Crystallisation-induced asymmetric transformation (CIAT) for the synthesis of dipeptides containing homophenylalanine

Article abstract of DOI:10.1016/j.tetasy.2010.11.015

A novel synthesis of highly enantioenriched dipeptides containing homophenylalanine is described. The process involves a crystallisation-induced asymmetric transformation (CIAT) in a Michael addition followed by exhaustive reduction. A unique example of a formally stereodivergent CIAT in conjugate addition of an achiral N-nucleophile to enantiomerically pure Michael acceptor has been discovered.

Full text of DOI:10.1016/j.tetasy.2010.11.015

Tetrahedron: Asymmetry 21 (2010) 2807–2815
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Crystallisation-induced asymmetric transformation (CIAT) for the synthesis
of dipeptides containing homophenylalanine
⇑
,
⇑
Pavol Jakubec , Dušan Berkeš
Department of Organic Chemistry, Institute of Organic Chemistry, Catalysis and Petrochemistry, Slovak University of Technology, Radlinského Street 9, SK-812 37 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel synthesis of highly enantioenriched dipeptides containing homophenylalanine is described. The
process involves a crystallisation-induced asymmetric transformation (CIAT) in a Michael addition fol-
lowed by exhaustive reduction. A unique example of a formally stereodivergent CIAT in conjugate addi-
tion of an achiral N-nucleophile to enantiomerically pure Michael acceptor has been discovered.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 29 September 2010
Accepted 22 November 2010
Available online 20 December 2010
1. Introduction
(CIAT)13–15 in the Michael addition of enantiomerically pure amines
7 to aroylacrylic acids 6 followed by reductive manipulation of oxo-
Enantiomerically pure homophenylalanines 1 (Hfe, R = H) are an
important class of non-proteinogenic -amino acids. They are
Hfe 8 (Scheme 1).16,17 We have further developed this transforma-
tion into a general synthetic tool for the production of modified
homophenylalanines, such as 4-oxo-, 4-hydroxy- and 2-methyl-
homophenylalanines and their derivatives.18 In connection to our
previous research, we believed a technically simple procedure for
a synthesis of oligopeptides containing Hfe could employ a CIAT
in the conjugate addition of tunable N-nucleophiles to suitable Mi-
chael acceptors 11 as a key step (retrosynthetic analysis, Scheme 1).
Such an approach could allow the rapid generation of dipeptides
containing homophenylalanine 9 via oxo-derivatives 10. We have
already successfully shown the power of CIAT in the conjugate addi-
tion of achiral nucleophiles as a key step in simple Hfe-containing
dipeptide synthesis.19 Herein we report our full findings into this
CIAT in the conjugate addition of achiral and chiral nucleophiles
to chiral Michael acceptors for the production of dipeptides con-
taining homophenylalanine.
a
abundant in cyclic peptides, such as cytotoxic dodecapeptides tych-
onamide A and B,1 depsipeptide kulokekahilide-1,2 schizopeptin
7913 isolated from natural sources and many other important phar-
maceutical compounds including protease inhibitors4–6 and recep-
tor ligands (Fig. 1).7 Arguably, the most important class, ACE
inhibitors,8 represented by enalapril 2, (Fig. 1) have been used
extensively as drugs with therapeutic significance for the treatment
of cardiovascular diseases, hypertension and related disorders.
Another important class of peptides contain homophenylalanine in-
hibit matrix metalloproteinases4 3 (R1 = Bn, iPr, tested for the treat-
ment of arthritis) and cysteine protease5 4 (R2 = iPr, CH2CH2CH2OH).
Tripeptides 5,6 which contain oxo-Hfe, have been proven to have
anti-inflammatory properties. Generally, the synthesis of Hfe de-
rived biologically active peptides requires the incorporation of a
single enantiomer of Hfe into a complex structure via classic amide
bond formation. Due to this being such an important part of many
biologically active substances, Hfe as an unnatural amino acid has
attracted significant attention from the synthetic community with
an efficient, scalable synthesis of both enantiomers being required.9
Since the first reported synthesis of a single enantiomer of Hfe via
classic resolution of rac-Hfe in 1938 by du Vigneaud and Irish,10
miscellaneous synthetic methods have been developed,9 including
metal-catalysed11 and enzymatic processes.12 One of the ap-
proaches allowing the production of both enantiomers of Hfe em-
2. Results and discussion
Over the last decade we and others presented successful exam-
ples of process CIAT in conjugate additions, where we had ob-
served a similar scenario. Miscellaneous 4-substituted (4-aryl,
heteroaryl, alkyl) 4-oxobutenoic acids (eg. 6) were reacted with
enantiomerically pure nucleophiles 7 represented by amines or
aminoalcohols (R = Me or CH2OH) to give rise highly enantio- and
diastereoenriched N-protected 4-aryl- and 4-alkyl-4-oxo-2-amin-
obutanoic acids (for example 8, Scheme 1). We observed 1,3-asym-
metric inductions, where the original stereogenic centre was
located two bonds away from the newly formed stereogenic cen-
tre. We are aware that the high diastereoselectivity of the Michael
addition originated from an arrangement in the crystal lattice and
does not rely on kinetic or thermodynamic control.13,14 Having
extensively investigated the addition of chiral nucleophiles to
ploys
a
crystallisation-induced asymmetric transformation
⇑
Tel./fax: +421 2 52968560 (D.B.).
E-mail addresses: pavol.jakubec@zoznam.sk (P. Jakubec), dusan.berkes@stuba.sk
(D. Berkeš).
Present address: Department of Chemistry, Chemistry Research Laboratory,
University of Oxford, Mansfield Road, Oxford OX1 3TA, UK.
0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.11.015

2808
P. Jakubec, D. Berkeš / Tetrahedron: Asymmetry 21 (2010) 2807–2815
COOH
O
O
N
COOH
SH
COOH
COOH
O
HN
NH2
COOH
HN
O
HN
O
NH
H
H
N
R
H
N
N
COOMe
NHPh
COOEt
O
O
R1
O
R2
5
2
3
4
1
(S)-Hfe (R = H)
matrix metalloproteinase inhibitors
enalapril
cystein protease inhibitors
Figure 1. Homophenylalanine and peptides containing Hfe.
Table 1
R
The synthesis of enantiomerically pure Michael acceptors 11a–f
R
H2N
+
Ph
Entry
Ar
13
R
Amide
Yield (%)
7
O
Michael addition
CIAT
O
HN
Ph
12a
12a
12a
12b
12b
12b
Ph
Ph
Ph
4-MeOPh
4-MeOPh
4-MeOPh
Phe
Phg
Leu
Phe
Leu
Val
CH2Ph
Ph
CH2CH(CH3)2
CH2Ph
CH2CH(CH3)2
CH(CH3)2
11a
11b
11c
11d
11e
11f
83
74
82
85
83
89
Ar
COOH
Ar
Ar
COOH
8
6
FGI
Dipetides containig Hfe
Hfe
NH2
H
NH2
N
COOH
Ar
COOH
9
1
CIAT,13,14 only limited solubility of the dipeptide ensured crystalli-
sation from a saturated solution. On the other hand, a certain
amount of solubility was necessary for an efficient epimerisation
in the solution. Based on our assumptions and previous experience
we chose EtOH as a solvent for our initial studies. Solvent EtOH
was quickly identified as being very effective for the CIAT
(Scheme 3). The stereochemical development in the conjugate
addition of benzylamine 7a to amide 11a was monitored by HPLC
analysis (Fig. 2). At an early stage of the reaction, poor diastereose-
lectivity was observed, originating from the low asymmetric
O
R
FGI
H2N
+
Ph
O
O
NHR
H
N
H
N
COOH
COOH
Ar
Ar
CIAT
10
O
R
O
R
11
Scheme 1. Retrosynthesis of dipeptides containing Hfe.
achiral Michael acceptors,16,18 we carried out the addition of achi-
ral nucleophiles to enantiomerically pure Michael acceptors for the
synthesis of Hfe-containing dipeptides. In order to study the feasi-
bility and assess the scope of the process CIAT in the Michael addi-
tion, suitable Michael acceptors 11a–f were synthesised on a gram
scale in two steps (Scheme 2).20 (E)-3-Aroylacrylic acids 6a,b21
were treated with DCC and N-hydroxysuccinimide at ꢀ20 °C in tet-
rahydrofuran for 24 h. The isolated active esters 12a,b were then
Ph
Ph
O
O
H
N
H2N
EtOH
H
N
7a
COOH
Ph
H3N
COO
Ph
Ph
Ph
O
O
11a
14
coupled to enantiomerically pure
thoxyethane. The amides 11a–f were of sufficient purity to be used
in subsequent conjugate additions without purification (Table 1).
a-amino acids 13a–d in dime-
Ph
O
Ph
O
O
HN
O
HN
H
N
H
N
COOH
Ph
(2S,2´S)-10a
Scheme 3. The Michael addition of BnNH2 to 11a.
COOH
Ph
Ph
Ph
O
O
O
i
O
Ar
COOH
6a,b
Ar
N
12a,b
O
(2S,2´R)-10a'
O
H2N
COOH
ii
13a-d
R
O
100
H
N
COOH
90
80
70
60
50
40
30
20
10
0
Ar
O
R
11a-f
Scheme 2. Reagents and conditions: (i) DCC, N-hydroxysuccinimide, THF, ꢀ20 °C,
24 h, Ar = Ph (80%), An (64%); (ii) NaHCO3, DME–H2O, rt, 1 h.
Initially, proof of principle studies were performed on a repre-
sentative unsaturated chiral amide 11a, and the reliable achiral
nucleophile-benzylamine 7a (Scheme 3). Both starting materials
are typical organic compounds with high solubility in organic sol-
vents, contrary to the product of the conjugate addition 10a in a
zwitterion form, which was expected to have low solubility in or-
ganic solvents. According to the essential criteria for a successful
1
1.5
2
3
4
18
24
Time [h]
27
45
51
66
72 138 214
Figure 2. Stereochemical development in the transformation of 11a to 10a; j-
(2S,20S)-10a, N-(2S,20R)-10a0.

P. Jakubec, D. Berkeš / Tetrahedron: Asymmetry 21 (2010) 2807–2815
2809
induction in the Michael addition (dr 10a:10a0 40:60 after 1 h).
Ph
However, as the reaction progressed, the (2S,20S)-diastereoisomer
10a became the major diastereoisomer in the reaction with a
simultaneous decline of the other (2S,20R)-diastereoisomer 10a0.
Simultaneously, a significant change in appearance of the reaction
mixture took place. The solution became saturated and 10a began
to crystallise out, resulting in a dramatic change in the ratio of dia-
stereoisomers. A simultaneous epimerisation of the other diaste-
reoisomer 10a0 in the solution shifted the equilibrium to the side
of the preferentially crystallising (2S,20S)-diastereoisomer 10a.
After 214 h (dr 10a:10a0 in the heterogeneous mixture 94:6), 10a
was isolated by simple filtration as virtually single diastereoisomer
(dr 96:4) in 53% yield. The other diastereomer 10a0 remained the
major isomer in the mother liquor after diastereoisomer 10a was
removed by filtration. Overall, CIAT overrode the initial 1,3-asym-
metric induction (10a:10a0 after 1 h in the homogeneous mixture
40:60) and the original major diastereomer 10a0 became the minor
isomer of the system after CIAT took place (10a:10a0 94:6 after
214 h in the heterogeneous mixture).
O
O
HN
i
H
N
H
N
COOH
COOH
COOH
Ar
Ph
O
O
R
R
O
R
R
10a-c
11a-c
Ph
ii
O
O
HN
*
O
H
N
H
N
Ar
O NH3Bn
Ar
15a-c
O
11c-e
Scheme 4. Reagents and conditions: (i) BnNH2 (1.1 equiv), H2O, 40 °C; (ii) BnNH2
(2 equiv), H2O, 40 °C.
100
90
80
70
60
50
40
30
20
10
0
Further optimisation of the reaction using water as the solvent
improved the chemical yield up to 83% (Table 2).
With a range of suitable substrates 11b–f in hand, the unsat-
urated amides 11b,c were then investigated in the conjugate
addition (Scheme 4, Table 2). Benzylamine 7a was added to a
suspension of amides 11b,c and the mixture was stirred at
40 °C until constant, high diastereoselectivity was achieved. The
desired Michael adducts 10b,c were isolated in high yield and
high diastereoselectivity (Scheme 4, Table 2). Under identical
reaction conditions, the 4-methoxyphenyl substituted Michael
acceptors 11d,e only created salts with benzylamine. However,
increasing the amount of benzylamine (2 equiv) facilitated the
formation of benzyl ammonium salts of Michael adducts 15b–c
in good chemical yield and excellent stereochemical purity
(Scheme 4, Table 2).
Thorough analysis of the diastereomeric development in the
addition of BnNH2 to amide 11c revealed the unique example of
process CIAT. The addition of BnNH2 7a to 11c is described above
and the stereochemical course of the addition is depicted in
Figure 3a. This result does not differ from the other examples
described. Under identical reaction conditions, except for doubling
the amount of nucleophile, noteworthy results were observed
(Scheme 6, Fig. 3b). In the heterogenic mixture, after only 3 h,
the diastereomeric salt 15a was the major component and after
120 h the (2S,20R)-diastereomer 15a was isolated by filtration in
high diastereomeric purity (dr 99:1). We assumed that we could
control the stereoselectivity of this Michael addition simply by
varying the amount of the nucleophile. Indeed, the addition of
1 equiv of BnNH2 to diastereomerically pure peptide 10c followed
by stirring of the reaction mixture at 40 °C resulted in nearly com-
plete inversion of the labile stereogenic centre and the synthesis of
the benzyl ammonium salt of its diastereomer 15a (Scheme 6,
Reaction C, Fig. 3c). Without any observable epimerisation, the dia-
stereomer (2S,20R)-10c0 was released from its benzylammonium
salt 15a with 4 M HCl (Scheme 6). This unique example of CIAT
1
2
3
4
7
9
11
24
48
72
Time [h]
Figure 3a. Stereochemical development in the transformation of 11c to 10c
(Reaction A) (2S,20S)-10c,
(2S,20R)-10c0.
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
6
8
16
23
28
120
Time [h]
Figure 3b. Stereochemical development in the transformation of 11c to 15a
(Reaction B) (2S,20S)-15a0, (2S,20R)-15a.
is the first of its kind to combine CIAT with a conjugate addition.
Moreover, it represents a rare example of inversion of configura-
tion via an elimination–addition mechanism, known from the
other examples of the CIAT processes in the conjugate addition of
N-nucleophiles published by our group.16,18 Also, it is supported
by our previous mechanistic studies on closely related CIAT pro-
cesses in the Mannich reaction.15d
Having investigated the addition of achiral nucleophiles, we
next investigated the addition of chiral nucleophiles with the
intention to synthesise analogues of matrix metalloproteinase.
From a range of possible nucleophiles we chose three easily
Table 2
The scope and limitations of the Michael addition of BnNH2 to the chiral Michael acceptors
Entry
Ar
R
Adduct
Time (days)
Yield (%)
Dr*
11a
11b
11c
11c
11d
11e
Ph
Ph
Ph
Ph
4-MeOPh
4-MeOPh
CH2Ph
Ph
CH2CH(CH3)2
CH2CH(CH3)2
CH2Ph
(2S,20S)-10a
(2S,20S)-10b
(2S,20S)-10c
(2S,20R)-15a
(2S,20S)-15b
(2S,20R)-15c
2
2
3
5
2
7
83
91
68
75
83
76
96:4
98:2
95:5
1:99
99:1
5:95
CH2CH(CH3)2
*
Ratio (2S,20S):(2S,20R).

2810
100
P. Jakubec, D. Berkeš / Tetrahedron: Asymmetry 21 (2010) 2807–2815
H
N
O
Ph
90
80
70
60
50
40
30
20
10
0
O
HN
H
N
i
H
N
Ph
11a + 17
COO H3N
Ph
Ph
O
O
18
Scheme 7. Reagents and conditions: (i) H2O, 40 °C, 24 h, 86%, dr 97:3.
0
1
2
4
6
24
42
48
54
72
Time [h]
Figure 3c. Stereochemical development in the transformation of 10c to 15a
(Reaction C) (2S,20S)-10c,
(2S,20R)-15a.
Table 3
The synthesis of dipeptides containing Hfe 9a–f
Entry
Ar
R
Dipeptide
Yield (%)
iii
i, ii
H
N
H
N
(2S,20S)-9a
(2S,20S)-9b
(2S,20S)-9c
(2S,20R)-9d
(2S,20S)-9e
(2S,20R)-9f
72
84
55
67
86
56
Ph
10a
10b
10c
15a
15b
15c
Ph
Ph
Ph
Ph
4-MeOPh
4-MeOPh
CH2Ph
Ph
CH(CH3)2
CH(CH3)2
CH2Ph
L-Ala
Ph
17
BocHN
16
H2N
O
O
Scheme 5. Reagents and conditions: (i) Boc2O, dioxane, H2O, rt, 1 h; (ii) ClCOOiBu,
Et3N, DCM, ꢀ5 °C, 5 min, then BnNH2, rt, 1 h, 58% over two steps; (iii) HCl, dioxane,
rt, 18 h, then 4M NaOH, 75%.
CH(CH3)2
available derivatives of alanine: the methyl and benzyl ester22 and
benzylamide, which was synthesised from alanine in two steps
(Scheme 5).
Ph
O
HN
i
H
N
COOH
After extensive screening of the reaction conditions (concen-
tration, temperature, solvents) for all nucleophiles, the benzyla-
mide of alanine 17 was found to be a suitable partner in
conjugate addition to amide 11a. The treatment of amide 11a
with 2 equiv of nucleophile 17 afforded adduct 18 in good yield
with excellent diastereoisomeric purity (86%, dr 97:3, Scheme 7).
To rationalise the observed stereoselectivity, we measured the
diastereomeric ratio in the reaction. The initial diastereomeric
ratio was low (dr 60:40 after 1 h), suggesting low asymmetric
induction in the Michael addition before CIAT took place. How-
ever, after 24 h (dr in the heterogeneous mixture 87:13) 18
was isolated by simple filtration as a virtually single diastereoiso-
mer (dr 97:3).23 We believe that such a dramatic change in
diastereoselectvity can be rationalised simply by CIAT occurring
in the conjugate addition. The Michael adduct 18 was isolated
as a salt with the benzylamide of alanine 17.
For the products of the conjugate addition 10a–c and 15a–c to
be of use in a synthesis of simple Hfe-containing dipeptides,
reductive manipulation of both the carbonyl and benzylic group
was required. The reductive removal of both functional groups
via standard catalytic hydrogenation was investigated initially.
Reduction of the carbonyl group and simultaneous debenzyla-
tion were accomplished by catalytic hydrogenation (Scheme 8,
Table 3). In the case of phenyl substituted derivatives 10a–c
(Ar = Ph), the stable dipeptides 9a–c were obtained by hydrogena-
tion in EtOH/water/HBr (Scheme 8, Table 3). The same process was
unsuccessful for 15a–c and led only to a mixture of unidentified
products. The best results for the benzylammonium salts 15a–c
Ar
O
R
NH2
*
10a-c
H
N
COOH
Ar
Ph
R
O
9a-f
O
HN
O
H
N
ii
Ar
O NH3Bn
*
O
R
15a-c
Scheme 8. Reagents and conditions: (i) H2, HBr, EtOH, 40 °C, 30–40 h; (ii) H2,
H2SO4, EtOH, 50 °C, 24–48 h.
(Ar = An, Ph) were obtained in the EtOH/H2SO4 system previously
used by Yamada (Scheme 8, Table 3) with only negligible epimer-
isation.16a In NMR spectra of compounds 9a–f only a single diaste-
reomer was the observed.
After hydrolysis of dipeptides 9a–f under standard conditions24
(6 M HCl, reflux, 30 h, Scheme 9), it was possible to confirm the
absolute configuration of the newly synthesised stereogenic centre
by means of HPLC using a chiral column CROWNPAK CR(+) and the
appropriate Hfe standard. Minimal racemisation was observed
during hydrolysis and the measured enantiomeric excess of hom-
ophenylalanine (ee of 1a,b ranged between 96–99%) corresponded
well with the diastereomeric ratio of Michael adducts 10a–c and
15a–c.
Ph
Ph
O
Ph
O
Reaction A
BnNH2 (1.1 eq.),
H2O
Reaction B
Ph
O
HN
O
O
HN
O
HN
H
N
H
N
H
N
H
N
HCl
BnNH2 (2.0 eq.),
H2O
COOH
H3N
COO
COOH
COOH
Ph
Ph
Ph
Ph
58%
10c
11c
15a
10c'
O
O
from 10c
BnNH2 (1.0 eq.)
H2O
Reaction C
Scheme 6. Formally stereodivergent Michael addition of BnNH2 to 11c involving CIAT.

P. Jakubec, D. Berkeš / Tetrahedron: Asymmetry 21 (2010) 2807–2815
2811
NH2
*
evaporated in vacuo. The crude product was recrystallised from
propan-2-ol.
NH2
i
H
N
+
COOH
H2N
COOH
Ar
Ar
COOH
*
O
R
R
4.2.1. 1-{[(2E)-4-Oxo-4-phenylbut-2-enoyl]oxy}pyrrolidine-2,5-
dione 12a
9a-f
1a,b
13a-c
According to general procedure A (for 160.1 mmol, 28.212 g of
6a) were used 168.1 mmol, 19.350 g of N-hydroxysuccinamide;
168.1 mmol, 34.684 g of DCC and 700 mL of THF. The crude prod-
uct was recrystallised from propan-2-ol (700 mL) to afford 12a
(35.210 g, 80%) as a yellow solid. Mp = 105–107 °C; 1H NMR
(DMSO-d6): 2.88 (s, 4H), 7.02 (d, 1H, J = 15.9), 7.57–7.76 (m, 5H),
8.29 (d, 1H, J = 15.9); 13C NMR (DMSO-d6): 25.4, 124.7, 129.0,
134.3, 135.5, 142.1, 161.1, 169.9, 188.5. Calcd for C14H11NO5: C,
61.54; H, 4.06; N, 5.13. Found: C, 61.14; H, 4.03; N, 5.31.
Scheme 9. Reagents and conditions: 6 M HCl, reflux, 30 h, Ar = Ph, An, 50–82%.
3. Conclusion
A simple, scalable method for the synthesis of enantiomerically
pure dipeptides containing Hfe involving CIAT in the Michael addi-
tion was developed. A unique example of stereodivergent CIAT in
the Michael addition was discovered and interconversion of one
diastereomer into the other one via CIAT was described. CIAT occu-
red in the Michael addition of BnNH2 and benzylamide of alanine
to chiral derivatives of aroylacrylic acids, thus extending our meth-
odology to 1,5-induction. Further development of this work and its
application in synthesis of matrix metalloproteinase inhibitors are
currently ongoing in our laboratory, and the work will be disclosed
in due course.
4.2.2. 1-{[(2E)-4-(4-Methoxyphenyl)-4-oxobut-2-enoyl]oxy}-
pyrrolidine-2,5-dione 12b
According to general procedure A (for 2.4 mmol, 0.500 g of 6b)
were used 2.50 mmol, 0.293 g of N-hydroxysuccinamide,
2.5 mmol, 0.525 g of DCC and 24 mL of THF. The crude product
was recrystallised from isopropanol (20 mL) to afford 12b
(0.471 g, 64%) as
a
yellow solid. Mp = 134–135 °C; 1H NMR
(DMSO-d6): 2.90 (s, 4H), 3.90 (s, 3H), 6.99 (d, 2H, J = 8.8), 7.02 (d,
1H, J = 15.8), 8.01 (d, 2H, J = 8.8), 8.13 (d, 1H, J = 15.2); 13C NMR
(DMSO-d4): 25.5, 55.5, 114.2, 125.3, 128.9, 131.4, 140.7, 161.1,
164.5, 168.8, 186.0. Calcd for C15H13NO6: C, 59.41; H, 4.32; N,
4.62. Found: C, 59.79; H, 4.33; N, 4.81.
4. Experimental
4.1. General
Reagents were used as received without further purification, un-
less otherwise stated. Melting points were obtained using Kofler hot
plate and are uncorrected. Optical rotations were measured with a
4.3. General procedure B for the synthesis of amides 11
POLAR L-lP polarimeter (IBZ Messtechnik) with a water-jacket
To a solution of ester 12 (18.3 mmol, 5.000 g) in dimethoxyeth-
10.000 cm cell at a wavelength of sodium line D (k = 589 nm). Spe-
cific rotations are given in units of 10ꢀ1 deg cm2 gꢀ1 and concentra-
tions are given in g/100 mL. 1H NMR spectra were recorded on a
Varian VXR-300 (299.94 MHz) spectrometer. Chemical shifts (d)
are quoted in parts per million and are referenced to tetramethylsil-
ane (TMS) as an internal standard (dMe = 0.00 for 299.94 MHz). Cou-
pling constant (J) are recorded in Hertz. The following abbreviations
were used throughout to characterise signal multiplicities: s (sin-
glet), d (doublet), t (triplet), m (multiplet), br (broad). Abbreviations
with quotation marks mean that the appearance of the signal is dif-
ferent from what is theoretically predicted. 13C NMR spectra were
recorded on a Varian VRX-300 (75.43 MHz) spectrometer. The mul-
tiplicities of the carbons were assigned from a broadband decou-
pled analysis used in a conjunction with either APT or DEPT.
Chemical shifts (d) are quoted in parts per million and are refer-
enced to tetramethylsilane (TMS) as an internal standard (d = 0.00
for 75.43 MHz). HPLC experiments were performed on a PYE UNI-
CAM chromatographic system (PU 4015 pump working in isocratic
mode). Multichannel detector PU 4021 was performed in SUM ABS
mode at wavelengths ranging from 210 to 310 nm. The following
ane (100 mL) was added a mixture of L-aminoacid 13 (1.1 equiv,
20.1 mmol) and sodium hydrogen carbonate (20.1 mmol, 1.691 g)
in water (150 mL) at rt. After 1 h, water (70 mL) was added and
the pH of the solution was adjusted to 2.0 using 4 M HCl. The
resulting emulsion was cooled to ꢀ20 °C and kept for 2 h at this
temperature. The crystallised solid was filtered off, washed with
cold water and dried to afford 11.
4.3.1. N-[(2E)-4-Oxo-4-phenylbut-2-enoyl]-L-phenylalanine 11a
According to general procedure B (for 18.3 mmol, 5.000 g of
12a) were used 20.1 mmol, 3.325 g of Phe 13a; 20.1 mmol,
1.691 g of NaHCO3; 100 mL of DME and 150 mL; 70 mL of water
were added during work-up. 11a (35.210 g, 83%) was isolated as
a yellow solid. Mp = 139–142 °C (AcOEt–CH2Cl2); ½a D25
¼ ꢀ4:8 (c
ꢂ
1.0, MeOH). 1H NMR (DMSO-d6): 2.92 (dd, 1H, J = 13.9, 10.3), 3.15
(dd, 1H, J = 13.9, 5.1), 4.55–4.62 (m, 1H), 7.03 (d, 1H, J = 15.4),
7.18–8.01 (m, 11H); 8.97 (d, 1H, J = 8.1), 12.94 (br s, 1H); 13C
NMR (DMSO-d6): 36.6, 53.8, 126.5, 128.2, 128.6, 128.9, 129.0,
132.4, 133.7, 135.4, 136.4, 137.3, 163.2, 172.5, 189.6. Calcd for
C19H17NO4: C, 70.58; H, 5.30; N, 4.33. Found: C, 69.73; H, 5.37;
columns and eluents were used for HPLC experiments: C18 5 lm
N, 4.43.
reverse phase column Phenomenex Luna C18 250 ꢁ 4.6 mm or
Phenomenex Luna Phenyl-Hexyl 250 ꢁ 4.6 mm or Waters Symme-
4.3.2. (2S)-{[(2E)-4-Oxo-4-phenylbut-2-
enoyl]amino}(phenyl)acetic acid 11b
try 5
l
m C18 250 ꢁ 4.6 mm. the eluent was a mixture of acetoni-
trile/water/Et3N 400:600:10 or 250:750:15 and the pH adjusted
to 2.9–3.5 by H3PO4. The mobile phase was pumped through the
system at 0.5–1.5 mL/min at room temperature.
According to general procedure B (for 7.3 mmol, 2.000 g of 12a)
were used 8.1 mmol, 1.217 g of Phg; 8.1 mmol, 0.680 g of NaHCO3;
40 mL of DME and 60 mL of water; 30 mL of water were added dur-
ing work-up. 11b (1.66 g, 74%) was isolated as a yellow solid.
4.2. General procedure A for the synthesis of esters 12
Mp = 87–90 °C (toluene); ½a D25
ꢂ
¼ ꢀ158:1 (c 1.0, MeOH); 1H NMR
A solution of acid 621 (1.00 mmol) and N-hydroxysuccinimide
(1.05 mmol) in dry THF (10 mL) was cooled to 0 °C and dicyclohex-
ylcarbodiimide (1.05 mmol) was added with stirring. The mixture
was kept in a refrigerator at ꢀ20 °C for 24 h. The separated N,
N-dicyclohexylurea was removed by filtration and the solvent
(DMSO-d6): 5.49 (d, 1H, J = 7.3), 7.21 (d, 1H, J = 15.4), 7.27–8.02
(m, 11H), 9.36 (d, 1H, J = 7.3); 13C NMR (DMSO-d6): 56.6, 127.6,
128.1, 128.4, 128.6, 128.9, 132.8, 133.6, 135.7, 136.5, 136.6,
163.1, 171.4, 189.9. Calcd for C18H15NO4: C, 69.89; H, 4.89; N,
4.53. Found: C, 69.37; H, 4.90; N, 4.64.

2812
P. Jakubec, D. Berkeš / Tetrahedron: Asymmetry 21 (2010) 2807–2815
4.3.3. N-[(2E)-4-Oxo-4-phenylbut-2-enoyl]-
To a solution of ester 12a (22.3 mmol, 6.100 g) in dimethoxy-
ethane (140 mL) was added mixture of Leu (23.4 mmol,
L
-leucine 11c
4.4. General procedure C for the Michael addition of BnNH2 to
amides 11
a
3.075 g) and sodium hydrogen carbonate (23.4 mmol, 1.966 g) in
water (210 mL) at rt. After 1 h, water (100 mL) was added and
the pH of the solution was adjusted to 2 using 4 M HCl. The result-
ing emulsion was extracted (AcOEt, 2 ꢁ 50 mL). The combined
organics were dried (Na2SO4) and concentrated in vacuo. The crude
product was recrystallised (toluene, 40 mL) to afford 11c (5.311 g,
A mixture of unsaturated amide (1 equiv) 11 and benzylamine
7a (excess) was stirred at 40 °C until HPLC analysis showed a con-
stant, high dr in the reaction mixture (typically 2–6 days). The sus-
pension was cooled to rt, at which point, the solid was filtered off,
washed (Et2O) and dried to obtain peptide 10 or 15 as a white
solid.
82%) as a white solid. Mp = 121–123 °C (toluene); ½a D25
¼ ꢀ26:4 (c
ꢂ
1.0, MeOH); 1H NMR (DMSO-d6): 0.87 (d, 3H, J = 5.9), 0.91 (d, 3H,
J = 5.9); 1.51–1.71 (m, 3H), 4.30–4.40 (m, 1H), 7.07 (d, 1H,
J = 15.4), 7.56–7.71 (m, 3H), 7.78 (d, 1H, J = 15.4), 7.96–8.03 (m,
2H); 8.86 (d, 1H, J = 7.3); 13C NMR (DMSO-d6): 21.2, 22.8, 24.3,
39.9, 50.6, 128.6, 128.9, 132.4, 133.7, 135.8, 136.5, 163.3, 173.6,
189.7. Calcd for C16H19NO4: C, 66.42; H, 6.62; N, 4.84. Found: C,
65.92; H, 6.61; N, 4.91.
4.4.1. N-[(2S)-2-(Benzylamino)-4-oxo-4-phenylbutanoyl]-L-
phenylalanine 10a
According to general procedure C (for 0.31 mmol, 0.100 g of
11a) were used 0.34 mmol, 37 L of BnNH2 7a and 4 mL of water;
l
2 days. 10a (0.110 g, 83%, dr 96:4) was isolated as a colourless so-
lid. Mp = 180–182 °C; ½a D25
ꢂ
¼ ꢀ18:3 (c 0.36, MeCN:1 M HCl 3:1); 1H
NMR (acetone-d6/DCl): 3.03 (dd, 1H, J = 14.1, 9.8), 3.26 (dd, 1H,
J = 14.1, 4.7), 3.86 (dd, 1H, J = 18.8, 6.4), 4.04 (d, 1H, J = 12.8), 4.06
(dd, 1H, J = 18.8, 6.0), 4.17 (d, 1H, J = 12.8), 4.54 (‘t’, 1H, J = 6.0,
6.0), 4.77 (dd, 1H, J = 9.8, 4.3), 7.02–7.64 (m, 16H); 13C NMR (ace-
tone-d6/DCl): 38.5, 40.8, 51.6, 55.7, 57.2, 128.3, 129.9, 130.1,
130.4, 130.6, 131.1, 132.1, 132.3, 135.6, 137.3, 138.9, 168.5,
173.5, 197.9.
4.3.4. N-[(2E)-4-(4-Methoxyphenyl)-4-oxobut-2-enoyl]-L-
phenylalanine 11d
According to general procedure B (for 16.5 mmol, 5.000 g of
12b) were used 18.1 mmol, 2.996 g of Phe; 18.1 mmol, 1.521 g of
NaHCO3; 100 mL of DME and 150 mL of water; 70 mL of water
added during work-up) 11d (4.950 g, 85%) was isolated as a pale-
yellow solid. Mp = 200–202 °C (AcOEt–EtOH); ½a D25
ꢂ
¼ 0:0 (c 0.5,
4.4.2. (2S)-2-((S)-2-(Benzylamino)-4-oxo-4-phenylbutanamido)-
2-phenylacetic acid 10b
According to general procedure C (for 0.64 mmol, 0.200 g of 11b
were used 0.71 mmol, 78 lL of BnNH2 7a and 6 mL of water;
MeOH);25 1H NMR (DMSO-d6): 2.93 (dd, 1H, J = 13.9, 9.5), 3.15
(dd, 1H, J = 13.9, 5.1), 3.85 (s, 3H), 4.55–4.62 (m, 1H), 7.01 (d, 1H,
J = 15.4), 7.08 (d, 2H, J = 8.8); 7.18–7.28 (m, 5H), 7.75 (d, 1H,
J = 14.6), 8.00 (d, 2H, J = 8.8); 8.98 (d, 1H, J = 8.1); 13C NMR
(DMSO-d6): 36.7, 53.9, 55.6, 114.3, 126.5, 128.2, 129.1, 129.4,
131.1, 132.4, 135.0, 137.5, 163.5, 163.7, 172.6, 187.6. Calcd for:
2 days). Compound 10b (0.245 g, 91%, dr 98:2) was isolated as a
colourless solid. Mp = 181–183 °C; ½a D25
¼ ꢀ57:8 (c 1.0, MeOH:1 M
ꢂ
HCl 3:1); 1H NMR (DMSO-d6/DCl): 3.71 (dd, 1H, J = 18.2, 5.9), 3.82
(dd, 1H, J = 18.2, 6.4), 4.04 (d, 1H, J = 12.9), 4.09 (d, 1H, J = 13.4),
4.48 (‘t’, 1H, J = 6.4, 5.9), 5.34 (s-broad, 1H), 7.33–7.95 (m, 15H),
9.32 (d, 1H, J = 7.0). 13C NMR (DMSO-d6/DCl): 49.4, 54.6, 57.0,
128.0, 128.4, 128.7, 129.0, 129.1, 129.3, 129.6, 130.5, 131.3,
134.4, 135.9, 136.5, 167.0, 171.3, 195.8 (a signal of 1C overlapped
by signal of DMSO-d6).
C20H19NO5: C, 67.98; H, 5.42; N, 3.96. Found: C, 68.04; H, 5.49;
N, 4.12.
4.3.5. N-[(2E)-4-(4-Methoxyphenyl)-4-oxobut-2-enoyl]-L-
leucine 11e
According to general procedure B (for 9.9 mmol, 3.000 g of
12b) were used 10.4 mmol, 1.362 g of Leu; 10.4 mmol, 0.874 g
of NaHCO3; 70 mL of DME and 100 mL of water; 50 mL of water
were added during work-up. 11d (2.640 g, 83%) was isolated as a
4.4.3. N-[(2S)-2-(Benzylamino)-4-oxo-4-phenylbutanoyl]-L-
leucine 10c
pale-yellow solid. Mp = 159–162 °C (AcOEt); ½a D25
ꢂ
¼ ꢀ26:6 (c 1.0,
According to general procedure C (for 2.2 mmol, 0.645 g of 11c
were used 2.5 mmol, 268 L of BnNH2 7a and 10 mL of water;
MeOH); 1H NMR (DMSO-d6): 0.87 (d, 3H, J = 5.8), 0.91 (d, 3H,
J = 5.8), 1.56–1.74 (m, 3H), 3.86 (s, 3H), 4.37 (dt, 1H, J = 8.8,
6.6), 7.05 (d, 1H, J = 15.4), 7.09 (d, 2H, J = 8.8), 7.83 (d, 1H,
J = 15.4), 8.02 (d, 2H, J = 8.8); 8.83 (d, 1H, J = 8.8); 13C NMR
(DMSO-d6): 21.2, 22.7, 24.3, 39.9, 50.6, 55.6, 114.2, 129.4,
131.1, 132.3, 135.1, 163.5, 163.6, 173.6, 187.6. Calcd for:
l
3 days) 10c (0.601 g, 68%, dr 95:5) was isolated as a colourless so-
lid. Mp = 162–165 °C; ½a D25
ꢂ
¼ ꢀ8:5 (c 0.5, MeOH:1 M HCl 3:1); 1H
NMR (acetone-d6/DCl): 0.88 (d, 3H, J = 6.0), 0.89 (d, 3H, J = 6.0),
1.57–1.71 (m, 3H), 3.99 (dd, 1H, J = 18.8, 5.6), 4.13 (dd, 1H,
J = 18.8, 6.0), 4.37 (dd, 1H, J = 9.8, 4.7), 4.46 (d, 1H, J = 13.7), 4.50
(d, 1H, J = 13.7), 4.66 (‘t’, 1H, J = 6.0, 5.6), 6.91–8.00 (m, 11H); 13C
NMR (acetone-d6/DCl): 22.4, 24.0, 26.2, 40.8, 41.2, 51.9, 52.9,
57.0, 129.9, 130.4, 130.6, 131.1, 132.2, 132.2, 135.6, 137.3, 168.8,
174.7, 198.1.
C17H21NO5: C, 63.94; H, 6.63; N, 4.39. Found: C, 63.25; H, 6.64;
N, 4.50.
4.3.6. N-[(2E)-4-(4-Methoxyphenyl)-4-oxobut-2-enoyl]-L-valine
11f
According to general procedure B (for 11.6 mmol, 3.530 g of
12b) were used 12.2 mmol, 1.429 g of Val; 12.2 mmol, 1.023 g
of NaHCO3; 50 mL of DME and 70 mL of water; 40 mL of water
were added during work-up. Compound 11f (3.150 g, 89%) was
isolated as a pale-yellow solid. Mp = 196–199 °C (AcOEt–EtOH);
4.4.4. Phenylmethanaminium (S)-2-((R)-2-(benzylamino)-4-
oxo-4-phenylbutanamido)-4-methylpentanoate 15a
According to general procedure C (for 6.9 mmol, 2.000 g of 11c
were used 13.8 mmol, 1.51 mL of BnNH2 7a and 30 mL of water;
5 days) 15a (2.613 g, 75%, dr 99:1) was isolated as a colourless so-
½
a 2D5
ꢂ
¼ ꢀ18:1 (c 1.0, MeOH); 1H NMR (DMSO-d6): 0.92 (d, 6H,
lid. Mp = 105–108 °C; ½a D25
ꢂ
¼ ꢀ20:2 (c 1.0, MeOH:1 M HCl 3:1); 1H
J = 7.3), 2.07–2.18 (m, 1H), 3.86 (s, 3H), 4.30 (dd, 1H, J = 8.5,
6.1), 7.09 (d, 2H, J = 8.5), 7.20 (d, 1H, J = 15.3), 7.79 (d, 1H,
J = 15.3), 8.02 (d, 2H, J = 8.5); 8.72 (d, 1H, J = 8.5), 12.77 (s-broad,
1H); 13C NMR (DMSO-d6): 17.9, 19.1, 29.8, 55.6, 57.6, 114.2,
129.4, 131.1, 132.4, 135.3, 163.6, 163.7, 172.5, 187.7. Calcd for:
NMR (acetone-d6/DCl): 0.76 (d, 3H, J = 6.1), 0.84 (d, 3H, J = 6.1),
1.51–1.82 (m, 3H), 4.00 (dd, 1H, J = 18.3, 7.9), 4.18 (dd, 1H,
J = 18.3, 4.3), 4.23 (s, 2H), 4.37 (dd, 1H, J = 10.4, 4.3), 4.40 (d, 1H,
J = 12.8), 4.47 (d, 1H, J = 12.8), 4.72 (dd, 1H, J = 7.9, 4.3), 7.32–8.00
(m, 16H); 13C NMR (acetone-d6/DCl): 22.0, 24.1, 26.1, 40.1, 41.0,
44.6, 51.3, 53.0, 57.4, 129.8, 130.1, 130.3, 130.4, 130.5, 130.6,
131.0, 132.1, 132.6, 134.6, 134.8, 135.5, 168.5, 175.1, 197.8.
C16H19NO5: C, 62.94; H, 6.27; N, 4.59. Found: C, 62.62; H, 6.33;
N, 4.65.

P. Jakubec, D. Berkeš / Tetrahedron: Asymmetry 21 (2010) 2807–2815
2813
4.4.5. Phenylmethanaminium (S)-2-((S)-2-(benzylamino)-4-(4-
methoxyphenyl)-4-oxobutanamido)-3-phenylpropanoate 15b
According to general procedure C (for 5.7 mmol, 0.200 g of 11d
and water (100 mL). The organic layer was dried (Na2SO4) and con-
centrated in vacuo. The crude product was recrystallised from tol-
uene (50 mL) to afford 16 (16.211 g, 58%) as a colourless solid.
Mp = 104–105 °C, lit.26 mp = 100–102 °C, lit.27 mp = 104–106 °C;
were used 11.3 mmol, 124 lL of BnNH2 7a and 10 mL of water;
2 days) 15b (0.271 g, 83%, dr 99:1) was isolated as a colourless so-
½
a 2D5
ꢂ
¼ ꢀ22:0 (c 1.0, CHCl3), lit.27
½
a 2D2
ꢂ
¼ ꢀ24:5 (c 1.9, CHCl3); 1H
lid. Mp = 129–131 °C; ½a D25
ꢂ
¼ ꢀ18:2 (c 1.0, MeOH:1 M HCl 3:1); 1
H
NMR (CDCl3): 1.36 (d, 3H, J = 7.3), 1.39 (s, 9H), 4.22 (br s, 1H),
4.41 (br s, 2H), 5.20 (s-broad, 1H), 6.83 (br s, 1H), 7.21–7.32 (m,
5H); 13C NMR (CDCl3): 18.4, 28.2, 43.3, 50.1, 80.1, 127.3, 127.5,
128.6, 138.1, 155.5, 172.7.
NMR (acetone-d6/DCl): 3.04 (dd, 1H, J = 14.1, 9.8), 3.27 (dd, 1H,
J = 14.1, 4.7), 3.81 (dd, 1H, J = 18.4, 6.8), 3.84 (s, 3H), 4.02 (d, 1H,
J = 12.8), 4.07 (dd, 1H, J = 18.8, 6.0), 4.16 (d, 1H, J = 12.8), 4.24 (s,
2H), 4.54 (‘t’, 1H, J = 6.4, 6.0), 4.78 (dd, 1H, J = 9.8, 4.7), 6.96–7.92
(m, 19H); 13C NMR (acetone-d6/DCl): 38.6, 40.3, 44.7, 51.6, 55.8,
57.0, 57.4, 115.7, 128.4, 130.3, 130.4, 130.5, 130.6, 130.7, 131.0,
131.1, 132.2, 132.5, 134.9, 135.1, 139.0, 166.0, 168.6, 173.6, 196.4.
4.4.9. N-Benzyl-L-alaninamide 17
A mixture of 16 (58.2 mmol, 16.200 g) and 4 M HCl (160 mL) was
stirred at rt. After 18 h the mixture was extracted with EtOAc
(100 mL) and the organic layer was discarded. The pH of the water
phasewasadjustedto10.0using4MNaOHandextractedwithAcOEt
(3 ꢁ 100 mL). The organics were dried (Na2SO4) and concentrated in
4.4.6. Phenylmethanaminium (S)-2-((R)-2-(benzylamino)-4-(4-
methoxyphenyl)-4-oxobutanamido)-4-methylpentanoate 15c
According to general procedure C (for 0.31 mmol, 0.100 g of 11e
vacuo to afford 17 (7.731 g, 75%) as colourless oil. ½a D25
¼ þ7:2 (c 1.0,
ꢂ
were used 0.63 mmol, 68 lL of BnNH2 7a and 5 mL of water; at
CHCl3), lit.28
½
a 2D0
ꢂ
¼ þ16:8 (c 1.41, benzene); 1H NMR (CDCl3): 1.37
40 °C, 7 days) 15c (0.127 g, 76%, dr 95:5) was isolated as a colour-
(d, 3H, J = 6.8), 1.77 (s-broad, 2H), 3.58 (s-broad, 1H); 4.44 (d, 2H,
J = 5.6); 7.22–7.36 (m, 5H, H-Ph); 7.65 (s-broad, 1H); 13C NMR
(CDCl3): 21.7, 43.0, 50.7, 127.3, 127.6, 128.6, 138.4, 175.4.
less solid. Mp = 82–86 °C; ½a D25
¼ ꢀ21:1 (c 1.0, MeOH:1 M HCl 3:1);
ꢂ
1H NMR (acetone-d6/DCl): 0.73 (d, 3H, J = 6.6), 0.83 (d, 3H, J = 6.6),
1.39–1.71 (m, 3H), 3.63 (dd, 1H, J = 17.5, 7.3), 3.71 (dd, 1H, J = 17.5,
5.1), 3.81 (s, 3H), 3.97 (br s, 2H), 4.10–4.14 (m, 2H), 4.26 (d, 1H,
J = 13.2), 4.33 (dd, 1H, J = 7.3, 5.1), 7.05 (d, 2H, J = 8.8), 7.31–7.55
(m, 10H), 7.91 (d, 2H, J = 8.8); 13C NMR (acetone-d6/DCl): 21.2,
23.1, 24.5, 38.1, 42.2, 48.8, 51.2, 55.1, 55.2, 55.9, 114.4, 128.7,
128.9, 129.0, 129.2, 129.4, 130.5, 130.7, 131.5, 134.0, 134.0,
163.9, 166.9, 173.7, 194.0.
4.4.10. (S)-1-(Benzylamino)-1-oxopropan-2-aminium (S)-2-(2-
((S)-1-(benzylamino)-1-oxopropan-2-ylamino)-4-oxo-4-
phenylbutanamido)-3-phenylpropanoate 18
A mixture of amide (3.1 mmol, 1.000 g) 11a and the benzyla-
mide of alanine 17 (2.1 equiv, 6.5 mmol, 1.157 g) was stirred at
40 °C until HPLC analysis had shown complete consumption of
the starting materials (24 h). The suspension was cooled to rt, at
which point, the solid was filtered off, washed (Et2O) and dried
to obtain peptide 18 (1.810 g, 86%, dr 97:3) as a white solid.
4.4.7. N-[(2R)-2-(Benzylamino)-4-oxo-4-phenylbutanoyl]-L-
leucine 10c0
A mixture of peptide 10c (0.9 mmol, 0.380 g, dr 96:4) and ben-
zylamine 7a (1.0 equiv, 0.9 mmol, 0.100 g, 102 L) in water (6 mL)
Mp = 165–166 °C; ½a D25
ꢂ
¼ ꢀ11:0 (c 1.0, MeOH:1 M HCl 3:1); 1H
l
NMR (DMSO-d6): 1.08 (d, 3H, J = 7.0), 1.29 (m, 3H), 2.90–2.96 (m,
2H), 3.03–3.12 (m, 2H), 3.16 (dd, 1H, J = 17.0, 8.8), 3.49 (dd, 1H,
J = 8.2, 3.5), 3.65–3.72 (m, 1H), 4.19 (dd, 1H, J = 14.6, J = 5.3), 4.26
(dd, 1H, J = 12.3, J = 6.4), 4.31 (d, 2H, J = 5.9), 4.36 (dd, 1H,
J = 14.6, J = 6.4), 7.14–7.88 (m, 20H), 8.05 (d, 1H, J = 7.0), 8.37 (‘t’,
1H, J = 5.9, 5.9), 8.76 (s-broad, 1H); 13C NMR (DMSO-d6): 18.9,
19.8, 37.2, 41.8, 41.9, 42.0, 48.9, 54.2, 55.9, 57.4, 125.9, 126.6,
126.8, 127.0, 127.1, 127.7, 127.9, 128.2, 128.2, 128.6, 129.4,
133.2, 136.5, 138.4, 139.0, 139.3, 171.9, 173.1, 174.1, 198.0.
was stirred at 40 °C until HPLC analysis had shown inversion of the
labile stereogenic centre (72 h). The suspension was cooled to rt, at
which point, the pH was adjusted to 2.5 by the addition of 4 M HCl
and the suspension was stirred at rt. After 2.5 h, the solid was fil-
tered off, washed (Et2O) and dried to obtain peptide 10c0
(0.210 g, 58%, dr 96:4) as a colourless solid. Mp = 133–137 °C;
½
a 2D5
ꢂ
¼ ꢀ16:6 (c 0.4, MeOH:1 M HCl 3:1); 1H NMR (acetone-d6/
DCl): 0.78 (d, 3H, J = 5.9), 0.85 (d, 3H, J = 5.9), 1.54–1.79 (m, 3H),
4.00 (dd, 1H, J = 18.2, 7.6), 4.18 (dd, 1H, J = 18.2, 4.1), 4.38 (dd,
1H, J = 10.3, 4.4), 4.43 (d, 1H, J = 12.9), 4.49 (d, 1H, J = 12.9), 4.70
(dd, 1H, J = 7.6, J = 4.1), 7.39–7.8.01 (m, 11H); 13C NMR (acetone-
d6/DCl): 22.1, 24.1, 26.2, 40.1, 41.1, 51.4, 53.0, 57.5, 129.9, 130.5,
130.6, 131.0, 132.2, 132.7, 135.6, 137.5, 174.0, 196.8.
4.5. General procedure D for the synthesis of dipeptides 9a–c
To a solution of Michael adduct 10 in EtOH, water and HBr
(2 equiv) was added Pd/C (20% of weight). The resulting heteroge-
neous mixture was stirred in an H2 atmosphere (1.1 atm) at 40 °C
until HPLC analysis showed the complete consumption of the start-
ing materials (typically 30 h). The suspension was cooled to rt, the
insoluble solid was filtered off, washed (EtOH). The filtrate was
concentrated to approximately 1/3 of the original volume, the pH
was adjusted to 6–6.5. The separated solid was filtered off, washed
(Et2O) and dried affording 9 as a white solid.
4.4.8. (tert-Butyl [(2S)-1-(benzylamino)-1-oxopropan-2-
yl]carbamate 16
To a solution of (S)-Ala (100.0 mmol, 8.900 g) in dioxane
(200 mL) and water (100 mL) was added Boc2O (1.1 equiv,
110.0 mmol, 24.010 g) at 0 °C and the resulting mixture was stirred
at rt. After 1 h the mixture was concentrated to a volume of 100 mL
and EtOAc (100 mL) was added. The pH was then adjusted to 2.0.
The organic layer was separated and the water phase was ex-
tracted with AcOEt (2 ꢁ 100 mL). The combined organic layers
were dried (Na2SO4) and concentrated in vacuo to afford Boc-Ala
(19.987 g, ꢃ100%) as a colourless solid. DCM (200 mL) was added
followed by the addition of Et3N (100.00 mmol, 13.94 mL). The
resulting solution was cooled to ꢀ5 °C. Isobutyl chloroformate
(100.00 mmol, 13.14 mL) was added and the mixture was stirred
for 5 min at ꢀ5 °C. A pre-cooled solution of BnNH2 (100.00 mmol,
10.92 mL) in DCM (80 mL) was added in one portion and the
resulting suspension was stirred at rt. After 1 h the mixture was
extracted subsequently with water (2 ꢁ 100 mL), NaHCO3 (5%
solution in water, 100 mL), HCl (1% solution in water, 3 ꢁ 50 mL)
4.5.1. N-[(2S)-2-Amino-4-phenylbutanoyl]-L-phenylalanine 9a
According to general procedure D (for 4.6 mmol, 2.000 g of 10a
were used 9.3 mmol, 1.600 g of 48% solution of HBr; 40 mL of EtOH,
8 mL of water and 0.400 g of Pd/C; 30 h) 9a (1.080 g, 72%, dr
>98:229) was isolated as a colourless solid. Mp = 276–279 °C
(EtOH–H2O); ½a 2D5
ꢂ
¼ þ20:7 (c 0.5, MeOH:1 M HCl 3:1); 1H NMR
(acetone-d6/DCl): 2.10–2.18 (m, 2H), 2.59–2.57 (m, 2H), 3.11 (dd,
1H, J = 14.1, 9.0), 3.28 (dd, 1H, J = 14.1, 5.6), 3.97 (t, 1H, J = 6.4),
4.50 (dd, 1H, J = 9.0, 5.6), 7.23–7.44 (m, 10H). 13C NMR (DMSO-
d6/DCl): 30.4, 33.6, 36.6, 52.2, 54.5, 126.6, 127.1, 128.7, 128.9,
129.0, 129.6, 137.7, 141.4, 168.9, 172.9.

2814
P. Jakubec, D. Berkeš / Tetrahedron: Asymmetry 21 (2010) 2807–2815
4.5.2. (S)-2-((S)-2-Amino-4-phenylbutanamido)-2-phenylacetic
acid 9b
mixture stirred for an additional 24 h under a H2 atmosphere at
50 °C) 9f (0.680 g, 56%, dr >98:229) was isolated as a colourless so-
According to general procedure D (for 8.4 mmol, 3.500 g of 10b
were used 16.8 mmol, 2.89 g of 48% solution of HBr; 70 mL of
EtOH, 14 mL of water and 0.700 g of Pd/C; 30 h) 9b (2.21 g, 84%,
dr >98:229) was isolated as a colourless solid. Mp = 240–244 °C
lid. Mp = 228–232 °C (EtOH–H2O, mp of a dried gel), ½a D25
¼ ꢀ24:7
ꢂ
(c 0.5, 0.1 M NaOH), 1H NMR (D2O/DCl): 0.94 (d, 3H, J = 6.0), 0.98 (d,
3H, J = 6.0), 1.65–1.80 (m, 3H), 2.27–2.34 (m, 2H), 2.67–2.84 (m,
2H), 3.83 (s, 3H), 4.29 (‘t’, 1H, J = 6.4, J = 6.0), 4.43 (dd, 1H, J = 5.5,
J = 9.0, H-2), 6.98 (d, 2H, J = 8.5), 7.27 (d, 2H, J = 8.5); 13C NMR
(D2O/DCl): 23.0, 24.8, 27.1, 31.9, 35.2, 41.6, 54.1, 55.6, 57.9,
116.8, 132.0, 134.9, 160.0, 172.2, 178.4.
(EtOH–H2O); ½a 2D5
ꢂ
¼ þ119:2 (c 0.5, 0.1 M NaOH); 1H NMR (D2O/
DCl): 2.24–2.32 (m, 2H), 2.75–2.92 (m, 2H), 4.10 (‘t’, 1H, J = 6.4,
6.0), 5.19 (s, 1H), 7.28–7.50 (m, 10H); 13C NMR (D2O/DCl): 30.9,
33.5, 53.7, 58.5, 127.7, 128.7, 129.5, 129.9, 130.3, 130.4, 135.6,
141.1, 170.1, 174.0.
4.7. General procedure F for the hydrolysis of peptides 9a–f24
4.5.3. N-[(2S)-2-Amino-4-phenylbutanoyl]-
L
-leucine 9c
A suspension of peptide 9 in 6 M HCl (5 mL per 0.3 mmol of 9)
was stirred at reflux until HPLC analysis showed the complete con-
sumption of the starting materials (typically 30 h). The mixture
was cooled to rt and the pH of the mixture was adjusted to 6.0–
6.5. The separated insoluble solid (50–82% yield, 95–99% ee) was
filtered off, washed (Et2O) and dried to afford 9 as a white solid.
A sample was analysed by HPLC [CROWNPAK CR (+) column, a
solution of HClO4 in H2O pH adjusted to 2.0 as an eluent, flow
1.0 mL/min].
According to general procedure D (for 3.0 mmol, 1.200 g of 10c
were used 6.1 mmol, 1.042 g of 48% solution of HBr; 40 mL of EtOH,
8 mL of water and 0.240 g of Pd/C; 40 h) 9c (0.480 g, 55%, dr
>98:229) was isolated as a colourless solid. Mp = 241–243 °C
(EtOH–H2O); ½a 2D5
ꢂ
¼ ꢀ2:0 (c 0.5, MeOH:1 M HCl 3:1); 1H NMR
(D2O/DCl): 0.92 (d, 3H, J = 5.3), 0.95 (d, 3H, J = 5.3), 1.62–1.79 (m,
3H), 2.15–2.29 (m, 2H), 2.65–2.87 (m, 2H), 4.15 (‘t’, 1H, J = 6.4,
5.9), 4.43 (dd, 1H, J = 5.9, J = 8.2), 7.27–7.42 (m, 5H). 13C NMR
(D2O/DCl): 23.6, 25.0, 27.3, 32.7, 35.6, 42.0, 54.6, 55.6, 129.6,
131.3, 131.8, 143.1, 172.5, 178.7.
Acknowledgements
Financial support by the Slovak Grant Agency No. 1/0629/08
and NMR measurements provided by the Slovak State Programme
Project No. 2003SP200280203 are gratefully acknowledged.
4.6. General procedure E for the synthesis of dipeptides 9d–f
To a solution of Michael adducts 15a–c in EtOH and H2SO4
(0.5 M solution) was added Pd/C (20% of weight). The resulting het-
erogeneous mixture was stirred in an H2 atmosphere (1.1 atm) at
50 °C until HPLC analysis showed the complete consumption of
the starting materials (typically 24 h). The suspension was cooled
to rt, and the insoluble solid was filtered off, washed (EtOH) and
the pH of the filtrate was adjusted to 12. After being stirred for
10 min, the pH of the solution was adjusted to 6.5. The separated
solid was filtered off, washed (Et2O) and dried affording 9 as a col-
ourless solid.
References
1. Mehner, C.; Müller, D.; Krick, A.; Kehraus, S.; Löser, R.; Gütschow, M.; Maier, A.;
Fiebig, H.-H.; Brun, R.; König, G. M. Eur. J. Org. Chem. 2008, 10, 1732–1739.
2. Kimura, J.; Takada, Y.; Inayoshi, T.; Nakao, Y.; Goetz, G.; Yoshida, W. Y. J. Org.
Chem. 2002, 67, 1760–1767.
3. Reshef, V.; Carmeli, S. J. Nat. Prod. 2002, 65, 1187–1189.
4. (a) Chapman, K. T.; Wales, J.; Sahoo, S. P.; Niedzwiecki, L. M.; Izquierdo-Martin,
M.; Chang, B. C.; Harrison, R. K.; Stein, R. L.; Hangmann, W. K. Bioorg. Med. Chem.
Lett. 1996, 6, 329–332; (b) Chapman, R. T.; Kopka, I. E.; Durette, P. L.; Esser, C.
K.; Lanza, T. J.; Izquierdo-Martin, M.; Niedzwiecki, L.; Chang, B.; Harrison, R. K.;
Kuo, D. W.; Lin, T.-Y.; Stein, R. L.; Hagmann, W. K. J. Med. Chem. 1993, 36, 4293–
4301.
5. (a) Roush, W. R.; Hernandez, A. A.; McKerrow, J. H.; Selzer, P. M.; Hansellb, E.;
Engel, J. C. Tetrahedron 2000, 56, 9747–9762; (b) Roush, W. R.; Hernandez, A. A.;
Zepeda, G. Synthesis 1999, 1500–1504.
6. Berrée, F.; Chang, K.; Cobas, A.; Rapoport, H. J. Org. Chem. 1996, 61, 715–721.
7. Ehrlich, P. P.; Ralston, J. W.; Michaelides, M. R. J. Org. Chem. 1997, 62, 2782–
2785.
8. Bommarius, A. S.; Riebel, B. R. Biocatalysis: Fundamentals and Applications;
Wiley-VCH Verlag GmbH: Weinheim, 2004.
4.6.1. N-[(2R)-2-Amino-4-phenylbutanoyl]-L-leucine 9d
According to general procedure E (for 4.0 mmol, 2.000 g of 15a
were used 120 mL of 0.5 M H2SO4, 40 mL of EtOH and 0.400 g of Pd/
C; 48 h) 9d (0.781 g, 67%, dr >98:229) was isolated as a colourless
solid. Mp = 274–279 °C (EtOH–H2O); ½a D25
¼ ꢀ40:3 (c 0.5, 0.1 M
ꢂ
NaOH); 1H NMR (D2O/DCl): 0.97 (d, 3H, J = 4.1), 1.00 (d, 3H,
J = 4.8), 1.62–1.82 (m, 3H), 2.28–2.35 (m, 3H), 2.72–2.89 (m, 2H),
4.25 (‘t’, 1H, J = 6.9, 6.2), 4.47 (dd, 1H, J = 8.9, 5.5), 7.33–7.46 (m,
5H); 13C NMR (D2O/DCl): 23.4, 25.3, 27.6, 33.2, 35.5, 42.0, 54.5,
56.0, 129.7, 131.1, 131.8, 142.8, 172.7, 179.0.
9. Ahmad, A. L.; Oh, P. C.; Abd Shukor, S. R. Biotechnol. Adv. 2009, 27, 286–296.
10. du Vigneaud, V.; Irish, O. J. J. Biol. Chem. 1938, 122, 349–370.
11. For a selected example see: Xie, Y.; Lou, R.; Li, Z.; Mi, A.; Jiang, Y. Tetrahedron:
Asymmetry 2000, 11, 1487–1494.
12. For a selected example see: Kao, C.-H.; Lo, H.-H.; Hsu, S.-K.; Hsu, W.-H. J.
Biotechnol. 2008, 134, 231–239.
4.6.2. N-[(2S)-2-Amino-4-(4-methoxyphenyl)butanoyl]-L-
13. For the definition of CIAT see: Eliel, E. L.; Wilen, S. H.; Mander, L. N.
Stereochemistry of Organic Compounds; John Wiley & Sons: New York, 1994.
14. For reviews on CIAT see: (a) Ryuzu, Y. Top. Curr. Chem. 2007, 269, 83–132; (b)
Brands, K. M. J.; Davies, A. J. Chem. Rev. 2006, 106, 2711–2733; (c) Anderson, N.
G. Org. Process Res. Dev. 2005, 9, 800–813.
phenylalanine 9e
According to general procedure E (for 7.5 mmol, 4.270 g of 15b
were used 180 mL of 0.5 M H2SO4, 60 mL of EtOH and 0.854 g of
Pd/C; 24 h) 9e (2.295 g, 86%, dr >98:229) was isolated as a colour-
less solid. Mp = 255–258 °C (EtOH–H2O); ½a D25
¼ þ35:3 (c 0.5,
ꢂ
15. For recent selected examples of CIAT see: (a) Xiao, S.; Lu, X.; Shi, X.-X.; Sun, Y.;
Liang, L.-L.; Yu, X.-H.; Dong, J. Tetrahedron: Asymmetry 2009, 20, 430–439; (b)
Kanomata, N.; Mishima, G.; Onozato, J. Tetrahedron Lett. 2009, 50, 409–412; (c)
Brands, J.; Pye, P.; Rossen, K. Asymmetric Synthesis, 2nd ed.; Wiley-VCH Verlag
0.1 M NaOH); 1H NMR (D2O/DCl): 2.09–2.17 (m, 2H), 2.55–2.73
(m, 2H), 3.12 (dd, 1H, J = 14.1, 8.5), 3.27 (dd, 1H, J = 14.1, 6.0),
3.83 (s, 3H), 4.00 (‘t’, 1H, J = 6.4, 6.0), 4.64 (dd, 1H, J = 8.5, 6.0),
6.97 (d, 2H, J = 9.0), 7.19 (d, 2H, J = 9.0), 7.30–7.42 (m, 5H); 13C
NMR (DMSO-d6/DCl): 29.0, 33.5, 36.1, 51.7, 53.9, 54.9, 113.7,
126.4, 128.2, 129.0, 132.8, 137.3, 157.5, 168.6, 172.4.
ˇ
GmbH&Co. KGaA: Weinheim, 2008; (d) Jakubec, P.; Petráš, P.; Duriš, A.; Berkeš,
D. Tetrahedron: Asymmetry 2010, 21, 69–74.
16. (a) Yamada, M.; Nagashima, N.; Hasegawa, J.; Takahashi, S. Tetrahedron Lett.
ˇ
ˇ
1998, 39, 9019–9022; (b) Kolarovic, A.; Berkeš, D.; Baran, P.; Povazanec, F.
Tetrahedron Lett. 2001, 42, 2579–2582.
17. For related classic syntheses of b-amino acids employing an aza-Michael
addition see: (a) Juaristi, E.; Escalante, J.; Lamatsch, B.; Seebach, D. J. Org. Chem.
1992, 57, 2396–2398; (b) Davies, S. G.; Smith, A. D.; Price, P. D. Tetrahedron:
Asymmetry 2005, 16, 2833–2891. and references cited therein..
4.6.3. N-[(2R)-2-Amino-4-(4-methoxyphenyl)butanoyl]-L-
leucine 9f
According to general procedure E (for 3.7 mmol, 2.000 g of 15c
were used 120 mL of 0.5 M H2SO4, 40 mL of EtOH and 0.520 g of
Pd/C; after 24 h, an additional 0.52 g of Pd/C was added; and the
ˇ
ˇ
18. (a) Kolarovic, A.; Berkeš, D.; Baran, P.; Povazanec, F. Tetrahedron Lett. 2005, 46,
ˇ
975–978; (b) Jakubec, P.; Berkeš, D.; Šiška, R.; Gardianová, M.; Povazanec, F.
Tetrahedron: Asymmetry 2006, 17, 1629–1637; (c) Jakubec, P.; Berkeš, D.;
ˇ
ˇ
Kolarovic, A.; Povazanec, F. Synthesis 2006, 23, 4032–4040; (d) Berkeš, D.;

P. Jakubec, D. Berkeš / Tetrahedron: Asymmetry 21 (2010) 2807–2815
2815
ˇ
Jakubec, P.; Winklerova, D.; Povazanec, F.; Daich, A. Org. Biomol. Chem. 2007, 5,
121–124.
24. Bodansky, M.; Bodansky, A. The Practice of Peptide Synthesis; Springer: Berlin
Heidelberg, 1994.
ˇ
19. Jakubec, P.; Berkeš, D.; Povazanec, F. Tetrahedron Lett. 2004, 45, 4755–4758.
25. The compound 11d is cryptochiral.
20. Spratt, T. E.; Kaiser, E. T. J. Am. Chem. Soc. 1984, 106, 6440–6442.
21. (a) Grummitt, O.; Becker, E. I.; Miesse, C. Org. Synth. 1949, 29, 11–13; (b)
Kameo, K.; Ogawa, K.; Takeshita, K.; Nakaike, S.; Tomisawa, K.; Sota, K. Chem.
Pharm. Bull. 1988, 36, 2050–2060.
26. Kim, J.-M.; Wilson, T. E.; Norman, T. C.; Schulz, P. G. Tetrahedron Lett. 1996, 37,
5309–5312.
27. Saito, Y.; Ouchiy, H.; Takahata, H. Tetrahedron 2008, 64, 11129–11135.
28. Kurganov, A.; Zhuchkoua, L.; Davankov, V. Liebigs Ann. Chem. 1980, 786–790.
29. Only one diastereomer was observed by 1H NMR.
22. Urbach, H.; Henning, R. Tetrahedron Lett. 1984, 25, 1143–1146.
23. All attempts to assign
a relative stereochemistry at the newly formed
stereogenic centre via chemical transformations were unsuccessful.