Chemical modification of usnic acid 2. Reactions of (+)-usnic acid with amino acids

Article abstract of DOI:10.1007/s11172-007-0189-7

The condensation of amino acids with (+)-usnic acid involves the 2-positioned acetyl group of the latter and gives derivatives containing an enamine fragment.

Full text of DOI:10.1007/s11172-007-0189-7

Russian Chemical Bulletin, International Edition, Vol. 56, No. 6, pp. 1249—1251, June, 2007
1249
Chemical modification of usnic acid
2.* Reactions of (+)ꢀusnic acid with amino acids**
O. A. Luzina,ꢀ M. P. Polovinka, N. F. Salakhutdinov, and G. A. Tolstikov
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry,
Siberian Branch of the Russian Academy of Sciences,
9 prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383) 330 9752. Eꢀmail: luzina@nioch.nsc.ru
The condensation of amino acids with (+)ꢀusnic acid involves the 2ꢀpositioned acetyl
group of the latter and gives derivatives containing an enamine fragment.
Key words: (+)ꢀusnic acid, amino acids, enamines.
(+)ꢀUsnic acid (1), which is a key secondary metaboꢀ
lite of some lichens, is known2,3 to exhibit antiviral, antiꢀ
biotic, fungicidal, analgesic, antituberculous, and other
types of biological activity. In order to obtain new bioꢀ
logically active compounds, previously,1 we carried out
chemical modification of acid 1 on treatment with
perfluoroolefins in the presence of bases and thus syntheꢀ
sized a number of its polyfluoroalkyl and perfluoroalkenyl
derivatives. We continued investigation of synthetic transꢀ
formations of usnic acid (1) by performing its condensaꢀ
tion with amino acids. It is known4—6 that introduction of
an amino acid fragment into a biologically active molecule
may enhance the activity. In addition, upon such a modiꢀ
fication, new compounds acquire a carboxy group, which
extends the range of possible transformations and can
increase water solubility, which is fairly important for
biological agents.
cine (6), Lꢀmethionine (7), and Lꢀserine (8), result in
enamine derivatives 9—15 (Scheme 1).*
Scheme 1
No data on the reactions of usnic acid (1) with amino
acids were reported in the literature. A publication7 deꢀ
scribes reactions of compound 1 with some primary
amines, resulting in the condensation products involving
the carbonyl groups of either the acetyl fragment at C(2)
or two acetyl fragments at C(2) and C(6) to give enamine
derivatives (more rarely).
R = C(16)H2C(17)OOH (Gly) (2, 9); C(16)H2C(17)H2C(18)OOH
(βꢀAla) (3, 10); C(16)H(C(17)OOH)C(18)H2Ph (Phe) (4, 11);
C(16)H(C(17)OOH)C(18)H(C(19)H3)(C(20)H3) (Val) (5, 12);
C(16)H(C(17)OOH)C(18)H2C(19)H(C(20)H3)(C(21)H3) (Leu) (6, 13);
C(16)H(C(17)OOH)C(18)H2C(19)H2SC(20)H3 (Met) (7, 14);
C(16)H(C(17)OOH)C(18)H2OH (Ser) (8, 15)
Results and Discussion
Unlike the condensation with primary amines deꢀ
scribed in the literature,7 where the reaction took place
on refluxing of reactants in ethanol, the reaction with
amino acids proceeded only in an alkaline medium.
We found that the reactions of compound 1 with a
number of available amino acids, specifically, glycine (2),
βꢀalanine (3), Lꢀphenylalanine (4), Lꢀvaline (5), Lꢀleuꢀ
* For Part 1, see Ref. 1.
** Dedicated to the memory of Academician N. N. Vorozhtsov
on the 100th anniversary of his birth.
* The atom numbering in the compounds is given for assignment
of NMR signals and does not always coincide with atom numꢀ
bering in the IUPAC name.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1203—1205, June, 2007.
1066ꢀ5285/07/5606ꢀ1249 © 2007 Springer Science+Business Media, Inc.

1250
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 6, June, 2007
Luzina et al.
dione (9) was obtained in the reaction with glycine (2). The
precipitate was collected on a filter, washed with hot water,
and dried in air. Yield 88%, m.p. 210 °C (dec.). 1H NMR
(DMSOꢀd6), δ: 1.65 (s, 3 H, C(15)H3); 1.97 (s, 3 H, C(10)H3);
2.55 (s, 3 H, C(12)H3); 2.64 (s, 3 H, C(14)H3); 4.48 (d, 2 H,
C(16)H2, J = 4.6 Hz); 5.90 (s, 1 H, H(4)); 12.21 (s, 1 H,
C(9)OH); 13.14 (br.s, 1 H, NH); 13.40 (s, 1 H, C(7)OH).
13C NMR (DMSOꢀd6), δ: 7.4 (C(15)); 18.7 (C(12)); 30.9
(C(14)); 31.6 (C(10)); 45.4 (C(16)); 56.3 (C(9b)); 100.8 (C(2));
101.8 (C(6)); 102.3 (C(4)); 105.1 (C(9a)); 106.3 (C(8)); 155.7
(C(5a)); 157.6 (C(9)); 162.5 (C(7)); 169.2 (C(11)); 172.9
(C(17)); 174.9 (C(4a)); 188.6 (C(3)); 197.5 (C(1)); 201.1
(C(13)). IR, ν/cm–1: 843, 1213, 1288, 1370, 1455, 1552,
1628, 1698, 1734, 3080, 3421. Highꢀresolution MS, found:
m/z 401.11090 [M]+. C20H19NO8. Calculated: M = 401.11105.
6ꢀAcetylꢀ2ꢀ[1ꢀ(2ꢀcarboxyethylamino)ethylidene]ꢀ7,9ꢀdihydrꢀ
oxyꢀ8,9bꢀdimethylꢀ1,2,3,9bꢀtetrahydrodibenzo[b,d]furanꢀ1,3ꢀ
dione (10) was obtained in the reaction with βꢀalanine (3). The
reaction mixture containing a finely dispersed precipitate was
extracted three times with ethyl acetate, the extract was dried
with MgSO4, and the solvent was removed on a rotary evaporaꢀ
tor. Compound 10 was isolated by column chromatography on
SiO2 (gradient elution with CHCl3—AcOEt, 0→50%). Yield
55%, m.p. 105—110 °C. 1H NMR (CDCl3), δ: 1.65 (s, 3 H,
C(15)H3); 2.04 (s, 3 H, C(10)H3); 2.61 (s, 3 H, C(12)H3); 2.64
(s, 3 H, C(14)H3); 2.79 (t, 2 H, C(17)H2, J = 6.1 Hz); 3.79 (dt,
2 H, C(16)H2, J = 6.1 Hz, J = 5.8 Hz); 5.80 (s, 1 H, H(4));
11.71 (s, 1 H, C(9)OH); 13.36 (br.s, 1 H, NH); 13.27 (s, 1 H,
C(7)OH). 13C NMR (CDCl3), δ: 7.5 (C(15)); 18.3 (C(12));
31.2 (C(14)); 32.0 (C(10)); 33.4 (C(17)); 39.4 (C(16)); 57.4
(C(9b)); 101.3 (C(6)); 102.0 (C(4)); 102.3 (C(2)); 104.9 (C(9a));
108.1 (C(8)); 155.8 (C(5a)); 158.1 (C(9)); 163.5 (C(7)); 173.7
(C(11)); 174.7 (C(17)); 175.5 (C(4a)); 190.5 (C(3)); 198.5
(C(1)); 200.7 (C(13)). IR, ν/cm–1: 668, 1068, 1289, 1372, 1464,
1559, 1626, 1695, 1727, 3023. Highꢀresolution MS, found:
m/z 415.12671 [M]+. C21H21NO8. Calculated: M = 415.12670.
6ꢀAcetylꢀ2ꢀ[1ꢀ(1ꢀcarboxyꢀ2ꢀphenylethylamino)ethylidene]ꢀ
7,9ꢀdihydroxyꢀ8,9bꢀdimethylꢀ1,2,3,9bꢀtetrahydrodibenzo[b,d]fuꢀ
ranꢀ1,3ꢀdione (11) was obtained in the reaction with Lꢀphenylꢀ
alanine (4). The filtrate was extracted three times with chloroꢀ
form, and the extract was dried with MgSO4, concentrated
in vacuo, and combined with the precipitate. Compound 11 was
isolated by column chromatography on SiO2 (gradient elution
with CHCl3—AcOEt, 0→50%). Yield 59%, m.p. 240 °C (dec.).
1H NMR (CDCl3), δ: 1.63 (s, 3 H, C(15)H3); 1.97 (s, 3 H,
C(10)H3); 2.39 (s, 3 H, C(12)H3); 2.63 (s, 3 H, C(14)H3); 3.15
(dd, 1 H, C(18)H2, J = 14 Hz, J = 7 Hz); 3.29 (dd, 1 H, C(18)H2,
J = 14 Hz, J = 4.5 Hz); 5.06 (dd, 1 H, H(16), J = 4 Hz, J =
7 Hz); 5.84 (s, 1 H, H(4)); 7.20—7.27 (5 H arom.); 12.02 (s,
1 H, C(9)OH); 13.35 (br.s, 1 H, NH); 13.35 (s, 1 H, C(7)OH).
13C NMR (CDCl3), δ: 7.5 (C(15)); 18.5 (C(12)); 31.0 (C(14));
31.6 (C(10)); 38.1 (C(18)); 56.4 (C(9b)); 57.3 (C(16)); 100.8
(C(2)); 101.7 (C(6)); 102.4 (C(4)); 105.0 (C(9a)); 106.4 (C(8));
127.1 (1 C), 128.4 (2 C), 129.5 (2 C), 135.4 (1 C) (C arom.);
155.7 (C(5a)); 157.6 (C(9)); 162.5 (C(7)); 170.8 (C(11)); 172.9
(C(17)); 174.5 (C(4a)); 188.8 (C(3)); 197.7 (C(1)); 200.9
(C(13)). IR, ν/cm–1: 848, 1070, 1201, 1287, 1372, 1460, 1542,
1632, 1701, 1738, 2926, 3042, 3436. Highꢀresolution MS, found:
m/z 491.15982 [M]+. C27H25NO8. Calculated: M = 491.15800.
6ꢀAcetylꢀ2ꢀ[1ꢀ(1ꢀcarboxyꢀ2ꢀmethylpropylamino)ethylidene]ꢀ
7,9ꢀdihydroxyꢀ8,9bꢀdimethylꢀ1,2,3,9bꢀtetrahydrodibenzo[b,d]fuꢀ
We found that compounds 9—15 are formed only in the
pH range of 9—10. When pH < 9, the condensation does
not occur, while at pH >10, usnic acid (1) decomposes.2
The method for isolation of the target product was dicꢀ
tated by the type of amino acid used. Indeed, in the reacꢀ
tion with glycine (2), the target condensation product 9
completely precipitated after acidification of the reaction
mixture (yield 88%). In the case of amino acids 3—7,
products 10—14 were distributed between the liquid and
solid phases; therefore, their isolation required column
chromatography. The yields of products 10—13 varied in
the range of 47—61%, and the yield of product 14 was
only 17%. The moderate yields are due to incomplete
conversion of usnic acid (1), which did not increase upon
the use of either longer reaction times and excess amino
acid or upon the variation of the pH. In the reaction with
Lꢀserine (8), the precipitate consisted of only recovered
usnic acid (1), while the condensation product 15 was
isolated from the filtrate in 29% yield by extraction.
No products of condensation of usnic acid with αꢀalaꢀ
nine, Lꢀlysine, Lꢀtyrosine, Lꢀhistidine, and glutamic acid
could be isolated, although they were present in the reacꢀ
tion mixtures in trace amounts, as shown by 1H NMR
spectroscopy.
The structures of obtained compounds 9—15 were esꢀ
tablished by 1H and 13C NMR and IR spectroscopies and
mass spectrometry. The configuration of the C(2)—C(11)
double bond in them was suggested based on published
data7 where analogous reaction of usnic acid with ammoꢀ
nia gave a product in which the enamine fragment conꢀ
figuration was established by Xꢀray crystallography.
Experimental
1
H and 13C NMR spectra were recorded on AMꢀ400
and AVꢀ300 Bruker spectrometers operating at 400.13 and
100.61 MHz, respectively. IR spectra were measured on a
Specord Mꢀ80 spectrometer (in KBr pellets), and mass spectra
(ionization electron energy 70 eV) were run on a Finnigan
MAT 8200 instrument. The conversion of usnic acid was moniꢀ
1
tored by H NMR spectroscopy. Column chromatography was
carried out using silica gel Merck (70—230 µ). Commercially
available amino acids were used. Usnic acid was isolated as
described previously.1
Reactions of usnic acid with amino acids (general procedure).
Amino acid (3 mmol) was dissolved in aqueous ethanol (10 mL,
1 : 1, v/v). Potassium hydroxide was added (to adjust pH ~9.5,
≥0.2 g) and the mixture was refluxed for 10 min on a water bath.
Then a suspension of (+)ꢀusnic acid (1) (344 mg, 1 mmol) in
ethanol (5 mL) was added in portions over a period of 30 min
and the mixture was refluxed for 3 h on a water bath, pH being
maintained at ~9.5. The mixture was cooled and dilute HCl was
added to pH ~5; this resulted in precipitation of a lightꢀyellow
solid. The subsequent workup of the reaction mixture depended
on the type of amino acid used.
6ꢀAcetylꢀ2ꢀ[1ꢀ(carboxymethylamino)ethylidene]ꢀ7,9ꢀdihydrꢀ
oxyꢀ8,9bꢀdimethylꢀ1,2,3,9bꢀtetrahydrodibenzo[b,d]furanꢀ1,3ꢀ

Enamines from usnic acid and amino acids
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 6, June, 2007
1251
ranꢀ1,3ꢀdione (12) was obtained in the reaction with Lꢀvaline (5).
The precipitate was filtered off and dried in air, and the product
was isolated by column chromatography on SiO2 (gradient eluꢀ
tion with CHCl3—AcOEt, 0→50%). Yield 61%, m.p. 210 °C
(dec.). 1H NMR (CDCl3), δ: 1.12, 1.16 (both d, 3 H each,
C(19)H3, C(20)H3, J = 6.8 Hz); 1.73 (s, 3 H, C(15)H3); 2.09 (s,
3 H, C(10)H3); 2.48 (d.sept, 1 H, H(18), J = 4.6 Hz, J = 6.8 Hz);
2.63 (s, 3 H, C(12)H3); 2.68 (s, 3 H, C(14)H3); 4.41 (dd, 1 H,
H(16), J = 4.6 Hz, J = 8.1 Hz); 5.88 (H(4)); 9.09 (C(17)OOH);
11.78 (br.s, 1 H, C(9)OH); 13.32 (s, 1 H, C(7)OH); 13.90 (br.s,
1 H, NH). 13C NMR (CDCl3), δ: 7.4 (C(15)); 18.7 (C(12));
17.6, 19.0 (C(19), C(20)); 31.1 (C(14)); 31.4 (C(10)); 38.9
(C(18)); 57.3 (C(9b)); 61.9 (C(16)); 101.3 (C(6)); 102.2, 102.3
(C(2), C(4)); 104.8 (C(9a)); 108.1 (C(8)); 155.7 (C(5a)); 158.0
(C(9)); 163.4 (C(7)); 172.7 (C(17)); 174.6 (C(11)); 175.1
(C(18)); 31.3 (C(14)); 31.6 (C(19)); 32.1 (C(10)); 54.9 (C(16));
57.4 (C(9b)); 101.4 (C(6)); 102.2 (C(4)); 102.8 (C(2)); 104.9
(C(9a)); 108.2 (C(8)); 155.8 (C(5a)); 158.1 (C(9)); 163.5 (C(7));
172.4 (C(11)); 174.8 (C(17)); 175.5 (C(4a)); 190.6 (C(3)); 198.7
(C(1)); 200.9 (C(13)). IR, ν/cm–1: 841, 1066, 1140, 1190, 1289,
1370, 1464, 1551, 1630, 1700, 1742, 2923, 3080. Highꢀresoluꢀ
tion MS, found: m/z 475.12480 [M]+. C23H25NO8S. Calcuꢀ
lated: M = 475.13007.
6ꢀAcetylꢀ7,9ꢀdihydroxyꢀ2ꢀ[1ꢀ(2ꢀhydroxyꢀ1ꢀcarboxyethylꢀ
amino)ethylidene]ꢀ8,9bꢀdimethylꢀ1,2,3,9bꢀtetrahydrodibenꢀ
zo[b,d]furanꢀ1,3ꢀdione (15) was obtained in the reaction with
Lꢀserine (8). According to TLC, the precipitate formed was the
recovered usnic acid (1). The aqueous filtrate was extracted
three times with ethyl acetate, and the extracts were dried with
MgSO4 and concentrated in vacuo. The residue was comꢀ
1
(C(4a)); 190.7 (C(3)); 198.6 (C(1)); 200.7 (C(13)). IR, ν/cm–1
:
pound 15, yield 29%, m.p. 105—108 °C. H NMR (CDCl3), δ:
850, 1067, 1132, 1186, 1287, 1373, 1475, 1555, 1627, 1701, 1741,
2963, 3109. Highꢀresolution MS, found: m/z 443.15820 [M]+.
C23H25NO8. Calculated: M = 443.15800.
1.58 (s, 3 H, C(15)H3); 1.97 (s, 3 H, C(10)H3); 2.60 (s, 3 H,
C(12)H3); 2.68 (s, 3 H, C(14)H3); 4.22, 4.34 (both d, ABꢀsystem,
2 H, C(18)H2, J = 11.5 Hz); 4.78 (m, 1 H, H(16)); 5.52 (s, 1 H,
H(4)); 11.32 (br.s, 1 H, C(9)OH); 13.24 (s, 1 H, C(7)OH);
13.58 (br.d, 1 H, NH, J = 7.6 Hz). 13C NMR (CDCl3), δ: 7.5
(C(15)); 19.2 (C(12)); 30.9 (C(14)); 31.8 (C(10)); 58.3 (C(16));
58.6 (C(9b)); 62.9 (C(18)); 101.0, 101.3 (C(6), C(4)); 102.7
(C(9a)); 103.9 (C(2)); 108.4 (C(8)); 155.2 (C(5a)); 157.4 (C(9));
163.7 (C(7)); 170.2 (C(17)); 174.4, 175.1 (C(11), C(4a));
190.5 (C(3)); 198.8 (C(1)); 200.3 (C(13)). IR, ν/cm–1: 844,
1065, 1119, 1190, 1286, 1371, 1465, 1557, 1629, 1698, 1745,
2928, 3393. Highꢀresolution MS, found: m/z 431.1222 [M]+.
C21H21NO9. Calculated: M = 431.1216.
6ꢀAcetylꢀ2ꢀ[1ꢀ(1ꢀcarboxyꢀ3ꢀmethylbutylamino)ethylidene]ꢀ
7,9ꢀdihydroxyꢀ8,9bꢀdimethylꢀ1,2,3,9bꢀtetrahydrodibenzo[b,d]fuꢀ
ranꢀ1,3ꢀdione (13) was obtained in the reaction with Lꢀleuꢀ
cine (6). The precipitate was filtered off and dried in air, and the
product was isolated by column chromatography on SiO2 (graꢀ
dient elution with CHCl3—AcOEt, 0→50%). Yield 47%, m.p.
1
105 °C. H NMR (CDCl3), δ: 0.99, 1.05 (both d, 3 H each,
C(21)H3, C(20)H3, J = 6.4 Hz); 1.74 (s, 3 H, C(15)H3); 1.85
(m, 1 H, H(19)); 1.94 (m, 2 H, H(18)); 2.10 (s, 3 H, C(10)H3);
2.65 (s, 3 H, C(12)H3); 2.69 (s, 3 H, C(14)H3); 4.53 (m, 1 H,
H(16)); 5.87 (s, 1 H, H(4)); 8.26 (br.s, 1 H, C(17)OOH); 11.76
(br.s, 1 H, C(9)OH); 13.34 (s, 1 H, C(7)OH); 13.80 (br.s, 1 H,
NH). 13C NMR (CDCl3), δ: 7.0 (C(15)); 18.3 (C(12)); 21.4,
22.2 (C(21), C(20)); 24.4 (C(19)); 30.8 (C(14)); 31.5 (C(10));
40.9 (C(18)); 54.7 (C(16)); 60.1 (C(9b)); 97.9 (C(2)); 100.9
(C(6)); 101.9 (C(4)); 104.4 (C(9a)); 107.8 (C(8)); 155.3 (C(5a));
157.7 (C(9)); 163.1 (C(7)); 173.6 (C(11)); 174.3 (C(4a)); 174.4
References
1. G. G. Furin, O. A. Luzina, R. I. Sokuev, M. P. Polovinka,
N. F. Salakhutdinov, and G. A. Tolstikov, Izv. Akad. Nauk.
Ser. Khim., 2007, 1198 [Russ. Chem. Bull., Int. Ed., 2007,
56, 1244].
2. K. Ingolfsdottir, Phytochemistry, 2002, 61, 729.
3. M. Cocchietto, N. Skert, P. L. Nimis, and G. Sava,
Naturwissenschaften, 2002, 89, 137.
(C(17)); 190.3 (C(3)); 198.3 (C(1)); 200.3 (C(13)). IR, ν/cm–1
:
844, 1064, 1135, 1189, 1289, 1371, 1468, 1553, 1629, 1699, 1745,
2960, 3090. Highꢀresolution MS, found: m/z 457.17270 [M]+.
C24H27NO8. Calculated: M = 457.17365.
4. D. Yu, Y. Sakurai, C.ꢀH. Chen, F.ꢀR. Chang, L. Huang,
Y. Kashiwada, and K.ꢀH. Lee, J. Med. Chem., 2006, 49, 5462.
5. L. Gros, S. O. Lorente, C. J. Jimenez, V. Yardley, L. Rattray,
H. Wharton, S. Little, S. L. Croft, L. M. RuizꢀPerez,
D. GonzalezꢀPacanowska, and I. H. Gilbert, J. Med. Chem.,
2006, 49, 6094.
6. A. Gangjee, Y. Zeng, J. J. McGuire, and R. L. Kisliuk, J. Med.
Chem., 2006, 48, 5329.
7. J. P. Kutney, I. H. Sanchez, and T. Yee, Can. J. Chem., 1976,
54, 2795.
6ꢀAcetylꢀ2ꢀ{1ꢀ[1ꢀcarboxyꢀ3ꢀmethylthio(propylamino)]ethylꢀ
idene}ꢀ7,9ꢀdihydroxyꢀ8,9bꢀdimethylꢀ1,2,3,9bꢀtetrahydrodibenꢀ
zo[b,d]furanꢀ1,3ꢀdione (14) was obtained in the reaction with
Lꢀmethionine (7). The precipitate was filtered off and dried in
air, and the product was isolated by column chromatography
on SiO2 (gradient elution with CHCl3—AcOEt, 0→50%). Yield
17%, m.p. 100—102 °C. 1H NMR (CDCl3), δ: 1.69 (s, 3 H,
C(15)H3); 2.04 (s, 3 H, C(10)H3); 2.12 (s, 3 H, C(20)H3); 2.35
(m, 2 H, C(18)H2); 2.64 (s, 3 H, C(12)H3); 2.65 (m, 2 H,
C(19)H2); 2.67 (s, 3 H, C(14)H3); 4.82 (m, 1 H, H(16)); 5.85 (s,
1 H, H(4)); 8.69 (br.s, 1 H, C(17)OOH); 11.66 (s, 1 H, C(9)OH);
13.27 (s, 1 H, C(7)OH); 13.61 (br.s, 1 H, NH). 13C NMR
(CDCl3), δ: 7.5 (C(15)); 15.3 (C(20)); 18.9 (C(12)); 29.8
Received February 28, 2007;
in revised form May 23, 2007