New chiral amino acid-derived α-acyloxynitroso reagents for asymmetric nitroso Diels-Alder reactions

Article abstract of DOI:10.1016/j.tetasy.2010.05.016

The preparation of new chiral α-acyloxynitroso derivatives 4a-e as chiral dienophiles for the nitroso Diels-Alder reaction is reported. These compounds are obtained from amino acid-derived iodobenzene dicarboxylates and ketoximes, and are stable and easy-

Full text of DOI:10.1016/j.tetasy.2010.05.016

Tetrahedron: Asymmetry 21 (2010) 1507–1510
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
New chiral amino acid-derived
nitroso Diels–Alder reactions
a-acyloxynitroso reagents for asymmetric
*
*
Hailing Li, Didier Gori, Cyrille Kouklovsky , Guillaume Vincent
Institut de Chimie Moléculaire et des Matériaux d’Orsay (UMR CNRS no. 8182), Bâtiment 410, Université de Paris-Sud 11, F-91405 Orsay, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation of new chiral
a-acyloxynitroso derivatives 4a–e as chiral dienophiles for the nitroso
Received 2 March 2010
Accepted 10 May 2010
Available online 12 June 2010
Diels–Alder reaction is reported. These compounds are obtained from amino acid-derived iodobenzene
dicarboxylates and ketoximes, and are stable and easy-to-handle reagents. Initial studies for their nitroso
Diels–Alder reactions with cyclohexadiene are also reported.
Ó 2010 Elsevier Ltd. All rights reserved.
Dedicated to Professor Henri Kagan on the
occasion of his 80th birthday
1. Introduction
exchange.17 It appeared to us that
a-amino acid derivatives would
be good candidates as source of chirality; indeed, the preparation
of amino acid-derived iodobenzene dicarboxylates has been re-
ported in the literature.18 Herein we report the preparation of chi-
Since first reported by Wichterle in 1947,1 the nitroso Diels–Al-
der cycloaddition reaction has been recognized as a valuable tool in
organic synthesis, and has been the subject of many mechanistical
investigations as well as synthetic applications.2 Several nitroso re-
agents such as arylnitroso, chloronitroso, acyl nitroso, or N-nitroso
reagents have been designed as dienophiles for cycloaddition reac-
tions. For some of these reagents, numerous chiral derivatives3,4 or
enantioselective catalytic systems5 have been developed for the
asymmetric nitroso Diels–Alder reaction, with very high levels of
enantioselectivity being achieved. Recently, we have introduced
ral, highly enantiomerically enriched
a-acyloxynitroso reagents
and preliminary results concerning their asymmetric nitroso
Diels–Alder reactions.
2. Results and discussion
According to the literature procedures, acetate exchange of ido-
benzene diacetate with N-phthaloyl aminoacids 1a–d19 derived
from alanine, valine, leucine, and phenylalanine was performed
at 50 °C in xylene under vacuum to give the intermediate chiral
iodobenzene dicarboxylate 2a–d. Additionally, N-Boc proline 1e
was treated under the same conditions to give 2e. The crude oily
iodobenzene dicarboxylates were not isolated but treated directly
with cyclohexanone oxime 3 to give the target nitroso reagents 4a–
e (Scheme 1, Table 1).
Compounds 4a–e were obtained as blue oils after chromato-
graphic purifications and could be stored for several months in a
freezer. HPLC analysis on a chiral column confirmed that all the
products were obtained in high enantiomeric excess (apart from
compound 4e for which enantiomers could not be separated), thus
confirming that the synthetic sequence was performed under mild
conditions and with no significant racemization.
a
-acetoxynitroso derivatives as a new family of nitroso dieno-
philes.6–8 These compounds, which are prepared by the treatment
of ketoximes with iodobenzene diacetate,9 are stable, easy-to-han-
dle, and reactive surrogates for the chloronitroso derivatives. They
react in cycloaddition reactions with a large panel of dienes, either
in anhydrous medium in the presence of a Lewis acid or in aqueous
medium (Fig. 1).6–8
Moreover
a-acyloxynitroso derivatives that were first re-
ported10 in 1956 have been the subject of several pharmacological
studies.11 Most notably, in 2006, King reported these compounds
as a new family of nitroxyl (HNO) donors.12 This is of importance
since the potential of nitroxyl donors in biochemistry has been
well documented13 notably for tubulin polymerization alteration14
or the treatment of heart failure.15
In the search for new
a-acyloxynitroso derivatives for their
We next investigated the possibility of performing diastereose-
pharmacological potential and especially for asymmetric nitroso
Diels–Alder reactions, we were interested in the preparation of chi-
ral nitroso reagents by replacing the acetoxy group with a chiral
carboxylic acid.16 Several reports from the literature mention the
preparation of chiral iodobenzene dicarboxylate by simple ligand
lective synthesis of chiral a-acyloxynitroso derivatives by reaction
of chiral idodobenzene dicarboxylates with prochiral ketoximes
such as 5. This type of reagent would possess a stereogenic center
closer to the nitroso function. Thus, the oxime was treated under
the same conditions with alanine or valine-derived iodobenzene
dicarboxylates 2a and 2b. Disappointingly, compounds 6a and 6b
were obtained as unseparable 1.2/1 mixtures of diastereomers
(Scheme 2). Moreover, the nitroso derivatives were obtained in
* Corresponding authors.
E-mail address: cyrille.kouklovsky@u-psud.fr (C. Kouklovsky).
0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.05.016

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H. Li et al. / Tetrahedron: Asymmetry 21 (2010) 1507–1510
O
N
H3O+
nitroso Diels-Alder
O
AcO
R
O
NH2
N
R
R
AcO
R
α-acetoxy nitroso
Figure 1. Nitroso Diels–Alder reactions of a-acetoxynitroso derivatives.
OH
N
NPht
O
O
N
3
O
PhI(OAc)2,
xylene, 55 ºC, 50 mbar
O
O
O
R
CH2Cl2, rt, 2h
I
R
R
R
O
O
OH
NPht
NPht
NPht
1a-d
2a-d
4a-d
3
O
PhI(OAc)2,
xylene, 55 ºC, 50 mbar
O
CH2Cl2, rt, 2h
O
N
CO2H
N
I
O
O
O
N
Boc
Boc
1e
N
N
O
Boc
Boc
2e
4e
Scheme 1. Synthesis of chiral
a-acyloxynitroso derivatives 4a–e.
low yields. Nevertheless, HPLC analysis of 6a and 6b proved that all
the stereoisomers were enantiomerically pure.
Table 1
Synthesis of chiral a-acyloxynitroso derivatives 4a–e
With a set of chiral
a-acyloxynitroso reagents 4a–e in hand,
Entry
Substrate
R
Product
Yield
ee
their Diels–Alder reactions with 1,3-cyclohexadiene were exam-
ined. The cycloaddition reaction leads to an aminoacetal 7 which
is not isolated, but hydrolyzed to give the bicyclic dihydrooxazine
8 together with the recovered aminoacid 1. Protection of 8 as its
Boc carbamate gave the final compound 9, which was analyzed
for its enantiomeric purity by HPLC (Scheme 3).
1
2
3
4
5
1a
1b
1c
1d
1e
Me
iPr
iBu
PhCH2
4a
4b
4c
4d
4e
53%
61%
74%
71%
56%
>93%
>96%
>96%
>94%
nd
Optimizations were carried out using proline-derived nitroso
dienophile 4e (Table 2). As expected, reactions were faster in protic
solvents especially in water (entry 3), giving better yields of cyc-
loadducts. The same conditions were applied to the other dieno-
philes 4a–e. The results are reported in Table 2.
Although the Boc-protected dihydrooxazine 9 was obtained in
moderate yields, enantiomeric excesses were rather low, indicat-
ing a low diastereoselectivity in the cycloaddition step. Interest-
ingly, cycloadditions with the proline-derived nitroso dienophile
4e gave the opposite enantiomer to those obtained with the other
non-cyclic amino acid-derived compounds. Comparison of specific
NPht
2a or 2b
CH2Cl2, rt, 2h
OH
Me
O
N
R
O
N
Me
1.2/1 mixture
of isomers
O
5
6a: R= Me 24%
6b: R= iPr 15%
Scheme 2. Synthesis of
a
-acyloxynitroso reagents from a prochiral ketoxime.
4a-e
nitroso
Boc2O,
NaOH
Diels-Alder
H3O+
O
O
N
Boc
O
NH2
N
RCO2
7a-e
8
9
R= aminoacid
Scheme 3. Nitroso Diels–Alder reactions of compounds 4a–e with 1,3-cyclohexadiene.

H. Li et al. / Tetrahedron: Asymmetry 21 (2010) 1507–1510
1509
Table 2
134.3, 131.8, 125.1, 123.6, 47.8, 29.2, 29.0, 24.5, 21.4, 21.3, 15.3;
IR (neat): 2942, 2865, 1755, 1722, 1561, 1468, 1451, 1391, 1220,
1147, 1076, 883; 721; HRMS (ESI): calcd: 353.1108 (C17H18N2O5-
Diastereoselective nitroso Diels–Alder reactions of chiral
4a–e with 1,1-cyclohexadiene
a-acyloxynitroso derivatives
Entry
Nitroso
Conditions
Time
Yield
eea
Na; M+Na); found: 353.1115; [a]D = +27.9 (c 0.66, CH2Cl2). HPLC:
ee = 93% (ADH; hexane/EtOH 90/10; 25 °C) rt major: 11.2 min;
minor: 45.2 min.
1
2
3
4
5
6
7
4e
4e
4e
4a
4b
4c
4d
EtOH, rt
6 h
6 h
3 h
3 h
3 h
3 h
3 h
36%
26%
32%
58%
43%
46%
49%
ꢀ10%
ꢀ10%
ꢀ17%
13%
10%
10%
Toluene, rt
H2O, 0 °C–rt
H2O, 0 °C–rt
H2O, 0 °C–rt
H2O, 0 °C–rt
H2O, 0 °C–rt
4.2. (2S)-(10-Nitroso-10-cyclohexyl)-3-methyl-2-phthalimido
butyrate 4b
11%
Yield: 61%. 1H NMR (CDCl3): 7.89 (dd, J = 5.7; 3.2 Hz, 2H), 7.75
(dd, J = 5.7; 3.2 Hz, 2H), 4.78 (d, J = 8.8 Hz, 1H), 2.93–2.79 (m,
1H), 2.09–1.28 (m, 10H), 1.22 (d, J = 6.4 Hz, 3H), 0.95 (d,
J = 7.0 Hz, 3H); 13C NMR (CDCl3): 167.8, 166.6, 134.4, 131.7,
125.1, 123.6, 58.2, 29.4, 28.9, 28.4, 24.5, 21.4, 21.3, 21.1, 19.6; IR
(neat): 2943, 2872, 1761, 1717, 1563, 1467, 1450, 1388, 1192,
1142, 721; HRMS (ESI): calcd: 381.1421 (C19H22N2O5Na; M+Na);
found: 381.1428; [a]D = +86.1 (c 0.55, CH2Cl2). HPLC: ee = 96%
(ADH; hexane/EtOH 95/5; 25 °C) rt major: 6.62 min; minor:
11.44 min.
a
The sign—means that the opposite enantiomer of 9 was obtained.
rotations with literature data2f indicated that cycloadducts ob-
tained from nitroso derivatives 4a–d have the (1R,4S)-configura-
tion for (+)-9 whereas the proline-derived nitroso compound 4e
led to the (1S,4R)-cycloadduct (ꢀ)-9 (Scheme 4).
We are currently investigating the influence of the amino acid
(structure, protecting group) in order to optimize the stereoselec-
tivity and to rationalize the sense of asymmetric induction.
4.3. (2S)-(10-Nitroso-10-cyclohexyl)-4-methyl-2-phthalimido
pentanoate 4c
3. Conclusion
In conclusion, we have prepared a set of new, chiral, amino
acid-derived nitroso reagents. These compounds were found to
be chemically and configurationally stable, and reactive toward
dienes such as 1,3-cyclohexadiene. The scope and applications of
this procedure, as well as mechanistic insights are currently in pro-
gress in our laboratory.
Yield: 74%. 1H NMR (CDCl3): 7.87 (dd, J = 5.7; 3.2 Hz, 2H), 7.74
(dd, J = 5.7; 3.2 Hz, 2H), 5.16 (dd, J = 11.4; 4.4 Hz, 1H), 2.52–2.39
(m, 1H), 2.06–1.27 (m, 12H), 0.99 (d, J = 7.0 H, 3H), 0.97 (d,
J = 6.9 Hz, 3H); 13C NMR (CDCl3): 167.7, 167.6, 134.3, 131.9,
125.0, 51.1, 37.4, 29.3, 29.1, 25.3, 24.6, 23.2, 21.5, 21.4, 21.2; IR
(neat): 2939, 2869, 1754, 1716, 1563, 1470, 1452, 1386, 1244,
1207, 1148, 720; HRMS (ESI): calcd: 395.1577 (C20H24N2O5Na;
4. Experimental
M+Na); found: 395.1589; [a]D = +32.4 (c 0.44, CH2Cl2). HPLC:
ee = 96% (ADH; hexane/EtOH 90/10; 25 °C) rt major: 5.02 min;
minor: 6.09 min.
General procedure for the preparation of a-acycloxynitroso re-
agents 4a–e: to a solution of the protected amino-acid 1a–e
(2.6 equiv) in xylene (10 mL/mmol) was added iodobenzene diace-
tate (1.2 equiv) and the suspension was concentrated at 50–60 °C
under 50 mbar with a rotovapor. The crude chiral iodobenzene
dicarboxylate 2a–e was redissolved in dichloromethane (10 mL/
mmol) and the solution was cooled to 0 °C. Cyclohexanone oxime
3 (1 equiv) was added portionwise over 5 min and the mixture
was stirred 30 min at 0 °C and 2 h at rt. The reaction was quenched
by the addition of a saturated aqueous NaHCO3 solution. The aque-
ous phase was extracted with CH2Cl2 and the combined organic
phases were dried over Na2SO4, filtered, and concentrated. The
blue residue was purified by flash chromatography on silica gel
4.4. (2S)-(10-Nitroso-10-cyclohexyl)-3-phenyl-2-phthalimido
propionate 4d
Yield: 71%. 1H NMR (CDCl3): 7.78 (dd, J = 5.7; 3.2 Hz, 2H), 7.67
(dd, J = 5.7; 3.2 Hz, 2H), 7.24–7.13 (m, 5H), 5.40 (t, J = 8.3 Hz, 1H),
3.67 (d, J = 8.3 Hz, 2H), 2.08–1.30 (m, 10H); 13C NMR (CDCl3):
167.4, 166.6, 136.6, 134.4, 134.2, 131.5, 128.9, 128.8, 128.6,
127.0, 125.4, 123.6, 123.5, 53.5, 34.6, 29.2, 29.0, 24.5, 21.4, 21.3;
IR (neat): 2940, 2864, 1755, 1717, 1562, 1454, 1387, 1232, 1149,
720; HRMS (ESI): calcd: 429.1421 (C23H22N2O5Na; M+Na); found:
429.1421; [
a
]
D = ꢀ76.5 (c 0.44, CH2Cl2). HPLC: ee = 94% (ADH; hex-
(EtOAc/cyclohexane = 5/95–10/90) to give the desired
a-[a-ami-
ane/EtOH 90/10; 25 °C) rt major: 8.67 min; minor: 17.74 min.
noacyloxy]nitroso compound 4a–e as a blue oil.
4.5. (2S)-(10-Nitroso-10-cyclohexyl)-1-tertbutyloxycarbonyl-1-
pyrrolidine-2-carboxylate 4e
4.1. (2S)-(10-Nitroso-10-cyclohexyl)-2-phthalimido propionate 4a
Yield: 53%. 1H NMR (CDCl3): 7.86 (dd, J = 5.6; 3.0 Hz, 2H), 7.73
(dd, J = 5.6; 3.0 Hz, 2H), 5.19 (q, J = 7.2 Hz, 1H), 2.01–1.29 (m,
10H), 1.78 (d, J = 7.2 H, 3H); 13C NMR (CDCl3): 167.5, 167.4,
Yield: 56%. 1H NMR (CDCl3, 2 Boc rotamers): 4.42–4.35 (m, 1H,
Boc rotamers), 3.61–3.35 (m, 2H, Boc rotamers), 2.42–1.45 (m,
14H, Boc rotamers), 1.47 (s, 4.5H, 1 rotamer), 1.45 (s, 4.5H, 1
NPht
O
O
N
O
N
O
O
N
Boc
R
O
4a-d
4e
O
Boc
N
N
O
(-)-9
Boc
(+)-9
Scheme 4. Stereodivergent cycloadditions of nitroso dienophiles 4a–e.

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H. Li et al. / Tetrahedron: Asymmetry 21 (2010) 1507–1510
rotamer); 13C NMR (CDCl3, 2 Boc rotamers): 170.8 (Boc rotamers),
154.4 (1 rotamer), 154.0 (1 rotamer), 124.2 (1 rotamer), 124.0 (1
rotamer), 80.3 (1 rotamer), 79.7 (1 rotamer), 59.3 (1 rotamer),
59.2 (1 rotamer), 46.7 (1 rotamer), 46.4 (1 rotamer), 31.3, 30.3,
29.5, 29.4, 29.3, 29.1, 28.5 (1 rotamer), 28.4 (1 rotamer), 24.7,
24.5, 23.5, 21.7, 21.5, 21.2; IR (neat): 2934, 2861, 1757, 1703,
1450, 1393, 1253, 1164, 1119, 935; HRMS (ESI): calcd: 349.1734
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(C16H26N2O5Na; M+Na); found: 349.1746; [
CH2Cl2).
a]D = ꢀ84.0 (c 0.608,
Acknowledgments
This research was supported by CNRS and Ministère de l0Ens-
eignement Supérieur et de la Recherche. We thank Dr. Denis Bou-
chu (Université Lyon I) for HRMS analyses.
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