Efficient synthesis of β'-amino-α, β-unsaturated ketones

Article abstract of DOI:10.3762/bjoc.9.52

A general and simple procedure to access chiral β'-amino-α, β-enones, in seven steps, from an α,β unsaturated ester has been described. The use of a Horner-Wadsworth-Emmons reaction as a key step for generating the β'-amino-α,β-enones, permits access to a range of substrates under mild conditions and in moderate to high yield.

Full text of DOI:10.3762/bjoc.9.52

Efficient synthesis of
β’-amino-α,β-unsaturated ketones
Isabelle Abrunhosa-Thomas1,2, Aurélie Plas2,3, Nishanth Kandepedu2,3,
Pierre Chalard1,2 and Yves Troin*1,2,§
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 486–495.
1Clermont Université, ENSCCF, Institut de Chimie de
Clermont-Ferrand (ICCF), BP 10448, F-63000 Clermont-Ferrand,
France, 2CNRS, UMR 6296, Institut de Chimie de Clermont-Ferrand
(ICCF), BP 80026, F-63171 Aubière, France and 3Clermont
Université, Institut de Chimie de Clermont-Ferrand, BP 10448,
F-63000 Clermont-Ferrand, France
doi:10.3762/bjoc.9.52
Received: 04 December 2012
Accepted: 01 February 2013
Published: 06 March 2013
Associate Editor: M. P. Sibi
Email:
Yves Troin* - yves.troin@ensccf.fr
© 2013 Abrunhosa-Thomas et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
§ Fax: (33)473407008
Keywords:
β’-amino-α,β-unsaturated ketones; Horner–Wadsworth–Emmons
reaction; stereoselective synthesis
Abstract
A general and simple procedure to access chiral β'-amino-α,β-enones, in seven steps, from an α,β unsaturated ester has been
described. The use of a Horner–Wadsworth–Emmons reaction as a key step for generating the β'-amino-α,β-enones, permits access
to a range of substrates under mild conditions and in moderate to high yield.
Introduction
Compounds incorporating β-amino ketone functionality are conducted under different protocols in which the stereose-
prevalent in many natural products of biological importance [1]. lectivity of the reaction can be introduced through the use of a
This versatile synthon has been extensively used in the chiral catalyst [9,10] (Lewis acid, Brønsted acids, L-proline,
construction of β-amino acids [2], β-amino alcohols [3], and Cinchona alkaloids derivatives, thioureas, etc.), or by the addi-
homoallylic amines [4,5], and can serve as building blocks for tion of chiral amines to α,β-unsaturated esters [11,12] or the
the preparation of nitrogen-containing molecules often found in reaction of chiral imines with enolates derived from Weinreb
medicinal chemistry [6-10]. Thus, the development of efficient amides [13,14]. In previous work on the asymmetric synthesis
and stereoselective reactions for a useful approach to chiral of 2,6-disubstituted piperidines by C–N bond formation, we
β-amino ketones is still of importance. One of the most demonstrated that intramolecular aza-Michael ”type” cyclisa-
powerful approaches is the Mannich reaction, which can be tion [15] using a β'-carbamate-α,β-unsaturated ketone predom-
486

Beilstein J. Org. Chem. 2013, 9, 486–495.
Scheme 1: Asymmetric synthesis of 2-methyl-6-phenyl piperidine.
inantly induces the formation of a piperidine ring with the 2,6- transformation of the ester moiety to a Weinreb amide [18] fol-
trans configuration (Scheme 1).
lowed by changing the nitrogen protecting group to a carbamate
furnished the key intermediate 6, which could be further
The relative stereochemistry of piperidine 2a was confirmed by alkylated with Grignard reagents to give β’-amino protected
further transformation to the known compound 3 [16,17] with α,β-enone 1 in good overall yield and high enantiomeric excess.
94% ee. In order to establish this new approach as a general As Grignard reagents did not allow the use of a wide range of
method for the preparation of chiral 2,6-disubstituted functional groups and sometimes gave bad overall yields, we
piperidines, we wish to report here a facile synthetic route to devised a general and simple method to easily access a variety
various β’-carbamate-α,β-unsaturated ketones in good overall of β’-amino-α,β-unsaturated ketones by a more convenient
yields and good enantioselectivities.
route using the Horner–Wadsworth–Emmons reaction [19,20]
as the key step, as described in Scheme 3.
Results and Discussion
In a preliminary approach, preparation of β-amino ketones was In order to gain access to phosphonates 13, in a general and
envisaged through a nucleophilic addition reaction of Grignard convergent process, two ways were investigated (Scheme 4). In
reagents to N-carbamoyl β-amino Weinreb amides (Scheme 2) route A, we planned to obtain the desired compound by using a
[18].
similar strategy to that which we have described previously (see
above): chiral induction was obtained through the addition of
Conjugate addition of (R)-N-benzyl-N-methylbenzylamide to Davies amine, furnishing 8. Hydrogenation of compound 8 fol-
methyl cinnamate under basic conditions led to β–aminoester 5 lowed by N-protection as a carbamate would furnish the
with high diastereoselectivity (dr >94%) [11,12]. Subsequent β-amino ester precursor of the phosphonate 13. In route B, the
Scheme 2: (a) Davies amine, BuLi, THF, −78 °C; dr ≥ 94% ; (b) H2, Pd(OH)2, MeOH; (c) Na2CO3, PhCH2CO2Cl, CH2Cl2/H2O; (d) NaOH 1 N, MeOH;
(e) CDI, N,O-dimethylhydroxylamine·HCl, (f) Mg, 1-bromo-2-propene, THF.
Scheme 3: Modified synthetic route to15.
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Beilstein J. Org. Chem. 2013, 9, 486–495.
Scheme 4: Possible pathways to obtain phosphonate 13 (a) Davies amine, BuLi, THF, −78 °C; dr ≥ 95%; (b) H2, Pd(OH)2/C, MeOH, 60 psi;
(c) Na2CO3, R2CO2Cl, CH2Cl2/H2O; (d) and (e) BuLi, (EtO)2P(O)Me, THF, −78 °C.
most convergent, the preparation of the phosphonate was envis- Thus, we focused on route A, and after optimization of the reac-
aged in the first step; then, hydrogenation would furnish amino tion conditions, we found that the transformation of 7a to 10a
phosphonate. In method A the last step to synthesize 13 would could be done without purification. Hence, the addition of enan-
be a nucleophilic addition of diethyl methylphosphonate under tiopure lithium N-benzyl-N-α-methylbenzylamide to α,β-unsat-
basic conditions, whereas for method B, the final step would be urated ester 7 followed by hydrogenation to the corresponding
the N-protection of the amino phosphonate as a carbamate.
primary amine and further protection as a carbamate gave the
β-amino methylester 10. At this stage, the ester function was
The two routes were then tested, starting from methyl crotonate transformed into the ketophosphonate 13 by treatment with 2.5
(7a, R1 = Me) as a model substrate (Scheme 4). While com- equivalents of the lithium anion of diethyl methylphosphonate
pounds 9 and 10a were obtained in reasonable yields (80 and [22-28] in THF at −78 °C, in moderate to good yields
70%, respectively) in route A, the hydrogenation of 11 (Scheme 6, Table 1).
(obtained in a 74% yield from 8) to 12 did not proceed to
provide the expected compound under various conditions Over the years, many examples of base-promoted
(methanol in acid conditions, using either Pd(OH)2/C under H2 Horner–Wadsworth–Emmons (HWE) reactions have been
pressure (60 psi) or Pd/C under reflux in the presence of ammo- reported in the literature, and various combinations of bases and
nium formate). Instead, the formation of 14 [21] was observed, solvents (K2CO3/CH3CN[23], DBU/THF[25], NaH/THF[29],
resulting from β-elimination and reduction of the transient Et3N/LiCl/CH3CN[30] or Ba(OH)2/(THF/H2O)[31]) have been
double bond (Scheme 5).
used. We subjected our substrate 13a to three of those mild sets
Scheme 5: Synthesis of compound 14.
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Beilstein J. Org. Chem. 2013, 9, 486–495.
Scheme 6: General synthesis of compound 13 (a) Davies amine, BuLi, THF, −78 °C; (b) H2, Pd(OH)2/C, MeOH; (c) Na2CO3, R2CO2Cl, CH2Cl2/H2O;
(d) BuLi, (EtO)2P(O)Me, THF, −78 °C.
Table 1: Formation of the ketophosphonates 13.
entry
1
ester 7
amino ester 10
yield (%)
phosphonate 13
yield (%)
57
68
7a
7a
10a
10b
13a
13b
2
3
62
77
57
65
7b
7c
10c
10d
13c
4
73
66
13d
13e
5
6
7
72
76
66
58
62
66
7d
7d
10e
10f
13f
7d
10g
13g
489

Beilstein J. Org. Chem. 2013, 9, 486–495.
of conditions (Scheme 7, Table 2), which, after reaction with with our model substrate. Those conditions were then applied to
the benzaldehyde, will furnish the chiral amino ketone 15a.
a wide range of functionalized aldehydes with phosphonate
13a–g, giving amino ketone 15a–z in good to excellent yields
and high E/Z ratio (≥ 95%). The results are presented in
Table 3.
Conclusion
In summary, a general methodology has been devised for the
asymmetric synthesis of β’-amino-protected-α,β enones, a valu-
able intermediate for the synthesis of trans 2,6-disubstituted
piperidines. The scope and limitation of the aza-Michael reac-
tion were studied with a range of substrates. We are currently
Scheme 7: Optimization of conditions for the
Horner–Wadsworth–Emmons reaction.
As illustrated in Table 2, we found that the use of 1.3 equiv of working on the application of this synthetic method to the
Ba(OH)2 THF/H2O (40/1) furnished the optimal yield of 95% preparation of piperidine natural products.
Table 2: Horner–Wadsworth–Emmons optimal conditions for 15a.
phosphonate 13a
aldehyde
conditions (a)
amino ketone 15a
yield (%)
1 equiv Et3N/LiCl/CH3CN,
3 h
75
80
95
1 equiv DBU/THF,
2 h
benzaldehyde
1.3 equiv Ba(OH)2/(THF/H2O),
1 h
Table 3: Formation of the β’-amino-α,β-unsaturated ketones 15 under Ba(OH)2 conditions.
entry
phosphonate 13
aldehyde
amino ketone 15
yield (%)
1
benzaldehyde
95
13a
13a
15a
15b
15c
2
3
o-nitrobenzaldehyde
m-nitrobenzaldehyde
89
86
13a
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Beilstein J. Org. Chem. 2013, 9, 486–495.
Table 3: Formation of the β’-amino-α,β-unsaturated ketones 15 under Ba(OH)2 conditions. (continued)
4
5
6
7
13a
13a
13a
13a
p-nitrobenzaldehyde
p-methoxybenzaldehyde
o-bromobenzaldehyde
p-bromobenzaldehyde
91
15d
79
15e
91
15f
89
15g
8
13a
2-chloro-5-nitrobenzaldehyde
95
15h
9
13a
pyridine-3-carboxaldehyde
87
15i
10
11
13a
13a
(E)-ethyl-4-oxo-2-butenoate
85
15j
ethylglyoxylate
53
15k
491

Beilstein J. Org. Chem. 2013, 9, 486–495.
Table 3: Formation of the β’-amino-α,β-unsaturated ketones 15 under Ba(OH)2 conditions. (continued)
12
13
13a
13a
butanal
nonanal
95
15l
88
15m
14
benzaldehyde
96
13b
13b
15n
15
16
decanal
78
15o
13b
dodecanal
91
15p
17
benzaldehyde
91
13c
13c
15q
18
p-nitrobenzaldehyde
84
15r
19
benzaldehyde
94
13d
15s
492

Beilstein J. Org. Chem. 2013, 9, 486–495.
Table 3: Formation of the β’-amino-α,β-unsaturated ketones 15 under Ba(OH)2 conditions. (continued)
20
21
13d
13d
ethanal
butanal
78
15t
95
15u
22
23
24
25
benzaldehyde
84
13e
13e
15v
ethanal
80
15w
13e
13e
butanal
95
15x
nonanal
89
15y
26
ethanal
81
13f
15z
27
ethanal
76
13g
1
493

Beilstein J. Org. Chem. 2013, 9, 486–495.
Experimental
3H), 3.62 (s, 3H), 2.46 (d, J = 6.9 Hz, 2H), 1.16 (t, J = 6.9 Hz,
Organic solutions were dried over MgSO4 or Na2SO4, and 3H), 1.15 (d, J = 6.6 Hz, 3H). Spectral data are identical to
filtered. When anhydrous solvents were used, they were those reported in [32].
prepared as follows: tetrahydrofuran (THF) was distilled under
N2 from sodium benzophenone ketyl and used immediately; an- General procedure for the synthesis of 13
hydrous acetonitrile was freshly distilled from CaH2. All 1H (R)-Ethyl [5-(diethoxyphosphoryl)-4-oxopentan-2-
and 13C NMR spectra were measured in CDCl3 or C6D6 and yl]carbamate 13a: To a solution of diethyl methylphosphonate
recorded on a Bruker 400 MHz (101 MHz for 13C) spectro- (5.8 mL, 39.7 mmol, 2.5 equiv) in anhydrous THF (15 mL) kept
meter with TMS as the internal standard. Chemical shifts are at −78 °C, was added dropwise n-butyl lithium (24.8 mL, 1.6 M
expressed in parts per million (ppm) and J-values are given in in hexane, 39.7 mmol, 2.5 equiv). After 20 min at −78 °C, a
hertz. The following abbreviations are used: singlet (s), doublet solution of 10a (3 g, 15.9 mmol, 1 equiv) in anhydrous THF
(d), doublet of doublets (dd), triplet (t), multiplet (m). High- (15 mL) was added dropwise. After addition, the temperature of
resolution mass spectroscopy (HRMS) was carried out in elec- the reaction was kept at −78 °C for 30 min and then allowed to
trospray mode and was performed by CRMP (Clermont- reach 0 °C over 1 h, and the reaction was quenched with a solu-
Ferrand, France). Monitoring of the reactions was performed by tion of ammonium chloride and extracted twice with ethyl
using silica-gel TLC plates (silica Merck 60 F254). Spots were acetate. After drying over MgSO4 and concentration under
visualized by UV light at 254 nm. Flash chromatography was vacuum, the crude oil was first distilled at low pressure to
performed by using silica gel 60 (70–230 mesh) or RP18 remove excess diethyl methylphosphonate, and the residue was
(25–40 lM) from Merck Chimie SAS (France) on a Flash II then purified by flash chromatography (eluent: cyclohexane/
apparatus (Armen Instrument, France).
EtOAc 2/1 to EtOAc) afforded 13a as a yellow oil (3.3 g, 68%
yield): [α]D25 +33.6 (c 1.17, CHCl3); 1H NMR (400 MHz,
CDCl3) δ 5.03 (br s, 1H,), 4.16–3.94 (m, 7H), 3.08 (dd, J =
General procedure for the synthesis of 10
(R)-Methyl 3-(ethoxycarbonylamino)butanoate 10a: To a 23.0, 14.0 Hz, 1H), 2.99 (dd, J = 22.6, 14.0 Hz, 1H), 2.84 (dd,
cold solution (0 °C) of (+)-(R)-N-benzyl-N-α-methylben- J = 17.1, 6.0 Hz, 1H), 2.71 (dd, J = 17.1, 5.7 Hz, 1H),
zylamine (23.0 mL, 110 mmol, 1.1 equiv) in dry THF (280 mL) 1.33–1.21 (m, 6H), 1.15–1.20 (m, 6H); 13C NMR (101 MHz,
was added n-butyllithium (75.0 mL, 1.6 M in hexane, CDCl3) δ 200.6, 155.8, 62.6 (d, J = 6.6 Hz), 62.5 (d,
120 mmol, 1.2 equiv) slowly under argon. The resultant pink J = 6.5 Hz), 60.5, 49.6, 43.5, 42.9 (d, J = 127.4 Hz), 20.7, 16.2,
solution of lithium amide was stirred for 30 min then cooled to 16.1, 14.6; HRMS-ESI (M + Na), m/z calcd. for
−78 °C before dropwise addition of a solution of methyl C12H24NO6PNa 332.1239, found 332.1239.
crotonate (10.0 mL, 100 mmol, 1 equiv) in dry THF (100 mL).
The mixture was stirred at –78 °C for 90 min. Then, a saturated General procedure for the synthesis of 15
aqueous solution of NH4Cl (100 mL) was added slowly, and the (R,E)-Ethyl [4-oxo-6-phenyl-hex-5-en-2-yl]carbamate 15a:
resulting solution was allowed to warm to room temperature. To a solution of 13a (0.5 g, 1.6 mmol, 1 equiv) in THF (7 mL),
Then, the solution was extracted twice with ethyl acetate. The was poured Ba(OH)2 (0.346 g, 2.0 mmol, 1.25 equiv) in one
combined organic extracts were dried over Na2SO4, filtered and batch at room temperature. After 30 min, a solution of benzal-
evaporated. The crude product was added to a suspension of dehyde (0.172 ml, 1.7 mmol, 1.05 equiv) in THF/H2O (40/1)
10% Pd/C (5.00 g) in methanol (200 mL). The mixture was (7 mL) was slowly added at room temperature. After 1 h, the
placed on a Parr apparatus and stirred under a hydrogen atmos- reaction mixture was quenched with ammonium chloride and
phere (60 psi) for 4 days. The catalyst was removed by filtra- extracted three times with ethyl acetate. Then the organic layer
tion on Celite®. The residue was concentrated in vacuum and was dried over MgSO4, concentrated under vacuum and puri-
dissolved in dichloromethane (200 mL) and water (200 mL). fied by flash chromatography (eluent: cyclohexane to cyclo-
Then, sodium carbonate (42.4 g, 400 mmol, 4.0 equiv) and hexane/EtOAc 8/2) to give 15a as a white solid (0.401 g, 95%):
ethyl chloroformate (28.5 mL, 200 mmol, 2 equiv) were added Mp 74 °C; [α]D25 +9.5 (c 1.21, CHCl3); 1H NMR (400 MHz,
dropwise. The resulting solution was stirred at room tempera- CDCl3) δ 7.50 (d, J = 16.7 Hz, 1H), 7.47 (dd, J = 7.8, 3.0 Hz,
ture for 3 h. The aqueous material was extracted with dichloro- 1H), 7.33–7.30 (m, 3H), 6.65 (d, J = 16.7 Hz, 1H), 5.14 (s, 1H),
methane and the combined organic extracts were dried over 4.14–4.06 (m, 1H), 4.02 (q, J = 6.9 Hz, 2H), 2.95 (dd, J = 15.9,
Na2SO4, filtered and concentrated in vacuo. Purification by 4.2 Hz, 1H), 2.71 (dd, J = 15.9, 6.5 Hz, 1H), 1.19 (d,
chromatography on silica gel (cyclohexane/EtOAc 9/1 to 5/5) J = 6.8 Hz, 3H), 1.14 (t, J = 6.9 Hz, 3H); 13C NMR (101 MHz,
afforded 10a as a yellow oil (21.4 g, 57% in three steps): [α]D25 CDCl3) δ 198.8, 155.9, 143.4, 134.3, 130.6, 128.9, 128.4,
−35.6 (c 0.99, CHCl3), lit.[32] [α]D25 −37.07 (c 1, CHCl3); 126.3, 60.6, 46.3, 44.1, 20.5, 14.6; HRMS-ESI (M + Na): calcd.
1H NMR (400 MHz, CDCl3) δ 5.03 (br s, 1H, NH), 4.03 (m, for C15H19NO3Na 284.1263, found 284.1275.
494

Beilstein J. Org. Chem. 2013, 9, 486–495.
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