Practical synthesis of peptide C-terminal aldehyde on a solid support

Article abstract of DOI:10.1016/j.tetlet.2013.06.103

We have investigated practical synthetic routes for the preparation of peptide aldehyde on a solid support. Peptide aldehyde was synthesized via efficient transformation of acetal/thioacetal structures.

Full text of DOI:10.1016/j.tetlet.2013.06.103

Tetrahedron Letters 54 (2013) 4848–4850
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Tetrahedron Letters
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Practical synthesis of peptide C-terminal aldehyde on a solid support
a
a
b
c
b
Hiroyuki Konno a, , Yoshihiro Sema , Manabu Ishii , Yasunao Hattori , Kazuto Nosaka , Kenichi Akaji
⇑
a Department of Biochemical Engineering, Graduate School of Science and Technology, Yamagata University, Yonezawa, Yamagata 992-8510, Japan
b Department of Medicinal Chemistry, Kyoto Pharmaceutical University, Yamashina-ku, Kyoto 607-8414, Japan
c Department of Chemistry, Hyogo College of Medicine, Nishinomiya, Hyogo 663-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We have investigated practical synthetic routes for the preparation of peptide aldehyde on a solid sup-
port. Peptide aldehyde was synthesized via efficient transformation of acetal/thioacetal structures.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 24 May 2013
Revised 18 June 2013
Accepted 21 June 2013
Available online 29 June 2013
Keywords:
Peptide aldehyde
Acetal
Thioacetal
Cysteine protease inhibitor
Peptide with C-terminal aldehyde is of interest due to its prop-
erty as a transition-state analogue toward numerous classes of pro-
teolytic enzymes; aspartyl1,2 and cysteine protease3,4 are inhibited
by peptide aldehyde. Since leupeptin5 is produced by actinomy-
cetes, which inhibit a variety of proteases potently, natural peptide
aldehydes6 are attractive targets for drug discovery. Peptide alde-
hyde can also be used as a key intermediate in the syntheses of
pseudo-peptides, particularly in the synthesis of reduced peptide
by reduced fragment condensation. Several methods for solid
phase synthesis of peptide aldehyde have been reported: reduction
of Weinreb amide,7,8 oxidation of alcohol,9 or ozonolysis10,11 of the
corresponding olefin. However Weinreb amides and ozonolysis are
limited to peptide aldehydes without reductant or oxidant labile
amino acid sequences. On the other hand, acetal linker,12,13 oxazo-
line linker,14,15 threonine-type linker,16 and semicarbazone
linker17 were developed for the attachment of aldehydes. Cleavage
of the peptide aldehydes from these linkers required strong acidic
conditions, HF, TFA, or AcOH, and so forth, which frequently caused
serious problems. In the course of our research regarding cysteine
protease inhibitors,18 we prepared several peptide aldehydes via a
thioacetal structure on a solid support since the aldehyde group
seems to be effective for the thiol functional group of cysteine pro-
tease.19 At that time, we found that the conversion of an acetal to
an aldehyde was quite slow but that the thioacetal can be effi-
ciently converted into the desired aldehyde by treatment with N-
bromo succinimide (NBS). In this case, the Fmoc-His(Trt)-H and
decane-1,2,10-triol linker12 were selected and peptide aldehydes
with a histidine residue at the P1 position were prepared using so-
lid phase synthesis. Subsequently, we discovered a tetrapeptide
with C-terminal aldehyde with potent inhibitory activity against
severe acute respiratory syndrome coronavirus 3C-like protease.20
Herein, we report a practical synthetic route for the preparation of
several peptide aldehydes with different amino acids and commer-
cially available linkers on a solid support.
Our synthetic plan centers on the transformation of acetal to
aldehyde via a thioacetal structure. Although the acetal linker
using decane-1,2,10-triol reported by Yao and Xu12 is stable and
useful for solid phase peptide synthesis, decane-1,2,10-triol is not
commercially available and the acetal is stable in TFA. When the
resin was treated with 95% TFA/H2O, the desired peptide aldehyde
was obtained in poor yield.12 We thought that the peptide acetal
(B), which was converted from the amino acetal (A) containing
commercially available alkyl triols by solid phase peptide synthe-
sis, transformed peptide thioacetal (C) by treatment with EtSH in
the presence of catalytic Lewis acid. As a final step, the thioacetal
(C) thus obtained could be treated with NBS in 10% CH2Cl2 aq to
give the desired peptide aldehyde (D). In addition, N-terminal pro-
tection was necessary to avoid the (hemi-) aminal formation be-
tween N-terminal amine and C-terminal aldehyde otherwise
complexed mixtures were given (Scheme 1).
To investigate the effect of the linker length, transformation of
acetal with two alkyl triols and BF3–Et2O complex as an acidic cat-
alyst was commenced in the present study.
According to the condition in the previous literature20 (entry 1),
the acetalization of Fmoc-Ala-H (1) with 1.5 equiv of hexane-1,2,6-
triol and 10 mol % BF3–Et2O in CH2Cl2 at room temperature for 3 h
obtained the desired hydroxyl acetal (2b) in 34% yield (entry 2).
⇑
Corresponding author. Tel./fax: +81 (0)238 26 3131.
E-mail address: konno@yz.yamagata-u.ac.jp (H. Konno).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.103

H. Konno et al. / Tetrahedron Letters 54 (2013) 4848–4850
4849
ture using CH2Cl2 to give 2c was a suspension because of the
insolubility of trimethylolethane (entry 6), THF was effective to en-
able the acetalization of 1 in 86% yield (entry 7) (Table 1).
After the oxidation of 2b by Jones’ condition, carboxylic acid
(3b) with DIPEA in DMF was loaded on 2-chloro trityl chloride re-
sin to give 4. After the conventional Fmoc-solid phase peptide syn-
thesis of 4 using diisopropylcarbodiimide/1-hydroxybenzotriazole
coupling and Fmoc deprotection by 20% piperidine/DMF, the treat-
ment of resin (5) with 95% TFA gave the desired peptide aldehyde,
Ac-Val-Leu-Ala-H (6) and acetal carboxylic acid (7) in 8% and 32%
yields, respectively. We found that the acetal carboxylic acid (7)
was smoothly converted to thioacetal (8) in 68% yield by treatment
with 10 equiv of EtSH and catalytic BF3–Et2O for several minutes at
room temperature. Subsequently, peptide aldehyde (6) was quickly
obtained from thioacetal (8) by NBS in 10% CH2Cl2 aq in 85% yield.
These results suggested that the cheap hexane-1,2,6-triol linker
compared favorably with Yao’s octane-1,2,10-triol linker and, fur-
thermore, it was possible to convert acetal to thioacetal quickly.
On the other hand, acetal (3c) oxidized from 2c was attempted
Scheme 1. Synthetic plan for peptide aldehyde via thioacetal.
After attempting several conditions, we optimized the condition
with 1.0 equiv of hexane-1,2,6-triol and 5 mol % BF3–Et2O in CH2-
Cl2 for 5 h to give 2b in 73% yield (entry 5). While the reaction mix-
Table 1
Investigation of acetal length
Entry
Triol linker (equiv)
Solvent
BF3–Et2O (mol %)
Time (h)
Alcohol (%)
1
2
3
4
5
6
7
Decane-1,2,10-triol (1.5)
Hexane-1,2,6-triol (1.5)
Hexane-1,2,6-triol (1.5)
Hexane-1,2,6-triol (1.0)
Hexane-1,2,6-triol (1.0)
Trimethylolethane (1.0)
Trimethylolethane (2.0)
CH2Cl2
CH2Cl2
THF
CH2Cl2
CH2Cl2
CH2Cl2
THF
10
10
10
100
5
3
3
25
2
5
6
2a (60)
2b (34)
2b (38)
2b (60)
2b (73)
2c (61)
2c (86)
100
5
18
Scheme 2. Synthesis of peptide aldehyde by direct acetal/thioacetal/aldehyde transformation.

4850
H. Konno et al. / Tetrahedron Letters 54 (2013) 4848–4850
Scheme 3. One-pot synthesis of peptide aldehyde (6).
Table 2
Sequences of synthetic peptide aldehydes and their chemical yields.
Entry
Thioactal (%)
Aldehyde (%)
1
2
3
4
5
6
Ac-Thr-Val-Phe(Hexahydro)-His-(SEt)2 (9)
Ac-Phe-Leu-Ala-(SEt)2 (10)
Ac-Ala-Val-Leu-Leu-(SEt)2 (11)
Ac-Leu-Ala-Phe-(SEt)2 (12)
Ac-Leu-Phe-Ser-(SEt)2 (13)
p-HBz-Val-Val-Arg-(SEt)2 (14)
(31)
(4)
(7)
(27)
(12)
(12)
Ac-Thr-Val-Phe(Hexahydro)-His-H (15)
Ac-Phe-Leu-Ala-H (16)
Ac-Ala-Val-Leu-Leu-H (17)
Ac-Leu-Ala-Phe-H (18)
Ac-Leu-Phe-Ser-H (19)
p-HBz-Val-Val-H (20)
(40)
(21)
(48)
(44)
(57)
(23)
to give 6 using the above protocol, which had similar results, giving
the corresponding peptide aldehyde (6) in poor yield. TFA-medi-
ated cleavage process from the resin with 3c afforded the trace
developed. Peptide acetals were efficiently converted to thioacetal
structures followed by treatment of NBS to give peptide C-terminal
aldehydes. Although it is difficult to apply this procedure to Trp/
Cys-containing peptides, general scope and limitations using sev-
eral amino acids are now underway.
amount of
6 with the corresponding acetal carboxylic acid
(ꢀ10%). Although each reaction via thiacetal (8) for the desired 6
proceeded to give the corresponding compounds, conversion of
acetal to thioacetal with EtSH and catalytic BF3–Et2O was extre-
mely slow because of the steric hindrance of acetal composed of
trimethylolethane. For this reason, we selected the hexane-1,2,6-
triol linker for the synthesis of other sequences (Scheme 2).
Although treatment of resin (5) with EtSH and catalytic BF3–
Et2O followed by the addition of NBS as a one-pot reaction afforded
Ac-Val-Leu-Ala-H (6) in 12% overall yield, the yields markedly de-
pended on the nature of the sequence, especially C-terminal amino
acid. The one-pot reaction using His, Arg, and b-(2-Thienyl)Ala at
the C-terminal position was unsuccessful in detecting the desired
products. We found that stepwise conversion was effective to ob-
tain the designed peptide aldehyde using isolated thioacetal
(Scheme 3).
This stepwise method was tested by synthesizing selected pep-
tide aldehydes (15)–(20). The yields of thioacetals (10)–(14) were
the overall yield from loading on resin, and conversion of acetal
to thioacetal proceeded smoothly with simultaneous deprotection
of side chains of the corresponding amino acid residues. Isolation
yield of Ac-Thr-Val-Phe(Hexahydro)-His-(OEt)2 (9) prepared using
Yao’s linker was 31% overall yield (entry 1).20 Peptide thioacetals
(10)–(14) were synthesized in moderate yield (entries 2–6). As a fi-
nal step, thioacetals were treated with NBS in 10% CH2Cl2 aq to af-
ford the desired peptide aldehydes within a few minutes and
Acknowledgments
This work was supported, in part, by a Grant-in-Aid for Scien-
tific Research 21689004 to H.K. from the Japan Society for the Pro-
motion of Science.
References and notes
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immediately the products caused some epimerization of the
a-
bearing the aldehyde and then decomposed. Therefore, it was
important to quench the reaction mixture quickly to purify it by
silica gel column chromatography or RP-HPLC. In order to check
chiral integrity, the peptide aldehydes were analyzed by 1H NMR
and the aldehyde appearing at the neighboring d 9.6 ppm was as-
sessed. In addition, we attempted the synthesis of tokaramide A
(20) using this methodology. Although aldehyde formation from
thioacetal (14) was moderate, it afforded tokaramide A (20) as a
cyclic structure (entry 6) (Table 2).18
20. Akaji, K.; Konno, H.; Mitsui, H.; Teruya, K.; Hattori, Y.; Ozaki, T.; Kusunoki, M.;
Sanjho, A. J. Med. Chem. 2011, 54, 7962–7973.
In conclusion, a very simple and cheap alkyl triol linker for
attaching Fmoc-aminals to solid phase peptide synthesis was