Menthols as Chiral Auxiliaries for Asymmetric Cycloadditive Oligomerization: Syntheses and Studies of β-Proline Hexamers

Article abstract of DOI:10.1021/acs.orglett.5b03154

To produce a novel class of structurally ordered poly-β-prolines, an emergent method for synthesizing chiral β-peptide molecular frameworks was developed based on 1,3-dipolar cycloaddition chemistry of azomethine ylides. Functionalized short β-peptides wi

Full text of DOI:10.1021/acs.orglett.5b03154

Letter
pubs.acs.org/OrgLett
Menthols as Chiral Auxiliaries for Asymmetric Cycloadditive
Oligomerization: Syntheses and Studies of β‑Proline Hexamers
,
†,‡
†
§
∥
Konstantin V. Kudryavtsev,* Polina M. Ivantcova, Claudia Muhle-Goll, Andrei V. Churakov,
†
⊥
⊥
#
Mikhail N. Sokolov, Artem V. Dyuba, Alexander M. Arutyunyan, Judith A. K. Howard,
∇
∇
†,‡
,§,○
̈
Chia-Chun Yu, Jih-Hwa Guh, Nikolay S. Zefirov, and Stefan Brase*
Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory 1/3, Moscow 119991, Russian Federation
‡Institute of Physiologically Active Compounds, Russian Academy of Sciences, Chernogolovka, Moscow Region 142432, Russian
Federation
†
§
Institute of Organic Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 6, Karlsruhe 76131, Germany
∥
Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii prosp. 31, Moscow 119991, Russian
Federation
⊥
A.N. Belozersky Institute of Physico-Chemical Biology, M.V. Lomonosov Moscow State University, Leninskie Gory 1/40, Moscow
1
19991, Russian Federation
#
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, U.K.
∇
School of Pharmacy, National Taiwan University, Linsen S. Rd. 33, Taipei 100, Taiwan
○
Institute of Toxicology and Genetics, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1,
Eggenstein-Leopoldshafen 76344, Germany
*
S Supporting Information
ABSTRACT: To produce a novel class of structurally ordered
poly-β-prolines, an emergent method for synthesizing chiral β-
peptide molecular frameworks was developed based on 1,3-dipolar
cycloaddition chemistry of azomethine ylides. Functionalized short
β-peptides with up to six monomeric residues were efficiently
synthesized in homochiral forms using a cycloadditive oligome-
rization approach. X-ray, NMR, and CD structural analyses of the
novel β-peptides revealed secondary structure features that were
generated primarily by Z/E-β-peptide bond isomerism. Anticancer
in cellulo activity of the new β-peptides toward hormone-refractory
prostate cancer cells was observed and was dependent on the absolute configuration of the stereogenic centers and the chain
length of the β-proline oligomers.
5
reation of artificial oligomers mimicking structure and
functions of natural peptides is an emerging area in
tion method differs conceptually from traditional protecting-
1
,3
C
group-use (PGU) β-peptide synthesis methods and is based on
biomedicinal and material sciences. One of the most efficient
strategies toward the functional unnatural oligomeric architec-
tures and scaffolds involves addition of a methylene unit between
carboxylic and amino functions of the peptide backbone that
a 1,3-dipolar cycloaddition of azomethine ylide and acrylamide
components. Despite that various chiral auxiliaries including
6
7
8
9
menthols, lactates, and optically active amines were
successfully used for diastereoselective synthesis of monomeric
pyrrolidine products, it is us who applied acrylamides, derived
1
results in a diverse and large family of β-peptides. Proline-rich
regions in native proteins are characterized by the absence of
fixed patterns of intra- or interchain hydrogen bonds that makes
corresponding protein regions more flexible and favors protein−
protein interactions. Few types of β-proline oligomers have
been developed, and different folding control factors have been
established for this specific class of β-peptides. Previously we
have developed a protecting-group-free (PGF) synthesis of
short β-peptides consisting of pyrrolidine-3-carboxylic acid (3-
PCA) or β-proline units that obtained well-defined, highly
ordered functionalized oligomers in both racemic and
10
from enantiomerically pure 3-PCA esters, as dipolarophiles for
the construction of chiral poly-β-proline framework. Moreover,
we observed that these unusual β-peptides influenced the
5
5
2
progression of tumor cell cycles and revealed potent anticancer
11
3
activity in PC-3 cell tests. In the present study, using L-menthol
and D-menthol as an accessible chiral pool for asymmetrical
induction, we synthesized for the first time both mirror sets of
4
Received: November 3, 2015
5
enantiopure forms. The developed cycloadditive oligomeriza-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03154
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
well-defined 3-PCA-oligomers with up to six pyrrolidine units
and studied their structural and biological properties.
alternating stereocenter configurations of adjacent pyrrolidine5
units with each new oligomeric chain enlargement reaction.
Consequent acryloylation and cycloaddition procedures led to
alternating oligomers L-(4−6) and D-(4−6) of a−c and a series,
respectively (Scheme 2). It should be noted that in each case we
Four series of trimeric compounds with various substituents
1
2
3
were synthesized from L-menthol (a: R = CH , R = R = R = H.
3
1
2
3
1
2
3
b: R = CH , R = R = R = 4-Br. c: R = t-Bu, R = R = R = H. d:
R = CH , R = 3-Cl, R = 4-MeO; R = H), and a and d sets of
3
1
2
3
3
trimers were synthesized from D-menthol (Scheme 1). The
Scheme 2. PGF Synthesis of Homochiral Higher 3-PCA
Oligomers
Scheme 1. PGF Synthesis of Homochiral 3-PCA Trimers (for
the Example of the L-Menthol Set); the D-3 Trimers Were
Synthesized by Analogous Transformations Starting from D-
Menthol
registered with NMR and chromatography methods only one
product of the cycloaddition step indicating an excellent
asymmetric induction of acrylamides L-3_A(D-3_A), L-
4
_A(D-4_A), and L-5_A(D-5_A) with 12, 15, and 18 well-
defined stereogenic centers, respectively. The absolute config-
uration of tetramer D-4a was confirmed by single crystal X-ray
analysis (Scheme 2, Supporting Information, Table S1, Figures
S4−S8). Two types of crystals grown from MeOH and EtOAc
were obtained for compound D-4a, representing a rare
starting monomers of L-1 and D-1 series were synthesized from
7a
L- and D-menthol, respectively, as reported by Grigg. The
absolute configurations of the L-menthol-derived pyrrolidine
monomers were determined by X-ray in the original Grigg’s
12
12
13
7
a
example of pseudo-isomorphism.
study, and we used these data to make structural assignments
and establish the homochiral integrity of the L-1 and D-1
compounds. Acryloylation transformed the nonracemic, diaster-
eomerically pure 5-arylpyrrolidine-2,4-dicarboxylates L-1 and D-
The secondary structures of tetramer D-4a, pentamer D-5a,
and hexamer D-6a in DMSO-d solutions were studied by NMR.
6
The configurations of the β-peptide bonds were determined by
α
β
analyzing the NOE-resonance H (i)/H (i − 1) distinguishing
1
to the chiral acrylamides L-1_A and D-1_A (Scheme 1). The
δ
for a Z- or cis-peptide bond and the NOE-resonance H (i + 1)/
last dipolarophiles contained complex chiral auxiliaries consisting
of the chiral all-cis-trisubstituted pyrrolidine moiety and adjusted
chiral menthol fragment that differs them from enantiopure 4-
tert-butyl 2-methyl N-acryloyl-5-arylpyrrolidine-2,4-dicarboxy-
β
H (i) distinguishing for an E- or trans-peptide bond (Figure
5,11
1
1
).
The H NMR signals of the dominant conformers of
5
,11
late with only three chiral centers in the initial research.
Fortunately, six stereogenic centers of olefins L-1_A and D-1_A
took concerted action, and their reactions with iminoesters
2
R PhCHNCH COOR in the presence of silver(I) acetate as a
2
Lewis acid for ylide generation led to the regio- and
stereospecific
5
,11
formation of the second pyrrolidine ring
(Scheme 1). Dimers L-2 and D-2 were isolated as individual
diastereomers in homochiral form and characterized by NMR.
Additional step of acryloylation of dimers L-2 and D-2 provided
dipolarophiles L-2_A and D-2_A with nine stereogenic centers,
and subsequent asymmetric cycloadditions led to alternating
trimers L-3 and D-3. The absolute configurations of compounds
D-2d and L-3a were confirmed by single crystal X-ray analysis
Figure 1. NOE interactions (green arrows) distinguishing between E-β-
and Z-β-peptide bonds (left). An n → π* interaction (red arrow, left)
stabilizing the Z-β-peptide bond configuration and its structural
parameters (right).
(
Scheme 1, Supporting Information, Table S1, Figures S1−
12
S3). These data also supported the previously observed
B
DOI: 10.1021/acs.orglett.5b03154
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Conformational Parameters of the L-n and D-n Oligomers
ε
δ
i
Oi‑1
C i
OMe
Oi
C
d (Å)
θ (deg)
Δ (Å)
Θ (deg)
L-3a (CCDC 981128)
ε
δ
O1
O2
C
OMe
OMe
O2
O3
C
C
2.656(2)
2.796(2)
98.09(13)
94.62(13)
0.034(2)
0.031(2)
4.0
3.7
2
2
ε
3
δ
3
C
D-4a (CCDC 981129)
ε
ε
δ
O1
O2
C
C
OMe
OMe
O2
O3
C
C
2.775(8)
2.684(7)
110.9(5)
99.3(4)
0.048(7)
0.021(7)
5.8
2.6
2
3
2
δ
3
a
From X-ray diffraction analysis of the crystalline compounds. Parameters are defined in Figure 1.
oligomers D-4a, D-5a, and D-6a were assigned unambiguously
using characteristic correlations observed in TOCSY and
NOESY or ROESY experiments (Table S2). Compound D-4a
and a negative Cotton effect at 198−201 nm were observed for
the dimer, tetramer, and hexamer of the D-na set series (Figure
2a). The corresponding magnitudes for the trimer and pentamer
of the D-na set series were shifted to 193−195 nm for the strong
positive Cotton effect, 205−214 nm for the first zero-cross and
224−225 nm for the second one, and 217−220 nm for the
negative absorption (Figure 2b). Odd D-na oligomers exhibited
additional strong negative absorption at 183−185 nm and zero-
cross at 186−190 nm (Figure 2b). The band positions and
intensities in the CD spectra of the L-na oligomers were inverted
relative to those described for the D-na compounds (Figure
2c,d). Another extraordinary feature of the studied oligomers was
the monotonic decrease observed in the CD signal intensities
when normalizing by the number of monomeric residues (Figure
S10). This decrease was not observed in the CD spectra for the
preferentially exists as a Z-Z-Z conformer in DMSO-d solution,
6
in contrast to its solid-state conformation. The Z-Z-E-Z
conformer of pentamer D-5a is more prevalent in DMSO-d6
solution. Hexamer D-6a has a strong tendency to adopt the Z-Z-
E-Z-Z configurations of the peptide bonds in both DMSO-d and
6
1
CD Cl solutions. Under elevated temperatures, the H signals of
2
2
the D-6a β-peptide backbone shift to high field, but the Z-Z-E-Z-
Z conformer dominates below 90 °C (Figure S9).
The strong preference for the Z-configuration of β-amide
bonds in the L-n and D-n oligomers may originate from the
interpenetration by the filled lone pair (n) of the carbonyl oxygen
ε
atoms of the β-amide group of the empty π* orbitals of C of the
3
methoxycarbonyl groups (Figure 1). Similar n → π* interactions
reported β-proline oligomers and is apparently attributable to
contribute up to 0.7 kcal/mol of stabilizing energy per
the mirror heterogeneity of the adjacent monomeric units. The
CD signals of the L-na and D-na oligomers were characterized by
increased intensities relative to the L-1a and D-1a monomers
(Figure S11) as well as different signs and bands positions,
indicating that secondary structure transition dipoles primarily
contribute to the CD absorbance of the oligomers. The CD
spectra of the D-na oligomers were fully matched to those of the
1
4
occurrence. Experimental evidence of n → π* interactions
14
was obtained from the short contacts (d) and pyramidalization
ε
14
of the C acceptors toward the O donors (Δ, Θ), which were
i‑1
detected by X-ray diffraction (Figure 1, Table 1). For all Z-
peptide fragments in the trimer L-3a (Z-Z) and the tetramer D-
4
a (Z-Z-E), the d-distances were substantially shorter than 3.22
Å, which is the sum of the van der Waals radii of oxygen and
5
original chiral 3-PCA oligomers, indicating prevailing influence
carbon (Table 1). The distortion parameter Θ, which represents
of the poly-β-proline backbone secondary structure on observed
Cotton effects and diastereomeric purity of menthol oligomers
14
strong evidence of an attractive n → π* interaction, amounts to
a substantial 2.6−5.8 degrees (Table 1).
(
Figure S12). The conformational stability of the alternating β-
Well-separated Cotton effect signals of the absorption peaks
were observed in circular dichroism (CD) experiments for the L-
na and D-na oligomers sets (Figure 2). Distinct differences were
observed between oligomers containing even numbers (Figure
peptide backbone under elevated temperatures observed by
NMR was confirmed by the stability of the pentamer D-5a CD
curves up to 80 °C (Figure S13). However, the L-nc oligomers,
which possess peripheral tert-butyl substituents, exhibited
different CD absorbance values in the 190−220 nm region
2a,c) or odd numbers (Figure 2b,d) of 3-PCA subunits. Strong
positive absorption at 187−190 nm, zero-cross at 193−196 nm,
(
Figures S12 and S14).
Chemotherapeutic treatment of hormone-refractory prostate
cancer (HRPC) was of little value until the emergence of novel
15
agents that target crucial tumor growth pathways. We evaluated
the effects of the synthesized homochiral 3-PCA oligomers on
the proliferation of the HRPC cell line PC-3. In our previous
work, where only one enantiomeric set of corresponding
oligomers was available, racemic oligomeric acrylamides were
11
the most active toward proliferation of PC-3 cells. Anti-
proliferative activity of several L-n and D-n compounds at low
micromolar concentrations was observed with the trimer D-3a
(
GI 4.4 μM) as the most active one (Table S3). In addition to
50
measuring growth inhibition (GI ), we determined the PC-3 cell
50
cycle phase in which progress stopped (Table S3). Whether the
absolute configuration of the 3-PCA oligomeric backbone
determines antiproliferative activity remains unclear; we noticed
that menthol residue at the C-end of β-peptides benefited the
11
Figure 2. CD spectra of D-na and L-na oligomers in acetonitrile
normalized to the compound concentration.
activity: the trimer (+)-3c with the backbone identical to the
trimer D-3a was inactive against PC-3 survival.
C
DOI: 10.1021/acs.orglett.5b03154
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
To conclude, we have developed an asymmetric protecting-
group-free method for the efficient synthesis of complex, well-
defined β-proline oligomers utilizing the stereospecific cyclo-
addition of nonracemic homochiral acrylamides to azomethine
ylides. For the first time we obtained homochiral alternating
poly-β-prolines with up to six monomeric units in both
enantiomeric forms. Several members of this novel β-peptide
class display cell-cycle directed antiproliferative activity in HRPC
cells that requires further investigation.
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ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03154.
(11) Kudryavtsev, K. V.; Yu, C.-C.; Ivantcova, P. M.; Polshakov, V. I.;
Churakov, A. V.; Brase, S.; Zefirov, N. S.; Guh, J.-H. Chem. - Asian J.
̈
2
(
015, 10, 383−389.
Experimental procedures, NMR spectra of the synthesized
compounds, X-ray data, and biological testing details
12) See Supporting Information for details.
(13) (a) Centore, R.; Fusco, S.; Jazbinsek, M.; Capobianco, A.; Peluso,
(
PDF)
A. CrystEngComm 2013, 15, 3318−3325. (b) Vogt, F. G.; Copley, R. C.
B.; Mueller, R. L.; Spoors, G. P.; Cacchio, T. N.; Carlton, R. A.;
Katrincic, L. M.; Kennady, J. M.; Parsons, S.; Chetina, O. V. Cryst.
Growth Des. 2010, 10, 2713−2733.
AUTHOR INFORMATION
Corresponding Authors
■
(14) (a) Newberry, R. W.; VanVeller, B.; Guzei, I. A.; Raines, R. T. J.
*
E-mail: kudr@med.chem.msu.ru (K.V.K.).
E-mail: stefan.braese@kit.edu (S.B.).
Am. Chem. Soc. 2013, 135, 7843−7846. (b) Newberry, R. W.; Raines, R.
*
T. ACS Chem. Biol. 2014, 9, 880−883.
Author Contributions
(15) (a) Persad, R. A.; Bahl, A. Br. J. Med. Surg. Urol. 2009, 2, 92−99.
b) Seng, S. M.; Tsao, C.-K.; Galsky, M. D.; Oh, W. K. Drug Discovery
(
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
Today: Ther. Strategies 2010, 7, 17−22.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The Russian Foundation for Basic Research (project Nos. 12-03-
2005-NNS_a, 14-03-00963-a), RAS Programme OXHM-3
■
9
“
Macromolecular compounds”, the National Science Council
of the Republic of China (NSC101-2923-B-002-008-MY3), and
the Ministry of Science and Art in Baden-Wurttemberg (ZO IV)
̈
are acknowledged for providing generous funding and support.
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DOI: 10.1021/acs.orglett.5b03154
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