A convenient synthesis of tricyclic β-lactams by chemo- and diastereoselective intramolecular CH insertion of metal carbenes generated by dirhodium(II) tetraacetate catalyzed decomposition of α-diazoamides 1a-c is reported. In the case of reagent 1b, in the presence of the (+)-menthyl

chiral auxiliary, the β-lactam is obtained with 76% e.e

Muroni, Daniele

Saba, Antonio

English

UnissResearch

General Papers ARKIVOC 2005 (xiii) 1-7 
ISSN 1424-6376 Page 1 ©ARKAT USA, Inc 
Quinoline β-lactams by Rh(II)-catalyzed highly steroselective intramolecular 
carbene insertion into a carbon-hydrogen bond 
 
Daniele Muroni and Antonio Saba* 
 
Dipartimento di Chimica, Facoltà di Scienze, Università di Sassari,  
Via Vienna 2, Sassari 07100, Italy 
E-mail: saba@uniss.it  
(received 11 Nov 04; accepted 16 Feb 05; published on the web 26 Feb 05) 
 
Abstract 
A convenient synthesis of tricyclic β-lactams by chemo- and diastereoselective intramolecular C-
H insertion of metal carbenes generated by dirhodium(II) tetraacetate catalyzed decomposition of 
α-diazoamides 1a-c is reported. In the case of reagent 1b, in the presence of the (+)-menthyl 
chiral auxiliary, the β-lactam is obtained with 76% e.e. 
 
Keywords: Metal-carbene, diazoamide, C-H insertion, intramolecular cyclization, β-lactam, 
chemoselectivity, stereoselectivity 
 
 
 
Introduction 
 
The intramolecular insertion of metal-coordinated carbenes, generated from α-diazocarbonyl 
compounds, into C-H bonds to form five- or six-membered heterocycles and carbocycles with a 
strong preference for the five-membered ring is a well-known process.1 Instead, due to the minor 
stability, the four-membered β-lactam ring obtaining by carbenic photochemical or catalytic 
intramolecular cyclization of diazoamides is limited and often characterized by low yields. 
Rhodium(II) acetate has been demonstrated to be the catalyst of choice to perform the majority 
of transition metal carbene reactions from α-diazocarbonyl compounds.2 Moreover, by changing 
the ligand, diversity in vast synthetic fields has been achieved,3 including asymmetric synthesis.4  
Following our interest in synthesis through stabilized metal-carbenes by its intramolecular 
cyclization reactions,5 here we report the results of this process performed in a series of 1,2,3,4-
tetrahydroquinolines 1a, 1b, and 1c bearing a proper α-diazoester or α-diazocarbonyl chain 
tethered to the N atom. In principle, two different outcome of fused carbocyclic ring size 
formation would be expected: the carbenoid C-H insertion at the methylene near the N atom, 
with β-lactam ring formation, or the aromatic formal substitution at C8 quinoline carbon to 
obtain the five-membered ring closure, normally the preferred reaction course.6 Metal carbene 
aromatic substitution in competition with C-H bond insertion was previous observed in catalytic 
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reactions of diazoamides and tentatively ascribed to the electronic character of the N-substituent 
or to steric factors.7 Moreover, the (+)-menthyl diazoamidoester 1b has been selected to control 
the stereochemical information transferred to the newly formed β-lactam carbon stereocenters by 
the chiral auxiliary.  
 
 
Results and Discussion 
 
The diazoamides 1a, b, c were prepared in convenient yields by diazo transfer reaction 
performed on the corresponding amides (tosyl azide, Et3N, MeCN) which were in turn easy 
obtained (without isolation) by reaction of the 1,2,3,4-tetrahydroquinoline with ethyl malonyl 
chloride, (+)-menthyl malonyl chloride and 2,2,6-trimethyl-4H-1, 3-dioxin-4-one, respectively. 
The Rh2(OAc)4-catalyzed decomposition of 1a, b, performed in refluxing benzene, resulted in 
the regiospecific intramolecular carbene insertion at the C-H bond of the methylene near to 
nitrogen, affording the corresponding β-lactams 2a, b respectively, in convenient yields. By 
using the same procedure, the diazoamide 1c gave a 3:1 mixture of the β-lactam 2c, and  the 
pyrroloquinoline 3 (Scheme 1). 
 
 
  
O 
O 
N 
O
O
N
N 
O 
O
N
OH
O 
RR 
N 
R
N2
2c     + 
R 
H 
3
2a, b
Rh2(OAc)4
 1
TsN3
1a R= OEt; 1b R= O-(+)-Ment; 1c  R= Me 
 
Scheme 1 
 
In the case of diazoamides 1a and 1b, the exclusive β-lactam ring 2 formation is probably 
ascribed, according with previous results reported for similar diazoamides,7c to the reaction site 
activation due to the presence of a neighbour heteroatom,2a,i concomitant with a conformational 
preference in which the metal-carbene center is placed in close proximity to the less sterically 
hindered amide substituent, as shown in Figure. In the case of diazocompounds 1a, b , this steric 
factor, probably due to destabilizing interaction between the rhodium carbenoid space and the H-
8 quinoline moiety, even if attenuated by the conformational mobility of the piperidine ring,8 
should be able to control the chemoselectivity. Moreover, the electronic effect of the ester 
General Papers ARKIVOC 2005 (xiii) 1-7 
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substituent on the carbene electroaffinity7c,9 would be in favour to this reaction course. Instead, in 
the catalytic decomposition of 1c, the presence of the pyrroloquinoline 3, deriving by the 
carbenoid attack to the aromatic ring, could be ascribed to the electronic influence of the 
carbonyl group which in this case may exert on the metal carbene center an effect able to 
partially contrast both the mentioned steric and site activation factors. 
 
N
H
LnM
O
OR N
..LnM
O
OR
H
H
H
..
 
 
Figure 1 
 
The previous observed diastereoselectivity of the carbenoid bond insertion into the pseudo-
equatorially oriented C-H of the methylene group is confirmed in all the cases investigated, 
being controlled by the ring conformational rigidity of the quinoline system.10 
The trans  relationship assigned to the four-membered ring hydrogens of 2  was based upon 
their 1H NMR coupling constants.7b,11 The amide 2b was obtained as 88/12 mixture of 
enantiomers. This measure was performed by using the chiral reagent Eu(hfc)3. Asymmetric 
induction by chiral auxiliaries  was previous obtained  in this type of intramolecular 
cyclizations.12 
When the decomposition reactions were performed in the presence of Cu(acac)2 as the 
catalyst, complex product mixtures were obtained. This is in agreement with the results obtained 
by Doyle and co-workers which have demonstrated rhodium(II) carboxylates to be the best 
catalysts for constructing the β-lactam ring, starting from diazoamides.7b,11c,d 
Since Corey’s methyl-6-aminopenicillinate13 pioneering synthesis, the diazo-amide 
decomposition process for β-lactam ring construction has been employed, unfortunately with low 
yields. 
In conclusion, the present work could be considered as a contribution in the effort to increase 
the chemoselectivity and the stereoselectivity in constructing β-lactam systems by the important 
strategy of the diazocarbonyl intramolecular cyclization process. 
 
 
Experimental Section 
 
General Procedures. Reactions were monitored by TLC on commercially available precoated 
plates (silica gel 60, F 254); products were visualized with ammonium molybdate solution. Silica 
gel 60, 230-400 mesh, was used for column chromatography. Petrol refers for light petroleum, bp 
30-40°C. Melting points were performed on a Büchi 510 capillary apparatus and are 
uncorrected.1H and 13C NMR spectra were recorded on a Varian VXR-300 spectrometer at 300 
General Papers ARKIVOC 2005 (xiii) 1-7 
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and 75 MHz, respectively, in CDCl3 solutions, with TMS as the internal standard. The IR spectra 
were performed on a Perkin-Elmer 983 spectrometer. Optical rotations were measured on a 
Perkin-Elmer 142 automatic polarimeter in a 1 dm tube for CHCl3 solutions whose 
concentrations are expressed in g/100mL. Elemental analyses were performed on a Perkin-Elmer 
240 B analyzer. 
 
2-Diazo-3-(3,4-dihydro-2H-quinolin-1-yl)-3-oxo-propionic acid ethyl ester (1a). To a stirred 
CH2Cl2 dry solution (10 mL)  of ethyl malonyl chloride (1.36 g, 9.04 mmol) a solution of 1,2,3,4-
tetrahydroquinoline (1.09 g, 8.2 mmol) in CH2Cl2 (10 mL) and Et3N (1.14 mL, 8.22 mmol) in 
CH2Cl2 (10 mL) were added dropwise and the mixture was stirred under argon for 2 h at 0° C. 
After washing with water (2X25 mL), the organic layer was dried (MgSO4) and the solvent 
evaporated  to furnish the 3-(3,4-dihydro-2H-quinolin-1-yl)-3-oxo-propionic acid ethyl ester 
(2.15 g, 81% yield). To a CH3CN solution (20 mL) of the crude ester (1.25 g, 5.07 mmol) tosyl 
azide14 (1.49 g, 7.60 mmol) and triethylamine (0.78 mL, 5.57 mmol) were added. The mixture 
was stirred for 12 h under argon at room temperature and the solvent was removed under reduced 
pressure. The residue was purified by flash chromatography on silica gel column (ethyl 
acetate/petrol 7:3) to give the diazo compound 1a (1.14 g, 83 % yield). Yellow oil; IR (neat): 
2947, 2125, 1713, 1687, 1624, 1579, 1489, 1380, 1289, 1106 cm-1; 1H NMR (CDCl3) δ: 7.25-
7.06 (m, 4H), 3.94 (q, 2H, J=7.2 Hz), 3.80 (t , 2H, J=6.6 Hz), 2.76 (t, 2H, J=6.6 Hz), 2.04-1.98 
(m, 2H), 1.07 (t, 3H, J=7.2 Hz); 13C NMR (CDCl3) δ: 161.7, 160.9, 139.2, 131.9, 128.4, 128.2, 
126.2, 124.8, 121.7, 61.3, 44.9, 26.6, 23.7, 14.0. Anal. Calcd. for C14H15N3O3: C, 61.53; H, 5.53; 
N, 15.38. Found: C, 61.40; H, 5.45; N, 15.16. 
2-Diazo-3-(3,4-dihydro-2H-quinolin-1-yl)-3-oxo-propionic acid (+)-menthyl ester (1b). To a 
stirred CH2Cl2 dry solution (10 mL) of (+)-menthyl malonyl chloride (2.76 g, 10.6 mmol) a 
solution of 1,2,3,4-tetrahydroquinoline (1.30 mL, 10.3 mmol) in CH2Cl2 (10 mL) and Et3N 
(1.35 mL, 9.1 mmol) in CH2Cl2 (10 mL) were added dropwise and the mixture was stirred under 
argon for 2 h at 0° C. After washing with water (2X25 mL), the organic layer was dried (MgSO4) 
and the solvent evaporated  to furnish the 3-(3,4-dihydro-2H-quinolin-1-yl)-3-oxo-propionic acid 
(+)-menthyl ester (2.92 g, 79 % yield). To a CH3CN solution (20 mL) of the crude ester (1.25 g, 
5.07 mmol) tosyl azide14 (1.49 g, 7.60 mmol) and triethylamine (0.78 mL, 5.57 mmol) were 
added. The mixture was stirred under argon for 36 h at room temperature and the solvent was 
removed under reduced pressure. The residue was purified by flash chromatography on silica gel 
column (ethyl acetate/petrol 7:3) to give the diazo compound 1b (2.15 g, yield 93.4%). Yellow 
crystals; mp 68-70 °C; [α]25D=  26.3 (c 0.1, CHCl3); IR (nujol): 2121, 1697, 1632, 1491, 1287, 
1266, 1105 cm-1; 1H NMR (CDCl3) δ: 7.23-7.02 (m, 4H), 4.50 (dt, 1H, J=4.5 and J=10.8 Hz), 
3.96-3.86 (m, 1H), 3.73-3.65 (m, 1H), 2.77 (t, 2H, J=6.6 Hz), 2.08-1.92 (m, 2H), 1.74-1.58 (m, 
4H), 1.39-1.71 (m, 2H), 1.01-0.58 (m, 12H); 13C NMR (CDCl3) δ: 161.3, 160.9, 139.5, 132.0, 
128.5, 126.3, 124.9, 121.7, 75.3, 46.8, 44.8, 40.5, 34.0, 31.2, 26.7, 26.3, 23.8, 23.4, 21.9, 20.6, 
16.4. Anal. Calcd. for C22H29N3O3: C, 68.90; H, 7.62; N, 10.96. Found: C, 68.78; H, 7.83; N, 
10.79. 
General Papers ARKIVOC 2005 (xiii) 1-7 
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2-Diazo-1-(3,4-dihydro-2H-quinolin-1-yl)-butane-1,3-dione (1c). To a CH3CN solution 
(20 mL) of 1-(3,4-dihydro-2H-quinolin-1-yl)-butane-1,3-dione15 (1.10 g, 5.07 mmol), tosyl azide 
(1.49 g, 7.60 mmol) and triethylamine (0.78 mL, 5.57 mmol) were added. The mixture was 
stirred for 12 h under argon at room temperature and the solvent was removed under reduced 
pressure. The residue was purified by flash chromatography on silica gel column (ethyl 
acetate/petrol 7:3) to give the diazo compound 1c (1.14 g, yield 92.6%). Yellow crystals; mp 63-
64 °C; IR (nujol): 2126, 1625, 1579, 1488, 1361, 1275, 1252 cm-1; 1H NMR (CDCl3) δ: 7.20-
7.12 (m, 4H), 3.80 (t, 2H, J=6.6 Hz), 2.71 (t, 2H, J=6.6 Hz), 2.40 (s, 3H), 2.04-1.98 (m, 2H); 13C 
NMR (CDCl3) δ: 188.0, 157.6, 135.1, 129.1, 125.9, 124.1, 122.5, 118.9, 41.2, 24.8, 23.5, 21.0. 
Anal. Calcd. for C13H13N3O2: C, 64.19; H, 5.39; N, 17.27 . Found: C, 64.35; H, 5.62; N, 17.01. 
Dirhodium tetraacetate catalysed reaction of the diazoamides (1a-c). The diazoamides 
(2.36 mmol) were dissolved in dry benzene under argon. Rh2(OAc)4 (5 mol %) was added to the 
solution and the mixture was stirred at 85 °C until the disappearance of the IR absorption of the 
diazo function. After filtration and evaporation, the residue was flash chromatographed on silica 
gel column (ethyl acetate/petrol 7:3). 
1-Oxo-2,2a,3,4-tetrahydro-1H-azeto[1,2-a]quinoline-2-carboxylic acid ethyl ester (2a). Yield 
75 %. Oil; IR (neat): 1758, 1721, 1487, 1357, 1318, 1251, 1194 cm-1; 1H NMR (CDCl3) δ: 7.48-
7.01 (m, 4H), 4.27 (q, 2H, J=7.2 Hz), 4.10 (dt, 1H, J=2.7 and J=11.7 Hz), 3.90 (d, 1H, J=2.7 Hz), 
2.88-2.85 (m, 2H), 2.41-2.36 (m, 1H), 1.70-1.56 (m, 1H), 1.33 (t, 3H, J=7.2 Hz); 13C NMR 
(CDCl3) δ: 166.7, 159.3, 133.6, 129.1, 127.2, 124.4, 123.9, 118.5, 61.8, 60.9, 51.3, 25.9, 25.0, 
14.1. Anal. Calcd. for C14H15NO3: C, 68.56; H, 6.16; N, 5.71. Found: C, 68.37; H, 5.90; N, 5.44. 
1-Oxo-2,2a,3,4-tetrahydro-1H-azeto[1,2-a]quinoline-2-carboxylic acid (+)-menthyl ester 
(2b). Yield 79 %. White crystals; mp 104-106 °C (petrol); IR (nujol): 1770, 1726, 1488, 1314, 
1253, 1206, 1195, 1157 cm-1; 1H NMR (CDCl3) δ: 7.49-6.98 (m, 4H), 4.84-4.75 (m, 1H), 4.10-
4.04 (m, 1H), 3.88 (d, 1H, J=2.5), 2.91-2.86 (m, 2H), 2.42-2.37 (m, 1H), 2.05-1.42 (m, 7H), 
1.11-0.76 (m, 12H); 13C NMR (CDCl3) δ: 166.3, 159.2, 133.7, 129.1, 127.3, 124.3, 123.8, 118.6, 
76.0, 61.2, 51.2, 46.8, 40.7, 34.1, 31.4, 26.3, 26.0, 25.1, 23.3, .21.9, 20.8, 16.2. Anal. Calcd. for 
C22H29NO3: C, 74.33; H, 8.22; N, 3.94 . Found: C, 74.55; H, 7.97; N, 4.08. 
2-Acetyl-2,2a,3,4-tetrahydro-1H-azeto[1,2-a]quinolin-1-one (2c). Yield 65%. White crystals; 
mp 120-122 °C (petrol-methylene chloride); IR (nujol): 1753, 1713, 1487, 1354, 1319, 1190, 
1130 cm-1; 1H NMR (CDCl3) δ: 7.44-6.98 (m, 4H), 4.16 (dt, 1H, J=2.7 and J=11.7 Hz), 4.05 (d, 
1H, J=2.7 Hz), 2.89-2.84 (m, 2H), 2.39 (s, 3H), 2.38-2.29 (m, 1H), 1.67-1.53 (m, 1H); 13C NMR 
(CDCl3) δ: 199.6, 160.4, 133.5, 129.0, 127.1, 124.5, 123.7, 118.3, 69.2, 49.4, 29.4, 25.9, 24.7. 
Anal. Calcd. for C13H13NO2: C, 72.54; H, 6.09; N, 6.51. Found: C, 72.22; H, 5.82; N, 6.33. 
1-(2-Hydroxy-5,6-dihydro-4H-pyrrolo[3,2,1-ij]quinolin-1-yl)ethanone (3). Yield 23%. 
Colourless crystals; mp 157-159 °C (petrol-methylene chloride); IR (nujol): 1655, 1607, 1359, 
1303, 1242 cm-1; 1H NMR (CDCl3) δ: 13.03 (s, 1H), 7.14-6.92 (m, 3H), 3.80 (t, 2H, J=5.86 Hz), 
2.80 (t, 2H, J=5.86 Hz), 2.38 (s, 3H), 2.07-1.99 (m, 2H); 13C NMR (CDCl3) δ: 172.6, 169.7, 
134.8, 123.7, 121.5, 120.5, 120.3, 117.4, 102.5, 38.3, 24.5, 21.2, 20.0. Anal. Calcd. for 
C13H13NO2: C, 72.54; H, 6.09; N, 6.51. Found: C, 72.35; H, 6.31; N, 6.69. 
General Papers ARKIVOC 2005 (xiii) 1-7 
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Acknowledgements 
 
Thanks are due to Mr. Mauro Mucedda for experimental assistence. Financial support by 
M.I.U.R. (ex-60% funds) and Regione Autonoma della Sardegna are gratefully acknowledged. 
 
 
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Quinoline beta-lactams by Rh(II)-catalyzed highly steroselective intramolecular carbene insertion into a carbon-hydrogen bond

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