One-pot conversion of benzyl carbamates into fluorenylmethyl carbamates

Article abstract of DOI:10.1016/S0040-4039(00)01811-6

A simple one-pot procedure smoothly converted N-benzyloxycarbonyl groups into N-fluorenylmethoxycarbonyl groups via hydrogenation with a poisoned catalyst in the presence of Fmoc-OSu. Functional groups such as t-butyl esters, t-butyl ethers, and N-Boc were stable under the reaction conditions. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01811-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9953–9956
One-pot conversion of benzyl carbamates into
fluorenylmethyl carbamates
Valerie Dzubeck† and Joel P. Schneider*
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716-2522, USA
Received 25 August 2000; accepted 18 September 2000
Abstract
A simple one-pot procedure smoothly converted N-benzyloxycarbonyl groups into N-fluorenyl-
methoxycarbonyl groups via hydrogenation with a poisoned catalyst in the presence of Fmoc-OSu.
Functional groups such as t-butyl esters, t-butyl ethers, and N-Boc were stable under the reaction
conditions. © 2000 Elsevier Science Ltd. All rights reserved.
In the course of de novo protein design it is often desirable to incorporate unnatural amino
acid residues into a protein to impart unique structural or functional features. While some
unnatural residues are commercially available, many must be prepared by the investigators
wishing to exploit their use. We have taken advantage of chemistry developed by Vederas in
which an Na-benzyloxycarbonyl (Cbz)-protected b-lactone derived from N-Cbz-Serine can be
opened at the b-carbon with a variety of nucleophiles to afford N-Cbz-protected amino acid
residues bearing unnatural side chains.1 In order to utilize the resulting unnatural residues in
Na-fluorenylmethoxycarbonyl (Fmoc)-based solid phase peptide synthesis (SPPS), the Cbz-pro-
tecting group must be replaced with the base labile Fmoc-protecting group.2
We present here, a simple one-pot procedure which smoothly converts N-Cbz groups into
N-Fmoc groups in very good yield via hydrogenation with a poisoned catalyst in the presence
of N-(9-fluorenylmethoxycarbonyloxy)succinimide (Fmoc-OSu).
* Corresponding author. Fax: (302)831-6335; e-mail: schneijp@udel.edu
† Undergraduate research participant.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01811-6

9954
In the past, the Cbz to Fmoc transformation had been effected in a multi-step procedure
involving catalytic hydrogenation of the Cbz-functionality, isolation of the liberated amine and
subsequent reaction of the amine with an Fmoc-donor reagent such as Fmoc-OSu, 9-fluorenyl-
methyl chloroformate, or 9-fluorenylmethyl pentafluorophenyl carbonate.
A one-pot procedure to effect this transformation should find utility not only for those
wishing to synthesize amino acid residues amenable to Fmoc-based SPPS but could potentially
find use in the combinatorial synthesis of Fmoc-protected small molecules. The Cbz protecting
group is advantageous in many respects to the Fmoc group for the combinatorial synthesis of
amines due to its marked stability under diverse reaction conditions. However, if libraries of
molecules are to be later incorporated into peptides or proteins, replacement of the Cbz group
with Fmoc is necessary and a one-pot transformation would facilitate this process by eliminating
additional steps.
To our knowledge, there have been no previous reports of a one-pot procedure for the
conversion of N-Cbz into N-Fmoc. This may be due to the fact that the Fmoc-protecting group
is only marginally stable under the conditions of catalytic hydrogenation. Williams has shown
that Fmoc-Gly is cleaved by catalytic hydrogenation under various conditions (t1/2=3–33 h).3
Therefore, a simple one-pot procedure involving removal of the Cbz group via Pd–C catalyzed
hydrogenation in the presence of an Fmoc donor reagent such as Fmoc-OSu is rendered
ineffective because cleavage of the desired Fmoc product, as well as the Fmoc-OSu reagent
competes with product formation.4
However, Hirota reported that olefin, benzyl ester, N-Cbz, and nitro functionalities could be
chemoselectively hydrogenated (5% Pd–C, H2) in the presence of a phenolic benzyl-protecting
group by poisoning the catalyst with sub-stoichiometric amounts of 2,2%-dipyridyl.5 We won-
dered whether a 2,2%-dipyridyl poisoned Pd–C catalyst could selectively reduce an Na-Cbz group
while leaving an Fmoc group intact. If this selectivity could be realized, then a simple one-pot
transformation of N-Cbz into N-Fmoc via catalytic hydrogenation in the presence of an
Fmoc-donor reagent could be possible.
Table 1
One-pot conversion of benzyl carbamates into fluorenylmethyl carbamates
Entry
Substrate
Product
Time (h)
Yielda (%)
% eeb
1
2
3
4
5
6
7
8
Na-Cbz-Ala-OMe
Na-Fmoc-Ala-OMe
3.5
15.0
4.5
15.0
3.5
7.0
4.0
4.0
90
88
89
89
79
88
84
83
]99
]99
–
]99
]99
–
Na-Cbz-Phe-OMe
Na-Fmoc-Phe-OMe
Na-Cbz-Asp(tBut)-OMe
Na-Cbz-Leu-OMe
Na-Fmoc-Asp(tBut)-OMe
Na-Fmoc-Leu-OMe
Na-Cbz-Pro-OMe
Na-Fmoc-Pro-OMe
Na-Cbz-Lys(Boc)-OMe
Na-Cbz-Gly-Gly-OMe
Na-Cbz-Thr(tBut)-Ala-OMe
Na-Fmoc-Lys(Boc)-OMe
Na-Fmoc-Gly-Gly-OMe
Na-Fmoc-Thr(tBut)-Ala-OMe
–
]99c
a Each one-pot conversion was performed in triplicate according to the procedure described in the text. Yields
represent the average isolated yield.
b % ee values were experimentally determined by chiral HPLC using either Chiralcel OD or Chiralpak AS columns
(0.46×25 cm). Products from the one-pot conversions were eluted by linear gradients of hexanes and isopropanol at
20°C and a flow-rate of 1 mL/min, monitoring at 280 nm. Fmoc-D-products were synthesized and used as standards
to demonstrate that resolution of possible enantiomers was achieved with the HPLC conditions employed.
c % ee value determined via RP-HPLC using a Vydac C4 peptide/protein column with water and 90% acetonitrile
in water each containing 0.1% TFA as elution solvents.

9955
Table 1 shows that the desired selectivity can be achieved. Treatment of Na-Cbz-protected
methyl esters of various amino acids and dipeptides with 10% Pd–C, H2, and 2,2%-dipyridyl in
the presence of Fmoc-OSu using methanol as solvent, efficiently converted the N-Cbz group into
N-Fmoc.
Substrates of varying hydrophobicity and size were transformed in very good yields with
convenient reaction times and tert-butyl ester, tert-butyl ether, and N-Boc functionalities were
perfectly stable under the reaction conditions. In addition, a simple workup affords a product
which is greater than 98% pure by NMR without performing column chromatography. A
convenient feature of this reaction is that Fmoc-OSu is sparingly soluble in methanol but goes
into solution upon reacting with the liberated amine, allowing one to visually detect when the
transformation is nearing completion.
All of the N-Fmoc protected products were characterized by 1H/13C NMR and high
resolution mass spectroscopy.6 Chiral HPLC was employed to assess racemization for amino
acids whose
D
-isomers were commercially available (Table 1). A representative chromatograph
(Fig. 1) is shown for the one-pot transformation of N-Cbz-Ala-OMe into N-Fmoc-Ala-OMe
demonstrating that greater than 99% ee had been achieved.
Figure 1. (a) Chiral HPLC analysis of Fmoc-
is no observable racemization of the Ca-carbon of the amino acid residue. (b) Conjection of isolated Fmoc-
OMe and Fmoc- -Ala-OMe. HPLC conditions are described in Table 1
L
-Ala-OMe isolated from a typical one-pot conversion reaction. There
L
-Ala-
D
The one-pot procedure presented here nicely complements the suite of previously established
one-pot conversions known to transform other N-protection groups commonly used in peptide
synthesis. For example, the conversions of Fmoc into Cbz7 or N-tert-butyloxycarbonyl (Boc)8
and Cbz into Boc9,10 are known. We are currently extending the scope of this reaction to include
the conversion of Cbz into 2,7-di-tert-butyl-Fmoc, a promising alternative to the Fmoc-protec-
tion group which has been reported to show enhanced solubility characteristics.11
Representative procedure: A 50 mL three necked round-bottomed flask equipped with a
rubber septum and two stopcock adapters (one used for balloon attachment and the other used
as a gas outlet) was charged with 2,2%-dipyridyl (24.8 mg, 0.16 mmol) and 3.8 mL of methanol.
The resulting solution was stirred and 10% Pd–C (9.2 mg, 2.7 mol%), a solution of Na-Cbz-
phenylalanine methyl ester (100 mg, 0.32 mmol) in 3.8 mL of methanol and Fmoc-OSu (107.6
mg, 0.32 mmol) were added. The flask was flushed with H2 and subsequently stirred under a
balloon of H2. TLC (ethyl acetate/hexanes, 1:1) was used to monitor the disappearance of

9956
Fmoc-OSu (15 h), after which time the reaction mixture was filtered and evaporated to dryness.
The resulting oily solid was dissolved in 20 mL of ethyl acetate and washed with 10% aqueous
HCl (3×20 mL), water (1×20 mL), dried (Na2SO4) and evaporated to afford 112.9 mg (0.28
mmol, 88%) of Na-Fmoc-phenylalanine methyl ester as a white solid. For residues containing
acid sensitive side chain protecting groups (entries 3, 6 and 8), 0.1 M aqueous citric acid is used
in the workup rather than 10% aqueous HCl.
Acknowledgements
We thank the Undergraduate Science and Engineering Scholars Program for support, as well
as Mr. John Dykins and Mr. Steve Bai for mass spectroscopy and NMR assistance, respectively.
References
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2. Carpino, L. A.; Han, G. Y. J. Org. Chem. 1972, 37, 3404–3409.
3. Atherton, E.; Bury, C.; Sheppard, R. C.; Williams, B. J. Tetrahedron Lett. 1979, 3041–3042.
4. In our hands, when Cbz-Ala-OMe (1.0 equiv.) is hydrogenated via 10% Pd–C (2.7 mol%), H2 (balloon) in the
presence of Fmoc-OSu (1.0 equiv.) using methanol as solvent, the desired Fmoc-protected product is formed in
very low yield. The main products of this reaction are H2N-Ala-OMe and 9-methylfluorene, which is probably
formed upon Fmoc-OSu decomposition to dibenzofulvene and subsequent saturation. Catalytic transfer hydro-
genation was also attempted under similar conditions with various hydrogen donors (1,4-cyclohexadiene, formic
acid, or ammonium formate) with little success due to either Fmoc-OSu instability and/or extremely slow
Cbz-deprotection rates.
5. Sajiki, H.; Kuno, H.; Hirota, K. Tetrahedron Lett. 1998, 39, 7127–7130.
6. Representative NMR and mass spectroscopic data is given for entries 2 and 8.
a
1
N -Fmoc-Phe-OMe: H NMR (CDCl3, 400 MHz) l 7.76 (d, J=7.5 Hz, 2H), 7.56 (t, J=6.7 Hz, 2H), 7.40 (td,
J=7.5, 0.6 Hz, 2H), 7.33–7.27 (m, 5H), 7.08 (d, J=6.5 Hz, 2H), 5.26 (d, J=8.1 Hz, 1H), 4.67 (m, 1H), 4.44 (dd,
J=10.6, 7.1 Hz, 1H), 4.34 (dd, J=10.6, 6.9 Hz, 1H), 4.20 (t, J=7.0 Hz, 1H), 3.73 (s, 3H), 3.11 (m, 2H); 13C
NMR (CDCl3, 400 MHz) l 172.1, 155.7, 144.0, 141.5, 135.9, 129.5, 128.8, 127.9, 127.4, 127.3, 125.3, 120.2, 67.1,
54.9, 52.6, 47.3, 38.4; HRMS (CI) m/z 402.1705 [(M+H)+, calcd for C25H24NO4, 402.1705].
a
1
N -Fmoc-Thr(tBut)-Ala-OMe: H NMR (CDCl3, 400 MHz) l 7.78 (br s, 1H), 7.74 (d, J=7.8 Hz, 2H), 7.58 (d,
J=7.4 Hz, 2H), 7.38 (m, 2H), 7.29 (m, 2H), 5.97 (d, J=5.0 Hz, 1H), 4.50 (t, J=7.1 Hz, 1H), 4.36 (d, J=7.3 Hz,
2H), 4.21–4.15 (m, 3H), 3.73 (s, 3H), 1.42 (d, J=7.2 Hz, 3H), 1.29 (s, 9H), 1.08 (d, J=6.4 Hz, 3H); 13C NMR
(CDCl3, 400 MHz) l 173.2, 169.4, 156.2, 144.0, 141.5, 127.9, 127.3, 125.4, 120.2, 75.8, 67.2, 66.9, 58.5, 52.6, 48.5,
47.4, 28.3, 18.4, 16.7; HRMS (CI) m/z 483.2476 [(M+H)+, calcd for C27H35N2O6, 483.2495].
7. Li, W. R.; Jiang, J. J.; Joullie, M. M. Synlett 1993, 362–362.
8. Li, W. R.; Jiang, J. J.; Joullie, M. M. Tetrahedron Lett. 1993, 34, 1413–1414.
9. Sakaitani, M.; Hori, K.; Ohfune, Y. Tetrahedron Lett. 1988, 29, 2983–2984.
10. Bajwa, J. S. Tetrahedron Lett. 1992, 33, 2955–2956.
11. Stigers, K. D.; Koutroulis, M. R.; Chung, D. M.; Nowick, J. S. J. Org. Chem. 2000, 65, 3858–3860.
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