New TFA-free cleavage and final deprotection in Fmoc solid-phase peptide synthesis: Dilute HCl in fluoro alcohol

Article abstract of DOI:10.1021/ol303124r

A novel method for cleaving from resin and removing acid-labile protecting groups for the Fmoc solid-phase peptide synthesis is described. 0.1 N HCl in hexafluoroisopropanol or trifluoroethanol cleanly and rapidly removes the tert-butyl ester and ether, Boc, trityl, and Pbf groups and cleaves the common resin linkers: Wang, HMPA, Rink amide, and PAL. Addition of just 5-10% of a hydrogen-bonding solvent considerably retards or even fully inhibits the reaction. However, a non-hydrogen-bonding solvent is tolerated.

Full text of DOI:10.1021/ol303124r

ORGANIC
LETTERS
2
012
Vol. 14, No. 24
346–6349
New TFA-Free Cleavage and Final
Deprotection in Fmoc Solid-Phase Peptide
Synthesis: Dilute HCl in Fluoro Alcohol
6
Pasquale Palladino and Dmitry A. Stetsenko*
Department of Chemistry, University of Reading, P.O. Box 224, Whiteknights,
Reading RG6 6AD, U.K.
d.stetsenko@imperial.ac.uk
Received November 13, 2012
ABSTRACT
A novel method for cleaving from resin and removing acid-labile protecting groups for the Fmoc solid-phase peptide synthesis is described. 0.1 N
HCl in hexafluoroisopropanol or trifluoroethanol cleanly and rapidly removes the tert-butyl ester and ether, Boc, trityl, and Pbf groups and cleaves
the common resin linkers: Wang, HMPA, Rink amide, and PAL. Addition of just 5À10% of a hydrogen-bonding solvent considerably retards or
even fully inhibits the reaction. However, a non-hydrogen-bonding solvent is tolerated.
6
requires liquid HF, 1 M trifluoromethanesulfonic acid
Peptides act as key molecules in diverse biological
processes. Their derivatives are used extensively for drug
design and drug discovery and as probes for molecular
imaging and disease diagnosis. After the advent of solid-
7
(TFMSA) in TFA, or neat methanesulfonic acid (MSA)
usually in the presence of additives such as anisole,
8
5
6
9
thioanisole, or dimethyl sulfide, for removing protect-
1
10
ing groups of benzyl or related types and cleaving from
solid supports such as 4-methylbenzhydrylamino resin
phase peptide synthesis (SPPS), peptides have attracted
considerable synthetic attention for both industrial man-
ufacturing and small-scale laboratory production. The
Fmoc SPPS has been established since the end of the
1
1
(MBHA) for peptide amides or phenylacetamido resin
1
2
(Pam) for peptide acids. In the Fmoc synthesis, trifluor-
oacetic acid (TFA) is usually employed as a deprotecting
agent at high concentration (90À95%). The remaining
5À10% are usually water and various scavengers such as
2
1
970s as a convenient way of making peptides. As the
3
Fmoc group is labile to bases, acid-labile protecting
groups for amino acid side chains in the Fmoc SPPS
could be removed under less stringent acidic conditions
5
than in the alternative Boc method. The latter commonly
4
13
14
1,2-ethanedithiol and triisopropylsilane. Scavengers
are used to minimize side reactions occurring with sensi-
tive residues, e.g. Trp in the presence of reactive cationic
intermediates generated by TFA from protecting groups.
Most acid-labile protecting groups used in the Fmoc
method to mask the side chains of amino acids belong to
(
(
1) Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149.
2) (a) Chang, C.-D.; Meienhofer, J. Int. J. Peptide Protein Res 1978,
11, 246. (b) Atherton, E.; Fox, H.; Harkiss, D.; Sheppard, R. C. J. Chem.
Soc. Chem. Commun. 1978, 537.
(
(
3) Carpino, L. A.; Han, G. J. J. Am. Chem. Soc. 1970, 92, 5748.
4) (a) Atherton, E.; Sheppard, R. C. In Solid Phase Peptide Synth-
esis: A Practical Approach, Atherton, E.; Sheppard, R. C., Eds.; IRL Press:
Oxford, 1989, Chapter 11, 149 and the references therein; (b) Chan, W. C.;
White, P. D. In Fmoc Solid Phase Peptide Synthesis:A Practical Approach,
Chan, W. C.; White, P. D., Eds.; IRL Press: Oxford, 2000, Chapter 3, 64 and
(8) Yajima, H.; Kiso, Y.; Ogawa, H.; Fujii, N.; Irie, H. Chem. Pharm.
Bull. 1975, 23, 1164.
(9) Tam, J. P.; Heath, W. F.; Merrifield, R. B. Tetrahedron Lett. 1982,
23, 4435.
ꢀ
the references therein.
(10) Isidro-Llobet, A.; Alvarez, M.; Albericio, F. Chem. Rev. 2009,
nd
(
5) Stewart, J. M.; Young, J. D. Solid Phase Peptide Synthesis, 2
Ed.; Pierce Chemical Co, 1984 and the references therein.
6) (a) Sakakibara, S.; Shimonishi, V. Bull. Chem. Soc. Jpn. 1965, 38,
412. (b) Lenard, J.; Robinson, A. B. J. Am. Chem. Soc. 1967, 89, 181.
7) (a) Yajima, H.; Fujii, N.; Ogawa, H.; Kawatani, H. J. Chem. Soc.
Chem. Commun. 1974, 107. (b) Yajima, H.; Fujii, N. J. Am. Chem. Soc.
981, 103, 5867. (c) Fujii, N.; Shimokura, M.; Nomizu, M.; Yajima, H.;
Shono, F.; Tsuda, M.; Yoshitake, A. Chem. Pharm. Bull. 1984, 32, 520.
109, 2455.
(11) Matsueda, G. R.; Stewart, J. M. Peptides 1981, 2, 45.
(12) Mitchell, A. R.; Erickson, B. W.; Ryabtsev, M. N.; Hodges,
R. S.; Merrifield, R. B. J. Am. Chem. Soc. 1976, 98, 7357.
(13) King, D. S.; Fields, C. G.; Fields, G. B. Int. J. Peptide Protein
Res. 1989, 36, 255.
(
1
(
1
(14) Pearson, D. A.; Blanchette, M.; Baker, M. L.; Guindon, C. A.
Tetrahedron Lett. 1989, 30, 2739.
1
0.1021/ol303124r r 2012 American Chemical Society
Published on Web 12/04/2012

2
hydrogen bonds and stabilize acid anions with HFIP
4
the tert-butyl family: tert-butyl ester for carboxylic acid
groupsof Asp and Glu, tert-butylether for hydroxygroups
of Tyr, Ser, and Thr, and Boc for the amino group of Lys
2
6
notably surpassing TFA in that ability. Noteworthy,
addition of TFE to superacids whether protic, Lewis, or
27
polymeric such as Nafion increases their “superacidity”.
1
5
and indole nitrogen of Trp. Also employed are the
triphenylmethyl (trityl, Trt) protecting group for the
amides of Asn and Gln, imidazole ring of His, and thiol
Fluoro alcohols are good solubilizing agents for peptides,
2
especially those forming secondary structures. Addition
8
1
5
group of Cys. Third, arenesulfonyl protecting groups
such as 2,2,4,6,7-pentamethyldihydrobenzofuran-5-sulfonyl
of 10À20% TFE to CH Cl improves yields of solid-phase
2
2
2
9
carbodiimide coupling. No premature Boc deprotec-
tion has been noted. However, preformed HFIP esters of
1
6
17
(
Pbf) or 2,2,5,7,8-pentamethylchroman-6-sulfonyl (Pmc)
R
are used for the guanidino group of Arg. Finally,
one should add acid-labile anchor groups that link the
C-terminal amino acid to the resin. For the peptide acids,
N -Boc- or Z-protected amino acids were found to be about
3
10 times less active acylating agents than the p-nitrophenyl
3
0
esters. Ester formation was thus a major drawback of the
use of fluoro alcohols in peptide coupling, which may be
31
alleviated by the addition of an appropriate cosolvent.
1
8
common linkers are p-benzyloxybenzyl ester (Wang resin),
19
p-hydroxymethylphenoxyacetyl ester (HMPA), and trityl
21
ester. For the peptide amides there are Rink amide and
PAL anchors. All the above linkers could be cleaved by
TFA at concentrations from as low as 1% in CH Cl for the
20
More success has been achieved by employing fluoro
alcohols for removing acid-labile protecting groups and
cleaving from resin. Some very acid-sensitive N-protecting
groups such as dicyclopropylmethoxycarbonyl are cleaved
22
2
2
least stable trityl ester to up to 95% TFA for most of the
others.
3
2
by HFIP. It has been also observed that the N-Trt group
could be removed selectively in the presence of other acid-
labile groups such as p-biphenylylisopropoxycarbonyl
(Bpoc) in 90% aq TFE by titration with conc aq HCl at
TFA is considered a milder deprotecting agent than
liquid HF, TFMSA, or MSA. However, TFA is an aggres-
sive, extremely corrosive chemical capable of inflicting
bodily harm through inhalation as well as skin contact
leaving hard to heal chemical burns. It readily attacks or
infiltrates many common materials and is relatively ex-
pensive, both for the initial purchase and for ultimate
disposal, especially when large-scale peptide synthesis re-
quires copious quantities of the chemical. Peptides for
medicinal use have to be freed from traces of TFA, which
is often used as a component of HPLC buffers as well. In
the latter case, the trifluoroacetate counterion has to be
exchanged with a biologically benign counterpart such as
3
3
ambient temperature. The Bpoc group could be cleaved
3
4
off by heating to 60 °C in 90% aq TFE. A mixture of
TFEÀCH Cl ÀAcOH (1:8:1 v/v) has been used for de-
2
2
taching fully protected peptides from very acid-labile
2
Barlos o-chlorotrityl resin. Similarly, a 4:1 (v/v) mixture
0
of CH Cl and more acidic HFIP was found to cleave the
2
2
o-chlorotrityl resin linkage within 15 minÀ1 h without any
3
5
damage to protecting groups of the tert-butyl type. The
only group affected to any significant extent was Trt on
His. However, peptides dissolved in HFIP were observed
2
3
36
chloride. Therefore, the problem of replacing TFA in
peptide synthesis with an equally or more effective equiva-
lent which would be less hazardous, less corrosive, and less
environmentally dangerous is worth investigating.
to lose tert-butyl ester over 24 h at ambient temperature.
Recently, both HFIP and TFE at elevated temperature or
(
27) (a) Taylor, S. K.; Dickinson, M. G.; May, S. A.; Pickering, D. A.;
Fluoro alcohols such as 2,2,2-trifluoroethanol (TFE) or
,1,1,3,3,3-hexafluoroisopropanol (HFIP) have been
Sadek, P. C. ChemInform 1998, 29, doi: 10.1002/chin.199848091; (b)
Prakash, G. K. S.; Mathew, T.; Marinez, E. R.; Esteves, P. M.; Rasul,
G.; Olah, G. A. J. Org. Chem. 2006, 71, 3952. (c) Zhang, S.; Xin, J.;
Zhang, Z.; Zhao, G.; Yan, D. Patent application CN 2010/1503827,
priority from 12.10.2010.
(28) (a) Yanagi, K.; Ashizaki, M.; Yagi, H.; Sakurai, K.; Lee, Y.-H.;
Goto, Y. J. Biol. Chem. 2011, 286, 23959. (b) Shen, F.; Tang, S.; Liu, L.
Science China: Chemistry 2011, 54, 110. (c) Miramon, H.; Cavelier, F.;
Martinez, J.; Cottet, H. Anal. Chem. 2010, 82, 394. (d) Chaudhary, N.;
Singh, S.; Nagaraj, R. J. Pept. Sci. 2009, 15, 675. (e) Chatterjee, C.;
Gerig, J. T. Biopolymers 2007, 87, 115. (f) Nilsson, M. R.; Nguyen, L. L.;
Raleigh, D. P. Anal. Biochem. 2001, 288, 76. (g) Buck, M. Q. Rev.
Biophys. 1998, 31, 297.
1
widely used as solvents and reagents in organic synthesis
in general and in peptide chemistry in particular. Typical
2
fluoroalcohols are acidic: the pK of TFE is12.4 and that
4
a
2
ofHFIPis9.3. Fluoroalcohols are known toform strong
5
(
15) Fields, G. B.; Noble, R. L. Int. J. Peptide Protein Res. 1990, 35, 161.
(16) Carpino, L. A.; Shroff, H.; Triolo, S. A.; Mansour, E.-S. M.E.;
Wenschuh, H.; Albericio, F. Tetrahedron Lett. 1993, 34, 7829.
(
(
(
17) Ramage, R.; Green, J. Tetrahedron. Lett. 1987, 28, 2287.
18) Wang, S. S. J. Am. Chem. Soc. 1973, 95, 1328.
19) Atherton, E.; Gait, M. J.; Sheppard, R. C.; Williams, B. J.
(29) (a) Yamashiro, D.; Blake, J.; Li, C. H. Tetrahedron Lett. 1976,
18, 1479. (b) Santangelo, F.;Montecucchi, P. C.;Gozzini, L.;Henschen, A.
Int. J. Peptide Protein Res. 1983, 22, 348.
Bioorg. Chem. 1979, 8, 351.
20) (a) Barlos, K.; Gatos, D.; Kallitsis, J.; Papaphotiu, G.; Sotiriu, P.;
Wenqing, Y.; Sch €a fer, W. Tetrahedron Lett. 1989, 30, 3943. (b) Barlos, K.;
Chatzi, O.; Gatos, D.; Stavropoulos, G. Int. J. Peptide Protein Res. 1991,
(30) Trzupek, S.; Go, A.; Kopple, K. D. J. Org. Chem. 1979, 44, 4577.
(31) (a) Nishino, N.; Mihara, H.; Makinose, Y.; Fujimoto, T. Tetra-
hedron Lett. 1992, 33, 7007. (b) Kuroda, H.; Chen, Y. N.; Kimura, T.;
Sakakibara, S. Int. J. Pept. Prot. Res. 1992, 40, 294. (c) Hinou, H.;
Hvugaji, K.; Garcia-Martin, F.; Nishimura, S.-I.; Albericio, F. RSC
Advances 2012, 2, 2729.
(
37, 513.
(
(
21) Rink, H. Tetrahedron Lett. 1987, 28, 3787.
22) Albericio, F.; Barany, G. Int. J. Peptide Protein Res. 1987,
(32) Carpino, L. A. Acc. Chem. Res. 1973, 6, 191.
3
0, 206.
23) Gaussier, H.; Morency, H.; Lavoie, M. C.; Subirade, M. Appl.
Environ. Microbiol. 2002, 68, 4803.
(33) Riniker, B.; Kamber, B.; Sieber, P. Helv. Chim. Acta 1975, 58,
1086.
(34) Sieber, P.; Kamber, B.; Hartmann, A.; J o€ hl, A.; Riniker, B.;
Rittel, W. Helv. Chim. Acta 1977, 60, 27.
(35) Bollhagen, R.; Schmiedberger, M.; Barlos, K.; Grell, E. J. Chem.
(
(24) Ballinger, P.; Long, F. A. J. Am. Chem. Soc. 1959, 81, 1050.
25) Middleton, W. J.; Lindsey, R. V. J. Am. Chem. Soc. 1964, 86,
(
948.
4
Soc. Chem. Commun. 1994, 2559.
(
26) F ꢁa rca s- iu, D.; Ghenciu, A.; Marino, G.; Kastrup, R. V. J. Mol.
Cat. A: Chem. 1997, 126, 141.
(36) Moretto, A.;Crisma, M.;Formaggio, F.;Kaptein, B.;Broxterman,
Q. B.; Keiderling, T. A.; Toniolo, C. Biopolymers 2007, 88, 233.
Org. Lett., Vol. 14, No. 24, 2012
6347

MW irradiation (100À150 °C) have been applied to remove
considerable negative effect on the rate of deprotection.
However, addition of a non-hydrogen-bonding solvent
such as CH Cl showed no appreciable slowdown. Simi-
larly, small quantities (1À2% v/v) of common scavengers
such as triisopropylsilane (TIS), thioanisole, or dimethyl-
sulfide were tolerated as well.
3
7
38
tert-butyl ester or carbonate or Boc group from a range
of compounds within several hours. Cleavage of fully
protected peptides from a trityl resin may be effected by
2
2
39
dilute HCl in aq DMF with optional TFE (up to 10% v/v).
We set out to investigate the applicability of fluoro alcohols
such as HFIP or TFE as potential reagents or additives for
cleaving acid-labile resin linkers and deprotecting acid-labile
protecting groups to produce fully unprotected peptides.
First, we have checked if neat HFIP at elevated tem-
perature would remove the tert-butyl ether group from
Next, we set out to elucidate ifeven the most robust of all
1
6
the TFA-cleavable protecting groups Pbf could be re-
moved as well. With 0.1 N HCl/HFIP we have observed
60.6% Pbf removal after 30 min and 84% after 1 h 40 min of
reaction, whereas 1 N HCl gave 92.4% conversion after
30 min and 95.3% after 2 h of reaction. Peptides with multiple
Arg residues may require prolonged treatment (18À24 h)
with dilute HCl in HFIP or a higher percentage of HCl, e.g.
1 N to remove all guanidino protecting groups quantitatively.
ε
Tyr, Ser, or Thr or the N -Boc from Lys, in line with the
3
7,38
published procedure.
vage of the tert-butyl ether from Fmoc-Tyr(tBu)-OH in
We have observed sluggish clea-
neat HFIP at 60 °C: 3.4% conversion after 1 h, 7.6% after
3
h, and 48% after 18 h of reaction. Some byproducts were
also evident: ca. 2% after 18 h at 60 °C and 7% after 4 h at
00 °C. Even slower was the unmasking of other protected
amino acids:Lys4.8%, Thr2.1%, and Ser0.7%after 4h at
0 °C. The use of HFIP at higher temperatures is discour-
1
Table 1. Deprotection of Amino Acid Derivatives by HCl in
Fluoro Alcohol
a
6
aged by its low boiling point (59 °C) and high volatility,
which require special glassware such as tightly capped
microwave tubes as screw-cap polypropylene tubes
with a rubber O-ring are unsuitable for HFIP. This has
prompted us to look for a fluoro alcohol deprotection that
could be carried out at ambient temperature in conventional
QuickFit glassware readily available in any chemical lab.
We have found that addition of as low as 0.1 N hydro-
Fmoc amino
acid
time,
min
yield,
b
c
d
product
conditions
%
Asn(Trt)
Asp(OtBu)
Arg(Pbf)
Asn
Asp
Arg
A
A
A
A
A
B
B
C
D
E
A
F
A
A
G
A
H
A
A
I
10
quant
>99
240
30
60.6
84.0
>99
100
240
30
92.4
95.3
0.8
120
70
3
chloric acid to neat HFIP (equiv to 1 cm of ca. 37% aq
3
HCl per 99 cm HFIP) improves the removal of acid-labile
90
98.5
12.3
>99
protecting groups quite dramatically. For example, Fmoc-
Ser(tBu)-OH, which is nearly resistant to neat HFIP at
15
Cys(Trt)
Cys
5
35
quant
>99
6
0 °C, gave 76.6% of Fmoc-Ser-OH after just 5 min of the
Gln(Trt)
Gln
Glu
15
reaction and, after1 h30min, produced95.7%conversion.
Similarly, all the protecting groups of the tert-butyl and
trityl families could be cleanly and rapidly removed from
all the protected amino acid derivatives commonly used in
Fmoc SPPS (Table 1). TFE is also effective but ca. 2À3
times slower than HFIP: 0.1 N HCl in TFE with Fmoc-
Ser(tBu)-OH gave 86.4% conversion after 2 h of reaction.
TFE is also less efficient for solubilizing Fmoc-amino acids.
The rate-enhancing effect of fluoro alcohols on the
removal of acid-labile protecting groups is highly specific
to fluoro alcohols. In hydrogen-bonding solvents whether
Glu(OtBu)
240
30
quant
quant
quant
4.8
e
f
His(Trt)
His
Lys(Boc)
Lys
240
240
5
Ser(tBu)
Ser
76.6
95.7
86.4
0.7
90
120
240
60
H
J
3.3
Thr(tBu)
Trp(Boc)
Tyr(tBu)
Thr
Trp
Tyr
A
H
A
A
A
H
H
H
240
240
30
quant
2.1
>99
i
protic, e.g. water, MeOH or Pr OH, or aprotic, e.g.
100
240
60
quant
quant
3.4
acetone, MeCN, 1,4-dioxane, THF, DMF, or 1-methyl-
pyrrolidin-2-one, 0.1 N HCl is nearly ineffective for the
tert-butyl removal. For Fmoc-Ser(tBu)-OH, 0.1 N HCl in
MeCN gave just 3.3% conversion after 1 h of reaction, in
180
1080
7.6
48.0
a
b
For full experimental conditions, see Supporting Information. An
1
2
,4-dioxane <0.2% after 25 min, in acetone <0.1% in
i
5 min, and in Pr OH less than 0.1% conversion was
R
c
N -Fmoc amino acid unless indicated otherwise. Conditions: (A) 0.1 N
HCl/HFIP, rt; (B) 1 N HCl/HFIP, rt; (C) 0.1 N HCl/HFIPÀDMF (9:1
observed after 1 h 30 min of reaction. Interestingly, addition
of just 5À10% (v/v) of such a solvent to HFIP produced a
v/v); (D) 0.1 N HCl/HFIPÀCH
2
Cl
2
(4:1 v/v); (E) 0.1 N HCl/HFIPÀ
i
Pr OH (95:5 v/v); (F) 0.1 N HCl/HFIPÀTIS (99:1 v/v), rt; (G) 0.01 N
HCl/HFIP, rt; (H) neat HFIP, 60 °C; (I) 0.1 N HCl/TFE, rt; (J) 0.1 N HCl/
MeCN, rt. Calculated as peak area. H-His(Trt)-OH. H-His-OH.
d
e
f
(
37) Choy, J.; Jaime-Figueroa, S.; Lara-Jaime, T. Tetrahedron Lett.
010, 51, 2244.
38) Choy, J.; Jaime-Figueroa, S.; Jiang, L.; Wagner, P. Synth.
Commun. 2008, 38, 3849. (b) Choy, J.; Jaime-Figueroa, S. Patent WO
010/142616, 2010.
39) Srivastava, K. S. US Patent 8,022,181, 2011.
2
(
Finally, we have investigated the cleavage of common
acid-labileresinlinkersfor theFmocSPPS. Cleavageof the
2
(
6
348
Org. Lett., Vol. 14, No. 24, 2012

4
0
trityl ester linkage of TentaGel S Trt resin by 0.1 N HCl
in HFIP is almost instantaneous and could be monitored
visually by the immediate appearance of the orange-brown
color of the resin-bound trityl cation upon contact with the
deprotection mixture. In practice, a 15 min period was suf-
ficient for the quantitative cleavage of a peptide from the
resin. In the case of TFE no color was observed due to a
scavenging effect of TFE. However, the cleavage was com-
plete within the same 15 min period as for HFIP. The
p-benzyloxybenzyl ester of Fmoc-Leu-Wang resin was
cleaved by 0.1 N HCl in HFIPÀCH Cl (1:4 v/v) more
TentaGelS Trt resin. After the completion of the assembly,
a sample of the resin was treated with 0.1 N HCl/HFIP
to cleave the peptide from the resin for 15 min and then
deprotect for 12 h at ambient temperature. The crude
peptide Fmoc-KTTKS-OH was obtained in high yield
and purity, and its integrity was confirmed by ESI
HRMS. The Fmoc group as expected was stable under
the deprotection conditions. Other peptide derivatives
were prepared as well (see Supporting Information).
Notably, similar results were obtained after treatment
with 0.1 N HCl/TFE for 12 h or even 4 h at ambient
temperature.
2
2
slowly: 91.6% release was seen after 3 h. Notably, the ratio of
HCl to the resin-bound ester was nearly equimolar: 0.1 mmol
3
An important side reaction in the new method would be
the TFE or HFIP ester formation with the free carboxyl
groupof e.g. Asp orGlu. To datewehave notobservedany
such ester formation in our experiments (Table 1).
To conclude, we have found that a low concentration of
aq HCl in HFIP, e.g. 0.1 N, removes common acid-labile
protecting groups of the tert-butyl, trityl, andarenesulfonyl
(Pbf) type from amino acid side chains and cleaves com-
mon acid-labile resin linkers used in the Fmoc SPPS such
as Trt ester, Wang, HMPA, Rink amide, or PAL. We
believe that the proposed composition of 0.1 N HCl in
HFIP may serve as a less hazardous substitute for TFA for
the Fmoc SPPS. Further studies on the general utility of
the described deprotection method for the synthesis of
longer and more complex peptides incorporating sensitive
amino acids are underway.
(
mixture) per 0.095 mmol of the resin-bound amino acid (0.1 g
10μL of concentrated ca. 37% aqueous HCl per 1 cm of the
À1
of the resin with the loading of 0.95 mmol g ). In con-
i
trast, substitution of Pr OH for HFIP in the mixture
produced no release of Fmoc-Leu-OH within 72 h. Addi-
tion of CH Cl was necessary for better swelling of Wang
2
2
resin asHFIPalone isa poor swelling agentforpolystyrene
supports such as Wang or MBHA resins. However, other
types of resins such as PEG-PS, TentaGel, or NovaPEG
have shown good swelling in neat HFIP or TFE.
The Fmoc-Gly-HMPA-PEG-PS resin had slightly lower
susceptibility to 0.1 N HCl in HFIP with 84.2% release
after 3 h. Addition of 1% (v/v) dimethylsulfide had no
appreciable accelerating effect on the cleavage. The two
common resins for peptide amides were also cleaved
successfully. Rink amide NovaPEG resin released 82%
Fmoc-glycinamide after 2 h, and PAL-PEG-PS lost 91%
of the same after 1 h of reaction. The type of resin had no
clear-cut influence on the rate of deprotection.
Acknowledgment. This work was supported by EPSRC.
The authors thank the mass spectral service of Imperial
College London for their help with obtaining HRMS data.
To test the general applicability of the new method in
a standard Fmoc SPPS, a collagen-stimulating peptide
Supporting Information Available. Experimental pro-
cedures and HPLC and MS data for amino acid and
peptide derivatives. This material is available free of
charge via the Internet at http://pubs.acs.org.
4
Fmoc-KTTKS-OH was prepared as described on a
1
42
(
40) Gr u€ bler, G.; Zimmermann, H.; Echner, H.; Stoeva, S.; Bernardi,
E.; Pourrias, B.; Voelter, W. In Innovation & Perspectives in Solid Phase
Synthesis, 3rd International Symposium; Epton, R., Ed.; Mayflower World-
wide Ltd.: Birmingham, 1994, 517.
(
42) Amblard, M.; Fehrentz, J.-A.; Martinez, J.; Subra, G. Mol.
Biotechnol. 2006, 3, 239.
(41) (a) Palladino, P.; Castelletto, V.; Dehsorkhi, A.; Stetsenko, D.;
Hamley, I. W. Chem. Commun. 2012, 9774. (b) P Palladino, P.; Castelletto,
V.; Dehsorkhi, A.; Stetsenko, D.; Hamley, I. W. Langmuir 2012, 28, 12209.
The authors declare no competing financial interest.
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