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Synthesis 2013; 45(4): 463-470
DOI: 10.1055/s-0032-1317984
paper
© Georg Thieme Verlag Stuttgart · New York

Synthesis of Cyclic α-Aminophosphonates through Copper-Catalyzed Enamine Activation

Junbin Han
a   Department of Chemistry, University of Louisville, Louisville, KY 40292, USA   Email: bo.xu@louisville.edu   Email: gb.hammond@louisville.edu
,
Robert S. Paton
b   Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, UK
,
Bo Xu*
a   Department of Chemistry, University of Louisville, Louisville, KY 40292, USA   Email: bo.xu@louisville.edu   Email: gb.hammond@louisville.edu
,
Gerald B. Hammond*
a   Department of Chemistry, University of Louisville, Louisville, KY 40292, USA   Email: bo.xu@louisville.edu   Email: gb.hammond@louisville.edu
› Author Affiliations
Further Information

Publication History

Received: 17 October 2012

Accepted after revision: 12 December 2012

Publication Date:
17 January 2013 (online)

 
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Abstract

A copper-catalyzed tandem-cyclization-triggered addition strategy that relies on electrophilic enamine activation has been used to synthesize various cyclic α-aminophosphonate derivatives in good to excellent yields. Both five- and six-membered rings can be generated under mild conditions with high regioselectivity. A mechanism based on copper-catalyzed enamine activation is proposed.


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Key words

copper - cyclic α-aminophosphonates - cyclization - enamines - hydroamination - nitrogen heterocycles - phosphorus

Medicinal chemists are continuously seeking better methods to prepare N-heterocycles of different ring sizes and various substitution patterns because of their important biological properties.[ 1 ] A case in point is the synthesis of cyclic α-aminophosphonates. α-Aminophosphonates and their corresponding α-aminophosphonic acids have received much attention in organic and medicinal chemistry because they are structural analogues of the corresponding α-amino acids and transition state mimics of peptide hydrolysis.[ 2 ] Moreover, α-aminophosphonates have broad applications due to their biological activity,[2d] [3] such as antibacterial,[ 2a,4 ] anti-cancer,[ 5 ] and antiviral activity and their action as enzyme inhibitors.[ 6 ] In this communication, we report the synthesis of cyclic α-aminophosphonates by a one-pot tandem hydroamination/phosphorylation strategy. The central tenet of our strategy is the electrophilic enamine activation shown in Scheme [1]: a cyclic enamine is formed first through a well-established transition-metal-catalyzed intramolecular hydroamination of alkynes,[ 7 ] then the same transition metal interacts with the in situ generated enamine, activating it as an electrophile in a reaction with an incoming nucleophile. We have successfully used this strategy for tandem amination/alkynylation, amination/cyanation, and amination/trifluoromethylation.[ 8 ] An added advantage of our approach is that all of these processes take place in one pot and one batch, without protection/deprotection steps.

Despite the importance of cyclic α-aminophosphonates, most literature syntheses of these compounds have significant limitations. For example, the most common method to prepare α-aminophosphonates – the Kabachnik–Fields reaction (the three-component coupling of a carbonyl, an amine and a hydrophosphoryl compound)[ 9 ] – is effective for the construction of acyclic (linear) products, but much less so in the generation of ring systems, because of the difficulty in preparing the corresponding starting material without extensive protection and deprotection steps (Scheme [2, a]).[ 3a ] Another method to prepare α-aminophosphonates is the oxidative addition of a dialkyl phosphonate to a cyclic amine. Li and Basle reported such a synthesis in 2009 using oxygen as the oxidant (Scheme [2, b]),[ 2d ] but this method suffers from a narrow substrate scope (a benzylic proton must be present). The third method is the decarboxylative coupling of natural α-amino acids and phosphites (Scheme [2, c]).[ 10 ] However, this method relies on natural cyclic amino acids as starting materials, and the pool of natural cyclic amino acids (e.g., l-proline) is limited. This problem is magnified when cyclic amino acids with ring sizes that natural amino acids lack, such as six-membered rings, are needed. Consequently, their syntheses require more steps. Similar to our strategy, the synthesis of α-aminophosphonates reported by Doye and co-workers is by hydroamination catalyzed by the early transition metal titanium, followed by addition of dialkyl phosphonate.[ 11 ] Because their method only works for primary amines, and our methodology mainly uses secondary amines, we believe our method is a good complement to Doye’s approach.

Zoom Image
Scheme 1 Tandem protocol for the construction of rings through enamine activation
Zoom Image
Scheme 2 Literature synthesis of cyclic α-aminophosphonate derivatives

The keystone of our strategy – transforming an enamine into a good electrophile – runs contrary to the ingrained teachings on enamine nucleophilicity. Although a handful of reports on the reactions of enamines with nucleophiles exist,[ 12 ] they pale in comparison to the overwhelming number of reports on enamines used as nucleophiles. Not only is the use of enamines as electrophiles uncommon in organic synthesis, even the mechanism of enamine activation is poorly understood; for example, unlikely interactions in the copper-catalyzed reaction of enamines have been proposed previously (Scheme [3a]).[12a] [b] [13] While enamines are known to be good nucleophiles they can also be made to act as a good electrophile. It is instructive to consider the oxygen analogue as a model for comparison: enol ethers are normally good nucleophiles, but if they are activated by a Brønsted acid, for example, then they become good electrophiles. One notable example of this transformation is the tetrahydropyranyl protection of alcohols (Scheme [3, b]). In theory, an enamine can also be protonated to generate an iminium intermediate (Scheme [3, c]). Most mechanistic studies have shown preferential attack at the nitrogen by a proton followed by rearrangement to the more stable C-protonated iminium form,[ 14 ] which can then act as an electrophile. As it happens, in the case of electron-rich enamines (e.g., R = alkyl), both the starting material and the product are rather basic (i.e., alkylamines), and therefore they combine to neutralize the acidity of the acid catalyst. Therefore, this method often does not work except for enamines with electron-withdrawing groups.

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Scheme 3 Activation of enamine by conversion into electrophile

One solution to this problem is to use a late transition metal Lewis acid, which normally has a relatively higher affinity for the electron-rich double bond. The electrophilic activation of this double bond facilitates nucleophilic attack (Scheme [3, d]). DFT calculations[ 15 ] predict the η2-bound copper–enamine π-complex to be more stable (by 2.2 kcal/mol) than the corresponding σ-complex (Figure [1]). Unsymmetrical coordination of copper(I) to the enamine π-bond leads to shortening of the C–N bond and increased positive charge at the α-carbon: calculated Mulliken charges show that the enamine transfers 0.32 units of charge to copper(I) bromide with a buildup at Cα by 0.13. We posit that in the presence of a suitable catalyst, even an electron-rich enamine can act as an electrophile. Specifically, our strategy could deliver N-heterocycles through a transition-metal-activated enamine that acts as an electrophile. The same metal catalyzes both the generation of the enamine in situ – by intramolecular amination of an alkyne – and its reaction with a nucleophile.

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Figure 1 Copper(I) bromide coordination to an enamine (B3LYP-D3/6-311+G(d,p) with LANL2DZ ECPs for Cu/Br; relative free energy at 298 K in kcal/mol)

It should be noted that cyclic enamines (where the nitrogen is part of the ring) are not trivial to prepare. Whereas acyclic enamines can be easily made by the reaction of an aldehyde or ketone with a secondary amine, the equivalent approach to synthesize a cyclic enamine (e.g., in Scheme [2]) would require placing an amino group and an aldehyde (or ketone) in the same molecule, a task that would add extra protection/deprotection steps.[ 16 ] An additional experimental difficulty of working with enamines, especially electron-rich enamines (e.g. N,N-dialkyl/arylenamines), is that they are sensitive to moisture and hydrolyze with relative ease.[ 17 ] This trait makes them difficult to isolate and purify. But our tandem chemistry overcomes this problem by using an in situ generated enamine, which reacts with another nucleophile simultaneously. Our method involves a tandem addition of two different nucleophiles to an alkyne. Literature reports on tandem addition of two different nucleophiles to an alkyne in one pot are not common.[ 18 ]

In a preliminary attempt, reaction of aminoalkyne 1a and diethyl phosphonate (2a) in the presence of copper(I) bromide (5 mol%) in 1,4-dioxane at room temperature for six hours gave the desired product 3a in 76% yield (Table 1, entry 1). In addition, pyrrolidine 4a was isolated in 21% yield (entry 1). Water strongly affected the ratio of the 3a to 4a. Two equivalents of water in 1,4-dioxane led to a decrease in the formation of 3a (58% yield) and an increase in the yield of 4a (entry 2). Most of the starting material was converted into pyrrolidine 4a in the presence of 20 mol% formic acid. The best choice of solvent was toluene, which provided 3a in better yield (entry 4). Further optimization revealed that the yield of 3a could be improved to 92% yield by addition of 3 Å molecular sieves (entry 5). Higher temperature led to faster reaction, but the yield was reduced slightly (entry 6). This reaction is not catalyzed by a strong Brønsted acid (entry 7).

With the optimized conditions in hand, we examined the scope of this reaction (Table 2). Various aminoalkynes were used in the reaction. Both benzylamines and aromatic amines were tolerated (Table 2). The aromatic amine 1i with a methoxy group in the para position produced 3i in good yield (entry 9). The reaction of substrate 1j with an electron-withdrawing chloro substituent on the aromatic ring did not take place, and most of the starting material was recovered (entry 10). Complete regioselectivity was observed in all cases. The regioselectivity obeyed Baldwin’s rules.[ 19 ] That is, cyclization of 3-ynamine 1a and 4-ynamine 1c gave five-membered ring products through 5-endo-dig and 5-exo-dig mechanisms, whereas the reaction of 5-ynamine (e.g., 1f) produced a six-membered ring through a 6-exo-dig pathway. When chiral aminoalkyne 1k was used, a negligible chiral induction was observed (dr = 1:1.1, entry 11). Dimethyl and dibenzyl phosphonates could also be employed under our reaction conditions (entries 12 and 13). Both of them gave the products in excellent yields.

The proposed mechanism of enamine activation is shown in Scheme [4]. The aminoalkyne 1 is first activated by coordination to the Lewis acidic copper center. Nucleophilic attack of the lone electron pair of the nitrogen atom generates the enamine 5 via intermediate 4. This enamine intermediate 5 can form a complex 6 with copper(I) bromide. Then the nucleophile (diethyl phosphonate in our case) – activated by base (amine starting material or product) – reacts with the activated enamine intermediate to give the final product. Alternatively, the enamine intermediate 5 can react with copper(I) bromide and the nucleophile first to generate an iminium intermediate and a nucleophile–copper species. Addition of the nucleophile–copper species to the iminium intermediate furnishes final product. This mechanism is based on the classic role of the iminium species (Scheme [4, ]A). At this time, it is difficult to verify which of the two possible mechanisms prevail. The iminium mechanism (mechanism A) involves generation of two relatively energetic species: a nucleophile–copper species and an iminium species (Scheme [4]), so we prefer the more concerted mechanism B.

Table 1 Optimization of the reaction condition for 3a a

Entry

Cat.

Additive

Solvent

Temp, time

Yieldb of 3a (4a) (%)

1

CuBr

none

1,4-dioxane

r.t., 6 h

76 (21)

2

CuBr

H2O (2 equiv)

1,4-dioxane

r.t., 6 h

58 (37)

3

CuBr

HCO2H

1,4-dioxane

r.t., 6 h

12 (63)

4

CuBr

none

toluene

r.t., 6 h

86

5

CuBr

3 Å MS

toluene

r.t., 6 h

92

6

CuBr

3 Å MS

toluene

microwave, 100 °C, 0.5 h

88

7

TfOH

none

toluene

r.t., 12 h

0

a Reaction conditions: 1a (0.15 mmol), 2a (0.3 mmol), CuBr (5 mol%), solvent (1 mL).

b Isolated yields.

Table 2 Scope of the Tandem Amination and Phosphorylation

Entry

Amine 1

Phosphite

Product

Yield (%)

2

R

1

1a

2a

Et

3a

92

2

1b

2a

Et

3b

94

3

1c

2a

Et

3c

93

4

1d

2a

Et

3d

88

5

1e

2a

Et

3e

67

6

1f

2a

Et

3f

83

7

1g

2a

Et

3g

86

8

1h

2a

Et

3h

89

9

1i

2a

Et

3i

91

10

1j

2a

Et

3j

0

11

1k

2a

Et

3k

88 (dr 1:1.1)

12

1l

2b

Me

3l

91

13

1m

2c

Bn

3m

94

a Reaction conditions: CuBr (5 mol%), 1 (0.15 mmol), 2 (0.3 mmol), 3 Å MS, toluene, r.t., 6 h.

b Isolated yield.

c The isolated product was contaminated with a small amount of dibenzyl phosphonate.

Zoom Image
Scheme 4 Proposed mechanism (Nu-H = dialkyl phosphonate)

In summary, a one-pot cyclization-triggered addition strategy has been successfully used to synthesize cyclic α-aminophosphonates in good to excellent yields. Both five- and six-membered rings can be generated under mild conditions. Further studies on the asymmetrical version are now under way.

1H, 13C, and 31P NMR spectra were recorded at 400, 100, and 162 MHz, respectively; CDCl3 was used as solvent. The chemical shifts are reported in δ (ppm) values relative to CHCl3 (δ = 7.26 for 1H and δ = 77.0 for 13C NMR) and 85% H3PO4 (δ = 0.00 ppm for 31P NMR). All air- and/or moisture-sensitive reactions were carried out under an argon atmosphere. Solvents (THF, Et2O, CH2Cl2, DMF) were chemically dried using a commercial solvent purification system. All other reagents and solvents were employed without further purification. The products were purified by using a commercial flash chromatography system or a regular glass column. TLC was carried out on Merck silica gel 60 F254 aluminum sheets. HRMS was carried out at the CREAM Mass Spectrometry Facility, University of Louisville. A microwave reactor (CEM, model Discovery) was used for the reactions carried out under microwave conditions.


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Diethyl (2-Butyl-1-phenylpyrrolidin-2-yl)phosphonate (3a); Typical Procedure

N-(Oct-3-ynyl)aniline (1a; 30 mg, 0.15 mmol), diethyl phosphonate (2a; 41 mg, 0.3 mmol), and toluene (1 mL) were charged into a small vial, and CuBr (1.1 mg, 5 mol%) was added to the mixture under stirring. Then the reaction mixture was stirred at r.t. for 6 h. The resulting mixture was directly concentrated in a rotary evaporator and purified by flash chromatography (silica gel, hexane–EtOAc, 3:1); this gave 3a.

Yield: 48 mg (92%); yellow liquid.

1H NMR (400 MHz, CDCl3): δ = 0.73 (t, J = 7.6 Hz, 3 H), 0.83–0.96 (m, 1 H), 1.12–1.34 (m, 9 H), 1.76–1.93 (m, 2 H), 1.98–2.15 (m, 2 H), 2.23–2.37 (m, 1 H), 2.51–2.62 (m, 1 H), 3.31–3.42 (m, 2 H), 3.88–4.09 (m, 4 H), 6.63–6.76 (m, 1 H), 7.16–7.22 (m, 4 H).

13C NMR (100 MHz, CDCl3): δ = 13.8, 16.5 (d, J = 3.9 Hz), 16.5 (d, J = 3.1 Hz), 22.7 (d, J = 1.6 Hz), 22.7, 24.4 (d, J = 10.1 Hz), 32.0 (d, J = 10.9 Hz), 35.7, 51.8 (d, J = 3.1 Hz), 61.4 (d, J = 7.8 Hz), 62.7 (d, J = 7 Hz), 66.2 (d, J = 154 Hz), 155.5, 117.4, 128.4, 146.4.

31P NMR (162 MHz, CDCl3): δ = 28.77.

HRMS (ESI+): m/z [M + H]+ calcd for C18H30NO3P: 340.2042; found: 340.2047.


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Diethyl (1-Benzyl-2-butylpyrrolidin-2-yl)phosphonate (3b)

Yield: 50 mg (94%); yellow oil.

1H NMR (700 MHz, CDCl3): δ = 0.91 (t, J = 7 Hz, 3 H), 1.21–1.37 (m, 8 H), 1.40–1.48 (m, 2 H), 1.61–1.72 (m, 1 H), 1.73–1.82 (m, 3 H), 1.83–1.94 (m, 1 H), 2.22–2.28 (m, 1 H), 2.65–2.74 (m, 2 H), 3.94 (d, J = 1.4 Hz, 2 H), 4.06–4.17 (m, 4 H), 7.14–7.31 (m, 5 H).

13C NMR (100 MHz, CDCl3): δ = 14.2, 16.7 (t, J = 8.9 Hz, 2C), 22.7 (d, J = 3 Hz), 23.4, 25.7 (d, J = 8.9 Hz), 31.9, 32.6 (d, J = 11.9 Hz), 51.5 (d, J = 3.7 Hz), 53.4, 61.1 (d, J = 7.4 Hz), 62.1 (d, J = 8.2 Hz), 65.2 (d, J = 143.7 Hz), 126.5, 128.1, 128.4, 140.9.

31P NMR (162 MHz, CDCl3): δ = 30.51.

HRMS (ESI+): m/z [M + H]+ calcd for C19H32NO3P: 354.2193; found: 354.2197.


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Diethyl (2-Methyl-1-phenylpyrrolidin-2-yl)phosphonate (3c)

Yield: 41 mg (93%); yellow oil.

1H NMR (400 MHz, CDCl3): δ = 1.23 (t, J = 7.2 Hz, 3 H), 1.30 (t, J = 7.2 Hz, 3 H), 1.56 (d, 14.4 Hz, 3 H), 1.78–1.96 (m, 2 H), 2.09–2.21 (m, 1 H), 2.66–2.76 (m, 1 H), 3.34–3.41 (m, 1 H), 3.43–3.49 (m, 1 H), 3.95–4.13 (m, 4 H), 6.72–6.78 (m, 1 H), 7.20–7.23 (m, 4 H).

13C NMR (100 MHz, CDCl3): δ = 16.5, 16.5, 21.5 (d, J = 10.9 Hz), 22.6 (d, J = 1.6 Hz), 40.4, 51.0 (d, J = 11.0 Hz), 63.6 (d, J = 7.8 Hz), 62.8 (d, J = 6.9 Hz), 62.9 (d, J = 158.8 Hz), 116.1 (d, J = 1.6 Hz), 117.6, 128.4, 146.0 (d, J = 1.5 Hz).

31P NMR (162 MHz, CDCl3) δ = 28.41.

HRMS (ESI+): m/z [M + H]+ calcd for C15H24NO3P: 298.1567; found: 298.1572.


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Diethyl (1-Benzyl-2-methylpyrrolidin-2-yl)phosphonate (3d)

Yield: 41 mg (88%); yellow oil.

1H NMR (400 MHz, CDCl3): δ = 1.26 (td, J = 7.2, 3.2 Hz, 6 H), 1.32 (d, J = 15.2 Hz, 3 H), 1.52–1.73 (m, 4 H), 2.31–2.48 (m, 2 H), 2.72–2.79 (m, 1 H), 3.55 (d, J = 13.2 Hz, 1 H), 4.03–4.19 (m, 4 H), 7.18–7.29 (m, 5 H).

13C NMR (100 MHz, CDCl3): δ = 16.6 (d, J = 5.4 Hz), 16.7 (d, J = 5.4 Hz), 18.8 (d, J = 10.9 Hz), 22.6 (d, J = 4.7 Hz), 36.7, 51.5 (d, J = 10.1 Hz), 54.2, 61.3 (d, J = 7.8 Hz), 62.7 (d, J = 7.7 Hz), 64.1 (d, J = 143.2 Hz), 126.6, 128.1, 128.5, 140.8.

31P NMR (162 MHz, CDCl3): δ = 28.77.

HRMS (ESI+): m/z [M + H]+ calcd for C16H26NO3P: 312.1723; found: 312.1724.


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Diethyl (1-Benzylpyrrolidin-2-yl)phosphonate (3e)

Yield: 30 mg (67%); yellow oil.

1H NMR (400 MHz, CDCl3): δ = 1.27 (td, J = 7.0, 3.5 Hz, 6 H), 1.52–1.74 (m, 2 H), 1.85–2.09 (m, 2 H), 2.12–2.19 (m, 1 H), 2.83–2.89 (m, 1 H), 2.92 (dd, J = 10.0, 6.0 Hz, 1 H), 3.34 (d, J = 13.2 Hz, 1 H), 4.07–4.20 (m, 4 H), 4.36 (d, 13.2 Hz, 1 H), 7.11–7.30 (m, 5 H).

13C NMR (100 MHz, CDCl3): δ = 16.6 (d, J = 5.4 Hz), 16.6 (d, J = 5.4 Hz), 24.4 (d, J = 5.4 Hz), 27.0 (d, 9.2 Hz), 54.3 (d, J = 15.5 Hz), 59.5 (d, J = 172.7 Hz), 60.2 (d, J = 2.3 Hz), 61.8 (d, J = 7.7 Hz), 62.6 (d, J = 7 Hz), 126.8, 128.0, 128.8, 139.4.

31P NMR (162 MHz, CDCl3): δ = 27.13.

HRMS (ESI+): m/z [M + H]+ calcd for C15H24NO3P: 298.1567; found: 298.1572.


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Diethyl (1-Benzyl-2-methylpiperidin-2-yl)phosphonate (3f)

Yield: 41 mg (83%); yellow oil.

1H NMR (400 MHz, CDCl3): δ = 1.33 (t, J = 7.2 Hz, 6 H), 1.39 (d, J = 14.4 Hz, 3 H),1.40–1.47 (m, 1 H), 1.54–1.61 (m, 3 H), 1.71–1.82 (m, 1 H), 2.05–2.13 (m, 1 H), 2.39–2.45 (m, 1 H), 2.77–2.89 (m, 1 H), 3.73 (dd, J = 14.4, 2.8 Hz, 1 H), 4.02 (d, J = 14.8 Hz, 1 H), 4.09–4.20 (m, 4 H), 7.04–7.04 (m, 1 H), 7.21 (m, J = 7.6 Hz, 2 H), 7.22–7.31 (m, 2 H).

13C NMR (100 MHz, CDCl3): δ = 16.7 (t, J = 6.1 Hz, 2 C), 21.0 (d, J = 3.8 Hz), 21.8 (d, J = 6.6 Hz), 26.0, 35.7 (d, J = 3 Hz), 46.9 (d, J = 3.8 Hz), 55.6, 58.2 (d, J = 130.6 Hz), 60.9 (d, J = 8.3 Hz), 61.6 (d, J = 7.6 Hz), 126.4, 128.0, 128.0, 141.2.

31P NMR (162 MHz, CDCl3): δ = 31.77.

HRMS (ESI+): m/z [M + H]+ calcd for C17H28NO3P: 326.1880; found: 326.1886.


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Diethyl (1-Benzyl-2-pentylpiperidin-2-yl)phosphonate (3g)

Yield: 49 mg (86%); yellow oil.

1H NMR (400 MHz, CDCl3): δ = 0.83 (t, J = 6 Hz, 3 H), 1.12–1.33 (m, 14 H), 1.52–2.08 (m, 4 H), 2.16–2.28 (m, 2 H), 2.58–2.72 (m, 2 H), 3.89 (s, 2 H), 3.92–4.18 (m, 4 H), 7.18–7.28 (m, 5 H).

13C NMR (100 MHz, CDCl3): δ = 14.1, 16.6 (t, J = 5.2 Hz, 2C), 18.7, 22.5, 22.7 (d, J = 3.8 Hz), 23.5 (d, J = 9.7 Hz), 30.0, 31.9, 32.9 (d, J = 11.9 Hz), 51.5 (d, J = 3.7 Hz), 53.4, 61.1 (d, J = 8.2 Hz), 62.1 (d, J = 7.4 Hz), 65.2 (d, J =143.6 Hz), 126.5, 128.1, 128.3, 140.9.

31P NMR (162 MHz, CDCl3): δ = 28.80.

HRMS (ESI+): m/z [M + H]+ calcd for C21H36NO3P: 382.2506; found: 382.2501.


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Diethyl (2-Pentyl-1-phenylpiperidin-2-yl)phosphonate (3h)

Yield: 49 mg (89%); yellow oil.

1H NMR (400 MHz, CDCl3): δ = 0.72 (t, J = 6.8 Hz, 3 H), 0.77–0.92 (m, 1 H), 0.96–1.22 (m, 7 H), 1.14 (t, J = 6.8 Hz, 3 H), 1.22 (t, J = 7.2 Hz, 3 H), 1.72–1.88 (m, 2 H), 1.92–2.11 (m, 2 H), 2.18–2.30 (m, 1 H), 2.46–2.58 (m, 1 H), 3.27–3.39 (m, 2 H), 3.80–4.05 (m, 4 H), 6.58–6.69 (m, 1 H), 7.06–7.16 (m, 4 H).

13C NMR (100 MHz, CDCl3): δ =13.9, 16.5 (t, J = 3.8 Hz, 2C), 22.2 (d, J = 9.8 Hz), 22.4, 22.7, 29.3, 31.5, 32.3 (d, J = 10.6 Hz), 35.6, 51.8, 61.5 (d, J = 7.6 Hz), 62.8 (d, J = 6.8 Hz), 66.2 (d, J = 154.1 Hz), 115.5, 117.4, 128.4, 146.4.

31P NMR (162 MHz, CDCl3): δ = 28.80.

HRMS (ESI+): m/z [M + H]+ calcd for C20H34NO3P: 368.2349; found: 368.2346.


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Diethyl [1-(4-Methoxyphenyl)-2-pentylpiperidin-2-yl]phosphonate (3i)

Yield: 54 mg (91%); yellow oil.

1H NMR (400 MHz, CDCl3): δ = 0.77 (t, J = 7.2 Hz, 3 H), 0.87–1.01 (m, 1 H), 1.03–1.22 (m, 7 H), 1.19 (t, J = 7.2 Hz, 3 H), 1.26 (t, J = 7.2 Hz, 3 H), 1.68–1.89 (m, 2 H), 1.95–2.01 (m, 2 H), 2.01–2.13 (m, 1 H), 2.46–2.57 (m, 1 H), 3.28–3.36 (m, 2 H), 3.73 (s, 3 H), 3.88–4.09 (m, 4 H), 6.78 (d, J = 8.8 Hz, 2 H), 7.17 (d, J = 9.2 Hz, 2 H).

13C NMR (100 MHz, CDCl3): δ = 13.9, 16.5 (t, J = 5.3 Hz, 2 C), 22.2 (d, J = 10.6 Hz), 22.4, 22.9, 29.3, 31.5, 32.4 (d, J = 10.7 Hz), 35.2, 52.1 (d, J = 3 Hz), 55.6, 61.3 (d, J = 7.6 Hz), 62.7 (d, J = 7.6 Hz), 66.1 (d, J = 154.8 Hz), 113.9, 117.4, 140.6, 152.2.

31P NMR (162 MHz, CDCl3): δ = 28.98.

HRMS (ESI+): m/z [M + H]+ calcd for C21H36NO4P: 398.2455; found: 398.2458.


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Diethyl {2-Methyl-1-[(R)-1-phenylethyl]pyrrolidin-2-yl}phosphonate (3k)

Yield: 43 mg (88%); yellow oil. The 31P NMR spectrum shows a mixture of diastereomers, dr 48:52.

31P NMR (162 MHz, CDCl3): δ = 28.40, 30.72.

HRMS (ESI+): m/z [M + H]+ calcd for C17H28NO3P: 326.1880; found: 326.1882.


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Dimethyl (1-Benzyl-2-methylpyrrolidin-2-yl)phosphonate (3l)

Yield: 39 mg (91%); yellow oil.

1H NMR (400 MHz, CDCl3): δ = 1.36–1.42 (m, 3 H), 1.67–1.81 (m, 3 H), 2.40–2.56 (m, 2 H), 2.77–2.85 (m, 1 H), 3.58 (d, J = 12.8 Hz, 1 H), 3.78 (d, J = 10.0 Hz, 3 H), 3.85 (d, J = 9.6 Hz, 3 H), 4.21 (d, J = 12.8 Hz, 1 H), 7.18–7.39 (m, 5 H).

13C NMR (100 MHz, CDCl3): δ = 18.6 (d, J = 10.9 Hz), 22.6 (d, J = 5.4 Hz), 36.8, 51.5 (d, J = 10 Hz), 53.2 (d, J = 7.7 Hz), 54.0 (d, J = 7.7 Hz), 54.2, 62.1 (d, J = 161.1 Hz), 126.6, 128.1, 128.5, 140.5.

31P NMR (162 MHz, CDCl3): δ = 31.24.

HRMS (ESI+): m/z [M + H]+ calcd for C14H22NO3P: 284.1410; found: 284.1412.


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Dibenzyl (1-Benzyl-2-methylpyrrolidin-2-yl)phosphonate (3m)

Yield: 61 mg (94%); yellow oil. The NMR spectra were contaminated with dibenzyl phosphite.

1H NMR (400 MHz, CDCl3): δ = 1.42 (d, J = 15.6 Hz, 3 H), 1.58–1.79 (m, 3 H), 2.42–2.58 (m, 2 H), 2.78–2.86 (m, 1 H), 3.63 (d, J = 12.8 Hz, 1 H), 4.25 (d, J = 13.2 Hz, 1 H), 4.86–5.15 (m, 4 H), 7.16–7.38 (m, 15 H).

13C NMR (100 MHz, CDCl3): δ = 18.6 (d, J = 11.6 Hz), 22.6 (d, J = 4.7 Hz), 36.8, 51.6 (d, J = 9.3 Hz), 54.3, 62.2 (d, J = 159.5 Hz), 66.8 (d, J = 7.7 Hz), 68.4 (d, 7.7 Hz), 126.6, 127.8, 127.8, 127.9, 128.0, 128.1, 128.1, 128.4, 128.5, 128.5, 128.6, 132.7, 133.5, 140.5.

31P NMR (162 MHz, CDCl3): δ = 31.24.

HRMS (ESI+): m/z [M + H]+ calcd for C26H30NO3P: 436.2036; found: 436.2037.


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2-Butyl-1-phenylpyrrolidine (4a)

Yellow oil.

1H NMR (400 MHz, CDCl3): δ = 0.85 (t, J = 7.2 Hz, 3 H), 1.24–1.37 (m, 5 H), 1.62–1.68 (m, 1 H), 1.73–1.78 (m, 1 H), 1.83–1.99 (m, 3 H), 3.06 (dd, J = 16.0, 9.2 Hz, 1 H), 3.33 (t, J = 6.8 Hz, 1 H), 3.55 (t, J = 6 Hz, 1 H), 6.48 (d, J = 8.4 Hz, 2 H), 6.56 (t, J = 6.8 Hz, 1 H), 7.11–7.18 (m, 2 H).

13C NMR (100 MHz, CDCl3): δ = 14.2, 22.8, 23.5, 28.9, 30.3, 32.8, 48.2, 58.6, 111.7, 115.1, 129.1, 147.3.

HRMS (ESI+): m/z [M + H]+ calcd for C14H21N: 204.1747; found: 204.1750.


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Acknowledgment

We are grateful to the National Institutes of Health for financial support (NIGMS-1R15GM101604-01). We thank Professor Dr. Bernhard Straub (Heidelberg University) for his insights on the reaction mechanism. We acknowledge the support provided by the CREAM Mass Spectrometry Facility (University of Louisville) funded by NSF/EPSCoR grant # EPS-0447479.

Supporting Information

    for this article is available online at http://www.thieme-connect.com.accesdistant.sorbonne-universite.fr/ejournals/toc/synthesis.
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Scheme 1 Tandem protocol for the construction of rings through enamine activation
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Scheme 2 Literature synthesis of cyclic α-aminophosphonate derivatives
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Scheme 3 Activation of enamine by conversion into electrophile
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Figure 1 Copper(I) bromide coordination to an enamine (B3LYP-D3/6-311+G(d,p) with LANL2DZ ECPs for Cu/Br; relative free energy at 298 K in kcal/mol)
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Scheme 4 Proposed mechanism (Nu-H = dialkyl phosphonate)