Asymmetric synthesis of 1-(2- and 3-haloalkyl)azetidin-2-ones as precursors for novel piperazine, morpholine, and 1,4-diazepane annulated beta-lactams

Article abstract of DOI:10.1021/jo0608319

A high-yielding, asymmetric synthesis of novel 4-formyl-1-(2- and 3-haloalkyl)azetidin-2-ones was developed as valuable starting materials for the synthesis of different enantiomerically enriched bicyclic azetidin-2-ones, such as piperazine, morpholine, a

Full text of DOI:10.1021/jo0608319

azetidin-2-ones by means of the Staudinger reaction could pave
the way for the preparation of a large variety of different bio-
logical interesting compounds such as bicyclic â-lactams and
other azaheterocycles. In addition, the fact that asymmetric
induction during the Staudinger reaction has been thoroughly
investigated in the past1,6,7 urged us to evaluate this methodology
for the synthesis of novel optically active 1-(2- and 3-haloalkyl)-
azetidin-2-ones and their conversion into different fused aza-
heterocycles such as piperazine, morpholine, and 1,4-diazepane
annulated â-lactam derivatives.
Asymmetric Synthesis of 1-(2- and
3-Haloalkyl)azetidin-2-ones as Precursors for
Novel Piperazine, Morpholine, and 1,4-Diazepane
Annulated Beta-Lactams
Willem Van Brabandt, Matthieu Vanwalleghem,
Matthias D’hooghe, and Norbert De Kimpe*
Department of Organic Chemistry, Faculty of Bioscience
Engineering, Ghent UniVersity, Coupure links 653,
B-9000 Ghent, Belgium
Results and Discussion
norbert.dekimpe@UGent.be
Whereas 4-formyl-â-lactams have already proven their poten-
tial in the stereocontrolled synthesis of heterocycles, amino acids,
amino sugars, etc.,8 their 1-(2- and 3-haloalkyl) derivatives have
not been synthesized and evaluated up to now. Therefore, (R)-
glyceraldehyde acetonide 1 was treated with different ω-ha-
loalkylammonium halides to prepare the corresponding func-
tionalized imines 2 and 4 (Scheme 1). When 2-chloroethylammo-
nium chloride and 3-bromopropylammonium bromide were
used, the condensation reaction with (R)-glyceraldehyde ac-
etonide 1 toward the corresponding chiral imines 2 and 4 could
be performed in almost quantitative yields. In contrast, when
2-bromoethylammonium bromide was used, a complex reaction
mixture was obtained, probably due to the lability of the
corresponding N-(2-bromoethyl)imine. Imines 2 and 4 had to
be handled with care, i.e., no heating above room temperature
and immediate further elaboration, as decomposition of the latter
compounds occurred relatively easy (after 1 day at room tem-
perature, the purity of the imines was reduced to less than 50%).
Imines 2 and 4 turned out to be valuable starting materials for
the preparation of 1-(ω-haloalkyl)-â-lactams 3 and 5 through a
Staudinger reaction. Treatment of the latter imines 2 and 4 with
1.3 equiv of benzyloxy-, phenoxy-, or methoxyacetyl chloride
in dichloromethane in the presence of triethylamine afforded
the optically active corresponding â-lactams 3 and 5 in high
yield and with high diastereomeric excess (Scheme 1, Table 1).
Azetidin-2-ones 3 and 5 could be easily converted into the
premised 4-formyl-1-(2- and 3-haloalkyl)azetidin-2-ones 7 and
9 by consecutive hydrolysis in THF in the presence of p-toluene-
sulfonic acid (PTSA) toward diols 6 and 8 and oxidation of the
resulting diols by sodium periodate in a two phase system of
water and dichloromethane (Scheme 2, Table 2). The e.e. values
of the 4-formyl-1-(2- and 3-haloalkyl)azetidin-2-ones 7 and 9
were equal to the d.e. values of their starting products 3 and 5
(e.e. values obtained by chiral GC).
ReceiVed April 20, 2006
A high-yielding, asymmetric synthesis of novel 4-formyl-
1-(2- and 3-haloalkyl)azetidin-2-ones was developed as
valuable starting materials for the synthesis of different
enantiomerically enriched bicyclic azetidin-2-ones, such as
piperazine, morpholine, and 1,4-diazepane annulated â-lac-
tam derivatives. Especially the hydride reduction of 4-imi-
doyl-1-(2- and 3-haloalkyl)azetidin-2-ones turned out to be
an efficient and straightforward method for the preparation
2-substituted piperazines and 1,4-diazepanes.
Introduction
The Staudinger reaction, in which an imine and an acid chlo-
ride react in the presence of a base, comprises a very reliable
and robust method for the preparation of â-lactam derivatives.1-4
Because â-lactams are widely recognized for their antibiotic
activity, in addition to macrolides and fluoroquinolones, research
in this area remains indispensable to provide new entries toward
azetidin-2-ones with potential biological activity.5 Up to now,
no examples of Staudinger reactions leading to 1-(2- and 3-halo-
alkyl)azetidin-2-ones have been reported, probably because of
the instability of the requisite N-alkylidene-(2- and 3-haloalkyl)-
amines. However, the synthesis of 1-(2- and 3-haloalkyl)-
Subsequently, the synthetic potential of 4-formyl-1-haloalkyl-
â-lactams 7 and 9, prepared by an asymmetric approach, toward
novel bicyclic â-lactam derivatives was investigated for the first
time. Thus, 1-(2-haloethyl)azetidin-2-ones 7a,b were treated
with 2 equiv of NaBH4 in methanol under reflux for 1 h, furnish-
ing the corresponding chiral 4-hydroxymethyl-â-lactams 10 in
86-89% yield (Scheme 3). Treatment of the latter alcohols 10
with NaH in DMSO resulted in a complex reaction mixture,
(1) Palomo, C.; Aizpurua, J. M.; Inaki, G.; Oiarbide, M. Eur. J. Org.
Chem. 1999, 3223.
(2) Cossio, F. P.; Ugalde, J. M.; Lopez, X.; Lecea, B.; Palomo, C. J.
Am. Chem. Soc. 1993, 115, 995.
(3) Singh, G. S. Tetrahedron 2003, 59, 7631.
(4) De Kimpe, N. In ComprehensiVe Heterocyclic Chemistry II, A reView
of the Literature 1982-1995; Katritzky, A. R., Rees, C. W., Scriven, E. F.
V., Padwa, A., Eds.; Pergamon: New York, 1996; Chapter on “Azetidines,
Azetines, and Azetes: Monocyclic”, pp 508-589.
(5) Fisher, J. F.; Meroueh, S. O.; Mobashery, S. Chem. ReV. 2005, 105,
395.
(6) Cossio, F. P.; Arrieta, A.; Lecea, B.; Ugalde, J. J. Am. Chem. Soc.
1994, 116, 2085.
(7) Palomo, C.; Cossio, F. P.; Cuevas, C.; Lecea, B.; Mielgo, A.; Roman,
P.; Luque, A.; Martinez-Ripoll M. J. Am. Chem. Soc. 1992, 114, 9360.
(8) Alcaide, B.; Almendros, P. Chem. Soc. ReV. 2001, 30, 226.
10.1021/jo0608319 CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/03/2006
J. Org. Chem. 2006, 71, 7083-7086
7083

SCHEME 1
SCHEME 4
TABLE 3. Synthesis of 4-Imidoyl-â-lactams 12 and 14 and of
Diazabicycloalkanones 13 and 15
TABLE 1. Synthesis of 1-(ω-Haloalkyl)-â-lactams 3a-c and 5a,b
R1
R2
n
compound (% yield)
compound (% yield)
compound
R1
n
X
yield (%)
d.e. (%)a
Bn
Bn
Me
Me
Me
Bn
Bn
Me
Me
Me
Allyl
iPr
1
1
1
1
1
2
2
2
2
2
12a (98)
12b (97)
12c (98)
12d (99)
12e (97)
14a (98)
14b (96)
14c (99)
14d (98)
14e (99)
13a (81)
13b (59)
13c (87)
13d (41)
13e (63)
15a (65)
15b (54)
15c (62)
15d (57)
15e (59)
3a
3b
3c
5a
5b
Bn
Ph
Me
Bn
Me
1
1
1
2
2
Cl
Cl
Cl
Br
Br
86
87
81
77
75
87
88
83
88
91
Allyl
tBu
Bn
Allyl
tBu
1
a Determined by means of H NMR and GC.
Allyl
iPr
Bn
SCHEME 2
suppressants, antidepressants, antitumor agents, antioxidants, and
antibiotics.10
In addition to the preparation of 4-oxa-1-azabicyclo[4.2.0]-
octan-8-ones 11, a synthesis of their nitrogen counterparts was
developed to broaden the scope of this methodology. Conversion
of 4-formyl-1-(ω-haloalkyl)-â-lactams 7 and 9 into the corre-
sponding novel 4-imidoyl-â-lactams 12 and 14 upon condensa-
tion with different primary amines, followed by reduction of
the latter azetidin-2-ones 12 and 14 with NaBH4 in refluxing
methanol or ethanol yielded the corresponding optically active
bicyclic â-lactams 13 and 15 in good to high yields in a one
step procedure (Scheme 4, Table 3).11 In a few cases, the corre-
sponding 4-aminomethyl-2-azetidinones were formed as minor
constituents (<20%). The reductive ring closure of monocyclic
R-(N-haloalkylamino)imines toward the corresponding diaza-
heterocycles comprises a new and very elegant methodology
in organic synthesis. In the literature, analogous piperazine
annulated â-lactams have been prepared either by cyclization
of Boc-protected 4-aminomethyl-1-(2-hydroxyethyl)-â-lactams
in a four-step approach, involving mesylation, N-deprotection,
base-induced ring closure and N-protection,12 or by intramo-
lecular dipolar cycloaddition of 4-vinyl-2-azetidinones.13 The
presented methodology comprises an elegant and efficient
alternative for these approaches. The yields appeared to be lower
when sterically hindering substituents were present at the imi-
TABLE 2. Synthesis of 4-(1,2-Dihydroxyethyl)azetidin-2-ones
6 and 8 and of 4-Formyl-â-lactams 7 and 9
compound
R1
n
X
yield (%)
d.e. (%)a
6a
6b
6c
8a
8b
7a
7b
7c
9a
9b
Bn
Ph
1
1
1
2
2
1
1
1
2
2
Cl
Cl
Cl
Br
Br
Cl
Cl
Cl
Br
Br
98
92
81
84
90
94
95
83
95
94
87
88
83
88
88
87
88
83
88
91
Me
Bn
Me
Bn
Ph
Me
Bn
Me
1
a Determined by means of H NMR and GC.
SCHEME 3
doyl nitrogen (R2 ) Bu, Pr). Because of the steric hindrance,
ring closure proceeded slower and reflux in ethanol instead of
methanol was needed for the cyclization. The fact that 1,4-
diazabicyclo[4.2.0]octan-8-ones 13 and 1,5-diazabicyclo[5.2.0]-
nonan-9-ones 15 cannot only be considered as novel bicyclic
â-lactam skeletons, but also as bicyclic piperazine and 1,4-
diazepane derivatives adds significant value to the biological
relevance of these target compounds. â-Lactams 13 are of
particular interest due to their structural resemblance to the
unsaturated isodethiaazacephems, which are known as potent
t
i
with no traces of the desired bicyclic â-lactams 11. On the other
hand, deprotonation of the alcohols 10 by means of NaH in
toluene (resulting in precipitation of the formed sodium salts),
followed by removal of solvent under reduced pressure and
addition of DMSO afforded optically active bicyclic â-lactams
11 after stirring overnight at 100 °C (Scheme 3). The bicyclic
â-lactams 11 were purified by column chromatography. An
analogous ring closure has been reported starting from 1-(1,2-
dibromopropyl)-4-hydroxymethyl-â-lactams.9 Our methodology
comprises a new asymmetric approach not only toward the 2-iso-
oxacepham skeleton, but also for the preparation of novel sub-
stituted morpholine derivatives, which can be applied as appetite
(10) Wijtmans, R.; Vink, M. K. S.; Schoemaker, H. E.; van Delft, F. L.;
Blaauw, R. H.; Rutjes, F. P. J. T. Synthesis 2004, 5, 641.
(11) See supporting information (Table 4) for detailed reaction conditions
for any specific case in the synthesis of azetidin-2-ones 13 and 15.
(12) Palomo, C.; Ganboa, I.; Cuevas, C.; Boschetti, C.; Linden, A.
Tetrahedron Lett. 1997, 38, 4643.
(9) Hakimelahi, G. H.; Jarrahpour, A. A. HelV. Chim. Acta 1989, 72,
1501.
(13) Murthy, K. S. K.; Hassner, A. Tetrahedron Lett. 1987, 28, 97.
7084 J. Org. Chem., Vol. 71, No. 18, 2006

antibacterial agents.14 Piperazine derivatives are frequently used
as antifungals, antidepressants, antivirals, and serotoninreceptor
(5-HT) antagonists/agonists,15 and carbon-monosubstituted pip-
erazines have been reported as farnesyl transferase inhibitors
and neurokinin-1 antagonists.16 Also, diazepanes in general and
1,4-benzodiazepines in particular have received a lot of attention
because of their value in psychotherapy (e.g., diazepam, the
active compound in Valium).17
In conclusion, a new asymmetric synthesis of novel 4-formyl-
1-(2- and 3-haloalkyl)azetidin-2-ones has been developed as
valuable starting materials for the synthesis of different optically
active bicyclic azetidin-2-ones, such as piperazine, morpholine,
and 1,4-diazepane annulated â-lactam derivatives. Especially
the reduction of 4-imidoyl-1-(2- and 3-haloalkyl)azetidin-2-ones
with NaBH4 turned out to be a promising method for the
synthesis of 2-substituted piperazines and 1,4-diazepanes as
interesting new targets with potential biological activity.
4-formyl-â-lactams 7 and 9 was performed according to a literature
procedure.20As a representative example, the synthesis of (2R,3R)-
3-benzyloxy-1-(2-chloroethyl)-4-oxoazetidine-2-carbaldehyde 7a is
described. To a solution of 3.39 g of (3R,4S)-3-benzyloxy-1-(2-
chloroethyl)-4-((4S)-2,2-dimethyl-1,3-dioxolan-4-yl)azetidin-2-
one 3a (10 mmol, 1 equiv) in THF/water (1:1, 200 mL) was added
2.28 g of p-TsOH‚H2O (12 mmol, 1.2 equiv) in a single portion.
The resulting clear solution was heated under reflux for 4 h. The
reaction mixture was allowed to cool to room temperature and was
then neutralized with solid NaHCO3. The mixture was extracted
with EtOAc (3 × 40 mL), the organic layer was dried (MgSO4),
and the solvent was removed under reduced pressure, yielding 2.93
g (9.8 mmol, 0.98 equiv) of (3R,4S)-3-benzyloxy-1-(2-chloroethyl)-
4-((1S)-1,2-dihydroxyethyl)azetidin-2-one 6a. In the next step, satu-
rated aqueous sodium hydrogen carbonate (980 µL) was added to
a solution of the diol (9.8 mmol) in dichloromethane (15 mL),
maintaining the temperature below 25 °C. Solid sodium periodate
(19.6 mmol) was added over a 10 min. period with vigorous stirring,
and the reaction was allowed to proceed for 2 h while the tem-
perature was maintained below 25 °C. The solid was removed by
filtration, and the filtrate was washed with 25 mL of water, dried
(MgSO4), and the solvent was removed under reduced pressure.
Further purification, although not necessary to proceed to the next
step, was performed by flash chromatography on silica gel or just
by elution of the filtrate (obtained after drying with MgSO4) over
a silica gel column, followed by evaporation of the solvent in vacuo.
(3R,4S)-3-Benzyloxy-1-(2-chloroethyl)-4-((1S)-1,2-dihydroxy-
ethyl)azetidin-2-one 6a. Colorless oil, 98% yield, TLC Rf 0.3
(EtOAc), [R]D ) +75° (c 1, CH2Cl2). 1H NMR (300 MHz,
CDCl3): δ 2.20-2.30 and 2.88-2.96 (2H, 2 × m), 3.51-3.88 (6H,
m), 3.90 (1H, t, J ) 5.0 Hz), 4.15 (1H, m), 4.70 (1H, d, J ) 11.7
Hz), 4.74 (1H, d, J ) 5.0 Hz), 4.95 (1H, d, J ) 11.7 Hz), 7.29-
7.40 (5H, m). 13C NMR (75 MHz, CDCl3): δ 41.2, 43.4, 59.2,
63.9, 71.2, 73.3, 80.5, 128.1, 128.3, 128.6, 136.5, 168.4. IR (NaCl,
cm-1): νCdO ) 1741. MS (70 eV) m/z (%): 302/0 (M++1, 100).
Anal. Calcd for C14H18ClNO4: C 56.10; H 6.05; N 4.67. Found:
C 56.36; H 6.21; N 4.50.
(2R,3R)-3-Benzyloxy-1-(2-chloroethyl)-4-oxoazetidine-2-car-
baldehyde 7a. White crystals, 94% yield, Mp: 54 °C, TLC Rf 0.4
(petroleum ether/ ethyl acetate 1/4), [R]D ) +133° (c 1, CH2Cl2).
1H NMR (300 MHz, CDCl3): δ 3.60-3.83 (4H, m), 4.38 (1H,
d×d, J ) 5.1 Hz, J ) 1.9 Hz), 4.65 (1H, d, J ) 11.7 Hz), 4.80
(1H, d, J ) 11.7 Hz), 5.01 (1H, d, J ) 5.1 Hz), 7.30-7.40 (5H,
m), 9.58 (1H, d, J ) 1.9 Hz). 13C NMR (75 MHz, CDCl3): δ
41.8, 43.5, 65.0, 73.5, 83.8, 128.3, 128.5, 128.7, 135.9, 166.5, 198.1.
IR (NaCl, cm-1): νCdO ) 1761 and 1733. MS (70 eV) m/z (%):
270/68 (M++1, 90), 222 (100). Anal. Calcd for C13H14ClNO3: C
58.32; H 5.27; N 5.23. Found: C 58.38; H 5.42; N 5.10.
Synthesis of 1-(2-Chloroethyl)-4-(hydroxymethyl)azetidin-2-
ones 10. As a representative example, the synthesis of (3R,4S)-3-
benzyloxy-1-(2-chloroethyl)-4-(hydroxymethyl)azetidin-2-one 10a
is described. In a 100 mL flask, 1.0 g (3.7 mmol, 1 equiv) of (2R,
3R)-3-benzyloxy-1-(2-chloroethyl)-4-oxo-2-azetidinecarbalde-
hyde 7a was dissolved in methanol (50 mL) and placed in an ice
bath. Then, 0.28 g (7.5 mmol, 2 equiv) of NaBH4 was added in
portions, and the resulting mixture was kept at reflux temperature
for 1 h. Water was added (50 mL), and the mixture was extracted
3 times with 40 mL of CH2Cl2. The combined organic fractions
were dried (MgSO4), filtered over silicagel, and the solvent was
evaporated in vacuo, yielding 0.89 g (3.3 mmol, 0.89 equiv) of
(3R,4S)-3-benzyloxy-1-(2-chloroethyl)-4-(hydroxymethyl)azetidin-
2-one 10a.
Experimental Section
(R)-Glyceraldehyde acetonide 1 was synthesized according to
literature procedures.18,19
Synthesis of 1-(2-chloroethyl)-â-lactams 3 and 1-(3-Bromo-
propyl)-â-lactams 5. As a representative example, the synthesis
of (3R,4S)-3-benzyloxy-1-(2-chloroethyl)-4-((4S)-2,2-dimethyl-1,3-
dioxolan-4-yl)azetidin-2-one 3a is described. In a 100 mL flask,
2.82 g (14.8 mmol, 1 equiv) of 2-chloro-N-(((S)-2,2-dimethyl-1,3-
dioxolan-4-yl)methylene)ethylamine 2 was dissolved in 50 mL dry
CH2Cl2, and 4.48 g (44.4 mmol, 3 equiv) of triethylamine was added
to the mixture, which was cooled to 0 °C. Subsequently, a mixture
of 3.54 g (19.2 mmol, 1.3 equiv) of benzyloxyacetyl chloride in
20 mL of dry CH2Cl2 was added dropwise. The reaction mixture
was allowed to warm slowly to room temperature (over a period
of about 2 h) and was stirred overnight at room temperature. Then,
water (50 mL) was added to the reaction mixture and the organic
layer was isolated. The aqueous fraction was additionally extracted
twice with 25 mL of CH2Cl2. The combined organic fractions were
dried (MgSO4), and the solvent was evaporated under vacuum.
Further purification was performed by flash chromatography on
silica gel (petroleum ether/EtOAc 6/1).
(3R,4S)-3-Benzyloxy-1-(2-chloroethyl)-4-((4S)-2,2-dimethyl-
1,3-dioxolan-4-yl)azetidin-2-one 3a. Colorless oil, 86% yield, TLC
Rf 0.05 (petroleum ether/ EtOAc 6/1), [R]D ) +59° (c 1, CH2Cl2).
1H NMR (300 MHz, CDCl3): δ 1.34 and 1.44 (6H, 2 × s), 3.57-
3.78 (6H, m), 4.15 (1H, d × d, J ) 8.8 Hz, J ) 6.6 Hz), 4.34 (1H,
d × t, J ) 9.1 Hz, J ) 6.3 Hz), 4.64 (1H, d, J ) 11.7 Hz), 4.67
(1H, d, J ) 5.2 Hz), 4.91 (1H, d, J ) 11.7 Hz), 7.26-7.40 (5H,
m). 13C NMR (75 MHz, CDCl3): δ 25.1, 26.8, 40.8, 42.8, 60.8,
66.7, 72.9, 76.9, 80.4, 109.6, 127.8, 128.1, 128.5, 136.8, 167.9. IR
(NaCl, cm-1): νCdO ) 1763. MS (70 eV) m/z (%): 342/0 (M++1,
100). Anal. Calcd for C17H22ClNO4: C 60.09; H 6.53; N 4.12.
Found: C 60.35; H 6.40; N 4.26.
Synthesis of 4-Formyl-â-lactams 7 and 9. The conversion of
4-((4S)-2,2-dimethyl-1,3-dioxolan-4-yl)azetidin-2-ones 3 and 5 into
(14) Hwu, J. R.; Tsay, S.-C.; Hakimelahi, S. J. Med. Chem. 1998, 41,
4681.
(15) ChemFiles, PriVileged Structures for Lead DiscoVery and Optimiza-
tion: Piperazines; Vol. 5, No. 5, p 2.
(16) Berkheij, M.; van der Sluis, L.; Sewing, C.; den Boer, D. J.; Terpstra,
J. W.; Hiemstra, H.; Iwema Bakker, W. I.; van den Hoogenband, A.; van
Maarseveen, J. G. Tetrahedron Lett. 2005, 46, 2369.
(17) Katritzky, A. R.; Rees, C. W. In ComprehensiVe Heterocyclic
Chemistry; Lwowski, W., Ed.; Pergamon Press: Oxford, 1984; Vol. 7, pp
608-620.
(18) Schmid, R.; Bryant, J. D. Org. Synth. 1993, 72, 6.
(19) Bianchi, P.; Roda, G.; Riva, S.; Danieli, B.; Zabelinskaja-Mackova,
A.; Griengl, H. Tetrahedron 2001, 57, 2213.
(3R,4S)-3-Benzyloxy-1-(2-chloroethyl)-4-(hydroxymethyl)aze-
tidin-2-one 10a. Colorless oil, 89% yield, [R]D ) +47° (c 1, CH2-
1
Cl2). H NMR (300 MHz, CDCl3): δ 2.38 (1H, broad s), 3.45
(20) Alcaide, B.; Almendros, P.; Salgado, N. R. J. Org. Chem. 2000,
65, 3310.
J. Org. Chem, Vol. 71, No. 18, 2006 7085

(1H, d × d × d, J ) 14.0 Hz, J ) 7.2 Hz, J ) 5.2 Hz), 3.64-3.69
(2H, m), 3.73-3.85 (1H, m), 3.88-3.93 (3H, m), 4.69 (1H, d, J )
11.6 Hz, 4.79 (1H, d, J ) 4.4 Hz), 4.94 (1H, d, J ) 11.6 Hz),
7.33-7.40 (5H, m). 13C NMR (75 MHz, CDCl3): δ 41.8, 42.3,
59.0, 59.7, 73.6, 81.8, 128.2, 128.4, 128.7, 136.4, 167.6. IR (NaCl,
cm-1): νCdO ) 1747. MS (70 eV) m/z (%): 272/0 (M++1, 100).
Anal. Calcd for C13H16ClNO3: C 57.89; H 5.98; N 5.19. Found:
C 58.10; H 5.84; N 4.99.
[R]D ) 43° (c 1, CH2Cl2). 1H NMR (300 MHz, CDCl3): δ 3.51-
3.73 (4H, m), 4.10 (2H, d × d, J ) 6.1 Hz, J ) 1.4 Hz), 4.36 (1H,
d × d, J ) 6.1 Hz, J ) 5.0 Hz), 4.61 (1H, d, J ) 11.7 Hz), 4.73
(1H, d, 11.7 Hz), 4.89 (1H, d, J ) 5.0 Hz), 5.07-5.22 (2H, m),
5.89-6.02 (1H, m), 7.28-7.38 (5H, m), 7.69 (1H, d × t, J ) 6.1
Hz, J ) 1.4 Hz). 13C NMR (75 MHz, CDCl3): δ 41.2, 43.0, 61.5,
63.3, 73.1, 83.3, 116.7, 128.0, 128.2, 128.5, 134.9, 136.4, 161.4,
167.0. IR (NaCl, cm-1): νCdO ) 1761, νmax) 2912, 2873, 1643,
1497. MS (70 eV) m/z (%): 307/9 (M++1, 100). Anal. Calcd for
C16H19ClN2O2: C 62.64; H 6.24; N 9.13. Found: C 62.40; H 6.39;
N 9.29.
Synthesis of 1,4-Diazabicyclo[4.2.0]octan-8-ones 13 and 1,5-
Diazabicyclo[5.2.0]nonan-9-ones 15. As a representative example,
the synthesis of (6S,7R)-4-allyl-7-benzyloxy-1,4-diazabicyclo[4.2.0]-
octan-8-one 13a is described. Depending on the steric hindrance
of the imidoyl substituent of the 4-imidoyl-â-lactams 12 and 14,
slightly different reaction conditions were required, presented in
Table 1. In a 100 mL flask, 1.11 g (3.6 mmol, 1 equiv) of (3R,4S)-
3-benzyloxy-1-(2-chloroethyl)-4-((E)-(2-propenylimino)methyl)-
azetidin-2-one 12a was dissolved in 40 mL of methanol and placed
in an ice bath. 0.27 g (7.2 mmol, 2 equiv) of NaBH4 was added,
and the mixture was warmed and kept at reflux temperature for
1 h. Water (50 mL) was added, and the resulting mixture was
extracted 3 times with 30 mL of CH2Cl2. The combined organic
fractions were dried (MgSO4), and the solvent was evaporated in
vacuo. Further purification was performed by flash chromatography
on silica gel (CH2Cl2/MeOH 95/5).
(6S,7R)-4-Allyl-7-benzyloxy-1,4-diazabicyclo[4.2.0]octan-8-
one 13a. Colorless oil, 81% yield, TLC Rf 0.2 (CH2Cl2/MeOH
95/5), [R]D ) +25° (c 1, CH2Cl2).1H NMR (300 MHz, CDCl3):
δ 2.00 (1H, d × d × d, J ) 11.6 Hz, J ) 11.3 Hz, J ) 4.1 Hz),
2.13 (1H, d × d, J ) 11.4 Hz, J ) 10.4 Hz), 2.67 (1H, d × d, J
) 11.4 Hz, J ) 4.4 Hz), 2.72 (1H, d × d, J ) 11.3 Hz, J ) 4.4
Hz), 2.91 (1H, d × d × d×d, J ) 13.1 Hz, J ) 11.6 Hz, J ) 4.4
Hz, J ) 1.1 Hz), 2.96-3.05 (2H, m), 3.54 (1H, d × t, J ) 10.2
Hz, J ) 4.3 Hz), 3.73 (1H, d × d, J ) 13.1 Hz, J ) 4.1 Hz), 4.53
(1H, d, J ) 11.8 Hz), 4.69 (1H, d×d, J ) 4.3 Hz, J ) 1.1 Hz),
4.79 (1H, d, J ) 11.8 Hz), 5.14-5.21 (2H, m), 5.71-5.84 (1H,
m), 7.26-7.35 (5H, m). 13C NMR (75 MHz, CDCl3): δ 38.9, 51.7,
51.8, 52.2, 61.7, 73.4, 82.7, 118.5, 128.2, 128.2, 128.5, 134.3, 136.8,
167.6. IR (NaCl, cm-1): νCdO ) 1760. MS (70 eV) m/z (%): 273
(M++1, 100). Anal. Calcd for C16H20N2O2: C 70.56; H 7.40; N
10.29. Found: C 70.32; H 7.58; N 10.38.
Synthesis of 4-Oxa-1-azabicyclo[4.2.0]octan-8-ones 11. As a
representative example, the synthesis of (6S,7R)-7-benzyloxy-4-
oxa-1-azabicyclo[4.2.0]octan-8-one 11a is described. In a 100 mL
flask, 0.89 g (3.3 mmol, 1 equiv) of (3R,4S)-3-benzyloxy-1-(2-
chloroethyl)-4-(hydroxymethyl)azetidin-2-ones 10a was dissolved
in 10 mL of toluene. NaH (0.08 g (3.3 mmol, 1 equiv)) was added,
and stirring was continued for 5 min at room temperature. Subse-
quently, the solvent was evaporated under reduced pressure and
30 mL of DMSO was added. The resulting reaction mixture was
stirred overnight at 100 °C. Afterward, 50 mL of water was added,
and the mixture was extracted 3 times with 40 mL of diethyl ether.
The combined organic fractions were washed 3 times with 20 mL
of water, dried (MgSO4), and the solvent was removed under
reduced pressure. Further purification was performed by flash chro-
matography on silica gel, yielding 0.19 g (0.8 mmol, 0.25 equiv)
of (6S,7R)-7-benzyloxy-4-oxa-1-azabicyclo[4.2.0]octan-8-one 11a.
(6R,7S)-7-Benzyloxy-4-oxa-1-azabicyclo[4.2.0]octan-8-one 11a.
Colorless oil, 25% yield, TLC Rf 0.2 (petroleum ether/ EtOAc 1/1),
[R]D ) +24° (c 1, CH2Cl2). 1H NMR (300 MHz, CDCl3): δ 2.98
(1H, d × d × d×d, J ) 13.5 Hz, J ) 11.6 Hz, J ) 4.7 Hz, J ) 1.4
Hz), 3.38 (1H, t×d, J ) 11.6 Hz, J ) 3.6 Hz), 3.57 (1H, t, J ) 9.9
Hz), 3.57-3.73 (3H, m), 3.79 (1H, d×d, J ) 11.6 Hz, J ) 4.7
Hz), 4.51 (1H, d, J ) 11.6 Hz), 4.69 (1H, d×d, J ) 4.1 Hz, J )
1.4 Hz), 4.82 (1H, d, J ) 11.6 Hz), 7.32-7.40 (5H, m). 13C NMR
(75 MHz, CDCl3): δ 38.6, 50.3, 65.7, 67.3, 73.7, 82.8, 128.3, 128.4,
128.6, 136.6, 167.7. IR (NaCl, cm-1): νCdO ) 1758. MS (70 eV)
m/z (%): 234 (M++1, 40), 222 (100). Anal. Calcd for C13H15-
ClNO3: C 66.94; H 6.48; N 6.00. Found: C 66.73; H 6.64; N 5.84.
Synthesis of 4-Imidoyl-â-lactams 12 and 14. As a representative
example, the synthesis of (3R,4S)-3-benzyloxy-1-(2-chloroethyl)-
4-((E)-(2-propenylimino)methyl)azetidin-2-one 12a is described.
In a 100 mL flask, 1.0 g (3.7 mmol, 1 equiv) of (2R,3R)-3-benzyl-
oxy-1-(2-chloroethyl)-4-oxo-2-azetidinecarbaldehyde 7a was dis-
solved in 50 mL of CH2Cl2, and 0.71 g (5.6 mmol, 1.5 equiv) of
MgSO4 was added. Subsequently, 0.21 g (3.7 mmol, 1 equiv) of
allylamine was added, and the resulting mixture was stirred at room
temperature for 1 h. MgSO4 was filtered off, and the solvent was
evaporated, yielding 1.11 g (3.6 mmol, 0.98 equiv) of (3R,4S)-3-
benzyloxy-1-(2-chloroethyl)-4-((E)-(2-propenylimino)methyl)aze-
tidin-2-one 12a. Although the resulting imines are obtained in high
purity, an additional purification can be performed by filtration of
the imine (dissolved in a mixture of CH2Cl2/Et3N (99/1)) over a
column of silica gel (previously treated with a mixture of CH2Cl2/
Et3N (99/1)).
Acknowledgment. We are indebted to the Fund for Scien-
tific Research-Flanders (FWO-Flanders) and Ghent University
(GOA) for financial support.
Supporting Information Available: General information and
all spectroscopic data for compounds 2, 3b-c, 4, 5a-b, 6b-c,
7b-c, 8a-b, 9a-b, 10b, 11b, 12b-e, 13b-e, 14a-e, and 15a-
e. This material is available free of charge via the Internet at
http://pubs.acs.org.
(3R,4S)-3-Benzyloxy-1-(2-chloroethyl)-4-((E)-(N-(2-prope-
nylimino))methyl)-azetidin-2-one 12a. Colorless oil, 98% yield,
JO0608319
7086 J. Org. Chem., Vol. 71, No. 18, 2006