A general synthesis of novel conformationally restricted arginine side- chain mimetics

Article abstract of DOI:10.1016/S0040-4039(00)00869-8

6-(Aminomethyl)-5,6,7,8-tetrahydro-2-quinazolinamine and 4,5,6,7- tetrahydro-2H-indazol-5-ylmethan-amine were synthesized as novel conformationally restricted arginine side-chain mimetics, designed for incorporation into trypsin-like serine protease inhibitors. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00869-8

Tetrahedron Letters 41 (2000) 5589±5592
A general synthesis of novel conformationally restricted
arginine side-chain mimetics
Lucija P. Masic and Danijel Kikelj*
Ï Ï
University of Ljubljana, Faculty of Pharmacy, AsÏkercÏeva 7, 1000 Ljubljana, Slovenia
Received 5 April 2000; accepted 23 May 2000
Abstract
-(Aminomethyl)-5,6,7,8-tetrahydro-2-quinazolinamine and 4,5,6,7-tetrahydro-2H-indazol-5-ylmethan-
amine were synthesized as novel conformationally restricted arginine side-chain mimetics, designed for
6
incorporation into trypsin-like serine protease inhibitors. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: arginine side-chain mimetics; serine proteases.
Trypsin-like serine proteases that cleave a polypeptide chain after the basic amino acids arginine
1) and lysine are involved in many physiological and pathophysiological processes, including
(
1
digestion, blood coagulation, ®brinolysis and in¯ammation. Thus, the speci®c inhibition of
certain trypsin-like serine proteases, e.g. thrombin is a therapeutically important target.2 Arginine
residues or mimetics thereof are common groups that have been incorporated into the P position
,3
1
of many thrombin inhibitors.4 It has been shown that selectivity of an inhibitor for a particular
�7
3
serine protease can be conferred based upon the structural moiety incorporated in the P position.
1
However, most of the inhibitors incorporating an arginine residue lack selectivity for the targeted
serine protease. It is also generally accepted that highly basic moieties such as the guanidino or
amidino groups are the main barriers to absorption of thrombin inhibitors after peroral applica-
tion.5 Thus, signi®cant e€ort has been focused on the design and preparation of arginine
mimetics and arginine side-chain mimetics that could confer selective inhibition for speci®c serine
proteases and possess reduced basicity. With regard to this, we have designed and synthesized
several novel conformationally restricted heterocyclic arginine side-chain mimetics, which were
,6
6
8
incorporated into tripeptidomimetic thrombin inhibitors shown to inhibit human thrombin.
*
Corresponding author. Fax: +386-61-1258031; e-mail: danijel.kikelj@€a.uni-lj.si
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00869-8

5
590
In this paper, we report a general synthesis of novel arginine side-chain mimetics 2 containing a
ve- or six-membered N-heterocyclic ring, optionally substituted by amino group(s) mimicking
®
the guanidino moiety of arginine and a saturated cyclohexane ring mimicking the arginine tri-
methylene side chain. When incorporated into potential inhibitors, the bulky cyclohexane ring of
2 could confer selectivity for certain types of serine proteases. Additionally, the aminomethyl
group bound to the cyclohexane ring of 2 could give to the inhibitors a substantial conforma-
tional freedom as compared to inhibitors comprising similar arginine side-chain mimetics in
9
which the amino group is bound directly to the cyclohexane ring.
At the beginning of our studies directed towards the design and synthesis of selective thrombin
inhibitors we envisaged the hitherto unknown 6-(aminomethyl)-5,6,7,8-tetrahydro-2-quinazolin-
amine (3) and 4,5,6,7-tetrahydro-2H-indazol-5-ylmethanamine (4) as interesting mimetics of the
arginine side-chain, which would be easily functionalized via the nucleophilic aminomethyl
group. Although several heterocyclic amines have recently been used as mimetics of arginine,1
0�13
partially saturated aminomethyl-substituted bicyclic heterocycles 2 have not been described so
far. Therefore, a general synthesis of arginine side-chain mimetics of the general structure 2
seemed highly desirable. A strategy originally designed by us to a€ord the arginine side-chain
mimetics 3 and 4 included the synthesis of the novel enamino ketone 10 as the key intermediate
(Scheme 1), which as a versatile synthon could be easily transformed by cyclocondensation and
subsequent hydrolysis to the bicyclic compounds 3 and 4.
ꢀ
Scheme 1. (a) TOSMIC, t-BuOK/DME, 0 C, 1 h, then rt, 2 h; (b) LiAlH
1
4
/THF, re¯ux, 2 h, then rt, 12 h; (c) Ac
2
O, rt,
h; (d) 90% HCOOH, rt, 16 h; (e) DMFDMA, re¯ux, 16 h
Our synthesis started with the preparation of the known 1,4-dioxaspiro[4.5]dec-8-ylmethanamine
7) by reductive cyanation of 1,4-cyclohexanedione monoethylene acetal (5) with tosylmethyl
(
isocyanide and subsequent reduction of the nitrile 6 with lithium aluminum hydride. After
14

5
591
protecting the amino group of 7, e.g. by acetylation,15 cleavage of the 1,3-dioxolane ring with
0% formic acid gave N-[(4-oxocyclohexyl)methyl]acetamide (9). The ketone 9 was converted by
9
condensation with dimethylformamide dimethyl acetal (DMFDMA) into novel enamino ketone
16
1
7
1
0
(Scheme 1), which is a useful intermediate for further transformations into di€erent hetero-
cyclic compounds 2.
The reaction of the enamino ketone 10, e.g. with guanidine hydrochloride in the presence of
sodium ethoxide gave the tetrahydroquinazoline derivative 11,18 which, upon hydrolysis under
basic conditions, produced the amine 3 (Scheme 2). Similarly, the enamino ketone 10 was
smoothly transformed with hydrazine hydrate into the tetrahydroindazole derivative 12,19 which,
after acid hydrolysis, provided 4,5,6,7-tetrahydro-2H-indazol-5-ylmethanamine (4) (Scheme 2).
Scheme 2. (a) Guanidine hydrochloride/NaOEt, abs. EtOH, re¯ux, 3 h; (b) aq. NaOH, MeOH, re¯ux, 6 h; (c)
NH NH
2
2
ÂH
2
O, rt, 16 h; (d) 6M HCl, re¯ux, 6 h
To summarize, we have succeeded in preparing 6-(aminomethyl)-5,6,7,8-tetrahydro-2-quinazolin-
amine (3) and 4,5,6,7-tetrahydro-2H-indazol-5-ylmethanamine (4) as novel conformationally
restricted arginine side-chain mimetics. These compounds are interesting peptidomimetic building
blocks for incorporation into various serine protease inhibitors. Starting from the key inter-
mediate, the novel enamino ketone 10, the method is generally applicable for the synthesis of
arginine side-chain mimetics 2,20 which are currently under investigation in our laboratory and
will be reported in due course.
Acknowledgements
This work was ®nancially supported by the Ministry of Science and Technology of the
Republic of Slovenia.
References
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0. St Laurent, D. R.; Balasubramanian, N.; Han, W. T.; Trehan, A.; Federici, M. E.; Meanwell, N. A.; Wright, J. J.;
Seiler, S. M. Bioorg. Med. Chem. 1995, 3, 1145±1156.
1
1
1. Bone, R.; Lu, T.; Illig, C. R.; Soll, R. M.; Spurlino, J. C. J. Med. Chem. 1998, 41, 2068±2075.
2. Rewinkel, J. B. M.; Lucas, H.; van Galen, P. J. M.; Noach, A. B. J.; van Dinther, T. G.; Rood, A. M. M.;
Jenneboer, A. J. S. M.; van Boeckel, C. A. A. Bioorg. Med. Chem. Lett. 1999, 9, 685±691.
3. Feng, D. M.; Gardell, S. J.; Lewis, S. D.; Bock, M. G.; Chen, Z.; Freidinger, R. M.; Naylor-Olsen, A. M.; Ramjit,
H. G.; Woltmann, R.; Baskin, E. P.; Lynch, J. J.; Lucas, R.; Schafer, J. A.; Dancheck, K. B.; Chen, I. V.; Mao,
S. S.; Krueger, J. A.; Hare, T. R.; Mulichack, A. M.; Vacca, J. P. J. Med. Chem. 1997, 40, 3726±3733.
4. Becker, D. P.; Flynn, D. L. Synthesis 1992, 1080±1082.
1
1
1
1
1
5. The application of other protecting groups is currently under study.
6. Abdulla, R. F.; Fuhr, K. H. J. Org. Chem. 1987, 43, 4248±4254.
^1 1
7. Compound 10: IR (KBr): ꢀ 3278, 2929, 1637, 1540, 1420, 1333, 1202, 1128, 1010 cm ; H NMR (300 MHz,
CDCl
(
3
): ꢁ 1.38±1.50 (m, 1H, CH), 1.73±1.90 (m, 2H, CH
2
), 2.01 (s, 3H, CH
3
2
), 2.25±2.50 (m, 3H, CH , CH), 3.09
s, 6H, 2ÂCH ), 2.80±2.92 (m, 1H, CH), 3.11±3.23, 3.30±3.39 (2Âm, 2H, CH NH), 5.75 (s br, 1H, NH), 7.49 (s,
3
2
+
1
H, CH-methylidene); MS (70 eV, EI): m/z (%) 224 (M , 56), 152 (100). Anal. calcd for C H N O : C, 64.26; H,
12 20 2 2
8
.99; N, 12.49. Found: C, 63.95; H, 8.89; N, 12.39.
1
8. (þ)-N-[(2-Amino-5,6,7,8-tetrahydro-6-quinazolinyl)methyl]acetamide (11): To a solution of sodium ethoxide (0.1
g, 4.46 mmol) in 20 ml of absolute ethanol, guanidine hydrochloride (0.43 g, 4.46 mmol) was added. After stirring
for 30 min, a solution of N-{3-[(dimethylamino)methylidene]-4-oxocyclohexyl}methyl)acetamide (10) (1.0 g, 4.46
mmol) in absolute ethanol was added and the reaction mixture was re¯uxed under an argon atmosphere for 3
hours. The separated solid was collected by ®ltration to give 0.75 g (76%) of white solid; IR (KBr): ꢀ 3304, 3084,
^1 1
2
944, 2859, 1674, 1649, 1602, 1560, 1493, 1421, 1376, 1265, 1211, 1096, 960, 786, 668 cm ; H NMR (300 MHz,
CDCl ): ꢁ 1.42±1.58 (m, 1H, CH), 1.88±2.08 (m, 2H, CH ), 2.04 (s, 3H, CH ), 2.25±2.38 (m, 1H, CH), 2.68±2.82
m, 3H, CH , CH), 3.32 (t, 2H, J=6.6 Hz, CH NH), 4.83 (s br, 2H, 2-NH ), 5.60 (s br, 1H, NH), 8.00 (s, 1H, 4-
3
2
3
(
2
2
2
+
16 4
CH); MS (70 eV, EI): m/z (%) 220 (M , 35), 148 (100). Anal. calcd for C11H N O: C, 59.98; H, 7.32; N, 25.44.
Found: C, 59.76; H, 7.06; N, 25.21.
19. (þ)-N-(4,5,6,7-Tetrahydro-2H-indazol-5-ylmethyl)acetamide (12): A solution of the enamino ketone 10 (1.0 g, 4.46
mmol) and NH NH ÂH O (0.25 ml, 5.0 mmol) in 20 ml of methanol was stirred at room temperature for 16
2
2
2
hours. The solvent was evaporated, and the product was puri®ed by column chromatography on Florisil using
ethyl acetate:methanol (2:1) as eluent to yield 0.56 g (65%) of 12; IR (KBr): ꢀ 3261, 2928, 1652, 1558, 1437, 1368,
^
1 1
1
1
1
293, 1088, 1035, 962, 782, 605 cm ; H NMR (300 MHz, CDCl ): ꢁ 1.49±1.60 (m, 1H, CH), 2.02 (s, 3H, CH ),
3
3
.89±2.05 (m, 2H, CH
2
), 2.18±2.28 (m, 1H, CH), 2.58±2.90 (m, 3H, CH NH), 3.51 (s,
H, NH), 5.60 (s broad, 1H, NH), 7.31 (s, 1H, 3-CH); MS (70 eV, EI): m/z (%) 193 (M , 40), 121 (100). Anal.
2
, CH), 3.20±3.40 (m, 2H, CH
+
2
calcd for C H N O: C, 62.15; H, 7.82; N, 21.74. Found: C, 61.88; H, 7.85; N, 21.44.
15
1
0
3
2
0. Compounds 3, 4, 10, 11 and 12 were obtained as racemates; experiments directed towards the preparation of pure
enantiomers of arginine side-chain mimetics 2 are in progress.