Enantioselective nucleophilic catalysis: The synthesis of aza-β-lactams through [2+2] cycloadditions of ketenes with azo compounds

Article abstract of DOI:10.1002/anie.200802439

(Chemical Equation Presented) Get ready for take off: A planar-chiral derivative of 4-pyrrolidinopyridine (1) mediates the title transformation (see scheme) in a convergent manner with good enantioselectivity, thus giving the first catalytic asymmetric synthesis of aza-β-lactams.

Full text of DOI:10.1002/anie.200802439

Communications
DOI: 10.1002/anie.200802439
Asymmetric Catalysis
Enantioselective Nucleophilic Catalysis: The Synthesis of Aza-b-
Lactams through [2 + 2] Cycloadditions of Ketenes with Azo
Compounds**
Jacob M. Berlin and Gregory C. Fu*
Even though aza-b-lactams have attracted interest because of
their biological activity[1] and their utility as intermediates in
organic chemistry (e.g., for the generation of a-amino acids
and hydantoins),[2–4] only limited progress has been reported
with regard to the enantioselective synthesis of this family of
heterocycles.[5] One attractive, convergent approach to the
formation of aza-b-lactams is the [2 + 2] cycloaddition of a
ketene with an azo compound [Eq. (1)].[6] To the best of our
knowledge, no stereoselective variants of this process have
yet been reported.
catalyst for the desired coupling, and generates the aza-b-
lactam in good yield and enantioselectivity (Table 1, entry 1; in
the absence of a catalyst there is no reaction: Table 1, entry 2).
Table 1: Effect of changing the “standard” reaction conditions (outlined
in the equation below) in the nucleophile-catalyzed enantioselective
synthesis of aza-b-lactams.
We have been exploring the use of chiral derivatives of
PPY (4-pyrrolidinopyridine; e.g., 1 and 2) as enantioselective
catalysts for an array of transformations,[7] including couplings
of ketenes with imines[8] or with aldehydes.[9,10] Although
there are no reports of nucleophilic catalysis for [2 +
2] cycloadditions of ketenes with azo compounds, we were
intrigued by the possibility that our planar-chiral pyridines
might be effective in this role. Herein,
Entry
Change from the “standard”
reaction conditions
ee [%]
Yield [%]
1
2
3
4
5
6
7
8
none
no (À)-1
86
–
89
<5
65
65
<5
85
81
20
<5
65
68
(À)-2, instead of (À)-1
(+)-3, instead of (À)-1
quinine, instead of (À)-1
R=CO2Et
À15[a]
<5
–
we establish that PPY derivative 1
effects the first catalytic asymmetric
synthesis of aza-b-lactams, through
[2 + 2] cycloadditions of ketenes with
azo compounds [Eq. (2)].
Initially, we examined the cyclo-
addition of phenyl ethyl ketene with
dimethyl azodicarboxylate (1.0 equiv).
We found that the planar-chiral PPY
derivative 1 serves as an effective
80
32
20
–
87
85
73
R=CO2iPr
R=CO2CH2CCl3
R=CO(piperidinyl)
ClCH2CH2Cl, instead of CH2Cl2
À308C
9
10
11
12
À108C
68
[a] A negative ee value signifies that the opposite enantiomer of the
product is formed preferentially.
[*] Dr. J. M. Berlin, Prof. Dr. G. C. Fu
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
Fax: (+1)617-324-3611
E-mail: gcf@mit.edu
Under the same reaction conditions, a related catalyst (2), as
well as a variety of chiral phosphines and cinchona alkaloids,
provide poor enantioselectivity or little of the cycloaddition
product (Table 1, entries 3–5).[11,12] The substituents of the azo
compound have a significant impact on the ee value and the
yield, with the methoxycarbonyl group affording the best
results (Table 1, compare entry 1 with entries 6–9). If
[**] We thank Dr. Xing Dai, Dr. Maximilian Dochnahl, and Takashi Nakai
for helpful discussions and for experimental assistance. Support
was provided by the NIH (NationalInstitute of GeneralMedical
Sciences: grant no. R01-GM57034), Merck Research Laboratories,
and Novartis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802439.
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Chemie
ClCH2CH2Cl rather than CH2Cl2 is employed as the solvent,
then the formation of the aza-b-lactam is less efficient (Table 1,
compare entry 1 with entry 10). The reaction temperature of
choice appears to be À208C (Table 1, compare entry 1 with
entries 11 and 12).[13]
The optimized reaction conditions can be applied to the
enantioselective synthesis of aza-b-lactams when starting
from a variety of ketenes (Table 2). If the alkyl group is
Table 2: Nucleophile-catalyzed enantioselective synthesis of aza-b-lac-
tams (see [Eq. (2)] for the reaction conditions).[a]
Entry
Ar
Alkyl
ee [%]
Yield [%][b]
Figure 1. Possible mechanism for the nucleophile-catalyzed synthesis
of aza-b-lactams.
1
2
3
4
5
6
7
8
Ph
Ph
m-tolyl
o-tolyl
o-anisylEt
Ph
Ph
Ph
Ph
Ph
Me
Et
Et
85
53
89
79
46
86 (>99)[c]
85
67
89
81
83
86
94
95
96
92
96
Et
In conclusion, we have developed a new process, the
nucleophile-catalyzed [2 + 2] cycloaddition of ketenes with
azo compounds, to generate aza-b-lactams. In addition, we
have established that planar-chiral PPY derivative 1 effects
this convergent transformation to give good enantioselectiv-
ity, thereby providing the first catalytic asymmetric synthesis
of this useful family of heterocycles.
93
Bn
iBu
cyclopentyl
cyclohexyl
iPr
iPr
iPr
iPr
73
87
84
90
91
91
90
90
9
10
11
12
13
p-anisyl
p-ClC6H4
3-thiophenyl
[a] All data are the average of two experiments. [b] Yield of isolated
product. [c] The ee value was determined after a single recrystallization
from isopropanol (overall yield: 71%).
Experimental Section
General procedure: Solutions of the ketene (0.68 mmol) and
dimethyl azodicarboxylate (100 mg, 0.68 mmol) in CH2Cl2 (49 mL),
and of the catalyst (À)-1 (13 mg, 0.035 mmol) in CH2Cl2 (0.8 mL)
were prepared in a glove box. Following removal from the glove box,
the solutions were cooled at À208C for 10 min, before the catalyst
solution was added to the solution of ketene/dimethyl azodicarbox-
ylate by syringe. After the reaction mixture was stirred for 2 h at
À208C, the solvent was removed in vacuo and the residue was
purified by column chromatography.
small (i.e., Me or a primary substituent), then the desired
heterocycle is generally produced with good (but not
excellent) enantioselectivity ( ꢀ 85% ee; Table 2, entries 1–
7). Fortunately, the ee values of the aza-b-lactam products is
readily enhanced by recrystallization (e.g., the product
generated from phenyl ethyl ketene can be obtained in >
99% ee after a single recrystallization; see Table 2, entry 2).
In the case of ketenes that bear a secondary alkyl group,
catalyst 1 typically furnishes the aza-b-lactam with very good
enantioselectivity and yield (> 90% ee; Table 2, entries 8–
13).[14]
A plausible mechanism for this new nucleophile-catalyzed
method for the synthesis of aza-b-lactams is illustrated in
Figure 1. Interestingly, the configuration at the quaternary
stereocenter is different from that produced in Staudinger
reactions that are catalyzed by 1 [Eq. (3); Ts = 4-toluenesul-
fonyl],[8b] and which are believed to proceed through a similar
pathway.[15]
Received: May 25, 2008
Published online: July 30, 2008
Keywords: asymmetric catalysis · azo compounds ·
.
heterocycles · homogeneous catalysis · lactams
[1] For an example, see: H. Morioka, M. Takezawa, H. Shibai, T.
Okawara, M. Furukawa, Agric. Biol. Chem. 1986, 50, 1757 –
1764.
[2] For leading references on the synthesis and utility of enantioen-
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Communications
[7] For references to early studies, see: a) G. C. Fu, Acc. Chem. Res.
[13] Notes: a) dimerization of the ketene is sometimes observed as an
undesired side reaction; b) the use of non-chlorinated solvents
can lead to significant changes in enantioselectivity.
2004, 37, 542 – 547; for recent studies, see: b) E. C. Lee, K. M.
McCauley, G. C. Fu, Angew. Chem. 2007, 119, 995 – 997; Angew.
Chem. Int. Ed. 2007, 46, 977 – 979; c) X. Dai, T. Nakai, J. A. C.
Romero, G. C. Fu, Angew. Chem. 2007, 119, 4445 – 4447; Angew.
Chem. Int. Ed. 2007, 46, 4367 – 4369.
[14] Notes: a) use of diethyl, rather than dimethyl, azodicarboxylate
for reactions of phenyl ethyl ketene and p-chlorophenyl
isopropyl ketene led to 85% yield with 80% ee (see Table 2,
entry 2) and 96% yield with 86% ee (see Table 2, entry 12),
respectively; b) this method is not highly air- or moisture-
sensitive: for a cycloaddition of phenyl ethyl ketene which was
carried out in air and with unpurified CH2Cl2, a fairly good yield
and ee value were observed (77% yield, 83% ee); c) a reaction
conducted on 1 g of phenyl ethyl ketene proceeded in 77% yield
with 84% ee; d) for the conversion of one of these aza-b-lactams
into a hydantoin and an a,a-disubstituted amino acid, see the
Supporting Information.
[15] Note: the ee value of the product correlates linearly with that of
the catalyst. For a review of non-linear effects in asymmetric
catalysis, see: H. B. Kagan, T. O. Luukas in Comprehensive
Asymmetric Catalysis (Eds.: E. N. Jacobsen, A. Pfaltz, H.
Yamamoto), Springer, New York, 1999, Chapter 4.1.
[8] a) E. C. Lee, B. L. Hodous, E. Bergin, C. Shih, G. C. Fu, J. Am.
Chem. Soc. 2005, 127, 11586 – 11587; b) B. L. Hodous, G. C. Fu, J.
Am. Chem. Soc. 2002, 124, 1578 – 1579.
[9] J. E. Wilson, G. C. Fu, Angew. Chem. 2004, 116, 6518 – 6520;
Angew. Chem. Int. Ed. 2004, 43, 6358 – 6360.
[10] For an overview on the chemistry of ketenes, see: T. T. Tidwell,
Ketenes, Wiley-Interscience, New York, 2006.
[11] For examples where phosphepine 3 is used as a chiral nucleo-
philic catalyst, see: a) R. P. Wurz, G. C. Fu, J. Am. Chem. Soc.
2005, 127, 12234– 12235; b) J. E. Wilson, G. C. Fu, Angew. Chem.
2006, 118, 1454 – 1457; Angew. Chem. Int. Ed. 2006, 45, 1426 –
1429.
[12] For a review of the use of chiral amines as enantioselective
nucleophilic catalysts, see: S. France, D. J. Guerin, S. J. Miller, T.
Lectka, Chem. Rev. 2003, 103, 2985 – 3012.
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