Diversity oriented synthesis of pyran based polyfunctional stereogenic macrocyles and their conformational studies

Article abstract of DOI:10.1021/ol3022275

A new approach to synthesize a homologous series of 14-, 15-, and 16-membered drug-like, macrocyclic glycoconjugates involving TBAHS promoted azide-propenone intramolecular cycloaddition in designed C-glycopyranosyl butenones from a simple sugar D-glucose and D-mannose is reported.

Full text of DOI:10.1021/ol3022275

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Diversity Oriented Synthesis of Pyran
Based Polyfunctional Stereogenic
Macrocyles and Their Conformational
Studies
Arya Ajay,† Shrikant Sharma,‡ Munna Prasad Gupt,† Vikas Bajpai,‡ Hamidullah,§
Brijesh Kumar,‡ Mahabir Prasad Kaushik, Rituraj Konwar,§ Ravi Sankar Ampapathi,*,‡
and Rama Pati Tripathi*,†
Medicinal and Process Chemistry Division, Sophisticated Analytical Instrumentation
Facility, Endocrinology Division, CSIR-Central Drug Research Institute,
Lucknow-226001, U.P., India, and Process Technology Division, Defence and Research
Development Establishment, Jhansi Road, Gwalior-474011, M. P., India
ravi_sa@cdri.res.in; rpt.cdri@gmail.com
Received April 26, 2012
ABSTRACT
A new approach to synthesize a homologous series of 14-, 15-, and 16-membered drug-like, macrocyclic glycoconjugates involving TBAHS promoted
azide-propenone intramolecular cycloaddition in designed C-glycopyranosyl butenones from a simple sugar D-glucose and D-mannose is reported.
Novel drug design and development relies on increased
structural complexity, stereogenicity, and rigidity in mole-
cules.1 Macrocycles with chiral center(s) are often used as
key molecules to address challenging problems such as
proteinꢀprotein interfaces.2 These structures are common
in many biologically active natural products and synthetic
compounds with important pharmacological properties.3
The unique feature of these macrocycles originates from
their structural complexity,1 rigidity,4 and ability to form
stable, intramolecular H-bonds in solution.5 In creating
molecular diversity and complexity in terms of chirality and
functionality, carbohydrates have played a significant role
in medicinal chemistry. In this context several leading groups
have reported the synthesis of carbohydrate derived macro-
cyles with interesting biological activity.6 Rademann’s group
has reported an elegant synthesis of triazolyl cyclopep-
tides as privileged protein binders using dipolar cyclo-
addition.7 Recently we have also reported 13-member cyclic
tetrapeptides containing C-linked carbo-β-amino acids
with preorganized conformations.8 However, identification
† Medicinal and Process Chemistry Division, CSIR-Central Drug Re-
search Institute.
‡ Sophisticated Analytical Instrumentation Facility, CSIR-Central Drug
Research Institute.
§ Endocrinology Division, CSIR-Central Drug Research Institute.
Defence and Research Development Establishment.
(5) Rezai, T.; Yu, B.; Millhauser, G. L.; Jacobsen, M. P.; Lokey, R. S.
J. Am. Chem. Soc. 2006, 128, 2510.
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U.S.A. 2004, 101, 15591.
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Rev. Drug Discovery 2008, 7, 608. (b) Wessjohann, L. A.; Ruijter, E.;
Garcia-Rivera, D.; Brandt, W. Mol. Diversity 2005, 9, 171. (c) Butler,
M. S. Nat. Prod. Rep. 2005, 22, 162.
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r
10.1021/ol3022275
XXXX American Chemical Society

reactors19 have been used. These are mainly associated
with the pseudodilution effect of heterogeneous catalysis.
Recently, Hein et al.20 reported a Cu-catalyzed, regiocon-
trolled synthesis of 5-iodo-1,2,3-triazole systems and used
it as an intermediate in macrocyclization, while James
et al.21 synthesized the same intermediate using their flow
reactor techniques toaccess a libraryof 1,4,5-trisubstituted
macrocyclic triazoles via Pd-assisted coupling protocols.
Marcaurelle et al.22 reported the synthesis of 1,5-disubsti-
tuted triazoles by Ru catalysis. Several other methods for
macrocylization using dipolar cycloaddition reactions
were also reported.23 Our protocol for intramolecular
macrocylization is devoid of dilution techniques or hetero-
geneous catalysis. Moreover, we have synthesized both
1,4-di- and 1,4,5-trisubstituted triazoles within the same
molecular framework without any postmodification in the
preorganized structure.
Our synthetic strategy for structurally unique and di-
verse macrocyclic glycoconjugates is based upon three
different scaffolds: (i) a polyfunctional pyran skeleton
derived from readily available D-glucose and D-mannose,
(ii) a 1,4-disubstituted triazole, and (iii) a 1,4,5-trisubstituted
triazole. Stereochemical diversity in this collection is high
as every molecule has five stereogenic centers, unaffected
throughout the synthetic maneuver. Furthermore, the
structural diversity and complexity were installed by
altering the substituents in the phenyl ring and/or the
length of the tether between the two triazole moieties.
The sugar based pyran substrates were prepared from
readily available sugar, D-glucose (1) and D-mannose (2)
(Schemes 1, 3). The glycopyranosyl butenones (3aꢀc, 4a)
were obtained by reaction of preformed 1-C-propanonyl
β-D-glycopyranosides with different aromatic aldehydes in
good yields (65ꢀ74%).24 The latter (3aꢀc, 4a) on reaction
with tosyl chloride (TsCl) in presence of Et3N at 0 °C in
pyridine led to the formation of respective 60-O-(tosyl)-β-
D-C-glycopyranosyl aryl butenones chemoselectively,
which on further acetylation (Ac2O/pyridine) in the same
pot afforded the corresponding 60-O-(tosyl)-tri-O-acetyl-
β-D-C-glycopyranosyl aryl butenones (5aꢀc, 6a) in good
yields (78ꢀ83%). Tosylated derivatives (5aꢀc, 6a) were
reacted separately with sodium azide in DMF at 80 °C to
give the respective 60-azido-60-deoxy glycopyranosyl deriv-
atives (7aꢀc, 8a) in good yields (78ꢀ85%). At first, a Cu-
catalyzed alkyneꢀazide cycloaddition was used to selec-
tively combine the azide of the monosaccharide with
different alkynols, to generate the desired triazolyl bute-
nones (9a, 10a, and 11b) in very good yields (83ꢀ87%).
Figure 1. Pyran and triazole based biologically active macrocycles.
of nonpeptidic inhibitors are more significant than the
peptidyl inhbibitors in drug design and may be achieved
either by a natural substrate mimic or via iso-
steric replacement of the amide bond in the peptide back-
bone.9 The 1,2,3-triazole unit is a surrogate of the peptide
bond, and introduction of this motif offers improved stabi-
lity, lipophilicity, and absorption to the molecules.10
In view of the continuous efforts to develop new che-
motherapeutic agents from sugars,11 we were inspired by
the biologically active macrolides with pyran skeleton such
as rapamycin,12 bryostatin,13 kendomycin,14 spongistatine,15
and triazole containing clicktophycin-5216 (Figure 1) to
undertake the synthesis, conformational studies, and prelim-
inary biological activity of pyran based macrocycles as
possible anticancer agents.
In spite of their remarkable properties, synthesis of
macrocycles has serious limitations due to poor yields
during intramolecular cyclization.17 To alleviate these
problems, solid supported Cu catalysts18 and Cu tube flow
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Srivastava, A. K.; Tripathi, R. P. Bioorg. Med. Chem. Lett. 2009, 19,
2699. (d) Pandey, J.; Sharma, A.; Tiwari, V. K.; Dube, D.; Ramachandran,
R.; Chaturvedi, V.; Sinha, S. K.; Mishra, N. N.; Shukla, P. K.; Tripathi,
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Cheng, H.; Dyke, K. T.; Xu, Q. J. Am. Chem. Soc. 2011, 133, 14042.
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S. T.; Horton, D. A.; Campitelli, M. R.; Craik, D.; Scanlon, M.; Smythe,
M. L. Org. Lett. 2003, 5, 2711.
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Langer, R.; Tirrell, D. A. Nature 2004, 428, 487. (c) Mann, J. Nat. Prod.
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P. L. Biochem. Pharmacol. 1999, 58, 1067.
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Fokin, V. V. Angew. Chem., Int. Ed. 2009, 48, 8018.
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W. D. Aust. J. Chem. 2002, 55, 147. (c) Bisht, S. S.; Pandey, J.; Sharma,
A.; Tripathi, R. P. Carbohydr. Res. 2008, 343, 1399. (d) Wang, J. f.; Lei,
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B
Org. Lett., Vol. XX, No. XX, XXXX

Scheme 1. Synthesis of Intermediatea
Scheme 3. Synthesis of Macrocyclic Glycoconjugatesa
a Isolated yields. b Mannose derived macrocycles are not deacetylated.
a Isolated yields.
macrocyclization (entry 2, Table S1). Furthermore, the
yield of macrocylization is slightly time dependent as
shown in Table S1.
The tosylation of 9a, 10a, and 11b with tosyl chloride in
CH2Cl2 in the presence of Et3N resulted in the desired
compounds 12a, 13a, and 14b only in very poor yields
(5ꢀ20%) (Scheme 1).
From the HMBC spectra of 21b, 22b, and 23b the long-
range correlations between C1H,H0 with C3 and C7 with
C8H,H0 confirmed that 18b, 19b, and 20b undergo 1,4 intra-
molecular cycloaddition (Supporting Information (Figures
S8, S9)). A reactivity comparison among 18aꢀc for macro-
cyclization with H, OMe, and Cl as phenyl ring substitu-
ents in the glycosyl aryl butenones reveals that 18a is more
reactive and required less reaction time (Figure 2). The
same trend was followed in the case of 19aꢀc and 20aꢀc.
Yields of 14-membered macrocycles (21aꢀd) are lower as
compared to 16-membered macrocycles (23aꢀd), sug-
gesting that the linker size plays a role during macro-
cyclization and aromatic substituents are insignificant in
controlling the yields of the macrocyles. The maximum
yields of the macrocyles were achieved with the substrates
20aꢀd having a 4-carbon tether to give the macrocyles
23aꢀd (largest ring in the series) respectively in 69ꢀ75%
yields (Figure 2). A notable trend in the cyclization reac-
tion of the substrates 18aꢀd, 19aꢀd, and 20aꢀd is the
gradual increase of the ring-closure efficiency from 14- to
16-membered rings.
To overcome the above-mentioned low yield problem,
an alternative strategy was adopted, where the 60-azido-60-
deoxy glycopyranosyl derivatives (7aꢀc, 8a) were reacted
with the preformed alkynyl tosylates (15ꢀ17) (Scheme 2)
to get the respective tosyloxy triazoles (12aꢀd, 13aꢀd, and
14aꢀd) in very good yields (81ꢀ89%) (Scheme 3). These
were then reacted separately with sodium azide in DMF at
80 °C to afford the requisite substrate 60-(400-azidoalkyl,
triazolyl) β-D-glycopyranose derivatives (18aꢀd, 19aꢀd,
and 20aꢀd) in good yields (70ꢀ77%). With these azido
derivatives, we were ready to attempt the key 1,3-dipolar
cycloaddition to generate the 1,4,5-trisubstituted triazoles
with concomitant construction of the macrocyclic archi-
tecture. Finally, the tetrabutylammonium hydrogen sul-
fate (TBAHS) promoted intramolecular cycloaddition
of the azide and alkenone in the above-mentioned
bifunctional systems afforded the respective 1,4,5-tri-
substituted triazolyl macrocyclic compounds (21aꢀd,
22aꢀd, and 23aꢀd) in good yields (61ꢀ76%).25 The
yield of the intramolecular macrocyclization is margin-
ally dependent on the substrate concentration. At 0.34 M
substrate concentration, we observed the maximum
Conformational preferences of the synthesized macro-
cycles (21b, 22b, and 23b) were evaluated by extenisve
NMR studies in CDCl3 with a concentration of 5ꢀ10 mM
at 300 K. Analysis of the NMR parameters suggest that the
macrocyclic conformation remains stable and is similar for
the major portion of the three macrocycles. For instance,
3
3
3
3
0
0
0
0
0
0
0
0
JC1 HꢀC2 H
,
JC2 HꢀC3 H
,
JC3 HꢀC4 H, and JC4 HꢀC5 H
Scheme 2. Synthesis of Alkynyl Tosylates
values of ∼10 Hz and the NOEs between C10HTC30H,
C30HTC50H, and C20HTC40H (Figures S1ꢀS7) suggest
that the pyranose ring exists in a chair conformation in all
(25) Singh, N.; Pandey, S. S.; Tripathi, R. P. Carbohydr. Res. 2010,
345, 1641.
Org. Lett., Vol. XX, No. XX, XXXX
C

Figure 3. Restrained MD structures of macrocycles. 15 lowest-
energy structures superimposed of (A) 21b, (B) 22b, and (C) 23b
respectively.
in its orientation between 21b and 24b and are represented
as a superimposed in Figure S21.
The biological activity of a few macrocycles (21aꢀc,
23aꢀc, 25a) were evaluated against breast cancer (MCF-7
cell line), and compounds 21c, 22c, and 23c were found to
have good to moderate activity with IC50 values of 14.26,
11.22, and 23.15 μM respectively (SI Table S7).
Several key synthetic challenges were addressed in the
synthesis of these macrocycles with the proposed route.
The noteworthy observations include no epi/anomeriza-
tion to attain the maximum stereochemical diversity and
regioselectivity during 1,3-dipolar cycloaddition to give
1,4-di- and 1,4,5-trisubsituted triazoles by substrate mani-
pulation for each macrocycle.
The current approach for the synthesis of these poly-
functional macrocyles with three points of stereochemical
and scaffold diversity holds potential to access a library of
biologically active macrocyles and natural product ana-
logs. The presence of an isolated carbonyl group in these
macrocyles enhances the potential in synthetic organic
chemistry. The current molecules may allow a means for
tuning the biological profile of any “hits” or enable label-
ing of the compounds for use as chemical probes.
Figure 2. Structure of the synthesized macrocycles (*isolated
yields).
three macrocycles. Further, 3JC5 HꢀC6 H >10 Hz and <3
0
0
Hz and the NOE betweeen C40HTC60H (Figures S1ꢀS7)
suggest the dihedral angle NꢀC60ꢀC50ꢀO is predomi-
nantly ∼60°.
Similarly, 3JC1 H C1H
-
>10 Hz and <1 Hz and the NOE
0
betweeen C20HTC1H0 indicate the preferred single con-
formation over the dihedral angle C(O)ꢀC1ꢀC10ꢀO.
Further, both 21b and 22b have a similar set of NOEs as
shown in Figures S1ꢀS4 and S7 (A). Along with these, an
additional NOE between Ar(ortho)TTriazole ring CH
(C16H) is characteristic for 23b (Figure S7 (B)). It indicates
that in 23b the phenyl ring is oriented closer to the
disubstituted triazole ring, creating more distance between
the two triazole rings. The NOEs and coupling constant
values used as distance and torsional constraints in the
restrained MD calculations performed with the Discovery
studio 3.0 version.26 Figure 3 shows that, in 23b, the
orientation of the trisubstituted triazole ring moves
away from the disubstituted triazole ring, compared to
21b and 22b.
Acknowledgment. A.A., S.S., and M.P.G. are thankful
to UGC and CSIR, NewDelhi respectively for SRFs.
Financial assistance from DRDO and CSIR (Thunder
project) New Delhi is acknowledged. We thank the SAIF
division, CSIR-CDRI for the analytical facilities. This is
CDRI communication No. 8306.
Conformational studies on water-soluble compounds
24b, 25b, and 26b in D2O were also carried out to show
the solvent dependencyonthe conformational preferences.
In 25b and 26b, the coupling constants and the NOE
pattern remained the same, comparedto studies conducted
in CDCl3, and thus the conformations were also same.
However for 24b in D2O, Ar(ortho) gave NOEs with C20H
and C40H instead of C10H as shown for the 14-membered
cycle studied in CDCl3. This was reflected in MD studies as
well where the trisubstituted triazole ring was varied 180°
Supporting Information Available. Experimental pro-
13
cedures; 1H, 13C, 2D-NOESY and 1Hꢀ C HMBC spec-
tra; HRMS data for all new compounds. This material
is available free of charge via the Internet at http://pubs.
acs.org.
(26) See SI Tables S2ꢀS4 for list of distance restriants.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX