Novel route to N-acyl-N′-formyl aminal

Article abstract of DOI:10.1080/00397910802398256

Oxidative decarboxylation of N-acyl amino acids induced by Ag+/S2O82- in formamide afforded N-acyl-N′-formyl aminal in 52-74% yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802398256

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Novel Route to N-Acyl-N′-
formyl Aminal
Hong Yue a , Hongmei Zhang b , Meiling Wang a &
Wenhua Huang a
a Department of Chemistry , Tianjin University ,
Tianjin, China
b Light Industry Branch of Shanxi General Vocational
College , Taiyuan, China
Published online: 08 Jan 2009.
To cite this article: Hong Yue , Hongmei Zhang , Meiling Wang & Wenhua Huang
(2009) Novel Route to N-Acyl-N′-formyl Aminal, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 39:3,
484-491, DOI: 10.1080/00397910802398256
To link to this article: http://dx.doi.org/10.1080/00397910802398256
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Synthetic Communications1, 39: 484–491, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802398256
Novel Route to N-Acyl-N0-formyl Aminal
Hong Yue,1 Hongmei Zhang,2 Meiling Wang,1 and Wenhua Huang1
1Department of Chemistry, Tianjin University, Tianjin, China
2Light Industry Branch of Shanxi General Vocational College, Taiyuan, China
Abstract: Oxidative decarboxylation of N-acyl amino acids induced by Agþ=
S2O28ꢀ in formamide afforded N-acyl-N0-formyl aminal in 52–74% yields.
Keywords: Aminal, formamide, N-acyl amino acid, oxidative decarboxylation,
persulfate, silver nitrate
Incorporation of an aminal moiety into a peptide chain, resulting in the
so-called retro-inverso peptide, is one of the most widely used peptidomi-
metic approaches for the design of novel bioactive molecules.[1–4] For
synthesis of aminal derivatives starting from amino acids, the most com-
monly used methods include Curtius[5–11] or Hofmann[12–15] rearrange-
ments. However, these methods require preconversion of amino acids to
a-aminoacyl azides or a-aminoamides and use expensive reagents such as
diphenyl phosphorazidate and bis(trifluoroacetate) iodobenzene. Other
methods, such as displacement of a-heteroatom of protected amino
acids[16–19] and benzotriazole-mediated Mannich reaction,[20] are also
known but require multiple-step manipulation.
In the course of our studies[21,22] on oxidative decarboxylation of 1
induced by Agþ=S2O82ꢀ using water as solvent, we found that N-acyl
amino acids can be converted into imides through hydrolysis and oxi-
dation of iminium 4 formed from oxidation of 1-amidoalkyl radical 3
(Scheme 1). If the reaction is carried out in formamide instead of water,
addition of formamide to iminium 4 will provide a facile access to N-acyl-
N0-formyl aminal 2 . Here we report our results on oxidative decarboxy-
lation of N-acyl amino acids in formamide.
Received April 21, 2008.
Address correspondence to Wenhua Huang, Department of Chemistry, Tianjin
University, 92 Weijin Road, Tianjin 300072, China. E-mail: huangwh@tju.edu.cn
484

Novel Route to N-Acyl-N0-formyl Aminal
485
Scheme 1. Oxidative decarboxylation of N-acyl amino acids in formamide.
When hippuric acid (1a) was treated with silver nitrate (10 mol%)
and 2 equiv of ammonium persulfate at 50 ꢁC in formamide, gas evolving
was observed, and the reaction completed within 30 min. After work-up,
the aminal product 2a was isolated in 70% yield (Table 1, entry 1). Its
1
structure was confirmed by H and 13C NMR as well as high resolution
mass spectrum (HRMS). Both electron-withdrawing and electron-donat-
ing substituents at the 4- or 2-position of benzoyl moiety were tolerated
(entries 2–5). Substrates bearing Cl and Br at the 4-position gave 2b and
2c in 67% and 57%, respectively. Introduction of an electron-donating
Table 1. Synthesis of N-acyl-N0-formyl aminala
Entry
R1
R2
1
2
Yieldb of 2 (%)
1
2
3
4
5
6
7
8
9
Ph
4-ClC6H4
4-BrC6H4
4-MeC6H4
2-MeOC6H4
Ph
H
H
H
H
H
Me
i-Pr
i-Bu
H
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
70
67
57
68
74
71
64
64
52
Ph
Ph
BnO
1g
1h
1i
2g
2h
2i
aReaction conditions: 1 (0.4 mmol), AgNO3 (0.04 mmol), and (NH4)2S2O8
(0.8 mmol), formamide (0.5 mL), 50 ꢁC, 30 min.
bIsolated yields by preparative TLC.

486
H. Yue et al.
group as in entries 4 and 5 led to a similar yield (68%) of 2d and even a
higher yield (74%) of 2e. Encouraged by these findings, we further
probed the versatility of the reaction for amino acids. Substrates derived
from alanine, valine, and leucine gave the corresponding products 2f, 2g,
and 2h in 71%, 64%, and 64% yields, respectively. Finally, N-carboben-
zyloxyglycine (1i) also gave 2i in a lower yield (52%) but attempts to
extend the reaction of N-carbobenzyloxy to other amino acids failed.
In summary, we have developed a novel route to N-acyl-N0-formyl
aminal by addition of formamide to iminium generated in situ from oxi-
dative decarboxylation of N-acyl amino acids induced by Agþ=S2O82ꢀ
.
This methodology features displacement of a carboxyl group of N-acyl
amino acid with a formamido group in a single step by using inexpensive
reagents. Further studies on transformation of the C-terminal amino acid
of a peptide into an aminal moiety using this method are in progress.
EXPERIMENTAL
Compounds 1b–i were prepared according to our previous proce-
dure.[21,22] Hippuric acid (1a) was purchased from GuangFu Chemical
Reagent Company, China. 1H and 13C NMR spectra were recorded on a
Varian Inova 500 NMR spectrometer. IR spectra were recorded on a
Thermo Nicolet Avatar 360 IR spectrometer. Electrospray ionization
mass spectra were obtained on an LCQ Advantage MAX (Finnigan)
instrument operating in a positive ion mode. Elemental analyses were
performed on a Heraeus Vanio-EL CHN analyzer. All melting points
were measured on a melting apparatus with a microscope and hot stage
and were uncorrected. Ammonium persulfate, silver nitrate, and forma-
mide were purchased from Jiangzhong Chemical Reagent Company,
China, and used as received. All reactions were carried out under nitrogen.
Typical Procedure
To a round-bottom flask (25 mL) with a magnetic stirrer, 1a (0.4 mmol),
AgNO3 (0.04 mmol), and (NH4)2S2O8 (0.8 mmol) were added. The flask
was protected under nitrogen and put in a preheated water bath
(50 ꢁC). Then formamide (0.5 mL) was added to the flask by a syringe,
and gas immediately evolved. The reaction mixture was extracted with
acetone (4 ꢂ 2 mL) after stirred vigorously for 30 min. The combined
extracts was evaporated in vacuo and separated by preparative thin-layer
chromatography (TLC) (CH2Cl2=MeOH,10=1, v=v) to give N-(formami-
domethyl)-benzamide (2a) as a white solid (50.1 mg, 70%); mp 147–149 ꢁC;
IR (KBr): 1671, 1639 cmꢀ1; 1H NMR (500MHz, DMSO-d6) of rotamer A

Novel Route to N-Acyl-N0-formyl Aminal
487
1
(3.2:1 mixture of two rotamers based on H NMR integration): d ¼ 4.63–
4.67 (m, 2 H), 7.43–7.49 (m, 2 H), 7.50–7.55 (m, 1 H), 7.87–7.90 (m, 2 H),
8.03 (d, 1 H, J ¼ 1.5Hz), 8.61–8.65 (m, 1 H), 9.17 (t, 1 H, J ¼ 6.0Hz); rota-
mer B: d ¼ 4.60–4.63 (m, 2 H), 7.43–7.49 (m, 2 H), 7.50–7.55 (m, 1 H), 7.84–
7.87 (m, 2 H), 8.20 (d, 1 H, J ¼ 10.5 Hz), 8.20–8.24 (m, 1 H), 9.13 (t, 1 H,
J¼ 5.5Hz); 13C NMR (125.7 MHz, DMSO-d6) of rotamer A: d ¼ 43.4,
128.0, 129.0, 132.2, 134.4, 161.9, 167.5; rotamer B: d ¼ 47.6, 127.9, 129.1,
132.3, 134.5, 166.4, 167.2; HRMS-ESI (positive mode): m=z [M þ Na]þ
calcd. for C9H10N2NaO2: 201.0634; found: 201.0634.
4-Chloro-N-(formamidomethyl)benzamide (2b)
A white solid; yield: 67%; mp 158 ꢁC (dec.); IR (KBr) 1697, 1643 cmꢀ1
;
1H NMR (500 MHz, DMSO-d6) of rotamer A (3.5:1 mixture of two rota-
1
mers based on H NMR integration): d ¼ 4.62–4.65 (m, 2 H), 7.50–7.53
(m, 2 H), 7.88–7.91 (m, 2 H), 8.02 (d, 1 H, J ¼ 2.0 Hz), 8.63–8.66 (m, 1
H), 9.25 (t, 1 H, J ¼ 6.0 Hz); rotamer B: d ¼ 4.59–4.62 (m, 2 H), 7.53–
7.55 (m, 2 H), 7.86–7.88 (m, 2 H), 8.19 (d, 1 H, J ¼ 11.5 Hz), 8.20–8.24
(m, 1 H), 9.19 (t, 1 H, J ¼ 5.5 Hz); 13C NMR (125.7 MHz, DMSO-d6)
of rotamer A: d ¼ 43.4, 129.1, 130.0, 133.1, 137.1, 161.9, 166.4; rotamer
B: d ¼ 47.7, 129.2, 129.9, 133.2, 137.1, 165.9, 166.1; HRMS-ESI (positive
mode): m=z [M þ Na]þ calcd for C9H9ClN2NaO2: 235.0243; found:
235.0245.
4-Bromo-N-(formamidomethyl)benzamide (2c)
A white solid; yield: 57%; mp 154 ꢁC (dec); IR (KBr): 1694, 1643 cmꢀ1; 1H
NMR (500 MHz, DMSO-d6) of rotamer A (3.6:1 mixture of two rotamers
based on 1H NMR integration): d ¼ 4.61–4.64 (m, 2 H), 7.65–7.70 (m, 2 H),
7.78–7.83 (m, 2 H), 8.02 (d, 1 H, J ¼ 1.5 Hz), 8.61–8.65 (m, 1 H), 9.25 (t, 1
H, J ¼ 6.0 Hz); rotamer B: d ¼ 4.58–4.62 (m, 2 H), 7.65–7.70 (m, 2 H),
7.78–7.83 (m, 2 H), 8.18 (d, 1 H, J ¼ 11.0 Hz), 8.18–8.23 (m, 1 H), 9.20
(t, 1 H, J ¼ 6.0 Hz); 13C NMR (125.7 MHz, DMSO-d6) of rotamer A:
d ¼ 43.4, 126.0, 130.2, 132.1, 133.5, 161.9, 166.5; rotamer B: d ¼ 47.7,
126.1, 130.0, 132.1, 133.6, 166.0, 166.4; HRMS-ESI (positive mode): m=z
[M þ Na]þ calcd. for C9H9BrN2NaO2: 278.9746; found: 278.9740.
N-(Fomamidomethyl)-4-methylbenzamide (2d)
A white solid; yield: 68%; mp 141–143 ꢁC; IR (KBr): 1667, 1630 cmꢀ1
1H NMR (500 MHz, DMSO-d6) of rotamer A (3.4:1 mixture of two
;

488
H. Yue et al.
rotamers based on 1H NMR integration): d ¼ 2.33 (s, 3 H), 4.61–4.65 (m,
2 H), 7.24–7.28 (m, 2 H), 7.77–7.80 (m, 2 H), 8.01 (d, 1 H, J ¼ 1.5 Hz),
8.59–8.62 (m, 1 H), 9.08 (t, 1 H, J ¼ 6.0 Hz); rotamer B: d ¼ 2.33 (s, 3
H), 4.58–4.61 (m, 2 H), 7.24–7.28 (m, 2 H), 7.76 (d, 1 H, J ¼ 8.5 Hz),
7.77–7.80 (m, 2 H), 8.18–8.20 (m, 1 H), 9.02–9.06 (t, 1 H, J ¼ 5.5 Hz);
13C NMR (125.7 MHz, DMSO-d6) of rotamer A: d ¼ 21.6, 43.3, 128.0,
129.5, 131.6, 142.1, 161.9, 167.3; rotamer B: d ¼ 21.6, 47.6, 127.9,
129.6, 131.7, 142.2, 166.4, 167.0; HRMS-ESI (positive mode): m=z
[M þ Na]þ calcd. for C10H12N2NaO2: 215.0792; found: 215.0791.
N-(Formamidomethyl)-2-methoxybenzamide (2e)
A white solid; yield: 74%; mp 142–143 ꢁC; IR (KBr): 1672, 1645 cmꢀ1; 1H
NMR (500 MHz, DMSO-d6) of rotamer A (3.2:1 mixture of two rota-
1
mers based on H NMR integration): d ¼ 3.87 (s, 3 H), 4.64–4.68 (m, 2
H), 7.01–7.05 (m, 1 H), 7.13 (d, 1 H, J ¼ 8.0 Hz), 7.46–7.50 (m, 1 H),
7.82 (dd, 1 H, J1 ¼ 2.0 Hz, J2 ¼ 7.5 Hz), 8.04 (d, 1 H, J ¼ 1.5 Hz), 8.64–
8.67 (m, 1 H), 8.74 (t, 1 H, J ¼ 6.0 Hz); rotamer B: d ¼ 3.87 (s, 3 H),
4.61–4.64 (m, 2 H), 7.01–7.05 (m, 1 H), 7.13 (d, 1 H, J ¼ 8.0 Hz), 7.46–
7.50 (m, 1 H), 7.79 (dd, 1 H, J1 ¼ 2.0 Hz, J2 ¼ 7.5 Hz), 8.06–8.12 (m, 1
H), 8.21 (d, 1 H, J ¼ 11.5 Hz), 8.77 (t, 1 H, J ¼ 6.0 Hz); 13C NMR
(125.7 MHz, DMSO-d6) of rotamer A: d ¼ 43.6, 56.6, 112.8, 121.2,
122.5, 131.4, 133.5, 157.9, 162.2, 166.3; rotamer B: d ¼ 47.6, 56.5,
112.7, 121.2, 122.8, 131.3, 133.4, 157.9, 165.9, 166.0; MS-ESI (positive
mode): m=z (%) ¼ 209 (7) [M þ H]þ, 231 (22) [M þ Na]þ, 438 (100)
[2M-H þ Na]þ, 439 (18) [2Mþ þ Na]. Anal. calcd. for C10H12N2O3: C,
57.68; H, 5.81; N, 13.45. Found: C, 57.54; H, 5.40; N, 13.39.
N-(1-Formamidoethyl)benzamide (2f)
A white solid; yield: 71%; mp 119–120 ꢁC; IR (KBr): 1654, 1634 cmꢀ1; 1H
NMR (500 MHz, DMSO-d6) of rotamer A (1.9:1 mixture of two rota-
mers based on 1H NMR integration): d ¼ 1.37 (d, 3 H, J ¼ 6.5 Hz),
5.73–5.80 (m, 1H), 7.43–7.47 (m, 2 H), 7.49–7.54 (m, 1 H), 7.84–7.88
(m, 2 H), 7.97 (d, 1 H, J ¼ 1.0 Hz), 8.34–8.36 (m, 1 H), 8.74 (d, 1 H,
J ¼ 7.5 Hz); rotamer B: d ¼ 1.42 (d, 3 H, J ¼ 7.0 Hz), 5.39–5.47 (m, 1
H), 7.43–7.47 (m, 2 H), 7.49–7.54 (m, 1 H), 7.84–7.88 (m, 2 H), 8.08–
8.13 (m, 1 H), 8.28 (d, 1 H, J ¼ 11.5), 8.89 (d, 1 H, J ¼ 7.0 Hz); 13C
NMR (125.7 MHz, DMSO-d6) of rotamer A: d ¼ 21.4, 52.3, 128.0,
128.9, 132.0, 134.8, 160.9, 166.1; rotamer B: d ¼ 21.3, 55.8, 128.1,
128.9, 132.1, 134.7, 165.3, 166.9; HRMS-ESI (positive mode): m=z
[M þ Na]þ calcd. for C10H12N2NaO2: 215.0789; found: 215.0791.

Novel Route to N-Acyl-N0-formyl Aminal
489
N-(1-Formamido-2-methylpropyl)benzamide (2g)
A white solid; yield: 64%; mp 170–170.5 ꢁC; IR (KBr): 1665, 1653 cmꢀ1
;
1H NMR (500 MHz, DMSO-d6) of rotamer A (2.2:1 mixture of two rota-
1
mers based on H NMR integration): d ¼ 0.89 (d, 3 H, J ¼ 6.5 Hz), 0.91
(d, 3 H, J ¼ 6.5 Hz), 1.98–2.98 (m, 1 H), 5.43–5.49 (m, 1 H), 7.41–7.55
(m, 3 H), 7.82–7.85 (m, 2 H), 8.03 (d, 1 H, J ¼ 1.0 Hz), 8.15–8.18 (m, 1
H), 8.58 (d, 1 H, J ¼ 8.0 Hz); rotamer B: d ¼ 0.94 (d, 3 H, J ¼ 2.0 Hz),
0.95 (d, 3 H, J ¼ 2.0 Hz), 1.98–2.98 (m, 1 H), 4.90–4.97 (m, 1 H), 7.41–
7.55 (m, 3 H), 7.85–7.88 (m, 2 H), 7.99–8.03 (m, 1 H), 8.20 (d, 1 H,
J ¼ 11.5 Hz), 8.78 (d, 1 H, J ¼ 8.0 Hz); 13C NMR (125.7 MHz, DMSO-
d6) of rotamer A: d ¼ 18.9, 19.2, 32.3, 60.2, 128.0, 128.9, 131.9, 135.0,
161.3, 166.4; rotamer B: d ¼ 19.6, 19.8, 31.5, 65.1, 128.1, 128.9, 132.1,
134.8, 165.5, 167.2; MS-ESI (positive mode): m=z (%) ¼ 220 (100) Mþ,
175 (24) [M ꢀ 5]þ. Anal. calcd. for C12H16N2O2: C, 65.43; H, 7.32; N,
12.72. Found: C, 65.20; H, 7.40; N, 12.80.
N-(Formamido-3-methylbutyl)benzamide (2h)
A white solid; yield: 64%; mp 130–131 ꢁC; IR (KBr): 1665, 1640 cmꢀ1; 1H
NMR (500 MHz, DMSO-d6) of rotamer A (2.0:1 mixture of two rota-
1
mers based on H NMR integration): d ¼ 0.88 (d, 3 H, J ¼ 3.0 Hz), 0.89
(d, 3 H, J ¼ 3.0 Hz), 1.53–1.69 (m, 3 H), 5.73–5.80 (m, 1 H), 7.43–7.55
(m, 3 H), 7.82–7.85 (m, 2 H), 7.97 (d, 1 H, J ¼ 1.0 Hz), 8.23–8.26 (m, 1
H), 8.66 (d, 1 H, J ¼ 8.0 Hz); rotamer B: d ¼ 0.88 (d, 3 H, J ¼ 3.0 Hz),
0.89 (d, 3 H, J ¼ 3.0 Hz), 1.53–1.69 (m, 3 H), 5.29–5.36 (m, 1 H), 7.43–
7.55 (m, 3 H), 7.85–7.88 (m, 2 H), 8.06–8.11 (m, 1 H), 8.27 (d, 1 H,
J ¼ 11.5 Hz), 8.83 (d, 1 H, J ¼ 8.0 Hz); 13C NMR (125.7 MHz, DMSO-
d6) of rotamer A: d ¼ 22.9, 23.0, 25.0, 43.7, 53.9, 128.0, 128.9, 132.0,
134.9, 160.9, 166.2; rotamer B: d ¼ 22.6, 22.8, 24.8, 42.7, 57.8, 128.1,
128.9, 132.2, 134.7, 165.4, 167.0; MS-ESI (positive mode): m=z
(%) ¼ 234 (100) Mþ, 189 (28) [M ꢀ 45]þ. Anal. calcd. for C13H18N2O2:
C, 66.64; H, 7.74; N, 11.96. Found: C, 66.33; H, 7.49; N, 12.07.
Benzyl Formamidomethylcarbamate (2i)
A white solid; yield: 52%; mp 112–113 ꢁC; IR (KBr) 1699, 1662 cmꢀ1; 1H
NMR (500 MHz, DMSO-d6) of rotamer A (3.1:1 mixture of two rota-
1
mers based on H NMR integration) : d ¼ 4.37–4.40 (m, 2 H), 5.03 (s,
2 H), 7.28–7.37 (m, 5 H), 7.95–7.97 (m, 1 H), 7.98 (d, 1 H, J ¼ 1.5 Hz),
8.56–8.59 (m, 1 H); rotamer B: d ¼ 4.33–4.36 (m, 2 H), 5.03 (s, 2 H),
7.28–7.37 (m, 5 H), 8.06 (d, 1 H, J ¼ 11.5 Hz), 8.17–8.22 (m, 1 H),

490
H. Yue et al.
8.50–8.53 (m, 1 H); 13C NMR (125.7 MHz, DMSO-d6) of rotamer A:
d ¼ 44.6, 66.1, 128.5, 128.5, 129.1, 137.6, 156.9, 161.8; rotamer B:
d ¼ 49.0, 66.2, 128.6, 128.6, 129.1, 137.5, 156.6, 166.1; HRMS-ESI (posi-
tive mode): m=z [M þ Na]þ calcd. for C10H12N2NaO3: 231.0740; found:
231.0743.
ACKNOWLEDGMENT
We thank Tianjin University for providing a starting research fund to W. H.
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