Determination of the absolute configuration of the enantiomers of dihydroquinolines, isolated by chiral chromatography, by non empirical analysis of circular dichroism spectra and X-ray analysis

Article abstract of DOI:10.1016/j.tetasy.2011.01.010

The enantiomers of racemic 3,4-dihydroquinolines with an acetal or thioacetal spiro ring and a quaternary stereogenic carbon have been isolated through semipreparative chiral chromatography using a polysaccharide-derived chiral stationary phase (Chiralpak

Full text of DOI:10.1016/j.tetasy.2011.01.010

Tetrahedron: Asymmetry 22 (2011) 270–276
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Determination of the absolute configuration of the enantiomers of
dihydroquinolines, isolated by chiral chromatography, by non empirical
analysis of circular dichroism spectra and X-ray analysis
Salvatore Caccamese a, , Giovanni Battista Barrano a, Mateo Alajarin b, Baltasar Bonillo b, Ángel Vidal b,
⇑
d
Giuseppe Bruno c, Giuseppe Mazzeo d, , Carlo Rosini
⇑
a Dipartimento di Scienze Chimiche, Università di Catania, Viale Andrea Doria 6, 95125 Catania, Italy
b Departamento de Quìmica Orgànica, Universidad de Murcia Campus de Espinardo, 30100 Murcia, Spain
c Dipartimento di Chimica Inorganica, Chimica Analitica e Fisica, Università di Messina, Salita Sperone 31, Vill. S. Agata, 98166 Messina, Italy
d Dipartimento di Chimica, Università della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The enantiomers of racemic 3,4-dihydroquinolines with an acetal or thioacetal spiro ring and a quaternary
stereogenic carbon have been isolated through semipreparative chiral chromatography using a polysaccha-
ride-derived chiral stationary phase (Chiralpak AD) and n-hexane/ethanol as a mobile phase. The absolute
configurations of the enantiomers of four compounds have been determined by a comparison of density
functional theory (DFT) calculations of their electronic circular dichroism (ECD) spectra with the experi-
mental ECD data. A detailed conformer population search to achieve a conformational average of these com-
pounds was crucial, due to the flexibility of these molecules. The conformer distribution was evaluated by
SPARTAN 02 and the structure of each of the conformers found within 4 kcal/mol energy range was optimized
with DFT. The final calculated ECD spectrum obtained after Boltzmann averaging was compared with the
ECD spectrum of the less well retained enantiomer and the correlation (R)/(ꢀ) was established for all com-
pounds. The monocrystals of both enantiomers of one compound were obtained from the HPLC eluates.
Their absolute configurations were determined by X-ray crystallographic analysis and confirmed by ECD
analysis. In all cases, the second-eluted enantiomer in chiral HPLC exhibits an (R)-configuration.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 8 December 2010
Accepted 12 January 2011
Available online 4 March 2011
Dedicated to the memory of Carlo Rosini
(1948–2010), an outstanding scientist, a
dear friend, and an unforgettable colleague
1. Introduction
cyclic nucleus of dihydroquinolines has provided potential drugs
for treating lipoxygenase-mediated disorders.5
Quinoline-containing natural products have attracted the atten-
tion ofsyntheticandmedicinalchemistsformany decades. Thequin-
oline ring system is found in many natural products of interest due to
its wide range of biological activities.1,2 Hydroquinolines constitute a
key structural motif present in many biologically-active compounds,
many of which have industrial and medicinal applications.
Some of us have recently reported6 the facile preparation of chiral
3,4-dihydroquinolines with only one stereogenic centre, its quater-
nary carbon 3, and bearing spiro-fused five and six-membered satu-
ratedheterocyclicrings atcarbon4. These compounds were obtained
in racemic forms due to the absence of absolute chirality sources in
thesyntheticsequencesusedfortheirpreparation,witha6p-electro-
Dihydroquinolines are generally sensitive intermediates in the
most common syntheses of their fully aromatized partners,
although over the last few decades, there has been a growing inter-
est in the preparation of stable dihydroquinoline derivatives.3
Whereas 1,2-dihydroquinolines are by far the most common mem-
bers of this class, the less abundant 3,4-dihydroquinolines are of
particular interest because a number of adequately substituted
derivatives have been shown to be potent inhibitors of the nitric
oxide synthases.4 The spiro-fusion of a third ring onto the hetero-
cyclization as the final key step, whereas alternative and efficient
enantioselectivesynthesesseem, atfirst sight, not easilyconceivable.
Herein our aim was to assign the absolute configuration of the
enantiomers of 3,4-dihydroquinolines 1–4 synthesized as racemic
mixtures.
X
X
X
X
CH3
CH3
N
N
⇑
Corresponding authors. Tel.: +39 95 7385029 (S.C.); +39 971 205478 (G.M.).
E-mail address: scaccamese@dpichi.unict.it (S. Caccamese).
1
3
X = O
X = S
4
X = O
X = S
Chemical Structures
2
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.01.010

S. Caccamese et al. / Tetrahedron: Asymmetry 22 (2011) 270–276
271
To perform this task, we isolated the single enantiomers of 1–4
through semipreparative enantioselective HPLC, employed an ab
initio calculation of the electronic dichroism spectra (ECD) of the
enantiomers and performed a comparison of the calculated and
experimental ECD spectra of the single enantiomers to assign their
absolute configurations. A critical point was the calculation of the
ECD spectra of stable conformer of an enantiomer, since the sign
for Cotton effects can be different in different conformers. Thus,
the overall calculated ECD spectrum originates from a Boltzmann
average of spectra of differently populated conformers. Subse-
quently, the XRD structures of (+)- and (ꢀ)-4 were solved and
the absolute configuration obtained with this method confirmed
the absolute configuration obtained by previous ECD calculations.
pure enantiomers of compound 4 by standing for a few days in a
screw-cap vial of an ethanolic solution afforded crystals, which
were used for XRD analysis. The ee values of the single enantio-
mers of 1–4 were checked by enantioselective HPLC using the con-
ditions described in Table 1, giving values greater than 98% with an
overall recovery of about 30%. The low recovery is due to the pres-
ence of sizeable amounts of impurities in the samples, as shown in
Figure 2 and from a partial retention of the compounds in the
column.
2.2. Electronic circular dichroism
The ECD spectra of the enantiomers of 1–4, isolated by semipre-
parative chiral HPLC as previously described, were measured using
4–9 ꢁ 10ꢀ3 solutions in ethanol. These spectra were mirror images
of each other, as shown in Figure 3 for compound 1, confirming
their enantiomeric nature.
2. Results and discussion
2.1. Enantioselective chromatography
The spectra also exhibited two clear Cotton effects. Remarkably,
as can be observed in the experimental ECD spectra of the second-
eluted enantiomers reported in the black line in Figure 6, the ellip-
ticity at longer wavelengths is higher for the thioacetals 2 and 3,
while the ellipticity is higher at shorter wavelengths for the corre-
sponding acetals 1 and 4. Thus, sulfur, due to a different electronic
configuration, significantly affected the ECD spectra. At this point,
due to the bisignate spectra, the presence of three different chro-
mophores in the molecule and the difference between acetal and
thioacetal spectra, it was worthwhile for us to carry out a careful
and detailed analysis of the ECD spectra of the single enantiomers
of 1–4 in order to assign their absolute configuration by the com-
parison of the predicted and experimental spectra. Indeed, our
experience in this field, with flexible molecules, was noteworthy.10
To obtain the pure enantiomers of racemic 1–4, we carried out a
preparative HPLC analysis using a polysaccharide-derived chiral
stationary phase (CSP) under normal phase conditions (n-hexane/
ethanol). These CSPs have a broad applicability and are widely used
also for preparative purposes.7 From several CSPs available in our
laboratory, we chose Chiralpak AD because amongst the four most
used polysaccharide-derived CSPs, it is the most efficient in terms
of enantioseparation according to statistics from over 1000 race-
mic discovery compounds.8 This observation does not leave out,
however, the possibility that the performances of various CSPs
can be very different with regard to the same analyte.9 Our choice
was satisfactory and the resolution was optimized by varying the
amount of the alcohol (the modifier). As shown in Table 1, the sep-
aration factor
a was good and it was slightly affected by the polar-
2.3. Ab initio ECD calculations and the absolute configuration
determination of 1–4
ity of the mobile phase. Also, the resolution factor Rs remained high
in all experiments.
Figure 1 shows the HPLC chromatograms of racemic 1–4, using
n-hexane/ethanol 90:10 as the mobile phase. On the basis of the
analytical chromatographic results, the preparative isolation of
the enantiomers of racemic 1–4 was carried out using experimen-
tal conditions that ensured reasonable elution times, with still
good Rs and adequate loading capacity. A 1.0 cm internal diameter
Chiralpak AD column was used and 10–15 replicate injections of
100–150 lL of a solution of 15 mg/mL in n-hexane/ethanol 80:20
of compounds 1, 3 and 4 and 5 mg/mL of compound 2 were carried
out.
Figure 2 shows the typical replicate separations of the enantio-
mers of ( )-1. Collection of the eluate corresponding to the two
major chromatographic peaks, centrifugation at 5000ꢁg for
10 min and rotoevaporation of the supernatants furnished about
3 mg of the single enantiomers of 1, 3 and 4 and 1.5 mg of the sin-
gle enantiomers of 2, all as white powders. Recrystallization of the
Nowadays the analysis of ECD spectra can be carried out using
ab initio techniques, leading to safe structural determinations, such
as the assignment of the molecular absolute configuration.11–13 We
decided to simulate the experimental ECD spectra of 1–4 by means
of the Time Dependent Density Functional Theory (TDDFT) meth-
od, using the widely used hybrid functional B3LYP,14 which, with
the 6-31G⁄ (or higher) basis set is being increasingly used by or-
ganic chemists for the determination of the absolute configuration
of small and complex molecules. Representative examples for
synthetic15 and natural compounds16 have recently appeared.
Theoretical ECD spectra were obtained by means of the GAUSSIAN09
package,17 both in the length and velocity representation, using the
lowest 50 states, from excitation energies and rotational strengths,
as a sum of gaussian functions centred at the wavelength of each
transition, with a parameter
r (width of the band at 1/e height)
of 0.15 eV.18
The ECD spectra were calculated both in the length and velocity
formalisms to guarantee origin independence and to evaluate the
quality of the molecular wave functions employed.19 These calcu-
lated spectra were almost coincident, indicating a good level of cal-
culation. For this reason in all figures, only the velocity-form
predicted spectra are reported. In practice, to simulate the experi-
mental ECD spectra of compounds 1–4, we arbitrarily assumed an
(R)-absolute configuration for the stereogenic centre.
Due to the flexibility of the molecules, conformational analysis
at a molecular mechanics level led to four most populated con-
formers for 1, seven for 2, six for 3 and seven for 4. The structures
of the conformers were then fully optimized at DFT/B3LYP/6-31G⁄
level. After free energy calculation we employed a Boltzmann dis-
tribution to calculate the conformer populations. In Figure 4,
we show the structures, Boltzmann distributions and relative
Table 1
Enantioselective HPLC resolution of 3,4-dihydroquinolines 1–4 on Chiralpak AD
Aa (%)
t1
t2
ac
Rs
d
k10
b
Compd
1
1
2
2
3
3
4
4
10
25
10
25
10
25
10
25
1.66
0.90
1.96
1.16
1.75
1.04
0.76
0.42
8.1
5.8
9.0
6.6
8.4
6.2
5.4
4.3
13.0
8.4
12.9
8.8
9.8
7.0
1.97
1.93
1.64
1.63
1.27
1.25
1.42
1.41
6.8
6.1
5.4
4.4
2.6
2.0
2.5
1.5
6.3
4.9
a
b
c
Percentage of ethanol in n-hexane at a flow rate of 1 mL/min; t0, min = 3.0.
Retention factor of the first eluted enantiomer.
Separation factor.
d
Resolution factor.

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S. Caccamese et al. / Tetrahedron: Asymmetry 22 (2011) 270–276
Figure 1. Typical separation of the enantiomers of compounds 1–4. Conditions as line 1 (1), line 3 (2), line 5 (3), line 7 (4) in Table 1.
1
[min]
0.00
20.00
40.00
60.00
80.00
Figure 2. Typical replicate separation of the enantiomers of compound 1 with a
Figure 3. CD spectra of isolated enantiomers of compd. 1 . First (1) and second (2)
HPLC peaks, in ethanol (4 ꢁ 10ꢀ3 M) and 1mm cell.
semipreparative Chiralpak AD column.
conformer energies (in kcal/mol) of the (R)-enantiomers of com-
pounds 1–4.
As shown, these conformers differ from each other by the sense
of the twist of the five and six membered acetal and thioacetal
rings and by the phenyl position (equatorial/axial) at the stereo-
genic carbon.
As expected, the equatorial phenyl conformer is the most stable.
In order to save computational effort and time, ECD calculations
were performed only on about 98% of the overall conformer popu-
lation. The direction of the electric transition dipole moments is
quite different in conformers where the phenyl ring at the stereo-
genic carbon is equatorial with respect to the conformers where
this phenyl is axial. As a consequence, the ECD spectra of the con-
formers strongly differ and the sign of the Cotton effect changes in
some cases, as shown for example for conformers 4a–4d in
Figure 5, for the same assumed (R)-configuration. With a TDDFT/
B3LYP/6-31G⁄ level of calculation we achieved a good agreement
between the experimental and calculated ECD saving computa-
tional effort and avoiding the use of larger basis sets. In Figure 6,
the comparisons between experimental ECD spectra (experimental
ECD, black line) together with the overall calculated ones (theoret-
ical ECD, grey line) of compounds 1–4 are displayed. The experi-
mental ECD spectra regard the (ꢀ)-enantiomer, which is the
second eluted one.
All calculations correctly reproduced the number and sign of
the observed Cotton effects at about 290 nm and 260 nm, even if
a clear red shift20 of the theoretical bands with respect to the
experimental spectra can be noticed. For 2 and 3, the theoretical
ECD spectra (Fig. 6) were reduced four times. The good agreement

S. Caccamese et al. / Tetrahedron: Asymmetry 22 (2011) 270–276
273
Figure 4. Structures of the most stable conformers of (R)-enantiomer of compounds 1–4 optimized at the B3LYP/ 6-31G⁄ level and their relative energies (in kcal molꢀ1) and
populations (in brackets, respectively). A: (R)-1 (1a-d); B: (R)-2 (2a-g); C: (R)-3 (3a-f); D: (R)-4 (4a-g).
between calculated and experimental spectra strongly supports
the (R)/(ꢀ) absolute configuration assignment.
can be obtained, free of charge, on an application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax +44(0)1223 336033 or
e-mail: deposit@ccdc.cam.ac.uk].
2.4. Single crystal X-ray analysis
3. Conclusions
The assignment of the absolute configuration of the enantio-
mers of 4 was then confirmed by X-ray analysis of single crystals
obtained in small amounts by enantioselective HPLC. Both (S)-4
and (R)-4 crystallize in the P212121 space group with a molecule
in the asymmetric unit. The corresponding Flack parameter for
(S)-4 was found to be 0.0(3), and for (R)-4 was determined to be
ꢀ0.2(4). The details of the crystal structure determination and
refinements for compounds (S)-4 and (R)-4 are given in Table 2.
An ORTEP view of both enantiomers is shown in Figure 7.
These values are all comparable with the corresponding struc-
tural parameters reported for similar compounds reported in the
Crystallographic Data Centre.21 Puckering parameters indicate a
distorted envelope conformation of the dihydropyridine hexatomic
ring, while the cyclic acetal ring shows an ideal chair conformation.
Molecular packing is mainly determined by Van der Waals interac-
tions and few weak hydrogen bonds involving both O1 and O2 ace-
tal oxygen atoms.
We have presented the isolation of the enantiomers of four
3,4-dihydroquinolines through semipreparative chiral HPLC. The
absolute configurations of the enantiomers were determined by
comparison of the measured and calculated ECD spectra. The flex-
ibility of these molecules made a detailed conformational distribu-
tion search necessary, including calculation of the ECD spectra of
several conformers and Boltzmann-weighted superimposition of
the ECD of the single conformers. We have shown that a good
agreement between experimental and calculated ECD can be
achieved using TDDFT calculations with a simple basis set at 6-
31G⁄ level and correct input geometry of several conformers. The
absolute configurations were then confirmed by X-ray crystal
structure determination of both enantiomers of one compound.
4. Experimental section
4.1. Starting materials
Furthermore, a weak intermolecular H–p hydrogen bond in-
volves a methyl hydrogen atom and the phenyl group bonded to
the stereogenic carbon atom at C17 [Hꢂ ꢂ ꢂPh-Centroid 2.816(3) Å].
Crystallographic data (excluding structure factors) for (S)-4 and
(R)-4 have been deposited with the Cambridge Crystallographic
Data Centre as Supplementary Publication Numbers CCDC
776100 [for (S)-4] and CCDC 776101 [for (R)-4]. Copies of the data
Racemic compounds 1 and 3 were synthesized as previously de-
scribed.6 Racemic compounds 2 and 4 were prepared following the
same synthetic procedure but using 1,3-propanedithiol and 1,3-
propanediol, respectively, as reagents in the first step of the
synthetic sequence, the acetalization of 2-azidobenzaldehyde.

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S. Caccamese et al. / Tetrahedron: Asymmetry 22 (2011) 270–276
25
25
15
5
Δε
Δε
4a
4b
15
5
220
240
260
280
300
320
340
220
240
260
280
300
320
340
-5
-15
-25
-5
RV
RL
-15
-25
RV
RL
25
15
5
25
15
5
Δε
Δε
4c
4d
220
240
260
280
300
320
340
230
250
270
290
310
330
350
-5
-5
RV
RL
-15
-15
RV
RL
-25
-25
Figure 5. Simulated ECD spectra of conformers (98.2% of overall populations) of compound 4, as obtained at the TDDFT/B3LYP/6-31G⁄ level, velocity (RV) and length (RL)
formalism, using the lowest 50 states on the DFT/B3LYP/6-31G⁄ geometry.
4
10
theor. ECD
2
2
1
5
0
0
-2
-4
-5
theor. ECD
exp. ECD
exp. ECD
-10
230
250
270
290
310
330
230
250
270
290
310
330
λ (nm)
λ (nm)
4
2
15
10
5
3
4
0
0
-5
-2
-4
theor. ECD
theor. ECD
-10
-15
exp. ECD
exp. ECD
230
250
270
290
λ (nm)
310
330
230
250
270
290
λ (nm)
310
330
Figure 6. Averaged calculated CD spectra of (R)-configuration of compounds 1–4 (grey line). Experimental CD spectra of the second eluted enantiomer in chiral HPLC of
compounds 1–4 (black line).

S. Caccamese et al. / Tetrahedron: Asymmetry 22 (2011) 270–276
275
Table 2
Crystal data and structure refinement for (S)-4 and (R)-4
Enantiomer
S
R
Empirical formula
Formula weight
C
19H19NO2
293.35
Crystal system, space group Orthorhombic, P212121 (N. 19)
Unit cell dimensions
a = 8.57590(10) Å
b = 11.9996(2) Å
c = 14.6026(2) Å
1502.72(4) Å3
a = 8.5759(2) Å
b = 11.9992(3) Å
c = 14.5998(3) Å
1502.38(6) Å3
Volume
Z
4
Density (calculated)
Absorption coefficient
F(0 0 0)
Crystal size
Theta range for data
collection
1.297 Mg/m3
0.084 mmꢀ1 (k = 0.71073 Å)
624
0.68 ꢁ 0.52 ꢁ 0.43 mm3 0.41 ꢁ 0.30 ꢁ 0.23 mm3
2.75–25.00°
Index ranges
Reflections collected
Independent reflections
10 ꢃ h ꢃ 10, ꢀ14 ꢃ k ꢃ 14, ꢀ17 ꢃ l ꢃ 17
43447
79943
2645 [R(int) = 0.0201] 2641 [R(int) = 0.0204]
99.7%
Full-matrix least-squares on F2
Completeness to h = 25.00° 99.8%
Refinement method
Data/restraints/parameters 2645/0/200
2641/0/200
1.014
Goodness-of-fit on F2
0.800
Final R indices [I > 2
R indices (all data)
Flack parameter
r
(I)]
R1 = 0.0259,
wR2 = 0.0747
R1 = 0.0262,
wR2 = 0.0754
0.0(3)
R1 = 0.0253,
wR2 = 0.0704
R1 = 0.0255,
wR2 = 0.0709
ꢀ0.2(4)
Largest diff. peak and hole 0.135/ꢀ0.114 e Åꢀ3
0.140/ꢀ0.101 e Åꢀ3
4.2. 30-Methyl-30-phenylspiro[1,3-dithiane-2,40(30H)-quinoline]
2
Figure 7. X-ray crystal structure of both enantiomers of compound 4 [(R) on the top
and (S) on the bottom] in their asymmetric unit showing the numbering scheme.
Thermal ellipsoids are drawn at the 50% of probability level while hydrogen size is
arbitrary.
Obtained by heating a solution of the corresponding keteni-
mine in anhydrous toluene for 24 h (60% yield). Colourless
prisms (diethyl ether). Mp: 141–143 °C. IR (Nujol)
1591 (m), 1311 (s), 1274 (m), 1213 (m), 1108 (w), 972 (w),
879 (w), 756 (s), 721 (s), 675 (m) cmꢀ1 1H NMR (300 MHz,
m: 1633 (vs),
4.4. Enantioselective HPLC
.
CDCl3, 25 °C) d: 1.64 (s, 3 H), 1.66–1.84 (m, 2 H), 2.15–2.32
(m, 2 H), 2.48–2.57 (m, 2 H), 7.25–7.39 (m, 5 H), 7.46 (dd, 1
H, J = 7.5, 1.5 Hz), 7.58–7.62 (m, 2 H), 7.95 (dd, 1 H, J = 7.5,
1.5 Hz), 8.16 (s, 1H). 13C NMR (75 MHz, CDCl3, 25 °C) d: 18.7,
23.1, 26.9, 27.0, 51.2 (s), 60.6 (s), 126.3, 127.6, 127.8, 128.1,
128.7, 130.0, 132.2 (s), 138.7 (s), 140.9 (s), 168.9. MS (70 eV,
EI): m/z (%): 325 (M+, 6), 251 (46), 250 (100), 219 (58), 218
(41), 217 (23), 204 (22), 103 (29), 77 (21). Elemental Anal. Calcd
for C19H19NS2 (325.50): C, 70.11; H, 5.88; N, 4.30. Found: C,
69.90; H, 5.80; N, 4.14.
The analytical resolution of 1–4 was performed on a polysaccha-
ride-derived column (250 mm ꢁ 4.6 mm i.d.) Chiralpak AD (amy-
lose tris[3,5-dimethylphenylcarbamate] coated on 5 lm silica gel.
For the semipreparative isolation of the enantiomers, a Chiralpak
AD column size 250 mm ꢁ 10 mm i.d. was used. Both columns
were purchased from Chiral Technologies (Illkirch, France). A col-
umn in-line filter with 0.5 lm stainless steel frit of 3 mm diameter
was used to protect the HPLC column. Column void volume (t0) was
measured by injection of tri-tert-butylbenzene as a nonretained
sample.22 The HPLC parameters (k,
a, and Rs) were those typically
employed.23 The HPLC system consisted of a PU 980 pump, a LG-
1580-02 low pressure mixer, and a DG-1580-54 in line degasser
4.3. 30-Methyl-30-phenylspiro[1,3-dioxane-2,40(30H)-quinoline] 4
with a Rheodyne injection valve equipped with 20 or 200 lL sam-
Obtained by heating a solution of the corresponding ketenimine
in anhydrous o-xylene for 24 h (81% yield). Colourless prisms
ple loops, a Uvidec 100-III UV spectrophotometric detector operat-
ing at 235 nm (all from Jasco). Chromatograms were acquired and
processed using a computer-based Jasco Borwin 2 software.
(diethyl ether). Mp: 154–155 °C. IR (Nujol)
m: 1618 (m), 1598
(w), 1496 (w), 1215 (s), 1178 (w), 1141 (m), 1118 (s), 1093 (vs),
1054 (w), 1033 (m), 1024 (m), 973 (m), 962 (m), 929 (w), 904
4.5. Chiroptical measurements
(w), 877 (w), 846 (w), 773 (vs), 761 (m) cmꢀ1
.
1H NMR
(300 MHz, CDCl3, 25 °C) d: 1.47 (s, 3 H), 1.55–1.66 (m, 1 H),
1.72–1.84 (m, 1 H), 3.17–3.25 (m, 1 H), 3.53–3.73 (m, 3 H), 7.24–
7.36 (m, 4 H), 7.37–7.40 (m, 1 H), 7.42–7.51 (m, 3 H), 7.63 (dd, 1
H, J = 7.4, 1.0 Hz), 8.06 (s, 1 H). 13C NMR (75 MHz, CDCl3, 25 °C)
d: 16.7, 24.7, 51.4 (s), 58.2, 59.7, 98.7 (s), 125.3, 127.1, 127.2,
127.6, 128.8 (s), 129.4, 129.5, 138.9 (s), 143.4 (s), 171.3. MS
(70 eV, EI): m/z (%): 293 (M+, 50), 251 (69), 250 (89), 234 (100),
220 (59), 206 (90), 130 (63), 103 (83), 77 (74). Elemental Anal.
Calcd for C19H19NO2 (293.37): C, 77.79; H, 6.53; N, 4.77. Found:
C, 77.98; H, 6.67; N, 4.65.
The ECD spectra were measured in ethanol on a Jasco 810 spec-
tropolarimeter using a 1 mm cell and scanning ten times from 350
to 230 nm at 50 nm/min.
4.6. Computational details
All calculations were carried out on a simple PC endowed with a
Intel(R) Xeon Quadcore E5420 at 2.50 GHz. The preliminary confor-
mational distribution search has been performed by the SPARTAN02
package24 using the MMFF94s molecular mechanics force field.

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S. Caccamese et al. / Tetrahedron: Asymmetry 22 (2011) 270–276
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The ‘Systematic’ and ‘Montecarlo’ options were used and search of
all possible conformers has been performed, considering the de-
grees of freedom of system and retaining only the structures with
an energy of not more than 4 kcal/mol above the most stable con-
former. The real minimum energy conformers found by molecular
mechanics have been further fully optimized at the DFT/B3LYP/6-
31G⁄ level as implemented in the GAUSSIAN09 package.17 All con-
formers are real minima, no imaginary vibrational frequencies
have been found and the free energy values have been calculated
and used to get the Boltzmann population of conformers at
298.15 K. The ECD calculations were carried out by means of
time-dependent DFT methods using the hybrid B3LYP functional
and the 6-31G⁄ as available within GAUSSIAN09. All calculations were
performed in gas phase avoiding more computational effort due to
application of solvation models.
9. Caccamese, S.; Chillemi, R.; Perna, F. M.; Florio, S. J. Chromatogr., A 2009, 1216,
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4.7. X-ray data collection and structure refinement
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As discussed in Section 2.1, enantioselective HPLC afforded pure
enantiomers. Colourless single crystals of both enantiomers were
selected for X-ray structural analysis from the crystals obtained
from enantioselective HPLC. Data were collected at room tempera-
ture with a Bruker APEX II CCD area-detector diffractometer and
graphite-monochromatized Mo K
a radiation (k = 0.71073 Å). Data
collection, cell refinement, data reduction and absorption correc-
tion by a multi-scan method were performed by the programmes
of the Bruker software. The structure was solved by direct methods
using XS97.25 The non-hydrogen atoms were refined anisotropi-
cally by the full-matrix least-squares method on F2 using
SHELXL-97.26 All H atoms were introduced in calculated positions
and constrained to ride on their parent atoms.
17. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.;
Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.;
Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven,
T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.;
Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.;
Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas,
O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. GAUSSIAN 09, Revision A.02;
Gaussian, Inc.: Wallingford, CT, 2009.
Acknowledgements
G.M. and C.R. are grateful to the MIUR, Rome for financial sup-
port (PRIN 2008 ‘Determination of the molecular absolute configu-
ration by means of chiroptical spectroscopies: development of new
efficient methods and their application to bioactive molecules’).
M.A., A.V. and B.B. thank the financial support of MICINN (Project
CTQ 2008-05827/BQU) and Fundacion Seneca-CARM (Project
08661/PI/08). S.C. thanks the University of Catania (Progetti di
Ricerca di Ateneo) for financial support.
18. Rosini, C.; Giorgio, E.; Tanaka, K.; Verotta, L.; Nakanishi, K.; Berova, N. Chirality
2007, 19, 434–445.
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