Total synthesis of pentosidine

Article abstract of DOI:10.1021/ol3021226

Pentosidine, a biologically important advanced glycation endproduct, has been accessed in a rapid, high-yielding manner. The synthesis was accomplished via a six-step sequence starting with 3-amino-2-chloropyridine and features a palladium-catalyzed tande

Full text of DOI:10.1021/ol3021226

ORGANIC
LETTERS
2012
Vol. 14, No. 17
4678–4681
Total Synthesis of Pentosidine
Adam J. Rosenberg and Daniel A. Clark*
Department of Chemistry, 1-014 Center for Science and Technology,
Syracuse University, Syracuse, New York 13244, United States
daclar01@syr.edu
Received July 31, 2012
ABSTRACT
Pentosidine, a biologically important advanced glycation endproduct, has been accessed in a rapid, high-yielding manner. The synthesis was
accomplished via a six-step sequence starting with 3-amino-2-chloropyridine and features a palladium-catalyzed tandem cross-coupling/
cyclization to construct the imidazo[4,5-b]pyridine core.
Pentosidine (1) is an advanced glycation endproduct
(AGE) containing an imidazo[4,5-b]pyridine core that has
attracted interest as a biochemical marker.1 It was discov-
ered as an extracellular protein cross-link by Monnier in
1989 and is one of only a handful of characterized AGEs.2
Pentosidine is a naturally occurring biological fluorophore
and, as such, has found use in noninvasive diagnostics. It
has been reported as a chemical marker of diabetic com-
plications, kidney dysfunction, oxidative stress, aging, and
age-related diseases.3À10 Recently, Veralight Inc. has be-
gun marketing a device that utilizes the spectroscopic
properties of AGEs to carry out a noninvasive method of
detecting type II diabetes.11À14 Biosynthetically, pentosi-
dine is believed to be a protein cross-link derived from a
post-translational Maillard reaction of arginine and lysine
residues with a pentose.15 There is also evidence that it is
both a singlet oxygen sensitizer and an antioxidant.16,17
Pentosidine is available commercially; however its low
supply and corresponding high price make it difficult to
study.18 Therefore, our goal was to develop a straightfor-
ward route to pentosidine that would allow us to explore
the properties of this molecule.
From a synthetic point of view pentosidine (1) presents
an interesting structural target. The imidazo[4,5-b]pyridine
core has attracted only casual interest in the literature, with
few direct efforts at preparation.19À27 The challenges
inherent in this molecular architecture are derived from
the need to generate this moiety in a cost efficient
and chemically straightforward manner. There have been
several published preparations of pentosidine to date,
(1) Dyer, D. G.; Dunn, J. A.; Thorpe, S. R.; Bailie, K. E.; Lyons, T. J.;
McCance, D. R.; Baynes, J. W. J. Clin. Invest. 1993, 91, 2463–2469.
(2) Sell, D.; Monnier, V. J. Biol. Chem. 1989, 264, 21597–21602.
(3) Sugimoto, K.; Yasujima, M.; Yagihashi, S. Curr. Pharm. Des.
2008, 14, 953–961.
(4) Tsukahara, H. Curr. Med. Chem. 2007, 14, 339–351.
(5) Cho, S.-J.; G. RoCman, F.; Yeboah; Konishi, Y. Curr. Med.
Chem. 2007, 14, 1653–1671.
€
€
(6) Wihler, C.; Schafer, S.; Schmid, K.; Deemer, E.; Munch, G.;
Bleich, M.; Busch, A.; Dingermann, T.; Somoza, V.; Baynes, J.; Huber,
J. Diabetologia 2005, 48, 1645–1653.
(14) Felipe, D. L.; Hempe, J. M.; Liu, S.; Matter, N.; Maynard, J.;
Linares, C.; Chalew, S. A. Diabetes Care 2011, 34, 1816–1820.
(15) Grandhee, S.; Monnier, V. J. Biol. Chem. 1991, 266, 11649–
11653.
(16) Deng, B.; Simental, A.; Lutz, P.; Shaheen, G.; Chaiken, J. Proc.
SPIE 8219, 82190D 2012.
(17) Deng, B.; Wright, C.; Lewis-Clark, E.; Shaheen, G.; Geier, R.;
Chaiken, J. Proc. SPIE 7560, 75600P 2010.
(18) Available from PolyPeptide Group, 2 mg for $414.00 as the TFA
salt.
(19) Senanayake, C. H.; Fredenburgh, L. E.; Reamer, R. A.; Liu, J.;
Larsen, R. D.; Verhoeven, T. R.; Reider, P. J. Tetrahedron Lett. 1994, 35,
5775–5778.
(7) Takahashi, M. Curr. Rheumatol. Rev. 2006, 2, 319–324.
(8) Treweek, J. B.; Dickerson, T. J.; Janda, K. D. Acc. Chem. Res.
2009, 42, 659–669.
(9) Thomas, M. C.; Forbes, J. The Maillard Reaction; The Royal
Society of Chemistry: 2010, pp 85À91.
(10) Sell, D. R.; Nagaraj, R. H.; Grandhee, S. K.; Odetti, P.; Lapolla,
A.; Fogarty, J.; Monnier, V. M. Diabetes Metab. Rev. 1991, 7, 239–251.
(11) Maynard, J. D.; Rohrscheib, M.; Way, J. F.; Nguyen, C. M.;
Ediger, M. N. Diabetes Care 2007, 30, 1120–1124.
(12) Ediger, M. N.; Maynard, J. D. Rev. Endocrinol. 2007 (June),
p 62.
(13) Conway, B.;Edmundowicz, D.;Matter, N.;Maynard, J.;Orchard,
T. Diabetes Technol. Ther. 2010, 12, 339–345.
(20) Batt, D. G.; Houghton, G. C. J. Heterocycl. Chem. 1995, 32,
963–969.
r
10.1021/ol3021226
Published on Web 08/21/2012
2012 American Chemical Society

including two synthetic routes. In 1989 Sell and Monnier
carried out a biomimetic synthesis by heating lysine,
arginine, and ribose in an aqueous environment and
then subjecting the mixture to HPLC purification to give
pentosidine in 0.02% yield.2,15 Recently Cravotto and
co-workers improved upon this by utilizing protected
amino acids under microwave irradiation.28 However
these methods, while requiring few synthetic steps,
require HPLC purification and are low yielding. The
first total synthesis of pentosidine was published by
Shioiri and co-workers in 199129,30 requiring 15 total
steps and HPLC purification. More recently Sayre’s
research group published an approach to the total
synthesis of pentosidine.31,32 Although this route is
shorter, the expensive 2,3-diaminopyridine was used as
a starting material.
ornithine and lysine residues, thus avoiding asymmetric
reactions and chiral auxiliaries by generating the stereo-
centers from the chiral pool. Recently we reported the
preparation of imidazo[4,5-b]pyridines using a cross-
coupling/cyclization strategy, which allowed the use of
3-amino-2-chloropyridine asourstartingmaterial.26 Using
this route, we would avoid using 2,3-diaminopyridine
which is known to be problematic to functionalize in a
regioselective manner.29,30,33
Scheme 2. Preparation of Imidazo[4,5-b]pyridine Core
Scheme 1. Retrosynthesis
Our synthesis started from commercially available ami-
no-2-chloropyridine 3,34 which was protected as its 2,4-
dimethoxybenzyl (DMB) amine (5) via a reductive amina-
tion (Scheme 2).33,35 We then applied our recently reported
palladium-catalyzed amide coupling/cyclization metho-
dology to generate the core imidazo[4,5-b]pyridine in a
single step.26 High yields have consistently been obtained
for this reaction on a 2.5À5 g scale. Compound 6 was then
chlorinated at the 2-position using hexachloroethane, giv-
ing chloro-azole 2 in 81% yield.36 This three-step sequence
rapidly assembles the activated imidazo[4,5-b]pyridine
core in high yield on a gram scale.
Ornithine residue 9 was prepared in excellent yield
from commercially available Boc-Orn(Z)-OH, by
esterification with isourea 8 followed by Cbz cleavage
with Pd/C and H2 (Scheme 3). The lysine fragment
can be prepared from Boc-Lys(H)-OH via alcohol 11
using the method of Adamczyk.37 Alcohol 11 was then
transformed to the desired iodide under Appel condi-
tions in 91% yield. This short, two-step route avoids the
seven-step sequence pursued by Shioiri. In addition the
stereocenter was installed from commercially available
lysine 10.29,30
Retrosynthetically we envisioned disconnections at
C2 and N4, leaving an imidazo[4,5-b]pyridine core with
an electron-donating protecting group at N1 (Scheme 1).
This protection scheme is required so that N4 will be
activated for selective alkylation. Without N1 or N3 being
blocked, a mixture of N1, N3, and N4 alkylation pro-
ducts is obtained.21 Additionally, Shioiri demonstrated
thatelectron-withdrawinggroups deactivatedthe imidazo-
[4,5-b]pyridine for alkylation at any position.29 These
disconnections would allow the introduction of protected
(21) Khanna, I. K.; Weier, R. M.; Lentz, K. T.; Swenton, L.; Lankin,
D. C. J. Org. Chem. 1995, 60, 960–965.
(22) Senanayake, C. H.; Fredenburgh, L. E.; Reamer, R. A.; Liu, J.;
Roberts, F. E.; Humphrey, G.; Thompson, A. S.; Larsen, R. D.;
Verhoeven, T. R.; Reider, P. J.; Sinkai, I. Heterocycles 1996, 42, 821–
830.
(23) Grimmett, M. R. Imidazole and Benzimidazole Synthesis; Aca-
demic Press: San Diego, 1997.
~
(24) Deghati, P. Y. F.; BierAugel, H.; Wanner, M. J.; Koomen, G.-J.
Tetrahedron Lett. 2000, 41, 569–573.
(25) Yıldırım, I.; Kandemirli, F.; Demir, E. Molecules 2005, 10, 559–
571.
(26) Rosenberg, A. J.; Zhao, J.; Clark, D. A. Org. Lett. 2012, 14,
1764–1767.
(27) Bukhryakov, K.; Kurkin, A.; Yurovskaya, M. Chem. Hetero-
cycl. Compd. 2011, 47, 533–557.
(28) Visentin, S.; Medana, C.; Barge, A.; Giancotti, V.; Cravotto, G.
(33) Leahy, D. K.; Desai, L. V.; Deshpande, R. P.; Mariadass, A. V.;
Rangaswamy, S.; Rajagopal, S. K.; Madhavan, L.; Illendula, S. Org.
Process Res. Dev. 2012, 16, 244–249.
(34) Available from Matrix Scientific for $133/250 g
(35) McLaughlin, M.; Palucki, M.; Davies, I. W. Org. Lett. 2006, 8,
3307–3310.
(36) Strekowski, L.; Kiselyov, A. S. e-EROS Encyclopedia of Re-
agents for Organic Synthesis; Wiley: New York, 2001.
(37) Adamczyk, M.; Johnson, D. D.; Reddy, R. E. Tetrahedron:
Asymmetry 1999, 10, 775–781.
(38) Hong, Y.; Senanayake, C. H.; Xiang, T.; Vandenbossche, C. P.;
Tanoury, G. J.; Bakale, R. P.; Wald, S. A. Tetrahedron Lett. 1998, 39,
3121–3124.
Org. Biomol. Chem. 2010, 8, 2473–2477.
(29) Sugiyama, H.; Yokokawa, F.; Shioiri, T.; Katagiri, N.; Oda, O.;
Ogawa, H. Tetrahedron Lett. 1999, 40, 2569–2572.
(30) Yokokawa, F.; Sugiyama, H.; Shioiri, T.; Katagiri, N.; Oda, O.;
Ogawa, H. Tetrahedron 2001, 57, 4759–4766.
(31) Liu, Y.; Zhang, W.; Sayre, L. M. J. Heterocycl. Chem. 2011, 48,
426–433.
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Org. Lett., Vol. 14, No. 17, 2012
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Table 1. Optimization of C2 Functionalization
Scheme 3. Preparation of Amino Acid Derived Side Chains
yield
(%)
entry
X
conditionsc
solvent
14:15
1
I
A
B
A
B
B
C
C
D
D
E
F
toluene
t-AmOH
toluene
t-AmOH
toluene
EtOH
10
0
1:0
À
2
I
3
Cl
Cl
Cl
I
13
39
44
0
0d
31
87
0
1:0
1:1
2:1
À
4
5
6
7
I
DMF
À
8a
9a
10
11
Cl
Cl
Cl
I
EtOH
2:1
1:0b
À
Functionalization of C2 with the ornithine side-chain
9 proved unexpectedly complicated (Table 1). We first
explored palladium catalyzed methods developed by
Senanayake for the synthesis of the 2-aminobenzimi-
dazole, Norastemizole.38,39 Unfortunately, only low
yields of the desired product 14 were obtained (entries
1À5). Having previously reported C2 functionalization
using an SNAr reaction with an iodide analogous to 13,
we returned to this for installation of the ornithine
residue.26 Initial attempts proved unsatisfactory due
to the propensity of ornithine 9 to cyclize and form
δ-lactam 15 (entry 6).40 Switching to DMF as the solvent
resulted in the dimethylamine adduct as the only isolable
addition product (entry 7). The halide was changed from
iodine to the more electronegative chlorine 2 to increase the
reactivity of the azole for nucleophilic attack. Use of 2
combined with a higher loading of ornithine 9 led to forma-
tion of desired product 14 in 87% yield (entries 8 and 9).
Efforts to lower the equivalents of ornithine 9 using either
fluoride41 or DABCO42À44 as nucleophilic catalysts did not
improve the yield or selectivity (entries 10 and 11). Using this
optimized procedure, we were able to synthesize >1 g of 14
in a single run.
n-BuOH
TGME
DMA
0
À
a 2.0 equiv of Orn. b After purification, 1:1 ratio prior to purification.
c Conditions A: 1.5 mol % Pd2(dba)3, 4.5 mol % BINAP, NaOt-Bu,
reflux. Conditions B: 1.5 mol % Pd2(dba)3, 4.5 mol % BippyPhos,
K3PO4, reflux. Conditions C: Na2CO3, reflux. Conditions D: EtN(iPr)2,
reflux. Conditions E: KF, 2,6-lutidine, 120 °C. Conditions F: cat.
DABCO, Na2CO3. d Dimethylamine addition observed.
Scheme 4. Final Assembly
As aforementioned, electron-donating substituents
installed at C2 and N1 activate N4 for alkylation. Iodide
12 and imidazo[4,5-b]pyridine 14 were refluxed in THF
to provide iodide salt 16, the fully protected pentosidine
(Scheme 4). Purification of pyridinium salts is known
to be problematic;45 however, to our delight, 16 was
easily purified using standard silica gel chromatogra-
phy. With pure protected pentosidine 16 in hand, a
(39) Hong, Y.; Tanoury, G. J.; Wilkinson, H. S.; Bakale, R. P.; Wald,
S. A.; Senanayake, C. H. Tetrahedron Lett. 1997, 38, 5607–5610.
(40) Chulin, A. N.; Rodionov, I. L.; Baidakova, L. K.; Rodionova,
L. N.; Balashova, T. A.; Ivanov, V. T. J. Pept. Sci. 2005, 11, 175–186.
(41) Senanayake, C. H.; Hong, Y.; Xiang, T.; Wilkinson, H. S.;
Bakale, R. P.; Jurgens, A. R.; Pippert, M. F.; Butler, H. T.; Wald,
S. A. Tetrahedron Lett. 1999, 40, 6875–6879.
global deprotection was accomplished by heating in
aqueous TFA. Following this protocol, pentosidine (1)
was obtained as the TFA salt without the need for
tedious HPLC purification.46 In our hands, we were
(42) Linn, J. A.; McLean, E. W.; Kelley, J. L. J. Chem. Soc., Chem.
Commun. 1994, 913–914.
(43) Shi, Y.-J.; Humphrey, G.; Maligres, P. E.; Reamer, R. A.;
Williams, J. M. Adv. Synth. Catal. 2006, 348, 309–312.
(44) Bita, B. Eur. J. Chem. 2010, 1, 54–60.
(45) Bluhm, L. H.; Li, T. Tetrahedron Lett. 1998, 39, 3623–3626.
4680
Org. Lett., Vol. 14, No. 17, 2012

able to prepare 112 mg of pentosidine in a single-run
over the six-step sequence.
Acknowledgment. We thank Syracuse University and
LighTouch Medical Inc. for generous financial sup-
port of this research. We also thank Professor Joe
Chaiken (Syracuse University) for his interest in this
research, bringing the pentosidine problem to our
attention, and many helpful discussions. Professors
Chisholm, Hahn, and Totah and the Clark research
group (Syracuse University) are also thanked for many
helpful discussions.
In summary, we report a short, rapid total synthesis
of pentosidine that utilizes a highly efficient synthesis of
imidazo[4,5-b]pyridine 2. Use of highly economical SNAr
and alkylation chemistry allows for a scalable and repro-
ducible synthetic route. This approach provides pentosi-
dine with only six steps in the longest linear sequence (ten
total steps) in 30.1% yield. The high efficiency of the
synthesis, low-cost of the starting materials, and lack of
toxic reagents make this a practical method for research
scale synthesis of pentosidine, thereby allowing further
studies of this interesting natural product.
Supporting Information Available. Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(46) The spectroscopic data (1H and 13C NMR as well as absorbance
and fluorescence) were found to be in agreement with the published
spectra obtained by Sell and Monnier. See ref 2.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 17, 2012
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