A new entry to α-alkylidene-β-lactams by 4-exo-dig cyclization of carbamoyl radicals

Article abstract of DOI:10.1016/j.tetlet.2009.03.071

A new synthetic method of α-alkylidene-β-lactam frameworks has been developed. Irradiation of carbamotelluroates 5 having a propargylic group at the nitrogen atom with visible light afforded α-telluromethylene-β-lactams 6 by 4-exo-dig cyclization of carba

Full text of DOI:10.1016/j.tetlet.2009.03.071

Tetrahedron Letters 50 (2009) 3628–3630
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new entry to
a-alkylidene-b-lactams by 4-exo-dig cyclization
of carbamoyl radicals
Shin-ichi Fujiwara a, Yoshihiko Shimizu b, Yuji Imahori b, Masashi Toyofuku b,
Tsutomu Shin-ike a, Nobuaki Kambe b,
*
a Department of Chemistry, Osaka Dental University, Hirakata, Osaka 573-1121, Japan
b Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new synthetic method of
amotelluroates 5 having a propargylic group at the nitrogen atom with visible light afforded
a
-alkylidene-b-lactam frameworks has been developed. Irradiation of carb-
-tellurom-
Received 16 January 2009
Revised 9 March 2009
Accepted 12 March 2009
Available online 16 March 2009
a
ethylene-b-lactams 6 by 4-exo-dig cyclization of carbamoyl radicals, generated by homolytic cleavage of
carbamoyl carbon–tellurium bond of 5, with internal C–C triple bond. Density functional theory (DFT)
calculations on cyclization step indicate that 4-exo-dig cyclization of carbamoyl radicals is kinetically
favored. The existence of a rapid equilibrium between carbamoyl and four-membered vinyl radicals
was suggested by control experiments using the corresponding carbamoselenoate.
Ó 2009 Elsevier Ltd. All rights reserved.
R'
The a-alkylidene-b-lactam framework is a common structural
R'
O
feature found in various synthetic intermediates1 and potent b-
lactamase inhibitors2 such as asparenomycins, Ro 15-1903, and
6-[(Z)-methoxymethylene]penicillanic acids. Classical methods
YPh
5 mol% Pd(PPh3)4
toluene, reflux
N
YPh
ð1Þ
R
N
R
for preparation of
a-alkylidene-b-lactam include (i) addition of
O
chlorosulfonyl isocyanate (CSI) to allenes,3 (ii) reaction of ket-
enes with imines,4 (iii) transition metal-catalyzed carbonylation
of allyl- and propargylamines,5 (iv) cyclization by nucleophilic
1: Y = S, Se
2
We also disclosed that carbamotelluroates 3 added to alkynes
in an intermolecular fashion under irradiation of visible light
SN2 attack of amino or amide nitrogen atom,6 (v)
a
-alkylidena-
tion by elimination,7 and several other reactions.8 As for the rad-
ical cyclization approach to -alkylidene-b-lactams, Pattenden
yielding b-telluroacrylamides
4 regioselectively provably via
photo-cleavage of the carbamoyl carbon–tellurium bond (Eq. 2).12
a
and co-workers reported that 4-exo-trig cyclization of a carbam-
oyl radical, generated by irradiation of carbamoyl cobalt(salo-
phen) complex with UV light, and subsequent b-elimination
O
O
R2N
TePh
R'
hν
60 °C, 3 h
+
R'
2 eq
ð2Þ
R2N
TePh
afforded
a
-methylidene-b-lactam in 25% yield.9 Ryu and co-
3
4
workers revealed that intramolecular attack of acyl radicals,
generated by carbonylation of azaenynes, at imino nitrogens
These results prompted us to examine 4-exo-dig cyclization of
carbamoyl radicals aiming at the construction of -alkylidene-b-
lactam framework according to the reaction pathway as shown
in Scheme 1. The first step is photo-induced generation of carbam-
oyl radicals 7 from carbamotelluroates 5 having a propargyl group
at the nitrogen atom. Subsequent 4-exo-dig cyclization gives vinyl
led to
a-alkylidene-b-lactams carrying a stannyl, silyl, or sulfe-
a
nyl substituent.10
Recently we developed an efficient method for construction
of this framework by palladium-catalyzed intramolecular addi-
tion of thio- and selenocarbamoyl groups to alkyne units (Eq.
1);11 however, this system cannot be applied to carbamotelluro-
ates under the same reaction conditions.
radicals 8. SH2 reaction at Te of 5 with 8 leads to
a-alkylidene-b-
lactams 6 and regenerates 7. If 5-endo-dig cyclization proceeds,
five-membered lactams 9 will be formed via vinyl radicals 10
Scheme 2.
Thus, we irradiated a toluene (2.5 mL) solution of 5a (R = Bu,
R0 = H, Ar = Ph; 0.5 mmol) in a Pyrex flask with a tungsten lamp
at 110 °C for 3 h. Usual workup of the resulting mixture using
* Corresponding author. Tel.: +81 6 68797388; fax: +81 6 6879 7391.
E-mail address: kambe@chem.eng.osaka-u.ac.jp (N. Kambe).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.071

S. Fujiwara et al. / Tetrahedron Letters 50 (2009) 3628–3630
3629
Representative results are summarized in Table 1.15 Electron-
donating substituent, methoxy group, at the para position of the
phenyl ring increased the yield of the product (run 3 vs runs 1, 2,
and 4); probably because of lower homolytic bond dissociation en-
ergy between carbamoyl carbon and tellurium. As for the substit-
uents on nitrogen atom, 5d,e having methyl and benzyl groups
also afforded the expected products (runs 4 and 5). It is noteworthy
that when phenyl group was introduced at the end of C–C triple
bond, desired b-lactam 6f was obtained in 83% yield due to stabil-
ization of a vinyl radical intermediate 8f (run 6), whereas, 5g hav-
ing ethyl group underwent 5-endo-dig cyclization, although as a
minor process, to afford five-membered lactam 9g in 12% yield
along with 31% of 6g (run 7).16–18
In order to gain insight into the effect of substituents at the
acetylenic terminus in cyclization step, we performed computa-
tional studies. Aa–c, Ba–c, and Ca–c are model compounds for
the reactions of 5a, 5f, and 5g, respectively, and TS-4a–c and TS-
5a–c are transition-state structures of 4-exo-dig and 5-endo-dig
cyclizations. All geometries were optimized at the UB3LYP/6-
311G(d,p) level using GAUSSIAN 03 program.19 Structures of Aa, Ba,
Ca, TS-4a, and TS-5a were also optimized at the UB3PW91/cc-pVTZ
level. These computations show that 4-exo-dig processes are fa-
vored kinetically than 5-endo-dig processes. For example, activa-
tion barrier of TS-4a is ca. 1.4 kcal/mol lower than that of TS-5a.
When Ph group was introduced at the acetylenic terminus, DDEa
increased to 4.5 kcal/mol, and on the contrary Ac (R0 = Me) reduced
DDEa to 0.8 kcal/mol. These results do not conflict with the exper-
imental results.
Under similar conditions, carbamoselenoate 1a did not undergo
photo-induced cyclization. So we attempted to generate carbamoyl
radical 7h by abstraction of PhSe moiety from 1a using silyl radical
that should lead to cyclization in the presence of hydrogen donor
and would provide information on the existence of equilibrium be-
tween carbamoyl radical 7 and vinyl radical 8. Thus, when a solu-
tion of 1a in toluene (0.047 M) was heated at reflux using
0.25 equiv of VAZOÒ (1,10-azobis(cyclohexanecarbonitrile)) as a
radical initiator and 2 equiv of (Me3Si)3SiH as a hydrogen donor,
b-lactam 11 was formed in 36% yield together with 20% of formam-
ide 12 (Scheme 3). These products 11 and 12 are considered to be
formed by hydrogenation of vinyl radical 8h and carbamoyl radical
7h, respectively. To suppress the formation of 12, the reaction was
carried out under more diluted conditions (0.010 M); however,
product ratio was not largely improved. These results suggest that
R'
TeAr
O
TeAr
R'
hν
N
TeAr
+
N
R
R'
N
R
R
O
5
O
6
9
SH2
SH2
5
5
R'
R'
4-exo-dig
N
R'
N
R
N
R
R
O
O
10
O
8
5-endo-dig
7
Scheme 1. Intramolecular cyclization of carbamoyl radicals.
R'
R'
N
N
g
i
H
d
o
H
O
x
n
R'
O
e
Ba
: 2.0 (-2.1)
TS-4a
TS-4b
: 12.8 (10.8)
: 9.2
Bb
Bc: 0.5
: -4.9
N
TS-4c: 12.7
e
H
d
o
d
O
i
g
0.0 kcal/mol
R'
H
N
N
H
Aa
: R' = H
Ab: R' = Ph
Ac
R'
: R' = Me
O
O
Ca
Cb
Cc: -13.4
TS-5a: 14.2 (13.1)
TS-5b: 13.7
TS-5c
: -14.9 (-20.1)
: -12.7
: 13.5
Scheme 2. Relative energies of radical intermediates calculated at UB3LYP/6-
311G(d,p) and those at UB3PW91/cc-pVTZ in parentheses.
preparative TLC gave the corresponding
a-methylidene-b-lactam
6a in 40% yield (E/Z = 61/39) in pure form (Table 1, run 1). Unlike
the case of intermolecular addition of carbamoyl radicals to
alkynes, cyclization needs heating at 100 °C or above to proceed.13,14
Table 1
4-exo-dig Cyclization of carbomoyl radicalsa
Ar
TeAr
O
Ar
R'
Ar
(Me3Si)3SiH (2 eq)
VAZO (0.25 eq)
R'
+
hν
N
H
N
SePh
N
N
TeAr
toluene, reflux, 9 h
R
N
O
toluene, reflux
3 h
O
12
O
1a
R
O
Ar = 4-ClC6H4
11
5
6
0.047 M
0.010 M
36% (E/Z = 51/49)
32% (E/Z = 46/54)
20%
11%
Run
5
R
R0
Ar
Isolated yield (E/Z ratiob)
1
2
5a
5b
5c
5d
5e
5f
Bu
Bu
Bu
Me
PhCH2
Bu
H
H
H
H
H
Ph
Et
Ph
6a, 40% (61/39)
6b, 39% (71/29)
6c, 58% (70/30)
6d, 42% (60/40)
6e, 50% (60/40)
6f, 83% (57/43)d
6g, 31% (83/17)
(Me3Si)3Si
p-MeC6H4
p-MeOC6H4
p-BrC6H4
p-MeOC6H4
p-MeOC6H4
Ph
3
(Me3Si)3SiSePPh
(Me3Si)3SiH
4c
5
6
7e
5g
Bu
Ar
Ar
a
4-exo-dig
Conditions:
5 (0.5 mmol), toluene (2.5 mL), reflux, 3 h, irradiation with a
tungsten lamp.
N
O
N
b
c
E/Z Ratio was determined by 1H NMR.
6 h.
O
7h
8h
d
e
Major/minor ratio.
Five-membered lactam 9g (12%) was formed.
Scheme 3. Reaction of carbamoselenoate 1a.

3630
S. Fujiwara et al. / Tetrahedron Letters 50 (2009) 3628–3630
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there might be a rapid equilibrium between vinyl radical 8h and
carbamoyl radical 7h in this reaction system.20
In contrast to 5- and 6-exo cyclization processes that have been
employed widely in organic synthesis,21 4-exo cyclization process
has been rarely involved in synthetic reactions due to unfavorable
torsional strain and rapid reverse processes; opening of four-mem-
bered rings.22 Since Araki reported the first example of 4-exo-trig
radical cyclization,23 this cyclization has been studied by several
groups.24 However only two systems are known for 4-exo-dig
cyclization so far.25 Here we revealed 4-exo-dig cyclization of
carbamoyl radicals onto alkynes proceeds efficiently to afford
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a
-alkylidene-b-lactams. Reduced ring strain by the incorporation
of two sp2 carbon atoms in the ring may facilitate this unique
4-exo-dig cyclization. Equilibrium between carbamoyl radicals
and vinyl radicals is also suggested.
11. Toyofuku, M.; Fujiwara, S.; Shin-ike, T.; Kuniyasu, H.; Kambe, N. J. Am. Chem.
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Acknowledgments
12. Fujiwara, S.; Shimizu, Y.; Shin-ike, T.; Kambe, N. Org. Lett. 2001, 3, 2085.
13. Cyclization did not proceed when the reaction was carried out at 80 °C.
14. Walton and co-workers also examined 4-exo-dig cyclization of carbamoyl
radical generation from N-benzyl-N-alkynyl-(1-methyl)cyclohexa-2,5-diene-1-
carboxamide, at 80 °C and mentioned that the only product isolated was
This research was partly supported by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan. M.T. expresses his special thanks to
JSPS for financial support in the form of a JSPS Research Fellowship
for Young Scientists.
formamide. However, the GC–MS chromatogram showed a small amount of
a-
alkylidene-b-lactam, although this was not confirmed by isolation and NMR
spectral characterisation, see: Bella, A. F.; Jackson, L. V.; Walton, J. C. Org.
Biomol. Chem. 2004, 2, 421.
15. Carbamotelluroates
5 were completely comsumed in all runs. Complex
Supplementary data
mixtures were obtained and five-membered lactams were not detected
except run 7.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.03.071.
16. For 5-endo-dig cyclization of carbon-centered radicals: (a) Johnson, J. P.;
Bringley, D. A.; Wilson, E. E.; Lewis, K. D.; Beck, L. W.; Matzger, A. J. J. Am. Chem.
Soc. 2003, 125, 14708; (b) Lewis, K. D.; Rowe, M. P.; Matzger, A. J. Tetrahedron
2004, 60, 7191; (c) Scott, J. L.; Parkin, S. R.; Anthony, J. E. Synthesis 2004, 161.
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Alabugin, I. V.; Manoharan, M. J. Am. Chem. Soc. 2005, 127, 9535; (b)
Alabugin, I. V.; Manoharan, M. J. Am. Chem. Soc. 2008, 130, 10984.
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