Tetrahydropyranyl: A Non-aromatic, Mild-Acid-Labile Group for Hydroxyl Protection in Solid-Phase Peptide Synthesis

Article abstract of DOI:10.1002/open.201600157

The use of the tetrahydropyranyl (Thp) group for the protection of serine and threonine side-chain hydroxyl groups in solid-phase peptide synthesis has not been widely investigated. Ser/Thr side-chain hydroxyl protection with this acid-labile and non-aromatic moiety is presented here. Although Thp reacts with free carboxylic acids, it can be concluded that to introduce Thp ethers at the hydroxyl groups of N-protected Ser and Thr, protection of the C-terminal carboxyl group is unnecessary due to the lability of Thp esters. Thp-protected Ser/Thr-containing tripeptides are synthesized and the removal of Thp studied in low concentrations of trifluoroacetic acid in the presence of cation scavengers. Given its general stability to most non-acidic reagents, improved solubility of its conjugates and ease with which it can be removed, Thp emerges as an effective protecting group for the hydroxyl groups of Ser and Thr in solid-phase peptide synthesis.

Full text of DOI:10.1002/open.201600157

DOI: 10.1002/open.201600157
Tetrahydropyranyl: A Non-aromatic, Mild-Acid-Labile
Group for Hydroxyl Protection in Solid-Phase Peptide
Synthesis
Anamika Sharma+,[a] Ivꢀn Ramos-Tomillero+,[b] Ayman El-Faham,[c, d] Hortensia Rodrꢁguez,[e]
Beatriz G. de la Torre,[a, f] and Fernando Albericio*[a, b, c, g, h]
The use of the tetrahydropyranyl (Thp) group for the protec-
tion of serine and threonine side-chain hydroxyl groups in
solid-phase peptide synthesis has not been widely investigat-
ed. Ser/Thr side-chain hydroxyl protection with this acid-labile
and non-aromatic moiety is presented here. Although Thp
reacts with free carboxylic acids, it can be concluded that to in-
troduce Thp ethers at the hydroxyl groups of N-protected Ser
and Thr, protection of the C-terminal carboxyl group is unnec-
essary due to the lability of Thp esters. Thp-protected Ser/Thr-
containing tripeptides are synthesized and the removal of Thp
studied in low concentrations of trifluoroacetic acid in the
presence of cation scavengers. Given its general stability to
most non-acidic reagents, improved solubility of its conjugates
and ease with which it can be removed, Thp emerges as an ef-
fective protecting group for the hydroxyl groups of Ser and
Thr in solid-phase peptide synthesis.
view, hydroxyl-containing amino acids can be introduced into
peptides without side-chain protection.[7] However, unprotect-
ed hydroxyl functionalities in Ser and Thr can undergo side re-
actions such as dehydration or O-acylation, followed by O!N
migration after amino group deprotection, particularly in the
presence of powerful activating agents such as carbodi-
imides.[7] This undesirable reaction is more predominant for
the primary alcohol of Ser than for the secondary alcohol of
Thr. Some studies have demonstrated peptide synthesis with-
out the protection of the hydroxyl side chains of Ser and Thr;
however, care must be taken in choosing the activating
agents.[8,9] This point is especially relevant in SPPS because this
method involves the use of an excess of activating agents.[10]
Thus, it can be concluded that the safest way to introduce hy-
droxylated amino acids is by protecting their side chains.
In peptide synthesis, hydroxyl functionalities are protected
as ethers, which are more stable than the corresponding car-
bamates. Moreover, ethers are less prone to taking part in side
reactions.[11] The most widely used hydroxyl protecting groups
for the common tert-butyloxycarbonyl (Boc) and 9-fluorenylme-
thyloxycarbonyl (Fmoc) strategies are benzyl and tert-butyl
(tBu), respectively. Trityl has been used less often for Ser and
Thr in SPPS[12,13] because trityl ethers are much more sensitive
to acidic conditions than the corresponding tBu ethers. Addi-
tionally, trityl-protected Ser and Thr can be selectively removed
in the presence of a tBu ether in SPPS with a low concentra-
tion of trifluoroacetic acid (TFA),[14] taking advantage of the re-
Since the first synthesis of a tetrapeptide on a solid support in
1963,[1] methodological advances in solid-phase peptide syn-
thesis (SPPS) have allowed the preparation of a vast number of
complex peptides.[2] Accordingly, a key factor in peptide sci-
ence is amino acid side-chain protection, a procedure used to
prevent undesired side reactions. In peptide and protein sci-
ence, serine (Ser) and threonine (Thr) play important roles as
their hydroxyl side chains are crucial as phosphorylation and
O-glycosylation sites,[3,4] as well as for the preparation of depsi-
peptides,[5,6] among other molecules. From a synthetic point of
[a] Dr. A. Sharma,+ Dr. B. G. de la Torre, Prof. F. Albericio
Catalysis and Peptide Research Unit, School of Health Sciences
University of KwaZulu-Natal, Durban 4001 (South Africa)
E-mail: albericio@ukzn.ac.za
[g] Prof. F. Albericio
School of Chemistry & Physics
University of KwaZulu-Natal, Durban 4001 (South Africa)
[h] Prof. F. Albericio
[b] Dr. I. Ramos-Tomillero,+ Prof. F. Albericio
CIBER-BBN, Networking Centre on Bioengineering
Biomaterials and Nanomedicine, Barcelona Science Park
Baldiri Reixac 10–12, 08028 Barcelona (Spain)
Inorganic and Organic Department, University of Barcelona
Martꢀ Franquꢁs 1–11, 08028 Barcelona (Spain)
[c] Prof. A. El-Faham, Prof. F. Albericio
[+] These authors contributed equally to the work
Department of Chemistry, College of Science
King Saud University, P.O. Box 2455, Riyadh 11451 (Saudi Arabia)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
open.201600157.
[d] Prof. A. El-Faham
Department of Chemistry, Faculty of Science, Alexandria University
P.O. Box 426, Ibrahimia, Alexandria 21321 (Egypt)
ꢂ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
[e] Dr. H. Rodrꢀguez
School of Chemistry, Yachay Tech
Yachay City of Knowledge, Urcuqui (Ecuador)
[f] Dr. B. G. de la Torre
School of Laboratory of Medicine & Medical Sciences
University of KwaZulu-Natal, Durban 4001 (South Africa)
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quirement for a concentration of at least 50% TFA for cleavage
of tBu ethers.[13]
DHP similarly to alcohols, the corresponding hemiacetal ester
is unstable, and during the work-up the Thp was cleaved from
the carboxyl group.[27,28] Thus, Fmoc-Thr/Ser-OH can be side-
chain protected without the need for protection of the carbox-
yl group. This is highly relevant when dealing with the protec-
tion of unnatural w-hydroxy amino acids that are difficult to
synthesize. Examples of such compounds include allo-Thr de-
rivatives and other b-hydroxy amino acids that are natural cy-
clodepsipeptides.[29,30] Furthermore, we demonstrated that
PTSA is a more convenient and more efficient catalyst than
PPTS, as the latter requires the reaction mixture to be heated
to nearly 608C for 10–12 h. PTSA was a more efficient catalyst
as the reaction rate increased (reaction time <60 min at room
temperature) compared to PPTS. The use of Thp as a protecting
group for the hydroxyl side chain in Fmoc-Ser/Thr-OH in-
creased the solubility of the amino acid, which is consistent
with our earlier reports.[18]
Tetrahydropyranyl (Thp)[15] is widely used as a hydroxyl pro-
tecting group in organic chemistry[16] due to its low cost, ease
of introduction, general stability to most non-acidic reagents,
good solubility, and the ease with which it can be removed if
the functional group it protects requires further manipula-
tion.[17] Accordingly, Thp has additional advantages over
benzyl-based protecting groups as it lacks aromaticity and is
characterized by atom economy and better solubility. In addi-
tion to producing more protected hydrophobic peptides, the
use of bulky or aromatic protecting groups in SPPS affects
inter-/intrachain interactions during peptide elongation and
could therefore dramatically affect the purity of the final prod-
uct.[18] The main drawback of using Thp in organic synthesis is
the formation of a new stereocenter, which leads to diastereo-
meric mixtures. However, if Thp is used as a protecting group
or linker in SPPS, its use is temporary, and therefore the forma-
tion of a new stereocenter is not a limitation.
In order to determine the acid stability of Thp as a protecting
group, Fmoc-Ser(Thp)-OH and Fmoc-Thr(Thp)-OH were studied
under a range of acidolytic conditions (Table 1). The lability of
the Thp group was greatly increased in the presence of triiso-
propylsilane (TIS) as a scavenger compared with H2O (Table 1,
entries 3 and 4). Furthermore, we prepared the model Thp-pro-
tected tripeptides Fmoc-Ala-Xxx(Thp)-Leu-O-2CTC (where
Xxx=Ser or Thr, 2CTC=2-chlorotrityl chloride) using N,N’-diiso-
propylcarbodiimide (3 equiv.) and ethyl (hydroxyimino)cyanoa-
cetate (Oxyma Pure, 3 equiv.) in N,N-dimethylformamide (DMF)
with a 3 min pre-activation time and 1.5 h coupling time at
258C. Both Thp-protected Ser and Thr were stable under the
coupling conditions tested and also to the Fmoc deprotection
in piperidine/DMF (1:4). In additional, the lability of the synthe-
sized tripeptides was also studied under the indicated cleavage
conditions (Scheme 1 and Table 2). During tripeptide elonga-
tion, the Thp group remained stable and was cleaved with
a low concentration of TFA to afford the deprotected tripep-
tide, as shown by the HPLC chromatograms (see the Support-
ing Information). Complete cleavage was obtained in the pres-
ence of TIS as a scavenger and at low TFA concentration
(Table 2, entries 2 and 7).
As Thp has received little attention as a protecting group in
SPPS, we decided to study its potential for protecting Ser and
Thr. There are only a few examples of the use of Thp to protect
the side chains of Ser[19–24] and Thr.[25,26] Furthermore, the meth-
ods described in the literature for the protection of Ser and
Thr hydroxyl groups involve prior protection of the carboxyl
group. Here we report an efficient method for Thp protection
of the hydroxyl side chains of Ser and Thr with free carboxyl
groups and its application in SPPS.
In 2006, Krishnamoorthy et al. introduced Thp as a protecting
group for the Thr side chain in the synthesis of callipeltin B.[25]
For the introduction of Thp into the side chain of Thr, the au-
thors used Fmoc-Thr-OAllyl as a precursor, which was reacted
with dihydropyran (DHP) in the presence of pyridinium p-tol-
uenesulfonate (PPTS) using dichloroethane as solvent. The re-
action was allowed to react at 608C for 12 h. Removal of the
allyl group afforded the desired Fmoc-Thr(Thp)-OH, which was
used in the next step without purification. However, we syn-
thesized Fmoc-Ser(Thp)/Thr(Thp)-OH in good yield using an
acid-catalyzed reaction between Fmoc-Ser/Thr-OH and DHP,
with p-toluenesulfonic acid (PTSA) as catalyst and dichlorome-
thane as solvent. We found that protection of the carboxyl
group was unnecessary. Although carboxylic acids reacts with
In addition to the aforementioned use of Thp as a protecting
group, the Thp moiety was also studied as a cleavable linker
for SPPS. Ellman resin has been found to be convenient for the
Table 1. Acid lability studies of Fmoc-Ser(Thp)-OH and Fmoc-Thr(Thp)-OH.
Entry
Compound
Deprotecting
cocktail
Reaction
time [min]
Deprotected
amino acid [%]
1
2
3
4
5
6
7
8
Fmoc-Ser(Thp)-OH
TFA/CH2Cl2 (1:99)
TFA/CH2Cl2 (2:98)
TFA/H2O/CH2Cl2 (2:0.5:97.5)
TFA/TIS/CH2Cl2 (2:0.5:97.5)
TFA/CH2Cl2 (5:95)
TFA/CH2Cl2 (10:90)
TFA/H2O/CH2Cl2 (10:2:88)
TFA/TIS/CH2Cl2 (10:2:88)
60
30
30
15
30
30
15
15
56
97
97
>99
98
98
>99
>99
9
10
Fmoc-Thr(Thp)-OH
TFA/CH2Cl2 (1:99)
TFA/CH2Cl2 (2:98)
60
15
93
>99
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Table 2. Acid lability studies of Fmoc-Ala-Ser(Thp)-Leu-OH and Fmoc-Ala-Thr(Thp)-Leu-OH.
Entry
Compound
Deprotecting
cocktail
Reaction
time [min]
Deprotected
peptide [%]
1
2
3
Fmoc-Ala-Ser(Thp)-Leu-OH
TFA/H2O/CH2Cl2 (2:0.5:97.5)
TFA/TIS/CH2Cl2 (2:0.5:97.5)
TFA/CH2Cl2 (3:97)
30
15
60
96
>99
97
4
5
6
7
Fmoc-Ala-Thr(Thp)-Leu-OH
TFA/CH2Cl2 (1:99)
TFA/CH2Cl2 (2:98)
TFA/H2O/CH2Cl2 (2:0.5:97.5)
TFA/TIS/CH2Cl2 (2:0.5:97.5)
60
15
15
15
92
>99
>99
>99
reveal Thp to be a valuable protecting group for Ser and Thr if
applied to SPPS using the Fmoc/tBu strategy. Given the solubil-
ity of its peptide ethers and ease of introduction, Thp might
be particularly useful for the effective synthesis of Ser- or Thr-
rich peptides and for the protection of unnatural w-hydroxy
amino acids (allo-Thr derivatives and other b-hydroxy amino
acids) that are difficult to synthesize.
Experimental Section
All reagents and solvents were obtained from commercial suppliers
and were used without further purification, unless otherwise
stated. 2-CTC resin and Fmoc-protected amino acids were pur-
chased from GL Biochem Pvt. Ltd. (Shanghai, China). Ellman resin
was supplied by Merck. NMR spectra (1H and 13C) were recorded
on a Bruker Avance III 400 MHz spectrometer. Chemical shift values
are expressed in parts per million (ppm).For shorter reaction times
(2–5 min), the reactions were manually stirred with a Teflon rod,
whereas for longer reactions times (>30 min), they were stirred on
a Unimax 1010 shaker (Heidolph Instruments). Solvents were re-
moved from the reacion under reduced pressure. All the reactions
were performed at room temperature (ꢀ258C). Each reaction step
was followed by washing of the peptide resin with DMF (4ꢃ1 min)
and CH2Cl2 (4ꢃ1 min). Analytical HPLC was performed on an Agi-
lent 1100 system using a Phenomenex C18 column (3 mm, 4.6ꢃ
50 mm; solvent A: 0.1% TFA in H2O; solvent B: 0.1% TFA in
CH3CN). LC–MS was performed on a Shimadzu 2020 UFLC-MS in-
strument using an YMC-Triart C18 column (5 mm, 4.6ꢃ150 mm; sol-
vent A: 0.1% formic acid in H2O; solvent B: 0.1% formic acid in
CH3CN). Data processing was carried out using LabSolution soft-
ware. High-resolution mass spectrometry (HRMS) was performed
using a Bruker ESI-QTOF mass spectrometer in positive-ion mode.
Scheme 1. Removal of the Thp group under acidic conditions.
synthesis of a wide range of hydroxyl-containing organic com-
pounds.[31] However, there are only a few reports on the use of
this resin for the synthesis of peptide alcohols.[32] Taking these
facts into consideration, we attempted to introduce Fmoc-Ser/
Thr-NH2 onto Ellman resin using PTSA as catalyst and CH2Cl2/
THF (1:4) as solvent. However, we were unable to anchor it
onto the resin due to solubility issues. Therefore, Fmoc-Ser-
OMe was attached to Ellman resin under similar reaction con-
ditions using PTSA as catalyst and THF as solvent. Cleavage
studies revealed that Fmoc-Ser-OMe anchored to Ellman resin
at the hydroxyl side chain even in THF.
In summary, we studied the PTSA-catalyzed Thp protection
of the hydroxyl side chains of Ser and Thr. Our findings reveal
that there is no need to protect the C-terminal carboxyl group,
as the corresponding hemiacetal ester is unstable, and Thp is
removed from the carboxyl group during aqueous work-up.
We also conclude that the deprotection of Thp from side
chains of Ser and Thr can be achieved using lower concentra-
tions of TFA (2%) in the presence of water and TIS as scaveng-
ers over short reaction times (approximately 15 min). Unex-
pectedly, Thp protection of the hydroxyl side chain of Thr was
slightly more unstable than that of Ser. Similar trends have
been found for model peptides using 2-CTC resin. The Thp
group was stable during peptide elongation and the Fmoc
elimination step on resin. Among the scavengers used, TIS
proved to be more efficient than water. Furthermore, we ana-
lyzed the incorporation of Fmoc-Ser-OMe to the Ellman resin
using PTSA as a catalyst and THF as the solvent. Our results
Synthesis of Fmoc-Ser(Thp)-OH
PTSA (14.5 mg, 0.077 mmol) was added to a suspension of Fmoc-
Ser-OH (500 mg, 1.52 mmol) and DHP (277 mL, 3.06 mmol) in
CH2Cl2. The mixture was allowed to react for 30 min at RT. After-
wards, the organic layer was washed with brine (3ꢃ20 mL) and
water (3ꢃ20 mL) and then dried over Na2SO4 and filtered. The sol-
vent was evaporated under reduced pressure. The crude product
was purified on a silica column using n-hexane/EtOAc (1:1). The
collected fractions were concentrated to afford a white solid
(478 mg, 76% yield). 1H NMR (400 MHz, DMSOd6): d=7.88 (d, J=
7.6 Hz, 2H), 7.73 (t, J=6.0 Hz, 2H), 7.63 (t, J=8 Hz, 1H), 7.41 (t, J=
7.6 Hz, 2H), 7.32 (t, J=7.6 Hz, 2H), 4.63–4.56 (m, 1H), 4.34–4.18 (m,
4H), 3.92–3.57 (m, 4H), 1.73–1.40 ppm (m, 6H); 13C NMR (100 MHz,
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DMSOd6): d=171.9, 143.8, 140.8, 127.5, 126.9, 125.2, 120.0, 98.5,
97.4, 66.0, 61.3, 54.4, 46.7, 29.9, 24.9, 18.8 ppm; HPLC: linear gradi-
ent of H2O/MeCN 5:95 to 0:100; tR: 10.4 min; LC–MS: m/z: calcd for
C23H25NO6: 411.4; found: 410 [MꢁH]+; HRMS: m/z: calcd for
C23H26NO6: 412.1760 [M+H]+; found: 412.1745.
MeOH (10 equiv.) to the mixture, which was stirred for 30 min at
258C. The resin was then washed with CH2Cl2 (4ꢃ5 mL) and DMF
(5ꢃ5 mL), and the Fmoc protecting group was removed by treat-
ing the resin with 20% piperidine in DMF (2ꢃ10 mL, each 10 min),
followed by washing with DMF (2ꢃ5 mL) and CH2Cl2 (2ꢃ5 mL).
The H-Leu-O-2CTC resin was divided into two portions and swelled
in CH2Cl2 (3ꢃ5 mL) and then DMF (3ꢃ5 mL). Fmoc-Ser(Thp)-OH
(3 equiv.) was added to one portion and Fmoc-Thr(Thp)-OH
(3 equiv.) was added to the other. Amino acids were coupled using
N,N’-diisopropylcarbodiimide (3 equiv.) and Oxyma Pure (3 equiv.)
in DMF with a 3 min pre-activation time for 1.5 h at 258C. The
resins were washed with CH2Cl2 (4ꢃ5 mL) and DMF (2ꢃ5 mL). The
Fmoc protecting group was removed by treating the resins with
20% piperidine in DMF (2ꢃ10 mL, each 10 min), and then Fmoc-
Ala-OH was incorporated as described above.
Synthesis of Fmoc-Thr(Thp)-OH
PTSA (14 mg, 0.073 mmol) was added to a suspension of Fmoc-
Thr-OH (500 mg, 1.46 mmol) and DHP (265 mL, 2.92 mmol) in
CH2Cl2. The mixture was allowed to react for 60 min at RT. After-
wards, the organic layer was washed with brine (3ꢃ20 mL) and
water (3ꢃ20 mL) and then dried over Na2SO4 and filtered. The sol-
vent was evaporated under reduced pressure. The crude product
was purified on a silica column using n-hexane/EtOAc (1:1). The
collected fractions were concentrated to afford a white solid
1
(453 mg, 73% yield). H NMR (400 MHz, DMSOd6): d=12.8 (s, 1H),
Cleavage Studies
7.88 (d, J=7.2 Hz, 2H), 7.77 (d, J=7.6 Hz, 2H), 7.41 (t, J=7.6 Hz,
2H), 7.32 (t, J=7.2 Hz, 2H), 7.16 (d, J=8.8 Hz, 1H), 4.77–4.62 (m,
1H), 4.33–4.18 (m, 3H), 4.17–3.98 (m, 2H), 3.85–3.66 (m, 1H), 3.45–
3.37 (m, 1H), 1.78–1.32 (m, 6H), 1.22–1.05 ppm (m, 3H); 13C NMR
(100 MHz, DMSOd6): d=171.7, 156.6, 143.9, 140.6, 127.6, 127.0,
125.3, 120.1, 99.4, 94.0, 69.5, 65.9, 60.9, 46.9, 30.1, 25.0, 20.3,
18.6 ppm; HPLC: linear gradient of H2O/MeCN 5:95 to 0:100; tR:
10.8 min; LC–MS: m/z: calcd for C24H27NO6: 425.47; found: 424
[MꢁH]+; HRMS: m/z: calcd for C24H28NO6: 426.1917 [M+H]+;
found: 426.1896.
For each cleavage study, samples of Fmoc-Ala-Xxx(Thp)-Leu-O-
2CTC resin (Xxx=Ser or Thr; 5 mg) were treated with mixtures of
TFA/H2O/TIS/CH2Cl2 (500 mL) at different percentages (Table 2) at
RT. The reaction was monitored by RP-HPLC [linear gradient of
H2O/MeCN (5:95) over 15 min] after 15, 30, and 60 min of treat-
ment. After each interval, an aliquot of 20 mL was taken, and the
solvent was then removed under a nitrogen stream and the resi-
due re-dissolved in MeCN (400 mL; Table 2).
Acknowledgements
Peptide Synthesis with Ellman (DHP) resin
Fmoc-Ser-OMe (50 mg, 0.059 mmol) in CH2Cl2 was added to Ellman
(DHP) resin (50 mg, f=1.18 mmolgꢁ1) after the latter was swelled
in CH2Cl2 for 10 min. PTSA (0.28 mg, 0.0015 mmol) in THF (1 mL)
was added to the resin. The reaction was stirred for 30 min and
the resin was then washed with CH2Cl2 (2ꢃ5 mL) and dried. The
dried resin was used to study the lability of Fmoc-Ser-OMe from
Ellman resin.
We thank Dr. Thomas Bruckdorfer from Iris Biotech (Germany) for
supporting this work and providing free samples of protected
amino acids. This work was funded in part by the following: Na-
tional Research Foundation (NRF) and the University of KwaZulu-
Natal (South Africa); the International Scientific Partnership Pro-
gram (ISPP#0061) at King Saud University (Saudi Arabia); and
MEC (CTQ2015-67870-P) and Generalitat de Catalunya (2014 SGR
137; Spain).
Lability Experiments
The protected Fmoc-Ser(Thp)-OH and Fmoc-Thr(Thp)-OH (1 mg)
were treated at RT with cleavage cocktails (200 mL), which were
CH2Cl2 solutions containing different percentages of TFA and scav-
engers (water and/or TIS; Table 1). The reaction was monitored by
RP-HPLC [linear gradient of H2O/MeCN (5:95) over 15 min] after 15,
30, and 60 min of treatment. After each interval, an aliquot of
20 mL was withdrawn, the solvent was removed under nitrogen
stream and the residue re-dissolved in MeCN (400 mL). Fmoc-
Ser(Thp)-OH, tR =10.46 min; Fmoc-Thr(Thp)-OH, tR =10.89 min.
Cleavage of Fmoc-Ser-OMe from the DHP resin was achieved using
2% TFA in the presence of TIS for 15 min.
Keywords: protecting groups · serine · solid-phase peptide
synthesis · tetrahydropyranyl group · threonine
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Peptide syntheses were performed manually on 2-CTC resin
(50 mg, f=1.60 mmolgꢁ1). Initially, the resin was activated over-
night using thionyl chloride (10% in CH2Cl2) and then washed with
CH2Cl2 (2ꢃ5 mL). Attachment of the first amino acid was per-
formed by treating the resin with Fmoc-Leu-OH (3 equiv.) and N,N-
diisopropylethylamine (10 equiv) in CH2Cl2, and allowing it to react
for 60 min at 258C. Thereafter, the resin was capped by adding
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Received: November 26, 2016
Revised: December 12, 2016
Published online on && &&, 2017
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These are not the final page numbers! ÞÞ

COMMUNICATIONS
A. Sharma, I. Ramos-Tomillero,
A. El-Faham, H. Rodrꢀguez,
B. G. de la Torre, F. Albericio*
Save the alcohol: The tetrahydropyran-
yl (Thp) group is characterized by its
ease of introduction, stability under
basic conditions, versatility and atom
economy. It has proved to be an effec-
tive non-aromatic acid-labile moiety for
the protection of the hydroxyl groups
of serine and threonine residues in
solid-phase peptide synthesis.
&& – &&
Tetrahydropyranyl: A Non-aromatic,
Mild-Acid-Labile Group for Hydroxyl
Protection in Solid-Phase Peptide
Synthesis
&
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ꢂ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!