Efficient resolution of racemic N-benzyl β3-amino acids by iterative liquid-liquid extraction with a chiral (salen)cobalt(iii) complex as enantioselective selector

Article abstract of DOI:10.1039/b711477j

The efficient (up to 93% ee) resolution of racemic N-benzyl β³-amino acids has been achieved by an iterative (two cycle) liquid-liquid extraction process using a lipophilic chiral (salen)cobalt(iii) complex [Co^III(1)(OAc)]. As a result of the resolution by extraction, one enantiomer of the N-benzyl β³-amino acid predominated in the aqueous phase, while the other enantiomer was driven into the organic phase by complexation to cobalt. The complexed amino acid was then quantitatively released into an aqueous phase, by a reductive (Co^III → Co ^II) counter-extraction using l-ascorbic acid. The reductive cleavage allowed for the recovery of the cobalt(ii) selector in up to 90% yield (easily re-oxidable to Co^III with air/AcOH). The Royal Society of Chemistry.

Full text of DOI:10.1039/b711477j

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Efficient resolution of racemic N-benzyl b3-amino acids by iterative
liquid–liquid extraction with a chiral (salen)cobalt(III) complex as
enantioselective selector†
Pawel Dzygiel,a Chiara Monti,a Umberto Piarulli*b and Cesare Gennari*a
Received 27th July 2007, Accepted 11th September 2007
First published as an Advance Article on the web 18th September 2007
DOI: 10.1039/b711477j
The efficient (up to 93% ee) resolution of racemic N-benzyl b3-amino acids has been achieved by an
iterative (two cycle) liquid–liquid extraction process using a lipophilic chiral (salen)cobalt(III) complex
[CoIII(1)(OAc)]. As a result of the resolution by extraction, one enantiomer of the N-benzyl b3-amino
acid predominated in the aqueous phase, while the other enantiomer was driven into the organic phase
by complexation to cobalt. The complexed amino acid was then quantitatively released into an aqueous
phase, by a reductive (CoIII → CoII) counter-extraction using L-ascorbic acid. The reductive cleavage
allowed for the recovery of the cobalt(II) selector in up to 90% yield (easily re-oxidable to CoIII with
air/AcOH).
with classical resolution via crystallization of diastereomeric salts:
1. Introduction
on production scale this is often the slowest step in the process.
Following the above concepts, we have recently disclosed the
resolution of racemic N-benzyl a-amino acids by liquid–liquid
extraction, using a lipophilic chiral (salen)cobalt(III) complex
[CoIII(1)(OAc)] (ligand 1 = N,Nꢀ-bis(3,5-di-tert-butylsalicylidene)-
1,2-cyclohexanediamine), in excellent yield and enantioselectivity.6
As a result of the resolution by extraction, one enantiomer of
the N-benzyl a-amino acid predominated in the aqueous phase,
while the other enantiomer was driven into the organic phase
by complexation to cobalt. The complexed amino acid was then
released by a reductive (CoIII → CoII) counter-extraction into an
aqueous phase. The original chiral complex [CoIII(1)(OAc)] could
be restored and reused with essentially no loss of reactivity and
selectivity.
b-Peptides are appealing substrates due to their stability to
proteolitic enzymes and their ability to form specific folded
structures and mimic a-peptides in peptide–protein and protein–
protein interactions.‡1 For these reasons, the practical production
of enantiopure b-amino acids is an important and challenging
endeavour for the chemical community. Numerous methodologies
for the enantioselective synthesis of these compounds have recently
emerged, which often require multistep processes and are usually
rather limited in scope.1b,2 On the other hand, the large-scale
separation of racemic mixtures of b-amino acids has been essen-
tially restricted to enzymatic resolution processes (using lipases,
aminoacylases, etc.),3 chiral HPLC separation being confined to
analytical purposes.4
An alternative and promising methodology is the enantiomeric
separation of racemic mixtures of hydrophilic substrates by liquid–
liquid extraction, where one enantiomer is driven into the organic
phase by selective coordination to a hydrophobic selector (e.g. a
chiral metal complex), leaving the uncomplexed enantiomer in the
aqueous phase.5 One of the attractions of this method is that it
circumvents the use of excessive solids handling that is associated
2. Results and discussion
Herein, we report on the highly enantioselective separation
of racemic N-benzyl b3-amino acids by iterative liquid–liquid
extraction using [CoIII(1)(OAc)]. A racemic mixture of N-benzyl
b3-homophenylglycine (N-Bn-b3-hPhg) (2 equiv) was dissolved in
water and added to a dichloromethane solution of [CoIII(1)(OAc)]
(1 equiv) at 10 ◦C (Scheme 1).§ The biphasic mixture was
thoroughly stirred, then the two phases were separated. Practically
one equivalent (≥0.95 equiv) of N-benzyl b3-homophenylglycine
was driven into the organic phase by complexation to the
chiral selector, and the unbound N-benzyl b3-homophenylglycine
present in the aqueous phase (≥0.95 equiv) was analysed by chiral
HPLC, showing a 55% enantiomeric excess (ee) in favour of the
R enantiomer. By evaporation of the dichloromethane phase,
a brown-green solid [CoIII(1)(N-Bn-b3-hPhg)] was isolated and
characterised by FT-IR, HRMS, 1H- and 13C-NMR spectroscopy.
aDipartimento di Chimica Organica e Industriale, Centro di Eccellenza
C.I.S.I., Universita` degli Studi di Milano, Via G. Venezian, 21, 20133,
Milano, Italy. E-mail: cesare.gennari@unimi.it; Fax: +39 02 5031 4072;
Tel: +39 02 5031 4091
bDipartimento di Scienze Chimiche e Ambientali, Universita` degli Studi
dell’Insubria, Via Valleggio, 11, 22100, Como, Italy. E-mail: umberto.
piarulli@uninsubria.it
† Electronic supplementary information (ESI) available: Chiral HPLC
traces of the N-benzyl b3-amino acids, NMR spectra (1H, 13C) of
the N-benzyl b3-amino acids and of the cobalt complexes. See DOI:
10.1039/b711477j
‡ Recently, a b3-dodecapeptide has been prepared and its secondary and
tertiary structures studied. The oligomer forms a 314-helix secondary
structure (characteristic of b3-amino acids) and crystallizes as an octamer
which is best described as a pair of tetrameric “hands”, see ref. 15. b3-
Amino acids are also efficient enantioselective catalysts for the aldol and
Mannich reactions, see ref. 16.
§ We have observed a moderate effect of the temperature in this extraction
and the best enantiomeric excesses were obtained operating between 5 and
10 ◦C (lower temperatures are not possible because of the solidification of
the reaction mixture).
3464 | Org. Biomol. Chem., 2007, 5, 3464–3471
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
Scheme 1 Resolution of a racemic mixture of N-benzyl b3-homophenylglycine by liquid–liquid extraction using the chiral complex [CoIII(1)(OAc)].
Unfortunately, no conclusive evidence regarding the diastere-
omeric composition of the complex could be obtained from the
1H- and 13C-NMR spectra (see also the ESI†).
(in favour of the S enantiomer). From the above discussion it is
evident that, while the separation yield is satisfactory (≥95%),
the ee’s are only moderate. To improve the enantiomeric excess to
a preparatively useful level, we decided to iterate the extraction
protocol once.
After the first separation step, which was performed using
1 equiv of the chiral selector [CoIII(R,R-1)(OAc)] and 2 equiv of
the racemic b-amino acids (see above), two alternative paths were
followed (Scheme 2). In the first procedure (see Scheme 2, upper
part, and Table 1), the aqueous phase (Aq1), which is enriched in
the R enantiomer (ee 55%), was treated with 0.5 equiv of [CoIII(S,S-
1)(OAc)] in order to complex the more abundant enantiomer. In
this way 0.48 equiv of N-Bn-b3-hPhg with 16% ee were found in
the aqueous phase (Aq3), while [CoIII(S,S-1)(N-Bn-b3-hPhg)] was
isolated from the organic phase (Org2), which, after cleavage with
L-ascorbic acid and resin purification, gave 0.47 equiv of (R)-N-
Bn-b3-hPhg in 90% ee (Aq4).
The release of N-benzyl b3-homophenylglycine from the
cobalt(III) complex was then studied. In the case of N-benzyl a-
amino acids,6 a reductive cleavage with aqueous sodium dithionite
(10 equiv) had been used. This caused the reduction of cobalt
and formation of complex [CoII(1)] with concurrent release of the
bound N-benzyl a-amino acid into the aqueous solution. While
this methodology allowed for the quantitative recovery of the
released amino acid, the yield of the reduced cobalt complex was
generally moderate (≤78%) and not always reproducible. Many
other reducing agents were then investigated and it was found
that L-ascorbic acid was definitely more efficient and reliable.7
In fact, reaction of complex [CoIII(1)(N-Bn-b3-hPhg)] with an
equimolar amount of L-ascorbic acid in methanol for 15 min
caused precipitation of the red [CoII(1)] complex, which was
isolated by filtration. In this way, up to 90% recovery of the
cobalt(II) complex was obtained (the average recovery yields were
in the range 85–90%). The methanolic filtrate was then applied
to a Dowex 50W-X8 resin, to eliminate dehydroascorbic acid and
excess L-ascorbic acid, and eluted with 2 M ammonia to give pure
N-benzyl b3-homophenylglycine in quantitative yield and 58% ee
The organic phase (Org1) was treated with L-ascorbic acid and
Dowex 50W-X8, to release the complexed amino acid; in this
way 0.95 equiv of N-benzyl b3-homophenylglycine (ee = 58% in
favour of the S enantiomer) was isolated (Aq2). Reaction with
0.5 equiv of [CoIII(R,R-1)(OAc)] yielded an aqueous phase (Aq5)
containing 0.48 equiv N-benzyl-b3-homophenylglycine (16% ee)
Scheme 2 Iterative extraction of N-Bn-b-amino acids with [CoIII(1)OAc]. In the upper part of the scheme the aqueous phases Aq1 and Aq2 are reacted
with the enantiomer of the cobalt complex which is more selective for the major enantiomer; in the lower part of the scheme the minor enantiomer of the
mixture is preferentially complexed. The HPLC traces in the scheme refer to the iterative extraction of N-Bn-b3-hPhg (see the ESI†).
This journal is
The Royal Society of Chemistry 2007
Org. Biomol. Chem., 2007, 5, 3464–3471 | 3465
©

View Article Online
Table 1 Iterative extraction of N-Bn-b3-hPhg using [CoIII(1)OAc]
Aqueous phasea
Selector (equiv)
N-Bn-b3-hPhg (equiv)
% Extract. yield
% Selector recoveryb
% eec,d (abs. conf.)
Aq1
Aq2
Aq3
Aq4
Aq5
Aq6
Aq3ꢀ
Aq4ꢀ
Aq5ꢀ
Aq6ꢀ
[Co(R,R-1)(OAc)] (1.0)
[Co(S,S-1)(OAc)] (0.5)
[Co(R,R-1)(OAc)] (0.5)
[Co(R,R-1)(OAc)] (0.5)
[Co(S,S-1)(OAc)] (0.5)
0.97e
0.95f
0.48e
0.47f
0.48e
0.47f
0.48e
0.47f
0.47e
0.46f
55 (R)
58 (S)
16 (R)
90 (R)
16 (S)
90 (S)
93 (R)
10 (S)
93 (S)
5 (R)
95f
94f
94f
93f
92f
90
82
85
84
87
a See Scheme 2 for the origin of the aqueous phases. b Based on the yield of [CoII(1)] after reductive cleavage with L-ascorbic acid. c Determined by chiral
HPLC analysis using a Chirobiotic T column. d The absolute configuration was determined by comparison of the optical rotation with reported values
(see the Experimental section). e The equiv were calculated after evaporation of the aqueous phase. f The equiv and yield were calculated after cleavage of
the cobalt(III) complex with L-ascorbic acid and purification on the Dowex 50W-X8 resin.
and the organic phase (Org3) which, after the usual work up (Aq6),
gave 0.47 equiv N-benzyl b3-homophenylglycine in 90% ee (in
favour of the S enantiomer). The aqueous phases Aq3 and Aq5
could be combined (to yield ca. 1 equiv of almost rac-N-benzyl
b3-homophenylglycine) and recycled into the feeding phase of the
process. In addition, the cobalt selector (in the form of the reduced
Co2+ species) was recovered (85–90%), oxidised (air/AcOH) and
re-used.
Following an alternative path (see Scheme 2, lower part, and
Table 1), the aqueous phase (Aq1) was extracted with 0.5 equiv
of [CoIII(R,R-1)(OAc)]; in this way, the minor component of the
mixture was complexed and the R-enantiomer (0.48 equiv) was
obtained uncomplexed in the aqueous phase (Aq3ꢀ) in 93% ee.
The organic phase (Org2ꢀ), treated in sequence with L-ascorbic
acid and Dowex W50-X8, yielded 0.47 equiv of N-Bn-b3-hPhg
in 10% ee in favour of the S-enantiomer (Aq4ꢀ). The aqueous
phase (Aq2) was subjected to extraction with [CoIII(S,S-1)(OAc)]:
the aqueous phase (Aq5ꢀ) gave 0.47 equiv of N-Bn-b3-hPhg in
93% ee for the S-enantiomer, while the organic phase (Org3ꢀ),
after the usual treatment, gave 0.46 equiv of N-Bn-b3-hPhg in
5% ee (Aq6ꢀ). The organic phases Org2ꢀ and Org3ꢀ could be
combined, treated in sequence with L-ascorbic acid and Dowex
W50-X8, to yield approximately 1 equiv of almost rac-N-benzyl
b3-homophenylglycine, which could be recycled into the feeding
phase of the process.
Due to the high extraction yields of the complexation steps
and release of N-Bn-b3-hPhg from the cobalt(III) complexes in
both schemes, the two methodologies are essentially equivalent in
terms of yields, separation efficiency and ease of operation.
The scope of the resolution by extraction, following the
procedure reported in Scheme 2, was then investigated with two
other N-benzyl b3-amino acids: N-benzyl b3-homoalanine (N-Bn-
b3-hAla, Table 2) and N-benzyl b3-homovaline (N-Bn-b3-hVal,
Table 3). The two cycle approach resulted in an efficient resolution
protocol (90% ee for N-Bn-b3-hAla; 93% ee for N-Bn-b3-hVal),
although the extraction yields in both cases were poorer than
those reported in Table 1 for N-Bn-b3-hPhg.
Conclusions
We have performed an efficient resolution of racemic N-benzyl
b3-amino acids by an iterative (two cycle) liquid–liquid extraction
using the chiral complex [CoIII(1)(OAc)]. As a result of the first
resolution by extraction, one enantiomer predominated in the
aqueous phase, while the other enantiomer was driven into the or-
ganic phase by complexation to cobalt. Two alternative procedures
may be followed for improving the moderate ee’s resulting from the
first extraction: (a) enrichment of the major enantiomer through
a second complexation to the appropriate chiral selector, or (b)
depletion of the minor enantiomer through a second complexation
Table 2 Iterative extraction of N-Bn-b3-hAla using [CoIII(1)OAc]
Aqueous phasea
Selector (equiv)
N-Bn-b3-hAla (equiv)
% Extract. yield
% Selector recoveryb
% eec,d (abs. conf.)
Aq1
Aq2
Aq3
Aq4
Aq5
Aq6
Aq3ꢀ
Aq4ꢀ
Aq5ꢀ
Aq6ꢀ
[Co(R,R-1)(OAc)] (1.0)
[Co(S,S-1)(OAc)] (0.7)
[Co(R,R-1)(OAc)] (0.3)
[Co(R,R-1)(OAc)] (0.7)
[Co(S,S-1)(OAc)] (0.3)
1.41e
0.59f
0.99e
0.31f
0.38e
0.17f
0.97e
0.34f
0.39e
0.16f
25 (S)
75 (R)
5 (S)
88 (S)
65 (R)
90 (R)
36 (S)
75 (R)
83 (R)
51 (S)
59f
44f
58f
49f
52f
84
85
83
82
84
a See Scheme 2 for the origin of the aqueous phases. b Based on the yield of [CoII(1)] after reductive cleavage with L-ascorbic acid. c Determined by chiral
HPLC analysis using a Chirobiotic T column. d The absolute configuration was determined by comparison of the optical rotation with reported values
(see the Experimental section). e The equiv were calculated after evaporation of the aqueous phase. f The equiv and yield were calculated after cleavage of
the cobalt(III) complex with L-ascorbic acid and purification on the Dowex 50W-X8 resin.
3466 | Org. Biomol. Chem., 2007, 5, 3464–3471
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
Table 3 Iterative extraction of N-Bn-b3-hVal using [CoIII(1)OAc]
Aqueous phasea
Selector (equiv)
N-Bn-b3-hVal (equiv)
% Extract. yield
% Selector recoveryb
% eec,d
Aq1
Aq2
Aq3
Aq4
Aq5
Aq6
Aq3ꢀ
Aq4ꢀ
Aq5ꢀ
Aq6ꢀ
[Co(R,R-1)(OAc)] (1.0)
[Co(S,S-1)(OAc)] (0.63)
[Co(R,R-1)(OAc)] (0.33)
[Co(R,R-1)(OAc)] (0.63)
[Co(S,S-1)(OAc)] (0.33)
1.25e
0.66f
0.72e
0.43f
0.42e
0.21f
0.74e
0.42f
0.40e
0.20f
36 (+)
76 (−)
10 (−)
90 (+)
64 (−)
93 (−)
80 (+)
40 (−)
92 (−)
5 (−)
66f
68f
64f
67f
61f
80
83
80
82
85
a See Scheme 2 for the origin of the aqueous phases. b Based on the yield of [CoII(1)] after reductive cleavage with L-ascorbic acid. c Determined by chiral
HPLC analysis using Chirobiotic T column. d The sign of the optical rotation is reported in brackets. e The equiv were calculated after evaporation of the
aqueous phase. f The equiv and yield were calculated after cleavage of the cobalt(III) complex with L-ascorbic acid and purification on the Dowex 50W-X8
resin.
to the appropriate selector. The complexed amino acid were
quantitatively released by a reductive (CoIII → CoII) cleavage, using
L-ascorbic acid as the reducing agent. The release of the complexed
N-Bn-b3-AA by reductive cleavage allowed for the recovery of the
cobalt(II) selector in up to 90% yield (easily re-oxidable to CoIII
with air/AcOH), thus rendering this process virtually catalytic
in the chiral cobalt complex. We are also actively investigating
the origin of the observed selectivity which, upon preliminary
inspection, appears to be governed by the relative thermodynamic
stability of the diastereomeric cobalt(III) N-benzyl b3-amino acid
complexes, rather than by a kinetic preference for their formation.
tert-butylsalicylidene)-1,2-cyclohexanediaminocobalt(II), (R,R)-
[CoII(1)], and (S,S)-N,Nꢀ-bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediaminocobalt(II), (S,S)-[CoII(1)], were purchased
from Sigma-Aldrich. Extractions were performed using HPLC
grade water and dichloromethane.
Synthesis of (rac)-3-benzylamino-3-phenylpropionic acid
(N-benzyl-b3-homophenylglycine, N-Bn-b3-hPhg)
(rac)-N-Benzyl-b3-homophenylglycine was prepared according to
the method reported by Nagao et al. as follows.8 Cinnamic acid
(2.96 g, 20 mmol) and benzylamine (2.14 g, 20 mmol) were added
◦
to dry pyridine (15 mL). The mixture was heated to 120–130 C
Experimental
under nitrogen and stirred for 1 h; the solvent was evaporated
under reduced pressure to afford a crude crystalline product.
Re-crystallization from methanol–water gave the desired product
General remarks
1
Proton NMR spectra were recorded on a spectrometer operating
at 400.13 MHz. Proton chemical shifts are reported in ppm (d)
with the solvent reference relative to tetramethylsilane (TMS)
employed as the internal standard (D2O d 4.70 ppm, CDCl3 d
7.26 ppm). The following abbreviations are used to describe spin
multiplicity: s = singlet, d = doublet, t = triplet, q = quartet,
m = multiplet, br = broad signal, dd = doublet of doublets.
Carbon NMR spectra were recorded on a 400 spectrometer
operating at 100.56 MHz, with complete proton decoupling.
Carbon chemical shifts are reported in ppm (d) relative to TMS
with the respective solvent resonance as the internal standard
(CDCl3, d 77.0). Infrared spectra were recorded on a standard
FT-IR; peaks are reported in cm−1. Optical rotation values were
measured on an automatic polarimeter with a 1 dm cell at the
sodium D line. HPLC determination of the enantiomeric excesses
was carried out on a Waters 515 HPLC machine equipped with
a Waters 996 PAD diode array detector (detection wavelength
254 nm) using a chiral stationary phase (ASTEC Chirobiotic
T chiral column, 250 × 4.6 mm). High resolution mass spectra
(HRMS) were performed on a hybrid quadrupole time of flight
mass spectrometer equipped with an ESI ion source. A Reserpine
solution 100 pg ll−1 (about 100 count s−1), 0.1% HCOOH–CH3CN
1 : 1, was used as reference compound (Lock Mass). All com-
mercially available reagents were used as received. Benzylamine
was distilled under nitrogen immediately prior to use, and pyri-
dine was dried by distillation over CaH2. (R,R)-N,Nꢀ-Bis(3,5-di-
(3.07 g, yield: 60%). H-NMR (D2O–NaOH): d = 7.45–7.12 (m,
10H, Ar-H), 3.96 (t, J = 7.4 Hz, 1H, CHN), 3.52–3.44 (AB system,
JAB = 13.2 Hz, dA = 3.50, dB = 3.46, 2H, CH2Ph), 2.65 (dd, J =
14.2 Hz, J = 6.7 Hz, 1H, CHH), 2.45 (dd, J = 14.2 Hz, J = 8.1 Hz,
1H, CHH) ppm. 13C-NMR (D2O–NaOH): d = 180.4 (CO), 142.7
(Ar-C), 139.9 (Ar-C), 129.3 (Ar-CH), 129.3 (Ar-CH), 129.2 (Ar-
CH), 128.4 (Ar-CH), 128.3 (Ar-CH), 127.9 (Ar-CH), 60.1 (CHN),
50.9 (CH2N), 45.8 (CH2) ppm. FT-IR (Nujol): m = 2854, 1469,
1375 cm−1. HRMS (ESI): calcd. for C16H18N1O2 256.1332 [M +
H]+; found: 256.1331. HPLC conditions: Chirobiotic T (250 ×
4.6 mm) 90 : 10 CH3OH–H2O, 1.0 mL min−1, S enantiomer tr =
8.2 min, R enantiomer tr = 9.2 min.
Synthesis of (rac)-3-benzylaminobutyric acid
(N-benzyl-b3-homoalanine, N-Bn-b3-hAla)
(rac)-N-Benzyl-b3-homoalanine was prepared according to the
method reported by Zilkha and Rivlin as follows.9 To a solution of
crotonic acid (0.43 g, 5.0 mmol) in dry pyridine (15 mL) was added
benzylamine (0.53 g, 5 mmol). The mixture was heated at 120–
◦
130 C under an argon atmosphere for 1.5 h. On cooling, white
(rac)-N-benzyl-b3-homoalanine crystallized, which was filtered
and washed with acetone to give the desired product. Yield: 0.83 g
(86%). 1H-NMR (D2O): d = 7.46–7.38 (m, 5H, Ar-H), 4.24–4.14
(AB system, JAB = 13.2 Hz, dA = 4.21, dB = 4.15, 2H, CH2Ph),
3.55–3.43 (m, 1H, CHN), 2.49 (d, J = 6.5 Hz, 2H, CH2), 1.32
This journal is
The Royal Society of Chemistry 2007
Org. Biomol. Chem., 2007, 5, 3464–3471 | 3467
©

View Article Online
(d, J = 6.6 Hz, 3H, CH3) ppm. 13C-NMR (D2O–NaOH): d =
178.3 (CO), 131.6 (Ar-C), 129.9 (Ar-CH), 129.8 (Ar-CH), 129.7
(Ar-CH), 51.9 (CHN), 48.4 (CH2N), 39.2 (CH2), 16.3 (CH3)
ppm. FT-IR (Nujol): m = 2921, 1457, 1376 cm−1. HRMS (ESI):
calcd. for C11H16N1O2 194.1176 [M + H]+; found: 194.1173. HPLC
conditions: Chirobiotic T (250 × 4.6 mm) CH3OH, 0.4 mL min−1,
R enantiomer tr = 34.6 min, S enantiomer tr = 38.6 min.
Synthesis of (S,S)-N,Nꢀ-bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediaminocobalt(III) acetate, (S,S)-[CoIII(1)(OAc)]
See the procedure above as for the synthesis of (R,R)-
[CoIII(1)(OAc)].
Procedure for the extraction of N-Bn-b3-hPhg
To a solution of (R,R)-[CoIII(1)(OAc)] (99.3 mg, 0.15 mmol) in
dichloromethane (40 mL) at 10◦◦C in a 250 mL round-bottom
flask, was added a pre-cooled (10 C) solution of racemic N-Bn-b3-
hPhg (76.8 mg, 0.30 mmol, 2 equiv) in H2O (80 mL). The biphasic
mixture was stirred vigorously for 24 h at 10 ◦C, then transferred
to a separatory funnel, the organic phase (Org1) removed and the
aqueous phase washed once with dichloromethane (10 mL). A
small aliquot (ca. 100 lL) of the aqueous phase was removed,
filtered on a micropore filter, and the ee was determined by
chiral HPLC as described above. The aqueous phase (Aq1) was
then evaporated under reduced pressure to give N-Bn-b3-hPhg
as a white powder (37.2 mg, yield: 97%, 55% ee in favour of
R enantiomer). The combined dichloromethane extracts were
washed once with H2O (10 mL). Volatiles were removed under
reduced pressure to leave the complex (R,R)-[CoIII(1)(N-Bn-b3-
hPhg)] as a green-brown powder. This was dissolved in methanol
(10 mL) and ascorbic acid (26.4 mg, 0.15 mmol) was added.7 The
mixture was stirred vigorously for 15 min until a red precipitate
was formed ([CoII(1)]). The red product was filtered, washed with
methanol (10 mL) and dried under vacuum to obtain [CoII(1)]
as a red powder (81.4 mg, yield: 90%). HRMS (ESI): calcd. for
C36H52N2O2Co 603.3360 [M]+; found: 603.3332. The combined
pale yellow methanolic filtrates were purified using Dowex 50W-
X8 resin (pre-washed with 1 M NaOH, water, 1 M HCl and water)
and the amino acid was eluted using 2 M ammonia to obtain
N-Bn-b3-hPhg as white powder (36.5 mg, yield: 95%, 58% ee in
favour of S enantiomer).
Synthesis of (rac)-3-benzylamino-4-methylpentanoic acid
(N-benzyl-b3-homovaline, N-Bn-b3-hVal)
N-Benzyl-b3-homovaline was prepared according to the method
reported by Williams et al. as follows.10 Ethyl isobutyrylacetate
(400 lL, 2.4 mmol, 1.0 equiv) was dissolved in benzene (4 mL)
and treated with benzylamine (276 lL, 2.4 mmol, 1.0 equiv). The
mixture was heated at reflux under nitrogen with azeotropic water
removal (Dean–Stark apparatus) for 8 h. The solvent was then
evaporated, the residue dissolved in acetic acid (4 mL) and treated
with sodium cyanoborohydride (702 mg, 10.8 mmol, 4.5 equiv).
The mixture was stirred at room temperature for 2 h. The solvent
was then evaporated, the residue taken up in ether and washed
with 1 N NaOH and brine. The ethereal solution was dried
over Na2SO4 and evaporated to give rac-ethyl-3-benzylamino-4-
methylpentanoate as an oil (506 mg, 82% yield over two steps).
This compound was dissolved in 2 M KOH–methanol (1.8 mL,
3.6 mmol, 1.5 equiv) and the mixture was stirred overnight at
room temperature. The resulting mixture was acidified to pH 5
with HCl–MeOH (about 1.8 mL used). Diethyl ether was added
and the precipitate was filtered off. The filtrate was evaporated to
afford N-benzyl-b3-homovaline as a hygroscopic solid. The solid
was dissolved in water and purified on a Dowex 50W-X8 resin,
eluting with 2 M NH4OH, to give pure N-benzyl-b3-homovaline
1
(382 mg, 84% yield). H-NMR (D2O): d = 7.42 (s, 5H, Ar-H),
4.20 (s, 2H, CH2Ph), 3.30–3.25 (m, 1H, CHN), 2.51 (dd, J =
16.8 Hz, J = 4.8 Hz, 1H, CHH), 2.36 (dd, J = 16.8 Hz, J =
7.6 Hz, 1H, CHH), 2.13–2.08 (m, 1H, CHMe2), 0.91 (d, J =
6.8 Hz, 3H, CH3), 0.87 (d, J = 6.8 Hz, 3H, CH3) ppm. 13C-NMR
(D2O): d = 178.7 (CO), 131.3 (Ar-C), 129.6 (Ar-CH), 129.5 (Ar-
CH), 129.3 (Ar-CH), 60.5 (CHN), 48.4 (CH2N), 32.4 (CH2), 28.2
(CHMe2), 18.3 (CH3), 15.8 (CH3) ppm. FT-IR (Nujol): m = 1632,
1562 cm−1. HRMS (ESI): calcd. for C13H19N1O2Na 244.1308 [M +
Na]+; found: 244.1306. HPLC conditions: Chirobiotic T (250 ×
4.6 mm) 90 : 10 CH3OH–H2O, 1.0 mL min−1, (+) enantiomer tr =
7.7 min, (−) enantiomer tr = 9.1 min.
Procedure for the iterative extraction of N-Bn-b3-hPhg (Scheme 2,
upper part)
To a solution of (R,R)-[CoIII(1)(OAc)] (49.6 mg, 0.075 mmol) in
dichloromethane (20 mL) at 10◦◦C in a 100 mL round-bottom
flask, was added a pre-cooled (10 C) solution of racemic N-Bn-b3-
hPhg (38.6 mg, 0.15 mmol, 2 equiv) in H2O (40 mL). The biphasic
mixture was stirred vigorously for 24 h at 10 ◦C, then transferred
to a separatory funnel, the organic phase (Org1) removed and the
aqueous phase (Aq1) washed once with dichloromethane (10 mL).
The combined dichloromethane extracts were washed once with
H2O (10 mL).
Synthesis of (R,R)-N,Nꢀ-bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediaminocobalt(III) acetate, (R,R)-[CoIII(1)(OAc)]
Processing of the aqueous phase (Aq1). The aqueous phases
were combined, concentrated to 40 mL and extracted again
with (S,S)-[CoIII(1)(OAc)] (24.8 mg, 0.0375 mmol, 0.5 equiv)
in dichloromethane (20 mL) for 24 h at 10 ◦C. The biphasic
mixture was transferred to a separatory funnel, the organic phase
(Org2) removed and the aqueous phase (Aq3) washed once with
dichloromethane (10 mL). The organic phase (Org2) was washed
with water (10 mL). The aqueous phases were combined and a
small aliquot (ca. 100 ll) was removed, filtered on a micropore
filter, and the ee was determined by chiral HPLC as described
above. The aqueous phase was evaporated to dryness to obtain
(R,R)-[CoIII(1)(OAc)] was prepared according to a literature pro-
cedure, with slight modifications.11 Glacial acetic acid (0.35 mL,
6.0 mmol) was added to a stirred solution of [CoII(1)] (180.9 mg,
0.30 mmol) in toluene (20 mL) under air. An immediate color
change from bright red to brown was observed. The solution
was stirred for a further 30 min before volatiles were removed
under reduced pressure to leave (R,R)-[CoIII(1)(OAc)] as a brown
powder, which was used directly in subsequent extractions without
further purification. Yield: 194.6 mg (98%). Spectroscopic data
were consistent with those previously reported.12
3468 | Org. Biomol. Chem., 2007, 5, 3464–3471
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
N-Bn-b3-hPhg as white powder (9.1 mg, yield: 95%, 16% ee in
favour of R enantiomer). The organic phase (Org2) was evaporated
to dryness under vacuum to yield a brown-green powder. This
was dissolved in methanol (10 mL) and treated with ascorbic acid
(6.6 mg, 0.0375 mmol) for 15 min, until a red precipitate of [CoII(1)]
was formed, which was filtered, washed with 10 mL methanol and
dried (18.2 mg, yield: 82%). The combined pale yellow methanolic
filtrates were purified on Dowex 50W-X8 resin to obtain (R)-N-
Bn-b3-hPhg as a white powder. Yield: 9.0 mg (94%).
tracts were washed once with H2O (10 mL). Volatiles were removed
under reduced pressure to leave the complex (R,R)-[CoIII(1)(N-
Bn-b3-hPhg)] as a green-brown powder. This was dissolved in
methanol (10 mL) and ascorbic acid (6.6 mg, 0.037 mmol) was
added. The mixture was stirred vigorously for 15 min until a red
precipitate was formed ([CoII(1)]), which was filtered, washed with
10 mL methanol and dried (18.9 mg, yield: 84%). The combined
pale yellow methanolic filtrates were purified on Dowex 50W-
X8 resin to obtain N-Bn-b3-hPhg as a white powder (8.9 mg,
yield: 93%, 10% ee in favour of S enantiomer). The aqueous
phases (Aq3ꢀ) were combined and a small aliquot (ca. 100 lL) was
removed, filtered on a micropore filter, and the ee was determined
by chiral HPLC as described above. The aqueous phase was then
evaporated under reduced pressure to give (R)-N-Bn-b3-hPhg as
Processing of the organic phase (Org1). Volatiles were removed
under reduced pressure to leave the complex (R,R)-[CoIII(1)(N-
Bn-b3-hPhg)] as a green-brown powder. This was dissolved in
methanol (10 mL) and ascorbic acid (13.2 mg, 0.075 mmol) was
added. The mixture was stirred vigorously for 15 min until a red
precipitate of [CoII(1)] was formed, which was filtered, washed with
10 mL methanol and dried (40.7 mg, yield: 90%). The combined
pale yellow methanolic filtrates were purified on a Dowex 50W-X8
resin. The white crude solid (18.2 mg, yield: 94%) was dissolved
in water (40 mL, Aq2), filtered and submitted to a second
extraction with (R,R)-[CoIII(1)(OAc)] (24.8 mg 0.075 mmol) in
dichloromethane (20 mL) for 24 hours at 10 ◦C with vigorous
stirring. The biphasic mixture was then transferred to a separatory
funnel, the organic phase (Org3) removed and the aqueous phase
(Aq5) washed once with dichloromethane (10 mL). The organic
phase (Org3) was washed with water (10 mL). The aqueous phases
were combined and a small aliquot (ca. 100 lL) was removed,
filtered on a micropore filter, and the ee was determined by
chiral HPLC as described above. The aqueous phase (Aq5) was
evaporated to dryness to obtain N-Bn-b3-hPhg as white powder
(8.7 mg, yield: 95%, 16% ee in favour of S enantiomer). The organic
phase (Org3) was washed with water (10 mL), and evaporated to
dryness under vacuum to yield a brown-green powder. This was
dissolved in methanol (10 mL) and treated with ascorbic acid
(6.6 mg, 0.037 mmol) for 15 minutes, until a red precipitate of
[CoII(1)] was formed, washed with 10 mL methanol and dried
(19.0 mg, yield: 85%). The combined pale yellow methanolic
filtrates were purified on Dowex 50W-X8 resin to obtain (S)-N-
Bn-b3-hPhg as a white powder. Yield: 8.6 mg (94%).
24
a white powder. Yield: 9.3 mg (96%). [a]D = +52.1 (c = 0.93 in
MeOH), lit.13 [a]D = +56.3 (c = 1 in MeOH), (R)-enantiomer.
24
Processing of the organic phase (Org1). Volatiles were removed
under reduced pressure to leave the complex (R,R)-[CoIII(1)(N-
Bn-b3-hPhg)] as a green-brown powder. This was dissolved in
methanol (10 mL) and ascorbic acid (13.2 mg, 0.075 mmol) was
added. The mixture was stirred vigorously for 15 min until a red
precipitate of [CoII(1)] was formed, which was filtered, washed with
methanol (10 mL) and dried (40.6 mg, yield: 90%). The combined
pale yellow methanolic filtrates were purified on Dowex 50W-X8
resin. The crude white solid (18.2 mg, yield: 95%) was dissolved in
water (40 mL, Aq2), filtered and submitted to a second extraction
with (S,S)-[CoIII(1)(OAc)] (24.8 mg, 0.037 mmol, 0.5 equiv) in
dichloromethane (20 mL) for 24 h, at 10 ◦C, with vigorous stirring.
The biphasic mixture was then transferred to a separatory funnel,
the organic phase (Org3ꢀ) removed and the aqueous phase (Aq5ꢀ)
washed once with dichloromethane (10 mL). After washing the
organic phase with water (10 mL), volatiles were removed under
reduced pressure to leave the complex (S,S)-[CoIII(1)(N-Bn-b3-
hPhg)] as a green-brown powder. This was dissolved in methanol
(10 mL) and ascorbic acid (6.6 mg, 0.037 mmol) was added. The
mixture was stirred vigorously for 15 min until a red precipitate of
[CoII(1)] was formed, which was washed with 10 mL methanol and
dried (19.5 mg, yield: 87%). The combined pale yellow methanolic
filtrates were purified on Dowex 50W-X8 resin to obtain N-Bn-b3-
hPhg as a white powder (8.3 mg, yield: 92%, 5% ee in favour of R
enantiomer, Aq6ꢀ). The aqueous phases (Aq5ꢀ) were combined and
a small aliquot (ca. 100 lL) was removed, filtered on a micropore
filter, and the ee was determined by chiral HPLC as described
above. The aqueous phase was then evaporated under reduced
pressure to give (S)-N-Bn-b3-hPhg as a white powder. Yield: 8.4 mg
(93%).
Procedure for the iterative extraction of N-Bn-b3-hPhg (Scheme 2,
lower part)
To a solution of (R,R)-[CoIII(1)(OAc)] (49.6 mg, 0.075 mmol) in
dichloromethane (20 mL) at 10◦◦C in a 100 mL round-bottom
flask, was added a pre-cooled (10 C) solution of racemic N-Bn-b3-
hPhg (38.6 mg, 0.15 mmol, 2 equiv) in H2O (40 mL). The biphasic
mixture was stirred vigorously for 24 h at 10 ◦C, then transferred
to a separatory funnel, the organic phase (Org1) removed and the
aqueous phase (Aq1) washed once with dichloromethane (10 mL).
The combined dichloromethane extracts were washed once with
H2O (10 mL).
(S,S)-[CoIII(1)(N-Bn-b3-hPhg)]. 1H-NMR (CDCl3): d = 7.64
(s, 1H), 7.46 (d, J = 2.8 Hz, 1H), 7.40–7.27 (m, 7H), 7.08 (d,
J = 2.4 Hz, 1H), 6.98 (d, J = 2.4 Hz, 1H), 6.95 (d, J = 7.2 Hz,
2H), 6.59–6.55 (m, 2H), 6.27 (s, 1H), 5.18 (d, J = 10.6 Hz, 1H),
4.71–4.55 (m, 2H), 4.21–4.10 (m, 1H), 3.87 (s, 1H), 3.10 (dd, J =
17.6 Hz, J = 7.4 Hz, 1H), 3.00–2.91 (m, 1H), 2.78–2.67 (m, 2H),
1.93 (d, J = 10.4 Hz, 2H), 1.87–1.80 (m, 1H) 1.77–1.70 (m, 1H),
1.61 (s, 9H), 1.48–1.43 (m, 1H), 1.45 (s, 9H), 1.36 (s, 9H), 1.32–1.22
(m, 2H), 1.09 (s, 9H) ppm. 13C-NMR (CDCl3): d = 175.0, 163.8,
162.7, 161.0, 158.6, 144.8, 140.7, 140.3, 137.8, 136.7, 134.7, 130.3,
129.9, 129.4, 129.2, 128.3, 128.0, 127.7, 122.5, 120.4, 117.3, 75.5,
70.5, 55.3, 54.5, 35.7, 35.6, 34.0, 33.7, 32.9, 31.6, 31.5, 30.2, 29.7,
Processing of the aqueous phase (Aq1). The aqueous phases
were combined, concentrated to 40 mL and extracted again
with (R,R)-[CoIII(1)(OAc)] (24.8 mg, 0.037 mmol, 0.5 equiv)
in dichloromethane (20 mL) for 24 h at 10 ◦C. The biphasic
mixture was transferred to a separatory funnel, the organic phase
(Org2ꢀ) removed and the aqueous phase (Aq3ꢀ) washed once with
dichloromethane (10 mL). The combined dichloromethane ex-
This journal is
The Royal Society of Chemistry 2007
Org. Biomol. Chem., 2007, 5, 3464–3471 | 3469
©

View Article Online
28.8, 25.0, 23.9 ppm. FT-IR (Nujol): m = 3375, 3173, 2903, 2850,
1714, 1461, 1376, 1306, 1167, 1155, 970 cm−1. HRMS (ESI): calcd.
for C52H68N3O4Co 858.4615 [M + H]+; found: 858.4685.
Procedure for the iterative extraction of N-Bn-b3-hAla (Scheme 2,
lower part)
To a solution of (R,R)-[CoIII(1)(OAc)] (99.3 mg, 0.15 mmol) in
dichloromethane (20 mL) at 10 ◦C in a 100 mL round-bottom
flask, was added a pre-cooled (10 ◦C) solution of racemic N-Bn-
b3-hAla (58.0 mg, 0.30 mmol, 2 equiv) in H2O (15 mL). The organic
phase (Org1) and the aqueous phase (Aq1) were obtained.
Procedure for the extraction of N-Bn-b3-hAla
To a solution of (R,R)-[CoIII(1)(OAc)] (99.3 mg, 0.15 mmol) in
dichloromethane (20 mL) at 10◦◦C in a 100 mL round-bottom
flask, was added a pre-cooled (10 C) solution of racemic N-Bn-b3-
hAla (58.0 mg, 0.30 mmol, 2 equiv) in H2O (15 mL). The biphasic
mixture was stirred vigorously for 24 h at 10 ◦C, then transferred
to a separatory funnel, the organic phase (Org1) removed and
the aqueous phase washed once with dichloromethane (10 mL).
A small aliquot (ca. 100 lL) of the aqueous phase was removed,
filtered on a micropore filter, and the ee was determined by chiral
HPLC as described above. The aqueous phase (Aq1) was then
evaporated under reduced pressure to give N-Bn-b3-hAla as a
white powder (40.6 mg, 25% ee in favour of S enantiomer). The
combined dichloromethane extracts (Org1) were washed once with
H2O (10 mL). Volatiles were removed under reduced pressure
to leave the complex (R,R)-[CoIII(1)(N-Bn-b3-hAla)] as a brown
powder, which was characterised by HRMS (ESI): calcd. for
C47H66N3O4Co 818.4277 [M + Na]+; found: 818.4289. (R,R)-
[CoIII(1)(N-Bn-b3-hAla)] was dissolved in methanol (10 mL) and
ascorbic acid (26.4 mg, 0.15 mmol) was added. [CoII(1)] was
obtained as a red powder (76 mg, yield: 84%). The methanolic
filtrate was purified using Dowex 50W-X8 resin, and pure N-Bn-
b3-hAla was obtained as a white powder (17.0 mg, yield: 59%,
75% ee in favour of the R enantiomer).
Processing of the aqueous phase (Aq1). Aq1 was extracted
again with (R,R)-[CoIII(1)(OAc)] (69.5 mg, 0.105 mmol). Aq 3ꢀ:
N-Bn-b3-hAla was obtained as a white powder (28 mg, 36% ee
in favour of S enantiomer). Org2ꢀ: (R,R)-[CoIII(1)(N-Bn-b3-hAla)]
was obtained as a brown powder, which was dissolved in methanol
(10 mL) and ascorbic acid (18.4 mg, 0.105 mmol) was added.
[CoII(1)] was obtained as a red powder (51.8 mg, yield: 82%). The
methanolic filtrate was purified using Dowex 50W-X8 resin, and
N-Bn-b3-hAla was obtained as white powder (10 mg, yield: 49%,
75% ee in favour of R enantiomer).
Processing of the organic phase (Org1). Pure N-Bn-b3-hAla,
obtained as a white powder (17.0 mg, yield: 59%, 75% ee in
favour of the R enantiomer) from the reductive cleavage and
work-up of Org1, was dissolved in water (15 mL, Aq2), filtered
and submitted to a second extraction with (S,S)-[CoIII(1)(OAc)]
(29.8 mg, 0.045 mmol). Aq 5: N-Bn-b3-hAla was obtained as
a white powder (11.5 mg, 83% ee in favour of R enantiomer;
[a]D = +18.0 (c 0.86 in H2O); lit.14 [a]D = +22.5 (c 1.20 in
H2O), (R)-enantiomer). Org3: (S,S)-[CoIII(1)(N-Bn-b3-hAla)] was
obtained as a brown powder, which was dissolved in methanol
(10 mL) and ascorbic acid (7.9 mg, 0.045 mmol) was added.
[CoII(1)] was obtained as a red powder (22.5 mg, yield: 84%).
The methanolic filtrate was purified using Dowex 50W-X8 resin,
and N-Bn-b3-hAla was obtained as white powder (4.5 mg, yield:
52.0%, 51% ee in favour of S enantiomer).
24
24
Procedure for the iterative extraction of N-Bn-b3-hAla (Scheme 2,
upper part)
Processing of the aqueous phase (Aq1). Aq1 was extracted
again with (S,S)-[CoIII(1)(OAc)] (69.5 mg, 0.105 mmol). Aq 3:
N-Bn-b3-hAla was obtained as a white powder (28.8 mg, 5% ee
in favour of S enantiomer). Org2: complex (S,S)-[CoIII(1)(N-Bn-
b3-hAla)] was obtained as a brown powder, dissolved in methanol
(10 mL) and ascorbic acid (18.4 mg, 0.105 mmol) was added.
[CoII(1)] was obtained as a red powder (53.8 mg, yield: 85%). The
methanolic filtrate was purified using Dowex 50W-X8 resin, and
N-Bn-b3-hAla was obtained as a white powder (9.0 mg, yield: 44%,
88% ee in favour of S enantiomer).
Procedure for the extraction of N-Bn-b3-hVal
The procedure used is similar to that described above for N-
Bn-b3-hPhg. To a solution of (R,R)-[CoIII(1)(OAc)] (62.9 mg,
0.095 mmol) in dichloromethane (20 mL) at 10 C in a 100 mL
◦
round-bottom flask, was added a pre-cooled (10 ◦C) solution
of racemic N-Bn-b3-hVal (42 mg, 0.19 mmol, 2 equiv) in H2O
(15 mL). The biphasic mixture was stirred vigorously for 24 h
at 10 ◦C, then transferred to a separatory funnel, the organic
phase (Org1) removed and the aqueous phase washed once with
dichloromethane (10 mL). A small aliquot (ca. 100 lL) of the
aqueous phase was removed, filtered on a micropore filter, and
the ee was determined by chiral HPLC as described above.
The aqueous phase (Aq1) was then evaporated under reduced
pressure to give N-Bn-b3-hVal as a white powder (26.2 mg, 36% ee
in favour of (+) enantiomer). The combined dichloromethane
extracts (Org1) were washed once with H2O (10 mL). Volatiles
were removed under reduced pressure to leave the complex
(R,R)-[CoIII(1)(N-Bn-b3-hVal)] as a brown powder, which was
characterised by HR-MS (ESI): calcd. for C49H71N3O4Co 824.4771
[M + H]+; found: 824.4771. (R,R)-[CoIII(1)(N-Bn-b3-hVal)] was
dissolved in methanol (10 mL) and ascorbic acid (26.4 mg,
0.15 mmol) was added. [CoII(1)] was obtained as a red powder
(45.5 mg, yield: 80%). The methanolic filtrate was purified using
Processing of the organic phase (Org1). Pure N-Bn-b3-hAla,
obtained as a white powder (17.0 mg, yield: 59%, 75% ee in
favour of the R enantiomer) from the reductive cleavage and
work-up of Org1, was dissolved in water (15 mL, Aq2), filtered
and submitted to a second extraction with (R,R)-[CoIII(1)(OAc)]
(29.8 mg, 0.045 mmol). Aq 5: N-Bn-b3-hAla was obtained as
a white powder (11.0 mg, 65% ee in favour of R enantiomer).
Org3: (R,R)-[CoIII(1)(N-Bn-b3-hAla)] was obtained as a brown
powder, which was dissolved in methanol (10 mL) and ascorbic
acid (7.9 mg, 0.045 mmol) was added. [CoII(1)] was obtained
as a red powder (22.5 mg, yield: 83%). The methanolic filtrate
was purified using Dowex 50W-X8 resin, and N-Bn-b3-hAla was
obtained as white powder (5.0 mg, yield: 58.0%, 90% ee in favour
of R enantiomer).
3470 | Org. Biomol. Chem., 2007, 5, 3464–3471
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
Dowex 50W-X8 resin, and pure N-Bn-b3-hVal was obtained as
a white powder [13.9 mg, yield: 66%, 76% ee in favour of (−)
enantiomer].
(10 mL) and ascorbic acid (5.6 mg, 0.032 mmol) was added.
[CoII(1)] was obtained as a red powder (16.4 mg, yield: 85%).
The methanolic filtrate was purified using Dowex 50W-X8 resin,
and N-Bn-b3-hVal was obtained as white powder [4.4 mg, yield:
61%, 5% ee in favour of (−) enantiomer].
Procedure for the iterative extraction of N-Bn-b3-hVal (Scheme 2,
upper part)
Attribution of the absolute configuration to N-Bn-b3-hVal: by
analogy with the complexation selectivity shown for N-Bn-b3-
hPhg and N-Bn-b3-hAla, we attribute to (+)-N-Bn-b3-hVal the (R)
Processing of the aqueous phase (Aq1). Aq1 was extracted
again with (S,S)-[CoIII(1)(OAc)] (39.7 mg, 0.06 mmol, 0.63 equiv).
Aq 3: N-Bn-b3-hVal was obtained as a white powder (15 mg,
10% ee in favour of (−) enantiomer). Org2: complex (S,S)-
[CoIII(1)(N-Bn-b3-hVal)] was obtained as a brown powder, dis-
solved in methanol (10 mL) and ascorbic acid (10.6 mg, 0.06 mmol)
was added. [CoII(1)] was obtained as a red powder (30.1 mg, yield:
83%). The methanolic filtrate was purified using Dowex 50W-X8
resin, and N-Bn-b3-hVal was obtained as a white powder [9.0 mg,
24
absolute configuration: [a]D = +35.6, c 0.80 in MeOH, 100% ee
in favour of (R)-enantiomer.
Acknowledgements
We thank the European Commission (Marie Curie postdoctoral
fellowship MEIF-CT-2005–006253 to P. Dzygiel and EST grant
“Foldamers” MEST-CT-2004–515968 to U. Piarulli) and Merck
Research Laboratories (Merck’s Academic Development Program
Award to C. Gennari) for financial support.
24
yield: 68%, [a]D = +31.9, c 0.78 in MeOH, 90% ee in favour of
(+) enantiomer].
Processing of the organic phase (Org1). Pure N-Bn-b3-hVal,
obtained as a white powder [13.9 mg, yield: 66%, 76% ee in
favour of the (−) enantiomer] from the reductive cleavage and
work-up of Org1, was dissolved in water (15 mL, Aq2), filtered
and submitted to a second extraction with (R,R)-[CoIII(1)(OAc)]
(21.2 mg, 0.032 mmol). Aq 5: N-Bn-b3-hVal was obtained as a
white powder [9.0 mg, 64% ee in favour of (−) enantiomer]. Org3:
(R,R)-[CoIII(1)(N-Bn-b3-hVal)] was obtained as a brown powder,
which was dissolved in methanol (10 mL) and ascorbic acid
(5.6 mg, 0.032 mmol) was added. [CoII(1)] was obtained as a red
powder (15.8 mg, yield: 80%). The methanolic filtrate was purified
using Dowex 50W-X8 resin, and N-Bn-b3-hVal was obtained as
Notes and references
1 (a) D. Seebach, D. F. Hook and A. Glattli, Biopolymers, 2006, 84, 23;
(b) D. Seebach, A. K. Beck and D. J. Bierbaum, Chem. Biodiversity,
2004, 1, 1111; (c) R. P. Cheng, S. H. Gellman and W. F. DeGrado,
Chem. Rev., 2001, 101, 3219, and references therein.
2 (a) A. R. Katritzky, H. Tao, R. Jiang, K. Suzuki and K. Kirichenko,
J. Org. Chem., 2007, 72, 407; (b) H. M. Huang, X. C. Liu, J. Deng, M.
Qiu and Z. Zheng, Org. Lett., 2006, 8, 3359; (c) J. Song, Y. Wang and L.
Deng, J. Am. Chem. Soc., 2006, 128, 6048; (d) A. L. Tillman, J. X. Ye and
D. J. Dixon, Chem. Commun., 2006, 1191; (e) C. Palomo, M. Oiarbide,
R. Halder, M. Kelso, E. Gomez-Bengoa and J. M. Garcia, J. Am. Chem.
Soc., 2004, 126, 9188; (f) M. Liu and M. P. Sibi, Tetrahedron, 2002, 58,
7991.
24
white powder [4.5 mg, yield: 64%, [a]D = −32.0, c 0.34 in MeOH,
3 (a) E. Forro´ and F. Fu¨lo¨p, Chem.–Eur. J., 2007, 13, 6397; (b) A. Liljeblad
and L. T. Kanerva, Tetrahedron, 2006, 62, 5831.
93% ee in favour of (−) enantiomer].
4 (a) R. Berkecz, A. Sztojkov-Ivanov, I. Ilisz, E. Forro´, F. Fu¨lo¨p, M. H.
Hyun and A. Peter, J. Chromatogr., A, 2006, 1125, 138; (b) A. Peter, A.
Arki, E. Vekes, D. Tourwe, L. Lazar, F. Fueloep and D. W. Armstrong,
J. Chromatogr., A, 2004, 1031, 171; (c) A. Peter, J. Chromatogr., A,
2002, 955, 141, and references therein.
5 (a) Y. Ihara, S. Kurose and T. Koyama, Monatsh. Chem., 2001, 132,
1433; (b) H. Tsukube, S. Shinoda, J. Uenishi, T. Kanatani, H. Itoh, M.
Shiode, T. Iwachido and O. Yonemitsu, Inorg. Chem., 1998, 37, 1585,
and references therein.
6 T. B. Reeve, J.-P. Cros, C. Gennari, U. Piarulli and J. G. de Vries, Angew.
Chem., Int. Ed., 2006, 45, 2449.
7 J. F. Larrow, K. E. Hemberger, S. Jasmin, H. Kabir and P. Morel,
Tetrahedron: Asymmetry, 2003, 14, 3589.
8 Y. Nagao, T. Kumagai, S. Tamai, H. Matsunaga, T. Abe and Y. Inoue,
Heterocycles, 1996, 42, 849.
9 A. Zilkha and J. Rivlin, J. Org. Chem., 1958, 23, 95.
10 R. M. Williams, B. H. Lee, M. M. Miller and O. P. Anderson, J. Am.
Chem. Soc., 1989, 111, 1073.
11 S. E. Schaus, B. D. Brandes, J. F. Larrow, M. Tokunage, K. B. Hansen,
A. E. Gould, M. E. Furrow and E. N. Jacobsen, J. Am. Chem. Soc.,
2002, 124, 1307.
12 L. P. C. Nielsen, C. P. Stevenson, D. G. Blackmond and E. N. Jacobsen,
J. Am. Chem. Soc., 2004, 126, 1360.
13 S. D. Bull, S. G. Davies, P. M. Kelly, M. Gianotti and A. D. Smith,
J. Chem. Soc., Perkin Trans. 1, 2001, 3106.
14 A. Griesbeck and D. Seebach, Helv. Chim. Acta, 1987, 70, 1326.
15 D. S. Daniels, E. J. Petersson, J. X. Qiu and A. Schepartz, J. Am. Chem.
Soc., 2007, 129, 1532.
16 P. Dziedzic and A. Cordova, Tetrahedron: Asymmetry, 2007, 18, 1033,
and references therein.
Procedure for the iterative extraction of N-Bn-b3-hVal (Scheme 2,
lower part)
Processing of the aqueous phase (Aq1). Aq1 was extracted
again with (R,R)-[CoIII(1)(OAc)] (39.7 mg, 0.06 mmol, 0.63 equiv).
Aq 3ꢀ: N-Bn-b3-hVal was obtained as a white powder (15.5 mg,
24
[a]D = +28.5, c 0.80 in MeOH, 80% ee in favour of (+)
enantiomer]. Org2ꢀ: complex (R,R)-[CoIII(1)(N-Bn-b3-hVal)] was
obtained as a brown powder, dissolved in methanol (10 mL)
and ascorbic acid (10.6 mg, 0.06 mmol) was added. [CoII(1)] was
obtained as a red powder (29.6 mg, yield: 82%). The methanolic
filtrate was purified using Dowex 50W-X8 resin, and N-Bn-b3-
hVal was obtained as a white powder [8.9 mg, yield: 67%, 40% ee
in favour of (−) enantiomer].
Processing of the organic phase (Org1). Pure N-Bn-b3-hVal,
obtained as a white powder [13.9 mg, yield: 66%, 76% ee in
favour of the (−) enantiomer] from the reductive cleavage and
work-up of Org1, was dissolved in water (15 mL, Aq2), filtered
and submitted to a second extraction with (S,S)-[CoIII(1)(OAc)]
(21.2 mg, 0.032 mmol). Aq 5ꢀ: N-Bn-b3-hVal was obtained as a
24
white powder [8.5 mg, [a]D = −32.6, c 0.51 in MeOH, 92% ee in
favour of (−) enantiomer]. Org3ꢀ: (S,S)-[CoIII(1)(N-Bn-b3-hVal)]
was obtained as a brown powder, which was dissolved in methanol
This journal is
The Royal Society of Chemistry 2007
Org. Biomol. Chem., 2007, 5, 3464–3471 | 3471
©