Pr(OTf)3-promoted Chichibabin pyridine synthesis of isodesmosine in H2O/MeOH

Article abstract of DOI:10.1016/j.tetlet.2014.09.097

1,2,3,5-Tetrasubstituted pyridinium amino acid isodesmosine is a crosslinking amino acid of elastin and is an attractive biomarker for the diagnosis of chronic obstructive pulmonary disease (COPD). Herein, we report an application of the Chichibabin pyrid

Full text of DOI:10.1016/j.tetlet.2014.09.097

Tetrahedron Letters 55 (2014) 6343–6346
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Tetrahedron Letters
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Pr(OTf)3-promoted Chichibabin pyridine synthesis of isodesmosine
in H2O/MeOH
⇑
Takanori Sugimura, Akira Komatsu, Yohei Koseki, Toyonobu Usuki
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102-8554, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
1,2,3,5-Tetrasubstituted pyridinium amino acid isodesmosine is a crosslinking amino acid of elastin and
is an attractive biomarker for the diagnosis of chronic obstructive pulmonary disease (COPD). Herein, we
report an application of the Chichibabin pyridine synthesis to the total synthesis of isodesmosine. Specif-
ically, the appropriate protected lysine and the corresponding aldehyde were reacted using Pr(OTf)3 in
H2O/MeOH.
Received 20 August 2014
Revised 18 September 2014
Accepted 22 September 2014
Available online 28 September 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Isodesmosine
Chichibabin pyridine synthesis
Total synthesis
Pr(OTf)3
In 1905, Chichibabin reported the thermal cyclo-condensation
between 1 equiv of ammonia and 3 equiv of aldehyde to form 2,3,
5-trisubstituted pyridines.1 The Chichibabin pyridine synthesis,
however, required intense conditions, such as high pressures, high
temperatures, and long reaction times.1,2 In 1997, Wang and co-
workers reported that the use of lanthanide trifluoromethanesulfo-
nates (triflates), Ln(OTf)3, as Lewis acids promoted the condensation
of amine hydrochlorides and aldehydes for the preparation of
1,2,3,5-tetrasubstituted dihydropyridinium and pyridinium deriva-
tives.3 In contrast to the original Chichibabin pyridine synthesis, this
reaction proceeded at room temperature in aqueous media to give
the corresponding products.
Isodesmosine (1, Fig. 1) and desmosine (2) are 1,2,3,5- and
1,3,4,5-tetrasubstituted pyridinium amino acids, respectively, that
are found only in the elastin matrix.4 It has been proposed that the
formation of the crosslinkers 1 and 2 occurs spontaneously via
Mannich reactions and aldol condensations after oxidative trans-
formation of the lysine residues of tropoelastin by lysyl oxidase.5
Elastin, the main component of elastic fibers, is an insoluble extra-
cellular matrix protein that consists of soluble precursor tropoelas-
tin monomers connected in a sophisticated three-dimensional
crosslinked network by desmosines.6,7
isodesmosine 1 and desmosine 2 can be measured specifically and
sensitively in clinical samples, such as plasma, urine, sputum, and
bronchoalveolar lavage fluid (BALF), using liquid chromatography–
mass spectrometry (LC–MS or LC–MS/MS). Therefore, desmosines
are attractive biomarkers for both drug discovery and the rapid
diagnosis of COPD.
We recently reported the total synthesis of 2 starting from 4-
hydroxypyridine or 3,5-dibromopyridine.10 We also reported the
total synthesis of both 1 and 2 using a room temperature Chichiba-
bin pyridine synthesis with Ln(OTf)3 in H2O.11 These syntheses
were achieved starting from amine hydrochloride and aldehyde,
and the poor solubility of the amine hydrochloride in H2O resulted
in low yields for these reactions. It was envisaged that the addition
of organic solvents such as methanol (MeOH) would enhance the
solubility of the substrates and potentially afford better yields of
the products. It was also thought that the Chichibabin pyridine
The irreversible degradation of elastin that occurs in chronic
obstructive pulmonary disease (COPD) is found to give rise to the
elastin crosslinkers 1 and 2.8,9 COPD is known as ‘tobacco disease’
and is currently the fourth leading cause of death worldwide. Both
⇑
Corresponding author. Tel.: +81 3 3238 3446; fax: +81 3 3238 3361.
E-mail address: t-usuki@sophia.ac.jp (T. Usuki).
Figure 1. Structures of isodesmosine (1) and desmosine (2).
http://dx.doi.org/10.1016/j.tetlet.2014.09.097
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

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T. Sugimura et al. / Tetrahedron Letters 55 (2014) 6343–6346
Scheme 1. Synthesis of the protected lysine 5.
Scheme 2. Synthesis of the aldehyde 10.
synthesis would allow for the exclusive formation of the 1,2,3,5-
tetrasubstituted pyridinium amino acid isodesmosine 1 in the
same manner as the original reaction.1–3 Herein, we report the
development of new methods for the preparation of the protected
lysine 5 and the corresponding aldehyde 10, as well as the total
synthesis of 11 via the reaction of these two compounds in a Chic-
hibabin pyridine synthesis using Pr(OTf)3 in H2O and MeOH.
Our synthesis commenced with the preparation of the protected
lysine 5 starting from commercially available 6-(benzyloxycarb-
ony)-amino-2-(S)-4-[(tert-butoxycarbonyl)amino]-hexanoic acid
(3) (Scheme 1). Protection of the carboxyl group as the
corresponding tert-butyl ester was achieved using O-tert-butyl-
N,N0-diisopropyl isourea, which was freshly prepared from
N,N0-diisopropylcarbodiimide.12 Compound 3 and the isourea were
reacted in dichloromethane (CH2Cl2) at room temperature to give
the tert-butyl ester 4 in 98% yield. Subsequent removalof the benzyl-
oxycarbonyl group under Pd/C hydrogenation conditions in MeOH
gave the desired amine 5 in 96% yield. The optical rotation of this
Table 1
Chichibabin pyridine synthesis of 11 from the protected lysine 5 and aldehyde 10
Entrya
H2O/MeOH ratio
Yieldb (%)
1
2
3
4
5
6
1:0
1:1
1:2
1:5
2:1
3:1
N.R.
Trace
Trace
Trace
29
26
a
Reactions between 1 equiv of 5 and 4 equiv of 10 were run at room temperature
for 24 h.
material ([
a]D +7.75) was in good agreement with the reported value
b
Isolated yield.
([a
]
D +6.81).13 Thus, the synthesis of the protected lysine 5, which is
a precursor of the Chichibabin reaction for the synthesis of 1, was
achieved in an overall yield of 94% in two steps from 3.
diethyl ether (Et2O) afforded the aldehyde 7 in 89% yield, which
was converted to the exo-olefin 8 in 68% yield via a Wittig reaction
using n-butyllithium (nBuLi) and methyltriphenylphosphonium
bromide (MePPh3Br) in tetrahydrofuran (THF). Formation of the
ylide was carried out at ꢀ78 °C rather than 0 °C, which resulted
in an improvement of the yield over these two steps from 47% to
61%.14 Hydroboration–oxidation of 8 using sodium borohydride
(NaBH4) and boron trifluoride etherate (BF3ꢁEt2O) in THF, followed
The synthesis of the aldehyde 10 was achieved in four steps
beginning with 1-benzyl-5-methyl-2-(S)-[bis-(tert-butoxycar-
bonyl)-amino]-pentanedioate (6) according to the route shown in
Scheme 2. Compound 6 was prepared from commercially available
2-(S)-[(tert-butoxycarbonyl)amino]pentanedioic acid 1-benzyl
ester according to literature procedures,14 and the overall yield
for the steps from 6 to 9 was slightly improved. Reduction of the
methyl ester 6 using diisobutylaluminum hydride (DIBAL-H) in

T. Sugimura et al. / Tetrahedron Letters 55 (2014) 6343–6346
6345
Scheme 3. Total synthesis of isodesmosine 1.
by sodium hydroxide (NaOH) and hydrogen peroxide (H2O2) then
afforded the anti-Markovnikov product 9 in 80% yield. Treatment
of the primary alcohol 9 with Dess–Martin periodinane (DMP) in
CH2Cl2 led to the desired compound 10 in 98% yield. It is notewor-
thy that the oxidation of the mono-Boc-protected version of 9 gave
the cyclized product.15 Thus, the aldehyde 10, which is a precursor
of the Chichibabin pyridine synthesis for the preparation of 1, was
prepared from 6 in an overall yield of 47% over four steps.
With the protected lysine 5 and aldehyde 10 in hand, we turned
our attention to the Pr(OTf)3-promoted Chichibabin pyridine syn-
thesis. Among the family of Ln(OTf)3 reagents,16,17 Pr(OTf)3 was
found to be the best reagent for the Chichibabin pyridine synthesis,
most likely because it possesses the appropriate level of Lewis
acidity for the Mannich and aldol condensation reactions that
occur.11 Given that the starting materials for the Chichibabin pyr-
idine synthesis were expected to be poorly soluble in H2O, the
effect of using MeOH as a co-solvent was investigated (Table 1).
Reactions between 1 equiv of 5 and 4 equiv of 10 were conducted
in the presence of 50 mol % Pr(OTf)3 at room temperature for 24 h.
When the reaction was performed in 100% H2O, it failed to afford
any of the desired products (Table 1, entry 1), likely due to the fact
that substrates 5 and 10 were insoluble in H2O. Different amounts
of MeOH were then added to the reaction (H2O/MeOH = 1:1, 1:2,
and 1:5 (v/v)) in an attempt to increase the solubility of the sub-
strates (Table 1, entries 2–4, respectively). However, only trace
amounts of the desired pyridinium compound 11 were observed
in these reactions, along with several byproducts, which had the
same m/z as 11 as determined by mass spectrometry (MS) analysis.
Pleasingly, when the reaction was run in a 2:1 (v/v) mixture of H2O
and MeOH, the isodesmosine-type Chichibabin product 11 was
obtained in 29% yield (Table 1, entry 5), along with several uniden-
tified byproducts, which were observed by thin layer chromatogra-
phy (TLC). Interestingly, and in contrast to the previous report,11
only a trace amount of the desmosine-type pyridinium compound
was identified in this particular case. Further increasing the ratio of
H2O to MeOH to 3:1 (v/v) resulted in a slight decrease in the yield
of 11 to 26% (Table 1, entry 6). Taken together, these results indi-
cated that the use of a 2:1 (v/v) mixture of H2O and MeOH (Table 1,
entry 5) was optimal for the Chichibabin pyridine synthesis in H2O/
MeOH and suggested that the solubility of starting materials 5 and
10 was a crucial factor for the success of this synthesis. These
conditions also promoted the selective synthesis of the isodesmo-
sine-type product.
completed in 27% yield (under the conditions of entry 5 in Table 1)
over five steps starting from commercially available 6-(benzyloxy-
carbony)-amino-2-(S)-4-[(tert-butoxycarbonyl)amino]-hexanoic
acid (3). The results obtained in the current study suggest that the
solubility of compounds 5 and 10 in the H2O/MeOH solvent system
was critical to the success of this Chichibabin pyridine synthesis.
Furthermore, this reaction system favored the selective formation
of the isodesmosine-type Chichibabin product, with only a trace
amount of the desmosine-type Chichibabin product being
detected. This selectivity implies that the formation of 1,3,4,5-tet-
rasubstituted pyridine was prevented in the presence of MeOH.
Further studies designed to elucidate the mechanisms involved
in the formation of the elastin crosslinkers isodesmosine 1 and des-
mosine 2 from lysine in nature are required in order to develop a
deeper understanding of these processes, and these studies are
currently underway in our laboratory. The preparation of isotopi-
cally labeled desmosines as internal standards for the analysis of
clinical COPD samples by the isotope-dilution LC–MS/MS9d is also
underway in our laboratory using the synthetic route described
above. This route is also being investigated for the preparation of
related crosslinking amino acids for the elucidation of the three-
dimensional crosslinking structure of elastin.18
Acknowledgments
We would like to thank Dr. Yong Y. Lin (Roosevelt Hospital Cen-
ter, New York) for many fruitful discussions. This work was sup-
ported by a Grant-in-Aid for Young Scientists (B) from the Japan
Society for the Promotion of Science (KAKENHI Grants 22710224
and 25750388).
Supplementary data
Supplementary data (experimental procedures and character-
ization data) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2014.09.097.
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