A. Beauchard et al. / Tetrahedron Letters 52 (2011) 5311–5314
5313
Cbz-
coupling Cbz-
was then removed quantitatively by hydrogenolysis, affording
the dipeptide -Leu-
-Ser(OBut)OMe 11. N-Cbz-b-cyanomethyl-
L
-Leu-
D
-Ser(OBut)OMe 10 was prepared in the usual way by
-Leu 8 with
-Ser(OBut)OMe 9 using PyBOP. Cbz
In this Letter, we introduce b-cyanomethyl-
L
-Ala (S-2-amino-4-
L
D
cyanobutanoic acid) as a potentially useful and perfectly atom-effi-
cient synthetic equivalent of -Gln in solution-phase peptide
synthesis. N -Cbz-b-cyanomethyl- -Ala 6 is generated in high yield
from Cbz- -Gln 5. The Cbz protection can be removed selectively by
L
a
L
D
L
-
L
Ala 6 coupled very effectively with this dipeptide 11 by the HATU
method to provide the protected tripeptide 12,20 demonstrating
that it can be attached easily to growing chains. The Cbz group
was then removed by hydrogenolysis under standard conditions
(H2, Pd/C) to give 13 quantitatively, with no evidence of any com-
L
hydrogenolysis, under conditions which preserve the nitrile com-
pletely. The nitrile is also stable to the acidic conditions required
for removal of Boc and OBut protecting groups and for condensa-
tions and to the basic conditions for removal of Fmoc. It is,
however, rapidly and quantitatively hydrated to the primary
peting reduction of the nitrile.21 Coupling with Cbz-
L
-LeuOH 8 then
led to the protected tetrapeptide 14 in good yield,22 showing that
peptide couplings to the N-terminus of b-cyanomethyl- -Ala are
efficient. Tetrapeptide 14 contains b-cyanomethyl- -Ala as
carboxamide of
L-Gln. Thus we propose b-cyanomethyl-L-Ala as a
L
synthon for -Gln; complete characterisation of its uses and limita-
L
L
a
tions will be presented in a later full paper.
masked Gln. The Gln was revealed quantitatively by mild hydra-
tion with hydrogen peroxide under basic conditions, forming 15,
with no evidence for over-hydrolysis to the carboxylic acid
(Glu).23 Thus, within a peptide sequence, regeneration of the
carboxamide is feasible and efficient. To test the stability of the ni-
trile under acidic deprotection conditions, 14 was treated with tri-
fluoroacetic acid, unmasking the Ser side chain in 16. From here,
acid-catalysed condensation with 2,2-dimethoxypropane fur-
nished the tetrapeptide 17, which contains both the C-terminal
Acknowledgments
We thank Dr. Timothy J. Woodman (University of Bath) for
many of the NMR spectra, Dr. Anneke T. Lubben (University of
Bath) for the mass spectra and Dr. Ghadeer A. R. Y. Suaifan (Univer-
sity of Jordan) for helpful discussions. M.D.L. and M.D.T. are mem-
bers of Cancer Research at Bath (CR@B). We are very grateful to the
Association for International Cancer Research (AICR Grant 08-
0058) and the Prostate Cancer Charity for funding this work.
D
-Dmo residue (which can be considered as a masked
as an analogue of -Pro) and the b-cyanomethyl- -Ala (masked
Gln). Analogously, the side-chain But protection was removed from
the
-Ser of the shorter peptide 12, giving 18,24 prior to formation
of the
-Dmo-containing tripeptide 19.25
Scheme 4 gives further demonstrations of the compatibility and
use of b-cyanomethyl- -Ala in solution-phase peptide synthesis.
Cbz-b-cyanomethyl-
D-Ser and
D
L
Supplementary data
D
D
Supplementary data associated with this article can be found, in
L
L
-AlaOH 6 was coupled to L-alanine methyl es-
ter by the PyBOP method to give protected dipeptide 23.26 In this
case, the hydrogenolytic cleavage of Cbz had to be monitored care-
fully, as over-reaction gave traces of the product of reduction of the
References and notes
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6. Henkel, B.; Zhang, L.; Goldammer, C.; Bayer, E. Z. Naturforschung B 1996, 51,
1339–1346.
nitrile (
cyanomethyl-Ala)-
Boc-
-AlaOH by the HATU procedure gave masked tripeptide 25.27
L
-Orn-L-AlaOMe) and of the diketopiperazine cyclo-
L-(b-
L-Ala. Immediate coupling of the amine 24 with
L
Careful hydration of the nitrile with hydroperoxide ion was
achieved without disruption of the N-Boc protection, generating
7. Gitu, P. M.; Yusuf, A. O.; Bhatt, B. M. Bull. Chem. Soc. Ethiopia 1998, 12, 35–43.
8. Franzén, H. F.; Någren, K.; Grehn, L.; Långström, B.; Ragnarsson, U. J. Chem. Soc.,
Perkin Trans. 1 1988, 497–502.
Boc-
L
-Ala-
L
-Gln-
L
-AlaOMe 26.28
9. Sieber, P.; Riniker, B. Tetrahedron Lett. 1991, 32, 739–742.
10. Lee, J. T.; Chen, D. Y.; Yang, Z.; Ramos, A. D.; Hsieh, J. J.-D.; Bogyo, M. Bioorg.
Med. Chem. Lett. 2009, 19, 5086–5090.
N
N
11. Sax, B.; Dick, F.; Tanner, R.; Gostelli J. Peptide Res. 1992, 5, 245–246.
12. Jelinski, M.; Hamacher, K.; Coenen, H. H. J. Labelled Compd. Radiopharm. 2002,
45, 217–229.
13. Ramesh, D.; Wieboldt, R.; Billington, A. P.; Carpenter, B. K.; Hess, G. P. J. Org.
Chem. 1993, 58, 4599–4605.
14. Ressler, C.; Ratzkin, H. J. Org. Chem. 1961, 26, 3356–3360.
15. Webber, S. E.; Okano, K.; Little, T. L.; Reich, S. H.; Xin, Y.; Fuhrman, S. A.;
Matthews, D. A.; Love, R. A.; Hendrickson, T. F.; Patick, A. K.; Meador, J. W.;
Ferre, R. A.; Brown, E. L.; Ford, C. E.; Binford, S. L.; Worland, S. T. J. Med. Chem.
1998, 41, 2786–2805.
i
H
N
CO2Me
CbzHN
CO2H
RHN
ii
O
6
23: R = Cbz
24: R = H
iii
16. Chen, Y.; Bilban, M.; Foster, C. A.; Boger, D. L. J. Am. Chem. Soc. 2002, 124, 5431–
5440.
17. Jao, S.-C.; Chen, J.; Yang, K.; Li, W.-S. Bioorg. Med. Chem. 2006, 14, 304–318.
18. Dubois, J.; Bory, S.; Gaudry, M.; Marquet, A. J. Med. Chem. 1984, 27, 1230–1233.
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721–734.
N
20. Synthesis of 12. Compound 6 (900 mg, 3.43 mmol) and L-Leu-D
-Ser(OBut)-OMe
O
H
11 (1.00 g, 3.43 mmol) were stirred with 2-(1H-7-azabenzotriazol-1-yl)-
1,1,3,3-tetramethyluronium hexafluorophosphate (HATU) (1.43 g, 3.77 mmol)
and Pri NEt (885 mg, 6.86 mmol) in CH2Cl2 (80 mL) for 20 h. Washing (H2O
(50 mL2), aq. citric acid (5%, 50 mL)), aq. NaHCO3 (5%, 100 mL), H2O (50 mL)),
drying, evaporation and chromatography (EtOAc, silica gel) gave 12 (1.14 g,
77%) as a white powder: mp 95–96 °C.
BocHN
N
CO2Me
Me
N
H
Me
O
25
26
iv
21. Synthesis of 13. Compound 12 (820 mg, 1.54 mmol) was stirred vigorously
Boc-L-Ala-L-Gln-L-AlaOMe
under H2 with Pd/C (10%, 80 mg) in MeOH (80 mL) for 16 h. Filtration (CeliteÒ
)
and evaporation gave 13 (640 mg, quant.) as a pale yellow oil.
22. Synthesis of 14. Cbz-L-LeuOH 8 (210 mg, 0.80 mmol) and 13 (320 mg, 0.80 mmol)
Scheme 4. Further example of the use of S-4-cyanobutanoic acid as a synthon for L-
Gln in solution-phase peptide synthesis; compatibility with NBoc protection.
were stirred with benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluoro
phosphate (PyBOP) (500 mg, 0.96 mmol) and Et3N (323 mg, 3.20 mmol) in
CH2Cl2 (15 mL) for 2 d. Washing (H2O (20 mL), aq. citric acid (5%, 20 mL)), aq.
NaHCO3 (5%, 20 mL), H2O (40 mL)), drying, evaporation and chromatography
Reagents and conditions: (i)
L
-AlaOMeꢀHCl, PyBOP, Pri NEt, DMF, 87%; (ii) H2, Pd/C,
2
MeOH; (iii) Boc-
L
-AlaOH, Pri NEt, HATU, DMF, 64% from 23; (iv) aq. H2O2, NaOH,
2
MeOH, 0 °C; 23%.