A novel asymmetric synthesis of 3-(1H-pyrrol-1-yl)-substituted β-lactams via a bismuth nitrate-catalyzed reaction

Article abstract of DOI:10.1002/hlca.201100202

The reaction of racemic α-keto β-lactams 5a-5c with the commercially available chiral compound trans-4-hydroxy-L-proline (6) in the presence of a catalytic amount of Bi(NO₃)₃·5 H ₂O in EtOH gave a diastereoisomer mixture o

Full text of DOI:10.1002/hlca.201100202

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839
A Novel Asymmetric Synthesis of 3-(1H-Pyrrol-1-yl)-Substituted b-Lactams
via a Bismuth Nitrate-Catalyzed Reaction
by Aarif L. Shaikh and Bimal K. Banik*
The University of Texas-Pan American, Department of Chemistry, 1201 West University Drive,
Edinburg, Texas 78539, USA (e-mail: banik@utpa.edu)
The reaction of racemic a-keto b-lactams 5a – 5c with the commercially available chiral compound
trans-4-hydroxy-l-proline (6) in the presence of a catalytic amount of Bi(NO3)3 · 5 H2O in EtOH gave a
diastereoisomer mixture of b-lactams with a pyrrole ring at C(3) (7 to 12). This is the first
enantioselective synthesis of optically active b-lactams (¼azetidin-2-ones) that possess a pyrrolyl residue
at C(3), in a single step.
Introduction. – The b-lactam (¼azeridin-2-one) skeleton is the key structural unit
of the most widely used b-lactam antibiotics1). New b-lactam antibiotics are necessary
for the treatment of bacterial diseases [2]. The b-lactam skeleton has also been
recognized as an important starting material by exploiting its strain energy associated
with the four-membered ring [3], and many efforts have been made to use
enantiomerically pure b-lactams as versatile intermediates for organic syntheses [3][4].
Like b-lactams, substituted pyrroles are also an important class of compounds with
various biological activities2). Methods for the synthesis of diversely substituted
pyrroles have been described [6]. Conjugate addition reactions [7], transition metal-
mediated reactions [8], reductive couplings [9], aza-Wittig reactions [10], and other
multistep operations [11] have been performed for the preparation of several pyrroles.
We disclose the synthesis of optically pure b-lactams substituted with a 1H-pyrrol-1-yl
moiety at C(3) of the azetidine-2-one ring using a novel bismuth nitrate-induced
catalytic method. To the best of our knowledge, this is the first report for the synthesis
of optically active pyrrolyl-substituted b-lactams.
The use of Bi salts in several organic transformations has received much attention.
Recently, our group has demonstrated a facile method for the synthesis of substituted
pyrroles [12a]3). In continuation of our endeavor for the synthesis and biological
evaluation of novel anticancer b-lactams [13], we have developed a new method for the
synthesis of novel b-lactams that have a pyrrol-1-yl ring at C(3) of the azetidine-2-one
ring. Although the chemistry of b-lactams is extremely rich, preparation of optically
active pyrrolyl-substituted azetidin-2-ones has not been described [14]. Asymmetric
synthesis of b-lactams has been performed by employing either chiral ketenes derived
1
)
)
)
For reviews on b-lactam antibiotics, see [1].
For some biologically active pyrroles, see [5].
For our other Bi(NO3)3-catalyzed reactions, see [12b – 12k].
2
3
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

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from acid precursors or chiral imines (derived from either chiral aldehydes or amines)
[15]. Here, we describe a novel method for the synthesis of optically active b-lactams
starting from racemic a-keto b-lactam.
Results and Discussion. – The Staudinger cycloaddition of 1-(acetoxy)acetyl
chloride (1) with imines 2 gave 3-acetoxy b-lactams 3 in 65 – 70% yields, which, on
careful hydrolysis with aqueous NaOH in THF, afforded the corresponding 3-hydroxy
b-lactams 4 in quantitative yields. Oxidation of the OH group was carried out by a
known procedure with P2O5 and DMSO [16] to give the desired a-keto b-lactams 5a –
5c [16] in 80 – 85% yield (Scheme 1).
Scheme 1
a) Dry Et3N, dry CH2Cl2, 08, 15 h. b) Aq. NaOH, THF, 08, 30 min; c) P2O5, dry DMSO, 24 h [16].
Initially, the racemic a-keto b-lactam 5a was reacted with trans-4-hydroxy-l-proline
(6) in the presence of a catalytic amount of Bi(NO3)3 · 5 H2O in EtOH at room
temperature. However, no product formation was observed. We then tried the same
reaction without Bi(NO3)3 · 5 H2O using only EtOH, but, no change of the reaction
mixture was observed in TLC, and the starting material was recovered. When a solution
of 5a and 6 was refluxed for 3 h in the presence of Bi(NO3)3 · 5 H2O and EtOH, a
dramatic change was observed (Scheme 2). The TLC indicated formation of two new
products. The 1H-NMR spectra of the crude product revealed the presence of a mixture
of two diastereoisomeric b-lactams.
Scheme 2
The two diastereoisomers were separated by flash column chromatography to
obtain pure cis- and trans-b-lactam 7 and 8, respectively, in a 3.5 :1 ratio with a pyrrolyl

Helvetica Chimica Acta – Vol. 95 (2012)
841
substituent at C(3). The stereochemical outcome and structures for 7 and 8 were
established by IR and NMR spectroscopy. The major diastereoisomer 7 showed a
characteristic absorption band at 1746 cmꢀ1 for the b-lactam C¼O group. The 1H-NMR
spectrum for 7 showed two doublets at 5.42 and 5.76 ppm for HꢀC(4) and HꢀC(3) of
the b-lactam ring, respectively. The coupling constant of these two doublets was J ¼
5.5 Hz, which confirmed the cis-configuration for 7 [17]. There were two triplets
observed at 5.88 and 6.47 ppm for four pyrrole H-atoms. The coupling constant (J ¼
2.1 Hz) of these two triplets confirmed the presence of the pyrrolyl moiety4). The
signal at 167.8 ppm in the 13C-NMR spectrum was also characteristic for a b-lactam
C¼O group.
The minor diastereoisomer 8 also gave rise to a characteristic absorption band at
1745 cmꢀ1 in the IR spectrum, and its 1H-NMR spectrum exhibited two doublets at 4.96
and 5.13 ppm for HꢀC(4) and HꢀC(3), respectively, of the b-lactam ring. The smaller
coupling-constant value (J ¼ 2.0 Hz) confirmed the trans-configuration for 8. Addi-
tionally, two triplets at 6.25 and 6.76 ppm with a coupling constant J of 2.19 Hz also
confirmed the presence of the pyrrole ring. A peak at 167.8 ppm in the 13C-NMR
spectrum was an additional evidence for the presence of a b-lactam C¼O group in 8.
The absolute configurations of b-lactams 7 and 8 were assigned on the basis of their
optical rotation data. The optical rotation for 7 was found to be [a]2D0:1 ¼ þ10.8 (c ¼ 2.0,
CHCl3), and that of compound 8 was [a]2D0:1 ¼ þ2.08 (c ¼ 1.0, CHCl3). It has been
established that (S)-configuration at C(4) of a b-lactam gives a positive optical rotation
[19]. The absolute configurations of 7 and 8 were also confirmed by NMR comparison
with those of known analogs using optically active shift reagents [19b – 19d]. It should
be noted that both products were obtained with (S)-configuration at C(4), indicating
that (R)- and (S)-5a or 13 undergo racemization, and only the (4S)-configured 5a or 13
react with 6 to give the products. Considering the yields and isomeric ratio of the
products, we assume that 5b and 5c follow similar pathways.
The NH group in 6 is ideally located to undergo a nucleophilic addition reaction
with the highly reactive C¼O group of the a-keto b-lactams 5a – 5c in the presence of
Bi(NO3)3 · 5 H2O to afford 13 via a series of unstable intermediates [20] (Scheme 3). In
the presence of an acid catalyst, the intermediate (probably the amino alcohol)
resulting from a nucleophilic reaction reacts with the carboxy group spontaneously to
form the cyclic intermediate 13. The hydroxy-proline derivative 6 reacts with the C¼O
group of 5 from the opposite side of the C(4)-substituent R1 present in the b-lactam
ring, probably because of steric hindrance. This nucleophilic reaction and subsequent
ring formation step at C(3) is crucial, since it dictates the absolute configuration at C(4)
of as shown in 13. At reflux temperature, intermediate 13 can form an azomethine ylide
14 via decarboxylation, and this ylide then is converted to 15 via elimination of H2O,
because of conjugative effects [18]. Proton transfer from 15 may then occur to afford
the products 7 and 8 (also 9 to 12) to gain aromatic character [18]. The cis/trans-isomer
ratios have been confirmed by 1H-NMR. The fragment in the mesomeric structures of
15 involved in the proton transfer is planar, and, as a result, protonation at the anionic
center C(3) may occur from the front or the reverse side. The minor trans-isomer 8
could also be formed because of thermal isomerization of 7, since the trans-isomer is
4
)
For synthesis of these related compounds, see [18].

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thermodynamically more stable. However, this pathway was ruled out, since no
isomerization of pure 7 was observed, when it was treated with Bi(NO3) · 5 H2O in the
presence of EtOH at reflux temperature for 2 h. We assume that an identical pathway is
conceivable with other substrates for the preparation of 9 – 12.
Scheme 3
Conclusions. – These results indicate the capability of Bi(NO3)3 as an acid activator
in catalyzing several processes. Optically active b-lactams described herein are new5).
The success of this method would be extremely useful to investigate the reactions
between other substituted a-keto b-lactams and hydroxy-proline derivatives.
Experimental Part
General. Anal. grade chemicals (SigmaꢀAldrich) were used. Silica gel (SiO2) was used for column
chromatography (CC). Deionized H2O was used for the preparation of all aq. solns. M.p.: Fisher
Scientific electrochemical Mel-Temp* manual melting-point apparatus (Model 1001). Optical Rotation:
Rudolph Polarimeter. FT-IR Spectra: Bruker IFS 55 Equinox FT-IR spectrophotometer, as KBr discs. 1H-
(300 MHz) and 13C-NMR (75.4 MHz) spectra: at r.t.; JEOL Eclipse-300 equipment; TMS as internal
standard; CDCl3 as solvent.
Synthesis of 3-(1H-Pyrrol-1-yl)-Substituted b-Lactams 7 – 12. 3-Keto b-lactam 5a (100 mg,
0.42 mmol) was suspended in 2 ml of EtOH under N2 [16]. trans-4-Hydroxy-l-proline (6; 60 mg,
0.46 mmol) and Bi(NO3)3 · 5 H2O (20 mg, 0.04 mmol) were added to the mixture. The mixture was
5
)
Recently, Tidwell and co-workers have reported the synthesis of racemic pyrrolyl-substituted b-
lactams using 2-(pyrrol-2-yl)acetic acid and imine by cycloaddition; e.g., see [21].

Helvetica Chimica Acta – Vol. 95 (2012)
843
heated to reflux for 3 h. The TLC indicated formation of two new products. Then, the mixture was diluted
with H2O (10 ml) and CH2Cl2 (25 ml). The CH2Cl2 layer was separated, washed with aq. NaHCO3 soln.
(5 ml), and dried (Na2SO4). The CH2Cl2 layer was evaporated under reduced pressure to give a crude
pale-yellow oil (85 mg, 83%). The 1H-NMR spectrum of the crude indicated a mixture of two
diastereoisomers: cis (78%) and trans (22%). The crude mixture was separated chromatographically (5 –
10% AcOEt/hexane) to afford pure trans-isomer 8 as colorless oil, followed by cis-isomer 7 as a white
solid. The structures of 7 and 8 were confirmed by spectroscopic and anal. data. An identical procedure
was applied for the preparation of compounds 9 – 12 (yield of 7 þ 8 219.2 mg (80%); of 9 þ 10 135 mg,
and of 11 þ 12 130 mg (85%)).
(3R,4S)-1,4-Diphenyl-3-(1H-pyrrol-1-yl)azetidin-2-one (7). White solid. Rf (10% AcOEt/hexane)
0.52. M.p. 138 – 1398. [a]2D0:1 ¼ þ10.8 (c ¼ 2.0, CHCl3). IR (KBr): 1387, 1499, 1599,1746, 2365, 2921.
1H-NMR: 5.42 (d, J ¼ 5.5, HꢀC(4)); 5.76 (d, J ¼ 5.5, HꢀC(3)); 5.88 (t, J ¼ 2.1, 2 H); 6.47 (t, J ¼ 2.1, 2 H);
7.13 – 7.71 (m, 10 arom. H). 13C-NMR: 61.1; 68.2; 109.2; 117.6; 119.5; 120.2; 128.7; 129.0; 131.0; 132.5;
137.2; 161.5; 167.8.
(3S,4S)-1,4-Diphenyl-3-(1H-pyrrol-1-yl)azetidin-2-one (8). Colorless oil. Rf (10% AcOEt/hexane)
0.82. [a]2D0:1 ¼ þ2.08 (c ¼ 1.0, CHCl3). IR (KBr): 1242, 1510, 1745, 2360, 2967. 1H-NMR: 4.96 (d, J ¼ 2.0,
HꢀC(4)); 5.13 (d, J ¼ 2.0, HꢀC(3)); 6.25 (t, J ¼ 2.2, 2 H); 6.76 (t, J ¼ 2.2, 2 H); 7.25 – 7.68 (m, 10 arom. H).
13C-NMR: 61.0; 68.2; 108.9; 109.5; 119.0; 121.0; 128.7; 129.5; 131.5; 132.5; 140.0; 161.7; 167.8.
(3R,4S)-1,4-Bis(4-methoxyphenyl)-3-(1H-pyrrol-1-yl)azetidin-2-one (11). White solid. Rf (10%
AcOEt/hexane) 0.48. M.p. 142 – 1438. [a]D20:1 ¼ þ10.11 (c ¼ 1.3, CHCl3). IR (KBr): 1247, 1513, 1748,
2400, 2999. 1H-NMR: 3.70, 3.76 (2s, 2MeO); 5.32 (d, J ¼ 5.2, HꢀC(4)); 5.69 (d, J ¼ 5.2, HꢀC(3)); 5.90 (t,
J ¼ 2.1, 2 H); 6.48 (t, J ¼ 2.1, 2 H); 6.67 – 7.36 (m, 8 arom. H).13C-NMR: 54.8; 55.3; 61.0; 68.0; 109.1; 113.6;
114.2; 114.7; 118.8; 119.0; 119.8; 120.6; 124.4; 127.8; 128.2; 130.9; 156.6; 159.8; 161.3; 167.9.
(3S,4S)-1,4-Bis(4-methoxyphenyl)-3-(1H-pyrrol-1-yl)azetidin-2-one (12). Colorless oil. Rf (10%
AcOEt/hexane) 0.75. [a]2D0:1 ¼ þ19.09 (c ¼ 0.3, CHCl3). IR (KBr): 1244, 1391, 1510, 1746, 2890.
1H-NMR: 3.74, 3.80 (2s, 2MeO); 4.88 (d, J ¼ 2.0, HꢀC(4)); 5.06 (d, J ¼ 2.0, HꢀC(3)); 6.24 (t, J ¼ 2.1, 2 H);
6.74 (t, J ¼ 2.1, 2 H); 6.77 – 7.71 (m, 8 arom. H). 13C-NMR: 55.2; 56.0; 60.8; 68.5; 109.7; 113.5; 114.6;
114.8; 118.5; 119.3; 119.8; 120.0; 125.4; 127.3; 128.8; 130.9; 132.5; 156.0; 159.8; 160.0; 167.9.
(3R,4S)-1-(4-Methoxyphenyl)-4-phenyl-3-(1H-pyrrol-1-yl)azetidin-2-one (9). White solid. M.p.
158 – 1598. Rf (10% AcOEt/hexane) 0.44. [a]2D0:1 ¼ þ10.0 (c ¼ 2.0, CHCl3). IR (KBr): 1171, 1242, 1296,
1389, 1453, 1745, 2360, 2967. 1H-NMR: 3.76 (s, MeO); 5.38 (d, J ¼ 5.2, HꢀC(4)); 5.74 (d, J ¼ 5.2, HꢀC(3));
5.87 (t, J ¼ 1.9, 2 H); 6.46 (t, J ¼ 1.9, 2 H); 7.12 – 7.71 (m, 9 arom. H). 13C-NMR: 55.7; 62.2; 67.8; 108.8;
109.2; 113.6; 114.3; 114.8; 118.8; 120.6; 126.9; 128.2; 128.4; 132.6; 156.7; 161.2; 167.8.
(3S,4S)-1-(4-Methoxyphenyl)-4-phenyl-3-(1H-pyrrol-1-yl)azetidin-2-one (10). Colorless oil. Rf (10%
AcOEt/hexane) 0.77. [a]2D0:1 ¼ þ3.75 (c ¼ 0.33, CHCl3). IR (KBr): 1240, 1389, 1510, 1745, 2340, 2967.
1H-NMR: 3.76 (s, MeO); 4.93 (d, J ¼ 1.9, HꢀC(4)); 5.10 (d, J ¼ 1.9, HꢀC(3)); 6.25 (t, J ¼ 2.1, 2 H); 6.76 (t,
J ¼ 2.1, 2 H); 6.82 – 7.68 (m, 9 arom. H). 13C-NMR: 53.5; 65.6; 70.4; 109.6; 113.4; 119.4; 126.5; 128.0;
128.5; 131.9; 156.6; 161.7; 166.08.
We gratefully acknowledge the funding support from the NIH-SCORE (Grant No. 2S06M008038-
37).
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Received May 16, 2011