
the use of anhydrous sodium sulfate.
(2R,3S)-(-)-3-p -Tolyl-2,3-d ih yd r o-1,4-b en zod ioxin -2-
ca r boxylic Acid [2-(2,6-Dim eth oxyp h en oxy)eth yl]a m id e
[(-)-4]. This was obtained from (+)-3 following the procedure
described for the enantiomer (+)-4 and purified by column
chromatography: 1.1 g (81% yield); mp 120-122 °C; [R]D
1
-3.96° (c 1, MeOH). The H NMR spectrum was identical to
that of (+)-4.
(2R,3R)-(+)-[2-(2,6-Dim eth oxyph en oxy)eth yl][(3-p-tolyl-
2,3-d ih yd r o-1,4-ben zod ioxin -2-yl)m eth yl]a m in e Oxa la te
Em ih yd r a te [(+)-2]. A solution of 10 M BH3‚CH3SCH3 (0.2
mL) in dry diglyme (1 mL) was added dropwise at room
temperature to a solution of (+)-4 (1.0 g, 2.2 mmol) in dry
diglyme (40 mL) with stirring under a stream of dry nitrogen
with exclusion of moisture. When the addition was completed,
the reaction mixture was heated at 120 °C for 12 h. After
cooling at 0 °C, excess borane was destroyed by cautious
dropwise addition of MeOH (5 mL). The resulting mixture was
left to stand for 5 h at room temperature, treated with HCl
gas for 10 min, and then heated at 120 °C for 4 h. Removal of
the solvent under reduced pressure gave a residue which was
dissolved in water. The aqueous solution was basified with
NaOH pellets and extracted with chloroform (4 × 30 mL).
Removal of dried solvent gave a residue which was purified
by column chromatography. Eluting with CHCl3-EtOAc (95:
5) afforded (+)-2 as the free base: 1H NMR (CDCl3) δ 2.25 (br
s, 1, NH, exchangeable with D2O), 2.38 (s, 3, CH3C6H4), 2.70-
2.84 (m, 4, CH2NCH2), 3.85 (s, 6, OCH3), 4.10 (t, 2, CH2O),
4.20 (m, 1, 2-H), 4.96 (d, J ) 7.91 Hz, 1, 3-H), 6.56-7.35 (m,
11, ArH); enantiomeric purity > 98% (detection limit), deter-
mined with (+)-MTPA as the chiral shift reagent.
(+)-2 free base was transformed into the oxalate salt by
treating an ether solution with oxalic acid. It was recrystal-
lized from 2-PrOH: 0.64 g (55% yield); [R]D +22.04° (c 1,
MeOH), CD (c 0.015, MeOH) [θ]281 +3369, [θ]275.5 +3768, [θ]229
-20140. The melting point was indefinite; fusion started at
147 °C and was complete at 157-158 °C. Anal. (C26H29NO5‚
H2C2O4‚0.5H2O) C, H, N.
(2S,3S)-(-)-[2-(2,6-Dim eth oxyph en oxy)eth yl][(3-p-tolyl-
2,3-d ih yd r o-1,4-ben zod ioxin -2-yl)m eth yl]a m in e Oxa la te
Hyd r a te [(-)-2]. This was obtained as free base from (-)-4
Resolu tion of (()-tr a n s-3-p-Tolyl-2,3-d ih yd r o-1,4-ben -
zod ioxin -2-ca r boxylic Acid (3). Racemic 312 (4.1 g, 15.0
mmol) in 95% EtOH (65 mL) was treated with a solution of
(S)-(-)-R-methylbenzylamine (1.82 g, 15.0 mmol) in 95% EtOH
(77 mL) and left at room temperature for 30 h. The white
crystals were crystallized twice from 95% EtOH: 1.4 g; mp
223.5-225.5 °C; [R]D -4.71° (c 1, MeOH).
The salt was dissolved in water (50 mL), and the ice-cooled
solution was made basic with 2 N NaOH (50 mL). The
resulting mixture was extracted with ether (3 × 30 mL) to
recover the amine and then acidified with cold 2 N HCl and
extracted with CHCl3 (3 × 30 mL). Removal of dried solvent
gave (2S,3R)-(-)-3-p-tolyl-2,3-dihydro-1,4-benzodioxin-2-car-
boxylic acid [(-)-3]: 0.85 g; mp 169-172 °C; [R]D -1.81° (c 1,
1
MeOH); H NMR (CDCl3) δ 2.37 (s, 3, CH3C6H4), 4.81 (d, J )
6.0 Hz, 1, 2-H), 5.24 (d, J ) 6.0 Hz, 1, 3-H), 6.82-7.30 (m, 8,
ArH), 8.50 (br s, 1, COOH). Anal. (C16H14O4) C, H.
The acid recovered from the mother liquors by a similar
alkaline treatment (3.25 g, 12.0 mmol) was dissolved in 95%
EtOH (39 mL) and treated with a solution of (R)-(+)-R-
methylbenzylamine (1.5 g, 12.0 mmol) in 95% EtOH (55 mL).
The solution was left at room temperature for 30 h and the
white crystals were crystallized twice from 95% EtOH: 1.1 g;
mp 223.5-224.5 °C; [R]D +5.48° (c 1, MeOH).
This salt was treated as described for the other enantiomer
to give (2R,3S)-(+)-3-p-tolyl-2,3-dihydro-1,4-benzodioxin-2-car-
boxylic acid [(+)-3]: 0.7 g; mp 169-172 °C; [R]D +1.64° (c 1,
1
MeOH); the H NMR spectrum was identical to that of (-)-3.
Anal. (C16H14O4) C, H.
(2S,3R)-(+)-3-p -Tolyl-2,3-d ih yd r o-1,4-b en zod ioxin -2-
ca r boxylic Acid [2-(2,6-Dim eth oxyp h en oxy)eth yl]a m id e

Synthesis of Enantiomers of Mephendioxan
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 11 2257
as described for the enantiomer (+)-2 and purified by column
Ack n ow led gm en t. This work was supported by a
grant from MURST. We express our thanks to Prof. G.
P. Spada (University of Bologna) for performing CD
measurements and for valuable discussions.
1
chromatography. The H NMR spectrum exhibited the same
resonances as that of the other enantiomer; enantiomeric
purity was >98% (detection limit), determined with (+)-MTPA
as the chiral shift reagent.
(-)-2 free base was transformed into the oxalate salt by
treating an ether solution with oxalic acid. It was recrystal-
lized from 2-PrOH: 0.76 g (60% yield); [R]D -23.23° (c 1,
MeOH), CD (c 0.015, MeOH) [θ]281 -3173, [θ]275.5 -3566, [θ]229
+21000. The mp was indefinite; fusion started at 96 °C and
was complete at 154-155 °C. Anal. (C26H29NO5‚H2C2O4‚H2O)
C, H, N.
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