LETTER
Synthesis of Ne-(Carboxymethyl)lysine
533
10).20 In general we had observed that the major byprod- pressure of five bar and MeOH as solvent.21 CML (1) was
uct in these reactions was the corresponding dialkylated isolated by simple filtration and evaporation in 96% yield.
product. We speculated that the formation of this byprod-
In summary, we have developed a facile, efficient, and
uct could be minimized by adding aldehyde 5 more slowly
convenient method for the synthesis of CML (1) on gram
to a solution of amine 3 and the reducing agent. Thus, by
scale. The synthesis progresses via benzyl ester- and Cbz-
adding aldehyde 5 (1.0 equiv) to a solution of amine 3 and
protected CML 6 by a simple hydrogenation to give, for
NaBH(OAc)3 (1.5 equiv) over 30 minutes, we were able
the first time, pure CML as the free zwitter ion in a crys-
to increase the yield to 47% while no formation of the di-
talline state. We expect that the strategy may be modified
alkylated product was observed (Table 1, entry 11). By in-
to enable access to suitably protected CML amino acid
building blocks for incorporation of CML in peptide
creasing the amount of added aldehyde 5 with 0.2
equivalents (Table 1, entry 12), the yield increased to
chains22 and thereby allow for the observation of AGE at
63%. Further increments in the amount of aldehyde and
work in natural, biological contexts.
increase in reaction time both resulted in a decrease in the
yield (Table 1, entries 13 and 14). Adaptation of the opti-
mal conditions to the use of NaBH3CN resulted in a de-
(S)-Benzyl 6-Amino-2-{[(benzyloxy)carbonyl]amino}hexanoate
crease of yield to 20% (Table 1, entry 15). Furthermore,
the use of BH3·pyridine or BH3·THF as reducing agents
(3)
To a solution of (S)-benzyl 2-{[(benzyloxy)carbonyl]amino}-6-
only furnished traces of the desired product (Table 1, en-
tries 16 and 17). Using the conditions from entry 12
(Table 1), we then synthesized 3.65 g of protected CML 6
in 62% yield.
[(tert-butoxycarbonyl)amino]hexanoate (5.50 g, 11.7 mmol) in
CH2Cl2 (63 mL) at 0 °C was added slowly TFA (42 mL). The result-
ing mixture was stirred at 0 °C for 1.5 h and was then concentrated
under reduced pressure at 0 °C. The residue was dissolved in
CH2Cl2 (250 mL) and was washed with sat. aq Na2CO3 (150 mL).
The aqueous layer was extracted with CH2Cl2 (250 mL), and the
combined organic layers were washed with H2O (250 mL), dried
over Na2SO4, filtered, concentrated at 0 °C and dried in vacuo,
yielding 3 (4.22 g, 98%) as a colorless oil. Rf = 0.40 (EtOAc–
With protected CML 6 in hand, we could then convenient-
ly complete the first synthesis of CML (1) as the free zwit-
ter ion by a simple hydrogenation step (Scheme 2). The
hydrogenation was performed using Pd/C under H2 with a
1
MeOH–concd aq NH3 = 75:20:5). H NMR (300 MHz, CDCl3):
d = 7.40–7.30 (m, 10 H), 5.55–5.40 (m, 1 H), 5.25–5.17 (m, 4 H),
4.48–4.37 (m, 1 H), 2.67–2.53 (m, 2 H), 1.93–1.58 (m, 2 H), 1.47–
1.20 (m, 4 H), 1.17–1.05 (s, 2 H) ppm. 13C NMR (75 MHz, CDCl3):
d = 172.3 (Cq), 155.8 (Cq), 136.2 (Cq), 135.3 (Cq), 128.5, 128.4,
128.4, 128.3, 128.1, 128.0 (10 CH), 67.0 (CH2), 66.9 (CH2), 53.8
(CH), 41.7 (CH2), 33.1 (CH2), 32.4 (CH2), 22.3 (CH2) ppm. NMR
spectra were in full accordance with those reported in literature.17
O
O
OH
H
N
O
Ph
Boc
OH
Ph
O
HN
OH
O
4
2
Cbz
a, b
c
(S)-Benzyl 6-{[(Benzyloxy)-2-oxoethyl]amino}-2-{[(benzy-
loxy)carbonyl]amino}hexanoate (6)
O
O
To a solution of 3 (4.20 g, 11.4 mmol) in DCE (75 mL) at r.t. was
added NaBH(OAc)3 (3.60 g, 17.1 mmol), and then a solution of 5
(2.20 g, 13.7 mmol) in DCE (75 mL) slowly over 30 min. After stir-
ring for 1 h further, H2O (130 mL) was added, the organic layer was
isolated, and the aqueous layer was extracted with CH2Cl2 (150
mL). The combined organic layers were dried over Na2SO4, filtered,
concentrated, and dried in vacuo. Flash column chromatography of
the crude product (SiO2, EtOAc) yielded 6 (3.65 g, 62%) as a color-
less oil. Rf = 0.30 (EtOAc). 1H NMR (300 MHz, CDCl3): d = 7.40–
7.28 (m, 15 H), 5.46–5.32 (m, 1 H), 5.22–5.08 (m, 6 H), 4.45–4.35
(m, 1 H), 3.42 (s, 2 H), 2.58–2.48 (m, 2 H), 1.93–1.60 (m, 2 H), 1.55
(s, 1 H), 1.50–1.40 (m, 2 H), 1.40–1.20 (m, 2 H) ppm. 13C NMR (75
MHz, CDCl3): d = 172.4 (Cq), 172.2 (Cq), 155.9 (Cq), 136.3 (Cq),
135.6 (Cq), 135.3 (Cq), 128.6, 128.5, 128.5, 128.4, 128.4, 128.3,
128.2, 128.1 (15CH), 67.1 (CH2), 66.9 (CH2), 66.6 (CH2), 53.9
(CH), 50.6 (CH2), 48.9 (CH2), 32.4 (CH2), 29.3 (CH2), 22.7 (CH2)
ppm. HRMS (TOF MS ES+): m/z calcd for C30H35N2O6 [M + H]+:
519.2490; found: 519.2488.
H2N
O
Ph
Ph
O
HN
O
5
Cbz
3
d
O
O
H
N
Ph
O
O
Ph
HN
Cbz
6
Scheme 1 Synthesis of protected CML 6. Reagents and conditions:
(a) BnBr, DIPEA, MeCN, r.t., 90%; (b) TFA–CH2Cl2 (2:3), 0 °C,
98%; (c) HIO4, Et2O, r.t., 64%; (d) reductive amination (see Table 1
for details).
O
O
O
O
a
H
H
N
N
HO
OH
BnO
OBn
NH2
HN
Cbz
6
1
Scheme 2 Hydrogenation of protected CML 6 leading to CML (1). Reagents and conditions: a) Pd/C, H2 (5 bar), MeOH, 96%.
© Thieme Stuttgart · New York Synlett 2012, 23, 531–534