The Betti base: Absolute configuration and routes to a family of related chiral nonracemic bases

Article abstract of DOI:10.1016/S0957-4166(98)00379-6

A detailed protocol for the resolution of the Betti base [i.e. 1-(α- aminobenzyl)-2-naphthol] was investigated and the absolute configuration of the two isomers was established by means of an X-ray diffractometric study. A series of optically active deriv

Full text of DOI:10.1016/S0957-4166(98)00379-6

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 3667–3675
The Betti base: absolute configuration and routes to a family of
related chiral nonracemic bases
Cosimo Cardellicchio,a Giuseppe Ciccarella,a Francesco Naso,a,∗ Emanuela Schingaro b and
Fernando Scordari b
aC.N.R., Centro di Studio sulle Metodologie Innovative di Sintesi Organiche, Dipartimento di Chimica, Università di Bari,
via Amendola 173, 70126 Bari, Italy
bDipartimento Geo-Mineralogico, Università degli Studi, via Orabona 4, 70125, Bari, Italy
Received 31 July 1998; accepted 16 September 1998
Abstract
A detailed protocol for the resolution of the Betti base [i.e. 1-(α-aminobenzyl)-2-naphthol] was investigated
and the absolute configuration of the two isomers was established by means of an X-ray diffractometric study. A
series of optically active derivatives was also prepared. The list includes the N-benzyl- and the N,N,O-trimethyl
derivatives. The N,N-dimethyl derivative was prepared in racemic form by means of the Betti reaction and was
resolved into the two enantiomers with an extremely easy and efficient procedure. The O-methyl derivative
was prepared in a racemic form and subjected to resolution. The configuration of each base was determined by
correlation with the configuration of the Betti amine. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
At the beginning of this century, Betti, a pioneer of asymmetric synthesis,1 reported a straightforward
condensation of 2-naphthol, ammonia and 2 equivalents of benzaldehyde.2–5 A product 1 is easily ob-
tained from these reagents. The so-called Betti base 2 is produced by acid treatment of this intermediate,
followed by the addition of KOH to the ammonium salt to yield the amine 26–11 (Scheme 1).
On the basis of IR data,12 the structure 1a is considered valid for the solid material, whereas an
equilibrium between 1a and 1b is established in solution.12
∗
Corresponding author. E-mail: naso@area.ba.cnr.it
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00379-6

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C. Cardellicchio et al. / Tetrahedron: Asymmetry 9 (1998) 3667–3675
Scheme 1.
A similar reaction can be performed using other naphthols13–16 or quinolinols17–20 or replacing
ammonia with amines.21 The enantiomers of the base 2 were resolved by Betti22,23 and used by the
same author to resolve 2-(p-anisyl)propanaldehyde24 and glyceraldehyde25 and to discriminate between
aldohexoses and ketohexoses.26 However, in spite of both availability and low cost of the base 2, no
further application was reported for the optically active compound. In recent times, the racemic Betti
base was used only for the transformation into products showing antibacterial activity.27 In addition, a
variety of racemic structures related to the Betti base have been prepared recently by preformed iminium
salts.28
In our opinion, the long silence over this optically active material was in sharp contrast with the high
popularity enjoyed by the chirality theme in recent decades. This appeared to us still more surprising
considering the bifunctionality of the base which is reminiscent of well recognized members of the
chirality pool (e.g. ephedrine derivatives, cinchona alkaloids), and can be considered a good requisite for
further elaborations.
With this background, an investigation aimed at re-evaluation of the Betti base seemed warranted. In
the present contribution, we report the determination of the absolute configuration of the two enantiomers
of 2, the synthesis of various derivatives of the base and the correlation of their configuration with the
configuration of the prototype 2.
2. Results and discussion
The synthesis of the base 2 was performed in a similar manner to that described by Betti.11 However,
some small but significant variations were introduced by us in the resolution of the base, since the
Betti original protocol was vague and fragmentary, expecially as far as the procedure to obtain (−)-2
is concerned. The crystalline material which was subjected to X-ray diffraction analysis was represented
by the hydrobromide 2·HBr·H2O, obtained by treating (+)-12,3,11 with aqueous hydrogen bromide.
As shown in Fig. 1, the absolute configuration of (+)-2 was found to be (S).

C. Cardellicchio et al. / Tetrahedron: Asymmetry 9 (1998) 3667–3675
3669
Fig. 1. Projection down the a-axis of the structure of the hydrobromide 2·HBr·H2O, obtained by treating (+)-1 with aqueous
hydrogen bromide
3. Synthesis of derivatives of the Betti base
The N-benzyl derivative 3 was easily obtained by reducing (S)-(+)-1 with hydrogen in the presence of
Pd/C, or with NaBH4 (Scheme 2).
Scheme 2.
In an equally straightforward manner, treatment of the (S)-(+)-2 base with NaOH/CH3I led to the
trimethyl derivative (S)-(−)-4 (Scheme 3).
Scheme 3.

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In order to obtain the N,N-dimethylamino derivative 5, in principle a chemoselective alkylation of
the amino group should be required. However, we found that the problem of chemoselection could be
skipped by adopting a variation of the synthetic strategy. Indeed, the N,N-dimethylamino derivative 5 can
be easily obtained by Betti reaction replacing ammonia with dimethylamine21 (Scheme 4).
Scheme 4.
Furthermore, resolution of the enantiomers of 5 was achieved in an extremely easy manner by
separation of the diastereoisomeric tartaric acid salts in acetone. The less soluble salt was found to
precipitate in an almost pure form. As a consequence, the more soluble salt was obtained, also almost
pure, by evaporation of the solvent. After the hydrolysis of the less soluble salt, the N,N-dimethylamino
derivative (+)-5 was treated with NaOH/CH3I to give (S)-(−)-4, thus allowing us to attribute the (S)
configuration to the compound (+)-5 (Scheme 5).
Scheme 5.
Finally, it was considered convenient to obtain the O-methyl derivative of 2. Difficulties in preparing
such a derivative were described by Ray and coworkers,29,30 who eventually synthesized the same product
by a multistep procedure.29 We found that methylation of the oxygen was possible by treatment of 1 with
NaOH/CH3I followed by hydrolysis of the intermediate 6 (Scheme 6).
Scheme 6.
Unfortunately, the procedure could be applied only to the synthesis of the racemic O-methyl derivative.
The extension of the method to the optically active material was plagued with the complete loss of optical
activity, very likely the result of an aza-allylic tautomerism.31 Indeed, compound 8 was found as a side
product in the methylation of 1 (Scheme 7).
However, this pitfall did not represent a special problem, since it is known that the O-methyl derivative
7 can be resolved by using malic acid.29 As a consequence, the optimized procedure could be represented
by the synthesis of the racemic compound, starting with 1, followed by resolution with this acid.

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3671
Scheme 7.
Careful methylation of the nitrogen atom of (+)-7 was found to give (S)-(−)-4. Thus, both compounds
must present the (S) configuration (Scheme 8).
Scheme 8.
4. Conclusion
Our work has shown that a family of interesting chiral nonracemic bases with a known configuration
can be obtained by using the Betti reaction and performing simple operations. Taking into account that
a scale-up can be easily envisaged,32 the obtained bases, in particular the base 5, appear very convenient
from an economical point of view. In addition, the products of our investigation are likely candidates
for a variety of applications dealing with enantioselective processes. Our contributions in this field will
appear in forthcoming papers.
5. Experimental section
1
The purified reaction products were characterized by their H- and 13C-NMR spectra, recorded at
500 and 125 MHz, respectively, and their mass spectra determined, when possible, by GC–MS analysis
(SE30, 30 m, capillary columns and mass selective detector, 70 eV).
5.1. Improved synthesis of 2
Crude 1,3-diphenyl-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazine 1a and 2·HCl were obtained according
to the Betti procedure11 and used in the preparation of 2 without further purification.
80 ml of 2 M Na2CO3 was added to a suspension of 2·HCl in water (15 ml). The mixture was stirred
for 1 h at rt and then extracted with diethyl ether and dried (anhydrous Na2SO4). The organic solvent was
evaporated under reduced pressure, producing a residue, 1-(α-aminobenzyl)-2-naphthol 2 (5.7 g, yield
85%), which can be further purified by washing with a small amount of diethyl ether. Mp 124–125°C
(lit.11 mp 124–125°C).

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5.2. Resolution of 2
A solution of (2R,3R)-tartaric acid (4.51 g, 30.0 mmol) in an ethanol 95%:methanol mixture (20:10
ml) was added dropwise to a solution of racemic 2 (7.49 g, 30.0 mmol) in ethanol 95% (240 ml). After
the addition, the mixture was stirred for 6 h at rt and then filtered. Methanol (30 ml) was added to the
less soluble salt and the suspension was stirred for 3 h at rt. The salt was filtered and collected (5.4 g,
13.5 mmol, 45%). The mother solution was evaporated to dryness under reduced pressure, producing an
orange residue, which was washed with methylene chloride, yielding a white crystalline powder (3.01 g,
7.54 mmol, 25%).
Each diastereomeric salt was suspended in water (10 ml/g salt) and a 2 M Na2CO3 solution was added
(10 ml/g salt) to this suspension. After 45 min, the mixture was extracted with diethyl ether and dried
(Na2SO4). The solvent was evaporated under reduced pressure (yield 95%) to give resolved 2 which was
further purified by washing with a small amount of diethyl ether.
The less soluble salt originated (+)-2, mp 136–137°C, [α]D +56 (c 4.4, benzene) (lit.22 [α]D +58.84
(c 5, benzene)). The salt recovered from the evaporation of the mother liquors originated (−)-2, mp
136–137°C, [α]D −56 (c 4.4, benzene) (lit.22 [α]D −58.9 (c 5, benzene)). The enantiomeric purity was
measured by 1H-NMR experiments with the addition of (R)-(−)-3,5-dinitro-N-(1-phenylethyl)benzamide
and was found to be >98%.
5.3. Synthesis of (+)-1
A solution of benzaldeyde (0.132 g, 1.24 mmol) in 2 ml of ethanol 95% was added dropwise to a
solution of (+) 2 (0.310 g, 1.24 mmol) in 14 ml of ethanol 95%. The mixture was stirred overnight at rt.
The solid was recovered by filtration (0.393 g, 1.16 mmol, yield 94%). (+)-1: mp 163–164°C, lit.33 mp
158°C, [α]D +169 (c 1; CHCl3), lit.33 [α]D +110.72 (c 4.734; benzene).
5.4. Absolute configuration of 2·HBr·H2O
2·HBr·H2O was obtained from (+)-1 as described for the racemic 2·HCl. The crystalline compound
was dissolved in ethanol and the solution was slowly evaporated to dryness. The crystalline salt was
subjected to a single crystal X-ray diffraction analysis. The data collection was carried out on a Nonius
CAD4 diffractometer at room temperature using MoKα radiation (λ=0.71067 Å). Relevant crystal data
are: crystal dimensions 0.16×0.17×0.55 mm; formula C17H16BrNO·H2O, Mr=348.24, orthorhombic,
space group P212121; a=5.198(1), b=16.474(2), c=17.999(3) Å, V=1541.3 Å3; Z=4, dx=1.48 g/cm3.
The cell constants were obtained from a least squares fit of the setting angle on 25 reflections in the
range 7≤θ≤13°. The intensity measurements were performed with the ω/2θ scan mode in the range
2≤θ≤28°, 0≤h≤6, 0≤k≤21, 0≤l≤23 for a total of 2162 reflections (1710 unique) and subsequently
corrected for Lorentz and polarization effects. An empirical absorption correction (ψ-scan) was also
applied.34
The structure was solved using direct methods (SIR92)35 and refined with the full matrix least squares
procedure (CRYSTALS’96).36 Atomic positions with isotropic equivalent temperature factors, bond
distances and bond angles are submitted separately. A sketch of the molecule with the Br− ion and a
water solvent molecule is shown in Fig. 1. The anisotropic refinement of 1179 reflections with I>2σ(I)
converged at R=0.053, Rw=0.038. All hydrogen atoms were located in calculated positions with the
exception of H and Hw, bound to the oxygen atoms O and Ow, respectively. These hydrogen atoms were
located by difference Fourier and then refined isotropically. Attempts to locate the second hydrogen atom

C. Cardellicchio et al. / Tetrahedron: Asymmetry 9 (1998) 3667–3675
3673
of the solvent molecule by difference Fourier were unsuccessful. Minimum and maximum map densities
of the difference Fourier after the last refinement step were −0.474 and 0.558 e−/Å3, respectively.
The refinement of the centrosymmetric structure led to R=0.070, Rw=0.050, implying that the molecule
illustrated in Fig. 1 is of the correct configuration. The absolute configuration of the molecule was also
determined according to the method proposed by Flack.37 The value of the refined Flack’s parameter,
using the coordinate data set calculated, is −0.06(4) (and 0.04(6) for all reflections), thus confirming that
Fig. 1 refers to the actual atomic coordinates for the studied compound. The determination of the absolute
configuration for 2·HBr·H2O yielded the (S) configuration for the (+)-2 enantiomer, and consequently
for the (+)-1 enantiomer, and the (R) configuration for the (−)-2 isomer.
5.5. 1-(α-N-Benzylaminobenzyl)-2-naphthol 3
Racemic 1 or (+)-1 in toluene were reduced by hydrogen in the presence of Pd/C (90% yield) or by
NaBH4 in THF (95% yield). Racemic 3, mp 136–137°C (lit.30 mp 143°C). (+)-3, mp 124–125°C, [α]D
+208 (c=1; benzene). 1H-NMR (CD3OD) δ 7.66–7.64 (m, 2H), 7.57–7.55 (m, 1H), 7.29–7.10 (m, 13H),
7.04–7.02 (m, 1H), 5.66 (s, 1H), 3.90 (d, J=13.3 Hz, 1H), 3.72 (d, J=13.3 Hz, 1H), 2.08 ppm (m, 1H).
13C-NMR (CDCl3) δ 156.71, 141.17, 137.82, 132.61, 129.76, 128.97, 128.80, 128.68, 128.66, 128.50,
127.99, 127.66, 126.43, 122.41, 121.07, 120.05, 112.91, 62.67, 52.62 ppm. The enantiomeric purity was
determined by chiral HPLC (Chiralcel OD, hexane:isopropanol=90:10, flow rate 1 ml/min, separation
factors for the racemic compound α=1.39) and was found to be >98%.
5.6. [(2-Methoxynaphth-1-yl)benzyl]dimethylamine 4
Powdered NaOH (1.8 g, 45.0 mmol) was added to a solution of 2 (2.50 g, 10,0 mmol) in THF (30
ml). After 10 min CH3I (6 ml) was dropped into the slurry. The mixture was stirred for 6 h and then
a solution of saturated NH4Cl was added. After extraction with diethyl ether, the organic extracts were
dried with anhydrous Na2SO4 and the solvent was removed under reduced pressure. The crude product
was crystallized from ethanol 95% yielding 2.62 g (90%) of racemic 4, mp 117–118°C. (−)-4, mp
98–99°C, [α]D −26 (c 1.09; CHCl3). MS 70 eV m/e (relative intensity) 291 (M+, 17), 247 (59), 215
(26), 134 (34), 91 (100). 1H-NMR (CDCl3) δ 9.23 (d-like, J=8.7 Hz, 1H), 7.69–7.66 (m, 2H), 7.63–7.61
(m, 2H), 7.44–7.40 (m, 1H), 7.29–7.17 (m, 4H), 7.08–7.04 (m, 1H), 5.40 (s, 1H), 3.97 (s, 3H), 2.28 ppm
(broad s, 6H). 13C-NMR (CDCl3) δ 154.57, 143.34, 132.61, 129.51, 129.14, 128.18, 127.96, 127.80,
126.16, 125.97, 125.86, 124.33, 123.40, 113.87, 68.15, 56.75, 45.2 ppm. Anal. calcd for C20H21NO: C,
82.44; H, 7.26; N, 4.81. Found C, 82.60; H, 7.41; N, 4.71.
5.7. Resolution of 1-(α-N,N-dimethylaminobenzyl)-2-naphthol 5
Racemic 5, mp 165–166°C (lit.21 mp 164–164.5°C) was prepared by the Betti reaction, as previously
shown for compound 2. Spectral data agreed with those reported.28 (2R,3R)-Tartaric acid (3.5 g, 23.44
mmol) in 100 ml of acetone was added to racemic 5 (6.5 g, 23.44 mmol) in acetone (150 ml). Upon
addition of the acid, the solution became pale yellow. After 5 h, the mixture was filtered and the less
soluble salt was washed with acetone (4.5 g, 10.5 mmol, 45%). The mother solution was evaporated to
dryness under reduced pressure, yielding a yellow residue which was crystallized from ethyl ether:ethyl
acetate 80:20 to a white powder (4.1 g, 9.6 mmol, 41%).
Each diastereomeric salt was suspended in water (10 ml/g salt) and a 2 M Na2CO3 solution was
added (10 ml/g salt) to the suspension. After 15 min, the mixture was extracted with ethyl acetate and

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dried (Na2SO4). The solvent was evaporated under reduced pressure (yield 95%). The less soluble salt
originated (+)-5, mp 158°C, [α]D +238 (c 0.5; ethanol). The salt recovered from the evaporation of the
mother liquors originated (−)-5, [α]D −238 (c 0.5; ethanol). The enantiomeric purity was determined by
chiral HPLC (Chiralcel OD, hexane:isopropanol=99.5:0.5, flow rate 1 ml/min, separation factors for the
racemic compound α=1.311) and was found to be >98%.
5.8. (2-Methoxynaphth-1-yl)benzylamine 7
Powdered NaOH (1.8 g, 45 mmol) was added to a solution of 1 (10.12 g, 30 mmol) in 60 ml of THF
and 80 ml of acetone at 0°C. After 10 min, 5 ml of methyl iodide was added and the mixture was stirred
at rt for 6 h. The reaction mixture was concentrated under reduced pressure, and then extracted with ethyl
acetate. The organic phase was dried (Na2SO4) and the solvent was evaporated under reduced pressure,
yielding a pale yellow mixture of 6 and 8.
The crude mixture was treated with 100 ml of HCl 2 N, heating with vigorous stirring at 70–80°C for 1
h. After cooling to room temperature, the solid phase was filtered, washed first with cold water and then in
a Soxhlet apparatus with acetone for 3 h. The salt was treated with a solution of Na2CO3 2 M for 1 h at rt,
and the mixture was extracted with diethyl ether. The organic phase was dried (Na2SO4) and the solvent
was evaporated under reduced pressure to yield 5.093 g (64%) of 7, mp 98–101°C (lit.29 mp 102°C). MS
1
70 eV m/e (relative intensity) 263 (M+, 36), 232 (16), 186 (100), 170 (38). H-NMR (CDCl3) δ 8.03
(d-like, J=8.6 Hz, 1H), 7.78–7.75 (m, 2H), 7.41–7.35 (m, 3H), 7.31–7.22 (m, 4H), 7.16–7.13 (m, 1H),
6.14 (s, 1H), 3.72 (s, 3H), 2.27 ppm (broad s, 2H). 13C-NMR (CDCl3) δ 154.64, 146.50, 131.86, 129.52,
129.00, 128.56, 127.75, 126.96, 126.35, 125.85, 125.65, 123.45, 123.23, 114.07, 56.27, 50.72 ppm. 7
was resolved by malic acid, according to a reported procedure.29 (+)-7: [α]D +196 (c 1.6; CHCl3) (lit.29
[α]D +197 (c 1.55; diethyl ether)). The enantiomeric purity was measured by 1H-NMR experiments with
the addition of (R)-(−)-3,5-dinitro-N-(1-phenylethyl)benzamide and was found to be >98%.
5.9. Correlations of configuration
5.9.1. Synthesis of (S)-(−)-4 from (+)-5
(+)-5 ([α]D +238 (c 0.5; ethanol)) was methylated according to the methylation procedure described
for (S)-2, yielding (S)-(−)-4, [α]D −26.2 (c 1.1; CHCl3).
5.9.2. Synthesis of (S)-(−)-4 from (+)-9
(+)-9 ([α]D +196 (c 1.6; CHCl3)) was methylated according to the methylation procedure described
for (S)-2, yielding (S)-(−)-4, [α]D −26.8 (c 1.1; CHCl3).
Acknowledgements
This work was financially supported in part by Ministero dell Università e della Ricerca Scientifica e
Tecnologica, Rome (National Project ‘Stereoselezione in Sintesi Organica. Metodologie e Applicazioni’)
and from the University of Bari. Dr. Antonio Cardone is gratefully thanked for preliminary experiments.
We are grateful to Prof. V. Bertolasi of the Centro di Strutturistica Diffrattometrica of the Ferrara
University for performing the data collection.

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3675
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