One-pot conversion of carbamates of unsaturated β-aminoesters into unsaturated β-lactams by use of trimethylsilyl iodide

Article abstract of DOI:10.1080/10426507.2016.1206105

Trimethylsilyl iodide (TMSI) is introduced as an efficient reagent for the one-pot and direct transformation of carbamates of unsaturated β-aminoesters into the corresponding α-methylene-β-lactams and α-arylidene-β-lactams. The mild reaction conditions, e

Full text of DOI:10.1080/10426507.2016.1206105

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
One-pot conversion of carbamates of unsaturated
β-aminoesters into unsaturated β-lactams by use
of trimethylsilyl iodide
Abed Badrian, Manouchehr Mamaghani & Sharif Kaamyabi
To cite this article: Abed Badrian, Manouchehr Mamaghani & Sharif Kaamyabi (2016): One-
pot conversion of carbamates of unsaturated β-aminoesters into unsaturated β-lactams by
use of trimethylsilyl iodide, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI:
10.1080/10426507.2016.1206105
To link to this article: http://dx.doi.org/10.1080/10426507.2016.1206105
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ONE-POT CONVERSION OF CARBAMATES OF UNSATURATED β-AMINOESTERS
INTO UNSATURATED -LACTAMS BY USE OF TRIMETHYLSILYL IODIDE
Abed Badrian1,*, Manouchehr Mamaghani2 and Sharif Kaamyabi3
1Department of Basic Science, Research Institution for Educational Studies (OERP), PO Box
15875-4874, Tehran, Iran.
2Department of Chemistry, Faculty of Science, The University of Guilan, PO Box 41335-1914,
Rasht, Iran.
3Mofateh Campus, the University of Farhangian, Tehran, Iran.
*Corresponding Author E-mail address :ab.badrian@gmail.com
Abstract
Trimethylsilyl iodide (TMSI) is introduced as an efficient reagent for the one-pot and direct
transformation of carbamates of unsaturated β-aminoesters into the corresponding α-methylene-
β-lactams and α-arylidene-β-lactams. The mild reaction conditions, excellent yields and easy
work-up procedures make it a useful alternative to previously applied procedures for the rapid
synthesis of β-lactams from easily available Baylis-Hillman adducts.
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Key words
Baylis-Hillman adducts, carbamates, unsaturated β-aminoesters, α-methylene-β-lactams, α-
arylidene-β-lactams, trimethysilyl iodide
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INTRODUCTION
2-Azetidinones, commonly known as β-lactams, are well-known heterocyclic
compounds.1 The activity of famous antibiotics such as penicillins, cephalosporins and
carbapenems are attributed to the presence of the 2-azetidinone ring in these compounds.2 Apart
from clinical use, β-lactams can also serve as good synthons in the synthesis of many
biologically active heterocycles. Several approaches have been developed for the synthesis of β-
lactams including cyclization of β-aminoesters to α-methylene-β-lactams3-6, Grignard reaction on
azetidin-2,3-diones followed by dehydration using PPh3/CCl4 reagent7, rhodium catalyzed olefin
cross metathesis8, thermal β-elimination of trans-3-allyl-3-sulfinyl-β-lactams9, copper-catalyzed
carbonylative coupling of cycloalkanes and amides10, synthesis of optically active α-methylene
β-lactams through lipase-catalyzed kinetic resolution11, synthesis of optically active spiro-β-
lactams by cycloadditions to α-alkylidene-β-lactams12 and other miscellaneous approaches such
as ring contraction and radical processes1. Recently Li and co-workers described the palladium-
catalyzed direct oxidative carbonylation of N-allylamines palladium13 and PPh3-catalyzed
umpolung cyclization of 2-propiolamidoacetates or α-propiolamido ketones14. Some of the
reported methods exhibit disadvantages such as: 1) multi-step synthesis with the use of
expensive or harmful reagents and 2) low yields. Thus, the development of simple, efficient and
general methods for the synthesis of biologically active β-lactams, in one-step would be highly
valuable and desirable. In continuation of our interest for the Baylis-Hillman adducts and their
transformation into a variety of natural and unnatural compounds,15-19 herein we wish to report
our results on the direct transformation of carbamates of unsaturated β-aminoesters into the
corresponding α-methylene-β-lactams and α-arylidene-β-lactams by use of trimethysilyl iodide.
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RESULTS AND DISCUSSION
We recently reported a mild and efficient one-pot easy conversion of Baylis-Hillman
adducts into carbamates of unsaturated β-amino esters by use of the reaction with the Burgess
reagent.20 Treatment of Baylis-Hillman adducts 1 with the Burgess reagent in dry THF at 20°C
gave sulfamate esters 2. When heated to 95°C, these sulfamate esters undergo pyrolysis with
elimination of SO3 providing the corresponding carbamates 3a-g in excellent yields. This
reaction takes place via an SN pathway and the alcohol moiety is displaced to form the urethane
(Scheme 1).
When the sulfamate esters 2 were treated with sodium hydride, the corresponding sodium
salt was obtained. However, when the sodium salt decomposed as a solid at 80°C and the
reaction mixture was treated with water, a substitution with an allylic rearrangment (SN')
occurred to give the carbamates 4a-g.
The ability to access β-aminoesters exclusively was exploited and its cyclization to α-
methylene-β-lactams and α-arylidene-β-lactams, which are valuable precursors for the synthesis
of substituted β-lactams, was investigated. A wide variety of methods are available to cyclize β-
aminoesters into the corresponding β-lactams. Conventional bases such as isopropyl magnesium
bromide,21 n-butyllithium,22 potassium tert-butoxide,23 lithium hydroxide, potassium hydroxide24
and bis[N,N-bis(trimethylsilyl)amino]tin(II),25 were reported earlier to have been used in such
cyclization reactios. Our interest in β-lactams stems from our work with trimethysilyl iodide
(TMSI). In particular, we have found that TMSI which has been previously used as a versatile
reagent for the mild dealkylation of carboxylic esters26, alkyl ethers27 and carbamates28 serves as
a new and useful source for cyclization of β-aminoesters into the β-lactams.
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To explore the scope and versatility of this method and optimize the reaction conditions,
the reaction of carbamate 3a (1 mol%) and TMSI (3 mol%, generated in situ from TMSCl and
NaI)29 was used as a model system based on reported procedures.26 When adduct 3a is mixed
with TMSI in an aprotic solvent, e.g. tetrachloromethane, and the solution is warmed to 50°C,
the reaction proceeds smoothly and cleanly under mild conditions and after work-up the reaction,
the product 5a produced in 5 h and in 65% yield. We also verified the amounts of TMSI in
preparation of 5a and the best result was obtained using 5 mol% TMSI at 50°C in
tetrachloromethane and the product 5a was produced in 3 h and in 92% yield.
Using the optimized conditions for the reaction of the adducts 3a-g and 4a-g with TMSI,
several α-methylene-β-lactams 5a-g and α-arylidene-β-lactams 6a-g were synthesized (Scheme
2). The results are summarized in Table 1. The structure of all products was established by
spectroscopic methods (1H NMR, IR) and elemental analyses. The reactions are clean and the
products are obtained in high yields except for the reaction of derivatives with Ar = 4-MeOPh
and Ar = 3-NO2Ph (entries c and g, Table 1) because the ether and nitro substituents react with
TMSI.
A proposed mechanism of this transformation is depicted in Scheme 3.26-28 When the
adduct 3c as a simple model substrate is treated with 3 mol% TMSI in CCl4 at 50°C, the
corresponding silylated intermediate 7 and methyl iodide was produced in quantitative yields.
When intermediate 7 is treated with excess TMSI for extended periods of time, the initially
formed trimethylsilyl carboxylate moiety is slowly and efficiently converted into the acyl iodide
8 in high yield. To form the β-lactam cycle, the acyl moiety of intermediate 8 is attacked by
exocyclic NH-group followed by the release of HI and produces the intermediate 9. This
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intermediate is converted directly into the product 5c by addition of methanol and heating the
reaction mixture. Methanol can cleave the O-Si bond of the trimethylsilyl carbamate to afford
methyl trimethylsilyl ether (MeOTMS) and the carbamic acid 10 which spontaneously
decarboxylates to produce the α-methylene-β-lactam 5c.
EXPERIMENTAL
Chemicals were purchased from Merck and Fluka. Melting points were measured on an
Electrothermal 9100 apparatus. Elemental analyses were performed by use of a Heraeus CHNO-
Rapid analyzer. IR spectra (νmax/cm1) were determined on a Shimadzo FT-IR-8900
1
spectrometer. H NMR spectra were recorded 500 MHz on a Bruker DRX-500 in CDCl3 as
solvent and with TMS as an internal standard. Preparative thin layer chromatography (TLC) was
performed with Merck Kieselgel 60 H, F254, Art No 7730. GC was carried out by using a Buck
Scientific 910 (capillary column, MXT-5, 15 m). All solvents used were dried and distilled
according to standard procedures. All reactions and manipulations were carried out under a
nitrogen atmosphere and/or by using standard vacuum line techniques. The Supplemental
Materials file contains sample 1H and 13C NMR spectra of products 6 (Figures S 1 -- S 14)
General procedure for the synthesis of α-methylene-β-lactams and α-arylidene-β-
lactams by reaction of carbamates of unsaturated β-aminoesters with TMSI.27
To a stirred solution of TMSCl (6 mmol) and NaI (6 mmol) in CCl4 (10 mL) was added
carbamate 3 or 4 (1 mmol in 5 mL of CCl4) dropwise within 5 min, and the resulting mixture was
vigorously stirred for 3-12 h at 50°C (reaction monitored by TLC or GC). Work-up involved
initial evaporation of the volatile component for removal of solvent and slight excess of TMSI,
addition of degassed MeOH for the solvolysis of the silyl carbamates, evaporation of volatile
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materials for removal of MeOH and decarboxylation. The mixture was then extracted with
EtOAc (3×10 mL). The combined extract was washed with brine (2× 3 mL) and dried over
Na2SO4 and then concentrated in vacuo. The crude product was purified by flash
chromatography (EtOAc/hexane 1:1 as eluent) to afford the adducts 5a-g and 6a-g as solids.
Some data of selected compounds are listed below.
3-Methylene-4-p-tolylazetidin-2-one (5b)
Pale yellow solid; M. p. 160-161°C; yield 87%; IR (neat): 3383, 3310 (NH), 1615 (CONH);
Anal. Calcd. for C11H11NO (Found: C, 76.26; H, 6.33; N, 7.97 requires C, 76.30; H, 6.36; N,
8.09%); 1H NMR: δ = 2.32 (s, 3H), 5.59 (s, 1H), 5.92 (s, 1H), 6.16 (s, 1H), 6.37 (d, J = 1.51 Hz,
br. NH), 7.21 (d, J = 7.43 Hz, 2H), 7.67 (d, J = 7.43 Hz, 2H) ppm.
4-(4-Hydroxyphenyl)-3-methyleneazetidin-2-one (5c)
Pale yellow solid; M. p. 162-163°C; yield 56%; IR (neat): 3441 (OH), 3402, 3324 (NH), 1613
(CONH); Anal. Calcd. for C10H9NO2 (Found: C, 68.48; H, 5.03; N, 7.91 requires C, 68.57; H,
5.14; N, 8.00%); 1H NMR: δ = 4.36 (s, br, OH), 5.14 (s, 1H), 5.78 (s, 1H), 5.92 (d, J = 1.50 Hz,
br, NH), 6.34 (d, J = 1.50 Hz, 1H), 6.94 (d, J = 6.93 Hz, 2H), 7.77 (d, J = 6.93 Hz, 2H) ppm.
3-Methylene-4-(pyridin-4-yl)azetidin-2-one (5d)
Yellow solid; M. p. = 165-167°C; yield 78%; IR (neat): 3405, 3325 (NH), 1609 (CONH); Anal.
Calcd. for C9H8N2O (Found: C, 67.41; H, 4.93; N, 17.41 requires C, 67.50; H, 5.00; N, 17.50%);
1H NMR: δ = 5.17 (s, 1H), 5.59 (s, 1H), 5.98 (d, J = 1.51 Hz, 1H), 6.45 (d, J = 1.51 Hz, br, NH),
7.36 (d, 2H, J = 7.87 Hz), 8.57 (d, 2H, J = 7.87 Hz) ppm.
4-[4-(Dimethylamino)phenyl]-3-methyleneazetidin-2-one (5e)
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Yellow solid; M. p. = 177-179°C; yield 78%; IR (neat): 3389, 3320 (NH), 1622 (CONH); Anal.
Calcd. for C12H14N2O (Found: C, 71.13; H, 6.85; N, 13.77 requires C, 71.28; H, 6.73; N,
1
13.86%); H NMR: δ = 3.12 (s, 6H) 5.21 (s, 1H), 5.51 (s, 1H), 5.93 (d, J = 1.51 Hz, br, NH),
6.26 (d, J = 1.51 Hz, 1H), 6.74 (d, J = 7.75 Hz, 2H), 7.77 (d, J = 7.75 Hz, 2H) ppm.
4-(4-Chlorophenyl)-3-methyleneazetidin-2-one (5f)
White solid; M. p. = 160-161°C; yield 87%; IR (neat): 3410, 3329 (NH), 1635 (CONH); Anal.
Calcd. for C10H8ClNO (Found: C, 61.91; H, 4.05; N, 7.17 requires C, 62.03; H, 4.13; N, 7.23%);
1H NMR: δ = 5.27 (s, 1H), 5.62 (s, 1H), 6.12 (d, J = 1.50 Hz, br, NH), 6.33 (d, J = 1.50 Hz, 1H),
7.54 (d, J = 7.55 Hz, 2H), 7.85 (d, J = 7.55 Hz, 2H) ppm.
3-Methylene-4-(3-nitrophenyl)azetidin-2-one (5g)
Yellow solid; M. p. = 192-193°C; yield 42%; IR (neat): 3395, 3320 (NH), 1635 (CONH); Anal.
Calcd. for C10H8N2O3 (Found: C, 58.73; H, 3.85; N, 13.70 requires C, 58.82; H, 3.92; N,
13.72%); 1H NMR: δ = 5.37 (s, 1H), 5.73 (s, 1H), 6.25 (d, J = 1.53 Hz, br, NH), 6.73 (d, J = 1.53
Hz, 1H), 7.80 (t, J = 7.66 Hz, 1H), 8.27 (d, J = 7.68 Hz, 1H), 8.51 (d, J = 7.78, 1H), 8.74 (s, 1H)
ppm.
3-Benzylideneazetidin-2-one (6a)
White solid; M. p. = 113-115°C; yield 94%; IR (neat): 3404, 3323 (NH), 1613 (CONH); Anal.
Calcd. for C10H9NO (Found: C, 75.40; H, 5.58; N, 8.71 requires C, 75.47; H, 5.66; N, 8.80%);
1H NMR: δ = 4.19 (d, J = 1.52 Hz, 2H), 6.24 (t, J = 1.52, br, NH), 7.27-7.43 (m, 5H), 7.49 (t, J =
1.71 Hz, 1H)) ppm.
3-(4-Methylbenzylidene)azetidin-2-one (6b)
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White solid; M. p. = 121-123°C; yield 88%; IR (neat): 3332, 3173 (NH), 1604 (CONH); Anal.
Calcd. for C11H11NO (Found: C, 76.20; H, 6.28; N, 8.00 requires C, 76.30; H, 6.36; N, 8.09%);
1H NMR: δ = 2.61 (s, 3H), 4.29 (d, J = 1.51 Hz, 2H), 6.25 (t, J = 1.51, br, NH), 7.21 (d, J = 7.75
Hz, 2H), 7.67 (d, J = 7.75 Hz, 2H), 7.86 (t, J = 1.73 Hz, 1H) ppm.
3-(4-Hydroxybenzylidene)azetidin-2-one (6c)
White solid; M. p. = 143-145°C; yield 55%; IR (neat): 3434, (OH), 3308 (NH), 1657 (CONH);
Anal. Calcd. for C10H9NO2 (Found: C, 68.49; H, 5.02; N, 7.91 requires C, 68.57; H, 5.14; N,
8.00%); 1H NMR: δ = 4.50 (d, J = 1.53 Hz, 2H), 5.08 (s, br, OH), 6.51 (t, J = 1.53 Hz, br, NH),
6.94 (d, J = 7.91 Hz, 2H), 7.77 d, J = 7.91 Hz, 2H), 7.96 (t, J = 1.72 Hz, 1H) ppm.
3-[(Pyridin-4-yl)methylene]azetidin-2-one (6d)
Pale yellow solid; M. p. = 132-134°C; yield 78%; IR (neat): 3424, 3314 (NH), 1643 (CONH);
Anal. Calcd. for C9H8N2O (Found: C, 67.42; H, 4.93; N, 17.41 requires C, 67.50; H, 5.00; N,
17.50%); 1H NMR: δ = 4.29 (d, J = 1.51 Hz, 2H), 6.25 (t, J = 1.51 Hz, br, NH), 7.36 (d, J = 8.38
Hz, 2H), 8.58 (d, J = 8.38 Hz, 2H), 8.63 (t, J = 1.70 Hz, 1H) ppm.
3-[4-(Dimethylamino)benzylidene]azetidin-2-one (6e)
Pale yellow solid; M. p. = 137-139°C; yield 78%; IR (neat): 3379, 3130 (NH), 1611 (CONH);
Anal. Calcd. for C12H14N2O (Found: C, 71.22; H, 6.83; N, 13.81 requires C, 71.28; H, 6.93; N,
1
13.86%); H NMR: δ = 3.11 (s, 6H), 4.24 (d, J = 1.50 Hz, 2H), 6.14 (t, J = 1.50 Hz, br, NH),
6.76 (d, J = 7.59 Hz, 2H), 7.78 d, J = 7.59 Hz, 2H), 7.95 (t, J = 1.71 Hz, 1H) ppm.
3-(4-Chlorobenzylidene)azetidin-2-one (6f): yellow solid; M. p. = 146-148°C; yield 95%;
IR (neat): 3337, 3148 (NH), 1606 (CONH); Anal. Calcd. for C10H8ClNO (Found: C, 61.92; H,
1
4.04; N, 7.11 requires C, 62.04; H, 4.13; N, 7.23%); H NMR: δ = 4.29 (d, J = 1.52 Hz, 2H),
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6.28 (t, J = 1.50 Hz, br, NH), 7.54 (d, J = 7.95 Hz, 2H), 7.85 d, J = 7.95 Hz, 2H), 7.98 (t, J =
1.75 Hz, 1H) ppm.
3-(3-Nitrobenzylidene)azetidin-2-one (6g): yellow solid; M. p. = 168-170°C; yield 42%;
IR: 3338, 3158 (NH), 1699 (CONH), 1309 (NO2); Anal. Calcd. for C10H8N2O3 (Found: C, 58.72;
1
H, 3.84; N, 13.63 requires C, 58.82; H, 3.92; N, 13.72%); H NMR: δ = 4.41 (d, J = 1.51 Hz,
2H), 6.81 (t, J = 1.50 Hz, br, NH), 7.77 (s, 1H, 7.81 (t, J = 7.66 Hz, 1H), 8.27 (d, J = 7.68 Hz,
1H), 8.51 (d, J = 7.64, 1H), 8.74 (s, 1H) ppm.
CONCLUSION
In summary, for the first time we showed that TMSI was an effective reagent for the
direct transformation of carbamates of unsaturated β-aminoesters into the corresponding α-
methylene-β-lactams and α-arylidene-β-lactams. The mild reaction conditions, excellent yields
and easy work-up procedures make it a useful alternative to previously applied procedures for
the rapid synthesis of β-lactams from easily available Baylis-Hillman adducts.
ACKNOWLEDGMENT
The authors are grateful to the Research Council of the University of Guilan for financial support
of this research work.
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Table 1. Conversation of carbamates of unsaturated β-aminoesters into the corresponding α-
methylene-β-lactams and α-arylidene-β-lactams.
β-lactam 5a
β-lactam 6a
Entry
Ar
Time (h)
Yieldb (%)
Time (h)
Yieldb (%)
a
b
c
d
e
f
C6H5
3
7
92c
87
56
78
78
94
42
3
7
94
88
55
78
78
95
42
4-Me-C6H4
4-MeO-C6H4
4-Pyridyl
10
7
12
7
4-(Me)2N-C6H4
4-Cl-C6H4
3-NO2-C6H4
6
6
3
3
g
12
12
1
aAll compounds have been fully characterized spectroscopy by H NMR, IR, and elemental
analyses.
bYields are based on isolated products.
cAnalysed by comparison of its spectroscopic data (1H NMR, IR) with those of an authentic
sample (Ref. 30).
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Scheme 1. Conversation of Baylis-Hillman adducts into the corresponding carbamates of
unsaturated β-aminoesters.
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Scheme 2. Synthesis of α-methylene-β-lactams and α-arylidene-β-lactams by reaction of
carbamates of unsaturated β-amino ester with TMSI.
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Scheme 3. A proposed mechanism for the formation of β-lactams.
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