(9H-fluoren-9-yl)methanesuIfonyl (Fms): An amino protecting group complementary to Fmoc

Article abstract of DOI:10.1002/ejoc.201000682

A sulfonamide-based protecting group (PG), (9H-fluoren-9yl)methanesulfonyl (Fms), which can be used in a similar way to the well-established Fmoc PG, was developed. The advantages of this new PG were demonstrated in the successful formation of a phosphonamide between an N-Fmsprotected a-phosphonoalanine monoester and secondary alkylamines, including (R)-2-phenylethylamine, (S)-phenylalanine iert-butyl ester (H-Phe-OtBu), H-Pro-Gly-OtBu, and H-Phe-Phe-OtBu, without formation of oxazaphospholine, which is a serious problem associated with the Fmoc PG. The success should pave the way to the solid-phase synthesis of unnatural peptides substituted with a-amino phosphonic acid (AP) at essentially any arbitrary position without significant modification of the Fmoc-based chemistry that has been accumulated since Carpino's report in 1970. The N-Fms-AP monomer would attract much attention in the field of peptide mimetics.

Full text of DOI:10.1002/ejoc.201000682

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000682
(9H-Fluoren-9-yl)methanesulfonyl (Fms): An Amino Protecting Group
Complementary to Fmoc
Yoshitaka Ishibashi,[a] Kengo Miyata,[a] and Masato Kitamura*[a]
Keywords: Protecting groups / Sulfonamides / Phosphorus / Peptides
A sulfonamide-based protecting group (PG), (9H-fluoren-9-
yl)methanesulfonyl (Fms), which can be used in a similar
way to the well-established Fmoc PG, was developed. The
advantages of this new PG were demonstrated in the suc-
cessful formation of a phosphonamide between an N-Fms-
protected α-phosphonoalanine monoester and secondary
alkylamines, including (R)-2-phenylethylamine, (S)-phenyl-
alanine tert-butyl ester (H-Phe-OtBu), H-Pro-Gly-OtBu, and
H-Phe-Phe-OtBu, without formation of oxazaphospholine,
which is a serious problem associated with the Fmoc PG. The
success should pave the way to the solid-phase synthesis of
unnatural peptides substituted with α-amino phosphonic acid
(AP) at essentially any arbitrary position without significant
modification of the Fmoc-based chemistry that has been ac-
cumulated since Carpino’s report in 1970. The N-Fms-AP
monomer would attract much attention in the field of peptide
mimetics.
This pathway becomes more significant in the synthesis of
α-amino phosphonic acid containing peptides, probably be-
cause the high affinity of oxygen for phosphorus facilitates
this cyclization, causing difficulty in the attachment of α-
substituted α-aminophosphonic acid onto the N-terminus
of peptides.[4] This paper communicates the development of
a new non-problematic amino PG that is removable under
the same conditions as those used for Fmoc cleavage, which
will enable utilization of the Fmoc-based synthetic chemis-
try that has been accumulated since 1970.[1a]
Introduction
Carpino’s Fmoc [(9H-fluoren-9-yl)methyloxycarbonyl]
group is established as a useful protecting group (PG) for
amines.[1] The Fmoc PG is easily detached, typically by
treatment with 5–20% piperidine in DMF, although it has
high stability toward HOCOCF3, HCl, HBr, and tertiary
amines.[1b,2] Its secondary amine lability, in addition to its
high acid-resistant properties, enhances its utility, particu-
larly as a temporary PG in the synthesis of peptides in both
the solid and solution phases.[1c] Fmoc removal, followed
by condensation with an N-Fmoc α-amino acid monomer,
smoothly elongates the peptide chain, but as shown in
Scheme 1, the activated monomer ester is sometimes
cleaved by nucleophilic attack of the carbamate C=O oxy-
gen atom onto A = O to give an oxazolone derivative.[3]
Results and Discussion
Our target PG is the (9H-fluoren-9-yl)methanesulfonyl
(Fms) group, which protects amines RRЈNH (1) as sulfon-
amide 2 by reaction with FmsCl. The expected characteris-
tics of 2 include (1) lower nucleophilicity of the sulfonamide
S=O group owing to the smaller contribution of N(+)=S–
O(–), as compared to N(+)=C–O(–), thereby avoiding the
above-mentioned cyclization problem;[5] (2) weaker metal-
coordinating ability of the sulfonamide group as compared
with carbamates, thereby increasing the applicability of
Fms-protected compounds in metal-catalyzed reactions;
and (3) the potential possibility of piperidine-assisted β-eli-
mination of sulfurous acid, thereby generating deprotected
amine 1, together with SO2 and 9-methylene-9H-fluorene
or its piperidine adduct.[1a,1b] FmsCl was easily synthesized
on a Ͼ30-g scale from (9H-fluoren-9-yl)methanol (3)
through chlorination (SOCl2, 80 °C, 4 h; 80% yield), sulfon-
ylation of 4 (Na2SO3, acetone/H2O, 90 °C, 8 h; 98% yield),
and chlorination of 5 (PCl5/POCl3, 25 °C, 18 h; 88%
yield).[6] The structure was confirmed by X-ray crystallo-
Scheme 1.
[a] Contribution from the Research Center for Materials Science
and the Department of Chemistry, Nagoya University,
Chikusa, Nagoya 464-8602, Japan
Fax: +81-52-789-2261
E-mail: kitamura@os.rcms.nagoya-u.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000682.
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Y. Ishibashi, K. Miyata, M. Kitamura
SHORT COMMUNICATION
graphic analysis. We found that FmsCl can be stored in a Table 1. Deprotection of amines 1a–e from N-Fms-protected com-
pounds 2a–e.[a]
general vial at room temperature for at least six months
without significant decomposition.
Entry N-Fms- [Sub.] [Piperidine] Solvent
t
%
amine
/ m
/ m
/ min
Conv.[b]
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
200
200
200
200
200
200
200
200
20
1000
200
200
200
200
1000
1000
1000
1000
1000
1000
500
DMF
CH3OH
dioxane
THF
Ͻ1
60
30
60
30
Ͼ99 (96)
Ͼ99 (91)
98 (90)
Ͼ99 (91)
Ͼ99 (94)
Ͼ99 (88)
Ͼ99
Ͼ99
Ͼ99
Ͼ99
CH2Cl2
toluene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
30
Ͻ1
Ͻ1
5
Ͻ1
Ͻ1
Ͻ1
Ͻ1
Ͻ1
250
100
9
10
11
12
13
14
5000
1000
1000
1000
1000
Ͼ99 (91)
Ͼ99 (79)
Ͼ99 (87)
Ͼ99 (84)
Simple amines 1a–e, in addition to various α-amino es-
ters (vide infra), were quantitatively protected with an iso-
lated yield of 80–95% by using FmsCl {[1] = 200 m,
[FmsCl] = 300 m, [N,N-diisopropylethylamine (DIEA)] =
400–600 m, CH2Cl2, 0–30 °C, 3–8 h}.[6] CH2Cl2, CHCl3,
THF, toluene, and CH3CN were the solvents of choice for
use with sterically demanding tertiary amines such as DIEA
and pentamethylpiperidine. Using DMF solvent or other
bases, such as triethylamine, pyridine, N-methylimidazole,
imidazole, triazole, pyrazine, and K2CO3, caused decompo-
sition of FmsCl into 9-methylene-9H-fluorene, decreasing
the yield of 2. Schötten–Baumann conditions were not ap-
plicable to the protection of α-amino acids.
[a] Reactions were carried out at 25 °C on a 0.2–0.5-mmol scale.
[b] Determined by H NMR spectroscopic analysis of the reaction
mixture after addition of an equimolar amount of HOCOCF3 for
piperidine.[6] The value in parentheses is the isolated yield after ben-
zoylation [2.5 equiv. of C6H5COCl, 2.5 equiv. of N(C2H5)3, 25 °C,
8 h], followed by silica gel column chromatography.
1
Removal of the Fms group from Fms-Phe-OH, as well as
the α-amino phosphonic ester Fms-AlaP(OCH3)-OCH3,[8]
could be efficiently attained.
Table 2. Removal of the Fms protecting group from α-amino
esters.[a]
Table 1 summarizes the results of Fms removal from 2a–
e to give 1a–e. Simple primary alkylamine 1a was quantita-
tively deprotected from 2a within 1 min under the standard
conditions for removal of Fmoc {[2a] = 200 m, [piper-
idine] = 1000 m, DMF, 25 °C; Table 1, Entry 1}. CH3OH,
dioxane, THF, CH2Cl2, and toluene could also be used, al-
though the reactivity was lowered by one to two orders of
magnitude (Table 1, Entries 2–6). Even with a 1.25-mol
amount of piperidine, the deprotection proceeded smoothly
without the formation of N-fluorenylmethylated 1a
(Table 1, Entry 8).[7] The substrate concentration could be
varied from 20 to 1000 m (Table 1, Entries 9 and 10). Not
only primary alkyl primary amine 1a but also secondary
and tertiary alkyl primary amines 1b and 1c were depro-
tected from 2b and 2c under the standard conditions
(Table 1, Entries 11 and 12). In a similar way, the Fms
group in Fms-protected arylamine 2d and secondary amine
2e was smoothly removed (Table 1, Entries 13 and 14).
The general applicability to N-Fms-protected α-amino
esters is demonstrated in Table 2.[6] Under the standard
conditions described above, Fms-Gly-OtBu[8] was quantita-
tively converted into H-Gly-OtBu within 10 min. α-Substi-
tuted α-amino carboxylic esters, such as H-Ala-OtBu, H-
Leu-OtBu, H-Phe-OtBu, H-Val-OtBu, H-Phg-OtBu, and
H-Pro-OtBu, were also completely deprotected and isolated
in 78–87% yield after converting the corresponding N-ben-
zoyl derivative. Lack of racemization was confirmed by
using H-Phg-OtBu: the S/R enantiomer ratio of 99:1 did
[a] Reactions were carried out on a 0.06–0.28-mmol scale in DMF
at 25 °C for 10 min with N-Fms-protected α-amino esters (200 m)
and piperidine (1000 m) unless otherwise specified. In all cases,
the conversion was Ͼ99%. [b] Isolated yield after benzoylation
[2.5 equiv. of C6H5COCl, 2.5 equiv. of N(C2H5)3, 25 °C, 8 h] fol-
lowed by silica gel column chromatography. In all cases, the conver-
sion was Ͼ99%. [c] No racemization of H-Phg-OtBu was con-
firmed by HPLC analysis [CROWNPAK CR, 200-nm detection,
NaClO4–HClO4 buffer (pH 2.0)] after converting into H-Phg-OH
not change during the protection/deprotection process. (HOCOCF3, 25 °C, 1 h).[6]
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An Amino Protecting Group Complementary to Fmoc
The base stability of Fms-Phe-OtBu was compared with Phe-Phe-OtBu, could also be successfully condensed with
that of Fmoc-Phe-OtBu under the conditions of [Fms- or Fms-AlaP(OCH3)-OH to give, after Fms removal, H-Ala-
Fmoc-protected amine] = 25 m, [amine] = 25 m, CDCl3, P(OCH3)-Pro-Gly-OtBu (12c) and H-AlaP(OCH3)-Phe-
3 h, and 25 °C, giving H-Phe-OtBu: piperidine [20 (Fms) vs. Phe-OtBu (13c) in 91 and 89% isolated yield, respectively.
2.2% (Fmoc)], piperazine (22 vs. 2.3%), morpholine (0.8 vs.
Ͻ0.1%), dicyclohexylamine (3.8 vs. Ͻ0.1%), diethylamine
(6.2 vs. Ͻ0.1%), dimethylaminopyridine (9.9 vs. 0.9%),
DIEA (Ͻ0.1 vs. Ͻ0.1%), pentamethylpiperidine (Ͻ0.1 vs.
Ͻ0.1%), triethylamine (19 vs. 0.8%), and proton sponge
(Ͻ0.1 vs. Ͻ0.1%).[6] We found that the Fms PG is more
labile than the Fmoc PG, which increases the efficiency of
Fms removal from the Fms-protected amines. This ten-
dency is, however, disadvantageous for chemoselective pro-
tection and deprotection; thus, selection of an appropriate
amine is recommended in the case of chemical transforma-
tions requiring bases. Both the Fms and Fmoc PGs were
stable under typical acidic conditions (HOCOCF3, neat,
25 °C, 24 h; 6  HCl, CH3OH, 25 °C, 24 h).[1a,2,6] Fmoc-
Gly-OH is known to be labile under hydrogenolysis condi-
tions (1 atm H2, 10 mol-% Pd/C, 1:4 HOCOCH3/CH3OH,
25 °C, 24 h).[9] Indeed, under these conditions, 35% of
Fmoc-Phe-OH was converted into H-Phe-OH, together
with (9H-fluoren-9-yl)methane. These conditions exerted
no effect on Fms-Gly-OH or Fms-Phe-OH:[6] even at
100 atm of H2, Fms-Phe-OH remained intact.
The Fms PG was confirmed to have the same efficiency
as the Fmoc PG in carboxylic acid based peptide synthesis
by liquid-phase synthesis of the simple tripeptide H-Pro-
Pro-Gly-OH, a collagen protein unit tripeptide.[6,10] H-Gly-
OtBu was combined with Fms-Pro-OH by the benzotriazol-
Conclusions
We have developed Fms, a new sulfonamide-based
PG[2,13] for the protection of various amines including pri-
mary, secondary, and tertiary alkyl primary and secondary
amines, α-amino carboxylic esters, and α-amino phos-
phonic esters. Smooth and quantitative removal of the Fms
PG can be attained under essentially identical conditions to
those used for Fmoc removal. The substitutability between
Fmoc and Fms has been demonstrated in the synthesis of
a tripeptide. The advantages of Fms over Fmoc have been
clearly shown by the successful dehydrative condensation
of an N-Fms-protected α-amino phosphoalanine monoester
with an α-substituted α-amino carboxylic ester and amide,
giving a phosphonamide-based peptide. Phosphonopeptide
chemistry, which attracts much attention from researchers
interested in mimetics of peptides and nucleotides, has de-
veloped mainly through the use of phosphonic acid ter-
minal peptides and residue-phosphorylated peptides.[11,14]
The successful synthesis of an α-amino phosphonic acid N-
terminal peptide demonstrated herein should provide a po-
tential tool for the diverse synthesis of α-amino phosphonic
acid containing peptides, which is our ongoing project.[15]
1-yl-oxytripyrrolidinophosphonium hexafluorophosphate
(PyBOP) method (CH2Cl2, 25 °C, 8 h), giving Fms-Pro-
Gly-OtBu (6). The Fms PG of the dipeptide was chemo-
selectively removed by treatment with piperidine (5 mol) in
DMF. The PyBOP condensation of H-Pro-Gly-OtBu with
Fms-Pro-OH, followed by N-Fms removal, afforded 7 in
85% total yield from 6. Treatment of 7 with HOCOCF3 at
25 °C for 1 h quantitatively yielded H-Pro-Pro-Gly-OH.
The utility of the Fms PG in the potential synthesis of
α-amino phosphonic acid containing peptides was con-
firmed by the successful condensation of Fms-Ala-
P(OCH3)-OH (8a)[8] with secondary alkylamines by using a
newly developed phosphonamidation system consisting of
1-hydroxy-7-azabenzotriazole (HOAt), DIEA, and N,NЈ-di-
isopropylcarbodiimide (DIC).[6] Reaction of 8a with (R)-
phenylethylamine ([8a] = [amine] = 100 m, CH3CN,
[HOAt] = [DIEA] = [DIC] = 200 m, 25 °C, 3 h)[11] af-
forded desired product 9a [31P NMR (CDCl3): δ = 29 ppm]
in 82% isolated yield. Under the same conditions, by con-
trast, Fmoc-AlaP(OCH3)-OH (8b) gave 9b in less than 20%
yield, and the main product in the reaction mixture was
oxazaphospholine derivative 11 [31P NMR (CD3CN): δ =
17 ppm].[12] Using H-Phe-OtBu in a condensation reaction
with Fms-AlaP(OCH3)-OH gave the dipeptide Fms-Ala-
P(OCH3)-Phe-OtBu (10a) in 90% isolated yield, the Fms
group of which was then quantitatively removed to give 10c
([10a] = 40 m, [piperidine] = 200 m, CDCl3, 25 °C, 1 h).
Experimental Section
Fms Protection: A 20-mL Young’s type Schlenk flask containing a
Teflon-coated magnetic stirring bar was charged with 2-phenyl-
ethylamine (1a; 126 µL, 1.00 mmol) and CH2Cl2 (5.0 mL). The
The N-terminus of the dipeptides, H-Pro-Gly-OtBu and H- whole mixture was cooled to 0 °C in an ice bath, and then DIEA
Eur. J. Org. Chem. 2010, 4201–4204
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Y. Ishibashi, K. Miyata, M. Kitamura
SHORT COMMUNICATION
[5]
For the theoretical calculation of the double bond character of
N–C(O) and N–S(O) in N-methoxycarbonyl- and N-methylsul-
fonyl-protected pyrrole derivatives, see: I. Chataigner, C. Panel,
H. Gérard, S. R. Piettre, Chem. Commun. 2007, 3288–3290.
The sulfonamide N atom has higher sp3 hybridization, re-
sulting in higher aromatic character and in a smaller N–S rota-
tion barrier.
(330 µL, 2.00 mmol) and FmsCl (418 mg, 1.50 mmol) were intro-
duced. The ice bath was removed, and the flask was then immersed
into a 30 °C water bath. After stirring the colorless solution for
3 h, water (2.0 mL) was added. The aqueous layer was extracted
with CH2Cl2 (2ϫ2.0 mL). The combined organic layer was washed
with brine (2.0 mL) and dried with Na2SO4. Filtration followed by
evaporation under a reduced pressure gave a yellow oil (580 mg),
which was then subjected to silica gel column chromatography
(25 g, CH2Cl2) to give 2a (345 mg, 95% yield) as a white solid.
[6]
[7]
For details see the Supporting Information.
N-Fluorenylmethylation sometimes occurs in the removal of
Fmoc PG during peptide synthesis. See ref.[1]
Fms Removal: A 10-mL Young’s type Schlenk flask containing a
Teflon-coated magnetic stirring bar was charged with 1-(9H-
[8]
α-Amino carboxylic acid abbreviation follows the IUPAC rule.
The N-PG and ester PG are denoted as a prefix and a suffix,
respectively. α-Amino phosphonic acid is denoted by the ad-
dition of “P” at the end of the three-letter abbreviation, e.g.,
H-AlaP(OH)-OH for 1-aminoethylphosphonic acid.
a) E. Atherton, C. Bury, R. C. Sheppard, B. J. Williams, Tetra-
hedron Lett. 1979, 20, 3041–3042; b) J. Martinez, J. C. Tolle,
M. Bodanszky, J. Org. Chem. 1979, 44, 3596–3598; c) for Fmoc
removal using Pd/C, see: H. Sajiki, T. Maegawa, K. Hirota,
Jpn. Kokai Tokkyo Koho Jpn. Pat. 8887, 2007.
fluoren-9-yl)-N-phenethylmethanesulfonamide
(2a;
182 mg,
0.500 mmol), mesitylene (60.1 mg, 0.500 mmol, internal standard),
and DMF (2.50 mL). To this was added piperidine (247 µL,
2.50 mmol), and the clear solution was stirred at 25 °C for 1 min.
Because of difficulty in efficient separation of 1a from piperidine
and the piperidine adduct, the isolated yield of 1a was indirectly
determined to be 96% after benzoylation followed by silica gel col-
umn chromatography.
[9]
[10]
J. Brinckmann, H. Notbohm, P. K. Müller (Eds.), Collagen
Primer in Structure, Processing and Assembly Series: Topics in
Current Chemistry, Springer, New York, 2005.
Supporting Information (see footnote on the first page of this arti-
cle): Details of the preparation of FmsCl, procedures for protec-
tion/deprotection, stability examination, synthesis of a tripeptide,
and condensation between N-Fms-protected α-phosphonoalanine
monoester and secondary alkyl primary amines.
[11]
[12]
Y. Ishibashi, M. Kitamura, Chem. Commun. 2009, 6985–6987.
This value is highly consistent with that reported for related
compounds. See: R. Hirschmann, K. M. Yager, C. M. Taylor,
W. Moore, P. A. Sprengeler, J. Witherington, B. W. Phillips,
A. B. Smith, J. Am. Chem. Soc. 1995, 117, 6370–6371. Com-
pound 11 could not be isolated by silica gel column chromatog-
raphy of the crude mixture. This may be due to the moisture
instability of 11. See, O. V. Korenchenko, A. Y. Aksinenko,
V. B. Sokolov, I. V. Martynov, Heteroat. Chem. 1992, 3, 147–
150.
Acknowledgments
This work was aided by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Science, Sports and Culture, Japan
(No. 25E07B212).
[13]
[14]
a) Nosyl: T. Fukuyama, C.-K. Jow, M. Cheung, Tetrahedron
Lett. 1995, 36, 6373–6374; b) dinitrobenzensulfonyl: T. Fukuy-
ama, M. Cheung, C.-K. Jow, Y. Hidai, T. Kan, Tetrahedron
Lett. 1997, 38, 5831–5834; c) SES: S. M. Weinreb, D. M.
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a) V. P. Kukhar, H. R. Hudson (Eds.), Aminophosphonic and
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[15]
Other carbamate-type PGs such as Nsc having a high inter-
changeablity with Fmoc could be extended to the correspond-
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chemistry.
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Received: May 12, 2010
Published Online: June 23, 2010
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Eur. J. Org. Chem. 2010, 4201–4204