Synthesis of unsaturated β-amino acid derivatives from carbamates of the Baylis-Hillman products

Article abstract of DOI:10.1016/S0040-4039(02)00233-2

By treatment with a catalytic amount of DBU in DCM, N-p-toluenesulphonyl carbamates 6a-c, prepared starting from the corresponding Baylis-Hillman adducts, gave (E)-2-(p-toluenesulphonylaminomethyl)propenoates 3a-c, exclusively. On the contrary, changing D

Full text of DOI:10.1016/S0040-4039(02)00233-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2199–2202
Synthesis of unsaturated b-amino acid derivatives from
carbamates of the Baylis–Hillman products
Marco Ciclosi,a Cristiana Fava,a Roberta Galeazzi,a Mario Orenaa,* and Jose´ Sepulveda-Arquesb
aDipartimento di Scienze dei Materiali e della Terra-Universita` di Ancona-Via Brecce Bianche, I-60131 Ancona, Italy
bDepartment of Organic Chemistry, Faculty of Pharmacy-University of Valencia, Avda. Vicent Andre´s Estelles s/n,
46100 Burjassot, Valencia, Spain
Received 15 January 2002; revised 30 January 2002; accepted 1 February 2002
Abstract—By treatment with a catalytic amount of DBU in DCM, N-p-toluenesulphonyl carbamates 6a–c, prepared starting from
the corresponding Baylis–Hillman adducts, gave (E)-2-(p-toluenesulphonylaminomethyl)propenoates 3a–c, exclusively. On the
contrary, changing DABCO for DBU, 2-methylene-3-p-toluenesulfonylamino esters 4a–f were obtained in good yield starting
from N-p-toluenesulphonyl carbamates 6a–f. In analogy, N-acyl carbamates 9a–f were treated with DABCO in DCM to give the
2-methylene-3-acylamino esters 5a–f. © 2002 Elsevier Science Ltd. All rights reserved.
The Baylis–Hillman reaction1 is a very useful process
whereby, via a base-catalyzed tandem Michael reaction-
enolate addition, followed by elimination, 2-methylene-
3-hydroxy alkanoates 1 are formed in a straightforward
manner (Scheme 1).2,3
As part of a program directed to prepare new, non-
proteinogenic amino acids which can induce conforma-
tional restrictions in oligopeptides,7–9 we devised to
prepare amino acid derivatives starting from the prod-
ucts of the Baylis–Hillman reaction. We report herein a
novel, convenient procedure for the preparation of both
unsaturated b-amino acid derivatives, 3 and 4,5 pro-
ceeding with high regioselection (Scheme 2).
The aza version of the reaction, i.e. exchanging the
aldehyde reactant for an aldimine and thus forming
2-methylene-3-aminocarbonyl compounds, 2, was also
reported (Scheme 1).4 In addition, compounds 2 can be
obtained by a simple substitution reaction on the
adducts 1, displacing the alcohol functionality with an
amine.5 However, a loss in selection generally results
when regioisomers are formed by competing SN2% reac-
tions or Michael additions on the allylic substrates.5,6
At first, N-tosyl carbamates 6a–c were prepared, start-
ing from the corresponding Baylis–Hillman adducts
1a–c and tosyl isocyanate (Scheme 3). These com-
pounds were treated in DCM at rt in the presence of a
catalytic amount of DBU (0.2 equiv.), and the corre-
sponding (E)-2-p-toluenesulphonylaminomethyl alken-
oates 3a–c were isolated in good yield,10,11 whose
configuration was determined following literature meth-
ods and NOE experiments.12 The (Z)-isomer which
might be present in trace amounts in the reaction
mixtures was never isolated.
The (E)-selective formation of 3, proceeding via a
tandem SN2%-decarboxylation sequence, can be easily
explained by inspection of molecular models supported
by molecular modeling since conformer A resulted to be
the preferred one for the intermediate anion (Scheme
4).13
Scheme 1.
On the contrary, starting from 6a–f, N-p-toluene-
sulphonylamino derivatives 4a–f were formed exclu-
sively in good yield when DABCO (0.2 equiv.) was
changed for DBU (Scheme 5).14,15
* Corresponding author. Tel.: +39 71 2204720; fax: +39 71 2204714;
e-mail: orena@popcsi.unian.it
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00233-2

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M. Ciclosi et al. / Tetrahedron Letters 43 (2002) 2199–2202
the anion of p-toluenesulphonylamide attacks the inter-
mediate cation to give the observed products 4 (Scheme
6).
Therefore, the reaction mechanism was easily proven
by treating the acetate 7 and p-toluenesulphonylamide
in DCM at rt in the presence of an excess of DABCO,
in order to generate both the intermediate cation and
the amide anion. In fact, under these conditions, the
acetate 7 was converted into the tosylamino derivative
4a in good yield, thus confirming the proposed mecha-
nism (Scheme 7).
Scheme 2.
The results observed on changing DABCO for DBU
can be ascribed to the higher basicity of DBU with
respect to DABCO.16 In fact, when DBU is employed,
the anion formation at the nitrogen is favored with
respect to the nucleophilic attack on the conjugate
double bond by DBU, so that the nucleophile is the
carbamidic nitrogen. On the contrary, DABCO acts as
the nucleophile, exclusively, leading to compounds 4,5
via a tandem SN2%–SN2% process, and competing forma-
tion of products 3, arising from the anion at the
carbamidic nitrogen, was never observed.
Scheme 3.
However, as a further development of this approach,
we devised to prepare amides of carboxylic acids which
can easily afford the corresponding free amino group.
In fact we considered as starting materials the N-acyl
carbamates 9, which can be easily synthesized simply by
treating the appropriate N-acyl isocyanates 8 with the
products 1 obtained from the Baylis–Hillmann reaction.
Thus, a number of N-acyl isocyanates were prepared
following literature methods,17 and directly reacted
without isolation, to give the corresponding N-acyl
carbamates 9 in very good yield. The subsequent reac-
tion of N-acyl carbamates 9, carried out in DCM in the
presence of a catalytic amount of DABCO (0.15
Scheme 4.
Scheme 6.
Scheme 5.
The observed results are due to a reaction mechanism
proceeding via a tandem SN2%–SN2% sequence involving
the initial formation of a quaternary ammonium ion,
followed by elimination of the N-p-toluenesulphonyl-
carbamate anion. Then, after loss of carbon dioxide,
Scheme 7.

M. Ciclosi et al. / Tetrahedron Letters 43 (2002) 2199–2202
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Scheme 8.
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M. Tetrahedron 1999, 55, 4029–4042; (c) Fava, C.;
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equiv.), gave the corresponding amides 5a–e in good
yield. It is worth mentioning that by using this method,
starting from the N-benzyloxycarbonyl carbamate 9f,
the corresponding N-Cbz derivative 5f was also pre-
pared in moderate yield (Scheme 8).18,19
In conclusion, starting from either N-p-toluenesulpho-
nyl 6 or N-acyl carbamates 9, both prepared starting
from the Baylis–Hillman products 1, a convenient
approach to either b-amino acid derivatives 3 and 4,5
was realized simply on changing DABCO for DBU.
The corresponding amino acids will be incorporated in
oligopeptides, with the aim to induce conformational
restrictions. Moreover work is currently underway in
order to prepare 4 and 5 in the enantiomerically pure
form, which can afford bioactive a-hydroxy-b-amino
acids.
Acknowledgements
10. General procedure for preparation of compounds 3: To a
solution containing the N-p-toluenesulphonyl carbamates
6a–c (5 mmol) in DCM (20 ml), DBU (0.15 g; 1 mmol)
was added and the mixture was stirred for 12 h at rt.
Then the mixture was diluted with ethyl acetate (150 ml)
and the organic layer washed with 1 M HCl (30 ml) and
brine (100 ml). After drying (Na2SO4), the solvents were
removed under reduced pressure and the residue was
purified by silica gel chromatography (cyclohexane:ethyl
acetate 80:20 as eluant), to give pure isolated 3a–c.
11. Selected data for compounds 3a–c. Compound 3a: 1H
NMR (200 MHz, CDCl3): 1.30 (t, 3H, J=7.0), 2.43 (s,
3H), 3.95 (d, 2H, J=6.4), 4.21 (q, 1H, J=7.0), 5.21 (t,
1H, J=6.4), 7.27 (d, 2 ArH, J=8.3), 7.40 (m, 5 ArH),
7.68 (d, 2 ArH, J=8.3), 7.74 (s, 1H). 13C NMR (50 MHz,
CDCl3): 14.6, 22.0, 41.1, 61.8, 126.9, 127.2, 127.7, 128.2,
129.0, 129.2, 130.0, 130.1, 134.4, 137.0, 143.7, 143.9,
We gratefully acknowledge financial support from
M.I.U.R. (Rome, Italy-PRIN 2000).
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1
167.7. Compound 3b: H NMR (200 MHz, CDCl3): 1.27
(t, 3H, J=7.1), 2.43 (s, 3H), 3.87 (d, 2H, J=6.6), 4.19 (q,
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2202
M. Ciclosi et al. / Tetrahedron Letters 43 (2002) 2199–2202
2H, J=7.1), 5.32 (t, 1H, J=6.6), 7.27 (d, 2 ArH, J=8.3),
7.35 (m, 4 ArH), 7.67 (s, 1H), 7.68 (d, 2 ArH, J=8.3). 13
55.7, 58.8, 61.4, 114.3, 127.4, 127.7, 128.3, 129.9, 131.3,
138.1, 139.6, 143.7, 159.5, 165.9. Compound 4f: 1H NMR
(200 MHz, CDCl3): 0.83 (d, 3H, J=6.4), 0.85 (d, 3H,
J=6.4), 1.22 (t, 3H, J=7.1), 1.31–1.67 (m, 3H), 2.40 (s,
3H), 4.05 (q, 2H, J=7.1), 4.05–4.19 (m, 1H), 5.38 (d, 1H,
NH, J=8.9), 5.48 (s, 1H), 5.91 (s, 1H), 7.23 (d, 2 ArH,
J=8.4), 7.65 (d, 2 ArH, J=8.4). 13C NMR (50 MHz,
CDCl3): 14.5, 21.9, 22.2, 22.9, 44.9, 55.3, 61.2, 127.1,
127.5, 127.7, 129.8, 138.5, 139.8, 143.5, 165.9.
C
NMR (50 MHz, CDCl3): 14.7, 22.0, 41.0, 62.0, 127.6,
127.7, 128.5, 129.5, 130.2, 131.3, 132.9, 136.0, 136.8, 142.3,
144.1, 167.4. Compound 3c: 1H NMR (200 MHz, CDCl3):
1.30 (t, 3H, J=7.2), 2.34 (s, 3H), 4.03 (d, 2H, J=6.2), 4.25
(q, 2H, J=7.2), 5.32 (t, 1H, NH, J=6.2), 7.12–8.08 (m, 11
ArH+1 H). 13C NMR (50 MHz, CDCl3): 14.7, 22.0, 41.3,
61.9, 127.1, 127.3, 127.7, 128.0, 128.1, 128.5, 128.8, 128.9,
129.3, 129.9, 130.1, 130.4, 131.9, 133.6, 133.9, 136.9, 143.8,
143.9, 167.8.
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Chem. 1987, 65, 996–1001; (b) Leffek, K. T.; Pruszynski,
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3742–3743; (b) Speziale, A. J.; Smith, L. R. J. Org. Chem.
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12. A similar behavior was observed when alkyl carbamates of
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14. General procedure for preparation of compounds 4: To a
solution containing the N-p-toluenesulphonyl carbamates
6a–f (5 mmol) in DCM (20 ml), DABCO (0.11 g; 1 mmol)
was added and the mixture was stirred for 12 h at rt. Then
the mixture was diluted with ethyl acetate (150 ml) and the
organic layer washed with 1 M HCl (30 ml) and brine (100
ml). After drying (Na2SO4), the solvents were removed
under reduced pressure and the residue was purified by
silica gel chromatography (cyclohexane:ethyl acetate 80:20
as eluant), to give pure isolated 4a–f.
18. General procedure for preparation of compounds 5: To a
solution containing the N-acyl carbamates (9) (5 mmol) in
DCM (20 ml), DABCO (0.11 g; 1 mmol) was added and
the mixture was stirred for 12 h at rt. Then the mixture was
diluted with ethyl acetate (150 ml) and the organic layer
washed with 1 M HCl (30 ml) and brine (100 ml). After
drying (Na2SO4), the solvents were removed under
reduced pressure and the residue purified by silica gel
chromatography (cyclohexane:ethyl acetate 80:20 as elu-
ant), to give pure isolated 5.
15. Selected data for compounds 4a–f. Compound 4a: 1H
NMR (200 MHz, CDCl3): 1.15 (t, 3H, J=7.0), 2.40 (s,
3H), 4.04 (q, 2H, J=7.0), 5.30 (d, 1H, J=8.8), 5.69 (d, 1H,
NH, J=8.8), 5.82 (s, 1H), 6.23 (s, 1H), 7.14–7.33 (m, 7
ArH), 7.68 (d, 2 ArH, J=8.4). 13C NMR (50 MHz,
CDCl3): 14.4, 22.0, 59.5, 61.5, 126.9, 127.0, 127.7, 128.0,
128.1, 128.7, 128.9, 129.0, 129.9, 130.0, 130.1, 138.1, 139.2,
19. Selected data for compounds 5a–f. Compound 5a: 1H
NMR (200 MHz, CDCl3): 1.22 (t, 3H, J=7.2), 4.16 (q,
2H, J=7.2), 6.00 (s, 1H), 6.24 (d, 1H, J=8.8), 6.41 (s, 1H),
7.21–7.57 (m, 9H, 8 ArH+NH), 7.81–7.89 (m, 2 ArH). 13
C
NMR (50 MHz, CDCl3): 14.5, 55.8, 61.6, 126.9, 127.6,
128.1, 129.1, 132.2, 134.7, 139.7, 140.2, 166.6, 166.8.
1
Compound 5b: H NMR (200 MHz, CDCl3): 1.24 (t, 3H,
1
139.3, 143.8, 165.8. Compound 4b: H NMR (200 MHz,
J=7.1), 3.78 (s, 3H), 4.17 (q, 2H, J=7.1), 5.97 (s, 1H),
6.17 (d, 1H, J=8.8), 6.38 (s, 1H), 6.86 (d, 2 ArH, J=8.8),
7.26 (d, 2 ArH, J=8.8), 7.33–7.59 (m, 4H, 3 ArH+NH),
7.80–7.85 (m, 2 ArH). 13C NMR (50 MHz, CDCl3): 14.5,
55.3, 55.7, 61.5, 114.5, 127.4, 127.5, 128.2, 129.1, 132.1,
132.3, 134.7, 139.9, 159.5, 166.7. Compound 5c: 1H NMR
(200 MHz, CDCl3): 1.24 (t, 3H, J=7.2), 4.18 (q, 2H,
J=7.2), 5.92 (d, 1H, J=8.9), 6.00 (s, 1H), 6.44 (s, 1H),
7.21–7.41 (m, 5 ArH), 8.14 (d, 1H, NH, J=8.9). 13C NMR
(50 MHz, CDCl3): 14.4, 57.3, 61.8, 126.7, 127.2, 128.5,
128.7, 129.0, 129.3, 138.4, 138.6, 161.5, 166.1. Compound
5d: 1H NMR (200 MHz, CDCl3): 1.24 (t, 3H, J=7.1), 3.79
(s, 3H), 4.17 (q, 2H, J=7.1), 5.86 (d, 1H, J=8.8), 5.96 (s,
1H), 6.41 (s, 1H), 6.88 (d, 2 ArH, J=8.8), 7.21 (d, 2 ArH,
J=8.8), 8.06 (d, 1H, NH, J=8.8). 13C NMR (50 MHz,
CDCl3): 14.5, 55.7, 56.9, 61.8, 114.7, 128.0, 128.4, 130.6,
CDCl3): 1.11 (t, 3H, J=7.2), 2.37 (s, 3H), 4.01 (q, 2H,
J=7.2), 5.29 (d, 1H, J=9.2), 5.78 (s, 1H), 6.17 (d, 1H, NH,
J=9.2), 6.18 (s, 1H), 7.04–7.20 (m, 6 ArH), 7.62 (d, 2 ArH,
J=8.3). 13C NMR (50 MHz, CDCl3): 14.4, 21.9, 58.5,
61.5, 127.6, 128.0, 128.7, 128.9, 129.9, 133.8, 138.0, 139.2,
143.9, 165.6. Compound 4c: 1H NMR (200 MHz, CDCl3):
1.11 (t, 3H, J=7.2), 2.30 (s, 3H), 4.02 (q, 2H, J=7.2), 5.53
(d, 1H, J=9.1), 5.90 (s, 1H), 6.14 (d, 1H, NH, J=9.1), 6.29
(s, 1H), 7.11 (d, 2 ArH, J=8.4), 7.22–7.28 (m, 1 ArH),
7.37–7.48 (m, 2 ArH), 7.54–7.58 (m, 1 ArH), 7.61–7.71 (m,
5 ArH). 13C NMR (50 MHz, CDCl3): 14.4, 21.9, 59.5,
61.5, 125.1, 126.2, 126.6, 126.7, 127.7, 128.0, 128.5, 128.8,
129.9, 133.2, 133.5, 136.5, 138.1, 139.4, 143.8, 165.8.
1
Compound 4d: H NMR (200 MHz, CDCl3): 1.10 (t, 3H,
J=7.1), 2.33 (s, 3H), 4.00 (q, 2H, J=7.1), 5.38 (d, 1H,
J=9.5), 5.81 (s, 1H), 6.20 (s, 1H), 6.36 (d, 1H, NH,
J=9.5), 7.18 (d, 2 ArH, J=8.3), 7.35 (d, 2 ArH, J=8.7),
7.63 (d, 2 ArH, J=8.3), 8.01 (d, 2 ArH, J=8.7). 13C NMR
(50 MHz, CDCl3): 14.3, 21.9, 58.8, 61.8, 123.9, 124.2,
127.6, 128.1, 129.1, 130.1, 130.2, 130.7, 137.8, 138.4, 144.2,
1
138.6, 159.8, 161.4, 166.2. Compound 5e: H NMR (200
MHz, CDCl3): 1.22 (t, 3H, J=7.2), 1.25 (s, 9H), 4.13 (q,
2H, J=7.2), 5.90 (s, 1H), 5.99 (d, 1H, J=8.3), 6.35 (s, 1H),
6.93 (d, 1H, NH, J=8.3), 7.21–7.42 (m, 5 ArH). 13C NMR
(50 MHz, CDCl3): 14.4, 27.9, 39.2, 55.2, 61.4, 126.7, 128.4,
1
1
146.8, 147.6, 165.3. Compound 4e: H NMR (200 MHz,
129.0, 129.1, 139.9, 140.3, 166.4, 177.9. Compound 5f: H
CDCl3): 1.15 (t, 3H, J=7.1), 2.40 (s, 3H), 3.74 (s, 3H), 4.05
(q, 2H, J=7.1), 5.25 (d, 1H, J=8.7), 5.66 (d, 1H, NH,
J=8.7), 5.80 (s, 1H), 6.19 (s, 1H), 6.74 (d, 2 ArH, J=8.8),
7.04 (d, 2 ArH, J=8.8), 7.23 (d, 2 ArH, J=8.3), 7.67 (d,
2 ArH, J=8.8). 13C NMR (50 MHz, CDCl3): 14.4, 22.0,
NMR (200 MHz, CDCl3): 1.18 (t, 3H, J=7.1), 4.12 (q,
2H, J=7.1), 5.15 (s, 2H), 5.78 (d, 1H, J=8.8), 5.83 (d, 1H,
NH, J=8.8), 6.39 (s, 1H), 7.25–7.43 (m, 10 ArH). 13C
NMR (50 MHz, CDCl3): 14.5, 57.2, 61.4, 67.5, 126.7,
127.8, 127.9, 128.8, 129.0, 139.8, 140.4, 166.5, 177.9.