Reversing the enantioselectivity of a peptidic catalyst by changing the solvent

Article abstract of DOI:10.1055/s-0030-1259523

The enantioselectivity of the peptidic catalyst H-Pro-Pro-Asp-NH(CH ₂)₁₁CH₃ is reversed in different solvents. One enantiomer forms preferentially in pure DMSO or MeOH, whereas the other is preferentially formed in mixture

Full text of DOI:10.1055/s-0030-1259523

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SUPPORTING INFORMATION
Supporting Information
for DOI: 10.1055/s-0030-1259523
© Georg Thieme Verlag KG Stuttgart · New York 2010
Thieme

Reversing the Enantioselectivity of a Peptidic Catalyst by
Changing the Solvent
Matthias Messerer and Helma Wennemers*
*Departement of Chemistry, University of Basel
St. Johanns-Ring 19, CH-4056 Basel (Switzerland)
Fax: +41-61-267-0976
e-mail: Helma.Wennemers@unibas.ch
Supporting Information
1.
2.
3.
4.
General aspects and materials
General procedure for direct aldol reaction of 4-nitrobenzaldehyde to cyclohexanone
Synthesis of peptidic catalyst 1a
NMR-spectra and HPLC-chromatograms

1. General aspects and materials
Materials and reagents were of the highest commercially available grade and used without
further purification. Reactions were monitored by thin layer chromatography using Merck
silica gel 60 F254 plates. Compounds were visualized by UV and Cer(IV)-sulfate stain.
Flash chromatography was performed using Fluka silica gel 60, particle size 40 – 63 µm.
1H and 13C NMR spectra were recorded on Bruker DPX 400 and DPX 500 spectrometer.
Chemical shifts are reported in ppm using TMS or the solvent residual peak as a
reference. Ion-exchange chromatography was performed on prepacked VaripureTM IPE
columns from Varian. Bruker Esquire 3000 Plus was used for electro spray ionisation
(ESI) mass spectrometry. The CD-spectra were recorded on an Applied Biophysics
Chirascan spectrometer. HPLC analyses were performed on an analytical HPLC with a
diode array detector from Shimadzu.
2. General procedure for the direct aldol reaction of 4-nitrobenzaldehyde with
cyclohexanone
The reactions were performed in 1.5 ml glass vials with screw caps that were agitated in a
thermomixer (700 rpm). The catalyst 1a (3.5 mg, 0.007 mmol, 0.05 eq.) and 4-
nitrobenzaldehyde (20.0 mg, 0.13 mmol, 1 eq.) were placed in a glass vial and mixed with
cyclohexanone (150 µl, 1.45 mmol, 11 eq.). The addition of the specified solvent (350 µl)
resulted in a clear solution after 1 min of agitation. The reaction mixture was agitated until
full conversion (3-48h, monitored by TLC-analysis: 20% EtOAc in cyclohexane), diluted
with CH2Cl2 (3 ml) and quenched with a half saturated solution of NH4Cl (2 ml). The
layers were separated, extracted with CH2Cl2 (3 x 2 ml) and the combined organic phases
were washed with sat. NaCl (3 ml) and dried over MgSO4. The filtrate was concentrated
in vacuo and purified by flash chromatography on silica gel (5.6 g, 25% EtOAc in
cyclohexane) to afford the aldol product as a colourless liquid that crystallized in long
needles upon standing. The enantiomeric excess (ee) and the diastereomeric excess (de)
1
were determined by chiral-phase HPLC analysis and by H NMR analysis, respectively.
Assignment of the stereoisomers was performed by comparison with published data1 and
chromatographic data obtained after separating the diastereoisomers.
1 (a) Mase, N.; Nakai, Y.; Ohara, N.; Yoda, H. Takabe, K.; Tanaka, F.; Barbas, III C. F. J. Am. Chem. Soc. 2006,
128, 734-735. (b) Tzeng, Z. H.; Chen, H. Y.; Reddy, R. J.; Huang, C. T.; Chen, K. Tetrahedron 2009, 65, 2879-
2888.

3. Synthesis of peptidic catalyst 1a
Boc-Pro-Pro-OMe (3)
To a solution of H-Pro-OMe (1.65 g, 12.8 mmol, 1 eq.) in dry CH2Cl2 (40 ml) was added
Boc-Pro-OH (2.69 g, 12.5 mmol, 0.98 eq.). The yellow solution was cooled to 0°C and
iPr2NEt (128 µl, 1.27 mmol, 0.1 eq.) was added followed by EDC·HCl (2.93 g, 15.31
mmol, 1.2 eq.). The flask was closed with a CaCl2-tube and the turbid solution was stirred
for one hour at 0°C and overnight at rt, by which time the solution had become clear
again. Removal of the solvent gave 7.3 g of the crude product that was dissolved in 15 ml
CH2Cl2, washed once with H2O (5 ml) and then with 1 M HCl (5 ml) and an additional
portion of water (5 ml). The organic phase was washed with sat. aq. NaHCO3 (8 ml), sat.
aq. NaCl (5 ml) solution and dried over MgSO4. Removal of the solvent under reduced
pressure yielded 3.58 g of an amber colored oil (11.0 mmol, 86%):
TLC Hex/EtOAc (1:3), Rf = 0.29;
s-cis/s-trans Conformers around the Pro-Pro amide bond are observed in a ratio of 1 : 0.75
in the 1H and 13C NMR spectra in CDCl3.
Major Isomer: 1H NMR (400MHz, CDCl3) į 4.58 (dd, J = 4.0 Hz, J = 8.8 Hz, 1H, HĮPro),
4.49 (dd, J = 3.2, J = 8.4 Hz, 1H, HĮPro), 3.79 – 3.35 (m, 4H, HįPro), 3.69 (s, 3H, OMe),
2.18 – 1.83 (m, 8H, HȕPro, HȖPro), 1.44 (s, 9H, OtBu).
13C NMR (100MHz, CDCl3) į 168.6, 166.8, 154.7, 81.2, 60.9, 60.1, 51.9, 46.8, 46.6, 30.9,
28.8, 24.0, 23.7.
MS (ESI) m/z (%) = 349 [M+Na]+ (100), 675 [2M+Na]+ (20), calcd for C16H26N2O5.
Boc-Pro-Pro-OH (4)
The dipeptide 3 (3.58 g, 11.0 mmol, 1eq) was dissolved in a mixture of THF/MeOH (1:1,
20 ml). An aqueous solution of NaOH (5 ml, 0.79 g, 19.8 mmol, 1.8 eq) was slowly added
to the peptide whereupon an increase of the reaction temperature was observed. The
solution was stirred at rt for 2.5 h. The organic solvents were removed under reduced
pressure and the aqueous phase was acidified to pH 2 by dropwise addition of conc. HCl
(ca. 2 ml). The aqueous solution was extracted with CH2Cl2 (2 x 10 ml) and the combined
organic layers were washed with H2O (10 ml), sat. aq. NaCl (10 ml) and dried over
MgSO4. After removal of the solvent a clear oil was obtained which solidified upon
drying (2.84 g, 9.1 mmol, 83%):

s-cis/s-trans Conformers around the Pro-Pro amide bond are observed in a ratio of 1 : 0.75
in the 1H and 13C NMR spectra in CDCl3.
Major isomer: 1H NMR (400MHz, CDCl3) į 4.63 (dd, J = 2.8 Hz, J = 8.0 Hz, 1H, HĮPro),
4.48 (dd, J = 3.6, J = 8.0 Hz, 1H, HĮPro), 3.75 (dd, J = 7.6 Hz, J = 9.6, 1H, HįPro), 3.58 –
3.36 (m, 3H, HįPro), 2.32 – 1.80 (m, 8H, HȕPro, HȖPro), 1.44 (s, 9H, OtBu).
13C NMR (100MHz, CDCl3) į 174.3, 173.7, 155.1, 80.3, 60.1, 58.2, 47.6, 47.4, 30.5, 29.7,
28.9, 25.4, 24.6.
MS (ESI) m/z (%) = 335 [M+Na]+ (100), 647 [2M+Na]+ (30), 213 [M-Boc]+ (30), calcd
for C15H24N2O5.
Cbz-Asp(OtBu)-NH(CH2)11CH3 (5)
Cbz-Asp(OtBu)-OH (3.50 g, 10.83 mmol, 1 eq.) and dodecylamine (1.97 g, 12.13 mmol,
0.98 eq.) were dissolved in CH2Cl2, the flask was closed with a CaCl2 tube and cooled
with an ice bath. iPr2NEt (40 µl) was added and the mixture was vigorously stirred while
EDC·HCl (2.12 g, 11.05 mmol, 1.2 eq.) was added. The resulting suspension was stirred
at 0°C for one hour and overnight at rt. The solvent was removed under reduced pressure
and the residue was taken up in EtOAc (30 ml). The organic phase was washed with H2O
(20 ml) and the aqueous phase was extracted with EtOAc (10 ml). The combined organic
layers were washed with 0.1 M HCl (20 ml), sat. NaHCO3 (15 ml), sat. NaCl (15 ml) and
dried over MgSO4. Removal of the solvent yielded 5 as a colorless solid (4.86 g, 9.91
mmol, 90%):
1H NMR (400MHz, CDCl3) į 7.35 (s, 4H, Cbz), 6.48 (s, 1H, Asp-NH), 5.98 (d, J = 7.2
Hz, NHCH2), 5.12 (s, 2H, Cbz), 4.47 (s, 1H, HĮAsp), 3.21 (dt, J = 6.8 Hz, J = 6.4 Hz,
NHCH2CH2), 2.89 (dd, J = 4.0 Hz, J = 16.8 Hz,1H, HȕAsp), 2.61 (dd, J = 6.8 Hz, J = 16.8
t
Hz,1H, HȕAsp), 1.43 (s, 11H, Bu, CH2CH3), 1.25 (s, 18H, 9xCH2), 0.87 (t, J = 6.0 Hz,
CH3).
13C NMR (100MHz, CDCl3) į 171.7, 170.6, 136.5, 129.0 128.7, 128.5, 82.2, 67.6, 51.5,
40.1, 37.9, 32.3, 30.1, 29.7, 28.4, 27.2, 23.1, 21.5, 14.6.
MS (ESI) m/z (%) = 677 (100), 514 [M+Na]+ (82), 492 [M+H]+ (6), calcd for C28H46N2O5.

H-Asp(OtBu)-NH(CH2)11CH3 (6)
Under a nitrogen atmosphere, MeOH (10 ml) was added to 10% (w/w) Pd/C (490 mg)
followed by Peptide 5 (4.86 g, 9.91 mmol, 1eq.). The suspension was stirred vigorously
and ammonium formate (5.00 g, 79.3 mmol, 8 eq.) was added, whereupon the mixture
evolved a gas. The mixture was stirred at rt for 3 h, filtered through a glass funnel and the
filtrate was washed with additional MeOH (3 x 15 ml). The solvent was removed under
reduced pressure to obtain an amber colored oil. The residue was dissolved in CH2Cl2 (40
ml), washed with sat. NaHCO3 (20 ml) and H2O (2 x 20 ml). The aqueous phase was
extracted with CH2Cl2 (2 x 15 ml) and the combined organic extracts were washed with
sat. NaCl (20 ml) and dried over MgSO4. The solvent was removed under reduced
pressure to obtain a colorless, sticky solid (3.37 g, 9.45 mmol, 95%):
1H NMR (400MHz, CDCl3) į 3.63 (dd, J = 4.0 Hz, J = 8.4 Hz, 1H, HĮAsp), 3.21 (dt, J =
5.2 Hz, J = 7.6 Hz, NHCH2CH2), 2.84 (dd, J = 4.0 Hz, J = 16.8 Hz, 1H, HȕAsp), 2.47 (dd,
J = 8.4 Hz, J = 16.8 Hz, 1H, HȕAsp), 1.43 (s, 11H, tBu, CH2CH3), 1.24 (s, 18H, 9xCH2),
0.87 (t, J = 6.4 Hz, CH3).
13C NMR (100MHz, CDCl3) į 173.7, 171.8, 81.5, 41.1, 39.7, 32.3, 30.0, 29.8, 28.5, 27.4,
23.1, 14.5.
MS (ESI) m/z (%) = 736 [2M+Na]+ (100), 379 [M+Na]+ (40), 357 [M+H]+ (10), calcd for
C20H40N2O5.
Boc-Pro-Pro-Asp(OtBu)-NH(CH2)11CH3 (7)
The alkylated amino acid 6 (2.46 g, 6.89 mmol, 0.98 eq.) was dissolved in CH2Cl2 (25 ml)
and dipeptide 4 (2.20 g, 7.03 mmol, 1 eq.) was added. The solution was cooled to 0°C,
EDC·HCl (1.62 g, 8.44 mmol, 1.2 eq.) and iPr2NEt (24 µl, 0.14 mmol, 0.02 eq.) were
added and the resulting suspension was stirred at 0°C for 1 h and for another 15 h at rt.
The reaction mixture was diluted with CH2Cl2 (10 ml), washed with H2O (15 ml) and the
aqueous phase was extracted with CH2Cl2 (3 x 10 ml). The combined organic extracts
were washed with 0.1 M HCl (20 ml), H2O (15 ml), sat. aq. NaHCO3 (15 ml), sat. aq.
NaCl (15 ml) and dried over MgSO4. All volatiles were removed under reduced pressure
to yield 3.60 g of the crude product. The oily residue was subjected to a column
chromatography (cyclohexane:EtOAc, 1:5) to yield 7 (2.74 g, 4.20 mmol, 61%) as a
colorless solid:

s-cis/s-trans Conformers around the Pro-Pro amide bond (1 : 0.10) and around the
Asp(OtBu) carbamate bond are observed in the 1H and 13C NMR spectra in CDCl3.
1
Major isomer: H NMR (400MHz, CDCl3) į 7.50 (t, J = 11.2 Hz, 1H, CONH), 6.96 (m,
1H, CONH), 4.74 – 4.18 (m, 3H, HĮPro, HĮPro, HĮAsp), 3.73 – 3.45 (m, 4H, HįPro)
3.25 – 3.09 (m, 2H, NHCH2CH2), 2.89 (dd, J = 5.6 Hz, J = 16.4 Hz,1H, HȕAsp), 2.63 (dd,
J = 4.8 Hz, J = 16.8 Hz,1H, HȕAsp), 2.28 – 1.80 (m, 8H, HȕPro, HȖPro), 1.52 – 1.34 (m,
20H, 2xtBu, CH2CH3), 1.32 – 1.16 (s, 18H, 9xCH2), 0.85 (t, J = 6.8 Hz, CH3).
Minor isomer: 1H NMR (400MHz, CDCl3) į 7.87 (d, J = 7.6 Hz, 1H, CONH), 4.64 (d, J =
8 Hz, 1H, HĮPro), 4.55 – 4.39 (m, 1H, HĮAsp), 4.32 (d, J = 8 Hz, 1H, HĮPro), 3.73 –
3.45 (m, 4H, HįPro), 3.25 – 3.09 (m, 2H, NHCH2CH2), 3.06 (dd, J = 4.0 Hz, J = 17.2 Hz,
1H, HȕAsp), 2.44 (dd, J = 5.6 Hz, J = 17.2 Hz, 1H, HȕAsp), 2.28 – 1.80 (m, 8H, HȕPro,
HȖPro), 1.52 – 1.34 (m, 20H, 2xtBu, CH2CH3), 1.32 – 1.16 (s, 18H, 9xCH2), 0.85 (t, J =
6.8 Hz, CH3).
13C NMR (100MHz, CDCl3) į 173.2, 171.5, 170.3, 155.0 81.7, 80.4, 62.0, 59.5, 58.2,
49.8, 47.6, 47.2, 40.3, 37.0, 32.3, 30.0, 29.7, 29.1, 28.8, 28.4, 27.3, 25.8, 24.8, 24.1, 23.1,
14.6.
MS (ESI) m/z (%) = 674 [M+Na]+ (100), 1325 [2M+Na]+ (26), calcd for C35H62N4O7.
H-Pro-Pro-Asp-NH(CH2)11CH3 (1a)
Under a nitrogen atmosphere, tripeptide 7 (2.63 g, 4.05 mmol, 1eq.) was dissolved in dry
CH2Cl2 (4.5 ml) and Et3SiH (1.62 ml, 10.13 mmol, 2.5 eq.) was added dropwise, followed
by TFA (4 ml, 52.60 mmol, 13 eq.) whereupon the solution evolved a gas. The mixture
was stirred at rt for 5 h and the solvent was removed under reduced pressure. The oily
residue was dissolved in MeOH (1 ml) and subjected to an ion-exchange column
chromatography. The column was washed with additional MeOH (5 x 2 ml) and the
combined portions were concentrated under reduced pressure. The concentrated solution
was added dropwise to a 100 fold excess of Et2O at 0°C under rapid stirring which caused
the immediate formation of a precipitate. The solvent was decanted and the precipitate
was dried under reduced pressure to yield 1a (1.90 g, 3.85 mmol, 95%) as a colorless
solid:
s-cis/s-trans Conformers around the Pro-Pro amide bond are observed in a ratio 1 : 0.10 in
the 1H and 13C NMR spectra in CDCl3.

1
Major isomer: H NMR (400MHz, CDCl3) į 8.14 (d, J = 8.0 Hz, 1H, CONH), 7.89 (m,
1H, CONH), 6.97 (m, 1H, CONH), 4.74 – 4.52 (m, 3H, HĮPro, HĮPro, HĮAsp), 3.72 –
3.38 (m, 4H, HįPro), 3.28 – 3.10 (m, 2H, NHCH2CH2), 2.88 – 2.66 (m, 2H, HȕAsp), 2.52
– 1.88 (m, 8H, HȕPro, HȖPro), 1.50 – 1.40 (m, 2H, CH2CH3), 1.32 – 1.20 (m, 18H,
9xCH2), 0.87 (t, J = 6.4 Hz, CH3).
1
Minor isomer: H NMR (400MHz, CDCl3) į 8.38 (d, J = 8.0 Hz, 1H, CONH), 7.64 (m,
1H, CONH), 6.72 (m, 1H, CONH), 4.92 (dd, J = 4.4 Hz, J = 8.8 Hz, 1H, HĮPro), 4.47 (d,
J = 8 Hz, 1H, HĮAsp), 4.26 (m, 1H, HĮPro), 3.72 – 3.38 (m, 4H, HįPro), 3.28 – 3.10 (m,
2H, NHCH2CH2), 2.88 – 2.66 (m, 2H, HȕAsp), 2.52 – 1.88 (m, 8H, HȕPro, HȖPro), 1.50 –
1.40 (m, 2H, CH2CH3), 1.32 – 1.20 (m, 18H, 9xCH2), 0.87 (t, J = 6.4 Hz, CH3).
13C NMR (100MHz, CDCl3) į 174.1, 172.1, 171.0, 168.1, 61.0, 59.2, 50.5, 47.9, 40.3,
32.3, 30.1, 29.8, 27.3, 25.5, 25.0, 23.1, 14.5.
MS (ESI) m/z (%) = 495 [M+Na]+ (100), 517 [M+Na]+ (15), 1013 [2M+Na]+ (17), calcd
for C26H46N4O5.

4. NMR-spectra and HPLC-chromatograms
H-Pro-Pro-Asp-NH-(CH2)11-CH3 (1a)
1H NMR (400 MHz, CDCl3, 25 °C)

1H NMR (400 MHz, 10% D2O in DMSO-d6, 25 °C)
1H NMR (500 MHz, DMSO-d6, 25 °C)

Direct aldol reaction in DMSO (Table2, entry 1):
(2R, 1’S)-2-[Hydroxy(4-nitrophenyl)methyl]cyclohexanone
HPLC analysis Chiracel AD-H (iPrOH/hexanes=20:80,
0.5 ml/min, 254 nm), tR = 32.0 min (major isomer).
1 (21.6 min, 10%, (2R,1`R)); 2 (22.9 min, 10%, (2S,1`S)); 3 (25.0 min, 17%, (2S,1`R)); 3
(32.0 min, 63%, (2R,1`S)).
Direct aldol reaction in DMSO + 10% H2O (Table 1, entry 4):
(2S, 1’R)-2-[Hydroxy(4-nitrophenyl)methyl]cyclohexanone
HPLC analysis Chiracel AD-H (iPrOH/hexanes=20:80,
0.5 ml/min, 254 nm), tR = 25.0 min (major isomer).
1 (21.6 min, 28%, (2R,1`R)); 2 (22.9 min, 14%, (2S,1`S)); 3 (25.0 min, 47%, (2S,1`R)); 3
(32.0 min, 12%, (2R,1`S)).