Site-Selective Esterifications of Polyol β-Hydroxyamides and Applications to Serine-Selective Glycopeptide Modifications

Article abstract of DOI:10.1021/acs.orglett.9b02809

The site-selective acylations of β-hydroxyamides in the presence of other hydroxyl groups are described. Central to the success of this modification is the metal-template-driven acylation using pyridine ketoxime esters as acylating reagents in combination

Full text of DOI:10.1021/acs.orglett.9b02809

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Site-Selective Esterifications of Polyol β‑Hydroxyamides and
Applications to Serine-Selective Glycopeptide Modifications
Kohei Takemoto, Yasuhiro Nishikawa,* Shohei Moriguchi, Yuna Hori, Yuki Kamezawa,
Takami Matsui, and Osamu Hara*
Faculty of Pharmacy, Meijo University, 150 Yagotoyama, Tempaku-ku, Nagoya, Aichi 468-8503, Japan
S
* Supporting Information
ABSTRACT: The site-selective acylations of β-hydroxyamides in the presence of other hydroxyl groups are described. Central
to the success of this modification is the metal-template-driven acylation using pyridine ketoxime esters as acylating reagents in
combination with CuOTf. This strategy enables β-hydroxyl groups to be site-selectively acylated in various derivatives, including
sterically hindered secondary β-alcohol. The utility of this methodology is showcased by the serine-selective modification of a
glycopeptide with unprotected sugar.
Scheme 1. Site-Selective Acylations by a Metal Template
Strategy
eptide-based drugs have attracted significant attention
Pbecause they exhibit therapeutic specificities and efficacies
similar to those of protein biologics.1 Recent representative
examples of peptide-based drugs are the glycopeptides used for
mucin antitumor vaccines, which induce specific immune
responses by inhibiting protein-binding events.2 To further
advance research into glycopeptide-based drugs, the develop-
ment of useful chemical modification methods that impart new
functionalities onto existing glycopeptides is essential. The
hydroxyl groups of the side chains of serine and threonine can be
used as targets when modifying peptides.3−5 In the case of
glycopeptides in which the attached sugars play important roles
in peptide/cell interactions, the selective functionalization of the
hydroxyl groups of Ser or Thr residues in the presence of the
sugar hydroxyl groups is problematic. Therefore, the develop-
ment of novel site-selective modification reactions for the Ser or
Thr residues in unprotected glycopeptides is highly important
for the development of glycopeptides as pharmaceuticals. We
also reported a site-selective acylation of the α-hydroxyamides in
polyols using pyridine aldoxime esters as acylating reagents
(Scheme 1a).6−10 In this system, a catalytic amount of
Zn(OTf)2 was used to moderately activate the pyridine
aldoxime ester. The resulting metal complex attracted an α-
hydroxyamide to its Lewis basic site, resulting in the formation
of a metal template for the facile acylation of α-hydroxyamide in
preference to other alcohols.11 Based on this precedent, the
application of this method for the selective acylation of the
hydroxyl group at the β-carbon to an amide carbonyl group was
pursued.12 The target hydroxyl groups are very attractive for
chemical modification as they are equivalent to the hydroxyl
groups on the side chains of Ser and Thr. Compared to α-
hydroxyamides that favor the formation of five-membered metal
complexes, β-hydroxyamides require the formation of the more
flexible six-membered ring with a Lewis acid, which results in the
slower formation of the metal template. Furthermore, the
differences in the steric and electronic environments of the
target hydroxyl group in a β-hydroxyamide and its competitors
Received: August 8, 2019
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.9b02809
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
are smaller than those of an α-hydroxyamide, which is
problematic from the perspective of distinguishing the target
from the other potential reactants. Herein, we report the
regioselective acylations of polyols with various β-hydroxyamide
structures, including the serine-selective modification of a
glycopeptide (Scheme 1b).
To optimize the site-selective acylation reaction conditions, β-
hydroxy-(N-ethyl-2-hydroxy)propionamide (1a) was selected
as a model substrate (Table 1). Initially, the acylation of 1a was
proceed in the absence of CuOTf·1/2 toluene (entry 3). Various
metal salts, such as Cu(OTf)2 and Zn(OTf)2, were next
examined, which revealed that CuOTf·1/2 toluene was more
efficient than the other catalysts (entries 4 and 5). The reaction
afforded lower yields when the reaction time was reduced to 3 h
(entry 6). Notably, the use of methyl 2-pyridyl ketoxime
provided higher yields and site-selectivities than the correspond-
ing 3-pyridyl or 4-pyridyl derivatives, indicating that the
bidentate nature of 2a was crucial in this reaction (entries 7
and 8). The higher reactivity of 2a compared to that of 2-pyridyl
aldoxime (2a-H) is attributable to the methyl group in 2a, which
facilitates the approach of the acyl group to the target hydroxyl
group in the transition state due to steric repulsion, thereby
enabling acylation (entry 9). No significant improvement in
yield was observed when the concentration of 1a was increased
(entry 10). The screening of various coordinating solvents
revealed that DMA was most favorable for this reaction (entries
11−13). The addition of water reduced the yield significantly
(entry 14), and the yield and selectivity did not change
significantly when a larger equivalence of 2a was used (entry 15).
In turn, conventional conditions using the corresponding acyl
chloride in the presence of pyridine afforded 3aA, 3aB, and
3aAB in comparable yields in a nonselective reaction, high-
lighting that the reactivities of two hydroxyl groups in 2a were
intrinsically equal but could be controlled in a desirable fashion
in our system (entry 16).
a
Table 1. Optimization Studies
With the optimized reaction conditions in hand, we next
examined the scope of the reaction (Scheme 2). β-Hydrox-
yamides bearing single hydroxyl groups on alkyl chains of
different length provided the corresponding monoacylated
products 3aA−3cA in good yields and excellent regioselectiv-
ities. The reaction of 1d, bearing a methyl group at the nitrogen
of the amide, afforded product 3da in 64% yield. Various amino
acid derivatives (1e−1h) were compatible with this reaction,
with the corresponding monoacylated products 3eA−3hA
obtained in 65%−78% yields. These results show that the
phenolic hydroxyl group in 1h and the β-hydroxyester in 1g are
sufficiently less reactive in this system compared to the β-
hydroxyamides (products 3gA and 3hA). Surprisingly, using our
strategy, sterically hindered secondary alcohols could be
acylated in preference to primary alcohols with much higher
reactivities to give 3iA and 3jA. Furthermore, the selective
monoacylations of Cbz-serine and Cbz-threonine derivatives 1k
and 1l bearing bulky substituents at their α-positions were
selectively acylated; the β-hydroxyl groups reacted selectively
even in sterically hindered environments to afford the desired
monoacylated products 3kA and 3lA. Likewise, our method
could be applied to dipeptide 1m to provide the monoacylated
product 3 mA with the hydroxyl group at the C-terminal intact.
Encouraged by these results, we decided to further explore the
pyridine ketoxime ester substrate range (Scheme 3). Benzoyl
and acetyl groups were also selectively introduced onto the β-
hydroxyl group in amide 1b, to afford products 4aA and 5aA in
high yields, respectively. Acylation with the bulky pivaloyl ester
donor 2d gave product 6aA, although in relatively low yield. The
reaction proceeded smoothly even when an acyl donor bearing a
terminal C−C double bond or the nucleophilic C2−C3 double
bond of indole was present in the molecule, to give 7aA and 8aA
as the target products. Subsequently, the introduction of amino
acid derivatives onto a target hydroxyl group was examined using
this method. When the reaction was carried out using acyl
donors 2g−2k derived from amino acids, the desired
monoacylated products 9aA−13aA were successfully obtained
b
yield (%)
entry
variations from standard conditions
3aA 3aAB 3aB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
none
86
53
0
5
0
0
5
24
0
10
17
0
5
10
8
-
-
-
-
-
-
8
11
0
-
-
-
-
-
-
15
CuOTf·1/2 toluene (20 mol %)
without CuOTf·1/2 toluene
Cu(OTf)2 instead of CuOTf·1/2 toluene
Zn(OTf)2 instead of CuOTf·1/2 toluene
3 h reaction time
iso-2a instead of 2a
iso-2a′ instead of 2a
2a-H instead of 2a
[1a] = 0.4 M instead of 0.2 M
DME as the solvent
CH3CN as the solvent
DMF as the solvent
10% H2O/DMA as the solvent
2a (2 equiv)
44
39
50
34
38
50
76
42
69
69
26
68
19
8
3
6
18
conventional conditions: acyl chloride
(1.0 equiv), pyridine (1.5 equiv), and DMA as
solvent
a
Standard reaction conditions: a mixture of 1a (0.2 mmol), 2a (0.24
mmol), and CuOTf·1/2 toluene (0.1 mmol) in N,N-dimethylaceta-
b
mide (DMA) was stirred at 25 °C for 18 h. Determined by 1H NMR
spectroscopy of the crude products.
attempted using the best conditions for the selective
monoacylation of an α-hydroxyamide, which resulted in
insufficient yield and selectivity for the desired product (3aA:
44%, 3aB + 3aAB: 7%).13 After considerable efforts, excellent
yield and good regioselectivity were obtained with CuOTf·1/2
toluene as the Lewis acid catalyst and 2-pyridyl ketoxime ester
(2a) in N,N-dimethylacetamide (entry 1). This reaction
provided 3aA in only 53% yield when performed using 20 mol
% of CuOTf·1/2 toluene (entry 2), and the reaction did not
B
DOI: 10.1021/acs.orglett.9b02809
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Scheme 2. β-Hydroxyamide Substrate Range for Site-
Selective Acylations
Scheme 3. Pyridine Ketoxime Ester Substrate Range for Site-
Selective Acylations
a
a
a
Standard reaction conditions: A mixture of 1b (0.2 mmol), 2 (0.24
mmol), and CuOTf·1/2 toluene (0.1 mmol) in DMA was stirred at
25 °C for 16−20 h. Yields are for the isolated products.14
Scheme 4. Serine OH Selective Acylation of Glycopeptide 1n
a
Standard reaction conditions: A mixture of 1 (0.2 mmol), 2a (0.24
mmol), and CuOTf·1/2 toluene (0.1 mmol) in DMA was stirred at
25 °C for 16−20 h. Yields are for the isolated products.14 bThe
reaction was performed at 40 °C.
in 64%−91% yield, without affecting the optical purity.
Furthermore, the introduction of a dipeptide into 1b was
examined. The C-terminus of a peptide is susceptible to
racemization through the formation of oxazolones during the
activation of the terminal carboxylic acid. Fortunately, no
racemization occurred during the preparation of oxime ester 2l,
and subsequent regioselective reaction with 1a afforded
dipeptide ester 14aA with 99% ee.
To demonstrate the applicability of our methodology to the
regioselective modifications of glycopeptides, glycopeptide 1n
was prepared as a model substrate bearing four competitive
hydroxyl groups for acylation.13 The β-O-glucosylated serine
residue in a peptide, adopted in 1n, is a biologically important
structure found in the epidermal growth factor domains and
notch receptors of various serum proteins.15 The optimized
conditions in our system feature the use of highly polar DMA as
the solvent, which is beneficial for dissolving hydrophilic
compounds such as 1n, thereby avoiding solubility issues in
these reactions (Scheme 4). The acylation of 1n under these
slightly modified conditions successfully afforded a mixture of
products. The reaction solution was analyzed by LC-MS to
ensure that all acylated products could be detected. The
extracted ion chromatogram (XIC) for the monoacylated
product ([M + Na]+ m/z 714.2481) revealed the presence of
three regioisomers (Figure 1). Subsequent preparative HPLC
followed by 2D NMR spectroscopy enabled the three
constituents to be assigned as 3n-3, 3n-6, and 3nA.13 The
Figure 1. Extracted ion chromatogram (XIC) of monoacylated
products ([M + Na]+ m/z 714.2481).
C
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relative proportions of these compounds were directly obtained
from the relative HPLC peak areas at λ = 210 nm (3n-3:3n-
6:3nA = 16:7:77; Figure 2, top). The selectivity for the serine-
ACKNOWLEDGMENTS
■
This work was partially supported by JSPS KAKENHI Grant
Number JP19K07007 (Y.N.) for Scientific Research (C). Y.N.
thanks Meijo University for financial support.
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Corresponding Authors
*E-mail: yasuhiro@meijo-u.ac.jp.
*E-mail: oshara@meijo-u.ac.jp.
ORCID
Yasuhiro Nishikawa: 0000-0003-1606-7288
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.9b02809
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