A versatile and efficient approach for the synthesis of chiral 1,3-nitroamines and 1,3-diamines via conjugate addition to new (S,E)-Γ-aminated nitroalkenes derived from L-α-amino acids

Article abstract of DOI:10.3762/bjoc.9.95

New chiral (S,E)-γ-N,N-dibenzylated nitroalkenes 2a-c were synthesized from natural L-(α)-amino acids in five steps with overall yields of 68-88%. The conjugate addition of hydride, methoxide, nitronate and azide nucleophiles to 2a-c led to the corresponding chiral 1,3-nitroamines in 74-90% yield. The conjugate addition of cyanide anion to 2a,b was followed by HNO₂ elimination affording chiral aminated acrylonitriles (73-98%). On the other hand, the azide anion reacted with 2a, in acetonitrile, via a [3 + 2]-cycloaddition in which HNO₂ was lost, providing the corresponding 1,2,3-triazole derivative. Direct reduction of 1,3- nitroamine derivatives 9a,b produced the corresponding 1,3-diamines in good yields.

Full text of DOI:10.3762/bjoc.9.95

A versatile and efficient approach for the synthesis of
chiral 1,3-nitroamines and 1,3-diamines via
conjugate addition to new (S,E)-γ-aminated
nitroalkenes derived from L-α-amino acids
Vera Lúcia Patrocinio Pereira*1,§, André Luiz da Silva Moura1,
Daniel Pais Pires Vieira1, Leandro Lara de Carvalho1,
Eliz Regina Bueno Torres1 and Jeronimo da Silva Costa1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 832–837.
1Núcleo de Pesquisas de Produtos Naturais, Laboratório de Síntese
Estereosseletiva de Substâncias Bioativas, Universidade Federal do
Rio de Janeiro, 21941-902, Rio de Janeiro, Brazil and 2Instituto
Federal de Educação, Ciência e Tecnologia do Rio de Janeiro,
26530-060, Nilópolis, RJ, Brazil
doi:10.3762/bjoc.9.95
Received: 20 January 2013
Accepted: 08 April 2013
Published: 30 April 2013
Email:
Associate Editor: B. Stoltz
Vera Lúcia Patrocinio Pereira* - patrocinio@nppn.ufrj.br
© 2013 Pereira et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
§ Tel.: +55 21 2562 6792; Fax: +55 21 2562 6512
Keywords:
amino alcohol; amino aldehyde; azide addition; Baylis–Hillman
reaction; cyanide addition; Michael addition
Abstract
New chiral (S,E)-γ-N,N-dibenzylated nitroalkenes 2a–c were synthesized from natural L-(α)-amino acids in five steps with overall
yields of 68–88%. The conjugate addition of hydride, methoxide, nitronate and azide nucleophiles to 2a–c led to the corresponding
chiral 1,3-nitroamines in 74–90% yield. The conjugate addition of cyanide anion to 2a,b was followed by HNO2 elimination
affording chiral aminated acrylonitriles (73–98%). On the other hand, the azide anion reacted with 2a, in acetonitrile, via a
[3 + 2]-cycloaddition in which HNO2 was lost, providing the corresponding 1,2,3-triazole derivative. Direct reduction of 1,3-
nitroamine derivatives 9a,b produced the corresponding 1,3-diamines in good yields.
Introduction
Nitroalkenes constitute a class of organic compounds that Baylis–Hillman reactions [15-18]. Furthermore, nitroalkenes
present exceptional versatility in organic synthesis [1-4]. They can react as dipolarophiles in [3 + 2] cycloadditions [19-22], as
are reactive in Michael reactions with a wide variety of nucleo- dienophiles in [4 + 2] cycloadditions [23-26], and as heterodi-
philes [5-10], in Friedel–Crafts alkylations [11-14], and enes in hetero-Diels–Alder reactions [27-31] and even partici-
832

Beilstein J. Org. Chem. 2013, 9, 832–837.
pate in cross-coupling reactions [32]. Due to this notable re- Results and discussion
activity, obtaining new chiral and achiral nitroalkenes is of great The preparation of 2a–c (Scheme 1) was initiated with the syn-
importance in synthetic organic chemistry. Achiral nitroalkenes, thesis of Reetz’s α-aminoaldehydes [69,70]. In this manner, the
specifically the chemically stable β-trans-nitrostyrene and its α-amino acids 1a–c were perbenzylated using a slight modifica-
phenyl-substituted analogues, besides the heteroaromatic tion in the experimental procedures described by Beaulieu [71]
nitroalkenes, have been employed widely in highly enantiose- and Warren [72] leading to tribenzylated esters 3a–c in 90–99%
lective syntheses promoted by chiral metallic catalysts [33-37] isolated yield. Next, reduction of 3a–c with LiAlH4 provided
and most recently by a great number of different chiral the desired alcohols 4a–c (87–95% yield), which were oxidized
organocatalysts [38-41] or combinations of both [42-44]. under Swern conditions to aldehydes 5a–c in 96–99% yield.
Nonetheless, the use of chiral nitroalkenes bearing a stereo- These were submitted to a Henry reaction, according to the
genic center in the γ-position in nonracemic diastereoselective modified conditions developed by Hanessian [73]. Thus, 5a–c
syntheses ("chiron" approach [45]) is less frequent in the litera- were reacted with nitromethane, at room temperature (instead of
ture. Notably, chiral nitroalkenes derived from carbohydrates 0 °C) in the presence of 0.2 equivalents of TBAF·3H2O (instead
[46-50] have been most often utilized as "chirons" in diastereo- of 1 equivalent) as base, furnishing an anti:syn-nitroalcohol
selective syntheses. Most nitroalkenes are synthesized from a mixture 6a–c in 91–95% yield and 68–86% d.e. The activation
nitroaldol product followed by elimination of the activated of the hydroxy groups of 6a–c by transformation into the
hydroxy group in the sulfonate or acetate form [51] although corresponding mesylates was followed by smooth elimination at
various routes have often been applied [52-54]. On the other −78 °C, leading to the desired nitroalkenes 2a–c in 95–99%
hand, chiral nitroamines [55,56] are important intermediates yield, as single E-diastereoisomers. It is worth mentioning that
directly transformed, via reduction of the nitro group, into di- the nitroalkenes 2a–c are obtained in high chemical and stereo-
amines [57] or via a Nef reaction in amino acids [58]. Specifi- chemical purity and can be stored in a freezer or even at room
cally, chiral 1,2-nitroamines have been mainly accessed by temperature for several months without decomposition occur-
direct nucleophilic addition of nitronate anions to imine deriva- ring.
tives by the use of chiral metal and organocatalysts [55,56]. In
contrast, chiral 1,3-nitroamines, despite constituting direct In order to verify the enantiomeric purity of the new
precursors of chiral 1,3-diamines, are scarcely synthesized by nitroalkenes 2a–c and to confirm their stereochemical stability,
conjugate addition of nitroalkanes to nitroalkenes. Chiral 1,3-di- a chiral HPLC analysis was conducted. Racemic (+/−)-2b was
amines present extraordinary opportunities in enantioselective synthesized from (D/L)-phenylalanine by the same route
synthesis as chiral auxiliaries, chiral catalysts, chiral reagents employed for (−)-2b and utilized as standard (Figure 1). The
and chiral ligands [59,60]. In our continuing interest in synthe- racemic nitroalkene (+/−)-2b showed a very good separation
sizing new chiral nonracemic nitro compounds [61-68] by a factor in the chromatograph chiral column used. Figure 1 shows
"chiron" approach [45], we intended to develop a route for that (−)-2b prepared from L-phenylalanine was enantiomeri-
obtaining γ-nitrogenated chiral nonracemic nitroalkenes 2a–c cally pure (enantiomeric excess > 99%).
from L-leucine (1a), L-phenylalanine (1b) and L-alanine (1c),
respectively, and investigate their reactivities in conjugate addi- HPLC analysis showed that the stereochemical integrity of 2
tions with different nucleophiles as a way of yielding chiral was preserved throughout the synthetic process. Next, the re-
nonracemic 1,3-nitroamine and its corresponding 1,3-diamine activity of the new chiral nitroalkenes was investigated with
derivatives.
respect to the conjugate addition of hydride, nitronate,
Scheme 1: Reagents and conditions: (i) K2CO3, EtOH/H2O 5:1, BnBr, 100 °C, 12 h (90–99%). (ii) (a) LiAlH4, THF dry, 100 °C, 8 h. (b) NaOHaq
(87–95%). (iii) (a) (COCl)2, CH2Cl2, DMSO, −78 °C, 30 min, then TEA, −78 °C to rt (96–99%). (iv) TBAF·3H2O (0.2 equiv), CH3NO2, THFdry, rt, 4 h
(91–95%). (v) (a) MsCl, CH2Cl2, −78 °C, then TEA 60 min, rt (95–99 %).
833

Beilstein J. Org. Chem. 2013, 9, 832–837.
Figure 1: Chiral HPLC analysis: Chiralpak AD-H; n-hexane/2-propanol 99:1 (0.6 mL/min), T = 25 °C; UV–vis detection at λ = 254 nm; retention times:
(+/−)-2 (11.13 and 11.60 min) and (−)-2b (10.99 min).
methoxide, cyanide, and azide anions in addition to benzyl- 2a,b led to the corresponding 2-nitromethyl-1,3-nitroamine
amine aiming to obtain directly 1,3-nitroamine derivatives. All derivatives 8a,b in good yields (Table 1, entries 2 and 3). Other
the experiments were carried out at room temperature and the nitronate anions also reacted adequately. On the other hand, a
results are summarized in Table 1.
literature search showed that the addition of an oxygenated
nucleophile to nitroalkenes has been scarcely studied due to the
The selective conjugate addition of hydride anion to 2b low reactivity of neutral nucleophiles associated with the reac-
produced the chiral 1,3-nitroamine 7b in 85% yield [74] tion reversibility [46,75,76]. Addition of methoxide anion to
(Table 1, entry 1). Similarly, the addition of nitromethane to 2a–c led to the corresponding chiral 2-methoxy-1,3-nitroamine
Table 1: Reactivity of 2a–c with different nucleophiles.
entry
nitroalkene
Nu
base/solvent/time (h)
adduct
yield (%)
anti/syna
1
2
2b
2a
2b
2a
2b
2a
2b
2c
2a
2b
2b
2c
2a
2a
2b
NaBH4
CH3NO2
CH3NO2
LiOMeb
–/CHCl3–iPrOH (16:3)/1
DBU 0.3 equiv/CH3CN/12
TBAF·3H2O/THF/8
–/MeOH/0.5
7b
8a
85
86
80
90
85
90
70
77
98
73
76
73
74
–
–
–
3
8b
–
4
9a
94:6
94:6
50:50
57:43
69:31
–
5
LiOMeb
–/THF/0.5
9b
6
LiOMeb
–/MeOH/24
9a
7
LiOMeb
–/MeOH/20
9b
8
LiOMeb
–/MeOH/12
9c
9
TMSCN
TMSCN
TMSN3
TBAF·3H2O/CH3CN/6
TBAF·3H2O/CH3CN/12
TBAF·3H2O/THF/16
TBAF·3H2O/THF/20
TBAF·3H2O/CH3CN/16
–/–/48
12a
12b
10b
10c
11a
–c
10
11
12
13
14
15
–
86:14
74:26
–
TMSN3
TMSN3
BnNH2 excess
BnNH2
–
DBU 0.5 equiv/CH3CN/8
–c
–
–
aDiastereoisomeric ratio. bSolution 1 M in MeOH. cNo reaction observed.
834

Beilstein J. Org. Chem. 2013, 9, 832–837.
derivatives 9a–c in 70–90% yield (Table 1, entries 4–8). A high Finally, the chiral 1,3-nitroamines 9a,b were easily trans-
d.r. was obtained when shorter reaction times were employed formed into the desired chiral 1,3-diamines 14a,b (80% yield)
(Table 1, entries 4 and 5). On the other hand, a severe decrease by treatment with NaBH4, NiCl2·6H2O in MeOH [87]
in the diastereoselectivity occurred with longer durations, (Scheme 2).
presumably due to an equilibration by a retro-conjugate addi-
tion [77] (Table 1, entries 6–8). Next, the conjugate cyanide ad-
dition to 2a,b promoted by TMSCN 1 equiv/TBAF·3H2O
1 equiv led to 12a,b in 98 and 73% respectively, instead of
2-cyano-1,3-nitroamines derivatives (Table 1, entries 9 and 10).
Products 12a,b were likely formed from the conjugate addition
of cyanide to 2a,b followed by HNO2 elimination. It is worthy
of note that the Michael addition of “HCN” equivalent to
Scheme 2: Synthesis of 1,3-diamines 14a,b. (i) NaBH4/NiCl2·6H2O/
MeOH/3 h/rt.
nitroalkenes is rare in the literature [78-83]. Paulsen [78] and
Sakakibara [79] were the first authors to report the cyanide ad-
dition to α,β-unsaturated nitroalkenes. In agreement with our
results, Sakakibara [79] and Spanevello [80] observed a total or Conclusion
partial HNO2 elimination from vicinal nitronitrile adducts. This An efficient, reproducible and versatile approach for the syn-
behavior was not observed in other findings reported [78,81-83] thesis of chiral nonracemic 2-substituted-1,3-nitroamines was
and the corresponding vicinal nitronitriles were the sole prod- developed. The high chemical and stereochemical stability of
ucts. It is important to note that 12a,b can in theory also be the new chiral nitroalkenes 2a–c and the variety of 1,3-
obtained by an aza-Baylis–Hillman reaction [15-18,68]. Thus, nitroamines synthesized provide robustness and generality to
this cyanide Michael addition to 2a,b constitutes a synthetic the methodology. Since the nitroamines 9a,b were efficiently
alternative to the cases where N-protected aza-Baylis–Hillman reduced to the corresponding chiral 2-substituted-1,3-diamines
adducts need to be obtained. Interestingly, the addition of 14a,b, the present route constitutes a versatile access to these
“HN3” promoted by TMSN3 1 equiv/TBAF·3H2O 1 equiv important classes of substances in high yields.
furnished either the 2-azido-1,3-nitroamine-derivatives 10b,c
(Table 1, entries 11 and 12) or the triazole derivative 11a
Supporting Information
(Table 1, entry 13) when THF or CH3CN were used as solvents,
respectively. The formation of chiral 1,2,3-triazole derivative
Supporting Information File 1
11a was proposed to be by way of a [3 + 2]-cycloaddition with
the azide group acting as 1,3-dipolarophile, followed by HNO2
elimination leading to aromatization. There are a few reports in
the literature where the azide anion is added to α,β-unsaturated
nitroalkenes [8,19-22] forming 1,4-adducts [8] or 1,2,3-triazole
derivatives [31-33]. Apparently, the reaction course depends on
the temperature and concentration of the azide reagent used. In
our case, we observed a dependence on the nature of the
solvent. However, an influence of the structure of the
nitroalkenes has not been independently investigated. Further
experiments must be performed for a better understanding of
the reaction course. On the other hand, the neat benzylamine ad-
General information, experimental procedure and
spectroscopic data of 3a–c, 6a–c, 2a–c, 7b, 8a, 8b, 11a,
10b–c, 9a–b, 12a, 14a,b.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-95-S1.pdf]
Supporting Information File 2
NMR, IR and MS spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-95-S2.pdf]
dition, or in the presence of DBU (0.5 equiv) as promoter [84], Acknowledgements
to 2a,b was unsuccessful (Table 1, entries 14 and 15). This We thank FAPERJ for the financial support and CNPq and
nonreactivity can be explained on the basis of the high ten- CAPES for the fellowship for some authors. We also thank Dr
dency toward reversibility presented in conjugate additions of C. B. Barreto, Jr. for assistance with the graphical layout.
amines to reactive nitroalkenes [15,85,86]. As a way of testing
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