Study of Squaric Acid-Based Peptidic Inhibitors
1
solid according to the general procedure. H NMR (300 MHz,
and the mixture was heated at reflux overnight. The solvent
was evaporated and the crude product was purified by column
chromatography (3:97 MeOH/CH2Cl2) giving 14 (0.032 g, 0.091
mmol, 67%). 1H NMR (300 MHz, CD3OD) δ 4.22 (s, 1 H), 3.32
(s, 1 H), 2.74 (s, 3 H), 1.69 (m, 3 H), 0.96 (s, 15 H); 13C NMR
(75 MHz, CD3OD) δ 196.3, 187.8, 180.7, 172.5, 171.9, 61.3,
55.8, 40.6, 34.5, 26.1, 25.1, 24.8, 22.5, 20.9; HRMS-ESI (M -
H+) calcd for C17H26N3O5 352.1872, found 352.1880.
CD3OD) δ 7.21 (m, 5 H), 4.63 (dd, J ) 9.6, 9.3 Hz, 1 H), 4.43
(d, J ) 8.6 Hz, 1H) 3.49 (s, 3 H), 3.12 (dd, J ) 6.3, 5.3 Hz, 1
H), 2.88 (dd, J ) 9.7, 9.3 Hz, 1 H), 2.71 (s, 3 H), 1.86 (m, 1 H),
1.52 (m, 1 H), 1.16 (m, 1 H), 0.90 (m, 6 H); 13C NMR (75 MHz,
CD3OD) δ 179.6, 178.9, 172.7, 171.7, 166.4, 166.2, 137.2, 129.2,
128.4, 126.7, 62.3, 54.9, 40.4, 38.1, 37.3, 25.4, 24.5, 14.5, 9.92;
HRMS-FAB (M + Na+) calcd for C21H28N4NaO5 439.1957,
found 439.1968.
3-(Hydroxymethylamino)-2-(L-isoleucine methyl ester)-
4-thioxo-2-cyclobuten-1-one (15). Compound 6a (0.100 g,
0.370 mmol) was dissolved in 2 mL of CH2Cl2 and the solution
was allowed to stir while Lawesson’s reagent (0.150 g, 0.370
mmol) was added. The reaction was monitored by TLC until
all the starting material had been consumed. The solvent was
then evaporated and the crude material was purified by
column chromatography (5:95 MeOH/CH2Cl2) providing 15 as
3-(Hydroxymethylamino)-4-(L-isoleucyl-L-phenylgly-
cine methyl amide)-3-cyclobutene-1,2-dione (12e). Com-
pound 12e (0.021 g, 0.053 mmol, 65%) was prepared from 8
(0.016 g, 0.08 mmol) and 11e (0.023 g, 0.083 mmol) according
1
to the general procedure. H NMR (300 MHz, CD3OD) δ 7.42
(m, 2 H), 7.35 (m, 3 H), 5.42 (s, 1 H), 4.6 (d, J ) 6 Hz, 1 H),
3.47 (s, 3 H), 2.73 (s, 3 H), 1.96 (m, 1 H), 1.63 (m, 1 H), 1.26
(m, 1 H), 1.0 (m, 6 H); 13C NMR (75 MHz, CD3OD) δ 179.1,
178.5, 171.1, 171.0, 166.1, 165.9, 137.3, 128.4, 128.0, 127.2,
61.8, 57.3, 40.0, 37.7, 25.0, 24.2, 14.1, 9.8; HRMS-FAB (M +
Na+) calcd for C20H26N4O5Na 425.1801, found 425.1810.
1
a yellow solid (0.062 g, 0.23 mmol, 62%). H NMR (300 MHz,
CDCl3) δ 9.02 (br s, 1 H), 8.19 (br s, 1 H), 5.93 (br s, 1 H), 3.76
(s, 3 H), 3.67 (s, 3 H), 2.20 (m, 1 H), 2.07 (m, 1 H), 1.48 (m, 1
H), 1.26 (m, 1 H), 1.00 (d, J ) 6.8 Hz, 3 H), 0.93 (t, J ) 7.3 Hz,
3 H); 13C NMR (75 MHz, CDCl3) δ 205.4, 201.6, 170.9, 170.8,
170.6, 61.2, 52.9, 39.4, 31.6, 25.3, 15.8, 12.2; ESI-MS (M + H+)
calcd for C12H19N2O3S2 271.1294, found 271.1287.
3-(Hydroxymethylamino)-4-(L-isoleucyl-L-trypto-
phan methyl amide)-3-cyclobutene-1,2-dione (12f). Com-
pound 12f (0.057 g, 0.125 mmol, 78%) was prepared from 8
(0.032 g, 0.16 mmol) and 11f (0.053 g, 0.16 mmol) according
3-(Hydroxymethylamino)-2-(L-isoleucyl-L-phenylala-
nine methyl amide)-4-thioxo-2-cyclobuten-1-one (16).
Compound 16 (0.093 g, 0.216 mmol, 71%) was prepared as a
yellow solid according to the procedure used to prepare 15
starting from 12d (0.127 g, 0.305 mmol) and Lawesson’s
reagent (0.123 g, 0.304 mmol). The crude material was purified
by column chromatography (3:97 MeOH/CH2Cl2). 1H NMR (300
MHz, CD3OD) 7.15 (m, 5 H), 5.56 (d, J ) 7.6 Hz, 1 H), 4.90 (s,
3 H), 4.68 (m, 1 H), 3.63 (s, 2 H), 3.10 (m, 1 H), 2.72 (d, J )
0.6 Hz, 3 H), 1.91 (m, 1 H), 1.49 (m, 1 H), 1.17 (m, 1 H), 0.87
(m, 6 H); 13C NMR (75 MHz, CD3OD) δ 206.7, 204.0, 172.7,
171.1, 170.7, 170.4, 137.1, 129.3, 128.4, 126.5, 60.8, 54.9, 38.3,
37.6, 30.4, 25.5, 24.4, 14.5, 10.3; ESI-MS (M + Na+) calcd for
C21H28N4O4SNa 455.1729, found 455.1738.
3-(Hydroxymethylamino)-2-(L-isoleucyl-L-leucine meth-
yl amide)-4-thioxo-2-cyclobuten-1-one (17). Compound 17
was prepared as a yellow solid (0.013 g, 0.032 mmol, 59%) from
12g (0.021 g, 0.055 mmol) and Lawesson’s reagent (0.022 g,
0.055 mmol) according to the procedure used to prepare
compound 15. The crude material was purified by column
chromatography (1.3:98.7 MeOH/CH2Cl2). 1H NMR (300 MHz,
CD3OD) δ 5.67 (d, J ) 6.8 Hz, 1 H), 4.39 (dd, J ) 9.5, 5.2 Hz,
1 H), 3.63 (s, 3 H), 2.73 (s, 3 H), 2.00 (m, 1 H), 1.56 (m, 4 H),
1.24 (m, 1 H), 0.96 (m, 12 H); 13C NMR (75 MHz, CD3OD) δ
206.6, 203.9, 173.5, 171.2, 170.9, 170.2, 60.5, 51.9, 40.6, 37.9,
29.9, 24.9, 24.5, 23.9, 22.0, 20.5, 14.2, 10.2; HRMS-FAB (M +
Na+) calcd for C18H30N4O4SNa 421.1885, found 421.1880.
1
to the general procedure. H NMR (400 MHz, CD3OD) δ 7.57
(d, J ) 7.8 Hz, 1 H), 7.31 (d, J ) 8.1 Hz, 1 H), 7.07 (m, 2 H),
6.99 (m, 1 H), 4.66 (dd, J ) 8.1, 6.4 Hz, 1 H), 4.46 (d, J ) 8.2
Hz, 1 H), 3.47 (s, 3 H), 3.25 (dd, J ) 14.6, 6.3 Hz, 1 H), 3.11
(dd, J ) 14.5, 8.1 Hz, 1 H), 2.67 (s, 3 H), 1.86 (m, 1 H), 1.50
(m, 1 H), 1.13 (m, 1 H), 0.88 (m, 6 H); 13C NMR (100 MHz,
CD3OD) δ 179.1, 178.5, 172.8, 171.3, 166.0, 165.8, 136.6, 127.3,
123.3, 120.9, 118.4, 117.9, 110.9, 109.3, 62.0, 54.3, 40.0, 37.3,
27.8, 25.0, 24.1, 14.1, 9.7; HRMS-FAB (M + Na+) calcd for
C23H29N5O5Na 478.2066, found 478.2060.
3-(Hydroxymethylamino)-4-(L-isoleucyl-L-leucine meth-
yl amide)-3-cyclobutene-1,2-dione (12 g). Compound 12g
(0.051 g, 0.133 mmol, 82%) was prepared from 8 (0.032 g, 0.16
mmol) and 11g (0.042 g, 0.163 mmol) according to the general
procedure. 1H NMR (400 MHz, CD3OD) δ 4.57 (d, J ) 8.1 Hz,
1 H), 4.41 (dd, J ) 9.4, 5.4 Hz, 1 H), 3.50 (s, 3 H), 2.74 (s, 3
H), 1.96 (m, 1 H), 1.63 (m, 3 H), 1.53 (m, 1 H), 1.22 (m, 1 H),
0.94 (m, 12 H); 13C NMR (100 MHz, CD3OD) δ 179.1, 178.6,
173.5, 171.6, 166.3, 165.9, 61.9, 51.9, 40.6, 40.1, 37.6, 25.0, 24.5,
24.3, 22.0, 20.7, 14.3, 9.9; HRMS-FAB (M + H+) calcd for
C18H31N4O5 383.2295, found 383.2304.
3-(Hydroxymethylamino)-4-(L-leucyl-L-tert-leucinemeth-
yl amide)-3-cyclobutene-1,2-dione (12h). Compound 12h
(0.028 g, 0.074 mmol, 74%) was prepared from 8 (0.02 g, 0.1
mmol) and 11h (0.026 g, 0.1 mmol) according to the general
1
procedure. H NMR (400 MHz, DMSO-d6) δ 4.76 (br s, 1 H),
4.2 (d, J ) 9.6 Hz, 1 H), 3.38 (s, 3 H), 2.57 (d, J ) 4.4 Hz, 3 H),
1.76 (m, 1 H), 1.62 (m, 1 H), 1.51 (m, 1 H), 0.87 (s, 15 H); 13C
NMR (75 MHz, CD3OD) δ 179.8, 178.9, 172.7, 171.9, 166.5,
166.3, 61.3, 56.6, 41.1, 40.4, 34.4, 26.1, 25.0, 24.9, 22.5, 20.8;
HRMS-FAB (M + Na+) calcd for C18H30N4O5Na 405.2114,
found 405.2105.
3-(L-Leucyl-L-tert-leucine methyl amide)-4-methoxy-3-
cyclobutene-1,2-dione (13). To a stirred solution of 2 (0.021
g, 0.15 mmol) in MeOH was added 11h (0.042 g, 0.165 mmol)
and the mixture was heated at reflux overnight. The solvent
was evaporated and the crude product was purified by column
chromatography (3:97 MeOH/CH2Cl2) giving compound 13
(0.052 g, 0.136 mmol, 95%). 1H NMR (300 MHz, CD3OD) δ
4.40 (s, 4 H), 4.25 (d, J ) 9 Hz, 1 H), 2.74 (s, 3 H), 1.68 (m, 3
H), 0.99 (d, J ) 6 Hz, 15 H); 13C NMR (75 MHz, CD3OD) δ
188.7, 187.9, 184.2, 183.7, 178.0, 176.9, 173.1, 172.6, 172.0,
171.9, 171.4, 60.8, 60.7, 59.9, 56.8, 56.1, 25.6, 24.6, 24.5, 22.1,
20.0; HRMS-ESI (M + Na+) calcd for C18H29N3O5Na 390.2005,
found 390.2010.
3-(Hydroxymethylamino)-2-(L-leucyl-L-tert-leucine
methyl amide)-4-thioxo-2-cyclobuten-1-one (18). Com-
pound 18 was prepared as a yellow solid (0.014 g, 0.035 mmol,
41%) from 12h (0.033 g, 0.086 mmol) and Lawesson’s reagent
(0.035 g, 0.086 mmol) according to the procedure used to
prepare compound 15. The crude material was purified by
column chromatography (1.3:98.7 MeOH/CH2Cl2). 1H NMR
(400 MHz, CD3OD) δ 5.92 (s, 1 H), 4.21 (s, 1 H), 3.62 (s, 3 H),
2.74 (s, 3 H), 1.72 (m, 3 H), 0.98 (m, 15 H); 13C NMR (100 MHz,
CD3OD) δ 206.1, 203.5, 171.7, 171.4, 170.7, 170.0, 61.2, 54.8,
40.7, 34.2, 30.2, 25.9, 24.8, 24.5, 22.2, 20.6; LRMS-FAB (M +
Na+) calcd for C18H30N4O4SNa 421.0, found 421.1.
3,4-Di(L-phenylalanine methyl ester)-3-cyclobutene-
1,2-dione (19). To a stirred solution of l-phenylalanine methyl
ester HCl salt (0.646 g, 3.0 mmol) in MeOH (10 mL) were
added KOH (0.168 g, 3.0 mmol) and compound 7 (0.256 g, 1.13
mmol). Formation of a white precipitate was observed. The
mixture was stirred for 3 h at room temperature, the solvent
was evaporated, EtOAc was added, and the solution was
washed with H2O. The organic layer was dried over MgSO4,
and the solvent was evaporated. The crude product was
recrystallized from MeOH. The resulting white crystals were
3-Hydroxy-4-(L-leucyl-L-tert-leucine methyl amide)-3-
cyclobutene-1,2-dione (14). To a stirred solution of 13 (0.05
g, 0.136 mmol) in MeOH (3 mL) was added 0.15 N HCl (1 mL)
J. Org. Chem, Vol. 70, No. 26, 2005 10801