Highly efficient stereoconservative amidation and deamidation of α-amino acids

Article abstract of DOI:10.1021/ol048771l

(Chemical Equation Presented) An overall stereoconservative protection and deprotection method of amino and carboxyl groups is presented. N-Phthaloyl N-alkyl secondary amides of α-amino acids can be generated from corresponding N-phthaloyl amino acids by coupling reaction of N-alkylamines using mixed anhydride method. These secondary amides can be transformed by thermal rearrangement of intermediate nitrosoamides to O-alkyl esters with retention of configuration and excellent yields.

Full text of DOI:10.1021/ol048771l

ORGANIC
LETTERS
2004
Vol. 6, No. 21
3675-3678
Highly Efficient Stereoconservative
Amidation and Deamidation of
Acids
r-Amino
Deepak M. Shendage, Roland Fro1hlich, and Gu1nter Haufe*
Organisch-Chemisches Institut, UniVersita¨t Mu¨nster, Corrensstrasse 40,
D-48149 Mu¨nster, Germany
haufe@uni-muenster.de
Received June 28, 2004
ABSTRACT
An overall stereoconservative protection and deprotection method of amino and carboxyl groups is presented. N-Phthaloyl N
′-alkyl secondary
amides of -amino acids can be generated from corresponding N-phthaloyl amino acids by coupling reaction of N-alkylamines using mixed
r
anhydride method. These secondary amides can be transformed by thermal rearrangement of intermediate nitrosoamides to O-alkyl esters
with retention of configuration and excellent yields.
We required a variety of amino acid amides for peptido-
mimetic studies. For this purpose, it was necessary to develop
procedures for amidation as well as deamidation of enantio-
pure R-amino acids. Also, we had the problem of stereo-
conservative deamidation of chiral R-amino acid secondary
amides in our recent synthesis of fluorinated amino acids.1
We could not find any chemical transformation of optically
active secondary amino acid amides to the corresponding
esters or acids without partial racemization. There are few
reports on strong basic deamidation of cyclopropyl amides2
and specific enzymatic amidation and deamidation of primary
amides.3
acid (AA) II and C-terminal N-alkyl amide VI. Further, the
peptide chain can be extended to the left side (N-terminal)
to XIV by repeating N-deprotection at VII and coupling
reaction with IX. However, it is always difficult to add a
new AA to the right side (C-terminal), which needs selective
secondary amide hydrolysis of VII to acid XI. Then, the
peptide XIII can be synthesized by coupling reaction of XI
and XII. Figure 1 shows the necessity of protection and
deprotection of the N-terminus as well as the C-terminus of
the corresponding amino acid building blocks.
Several decades ago, an impressive and convincing work
by White4 showed that N-alkyl secondary amides of simple
carboxylic acids can be transformed to N-nitrosoamides,
which were then thermally rearranged to O-alkyl esters with
For synthetic peptides (peptide design), several protection
and deprotection steps are required. The dipeptide VII can
be synthesized by coupling reaction of N-protected amino
(3) For enzymatic amidation/deamidation, see:(a) Trauthewein, H.; May,
O.; Dingerdissen, U.; Buchholz, S.; Drauz, K. Tetrahedron Lett. 2003, 44,
3737-3739. (b) Wolf, L. B.; Sonke, T.; Tjen, K. C. M. F.; Kaptein, B.;
Broxterman, Q. B.; Schoemaker, H. E.; Rutjes, F. P. J. T. AdV. Synth. Catal.
2001, 343, 662-674. (c) Cerovsky, V.; Kula, M.-R. Angew. Chem. 1998,
110, 1986-1989; Angew. Chem., Int. Ed. 1998, 37, 1885-1887. (d) Steinke,
D.; Kula, M.-R. Angew. Chem. 1990, 102, 1204-1206; Angew. Chem., Int.
Ed. 1990, 29, 1139-1140.
* To whom correspondence should be addressed. Phone: ++49-251-
8333281. Fax: ++49-251-83-39772.
(1) Shendage, D. M.; Fro¨hlich, R.; Bergander, K.; Haufe, G. Eur. J. Org.
Chem. 2004, submitted.
(2) Zhang, X.; Hodgetts, K.; Rachwal, S.; Zhao, H.; Wasley, J. W. F.;
Craven, K.; Brodbeck, R.; Kieltyka, A.; Hoffman, D.; Bacolod, M. D.;
Girard, B.; Tran, J.; Thurkauf, A. J. Med. Chem. 2000, 43, 3923-3932.
10.1021/ol048771l CCC: $27.50
© 2004 American Chemical Society
Published on Web 09/24/2004

Figure 2. Mechanism of mixed anhydride coupling reaction.
Figure 1. Synthetic strategy for polypeptides; PG ) protecting
group; Xaa ) Yaa ) Zaa ) AA ) amino acids.
methyl morpholine (NMM) and isobutyl chloroformate
(IBCF) in THF at -20 °C in quantitative yields. Several
different Boc-protected (Table 2, entries 1-6) and phthal-
elimination of nitrogen. There is also a recent report on
transformation of similar N-aryl amides to O-aryl esters.5
The issue of competing modes of decomposition of N-
nitrosoamides has been debated for decades and was recently
summarized by Darbeau and co-workers, who concluded that
the nitrosoamides decompose principally along deaminative
pathways (-N2), leading to rearranged products, and de-
nitrosative pathways (-“NO+”), leading to starting material,
depending on pH and temperature.6
Table 2. N′-Methyl Amidation of Protected Amino Acids 2
entry
(S)-2
PG
R
(S)-3
% yielda
This methodology has not yet been applied to chiral amino
acids. Thus, several amino acids 1 have been N-protected
by tert-butyloxycarbonyl (Boc) or phthaloyl (Pht) protecting
groups using Boc2O/NaHCO3/THF-H2O or PhtA/H2O in
nearly quantitative yields (Table 1).
1
2
3
4
5
6
7
8
9
a
b
d
e
f
g
h
i
Boc
Boc
Boc
Boc
Boc
Boc
Pht
Pht
Pht
Me
a
b
d
e
f
g
h
i
>98
>99
>99
>99
>99
>99
>99
>98
>99
N-(CH2)3-b
Phc
Bn
F-allyld
allyld
Me
iPr
Bn
j
j
Table 1. N-Protections of Amino Acids 1
a Yields of isolated products. b (S)-Proline. c (R)-Phenylglycine. d Race-
mate.
imide-protected (Table 2, entries 7-9, and Table 3, entries
1-5) amino acids 2 were derivatized to the respective N′-
alkyl amides 3 in practically quantitative yields using
corresponding N-alkylamines.8
entry
(S)-1
R
PG
(S)-2
% yielda
1
2
3
4
5
f
F-allylb
allylb
Me
Boc
Boc
Pht
Pht
Pht
f
>99
>99
>99
95
g
h
i
g
h
i
iPr
Bn
j
j
>98
Table 3. N′-Alkyl Amidation of N-Phthalimido Alanine (2h)
a Yields of isolated products. b Racemate; PhtA ) phthalic anhydride;
MW ) microwave; F-allyl ) 2-fluoroallyl.
The N′-alkyl amides 3 were synthesized by selective N′-
alkyl amidation of protected amino acids 2 followed by
mixed anhydride coupling reaction7 (Figure 2) using N-
entry
(S)-2
R
(S)-3
% yielda
1
2
3
4
5
k
l
m
n
o
nBu
iBu
tBu
Ph
k
l
m
n
o
>99
>98
>99
>99
>99
(4) (a) White, E. H. J. Am. Chem. Soc. 1954, 76, 4497-4498. (b) White,
E. H. J. Am. Chem. Soc. 1955, 77, 6008-6010. (c) White, E. H. J. Am.
Chem. Soc. 1955, 77, 6011-6014. (d) White, E. H. J. Am. Chem. Soc.
1955, 77, 6014-6022. (e) White, E. H.; Elliger, C. A. J. Am. Chem. Soc.
1967, 89, 165-167. (f) White, E. H. Organic Syntheses; Wiley: New York,
1973; Collect. Vol. V, pp 336-339. (g) Le Noble, W. J.; White, E. H.;
Dzadzic, P. M. J. Am. Chem. Soc. 1976, 98, 4020-4221.
Bn
a Yields of isolated products.
3676
Org. Lett., Vol. 6, No. 21, 2004

The N-protected amides 3 were subsequently hydrolyzed
to free amides 4 (peptide building blocks) using TFA/CH2Cl2
(Table 4, entries 1-2) or to the corresponding HCl-salts
(Table 4, entries 3-4) by HCl-MeOH in anhydrous MeOH
with high yields.
Table 4. N-Deprotection of Amides 3
Figure 3. ORTEP diagram of 3p.
obtain corresponding methyl esters 5 in good to excellent
yields (Table 5). The rearrangement of N′-tert-butyl or N′-
phenyl amide5 3m-n (Table 5, entries 6-7) did not occur,
and most of the starting material was recovered. This might
be due to decomposition of the intermediary nitrosoamide
following the denitrosative pathway.6
Interestingly, also the N′-methyl amide 3b was similarly
deamidated11 (86% yield), demonstrating the utility of the
procedure for N,N-dialkyl carbamates (Scheme 1).
entry
(S)-3
PG
R
(S)-4
% yielda
1
2
3
4
a
b
f
Boc
Boc
Boc
Boc
Me
a
b
f
98
>98
98
N-(CH2)3-b
F-allylc
allylc
g
g
82
a Yields of isolated products. b (S)-Proline. c Racemate.
Several lipases and esterases have been screened in order
to hydrolyze racemic amides 4f and 4g to the corresponding
free acids, e.g., 1f and 1g enzymatically, however, without
success.9 Thus, the N′-methyl amide (S)-4p1 was protected10
to PhtN-L-Fap-NHMe 3p (PG ) Pht, R ) F-allyl) and
converted11 to its methyl ester 5p (Table 5, entry 9) by
No racemization was observed for this deamidation
procedure by comparison of specific rotation of 2j ([R]D
(8) General Procedure for N-Alkyl Amidation. N-Methyl morpholine
(10 mmol) and isobutyl chloroformate (10 mmol) were successively added
to a solution of Boc-Xaa-OH 2a-g (10 mmol) or PhtN-Xaa-OH 2h-n (10
mmol) in THF (20 mL) at -20 °C. After an activation period of 3 min,
40% aqueous methylamine (12-50 mmol) or N-alkylamine (13 mmol) in
THF (5 mL) was added to above solution, and the resulting solution was
stirred for 1 h at -20 °C prior to the addition of 5% NaHCO3 (20 mL).
After 30 min at room temperature, the aqueous phase was extracted with
CH2Cl2 (three times). The combined organic layer was washed with 5%
NaHCO3 (two times) and dried (Na2SO4). Evaporation of solvents under
reduced pressure gave Boc-Xaa-NHMe 3a-g or PhtN-Xaa-NHR 3h-n.
The crude products were purified by flash chromatography (EtOAc/
cyclohexane, 1:2) or by crystallization (EtOAc/pentane or CH2Cl2/pentane).
(9) In collaboration with Prof. Dr. A. Liese, Institute of Biochemistry,
University of Mu¨nster, Germany, and Prof. Dr. F. P. J. T. Rutjes, Department
of Organic Chemistry, University of Nijmegen, Nijmegen, The Netherlands.
(10) To a solution of H-L-Fap-NHMe 4 (2 mmol) in CHCl3/MeOH (2:
1, 20 mL) was added phthalic anhydride (2.4 mmol, freshly recrystallized
from CHCl3) at 0 °C. After 10 min, oxalyl chloride (3 mmol) was added
dropwise at 0 °C. Then, the solution was refluxed for 5 h and cooled to
room temperature. The solvent was evaporated under vacuum, and the
residue was recrystallized from CH2Cl2/pentane in a freezer.
(11) General Procedure for Deamidation. To a solution of the desired
N-phthaloyl N′-alkyl amide 3 (5 mmol) in a 2:1 mixture of Ac2O and AcOH
(25 mL) was added granular NaNO2 (100 mmol) in portions over 2 h at
0-4 °C. Evolution of a brown gas occurred, and the solution changed color;
sometimes a solid precipitated. After 14-16 h, the mixture was warmed to
room temperature within 20 min, added into ice-water (25 mL), and
extracted with Et2O or CH2Cl2 (three times). The combined organic layer
was washed (carefully!) with 5% Na2CO3 (three times) and then H2O and
dried (Na2SO4). Evaporation of solvents gave a yellowish liquid. GC showed
complete conversion of phthalimido-amides. To this residue was added
anhydrous 1,4-dioxane (25 mL), and the solution was refluxed. The
yellowish color of the solution disappeared in the first 1 h, but reflux was
continued for 5 h. Then, the solution was cooled to room temperature and
the solvent was removed under vacuum to give colorless oils. GC of the
crude product as such shows high purity.
Table 5. Deamidation of Amides 3
entry
(S)-3
R
R′
(S)-5
% yielda
1
2
3
4
5
6
7
8
9
h
i
j
k
l
m
n
o
p
Me
iPr
Bn
Me
Me
Me
Me
Me
Me
Me
Me
nBu
iBu
tBu
Ph
h
i
j
k
l
m
n
o
p
96
>98
>98
>98
97
0
0
98
91
Bn
Me
F-allyl
a Yields of isolated products; F-allyl ) 2-fluoroallyl.
nitrosoamide decomposition reaction4 in 91% yield. The
stereochemistry of compound 3p was confirmed from X-ray
crystal structure (Figure 3).12
The deamidation procedure11 was developed and applied
for a variety of phthaloyl-protected amino acid amides 3 to
(12) X-ray crystal structure analysis for 3p: formula 2C14H13FN2O3 ×
CH2Cl2, M ) 637.45, colorless crystal 0.45 × 0.30 × 0.10 mm, a )
7.778(1), b ) 17.538(1), c ) 22.402(1) Å, V ) 3055.9(5) Å3, Fcalcd ) 1.386
g cm-3, µ ) 24.37 cm-1, empirical absorption correction (0.407 e T e
0793), Z ) 4, orthorhombic, space group P212121 (No. 194), λ ) 1.54178
Å, T ) 223 K, ω and æ scans, 13 926 reflections collected ((h, (k, (l),
[(sin θ)/λ] ) 0.59 Å-1, 5039 independent (Rint ) 0.042) and 4564 observed
reflections [I g 2 σ(I)], 396 refined parameters, R ) 0.047, wR2 ) 0.137,
Flack parameter -0.03(2), max residual electron density 0.51 (-0.43) e
Å-3; hydrogens at nitrogen atoms N2 were obtained from difference Fourier
calculations, others were calculated and all refined as riding atoms.
(5) Glatzhofer, D. T.; Roy, R. R.; Cossey, K. N. Org. Lett. 2002, 4,
2349-2352 and references therein.
(6) Darbeau, R. W.; Perez, E. V.; Sobieski, J. I.; Rose, W. A.; Yates,
M. C.; Boese, B. J.; Darbeau, N. R. J. Org. Chem. 2001, 66, 5679-5686
and references therein.
(7) Perich, J. W.; Johns, R. B. J. Org. Chem. 1988, 53, 4103-4105.
Org. Lett., Vol. 6, No. 21, 2004
3677

Scheme 1. Deamidation of Boc-L-Pro-NHMe 3b
-162.8 (c 1.01, CH2Cl2)) prepared from either 1j or
phenylalanine methyl ester 5j. Moreover, X-ray structures
of the PhtN-L-Ala-NHMe 3h13 (Figure 4) and its deamidation
Figure 5. ORTEP diagram of 5h.
The methyl esters 5 can be hydrolyzed to phthaloyl amino
acids 2 under neutral conditions.15 Finally, the phthaloyl
protection can be removed by well-known procedures.16
The spectroscopic evidence and corresponding literature
for reported compounds is given as Supporting Information.
In conclusion, we developed a highly efficient and
stereoconservative amidation/deamidation protocol for chiral
R-amino acids. Amidation and N-deprotection procedures
lead to important building blocks for synthetic peptides. The
deamidation procedure extends the peptide chain at the
C-terminus. The mild and efficient nitrosoamide decomposi-
tion reaction in the presence of an N,N-dialkyl carbamate
function could extend its use for other functional groups.
Figure 4. ORTEP diagram of 3h.
product PhtN-L-Ala-OMe (5h)14 (Figure 5) prove the de-
amidation reaction without racemization since there is no
intermediate in the mechanism,6 which could force racem-
ization of the R-position.
Acknowledgment. We thank the International Graduate
School of Chemistry, Mu¨nster, Germany (GSC-MS) for
financial support of this work. We also thank Prof. Dr. A.
Liese and Prof. Dr. F. P. J. T. Rutjes for enzymatic screening
and valuable discussions.
(13) X-ray crystal structure analysis for 3h: formula C12H12N2O3, M )
232.24, colorless crystal 0.40 × 0.15 × 0.10 mm, a ) 10.929(1), b )
11.064(1), c ) 9.906(1) Å, â ) 96.12(1)°, V ) 1191.0(2) Å3, Fcalcd ) 1.295
g cm-3, µ ) 7.87 cm-1, empirical absorption correction (0.744 e T e
0.925), Z ) 4, monoclinic, space group Cc (No. 9), λ ) 1.54178 Å, T )
223 K, ω and æ scans, 3222 reflections collected ((h, (k, (l), [(sin θ)/λ]
) 0.58 Å-1, 1594 independent (Rint ) 0.031) and 1514 observed reflections
[I g 2 σ(I)], 159 refined parameters, R ) 0.041, wR2 ) 0.114, Flack
Supporting Information Available: Experimental pro-
cedures; complete results of NMR and optimization studies,
and full characterization of all compounds; NMR (1H, 13C,
19F) spectra of new compounds and complete X-ray data for
3b, 3h, 3p, 5h. This material is available free of charge via
the Internet at http://pubs.acs.org.
parameter 0.4(3), max residual electron density 0.22 (-0.12) e Å-3
;
hydrogen at nitrogen atom N2 was obtained from difference Fourier
calculations, others were calculated and all refined as riding atoms.
(14) X-ray crystal structure analysis for 5h: formula C12H11NO4, M )
233.22, colorless crystal 0.50 × 0.05 × 0.03 mm, a ) 10.300(1), b )
15.373(1), c ) 7.235(1) Å, â ) 98.49(1)°, V ) 1133.0(2) Å3, Fcalcd ) 1.367
g cm-3, µ ) 8.74 cm-1, empirical absorption correction (0.669 e T e
0.974), Z ) 4, monoclinic, space group P21/c (No. 14), λ ) 1.54178 Å, T
) 223 K, ω and æ scans, 10254 reflections collected ((h, (k, (l), [(sin
θ)/λ] ) 0.59 Å-1, 1609 independent (Rint ) 0.071) and 928 observed
reflections [I g 2 σ(I)], 156 refined parameters, R ) 0.076, wR2 ) 0.208,
max residual electron density 0.39 (-0.26) e Å-3, hydrogens were calculated
and refined as riding atoms.
OL048771L
(15) For review, see: Olah, G. A.; Narang, S. C. Tetrahedron 1982, 38,
2225-2277 and references cited therein.
(16) Kocienski, P. J. In Protecting Groups; Enders, D., Noyori, R., Trost,
B. M., Eds.; Thieme: Stuttgart, 1994; Thieme Foundations of Organic
Chemistry Series.
3678
Org. Lett., Vol. 6, No. 21, 2004