The synthesis of 6-trifluoroethyl-L-lysine: A method to introduce functionality at C-6 of L-lysine

Article abstract of DOI:10.1016/j.tetlet.2003.07.001

Described is a method of introducing trifluoroalkyl groups at C-6 of lysine. This chemistry has the potential to introduce a variety of functionality at C-6 of lysine.

Full text of DOI:10.1016/j.tetlet.2003.07.001

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 7345–7347
The synthesis of 6-trifluoroethyl-
L
-lysine: a method to introduce
functionality at C-6 of
L-lysine
E. Ann Hallinan,a,* Clifford R. Dorn,a William M. Moore,b Gina M. Jerome,b Pamela T. Manningc
and Barnett S. Pitzelea
aPharmacia, 4901 Searle Parkway, Skokie, IL 60077, USA
bPharmacia, 800 North Lindbergh Boulevard, St. Louis, MO 63167, USA
cPharmacia, 700 Chesterfield Parkway, St. Louis, MO 63198, USA
Received 2 July 2003; revised 25 July 2003; accepted 28 July 2003
Abstract—Described is a method of introducing trifluoroalkyl groups at C-6 of lysine. This chemistry has the potential to
introduce a variety of functionality at C-6 of lysine.
© 2003 Elsevier Ltd. All rights reserved.
In the course of our research on selective inhibitors of
induced nitric oxide synthase (iNOS), iminoethyl-
L-NIL is an analog of lysine (Lys, 2) where the o-amine
has been functionalized as an amidine. The amidine
moiety is employed as an isostere for the guanidine
group of arginine, which is the endogenous substrate
for the NOS enzymes. Enhancing the desirable biologi-
cal properties of L-NIL by the incorporation of fluorine
into L-NIL was investigated.5 Introduction of fluorine
into the lysine side chain was one of our strategies to
modulate the lysine framework. A variation on this
strategy was incorporation of fluoro-alkyl groups at
C-6 to provide novel lysine analogs. The literature is
virtually devoid of methodology on alkylation at C-6 of
lysine.6 Our strategy was to exploit commercially avail-
L
-
lysine (L-NIL, 1)1 was identified as a potent selective
inhibitor of iNOS.2 Low levels of NO generated by the
constitutive endothelial nitric oxide synthase (eNOS)
regulate blood pressure, platelet adhesion, gastrointesti-
nal motility, and bronchomotor tone. Whereas, low
levels of NO generated by constitutive neuronal nitric
oxide synthase (nNOS) regulate neurotransmission.3
Elevated levels of nitric oxide (NO) generated by the
action of iNOS on
L-arginine and the resulting NO
derived-metabolites cause cellular cytotoxicity and tis-
sue damage and are thought to contribute to the patho-
physiology of a number of human diseases.4
able L-lysine derivatives as opposed to de novo assem-
Scheme 1. Protection of lysinol. Reagents and conditions: (a) 2,2-dimethoxypropane, TsOH, 96 h, rt, 100%; (b) t-BuPh2SiCl,
imidazole, DMF, rt, 96 h, clear oil, 100%.
* Corresponding author. Present address: 135 Barton Ave., Evanston, Il 60202. Tel.: 847-567-2351 (jones); e-mail: eahallinan@comcast.net
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.07.001

7346
E. A. Hallinan et al. / Tetrahedron Letters 44 (2003) 7345–7347
bly of the side chain skeleton. As shown in Scheme 1,
No diastereoselectivity was seen under these reduction
conditions. Had 14 showed any biological promise, we
would have explored the stereoselective reduction of 8.
The silyl protecting group was removed using tetra-n-
butylammonium fluoride and the alcohol was success-
fully oxidized using pyridinium dichromate to give the
nitro-amino acid 10. Catalytic hydrogenation of the nitro
group was achieved with the use of Pearlman’s catalyst
initiation of the synthesis of 6-trifluoroethyl-L-lysine
began with commercially available protected lysinol.7
Two protecting group strategies of the alcohol were
explored. N,O-Acetal 3 worked but the yields in subse-
quent chemistry were low. The alcohol protected as
t-butyldiphenylsilylether 4 proved more robust under the
reaction conditions used to install a trifluoroethyl group
at C-6.
to yield 11. The 6-trifluoroethyl-
-lysine (12)10 was iso-
L
lated upon treatment of 11 with anhydrous hydrochloric
acid.
As illustrated in Scheme 2, the benzyloxycarbonyl group
was removed under standard catalytic hydrogenation
conditions. The o-amine was successfully oxidized to the
nitro group using technical grade m-chloroperbenzoic
acid. Initially, model studies were performed with 1-
nitrohexane, trifluoroacetaldehyde ethyl hemiacetal,8
and a variety of bases. The most effective base for the
Henry reaction was DBU. Thus, with nitro-intermediate
6 in hand, the Henry reaction with DBU and trifluoroac-
etaldehyde ethyl hemiacetal successfully introduced a
trifluoroethyl group yielding 7. Acetylation of the
hydroxy group gave the desired elimination product 8 in
a single step. Reduction of the olefin was achieved with
sodium borohydride in the presence of water to give 9.9
Since our strategy was to identify L-NIL analogs, the
amidine of 6-trifluoroethyl-L-lysine was synthesized as
shown in Scheme 3. The amidine was formed as described
previously using methyl acetimidate hydrochloride.11
The t-butyloxycarbonyl group was removed with anhy-
drous hydrogen chloride as illustrated in Scheme 2
yielding 14.12
Interestingly, the amidine 14 had virtually no activity
against the NOS enzymes where the amino acid 12
showed modest nNOS activity and selectivity as shown
in Table 1.
Scheme 2. Synthesis of 6-(2,2,2-trifluoroethyl)-L-lysine (12). Reagents and conditions: (a) H2, Pd/C, EtOH, 5 psi, quantitative; (b)
ꢀ77% mcpba, 1,2-C2H6Cl2, D, 3 h, 62%; (c) CF3CH(OH)OEt, CH3CN, DBU, rt, 18 h, 75%; (d) acetic anhydride, pyridine, rt,
48 h, 66%; (e) NaBH4, THF/H2O, rt, 3 h, 91%; (f) n-Bu4N+F−, THF, 17 h, 95%; (g) pyridinium dichromate, DMF, 20 h, rt, 65%;
(h) 20% Pd(OH)2, EtOH, 60 psi, 40°C, 16 h, 90%; (j) HCl/dioxane, HOAc, 1 h, rt, quantitative.

E. A. Hallinan et al. / Tetrahedron Letters 44 (2003) 7345–7347
7347
Scheme 3. Amidine synthesis. Reagents and conditions: (a) methyl acetimidate hydrochloride, TEA, DMF, 44%; (b) HCl/dioxane,
HOAc, 1 h, rt, quantitative.
Table 1. NOS Inhibition (IC50, mM)13
Compound no. i-NOS (mM)
e-NOS (mM)
n-NOS (mM)
Selectivity e-NOS/i-NOS
Selectivity n-NOS/i-NOS
L-NIL (1)
12
14
4.9
136
>4000
128
88.8
3121
47.1
15.6
2317
26
0.65
Na
10
0.11
Na
In this paper we have described a method for function-
alizing C-6 of lysine starting with a commercially avail-
able protected lysine derivative. Nitro intermediate 6
allows for the possibility of a variety of interesting
compounds.
2.40–2.55 (m, 1H), 2.98–3.14 (m, 1H), 3.54–3.75 (m, 3H),
4.63–4.78 (m, 2H), 7.37–7.40 (m, 6H), 7.62–7.68 (m, 4H).
IR 1588, 1350 cm−1. Anal. calcd for C28H41N2O5F3Si
(582.72): C, 59.77; H, 7.08; N, 4.60. Found: C, 59.59; H,
7.18; N, 4.60.
10. 1H NMR (400 MHz, D2O, TSP) l 1.45–1.62 (m, 2H),
1.70–1.89 (m, 2H), 1.91–1.95 (m, 2H), 2.58–2.79 (m, 2H),
3.69–3.78 (m, 1H), 3.82 (t, J=6.0 Hz, 1H). 19F (376
MHz, D2O, CFCl3) l −63.0 (q, J=6.7 Hz). Anal. calcd
for C8H15N2O2F3·2.2 HCl·1.2 H2O (330.05): C, 29.11; H,
5.99; N, 8.49. Found: C, 29.49; H, 5.65; N, 8.19.
11. Hallinan, E. A.; Tsymbalov, S.; Dorn, C. R.; Pitzele, B.
S.; Hansen, D. W., Jr.; Moore, W. M.; Jerome, G. M.;
Connor, J. R.; Branson, L. F.; Widomski, D. L.; Zhang,
Y.; Currie, M. G.; Manning, P. T. J. Med. Chem. 2002,
45, 1686–1689.
References
1. Plapp, B. V.; Kim, J. C. Anal. Chem. 1974, 62, 291–294.
2. Moore, W. M.; Webber, R. K.; Jerome, G. M.; Tjoeng,
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37, 3886–3888.
3. Mayer, B.; Hemmens, B. Trends Biochem. Sci. 1997, 22,
477–481.
4. Hobbs, A. J.; Higgs, A.; Moncada, S. Annu. Rev. Phar-
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5. Hallinan et al., submitted for publication.
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Chem. 2003, 68, 2853–2860 and references cited therein.
9. 1H NMR (400 MHz, CDCl3, TMS) l 0.98 (s, 9H),
1.22–1.42 (m, 4H), 1.46 (s, 9H), 1.50–1.61 (m, 2H),
12. 1H NMR (400 MHz, D2O, TSP) l 1.30–1.62 (m, 2H),
1.63–1.85 (m, 2H), 1.91 (br q, 2H), 2.24 (s, 3H), 2.44–2.59
(m, 2H), 3.93–4.02 (m, 1H), 4.03–4.30 (m, 1H). Anal.
calcd for C10H18N3O2F3·2 HCl·1 H2O (360.20): C, 33.35;
H, 6.16; N, 11.67. Found: C, 33.46; H, 5.99; N, 11.33.
13. Moore, W. M.; Webber, R. K.; Fok, K. F.; Jerome, G.
M.; Connor, J. R.; Manning, P. T.; Wyatt, P. S.; Misko,
T. P.; Tjoeng, F. S.; Currie, M. G. J. Med. Chem. 1996,
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