Synthesis of stable isotope-labelled monolysyl advanced glycation endproducts

Article abstract of DOI:10.1007/s00726-013-1498-9

Advanced Glycation Endproducts (AGEs) are modified amino acids that form on proteins and are known to be implicated in the pathogenesis of diabetes and related diseases. Ready access to synthetic stable isotope-labelled AGEs allows for quantitative mass spectrometry studies to be undertaken, providing key insights into the roles AGEs play in the progression of such diseases. However, the majority of current syntheses of these compounds suffer from poor yields and lengthy procedures and are not suitable for the purposes required here. Here, we report robust syntheses of stable isotope-labelled monolysyl AGEs, N ^ε-(carboxymethyl)lysine, N^ε-(carboxyethyl)lysine and pyrraline, that provide straightforward access to these compounds for quantitative amino acid analysis. This work will facilitate future investigations with these compounds and lead to a better understanding of the roles they play in diabetes and related diseases.

Full text of DOI:10.1007/s00726-013-1498-9

Amino Acids (2013) 45:319–325
DOI 10.1007/s00726-013-1498-9
ORIGINAL ARTICLE
Synthesis of stable isotope-labelled monolysyl advanced glycation
endproducts
•
Tom M. Woods Garth J. S. Cooper
•
Margaret A. Brimble
Received: 4 March 2013 / Accepted: 5 April 2013 / Published online: 16 April 2013
Ó Springer-Verlag Wien 2013
Abstract Advanced Glycation Endproducts (AGEs) are
modified amino acids that form on proteins and are known
to be implicated in the pathogenesis of diabetes and related
diseases. Ready access to synthetic stable isotope-labelled
AGEs allows for quantitative mass spectrometry studies to
be undertaken, providing key insights into the roles AGEs
play in the progression of such diseases. However, the
majority of current syntheses of these compounds suffer
from poor yields and lengthy procedures and are not suit-
able for the purposes required here. Here, we report robust
syntheses of stable isotope-labelled monolysyl AGEs, Ne-
(carboxymethyl)lysine, Ne-(carboxyethyl)lysine and pyrr-
aline, that provide straightforward access to these com-
pounds for quantitative amino acid analysis. This work will
facilitate future investigations with these compounds and
lead to a better understanding of the roles they play in
diabetes and related diseases.
Introduction
The Advanced Glycation Endproducts (AGEs) are a large
and structurally diverse group of compounds that result
from post-translational non-enzymatic modification of
proteins with reducing sugars (Henle 2005; Rabbini and
Thornalley 2012; Ledl and Schleicher 1990). These mod-
ifications usually occur at lysine and arginine residues and
are most problematic on long-lived proteins, such as col-
lagens and lens crystallins (Verzijl et al. 2000a; b). The
formation and accumulation of AGEs in the body is
thought to play a significant role in the pathogenesis of
many debilitating diseases including diabetes (Ahmed et al.
2007; Stitt 2001; Peppa and Vlasarra 2005), atherosclerosis
(Basta et al. 2004) and age-related neurodegenerative dis-
eases, such as Alzheimer’s disease (Mu¨nch et al. 2012).
As part of an ongoing research program investigating
the roles of AGEs in the pathogenesis of diabetes, we
required readily available access to stable isotope labelled
AGE standards for quantitative amino acid analysis using
mass spectrometry. Several other groups have also required
access to these types of compounds in the course of their
studies and have synthesized them according to known
literature procedures (Thornalley et al. 2003; Teerlink et al.
2004). However, upon consulting the literature, we found
that the majority of these routes suffered from either dif-
ficult and lengthy procedures and/or low yields and did not
represent attractive routes. Consequently, there is a need
for the development of improved protocols/procedures for
the preparation of AGEs in their free amino acid forms.
Recently, we reported the synthesis of the three monolysyl
AGEs, Ne-(carboxymethyl)lysine 1 (CML), Ne-(carboxy-
ethyl)lysine 2 (CEL) and pyrraline 3 as both free amino
acids and Fmoc-protected analogues suitable for incorpo-
ration into peptide synthesis (Woods et al. 2012). In this
Keywords Stable isotope labels Á Synthesis Á Advanced
glycation endproducts Á Internal standards Á Diabetes Á
Glycation
T. M. Woods Á M. A. Brimble (&)
School of Chemical Sciences, The University of Auckland,
23 Symonds Street, Auckland, New Zealand
e-mail: m.brimble@auckland.ac.nz
G. J. S. Cooper
Centre for Advanced Discovery and Experimental Therapeutics
(CADET), Central Manchester University Hospitals NHS
Foundation Trust, and School of Biomedicine, The University
of Manchester and Manchester Academic Health Sciences
Centre, Manchester, UK
123

320
T. M. Woods et al.
Fig. 1 The stable isotope-
labelled monolysyl AGEs
synthesized
O
O
O
O
H
H
∗
∗
∗
∗
N
∗
N
∗
∗
∗
∗
∗
HO
OH
HO
OH
∗
∗
∗
∗
NH2
NH2
∗
∗
CML, 1
CEL, 2
HO
N
O
∗
∗
∗
∗
∗
∗
OH
∗
H
NH2
denotes 13C or 15
N
∗
∗
O
Pyrraline, 3
communication, we report efficient and straightforward
syntheses of the stable isotope labelled forms of these three
important AGEs (Fig. 1), which are suitable as internal
standards for mass spectrometry studies.
containing [U-13C6, 99 %, U-15N2, 99 %]-L-lysine dihy-
drochloride 4 (500 mg, 0.22 mmol, 1.0 eq.) and the reac-
tion was allowed to warm to room temperature and stirred
for 30 min. The reaction mixture was concentrated under
reduced pressure then dried under high vacuum affording
the free base as a white solid. This solid was added to a
refluxing solution of 9-BBN dimer (322 mg, 1.32 mmol,
0.6 eq) in MeOH (8 mL, HPLC grade) and the reaction
mixture refluxed for 1.5 h (the reaction mixture becomes
homogeneous after 30 min) before cooling to room tem-
perature. The MeOH was removed under reduced pressure
affording a gummy solid that was triturated in hot hexanes
(2 9 5 mL) then hot diethylether (2 9 5 mL), collected by
filtration and dried under high vacuum affording the title
compound as a white solid (895 mg). TLC Rf = 0.18
(2:18:80 NH4OH/MeOH/DCM). HRMS [M ? H]?
275.2385, 12C183C6H28B15N2O2 requires M? 275.2380. This
compound was used without further purification.
Experimental
General
[U-13C6, U-15N2]-L-lysine.2HCl was purchased from
Cambridge Isotope Laboratories. Unless stated, all solvents
and reagents were used as supplied from commercial
sources. All moisture sensitive reactions were performed in
an inert, dry atmosphere of nitrogen in oven-dried glass-
ware. Analytical thin-layer chromatography (TLC) was
performed using Kieselgel F254 0.2 mm (Merck) silica
plates with visualisation by ultraviolet irradiation (254 nm)
followed by staining with either potassium permanganate
or ninhydrin. Flash chromatography was performed using
Kieselgel S 63–100 lm (Riedel-de-Hahn) silica gel. The
solvent compositions reported for all chromatographic
separations are on a volume/volume (v/v) basis. Infrared
(IR) spectra were recorded on a PerkinElmer Spectrum 100
9BBN-Ns-lysine-6: to a cold (0 °C) solution of 5
(100 mg, 0.25 mmol, 1.0 eq., calculated from lysine used
in previous step) in 1 M K2CO3 aq. (1 mL) and dioxane
(1 mL) was added NsCl (81 mg, 0.37 mmol, 1.5 eq.) as a
solution in dioxane (0.5 mL). The reaction was allowed to
warm to room temperature and stirred for 2 h. The reaction
mixture was partitioned between ethyl acetate (2 mL) and
H2O (2 mL) and the layers separated. The aqueous layer
was extracted with ethyl acetate (2 9 2 mL) and the
combined organics washed with brine, dried over MgSO4,
filtered and concentrated under reduced pressure. Purifi-
cation by flash chromatography (1:1 ethyl acetate/hexanes
then ethyl acetate) afforded the title compound as a pale
yellow solid (100 mg, 0.22 mmol, 87 %). TLC Rf =
0.32 (8:2 ethyl acetate/hexanes). [a]2D2 = -10.7 ° (c =
0.169, MeOH). MP = sinters and decomposes. IR (ATR) t
3,419–3,011, 2,850, 1,660, 1,595, 1,537, 1,324, 1,249,
1,218, 1,161, 1,126, 1,057, 1,004, 962, 853, 781, 733,
654 cm-1. 1H NMR (400 MHz, MeOH-d4) d 8.08 (1H, m),
7.87–7.77 (3H, m), 6.35 (0.22H, dm, JNH = 74 Hz, NH),
5.62 (0.73H, dm, JNH = 72 Hz, NH), 3.60 (1H, dm,
JCH = 141 Hz), 3.07 (2H, dm, JCH = 139 Hz), 2.07 (0.5H,
1
FT-IR Spectrometer. Unless stated, H-NMR spectra were
recorded on 400 MHz Bruker spectrometer and are repor-
ted in parts per million (ppm) on the d scale relative to
CDCl3 (d 7.26) or TMS (0.00) and 13C-NMR spectra were
recorded on a 100 MHz Bruker spectrometer and are
reported in ppm on the d scale relative to CDCl3 (d 77.16).
1
The multiplicities of H signals are designated by the fol-
lowing abbreviations: s = singlet, d = doublet, t = triplet,
q = quartet, m = multiplet, br = broad, dd = doublet of
doublets, dt = doublet of triplets and dm = doublet of
multiplets. All coupling constants J = are reported in
hertz. Mass spectra were obtained by electrospray ionisa-
tion in positive ion mode.
Procedures for amino acid synthesis
9BBN-lysine-5: ammonium hydroxide solution (28–30 %,
21.0 mL) was cooled in an ice bath then added to a flask
br),
1.25–1.95
(17.5H,
m),
0.55
(2H,
br).
13C NMR (100 MHz, MeOH-d4) d 177.3 (C = O, d,
123

Advanced glycation endproducts
321
JCC = 53 Hz), 149.6 (quat), 135.0 (CH), 134.8 (quat),
133.6 (CH), 131.5 (CH), 125.9 (CH), 56.2 (CH, m), 43.9
(CH2, dt, JCC = 36 Hz, JNC = 5 Hz), 32.63 (CH2), 32.58
(CH2), 32.3 (CH2), 32.2 (CH2), 31.5 (CH2, tt,
JCC = 36 Hz, JNC = 5 Hz), 30.4 (CH2, td, JCC = 36 Hz,
JNC = 4 Hz), 25.6 (CH2), 25.3 (BCH), 25.2 (CH2) 24.1
partitioned between ethyl acetate (2 mL) and H2O (3 mL)
and the layers separated. The aqueous layer was extracted
with ethyl acetate (3 9 3 mL) and the combined organics
washed with H2O (5 9 1 mL), dried over MgSO4, filtered
and concentrated under reduced pressure. Purification by
flash chromatography (1:99 then 5:95 MeOH/DCM)
afforded the title compound as a colourless oil (52 mg,
0.14 mmol, 64 %). TLC Rf = 0.35 (1:9 MeOH/DCM).
[a]2D3 = -19.4° (c = 0.25, CHCl3). MP = Sinters and
decomposes. IR (ATR) t 3,600–2,500, 2,920, 2,843, 1,719
(C = O), 1,659 (C = O), 1,452, 1,357, 1,251, 1,215,
(BCH), 24.0 (CH2, t, JCC = 35 Hz). HRMS [M ? Na]?
12 13
14
482.1966,
482.1983.
C C6H30BO6S14N15N2Na requires M?
9BBN-Et-CML-7a: to a cold (0 °C) suspension of 6
(112 mg, 0.24 mmol, 1 eq.) and K2CO3 (135 mg,
0.98 mmol, 4 eq.) in dry DMF (0.6 mL) was added ethyl-
2-bromoacetate (54 lL, 0.49 mmol, 2 eq.) as a solution in
DMF (0.2 mL). The reaction was allowed to warm to room
temperature and stirred at room temperature for 18 h. The
reaction mixture was cooled to 0 °C and thiophenol
(130 lL, 1.22 mmol, 5 eq.) was added dropwise. The
reaction mixture was allowed to warm to room temperature
and stirred for a further 2 h (complete by TLC). The
reaction mixture was partitioned between ethyl acetate
(2 mL) and H2O (3 mL) and the layers separated. The
aqueous layer was extracted with ethyl acetate (3 9 3 mL)
and the combined organics washed with H2O (5 9 1 mL),
dried over MgSO4, filtered and concentrated under reduced
pressure. Purification by flash chromatography (5:95
MeOH/DCM) afforded the title compound as a colourless
oil (68 mg, 0.19 mmol, 77 %). TLC Rf = 0.38 (1:9
MeOH/DCM). [a]2D2 = -15.6 ° (c = 0.192, MeOH).
MP = sinters and decomposes. IR (ATR) t 3,700–2,500,
2,922, 2,845, 1,734 (C = O), 1,660, (C = O), 1,607,
1
1,144, 1,066, 1,003, 962, 835 cm-1. H NMR (400 MHz,
CDCl3) d 5.56 (1H, dm, JNH = 73 Hz), 4.57 (1H, m), 4.17
(2H, m), 3.78 (1H, dm, JCH = 143 Hz), 3.34 (1H, m), 2.62
(1H, dm, JCH = 135 Hz), 2.57 (1H, dm, JCH = 135 Hz),
2.11–2.33 (2H, m), 1.20–2.08 (24H, m), 0.56 (2H, br).
13C NMR (100 MHz, CDCl3) d 176.1, 176.0, 174.3
(C = O, d, J = 53 Hz), 61.19, 61.15, 56.9, 56.8, 55.5 (m),
46.9 (dt, J1 = 36 Hz, J2 = 4 Hz), 30.7 (t, J = 35 Hz),
30.0 (t, J = 35 Hz), 28.8 (m), 24.5, 24.0, 23.2
(t, J = 35 Hz), 22.6 (t, J = 35 Hz), 19.1, 14.4. HRMS
12 13
(ESI?) found [M ? H]? 375.2904, C13C6H36BO415N2Na
requires M? 375.2905.
CML bis-hydrochloride-8a (method 1): to a flask con-
taining 7a (35 mg, 0.97 9 10-4 mol, 1 eq.) was added
6 M HCl aq. (1 mL) and the mixture was stirred at reflux
for 1 h. After cooling to room temperature, the mixture was
extracted with ethyl acetate (1 9 2 mL) and the layers
separated. The aqueous layer was loaded directly onto a
C-18 reverse phase plug and eluted with 1 % MeCN/H2O.
Fractions of 1 mL were collected and were visualised by
ninhydrin stain. The fractions that stained positive were
combined and lyophilised affording the title compound as a
white solid (28 mg, quant.).
1
1,455, 1,214, 1,115, 1,003, 964, 920, 843 cm-1. H NMR
(400 MHz, CDCl3) d 5.60 (1H, dm, JNH = 73 Hz), 4.90
(1H, dm, JNH = 72 Hz), 4.17 (2H, q, J = 7.2 Hz), 3.79
(1H, dm, JCH = 141 Hz), 3.40 (2H, d, J = 4.7 Hz), 2.68
(1H, dm, JCH = 134 Hz), 2.61 (1H, dm, JCH = 134 Hz),
2.20 (0.5H, m), 2.00 (1H, m), 1.34–1.98 (17.5H, m), 1.28
(3H, t, J = 7.1 Hz), 0.56 (2H, br). 13C NMR (100 MHz,
CDCl3) d 174.4 (C = O, d, JCC = 53 Hz, 172.8 (C = O),
61.2, 55.4 (m), 50.9, 48.4 (d, JCC = 37 Hz), 31.9, 31.6,
31.5, 31.3, 29.8 (t, JCC = 35 Hz), 28.3 (t, JCC = 35 Hz),
24.5, 24.1, 23.3 (BCH), 23.2 (BCH), 22.6 (t,
JCC = 35 Hz), 14.3. HRMS (ESI?) found [M ? H]?
CML bis-hydrochloride-8a (method 2): a solution of 5
(100 mg, 0.25 mmol, 1 eq., based on lysine used in pre-
vious step) and glyoxylic acid hydrate (140 mg, 1.5 mmol,
6 eq.) in water (1 mL) was heated at 40 °C for 22 h after
which time almost no starting material remained by TLC
analysis. The reaction mixture was cooled to room tem-
perature and extracted with ethyl acetate (3 9 2 mL), the
combined organic layers were washed with saturated brine
(1 9 2 mL), dried over MgSO4, filtered and concentrated
to a pale yellow oil (76 mg). Rf = 0.32 (1:19:80 AcOH/
MeOH/DCM). 1 M aq. HCl (1.5 mL) was added to this
intermediate (insoluble) and the reaction mixture was
refluxed for 2 h after which time a pale yellow homoge-
neous solution was observed and the intermediate had been
consumed (TLC). The reaction mixture was cooled to room
temperature and partitioned between DCM (2 mL) and
H2O (2 mL) and the layers separated, the DCM layer was
washed with H2O (1 9 1 mL) and the combined aqueous
layers lyophilised affording the crude material as a pale
12 13
361.2748, C12C6H34BO145N2 requires M? 361.2750.
9BBN-Et-CEL-7b: to
a mixture of 6 (100 mg,
0.22 mmol, 1 eq.) and K2CO3 (180 mg, 1.31 mmol, 6 eq.)
in dry DMF (0.6 mL) was added ethyl-2-bromopropionate
(85 lL, 0.65 mmol, 3 eq.) as a solution in DMF (0.2 mL).
The reaction was allowed to warm to room temperature and
stirred at room temperature for 18 h. The reaction mixture
was cooled to 0 °C and thiophenol (130 lL, 1.22 mmol,
5 eq.) was added dropwise. The reaction mixture was
allowed to warm to room temperature and stirred for a
further 2.5 h (complete by TLC). The reaction mixture was
123

322
T. M. Woods et al.
brown solid. The crude material was dissolved in H2O
(1 mL) and passed down a short C-18 reverse phase col-
umn eluting with 1 % MeCN/H2O and 1 mL fractions
collected. The fractions that stained positive by ninhydrin
reagent were combined and lyophilised affording the title
compound as an amorphous white solid (46 mg,
0.16 mmol, 66 %). This compound is extremely hygro-
scopic. [a]2D3 = ?10.8° (c = 0.166, H2O). IR (ATR) t
3,660–2,220, 1,743 (C = O), 1,697 (C = O), 1,582, 1,496,
1,412, 1,196, 812 cm-1. 1H NMR (400 MHz, D2O) d 4.06
(1H, dm, JCH = 147 Hz), 3.93 (2H, d, J = 2 Hz), 3.11
(2H, dm, JCH = 144 Hz), 2.13 (1H, m), 1.50–2.0 (4H, m),
1.35 (1H, m). 13C NMR (100 MHz, D2O) d 171.9
(d, J = 59 Hz), 169.0, 52.5 (m), 47.2, 47.0 (dt, J1 =
35 Hz, J2 = 4 Hz), 29.1 (td, J1 = 34 Hz, J2 = 5 Hz), 24.8
saturated brine (2 mL), dried over MgSO4, filtered and
concentrated under reduced pressure. Purification by flash
chromatography (1. 25, 2. 50, 3. 75 % ethyl acetate/hexanes
with 0.25 % AcOH) afforded recovered enone starting
material (24 mg, 35 %) and the title compound as an
amorphous off-white solid (85 mg, 1.42 9 10-4 mol,
54 %). All the lysine starting material was consumed in the
reaction. TLC Rf = 0.39 (10 % MeOH/DCM, UV and nin-
hydrin). MP = sinters and decomposes above 40 °C.
[a]2D4 = ?15.0° (c = 0.240, DCM). IR (ATR)
3,500–2,400, 2,930, 2,857, 1,692, 1,656, 1,451, 1,251, 1,062,
836, 776, 759, 738 cm-1 1H NMR (400 MHz, CDCl3)
t
.
d 9.42 (1H, d, J = 4 Hz, CHO), 7.75 (2H, d, J = 7 Hz), 7.60
(2H, t, J = 7 Hz), 7.39 (2H, t, J = 7 Hz), 7.30 (2H, t,
J = 7 Hz), 6.87 (1H, t, J = 4 Hz), 6.15 (1H, t, J = 4 Hz),
5.49 (1H, dd, J1 = 91 Hz, J2 = 8 Hz, NH), 4.66 (2H, d,
J = 2 Hz), 4.05–4.65 (5H, m), 4.22 (1H, t, J = 7 Hz),
1.15–2.22 (6H, m), 0.988 (9H, s), 0.06 (6H, s). 13C NMR
(100 MHz, CDCl3) d 179.6, 175.8 (d, J = 59 Hz), 156.3 (d,
J = 24 Hz), 144.0, 143.9, 142.5 (d, J = 10 Hz), 141.5,
132.1 (d, J = 10 Hz), 127.9, 127.2, 125.3 (2 9 CH), 120.1,
110.5, 67.3, 57.4, 53.8 (m), 47.3, 45.6 (m), 31.9 (t,
J = 26 Hz), 30.6 (t, J = 26 Hz), 25.9, 22.6 (t, J = 26 Hz),
18.4, -5.2. HRMS (ESI?) found [M ? Na]? 621.2836,
(td, J1 = 35 Hz, J2 = 4 Hz), 21.4 (t, J1 = 34 Hz). HRMS
15
17
(ESI?) found [M ? H]? 213.1330, 12C213C6H N2O4
requires M? 213.1325.
CEL bis-hydrochloride-8b: to a flask containing 7b
(52 mg, 0.14 mmol, 1 eq.) was added 6 M HCl aq. (1 mL)
and the mixture was stirred at reflux for 1 h. After cooling
to room temperature, the mixture was extracted with ethyl
acetate (1 9 3 mL) and the layers separated. The aqueous
layer was loaded directly onto a C-18 reverse phase plug
and eluted with 1 % MeCN/H2O. Fractions of 2 mL were
collected and were visualised by ninhydrin stain. The
fractions that stained positive were combined and lyophi-
lised affording the title compound as an amorphous white
solid (40 mg, 0.13 mmol, 96 %). This compound is very
hygroscopic. [a]2D3 = ?8.5° (c = 0.188, H2O). IR (ATR) t
3,670–2,174, 1,740 (C = O), 1,701 (C = O), 1,577, 1,451,
12 13
15
42
C C6H N2O6SiNa requires M? 621.2846.
27
Fmoc-pyrraline-13:
a
solution of 12 (31 mg,
0.52 9 10-4 mol, 1 eq.) in a 1 % conc. HCl solution in
THF/H2O (9:1, 1 mL) was stirred at room temperature for
8 h. The reaction was diluted with H2O (2 mL) and
extracted with ethyl acetate (3 9 2 mL). The combined
organics were washed with saturated brine (2 mL), dried
over MgSO4, filtered and concentrated under reduced
pressure. Purification by flash chromatography (1. 5, 2.
10 % MeOH/DCM with 0.25 % AcOH) afforded the title
compound as an amorphous white solid (24 mg,
0.50 9 10-4 mol, 96 %). TLC Rf = 0.13 (10 % MeOH/
DCM). MP = sinters and decomposes above 40 °C.
1
1,388, 1,191, 1,107, 798, 732 cm-1. H NMR (400 MHz,
D2O) d 4.02 (1H, dm, J = 146 Hz), 3.96 (1H, qd,
J1 = 7 Hz, J2 = 2 Hz), 3.08 (2H, dm, J1 = 144 Hz),
1.46–1.99 (4H, m), 1.53 (3H, dd, J1 = 7 Hz, J2 = 3 Hz),
1.35 (1H, m). 13C NMR (100 MHz, D2O) d 172.1 (d,
J = 59 Hz), 171.9, 55.7, 52.6 (m), 45.4 (dt, J1 = 35 Hz,
J2 = 4 Hz), 29.2 (td, J1 = 34 Hz, J2 = 5 Hz), 25.2 (td,
J1 = 35 Hz, J2 = 4 Hz), 21.4 (t, J1 = 34 Hz), 14.1.
[a]2D3 = ?3.5° (c = 0.40, CHCl3). IR (ATR)
t
3,650–2,326, 2,931, 1,688, 1,645, 1,501, 1,449, 1,325,
1,191, 1,023, 760, 739 cm-1. 1H NMR (400 MHz, CDCl3)
d 9.40 (1H, d, J = 4 Hz, CHO), 7.74 (2H, d, J = 7 Hz),
7.56 (2H, t, J = 7 Hz), 7.38 (2H, t, J = 7 Hz), 7.29 (2H, t,
J = 7 Hz), 6.83 (1H, t, J = 4 Hz), 6.17 (1H, t, J = 4 Hz),
5.63 (1H, dd, J1 = 92 Hz, J2 = 8 Hz, NH), 4.00–4.78 (6H,
m), 4.37 (1H, d, J = 7 Hz), 4.18 (1H, t, J = 7 Hz),
1.10–2.17 (6H, m). 13C NMR (100 MHz, CDCl3) d 179.8,
175.5 (d, J = 59 Hz), 156.7 (d, J = 24 Hz), 144.0, 143.8,
141.9 (d, J = 13 Hz), 141.4, 132.3 (d, J = 13 Hz), 127.9,
127.3, 125.2 (2 9 CH), 120.1, 110.9, 67.3, 56.3, 53.4 (m),
47.3, 45.6 (m), 31.9 (t, J = 34 Hz), 30.4 (t, J = 34 Hz),
22.3 (t, J = 34 Hz). HRMS (ESI?) found [M ? Na]?
HRMS (ESI?) found [M ? H]? 227.1485, 12C313C6H
15
19
N2O4 requires M? 227.1481.
Fmoc-TBS-pyrraline-12: to a solution of Fmoc-Lys TFA
salt (140 mg, 0.29 mmol, 1.0 eq.) in THF (5 mL) was
added DIPEA (51 lL, 0.29 mmol, 1.0 eq.) and the reaction
stirred at room temperature for 3 h. A white precipitate
starts forming immediately. After 3 h, the reaction mixture
was concentrated to dryness and then dissolved in a THF/
H2O mixture (1:1, 1 mL), the enone 11 was then added
(68 mg, 0.29 mmol, 1 eq.) and the reaction was heated at
40 °C for 18 h. The reaction mixture was diluted with H2O
(2 mL) and extracted with ethyl acetate (3 9 2 mL). The
combined organics were washed with H2O (2 mL) then
12 13
15
28
507.1985, C21C6H N2O6Na requires M? 507.1982.
123

Advanced glycation endproducts
323
Pyrraline-14:
a
solution of 13 (53 mg, 1.09 9
lysine 5. With 5 in hand, the desired nosyl group was
cleanly introduced using standard conditions and furnished
BBN-Ns-lysine 6 in an 89 % yield over two steps from 4.
At this stage, a one-pot alkylation/deprotection reaction
was developed. Hence, a solution of 6 and K2CO3 in DMF
was treated with either ethyl bromoacetate or ethyl
bromopropionate and stirred for 24 h after which time
thiophenol was added and the reaction was stirred for
an additional 2 h affording BBN-OEt-CML 7a and BBN-
OEt-CEL 7b in 77 and 64 % yields, respectively. As
envisaged, use of the 9-BBN and ethyl ester protecting
groups enabled the final global deprotection step to be
carried out by simply refluxing 7a or 7b in 6 M aq HCl for
1 h. After cooling to room temperature, the reaction mix-
ture was partitioned between H2O and ethyl acetate to
remove the organic by-products (BBN) and the aqueous
layer was freeze-dried. The crude material was purified
using a short reverse phase C-18 column eluting with
0.5 % MeCN/H2O to remove any remaining organic by-
products. Finally, the dihydrochloride salts of CML 8a and
CEL 8b were isolated by lyophilization in 99 and 98 %
yields and overall yields of 68 and 56 %, respectively. It is
noteworthy that there are no difficult or lengthy isolation or
purification steps required in these syntheses.
10-4 mol, 1 eq.) in 5 % piperidine/DCM (1.5 mL) was
stirred at room temperature for 1.5 h. The reaction mixture
was concentrated under reduced pressure then under high
vacuum (1 h). Purification by silica flash chromatography
(1. 9:2:1, 2. 9:2:2 MeCN/EtOH/H2O) afforded the title
compound as a pale yellow solid (23 mg, 0.88 9
10-4 mol, 80 %). TLC Rf = 0.25 (9:2:2 MeCN/EtOH/
H2O). [a]2D2 = ? 3.5° (c = 0.40, H2O). IR (ATR) t
3,600–2,400, 1,656, 1,541, 1,392, 1,343, 1,153, 1,000, 779,
722 cm-1 1H NMR (400 MHz, D2O) d 9.35 (1H, d,
.
J = 4 Hz, CHO), 7.15 (2H, d, t = 4 Hz), 6.39 (1H, t,
J = 4 Hz), 4.70 (2H, d, J = 2 Hz), 4.31 (2H, dm,
J1 = 140 Hz), 3.73 (1H, dm, J1 = 145 Hz), 1.88 (2H, bd,
J = 130 Hz), 1.76 (2H, bd, J = 130 Hz), 1.49 (2H, bd,
J = 130 Hz). 13C NMR (100 MHz, D2O) d 181.4, 174.7
(d, J = 54 Hz), 143.4 (d, J = 14 Hz), 131.7 (d,
J = 14 Hz), 126.3, 111.0, 54.8, 54.7 (ddt, J1 = 54 Hz,
J2 = 34 Hz, J3 = 5 Hz), 44.9 (m), 30.3 (td, J1 = 34 Hz,
J2 = 4 Hz), 30.1 (td, J1 = 34 Hz, J2 = 4 Hz), 21.6 (t,
J = 34 Hz). HRMS (ESI?) found [M ? Na]? 285.1304,
12C163C6H N2O4Na requires M? 285.1301.
15
18
Results and discussion
A second, more direct synthesis of CML using BBN
protected lysine 5 was also developed (Scheme 2) and
employed an approach similar to that reported by Kilhberg
et al. (1983). The BBN group was introduced as described
above and the BBN protected lysine 5 was then treated with
an excess of glyoxylic acid in H2O at 40 °C for 22 h. After
cooling to room temperature, the mixture was partitioned
with ethyl acetate and the organic layer concentrated to
dryness affording the formylated intermediate 9, which was
then refluxed in 1 M aq HCl to cleave the formyl and BBN
groups. The only purification required in this route is a
simple filtration after the final step using a short C-18 col-
umn as described above, which furnished the desired CML
dihydrochloride 8a in an overall yield of 66 % from 4.
Unfortunately, attempts to synthesize CEL by replacing
glyoxylic acid with pyruvic acid were unsuccessful.
The first consideration when planning our synthesis was
that the desired compounds have a sufficient mass differ-
ence from their non-labelled analogues to easily distinguish
the peaks in the mass spectrum. Therefore, the synthesis of
the three isotopically labelled monolysyl AGEs com-
menced with [U-13C6, 99 %, U-15N2, 99 %]-L-lysine
dihydrochloride 4, providing a mass difference of 8 amu
from the non-labelled parent analogue.
For the synthesis of stable isotope labelled CML 1 and
CEL 2 it was decided to employ the 9-BBN group to
concomitantly protect the alpha amino and carboxylic acid
groups in lysine according to the method of Dent (Dent
et al. 2002). This approach had the advantage of selectively
protecting the alpha-NH2 group in the presence of the
epsilon-NH2, whilst reducing the number of protection/
deprotection steps required in the syntheses. More impor-
tantly, we envisaged that when used with other acid labile
protecting groups, the BBN group would enable a final
global deprotection step to access the desired free amino
acids, while providing a tractable organic compound up to
this final step. In addition, it was decided to use the nosyl
(Ns) group alkylation methodology as a reliable and effi-
cient method to introduce the required side chain func-
tionality (Kan and Fukuyama 2004).
Our strategy for the synthesis of pyrraline (Scheme
3) relied on the condensation of dihydropyranone 11 with
the side chain amine of a lysine derivative. We had hoped
to utilise BBN-protected lysine 5 as the starting material
for each AGE synthesis, however, despite several attempts
the condensation between 5 and 11 proved unsuccessful.
Therefore, we resorted to the use of Fmoc-lysine and the
required [U-13C6, 99 %, U-15N2, 99 %]-L-Fmoc-lysine
trifluoroacetate 10 was prepared from [U-13C6, 99 %,
U-15N2, 99 %]-L-lysine dihydrochloride 4 according to a
literature procedure (Wiejak et al. 2001).
Our synthesis (Scheme 1) started with treatment of
[U-13C6, 99 %, U-15N2, 99 %]-L-lysine dihydrochloride 4
with 9-BBN dimer in MeOH affording BBN protected
Thus, the TFA lysine derivative 10 was treated with
diisopropylethylamine and the resulting free base reacted
123

324
T. M. Woods et al.
Scheme 1 Synthesis of the
dihydrochloride salts of CML
(8a) and CEL (8b)
O
O
∗
∗
NsCl, K2CO3
9-BBN
MeOH
∗
∗
∗
∗
H2N
∗
H2N
∗
∗
∗
OH
O
∗
∗
∗
∗
Dioxane,
89%
(2 steps)
NH2
H2N
B
.2HCl
∗
∗
4
5
O
Br
O
O
O
EtO
∗
H
∗
∗
∗
∗
R
NsHN
N
∗
∗
∗
∗
∗
EtO
K2CO3, rt, 24 h
O
B
O
∗
∗
∗
∗
H2N
R
H2N
B
∗
∗
then
thiophenol
rt, 2 h
6
7a, R = H (77%)
7b, R = Me (64%)
O
O
6 M aq HCl
reflux, 1 h
H
∗
∗
N
∗
8a, R = H (99%)
8b, R = Me (98%)
∗
∗
HO
OH
∗
∗
R
NH2
.2HCl
∗
Scheme 2 More direct
synthesis of CML hydrochloride
(8a) using glyoxylic acid
1) 9-BBN,
MeOH
reflux, 1.5 h
H
O
O
O
O
∗
∗
∗
∗
∗
∗
H2N
∗
N
∗
OH
∗
∗
HO
∗
∗
O
∗
∗
2) Glyoxylic acid
H2O
NH2
H2N
.2HCl
B
∗
∗
40°C, 22 h
4
9
O
O
H
N
∗
1 M aq HCl
∗
∗
∗
∗
HO
OH
∗
∗
reflux, 4h
66%
(2 steps)
NH2
.2HCl
∗
8a
Scheme 3 Synthesis of
pyrraline (3)
O
TBSO
N
O
∗
∗
THF/H2O
∗
O
H2N
∗
+
∗
∗
∗
OH
O
∗
∗
∗
∗
40 °C, 18 h
54%
∗
∗
OH
∗
NHFmoc
.TFA
OH
∗
NHFmoc
O
∗
OTBS
11
12
10
HO
N
HO
HCl
THF/H2O
O
O
Piperidine
∗
∗
∗
∗
∗
N
∗
∗
∗
∗
∗
OH
∗
∗
OH
∗
∗
8 h
96%
DCM
rt, 1.5 h
80%
NH2
NHFmoc
O
∗
O
∗
13
3
with dihydropyranone 11 (Geng et al. 2012) affording the
desired protected pyrraline 12 in a 54 % yield. It is worth
noting that this compound is a suitable building block for
incorporation into peptides using automated solid phase
peptide synthesis. With 12 in hand, removal of the TBS
group was easily achieved by treatment with 1 M aq HCl in
THF (1:9 v:v) furnishing 13 in excellent yield (96 %).
Finally, the Fmoc group was removed under standard
conditions affording the desired free amino acid of pyrra-
line 3 in a good 80 % yield. As with CML and CEL, there
are no difficult isolation or purification steps required in
this route.
123

Advanced glycation endproducts
325
Kan T, Fukuyama T (2004) Ns strategies: a highly versatile synthetic
method for amines. Chem Commun 4:353–359
Conclusion
Kihlberg J, Bergman R, Wickberg B (1983) Synthesis of strombine: a
new method for monocarboxymethylation of primary amines.
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Ledl F, Schleicher E (1990) The Maillard reaction in food and in the
human body—new results in chemistry, biochemistry and
medicine. Angew Chem Int Ed 102:597–626
Mu¨nch G, Westcott B, Menini T, Gugliucci A (2012) Advanced
glycation endproducts and their pathogenic roles in neurological
disorders. Amino Acids 42:1221–1236
Peppa M, Vlasarra H (2005) Advanced glycation end products and
diabetic complications: a general overview. Hormones 4:28–32
Rabbani N, Thornalley PJ (2012) Glycation research in amino acids: a
place to call home. Amino Acids 42:1087–1096
In summary, we have developed robust and straightforward
syntheses of 13C/15N stable isotope-labelled monolysyl
AGEs CML 1, CEL 2 and pyrraline 3 in overall yields of
68, 56 and 41 %, respectively. These routes are noteworthy
for their practicality, requiring no difficult or lengthy iso-
lation or purification steps. The final compounds are suit-
able as standards for quantitative mass spectrometry
studies and will facilitate investigations into the role that
these AGEs play in the pathogenesis of diabetes and related
diseases. These studies are important steps towards the
development of new and improved therapeutic strategies to
combat these debilitating diseases.
Stitt AW (2001) Advanced glycation: an important pathological event
in diabetic and age related ocular disease. Br J Ophthalmol
85:746
Teerlink T, Barto R, ten Brink HJ, Schalwijk CG (2004) Measure-
ment of N’’-(carboxymethyl)lysine and N’’-(carboxyethyl)lysine
in human plasma protein by stable-isotope-dilution tandem mass
spectrometry. Clin Chem 50:1222–1228
Thornalley PJ, Battah S, Ahmed N, Karachalias N, Agalou S, Babaei-
Jadidi R, Dawnay A (2003) Quantitative screening of advanced
glycation endproducts in cellular and extracellular proteins by
tandem mass spectrometry. Biochem J 375:581–592
Acknowledgments We thank the Maurice Wilkins Centre for
Molecular Biodiscovery for generous financial support.
Conflict of interest The authors declare that they have no conflict
of interest.
Verzijl N, DeGroot J, Oldehinkel E, Bank RA, Thorpe SR, Baynes
JW, Bayliss MT, Bijlsma JWJ, Lafeber FPJG, TeKoppele JM
(2000a) Age-related accumulation of Maillard reaction products
in human articular cartilage collagen. Biochem J 350:381
Verzijl N, DeGroot J, Thorpe SR, Bank RA, Shaw JN, Lyons TJ,
Bijlsma JWJ, Lafeber FPJG, Baynes JW, TeKoppele JM (2000b)
Effect of collagen turnover on the accumulation of advanced
glycation end products. J Biol Chem 275:39027–39031
Wiejak S, Masiukiewicz E, Rzeszotarska B (2001) Improved scalable
synthesis of mono- and bis-urethane derivatives of ornithine.
Chem Pharm Bull 49:1189–1191
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