Hexafluoroacetone as protection and activation reagent in amino acid and peptide chemistry regiospecific α-functionalization of aspartic acid

Article abstract of DOI:10.3987/COM-02-S17

A highly efficient method for regiospecific α-functionalization of aspartic acid is described. Key step is the synthesis of a N-protected and regioselectively α-carboxy-activated heterocyclic intermediate from aspartic acid and hexafluoroacetone. The new strategy offers i.a. a two step access to the sweetener Aspartame and to libraries of aspartame analogues.

Full text of DOI:10.3987/COM-02-S17

HETEROCYCLES, Vol. 59, 2003, No. 1, pp. 189 - 198, Received, 17th June, 2002
HEXAFLUOROACETONE AS PROTECTION AND
ACTIVATION REAGENT IN AMINO ACID AND
PEPTIDE CHEMISTRY
REGIOSPECIFIC
ASPARTIC ACID
α-FUNCTIONALIZATION
OF
Klaus Burger*, Torsten Lange, and Martin Rudolph
Department of Organic Chemistry, University of Leipzig, Johannisallee 29, D-04103 Leipzig,
Germany
Abstract - A highly efficient method for regiospecific α-functionalization of
aspartic acid is described. Key step is the synthesis of a N-protected and
regioselectively α-carboxy-activated heterocyclic intermediate from aspartic acid
and hexafluoroacetone. The new strategy offers i.a. a two step access to the
sweetener Aspartame and to libraries of aspartame analogues.
INTRODUCTION
Regioselective derivatization of multifunctional compounds like α- and/or ω-functionalization of ω-
carboxy-α-amino acids requires sophisticated protection/activation concepts.1,2 Therefore, syntheses of
relatively simple target molecules like aspartic acid α-esters,3 aspartic acid α-amides (isoasparagine),4 e.g.
of the sweetener Aspartame5 are surprisingly laborious more-step procedures: The classical syntheses of
Asp-α-OMe is a five-step,6 and of Aspartame a seven step procedure.7
The state of the art synthesis of α-functionalized aspartic acid derivatives consists of a N-protection step,
preferentially the introduction of an urethane protective group, and activation via an intramolecular
anhydride.8 The main drawback of this concept is the lack of regiospecifity of the aminolysis.
Consequently, a separattion of regioisomers is neccessary. The final step involves N-deprotection.
__________
* Corresponding author. Tel: (49) 314-9736529; FAX (49) 314-9736599; E-mail address: burger@organik.chemie.uni-
leipzig.de
** Dedicated to Prof. Yuichi Kanaoka on the occasion of his 75th birthday.

Therefore, the development of new strategies, where activation of the α-carboxylic group and protection
of the adjacent amino function can be accomplished in one step is of current interest. This challenge can
be met by the „heterocyclic route“ via oxazolidinones,9,10 N-thiocarboxy anhydrides11, and Leuch´s
anhydrides.12 However, most oxazolidinones derived from aldehydes like formaldehyde,13 require
additonal N-protection and N-deprotection. We now report on an approach where separate N-protection
and N-deprotection can be avoided.14
RESULTS AND DISCUSSION
α-Amino acids and hexafluoroacetone readily react in dimethyl sulfoxide at room temperature to give
2,2-bis(trifluoromethyl)-1,3-oxazolidin-5-ones in excellent yields.15 With formation of a five-membered
heterocyclic system, amino group protection and selective activation of the α-carboxy group are achieved
in one step. This reaction tolerates additional functional groups in the side-chain, like ω-carboxy groups.
On reaction of aspartic acid (1) with hexafluoroacetone (2) the five-membered lactone (3) is formed
exclusively. No trace of a six-membered ring systems could by identified on monitoring the reaction by
19F NMR spectroscopy.
H
O
HOOC
O
COOH
NH2
HOOC
HN
O
+
H
F3C
CF3
F3C
CF3
1
2
3
Scheme 1
The cleavage of the lactone moiety of compound (3) by nucleophiles like water, alcohols, amines and
hydrazines occurs regioselectively to give α-functionalized derivatives. Simultaneously, with the ring
cleavage, deprotection of the α-amino group takes place. Therefore, the new general approach offers a
two step route to α-functionalized aspartic acid derivates as well as to their homologues with enormous
structural variability. Compounds (4 - 8) represent only a small selection of the synthetic potential of the
new strategy.
This route offers the shortest possible and the most efficient way to α-functionalized ω-carboxy α-amino
acid derivatives known so far. Even enzymatic strategies16 can not beat the minimum of two steps. The
new strategy is applicable to the L- as well as to the D-series. Optimal yields are obtained, when solvent
systems are used, where the products crystallize spontaneously. Furthermore, the new strategy can also be
applied to the regioselective α-functionalization of α-hydroxy-17 and α-mercapto-α,ω-dicarboxy acids.18

O
HOOC
O CH3
H
NH2
4
O
COOH
NH2
HOOC
HOOC
NH2
H
H
NH2
1
methanol 89%
5
H
HOOC
O
HN
O
F3C
CF3
3
5-aminomethyl-2-
phenyl-4-(trifluoro-
methyl)thiazole
48%
O
H
O
N
OH
HOOC
N
R
HOOC
N
H
H
NH2
H
H
NH2
O
6
8 a/b
S
HOOC
N
H
H
NH2
N
F3C
7
Scheme 2
Aminolysis of lactone (3) with amino acid esters results in the formation of dipeptide esters in acceptable
yields. 19F NMR spectral analysis indicates that the reactions are complete within 12 h at room
temperature. From solvents like ether and ethyl acetate the dipeptide esters usually crystallize after a short
induction period. The reason for the low solubility of the α-amides in these solvents is the formation of a
betain structure by an internal protonation of the amino group by the β-carboxy moiety. In most cases the
products are already analytically pure after filtration and careful trituration with ether. From NMR
spectral data and optical rotation values of the crude products we conclude that epimerization is < 3%.
The new strategy offers an elegant two step route of the sweetener Aspartame in an over-all yield of 62%.
Furthermore, the reaction sequence offers an efficient, preparative simple access to libraries of Aspartame
analogues.19 The new approach seems to be superior to recently published strategies. However, a major
drawback of the new Aspartame synthesis is the high price of hexafluoroacetone and its toxicity. On
reaction with alkoxycarbonylhydrazines, 3 can be transformed into dipeptide fragments of type Asp-
AXaa-OR (3 → 14), and tripeptide fragments like Asp-Agly-ψ(CH2NH)Xaa-OR (3 → 15) which
represent valuable buildings blocks for the synthesis of peptide surrogates.20

O
H
CH3
O
L-alanine ethyl ester
71%
OCH2CH3
HOOC
N
9
H
NH2
H
CH3
O
H
CH3
O
L-valine methyl ester
77%
HOOC
N
CH3
10
H
H
NH2
O
O
H
CO2CH3
L-phenylalanine methyl ester
72%
HOOC
N
NH2
H
H
11 a
11 b
12
Aspartame
O
H
CO2C(CH3)3
L-phenylalanine tert-buthyl ester
HOOC
N
NH2
60%
H
H
H
O
HOOC
CH3
HN
O
N
S
F3C
CF
3
L-3-(2-methylthiazol-4-yl)-
alanine methyl ester
O
H
3
O
57%
HOOC
N
CH3
H
NH2
H
O
O
H
COOCH2C6H5
L-proline benzyl ester
68%
HOOC
N
13
H
NH2
O
H
methoxycarbonylhydrazine
67%
N
O
HOOC
N
CH3
14 a
H
NH2
H
O
O
H
tert-butoxycarbonylhydrazine
N
O
HOOC
N
C(CH3)3
14 b
15
48%
H
NH2
H
O
O
O
H
N-benzoxycarbonylglycine hydrazide
N
HOOC
N
N
O
95%
H
NH2
H
H
O
Scheme 3

Compound (3) derived from aspartic acid is a reactive species, reacting fastly and cleanly to give high
yields of the corresponding α-amides. On monitoring aminolysis of the lactone derived from glutamic
acid, surprisingly we found by-products i.a. via a fragmentation process starting with a base induced
abstraction of the NH-hydrogen of the 2,2-bis(trifluoromethyl)-1,3-oxazolidin-5-one ring.21
On regioselective ω-functionalization ω-carboxy-α-amino acids and on regioselective α- and/or ω-
functionalization of ω-carboxy-α-hydroxy and ω-carboxy-α-mercapto acids we report elsewhere.
EXPERIMENTAL
1
IR spectra were obtained on a Genesis ATI Mattson/Unicam FTIR spectrophotometer. H NMR spectra
were recorded at 300 MHz and 400 MHz. Chemical shifts were reported in ppm relative to
tetramethylsilane (TMS, δ = 0 ppm); J values are given in Hertz (Hz). 13C NMR spectroscopy was
19
performed at 75 MHz, 101 MHz and 151 MHz. F NMR spectra were recorded at 282 MHz and 376
MHz with trifluoroacetic acid (TFA, δ = 0) as external standard. Optical rotations ([α]D) were measured
using a Polatronic polarimeter (Schmidt & Haensch) in a 5 cm cell. Melting points were determined on a
Boetius heating table. For C, H, N analyses a CHNO-Rapid-Elemental-Analyser (Hereaus) was used. For
flash chromatography, silica gel (32-63 µm) was used with solvent systems given in the text. Organic
solvents were dried and distilled prior to use.
[(4-L)-2,2-Bis(trifluoromethyl)-5-oxo-1,3-oxazolidin-4-yl]acetic acid (3). A solution of L-aspartic acid
(1) (30.0 g, 225 mmol) in DMSO (100 mL) was stirred in an atmosphere of hexafluoroacetone (>2
equivalents) at rt. The apparatus was sealed with a dry-ice condenser. After completion of the reaction,
the mixture was quenched with an ice/water mixture (500 mL) and the product was extracted with
dichloromethane (3 x 150 mL). The combined organic layer was washed with ice-cold water (3 x 100
mL) and dried over MgSO4, then the solvent was evaporated in vacuo. Yield 86% (54.5 g) 3, mp 45 °C
(from CHCl3/hexanes). For data see lit.22
Hydrolysis of compound (3). 3 (2.81 g, 10 mmol) was heated in a H2O/THF mixture (1:1, 20 mL) under
reflux for 12 h. The suspension formed was evaporated to dryness in vacuo. The residue was carefully
triturated with ether and recrystallized from water. Yield 84% (1.12 g) L-aspartic acid (1), mp 275 - 278
oC (decomp), lit.,23 mp 270 oC (decomp), [α]D = +7.0 o (c = 2, H2O), lit.,1 [α]D = +6.7 o (c = 2, H2O).
L-Aspartic acid α-methyl ester (4). A solution of 3 (2.81 g, 10 mmol) in dry methanol (20 mL) was
heated under reflux. The ester (4) began to crystallize within minutes, after 30 min the reaction was
o
complete. The sample was cooled down to 0 C and the precipitate was filtered off. After careful

trituration with ice-cold ether and drying over P2O5 in vacuo compound (4) was analytically pure. Yield
89% (1.30 g), mp 183 – 186 °C (decomp), lit.,24 mp 183 – 184 °C (decomp), [α]D = +1.54 o (c =1.3, H2O).
Aminolysis of 3. General method: A solution of 3 in dry ether (30 mL) was stirred with the
corresponding amine (12 mmol) at rt. Within a few minutes the product began to crystallize. The reaction
o
was complete within 24 h (19F NMR spectral analysis). After cooling the reaction mixture to 0 C the
precipitate was filtered off, washed carefully with ice-cold ether and dried over P2O5 in vacuo.
L-Aspartic acid α-amide (isoasparagine) (5). A solution of 3 (2.81 g, 10 mmol) in dry ether (30 mL)
was stirred with the corresponding amine (12 mmol) for 24 h at rt. Within a few minutes the product
began to crystallize. Yield 79% (1.03 g) 5, mp 212 - 214 oC (water/ethanol) (decomp), lit.,25 mp 204 -205
oC (water/methanol), [α]D = +16.5 o (c = 2, H2O), lit.,4b [α]D = +17 o (c = 1, aq HCl).
L-Aspartic acid α-(β-hydroxyethylamide) (6). Ethanolamine (0.60 g, 10.0 mmol) was added to 3 (2.81
g, 10.0 mmol) in 40 mL of isopropanol. The reaction was complete within 24 h (19F NMR spectral
analysis). The precipitate was filtered off, washed carefully with ice-cold ether and purified by flash
chromatography (eluent: methanol). Yield 79% (1.39 g) 6, oil, [α]D = -2 o (c = 1.0, CH3OH). IR (film): ν
= 3600 - 3200, 1712, 1414, 1217. 1H NMR (D2O, 200 MHz) δ 2.46 (2H, m), 3.05 (2H, m), 3.36 (2H, m),
13
3.90 (1H, dd, J = 7.5 Hz, J = 5.5 Hz). C NMR (D2O, 75 MHz) δ 41.4, 45.9, 55.2, 64.1, 173.9, 180.3.
Anal. Calcd for C7H14N2O4 x 2 H2O: C, 37.16; H, 8.02; N, 12.38, Found: C, 37.20; H, 8.18; N, 11.99.
L-Aspartic acid α-[(2-phenyl-4-trifluoromethylthiazol-4-yl)methyl]amide (7). 3 (1.50 g, 5.3 mmol)
o
and 5-aminomethyl-2-phenyl-4-trifluoromethyl-1,3-thiazole react to give 48% (0.94 g) 7, mp 170 C,
o
1
[α]D = -7.0 (c = 1.0, DMSO). IR (KBr): ν = 3700 - 2200, 1670, 1560 cm-1. H NMR (DMSO-d6, 300
MHz) δ 2.39 (1H, dd, J = 16.5 Hz, J = 8.5 Hz), 2.55 (1H, dd, J = 16.5 Hz, J = 5.0 Hz), 3.87 (1H, dd, J =
8.5 Hz, J = 5.0 Hz), 4.60 (2H, m), 6.28 (3H, br s), 7.57 (3H, m), 7.99 (2H, m), 9.34 (1H, br s). 13C NMR
(DMSO-d6, 75 MHz) δ 33.2, 37.7, 50.8, 121.0 (q, J = 267 Hz), 125.4, 126.3, 126.5 (q, J = 43.0 Hz),
129.3, 131.6, 149.7, 160.6, 171.1, 172.6. 19F NMR (DMSO-d6, 282 MHz) δ 17.0 (3F, s). Anal. Calcd for
C7H14N3O3F3S: C, 48.26; H, 3.78; N, 11.25. Found: C, 48.99; H, 4.09; N, 11.08.
L-Aspartic acid α-(N-benzoylhydrazide) (8a). Benzoylhydrazine (2.52 g, 18.5 mmol) was added to a
solution of 3 (5.20 g, 18.5 mmol) in ethyl acetate (50 mL) at rt with stirring. After 16 h the starting
material (3) was consumed (19F NMR spectral analysis). The precipitate was filtered off, carefully washed
o
with cold ether and dried over P2O5 in vacuo. Yield 98% (4.54 g) 8a, mp 211 - 212 C (decomp); [α]D =

1
+28.5 o (c = 2.0, 1 N NaOH). IR (KBr): ν = 3350 - 2900, 1685, 1655, 1615 cm-1. H NMR (D2O, NaOD,
300 MHz) δ 3.14 (1H, m), 3.37 (1H, m), 4.47 (1H, m), 8.17 - 8.20 (3H, m), 8.54 - 8.58 (2H, m). 13C NMR
(D2O, NaOD, 75 MHz) δ 45.7, 54.9, 129.6, 131.1, 133.0, 138.6, 168.6, 173.0, 182.2. Anal. Calcd for
C11H13N3O4: C, 52.59; H, 5.22; N, 16.72. Found: C, 52.55; H, 5.21; N, 16.60.
L-Aspartic acid α-(N-pyridoylhydrazide) (8b). Reaction of 3 (3.09 g, 11 mmol) and pyridoylhydrazine
o
(1.43 g, 10.4 mmol) in ethyl acetate (50 mL) gave 78% (2.06 g) 8b, mp 217 - 219 C (decomp) after
purification by flash chromatography (eluent: methanol), [α]D = +26.0 o (c = 2.0, 1N NaOH). IR (KBr) ν
3500 - 2800, 1720, 1660, 1615 cm-1. 1H NMR (DMSO-d6, 300 MHz) δ 2.38 (1H, dd, J = 16.4 Hz, J = 9.0
Hz), 2.58 (1H, dd, J = 16.4 Hz, J = 4.6 Hz), 3.87 (1H, dd, J = 9.0 Hz, J = 4.6 Hz), 7.77 (2H, m), 8.74 (2H,
m). 13C NMR (DMSO-d6, 75 MHz) δ 38.0, 49.9, 121.3, 139.3, 150.4, 163.8, 171.0, 172.0. Anal. Calcd for
C10H12N4O5: C, 47.62; H, 4.80; N, 22.21. Found: C, 47.51; H, 4.84; N, 22.00.
L-Aspartyl-L-alanine ethyl ester (9). A solution of 3 (1.40 g, 5 mmol) in ether (20 mL) was stirred with
L-alanine ethyl ester (0.64 g, 55 mmol) at rt until the starting material was consumed (19F NMR spectral
analysis). Then the precipitate was filtered off, carefully washed with cold ether and dried over P2O5 in
vacuo. Yield 71% (0.82 g) 9, mp 166 - 167 oC (from ethanol/water), [α]D = -27.5 o (c = 2, H2O). IR (KBr)
ν 3310, 2990, 1735, 1665, 1630-1540 cm-1. 1H NMR (DMSO-d6, 200 MHz) δ 1.16 (3H, t, J = 7.0), 1.27
(3H, d, J = 7.2 Hz), 2.24 (1H, dd, J = 16.4 Hz, J = 9.8 Hz), 2.47 (1H, dd, J = 16.4 Hz, J = 4.0 Hz), 3.73
(1H, dd, J = 9.8 Hz, J = 4.0 Hz), 4.06 (2H, m), 4.25 (1H, m), 8.71 (1H, d, J = 5.2 Hz). 13C NMR
(DMSO-d6, 75 MHz) δ 14.0, 16.8, 37.5, 47.8, 50.4, 60.6, 170.0, 172.1, 173.1. Anal. Calcd for C9H16N2O5:
C, 44.81; H, 7.10; N, 11.61. Found; C, 44.60; H, 6.83; N, 12.13.
L-Aspartyl-L-valine methyl ester (10). To a solution of 3 (2.73 g, 9.7 mmol) in ethyl acetate (20 mL) a
solution of L-valine methyl ester (1.27 g, 9.7 mmol) in ethyl acetate (20 mL) was added with stirring.
Crystallization of the dipeptide ester started within a few minutes. After 24 h the precipitate was filtered
off. The product was purified by RPC (eluent: methanol/water = 3:1) and dried in vacuo. Yield 77%
(1.83 g) 10, mp 105 - 107 oC, [α]D = -12.0 o (c = 1, H2O). IR (KBr): ν = 3309, 2966, 1744, 1670, 1555 cm-1.
1H NMR (CD3OD, 200 MHz) δ 0.96 (3H, d, J = 6.8 Hz), 0.97 (3H, d, J = 6.9 Hz), 2.17 (1H, m), 2.53
(1H, dd, J = 17.0 Hz, J = 8.8 Hz), 2.75 (1H, dd, J = 17.0 Hz, J = 5.1 Hz), 3.71 (3H, s), 4.12 (1H, dd, J =
8.8 Hz, J = 5.1 Hz), 4.35 (1H, d, J = 5.7 Hz). 13C NMR (CD3OD, 75 MHz) δ 18.4, 19.5, 31.6, 39.2, 52.4,
52.6, 59.4, 171.7, 173.1, 176.8. Anal. Calcd for C10H18N2O5 x 1/2 H2O: C, 47.05; H, 7.50; N, 10.97.
Found: C, 47.16; H, 7.35; N, 10.99.

L-Aspartyl-L-phenylalanine methyl ester (Aspartame®) (11a). 3 (5.62 g, 20 mmol) and L-
phenylalanine methyl ester (4.25 g, 24 mmol) were reacted in ether (30 mL) to give 72% (4.24 g) 11a, mp
o
o
o
1
250 C, lit.,11 mp 243 C, [α]D = +27.5 (c = 1.0, acetic acid). IR (KBr): ν 3320, 1735, 1665 cm-1. H
NMR (D2O, 200 MHz) δ 2.66 (1H, dd, J = 17.5 Hz, J = 8.0 Hz), 2.77 (1H, dd, J = 17.5 Hz, J = 5.5 Hz),
3.06 (1H, dd, J = 14.0 Hz, J = 9.0 Hz), 3.24 (1H, dd, J = 14.0 Hz, J = 6.0 Hz), 3.73 (3H, s), 4.17 (1H, dd,
J = 8.0 Hz, J = 5.5 Hz), 4.73 (1H, m), 7.26 - 7.41 (5H, m). 13C NMR (DMSO-d6, 75 MHz) δ 36.6, 37.2,
50.7, 51.9, 53.6, 126.6, 128.1, 129.1, 136.9, 171.4, 171.6, 172.4. Anal. Calcd for C14H18N2O5: C, 55.44;
H, 6.26; N, 9.24. Found: C, 55.59; H, 6.24; N, 9.20.
L-Aspartyl-L-phenylalanine tert-butyl ester (11b). 3 (5.62 g, 20 mmol) and L-phenylalanine tert-butyl
ester (6.70 g, 30 mmol) were reacted in ether (80 mL) to give 60% (4.04 g) 11b, mp 138 oC, [α]D = -7.5 o
(c = 2.0, DMSO). IR (KBr): ν 3700-3100, 1735, 1670, 1585, 1540 cm-1. 1H NMR (DMSO-d6, 200 MHz)
δ 1.33 (9H, s), 2.23 (1H, dd, J = 16.5 Hz, J = 9.5 Hz), 2.49 (1H, dd, J = 16.5 Hz, J = 4.0 Hz), 2.90 (1H,
dd, J = 14.0 Hz, J = 8.5 Hz), 2.98 (1H, dd, J = 14.0 Hz, J = 6.0 Hz), 3.68 (1H, dd, J = 9.5 Hz, J = 4.0
Hz), 4.36 (1H, m), 7.15 - 7.30 (5H, m), 8.66 (1H, m). 13C NMR (DMSO-d6, 75 MHz) δ 27.5, 36.8, 37.7,
50.5, 54.2, 80.9, 126.5, 128.2, 129.2, 137.0, 170.1, 171.0, 172.5. Anal. Calcd for C17H24N2O5: C, 60.70;
H, 7.18; N, 8.33. Found: C, 59.15; H, 7.18; N, 7.77.
L-Aspartyl-L-3-(2-methylthiazol-4-yl)alanine methyl ester (12). 3 (1.41 g, 5 mmol) and L-3-(2-
methylthiazol-4-yl)alanine methyl ester (1.30 g, 6.5 mmol) were reacted in ether (30 mL) to give 57%
o
o
(0.90 g) 12, mp 151 C (from methanol), [α]D = -11.0 (c = 1.0, H2O). IR (KBr) ν 3660 - 2800, 1730,
1660 cm-1. 1H NMR (D2O, 200 MHz) δ 2.66 (3H, s), 2.67 (1H, dd, J = 17.5 Hz, J = 8.0 Hz), 2.77 (1H,
dd, J = 17.5 Hz, J = 5.5 Hz), 3.25 (2H, m), 3.74 (3H, s), 4.22 (1H, dd, J = 8.0 Hz, J = 5.5 Hz), 4.78 (1H,
m), 7.11 (1H; s). 13C NMR (D2O, 75 MHz) δ 20.4, 34.3, 39.5, 53.1, 55.7, 55.8, 119.3, 152.1, 171.5,
172.0, 175.3, 178.3. Anal. Calcd for C12H17N3O5S x H2O: C, 43.20; H, 5.70; N, 12.60. Found: C, 43.78;
H, 5.98; N, 12.47.
L-Aspartyl-L-proline benzyl ester (13). 3 (2.81 g, 10 mmol) and L-proline benzyl ester (2.90 g, 14
mmol) were reacted in ether (30 mL) to give 68% (2.24 g) 13, mp 124 oC (methanol/water), [α]D = -76.0 o
1
(c = 1.0, DMSO). IR (KBr): ν 3600 - 3300, 2980, 1740, 1640 cm-1. H NMR (D2O, 200 MHz) δ 1.95 -
2.04 (3H, m), 2.30 (1H, m), 2.46 (1H, dd, J = 17.5 Hz, J = 10.5 Hz), 2.72 (1H, dd, J = 17.5 Hz, J = 3.0
Hz), 3.60 (1H, m), 3.70 (1H, m), 4.49 (1H, dd, J = 10.5 Hz, J = 3.0 Hz), 4.56 (1H, m), 5.17 (1H, d, J =
13
12.0 Hz), 5.23 (1H, d, J = 12.0 Hz), 7.45 (5H, m). C NMR (D2O, 75 MHz) δ 27.3, 31.2, 37.8, 50.1,

52.5, 62.4, 70.5, 131.1, 131.5, 131.6, 137.8, 170.4, 175.8, 178.0. Anal. Calcd for C16H20N2O5: C, 59.98;
H, 6.29; N, 8.74. Found: C, 59.70; H, 6.31; N, 8.96.
L-Aspartyl-azaglycine methyl ester (14a). 3 (1.62 g, 5.8 mmol) and methoxycarbonylhydrazine (0.51 g,
o
5.8 mmol) were reacted in ethyl acetate (20 mL) to give 67% (0.80 g) 14a, mp 145 C (decomp) after
purification by RPLC (eluent: water), [α]D = -55.0 o (c = 1.0, H2O). IR (KBr) ν 3400 - 2900, 1730, 1700,
1605 cm-1. 1H NMR (DMSO-d6, 200 MHz) δ 2.34 (1H, dd, J = 16.4 Hz, J = 9.0 Hz), 2.51 (1H, m), 3.59
(3H, s), 3.83 (1H, dd, J = 9.0 Hz, J = 4.4 Hz). 13C NMR (DMSO-d6, 75 MHz) δ 37.7, 49.6, 52.0, 156.5,
170.6, 172.6. Anal. Calcd for C6H11N3O5: C, 35.13; H, 5.40; N, 20.48. Found: C, 34.80; H, 5.61; N,
20.61.
L-Aspartyl-azaglycine tert-butyl ester (14b). 3 (3.08 g, 10.9 mmol) and BOC-hydrazine (1.44 g, 10.9
o
mmol) were reacted in ether (50 mL) to give 48% (1.30 g) 14b, mp 126-128 C (after trituration with
1
ether). IR (KBr) ν 3500 - 2800, 1760-1550 cm-1. H NMR (DMSO-d6, 200 MHz) δ 1.46 (9H, s), 2.59
(1H, m, J = 16.8 Hz, J = 9.7 Hz), 2.75 (1H, m, J = 16.8 Hz, J = 4.4 Hz), 4.08 (1H, dd, J = 9.7 Hz, J = 4.4
Hz). 13C NMR (DMSO-d6, 75 MHz) δ 28.0, 37.7, 49.6, 79.4, 155.1, 170.6, 172.4. Anal. Calcd for
C9H17N3O5: C, 43.72; H, 6.93; N, 16.99. Found: C, 44.04; H, 6.75; N, 17.08.
L-Aspartyl-azaglycyl-retro(CH2NH)glycine benzyl ester (15). 3 (1.93 g, 6.8 mmol) and N-benzoxy-
carbonylglycine hydrazide (1.53 g, 6.8 mmol) were reacted in ether (40 mL). The precipitate was
o
carefully washed with cold ether and dried in vacuo. Yield 95% (2.20 g) 15, mp 166 - 168 C (after
trituration with ether), [α]D = -6.5 o (c = 2.0, DMSO). IR (KBr) ν 3210, 3034, 1695, 1618, 1538 cm-1. 1H
NMR (CD3OD, 300 MHz) δ 2.68 (1H, dd, J = 17.2 Hz, J = 8.9 Hz), 2.81 (1H, dd, J = 17.2 Hz, J = 4.8
13
Hz), 3.91 (2H, s), 4.21 (1H, dd, J = 8.9 Hz, J = 4.8 Hz), 5.10 (2H, s), 7.30 - 7.36 (5H, m). C NMR
(DMSO-d6, 75 MHz) δ 38.0, 42.0, 49.8, 65.5, 127.7, 128.4, 137.0, 156.5, 167.9, 169.9, 172.3. Anal.
Calcd for C14H18N4O6: C, 49.70; H, 5.36; N, 15.56. Found: C, 49.25; H, 5.45; N, 15.40.
ACKNOWLEDGEMENTS
The authors are grateful to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen
Industrie for financial support.
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