Synthesis of Fmoc-protected amino ketones bearing tert-butyl based side-chain protecting groups

Article abstract of DOI:10.1016/S0040-4039(02)01805-1

A variety of Fmoc-protected amino ketones have been prepared in good yields from amino acids by their transformation into thioesters of 2-mercaptopyridine and reaction of the resulting products with dialkylcuprates.

Full text of DOI:10.1016/S0040-4039(02)01805-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 7499–7502
Synthesis of Fmoc-protected amino ketones bearing tert-butyl
based side-chain protecting groups
Jesu´s Va´zquez and Fernando Albericio*
Barcelona Biomedical Research Institute-Barcelona Science Park and Department of Organic Chemistry, University of Barcelona,
E-08028 Barcelona, Spain
Received 12 August 2002; revised 23 August 2002; accepted 26 August 2002
Abstract—A variety of Fmoc-protected amino ketones have been prepared in good yields from amino acids by their transforma-
tion into thioesters of 2-mercaptopyridine and reaction of the resulting products with dialkylcuprates. © 2002 Elsevier Science
Ltd. All rights reserved.
Proteases, notably cysteine proteases, are implicated in
the pathogenesis of many diseases; hence they have
become targets of great pharmaceutical interest in
recent years.1 Several types of diseases are treated by
inhibition of these enzymes, the most common strategy
utilizing a peptide that can interact with the reactive
centre of an enzyme’s active site.2–5 Both reversibly or
irreversibly binding molecules can be used as cysteine
protease inhibitors, although in almost all cases they
feature an electrophilic functionality such as a carbonyl
group or Michael acceptor that can react with the
nucleophilic cysteine residue. Thus, ketones,2 fluoroke-
tones,3 aldehydes4 and sulfones5 have been reported as
cysteine protease inhibitors.
peptides in which the C-terminal amino acid is a
modified aspartic acid containing electrophilic groups
such as ketones. Combinatorial chemistry has been
used to discover caspase inhibitors by screening both
amino acids and electrophillic groups. However, effec-
tive libraries require appropriate building blocks such
as novel amino ketones.
While amino ketones have been synthesized in both
solution and solid phase they have historically been
simple molecules, such as amino acids with minimally
functionalized side chains, and have required the use of
protecting groups that are stable in basic media.8
An optimal strategy for the solid-phase synthesis of
molecules containing a modified aspartic acid would
involve the incorporation of an Fmoc-protected amino
ketone to either Wang or Barlos resin through the
b-carboxylic acid. Therefore, a convenient method for
the preparation of Fmoc-derivatives of the modified
aspartic acid would be useful.
Caspases are a family of cysteine proteases that are
involved in apoptotic cell death, and pharmacological
caspase inhibition has been demonstrated to prevent
neuronal cell death.6 The syntheses of various inhibitors
of caspases have been reported.7 Among these
molecules are compounds based on a family of tetra-
Scheme 1. Reagents and conditions: (i) 1-ethyl-3-(3%-dimethylaminopropyl)carbodiimide (1 equiv.), 2-mercaptopyridine (1.1 equiv.),
DCM, rt, 6 h; (ii) R%2CuMgX (3 equiv.), THF, 0°C, 1–3 h.
t
Abbreviations: Bu, tert-butyl; Fmoc, 9-fluorenylmethyloxycarbonyl; Boc, tert-butoxycarbonyl; DCM, dichloromethane; EtOAc, ethyl acetate; Ph,
phenyl; THF, tetrahydrofuran; Amino acid symbols denote the L-configuration.
Keywords: amino ketones; ketone synthesis; combinatorial chemistry; high-throughput synthesis.
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01805-1

7500
J. Va´zquez, F. Albericio / Tetrahedron Letters 43 (2002) 7499–7502
In our search for ketone-forming conditions in which
both the Fmoc and Bu ester protecting groups of
of Fmoc-protecting groups. We report here a general
method for the preparation of Fmoc-protected amino
ketones from amino acids in good yields (Scheme 1).
t
Fmoc-Asp(OtBu)-OH would be stable, we initially
explored Grignard chemistry on Weinreb amides. As
one would expect, the Fmoc group proved to be
unstable under these conditions.9,10 Although we were
able to obtain the ketone product using the Weinreb
derivative of Boc-Asp(tBu)-OH,9 this process required
the elimination of the Boc group and subsequent Fmoc
protection of the amine.
We based our studies on thioesters because the tradi-
tional preparation of acid chlorides is incompatible with
the presence of Boc or tBu protecting groups.11 Further-
more, in solution the PPh3/CCl4 procedure generates
triphenylphosphine oxide, which is difficult to separate
from the desired product, and the alternative use of
PPh3 resin is rather expensive.
Organocuprates, which are used in the synthesis of
ketones from acid chlorides or thioesters, are widely
regarded as weak bases and so we hypothesized that
they would be compatible with syntheses in the presence
We chose 2-mercaptopyridine as the thiol since it is
odorless, relatively inexpensive and generates highly
reactive thioesters. The synthesis of these thioesters was
Table 1. Preparation of Fmoc-protected amino ketones from Fmoc-protected amino thioesters

J. Va´zquez, F. Albericio / Tetrahedron Letters 43 (2002) 7499–7502
7501
carried out with 1-ethyl-3-(3%-dimethylamino propyl)-
carbodiimide because its urea byproduct can be easily
removed by aqueous extraction.12 Treatment of
thioesters 2 with dialkylcuprates in anhydrous THF
under an Ar atmosphere afforded the Fmoc-protected
amino ketones 3 in good yields (Table 1). Besides the
Asp derivatives, this methodology was also used for the
preparation, with similar results (Table 1), of Tyr and
Lys derivatives from the corresponding Fmoc/tBu and
Fmoc/Boc protected amino acids.13
with diethyl ether (30 mL). After several washes with
saturated aqueous ammonium chloride (2×20 mL), 10%
aq. Na2CO3 (2×20 mL) and sat. brine (1×20 mL) the
THF–ether solution was dried over MgSO4, filtered and
concentrated by rotary evaporation. Purification by
column chromatography (silica gel, hexane–EtOAc 8:2)
gave 342 mg of the desired product (73%) as a yellow
oil. See Table 1 for NMR data. MS (ESI, positive ion):
m/z calcd: 513.3 [M+1]+ Found: 513.6 [M+1]+.
Acknowledgements
In conclusion, we have developed a general and
straightforward methodology to prepare Fmoc-pro-
tected amino ketones. These derivatives, after removal
This work was partially supported by CICYT
(BQU2000-0235), Generalitat de Catalunya [Grup Con-
solidat, and Centre de Refere`ncia en Biotecnologia].
t
of the Bu-based protecting groups with TFA, are cur-
rently being incorporated into solid supports through
the side-chain for the preparation of libraries.14 The
results of these studies will be published elsewhere.
References
Experimental
1. Otto, H.-H.; Schirmeister, T. Chem. Rev. 1997, 97, 133–
171.
2. Semple, G.; Ashworth, D. M.; Batt, A. R.; Baxter, A. J.;
Benzies, D. W. M.; Elliot, L. H.; Evans, D. M.; Franklin,
R. J.; Hudson, P.; Jenkins, P. D.; Pitt, G. R.; Rooker, D.
P.; Yamamoto, S.; Isomura, Y. Bioorg. Med. Chem. Lett.
1998, 8, 959–964.
3. Morris, T. S.; Frormann, S.; Sheckosky, S.; Lowe, C.;
Lall, M. S.; Gauss-Mu¨ller, V.; Purcell, R. H.; Emerson,
S. U.; Vederas, J. C.; Malcolm, B. A. Bioorg. Med. Chem.
1997, 5, 797–807.
General. THF was distilled from sodium/benzophenone
immediately before use. Column chromatography was
performed with silica gel (Silica 60 A, 35–70 mm).
Thin-layer chromatography (TLC) was performed on
aluminum-backed silica plates (Merck 60/F254) to fol-
1
low the reactions. H NMR spectra were obtained on a
Varian Gemini 200 MHz spectrometer in CDCl3 unless
stated otherwise. Chemical shifts are reported in ppm (l
units) downfield from internal tetramethylsilane or the
appropriate solvent signal [(CD3)2CO].
4. Yasuma, T.; Oi, S.; Choh, N.; Nomura, T.; Furuyama,
N.; Nishimura, A.; Fujisawa, Y.; Sohda, T. J. Med.
Chem. 1998, 41, 4301–4308.
Preparation of thioesters. A sample procedure is as
follows: To a stirred suspension of Fmoc-Asp(OtBu)-
OH (1.43 mmol) in DCM (20 mL) was added 1-ethyl-3-
(3%-dimethylaminopropyl)carbodiimide (1.43 mmol).
Upon complete dissolution, 2-mercaptopyridine (1.57
mmol) was added to the reaction and stirring was
continued for 6 h. The resulting mixture was diluted
with DCM (20 mL) and the organic phase was washed
with 0.1 N aq. HCl (2×20 mL), 10% aq. Na2CO3 (2×20
mL) and sat. brine (1×20 mL), dried over MgSO4 and
filtered. The solvent was removed (rotary evaporation)
5. Palmer, J. T.; Rasnick, D.; Klaus, J.; Bro¨mme, D. J.
Med. Chem. 1995, 38, 3193–3196.
6. (a) Thornberry, N. A.; Lazebnick, Y. Science 1998, 281,
1312–1316; (b) Mjalli, A. M.; Chapman, K. T.; Zhao, J.
J.; Thornberry, N. A.; Peterson, E. P.; MacCoss, M.
Bioorg. Med. Chem. Lett. 1995, 5, 1405–1408; (c) Hurel-
brink, C. B.; Armstrong, R. J. E.; Luheshi, L. M.;
Dunnett, S. B.; Rosser, A. E.; Barker, R. A. Exp. Neurol.
2001, 171, 46–58; (d) Lee, D.; Long, S. A.; Murray, J. H.;
Adams, J. L.; Nuttall, M. E.; Nadeau, D. P.; Kikly, K.;
Winkler, J. D.; Sung, C.-M.; Ryan, M. D.; Levy, M. A.;
Keller, P. M.; DeWolf, W. E. J. Med. Chem. 2001, 44,
2015–2026.
7. (a) Thornberry, N. A.; Rano, T. A.; Peterson, E. P.;
Rasper, D. M.; Timkey, T.; Garc´ıa-Calvo, M.;
Houtzager, V. M.; Nordstrom, P. A.; Roy, S.; Vaillan-
court, J. P.; Chapman, K. T.; Nicholson, D. W. J. Biol.
Chem. 1997, 272, 17907–17911; (b) Garc´ıa-Calvo, M.;
Peterson, E. P.; Leiting, B.; Ruel, R.; Nicholson, D. W.;
Thornberry, N. A. J. Biol. Chem. 1998, 273, 32608–
32613.
8. (a) Yamashita, D. S.; Dong, X.; Oh, H.-J.; Brook, C. S.;
Tomaszeck, T. A.; Szewcsuck, L.; Tew, D. G.; Veber, D.
F. J. Comb. Chem. 1999, 1, 207–215; (b) Ferna´ndez-
Meg´ıa, E.; Iglesias-Pintos, J. M.; Sardina, J. J. Org.
Chem. 1997, 62, 4770–4779; (c) De Luca, L.; Giacomelli,
G.; Porcheddu, A. Org. Lett. 2001, 3, 1519–1521; (d)
Paris, M.; Douat, C.; Heitz, A.; Gibbons, W.; Martinez,
J.; Fehrentz, J.-A. Tetrahedron Lett. 1999, 40, 5179–5182;
(e) Dinh, T. Q.; Armstrong, R. W. Tetrahedron Lett.
1996, 37, 1161–1164.
1
to give a pale yellow foamy solid (598 mg, 83%). H
NMR l 8.63 (d, J=5.9 Hz, 1H) 7.8–7.2 (m, 11H) 6.10
(d, J=9.5 Hz, 1H) 4.85 (ddd, J=9.5, 5.1, 4.4 Hz, 1H)
4.65–4.25 (m, 3H) 3.04 (dd, J=5.1, 17.6 Hz, 1H) 2.74
(dd, J=4.4, 16.8 Hz, 1H) 1.46 (s, 9H); MS (MALDI-
TOF): m/z calcd: [M+Na]+=527.16 [M+K]+=543.27.
Found: [M+Na]+=527.39 [M+K]+=543.39.
Preparation of ketones. A sample procedure is as fol-
lows: In a dry, Ar-purged flask was added anhydrous
THF (15 mL) to CuI (567 mg, 2.98 mmol) and the
resulting suspension cooled to −78°C. The Grignard
reagent of 1-bromo-3-phenylpropane (1 M in THF, 6
mL, 5.95 mmol) was added. Once the addition was
finished the reaction was heated to 0°C and stirring was
continued for 2 h. A solution of the previously pre-
pared thioester (500 mg, 0.99 mmol) in anhydrous THF
(15 mL) was then added and stirring was continued for
3 h. The reaction mixture was quenched with saturated
aqueous ammonium chloride (2×20 mL) and diluted

7502
J. Va´zquez, F. Albericio / Tetrahedron Letters 43 (2002) 7499–7502
9. Reactions with commercial phenyl and tolyl Grignard
11. Carpino, L. A.; Beyermann, M.; Wenschuh, H.; Bienert,
M. Acc. Chem. Res. 1996, 29, 268–274.
derivatives should be carried out for 16 h at 0°C for
complete conversion of the Weinreb amide into the
ketone.
12. These thioester derivatives are not stable in the presence
of TFA, as demonstrated when an attempt was made to
remove the tBu group with the aim of anchoring the
aspartic derivative through the side chain to a solid
support to carry out the Grignard reaction on solid
phase.
10. Research at Merck Frosst Canada & Co has been pub-
lished in
a patent [WO 00/32620 (Chem. Abstr.
133:17829)], which describes a longer, more complicated
method for the preparation of Fmoc-Asp(OtBu)-R:
13. Reaction of derivatives 2 with Grignard reagents, first, at
−78°C for 2 h and then at −30°C for 1.5–3 h, led to
intractable mixtures of crude products.
14. (a) Kates, S. A.; Sole´, N. A.; Johnson, C. R.; Hudson,
D.; Barany, G.; Albericio, F. Tetrahedron Lett. 1993, 34,
1549–1552; (b) Alsina, J.; Rabanal, F.; Chiva, C.; Giralt,
E.; Albericio, F. Tetrahedron 1998, 54, 10125–10152.