Spectroscopic characterization of the 1-substituted 3,3-diphenyl-4-(2′-hydroxyphenyl)azetidin-2-ones: Application of 13C NMR, 1H-13C COSY NMR and mass spectroscopy

Article abstract of DOI:10.1016/j.saa.2007.08.003

The article deals with spectroscopic characterization of azetidin-2-ones. The presence of substituents like hydroxyl, fluoro, methoxy and benzhydryl, etc., on the azetidin-2-one ring significantly affects the IR absorption and ¹³C NMR frequenci

Full text of DOI:10.1016/j.saa.2007.08.003

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 595–600
Spectroscopic characterization of the 1-substituted
3,3-diphenyl-4-(2ꢀ-hydroxyphenyl)azetidin-2-ones: Application
of 13C NMR, 1H–13C COSY NMR and mass spectroscopy
Girija S. Singh∗, Tshepo Pheko
Department of Chemistry, University of Botswana, Private Bag 0022, Gaborone, Botswana
Received 8 June 2007; received in revised form 18 July 2007; accepted 2 August 2007
Abstract
The article deals with spectroscopic characterization of azetidin-2-ones. The presence of substituents like hydroxyl, fluoro, methoxy and
benzhydryl, etc., on the azetidin-2-one ring significantly affects the IR absorption and 13C NMR frequencies of the carbonyl group present
in these compounds. The presence of an ester carbonyl group or too many methine protons in the molecule has been observed to limit the
1
scope of IR and H NMR spectroscopy in unambiguous assignment of the structure. The application of 13C NMR, 2D NMR (1H–13C COSY)
and mass spectroscopy in characterization of complex azetidin-2-ones is discussed. An application of the latter two techniques is described in
deciding unequivocally between an azetidin-2-one ring and chroman-2-one ring structure for the product obtained by treatment of the 1-substituted
3,3-diphenyl-4-[2ꢀ-(O-diphenylacyl)hydroxyphenyl]-2-azetidinones with ethanolic sodium hydroxide at room temperature.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Azetidin-2-ones; Chroman-2-ones; IR; 1H NMR; 13C NMR; Mass spectroscopy; 2D NMR
1. Introduction
cates the cis-relationship whereas a coupling constant in the
range of 2–2.5 Hz indicates the trans-relationship. During long
continued research by our group on diverse types of azetidin-
2-ones, we encountered many such examples, which could
Azetidin-2-ones, commonly known as ␤-lactams, consti-
tute a well-known class of heterocyclic compounds. This class
of cyclic amides has witnessed extensive investigation in last
two decades from synthetic, mechanistic and medicinal chem-
istry viewpoints [1–9]. Spectroscopy is the most important
experimental technique to determine the structures of azetidin-
2-ones [1]. A strong absorption band around 1750 cm−1 in
IR spectra characterizes the azetidin-2-ones. The structures of
many azetidin-2-one derivatives, containing another carbonyl
group especially ester besides the azetidinone ring carbonyl
group, may not be established only by the IR spectroscopy and
require application of 13C NMR spectroscopy. The ␤-lactam
carbonyl carbons have been observed to resonate from δ 166
1
not be characterized simply by IR or H NMR spectroscopy,
and required application of 13C NMR [10,11], Mass spec-
troscopy and 2D NMR techniques [12]. Many examples of
azetidin-2-ones are known from our group and others as well
in which the presence of substituents such as hydroxyl, flu-
oro, amino, nitro, benzyl and benzhydryl, etc., influences the
spectral patterns significantly and pose challenging problems in
characterization of such compounds. These examples have not
been discussed separately from spectral characteristics point of
view.
The present article, therefore, describes the general appli-
cation of spectroscopy in characterization of azetidin-2-ones,
the significant effect of some substituents on the spectral
characteristics, and scope and limitations of the different spec-
troscopictechniquesinstructureelucidationofsuchcompounds.
An application of 2D NMR (1H–13C COSY) and signifi-
cance of mass spectroscopy are described in characterization of
azetidin-2-one versus chroman-2-one structure for the product
obtained from treatment of the 1-substituted 3,3-diphenyl-4-[2ꢀ-
1
to 170 ppm. The H NMR spectroscopy is used for determin-
ing the stereochemical relationship between vicinal protons
at C(3) and C(4) besides for accounting the total number of
protons. A coupling constant in the range of 4.5–6 Hz indi-
∗
Corresponding author. Tel.: +267 3552501; fax: +267 3552836.
E-mail address: singhgs@mopipi.ub.bw (G.S. Singh).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.08.003

596
G.S. Singh, T. Pheko / Spectrochimica Acta Part A 70 (2008) 595–600
(O-diphenylacyl)hydroxyphenyl]-2-azetidinones with ethanolic
sodium hydroxide at room temperature.
the presence of two functional groups-amide carbonyl and ester
carbonyl in such compounds. Thus, 13C NMR study became
a necessity for characterization of these compounds. Similarly,
the structures of compounds 2 could not be established unam-
biguously only on the basis of IR spectroscopy because another
possible product chroman-2-one 3 from the reaction of azetidin-
2-ones 1 with alkali was also expected to absorb in the same
region.
2. Results and discussion
2.1. IR spectra of azetidin-2-ones
In the series of 1-substituted 3,3-diphenyl-4-[2ꢀ-(O-
diphenylacyl)-hydroxyphenyl]azetidin-2-ones1, synthesizedby
the Staudinger ketene-imine [2 + 2] cycloaddition, the com-
pound 1a with a phenyl group on nitrogen showed an absorption
band at 1754 cm−1 (Table 1) [12]. The substitution of the phenyl
ring by electron-donating groups such as methyl, methoxy or
ethoxy group lowered the absorption frequencies by 6–8 cm−1
whereas the substitution by an electron-withdrawing nitro group
increased the absorption frequency by 6 cm−1. The presence of
alkyl groups such as diphenylmethyl or benzyl group on ring
nitrogen also led to an increase in absorption frequencies by
5 cm−1. Thus, the IR absorption frequency of ␤-lactam carbonyl
group depends on the nature of substituents at adjacent nitro-
gen atom. N-Alkyl substituted azetidin-2-ones usually absorb
at higher frequency than N-aryl substituted azetidin-2-ones. A
similar trend in IR absorption frequency is reported by Lacroix
et al. [15].
2.2. 1H NMR spectra of azetidin-2-ones
The protons at C-3 and C-4 positions of the ring have been
observed to resonate from 4 to 6 ppm. The 1H NMR spec-
troscopy is the most powerful tool for the determination of
relative stereochemistry at C-3 and C-4 positions of azetidin-
2-ones. The coupling constant for vicinal protons at C-3 and
C-4 is 4.5–6.0 Hz for cis derivatives and 2.0–2.5 Hz for trans
derivatives [1,9].
The cleavage of ester group from C-4 substituent of 1 in the
presence of alkali afforded white crystalline products. These
compounds showed IR absorptions corresponding to carbonyl
groups lowered by 19–32 cm−1 in comparison to the corre-
sponding parent azetidin-2-ones 1 (Table 1). The IR spectra also
1
The H NMR spectra of azetidin-2-ones 1 accounted for
1
the total number of protons in the compounds [12]. The H
NMR spectroscopy, however, has some limitations in correct
assignments of two protons of similar nature resonating nearby
each other. For example, all the compounds of series 1 have
two methine protons except 1h, which is even more compli-
cated with three methine protons. The 1H NMR spectra of
these compounds showed two singlets around δ 5.8 and 5.5 ppm
(three in case of 1h at δ 5.61, 5.50 and 5.21) corresponding to
these protons. These signals, especially in case of 1h, cannot
be assigned unambiguously without application of the 2D NMR
spectroscopy.
showed a weak and broad absorption band around 3300 cm−1
.
These compounds have been characterized as 1-substituted 3,3-
diaryl-4-(2ꢀ-hydroxyphenyl)azetidin-2-ones 2. The lowering of
the absorption frequencies has been attributed to the possi-
ble hydrogen bonding ability of the hydroxyl group. A similar
observation has been reported recently when the o-aminophenyl
group in which amino group may form hydrogen bond replaced
the o-nitrophenyl group on C-4 of the azetidin-2-one ring
[13].
1
The H NMR spectra of the compounds 2 showed 10 aro-
matic protons less than the corresponding 1. However, all the
spectra still showed two singlet signals (three in case of 2h)
each corresponding to one proton. It was quite a puzzling
observation because loss of 10 aromatic protons, meaning two
aromatic rings, indicated the loss of diphenylketene moiety
from the molecule. If this would be the case the product would
be the imines (Scheme 1). It is worthy to mention here that
the fragmentation of azetidin-2-ones to ketenes and imines has
been reported previously [14]. However, neither the IR spec-
tra showed absorption band corresponding to imine linkage nor
the 1H NMR spectra showed the downfield singlet signal corre-
sponding to azomethine proton. At this stage, it was considered
pertinent to record the 1H NMR spectra in the presence of
D2O. In the presence of D2O one of the singlet signals con-
sidered initially as methine proton was observed to exchange.
This led to conclusion that one of the two singlets was either
due to OH or NH and the diphenylacetyl moiety got elimi-
nated from compounds 1 leading to possible formation of either
2 or 3.
Furthermore, the compounds of series 1 are the most suitable
examples to show the limitation of IR spectroscopy in unam-
biguous characterization of these compounds. The ester linkage
present in these compounds absorbed in the same region as the
lactam carbonyl linkage and it was impossible to show clearly
Table 1
IR absorption frequencies (KBr, cm−1) of carbonyl groups in azetidin-2-ones 1
and 2
R
Azetidin-2-one 1
Azetidin-2-one 2
(a) Ph
1754
1748
1746
1747
1760
1750
1759
1759
1721
1713
1715
1715
(b) 4-MeC6H4
(c) 4-MeOC6H4
(d) 4-EtOC6H4
(e) 4-O2NC6H4
(f) 4-ClC6H4
(g) CH2Ph
1720
1730
1740
(h) CHPh2

G.S. Singh, T. Pheko / Spectrochimica Acta Part A 70 (2008) 595–600
597
Scheme 1.
2.3. 13C NMR spectra of azetidin-2-ones
group resonated almost at same frequency as with the one having
phenyl ring. This trend has also been observed in azetidin-2-ones
2.
The 13C NMR spectra of azetidin-2-ones show the typical
carbonyl resonance at δ 166–170 ppm. However, the values out-
side this range are possible if strong electron withdrawing or
electron donating groups are present on the adjacent carbon
atoms. For example, the 13C NMR spectra of N-substituted
3,3-difluoroazetidin-2-ones showed the carbonyl carbon signal
in the range of δ 157–160 ppm where as the carbonyl group
in 3-hydroxyazetidin-2-one resonated at δ 174.3 ppm [15,16].
The effect of nature of substituents at nitrogen has also been
observed on 13C NMR frequency of carbonyl group. The lac-
tam carbonyl carbon in which nitrogen was alkyl substituted
(e.g. CH2Ph and CHPh2) resonated at about 2.5–3.0 ppm higher
frequency than those in which it was substituted by aromatic
substituents [12,15]. Among the N-aryl-substituted azetidin-2-
ones, the electron donating groups on phenyl ring slightly led to
an upfield shift of the carbonyl absorption whereas the electron-
withdrawing nitro group caused a slight downfield shift [12].
The carbonyl carbon in azetidin-2-ones 1 with 4-chlorophenyl
The 13C NMR spectroscopy proved to be a valuable tool in
characterization of the azetidin-2-ones 1. The 13C NMR spectra
of these compounds showed two downfield signals-one around δ
166 ppmandtheotheraroundδ171 ppmconfirmingthepresence
of two carbonyl groups in them. The former signal was assigned
to azetidin-2-one carbonyl carbon whereas the latter one to the
ester carbonyl carbon (see the succeeding section). However in
azetidin-2-one 1h, the two downfield signals were quite close-
one at δ 170.8 and the other at δ 169.9, which necessitated the
application of 2D NMR spectroscopy for correct assignment.
Similarly, the methine carbon signals appearing upfield in 13C
NMR spectra of 1, and the methoxy carbon signal of 1c also
appearing in the same region required 2D NMR spectroscopy
for correct assignment. The 13C NMR spectrum of the azetidin-
2-one 1c showed two comparatively downfield signals at δ 156.3
and 149.3 ppm in the aromatic region due to aromatic carbons
bearing OCOCHPh2 and OMe groups.
Fig. 1. Selected 1H–13C correlationships in azetidin-2-ones.

598
G.S. Singh, T. Pheko / Spectrochimica Acta Part A 70 (2008) 595–600
The 13CNMRspectraof 2cshowed two comparatively down-
field signals at δ 155.5 and 153.2 ppm in the aromatic region
due to aromatic carbons bearing OH and OMe groups. Only
proton–carbon correlation studies can lead to convincing assign-
ments of these signals to particular carbon. Furthermore, the
methoxy carbon in c also appeared in the same region as the two
methine carbon, which required assignment by the 2D NMR
spectroscopy. The application of the latter technique was also
necessitated to confirm whether the ring is an azetidin-2-one
or chroman-2-one because the 13C NMR spectra of both the
structures would have similar resonating patterns.
2.4. HMQC and HMBC studies using 2D NMR spectra
The 2D NMR studies led to assignment of signals to carbons
and protons in the azetidin-2-ones of series 1 and 2. The applica-
tion of HMQC and HMBC using 1H–13C COSY NMR spectra
in characterization of such compounds is discussed in succeed-
ing paragraphs by taking representative examples of 1c, 1h and
2c (Fig. 1).
Fig. 2. 1H–13C COSY NMR spectra of 1h (R = CHPh2) showing HMBC.
group and the carbon signals at δ 170.8 and 57.1 ppm have
been assigned to ester carbonyl carbon and methane carbon
in OCOCH group. Since the proton at δ 5.50 ppm was having
HMBC correlation with carbon signal at δ 169.9, 149.4 and
72.3 ppm (due to C-3 of azetidin-2-one ring), it was assigned
to methine proton at C-4. The carbon signals at δ 61.1 and
169.9 ppm were thus assigned to C-4 ring carbon and azetidin-2-
one carbonyl carbon. The remaining methine proton δ 5.61 ppm
andcarbonat62.6 ppmwerethusassignedtoN-CHgroupproton
and carbon.
As stated in the 1H and 13C NMR subsections, the character-
ization of 1c required assignment of proton signals at δ 5.79 and
5.42 ppm, and the carbon signals at 171.1, 166.3, 156.3, 149.3,
60.8, 57.2 and 55.4 ppm. The HMQC spectrum of 1c showed
the correlation of the methoxy proton signal at δ 3.76 ppm with
carbon signal at δ 55.4 ppm, which led to the assignment of
this signal to the methoxy group carbon. The HMQC spectrum
showed the correlation of proton signals at δ 5.79 and 5.42 ppm
with carbon signals at δ 60.8 and 57.2, respectively. The proton
at δ 3.76 ppm showed correlation with the carbon signal at δ
156.3 ppm in HMBC spectrum and thus the latter signal could
be assigned to the N-phenyl ring carbon bearing the methoxy
group. The signal at δ 149.3 ppm was assigned to the C-4 phenyl
ring carbon bearing the OCOCHPh2 group. This signal showed
correlation with the singlet at δ 5.79 ppm, which in turn showed
correlation with carbon signal at δ 72.6 ppm due to C-3, and with
166.3. The signal at δ 5.79 ppm, thus, could be assigned to C-4
methine proton and signal at d 166.3 ppm could be assigned to
azetidin-2-one ring carbonyl. Since this signal had shown corre-
lation with the carbon signal at δ 60.8 ppm in HMQC, the latter
signal has been assigned to the ring C-4. The other methine pro-
ton signal at δ 5.4 ppm and the carbon signal at δ 57.2 ppm could
thus be assigned to acyl proton and the carbon bearing it. Since
the methine proton signal at δ 5.4 ppm showed correlation to the
carbon signal at δ 171 ppm, the latter signal has been assigned
to ester carbonyl group.
The 1H–13C COSY spectrum (HMQC) of the compounds 2
showed no correlation of D2O exchangeable proton signal again
confirming that this proton was attached either to nitrogen or to
1
hydrogen. The H–13C COSY spectrum (HMBC) showed the
correlation of this proton signal with carbon signal at around δ
Table 2
Mass spectral data of azetidin-2-ones 2
R
m/z (r.i.)
2a
Ph
391 (M+, 30), 272 (90, Ph(OH)CH CPh2), 197 (65,
Ph(OH)CH
N
Ph), 194 (100, Ph2C
C O), 166
(Ph2C), 119 (15, Ph
N
C
O), 77 (Ph)
The 1H NMR spectra of 1h showed three singlets, each inte-
grating for one proton at δ 5.61, 5.50 and 5.21 ppm, which
required to be assigned unambiguously. In the 13C NMR spec-
trum of azetidin-2-one 1h, two downfield signals at δ 170.8 and
δ 169.9 ppm, and three upfield signals at δ 62.6, 61.1 and 57.1
2b
2c
2d
2f
4-MeC6H4
4-MeOC6H4
4-EtOC6H4
4-ClC6H4
CH2Ph
405 (M+, 10), 272 (70), 211 (100), 194 (75), 166
(90), 91 (48), 77 (20)
421 (M+, 26), 272 (45), 227 (100), 194 (58), 166
(70), 134 (15), 77 (35)
435 (M+, 2), 272 (35), 241 (100), 194 (34), 166
(95), 77 (5)
1
required unambiguous assignment. The H–13C COSY spec-
425 (M+, 8), 272 (82), 231 (95), 194 (100), 166
(90), 152 (5), 139 (15), 111 (40), 77 (85)
405 (M+), 272 M+ PhCH2NCO, 194
trum using HMQC showed correlation of protons at δ 5.61, 5.50
and 5.21 ppm with carbon signals at δ 62.6, 61.1 and 57.1 ppm,
respectively. The proton at δ 5.21 ppm was having HMBC cor-
relation with carbon signal at δ 170.83 (Fig. 2). Accordingly,
the signal at δ 5.21 ppm was assigned to proton in OCOCH
2g
2h
(Ph2C
481 (M+), 285, 272 (M+ Ph2CHNCO), 194
(Ph2C O), 165, 77
C O), 165, 77
CHPh2
C

G.S. Singh, T. Pheko / Spectrochimica Acta Part A 70 (2008) 595–600
599
Fig. 3. Mass spectrum of azetidin-2-one 2a (R = Ph).
Table 3
1H and 13C NMR spectral data of azetidin-2-ones 2
R
δ (ppm) (CDCl3)
1H NMR
13C NMR
Ph
7.97–6.57 (19H, aromatic), 6.32 (s, 1H, CH), 5.81 (s, 1H, OH, D2O
exchangeable)
167.6 (C O), 153.3 (C OH), 140.7, 138.0, 137.5, 129.0, 128.9,
128.5, 128.4, 128.3, 127.9, 127.85, 127.8, 127.3, 126.7, 124.1,
121.5, 120.6, 117.5, 115.0, 72.5 (C3), 60.7 (C4)
4-MeC6H4
4-MeOC6H4
7.98–6.57 (18H, aromatic), 6.29 (s, 1H, CH), 5.60 (s, 1H, OH, D2O
exchangeable), 2.29 (s, 3H, CH3)
167.3 (C O), 153.2 (C OH), 140.8, 138.1, 135.1, 133.7, 129.5,
128.8, 128.5, 128.4, 128.3, 127.9, 127.8, 127.7, 127.2, 126.7,
120.6, 117.5, 114.9, 72.4 (C3), 60.6 (C4), 20.9
167.0 (C O), 156.1 (C OMe), 153.4 (C OH), 140.9, 138.1,
131.1, 128.9, 128.6, 128.4, 128.3, 127.9, 127.8, 127.7, 127.2,
126.7, 121.6, 120.5, 118.8, 114.9, 114.3, 72.4 (C3), 60.7 (C4), 55.6
(C OMe)
7.98–6.17 (18H, aromatic), 6.25 (s, 1H, CH), 5.66 (s, 1H, OH, D2O
exchangeable), 3.76 (s, 3H, OCH3)
4-EtO C6H4
4-Cl C6H4
CH2Ph
7.95–6.57 (18H, aromatic), 6.25 (s, 1H, CH), 5.66 (s, 1H, OH, D2O
exchangeable); 3.97 (q, 2H, CH2, J = 6.9 Hz), 1.38 (t, 3H, CH3,
J = 6.9 Hz)
7.96–6.58 (18H, aromatic), 6.28 (s, 1H, CH), 5.66 (s, 1H, OH, D2O
exchangeable)
166.8 (C O), 155.5 (C OMe), 153.2 (C OH), 140.9, 138.2,
131.0, 128.8, 128.5, 127.9, 127.8, 127.7, 127.2, 126.6, 121.7,
120.6, 118.7, 114.9, 72.5, 63.7, 60.7, 14.8
167.5 (C O), 153.3 (C O), 140.6, 137.8, 136.0, 129.2, 129.1,
128.6, 128.3, 128.2, 127.9, 127.8, 127.7, 127.4, 126.8, 121.1,
120.7, 118.8, 115.1, 72.9, 60.8
170.7 (C O), 154.1 (C OH), 140.8, 138.4, 135.2, 128.8, 128.7,
128.5, 128.4, 128.2, 127.9, 127.8, 127.7, 126.5, 121.8, 120.2,
115.2, 73.4 (C3), 60.2 (ring CH), 44.7 (CH2)
7.76 (m, 2H, arom), 7.31–6.94 (m, 14H, arom), 6.80 (dd, 1H, arom,
J = 1.5, 7.5 Hz), 6.73 (dd, 1H, arom, J = 0.9, 8.1 Hz), 6.64 (t, 1H,
arom, J = 7.5 Hz), 6.23 (s, 1H, ring CH), 5.75 (s, 1H, D2O
exchangeable), 5.02 and 4.01 (two d, 2H, CH2, J = 15.0 Hz)
9.07 (s, 1H, OH, D2O exchangeable), 7.89 (dd, 2H, arom, J = 1.5,
8.7 Hz), 7.49 (m, 2H, arom), 7.47–7.15 (m, 12H, arom), 7.00–6.93
(m, 4H, arom), 6.82 (m, 3H, arom), 6.40 (m, 1H, arom), 6.00 (s,
1H, ring CH), 5.60 (s, 1H, N CH)
CHPh2
169.6 (C O), 155.4 (C OH), 141.9, 140.0, 139.6, 139.4, 128.7,
128.5, 128.3, 128.2, 128.1, 127.8, 127.6, 127.4, 127.3, 126.9,
126.3, 122.4, 118.9, 114.8, 72.0 (C3), 63.0, 61.3

600
G.S. Singh, T. Pheko / Spectrochimica Acta Part A 70 (2008) 595–600
150 ppm due to the aromatic carbon attached directly to oxygen,
which provided a vital clue in determination of the azetidin-2-
one ring structure 2 and not the chroman-2-one structure 3. If the
chroman-2-one structure is taken into consideration, this carbon
would be separated by four bonds from the proton (N–H) that is
not correlated to any carbon in HMQC, and its correlation with
the NH proton would not be justified. However, if the azetidin-2-
one structure is taken into consideration, the D2O exchangeable
proton that is not correlated to any carbon atom becomes the
hydroxyl proton and shows explainable correlations.
3.1. Reactions of azetidin-2-ones with sodium hydroxide
An equimolar solution of the 2-azetidinones (0.05 g) 1 and
sodiumhydroxidein10 mlofethanolwasstirredatroomtemper-
ature for 30 min. The solution was neutralized with hydrochloric
acid and extracted with dichloromethane. The organic layer
was dried over anhydrous sodium sulphate. The solvent was
evaporated under reduced pressure. The fractional crystalliza-
tion of the residue with ethanol afforded the white crystalline
2-azetidinones 2. The 1H and 13C NMR data is shown in Table 3.
2.5. Mass spectroscopy
Acknowledgement
The mass spectra of the compounds 2 (Table 2) showed
the molecular ion peak corresponding to the particular com-
pound. The fragmentation of the azetidin-2-ones leading to
the diphenylketene and the corresponding imines as shown
in Scheme 1 is observed as the main fragmentation path
(Fig. 3). The mass spectra provided further vital clue in elu-
cidation of the structure of the azetidin-2-ones 2. All the
compounds showed a fragment at 272 with considerable abun-
dance. This fragment has been identified as alkene radical ion
[Ph(OH)CH CPh2]+, formed as a result of cleavage of the
corresponding isocyanates from the azetidin-2-ones 2. The frag-
mentation of azetidin-2-ones to alkenes and isocyanates is a
reported phenomenon [14]. No fragmentation route of 3 can
lead to a fragment at 272, and thus eliminates the possibility of
this structure.
Thanks are due to the Head, Department of Chemistry and to
the ORD at University of Botswana, for providing the necessary
research facilities and financial assistance, respectively.
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The IR spectra were recorded on a Perkin-Elmer-781 IR spec-
1
trophotometer using KBr disc of the sample. The H and 13C
NMR spectra were recorded either in a CDCl3 solution at 300
and 75.4 MHz, respectively, on a BrukerTM 300 MHz spectrom-
eter. The mass spectra were recorded on Finnignan LC QDECA
mass spectrometer.
The 2-azetidinones 1 were synthesized by the 2.2:1 molar
reaction of the diarylketenes with N-salicylideneamines accord-
ing to the reported method [12].
[16] H. Kiyota, T. Takai, M. Saitoh, O. Nakayama, T. Oritani, S. Kuwahara,
Tetrahedron Lett. 45 (2004) 8191–8194.