Diketopiperazines from Costa Rican endolichenic fungus Colpoma sp. CR1465A

Article abstract of DOI:10.1016/j.bmcl.2016.03.115

Three new diketopiperazines (1-3), cyclo(l-Pro-d-trans-Hyp) (1), cyclo(l-Pro-d-Glu) (2), and cyclo(d-Pro-d-Glu) (3) and five known diketopiperazines (4-8) were isolated from the endolichenic fungus Colpoma sp. CR1465A identified from the Costa Rican plant Henriettea tuberculosa (Melatomataceae). The structures of the new compounds 1-3 were elucidated using a combination of extensive spectroscopic analyses, including 2D NMR and HR-MS, and their absolute configurations were determined by a combination of NOESY analysis and Marfey's method. Cyclo(l-Pro-d-allo-Thr) (4) was recently isolated from a South China Sea marine sponge Callyspongia sp., but its NMR spectroscopic data were not reported, and cyclo(l-Pro-l-Asp) (5) was previously reported but only as a synthetic product. The NMR data assignments of compounds 4 and 5 are reported for the first time. All of the isolated compounds were tested for antifungal and antimicrobial properties.

Full text of DOI:10.1016/j.bmcl.2016.03.115

Bioorganic & Medicinal Chemistry Letters 26 (2016) 2438–2441
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Diketopiperazines from Costa Rican endolichenic fungus Colpoma sp.
CR1465A
a,
Seulah Lee a, Giselle Tamayo-Castillo b, Changhyun Pang c, Jon Clardy d, Shugeng Cao e, , Ki Hyun Kim
⇑
⇑
a School of Pharmacy, Sungkyunkwan University, Suwon, Gyeonggi-do, 440-746, Republic of Korea
b Unidad Estratégica de Bioprospección, Instituto Nacional de Biodiversidad (INBio), Santo Domingo de Heredia, Costa Rica & CIPRONA-Escuela de Química, Universidad de Costa
Rica, 2060 San Pedro, Costa Rica
c School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do, 440-746, Republic of Korea
d Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA
e Department of Pharmaceutical Sciences, Daniel K Inouye College of Pharmacy, University of Hawaii at Hilo, 924 Stainback Hwy, Hilo, HI 96720, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new diketopiperazines (1–3), cyclo(L-Pro-D-trans-Hyp) (1), cyclo(L-Pro-D-Glu) (2), and cyclo(D-Pro-
Received 12 February 2016
Revised 24 March 2016
Accepted 31 March 2016
Available online 2 April 2016
D
-Glu) (3) and five known diketopiperazines (4–8) were isolated from the endolichenic fungus Colpoma
sp. CR1465A identified from the Costa Rican plant Henriettea tuberculosa (Melatomataceae). The
structures of the new compounds 1–3 were elucidated using a combination of extensive spectroscopic
analyses, including 2D NMR and HR-MS, and their absolute configurations were determined by a
combination of NOESY analysis and Marfey’s method. Cyclo(
from a South China Sea marine sponge Callyspongia sp., but its NMR spectroscopic data were not reported,
and cyclo( -Pro- -Asp) (5) was previously reported but only as a synthetic product. The NMR data
assignments of compounds 4 and 5 are reported for the first time. All of the isolated compounds were
tested for antifungal and antimicrobial properties.
L-Pro-D-allo-Thr) (4) was recently isolated
Keywords:
Colpoma sp.
Henriettea tuberculosa
Diketopiperazines
Marfey’s method
L
L
Ó 2016 Elsevier Ltd. All rights reserved.
Fungi have long been recognized as one of the largest natural
sources for a wide variety of drugs, including antibiotic, immuno-
suppressant and anti-cancer agents.1,2 Despite their potential
application in drug discovery, fungi remain relatively unexplored,
as only a small portion of the world’s estimated 1.5 million fungal
species has contributed pharmaceutically significant compounds.
Only ꢀ5% of the world’s fungal species have been studied and even
fewer have been explored for their chemical potential.3–5 Among
them, endosymbiotic fungi, which live inside vascular plants and
other organisms such as lichens, are some of the most under
explored.
In our ongoing effort to search for biologically active natural
products, we investigated endosymbiotic fungi from Costa Rica’s
tropical rainforests as part of an International Cooperative
Biodiversity Group program.6–9 In this study, we focused on an
endolichenic fungal species, designated as CR1465A, which was
isolated from Henriettea tuberculosa (Melatomataceae) collected
at Área de Conservación Cordillera Volcánica Central in Costa Rica.
CR1465A closest relative, based on DNA sequencing, is a Colpoma
sp. and was subjected to large-scale fermentation and secondary
metabolite analyses. The chemical investigation of CR1465A led
to the isolation of three new diketopiperazines (1–3), together
with five known diketopiperazines (4–8) (Fig. 1).10 The structures
of the new compounds were elucidated by extensive spectroscopic
analyses, including 1D, 2D NMR and HR-MS, and their absolute
configurations were determined by a combination of NOESY anal-
ysis and Marfey’s method. Compound 4 was recently isolated from
South China Sea marine sponge Callyspongia sp. without NMR data,
and compound 5 was previously reported but only as a synthetic
product. The NMR data assignments of compounds 4 and 5 are
reported for the first time. This Letter describes the isolation and
structural elucidation of compounds 1–8, as well as their
antimicrobial activities.
Compound 1 was isolated as a white amorphous powder with a
25
negative specific rotation value, [
a]
À30.8 (c 0.05, H2O). Its
D
molecular formula was determined to be C10H14N2O3 from the
[M+H]+ peak at m/z 211.1087 (calcd. for C10H15N2O3, 211.1083)
in the positive-ion HR-ESI-MS spectrum. The IR spectrum of 1
showed a broad hydroxy band at 3387 cmÀ1 and a carbonyl
absorption band at 1670 cmÀ1. The 1H NMR spectrum (Table 1)
of 1 showed signals for three down-field shifted methine protons
at dH 4.18 (dd, J = 7.5, 7.0 Hz), 4.52 (dd, J = 4.5, 4.0 Hz), and 4.58
⇑
Corresponding authors. Tel.: +1 808 4405208; fax: +1 808 5862970 (S.C.), tel.:
+82 290 7700; fax: +82 290 7730 (K.H.K.).
E-mail addresses: scao@hawaii.edu (S. Cao), khkim83@skku.edu (K.H. Kim).
http://dx.doi.org/10.1016/j.bmcl.2016.03.115
0960-894X/Ó 2016 Elsevier Ltd. All rights reserved.

S. Lee et al. / Bioorg. Med. Chem. Lett. 26 (2016) 2438–2441
2439
O
O
O
O
O
O
OH
O
O
H
7
H
H
H
HO
12
1
4
12
N
9
HO
N
H
6
N
11
HO
N
H
N
H
8
3
8
HN
HN
9
HN
10
H
10
9
O
O
3
1
2
4
O
O
O
O
H
O
11
N
H
N
8
N
H
N
HN
OH HN
HN
HN
9
H
H
O
O
O
O
5
6
7
8
Figure 1. Chemical structures of compounds 1–8.
Table 1
1H and 13C NMR data of compounds 1–3 in D2Oa
modeling (Fig. 3). The absolute configuration of the proline residue
was determined by Marfey’s method.11–13 A small sample of 1
(0.2 mg) was hydrolyzed with 6 N HCl (1 mL) at 110 °C for 15 h
Position
1
2
3
and derivatized with Marfey’s reagent (N -(2,4-dinitro-5-fluo-
a
dH
dC
dH
dC
175.0
dH
dC
rophenyl)-
Marfey’s derivative derived from 1 with Marfey’s derivatives
prepared from - or -proline showed that compound 1 contained
-proline, indicating the absolute configuration of the proline resi-
L-alaninamide; L-FDAA). An LC–MS comparison of the
1
2
3
173.7
175.7
D
L
4.18 dd (7.5,
7.0)
61.5 4.20 dd
(7.5, 7.0)
172.3
61.7 4.55 dd
(9.5, 6.0)
169.8
59.3
172.4
57.6
L
4
5
6
due a-position as 3S. This analysis simultaneously determined the
6R and 8S configurations based on the relative stereochemistry
4.58 dd (11.5,
6.5)
2.22 m; 2.05 m
4.52 dd (4.5,
4.0)
3.58 dd (13.0,
4.5)
3.38 br d (13.0)
2.20 m; 1.83 m
61.9 4.25 dd
(5.0, 4.5)
38.8 2.06 m
71.0 2.32 m
57.5 4.22 dd
(5.0, 4.5)
28.3 2.05 m
34.4 2.34 m
previously assigned by NOESY. Thus, the structure of 1 was
established as a cyclic dipeptide called cyclo(
Compound 2 was isolated as a white amorphous powder with a
L-Pro-D
-trans-Hyp).14
7
8
28.2
34.2
25
negative specific rotation value, [
a]
À5.0 (c 0.11, H2O) and its
D
9
54.9
183.6
183.4
molecular formula was determined as C10H14N2O4 from the [M
+H]+ peak at m/z 227.1034 (calcd. for C10H15N2O4, 227.1032) in
the positive-ion HR-ESI-MS spectrum. The 1H NMR spectrum
(Table 1) of 2 showed signals for two down-field shifted methine
protons at dH 4.20 (dd, J = 7.5, 7.0 Hz) and 4.25 (dd, J = 5.0,
4.5 Hz). The 13C NMR data (Table 1) of 1 exhibited signals for
two carbonyl resonances at dC 175.0 and 169.8. An analysis of
the 2D NMR data (COSY, HSQC, and HMBC data) of 2 revealed
the presence of glutamic acid and proline (Fig. 2), and the HMBC
10
11
12
31.7 2.21 m;
1.82 m
25.0 1.96 m;
1.84 m
47.8 3.43 m;
3.41 m
30.6 2.46 m;
2.08 m
24.7 1.88 m;
1.80 m
48.0 3.57 m;
3.48 m
31.8
24.8
48.5
1.95 m; 1.88 m
3.45 dd (11.5,
8.5); 3.33 m
1H and 13C NMR data were recorded at 600 and 150 MHz, respectively. Coupling
constants (in Hz) are given in parentheses.
a
correlations between the a-protons of the two amino acid residues
at dH 4.20 and 4.25 and the two carbonyl carbons at dC 175.0 and
169.8 enabled us to establish connectivity to a diketopiperazine
consisting of glutamic acid and proline (Fig. 2). On the other hand,
the 1H and 13C NMR data (Table 1) of 3 were very similar to those of
2, with apparent slight differences in the chemical shifts and
coupling constants for H-3/C-3 and C-4 in 1 compared to those in
2, suggesting that compound 3 was a stereoisomer of compound
2. The connectivity of glutamic acid and proline in 3 was also
(dd, J = 11.5, 6.5 Hz). The 13C NMR data (Table 1) of 1, which were
assigned by HSQC and HMBC analyses, displayed signals for two
carbonyl resonances at dC 173.7 and 172.3. Analyses of the 1H–1H
COSY, HSQC, and HMBC data obtained for 1 revealed the presence
of proline and 4-hydroxyproline (Fig. 2). The HMBC correlations of
the a-protons of two amino acid residues at dH 4.18 and 4.58 with
two carbonyl carbon resonances at dC 173.7 and 172.3 indicated
that compound 1 was a diketopiperazine composed of the amino
acids proline and 4-hydroxyproline (Fig. 2). The full NMR assign-
ments of 1 were determined by 1H–1H COSY, HSQC, and HMBC
analyses of the spectroscopic data (Table 1). The stereochemistry
of 1 was determined using a combination of NOESY analysis,
degradative reaction, and derivatization. The relative configuration
of the three stereogenic centers in 1 was established as 3S⁄, 6R⁄,
and 8S⁄ by NOESY spectral analysis combined with molecular
O
H
H
O
O
H
H
H
N
H
HO
H
N
H
HO
N
HN
H
O
O
1
2
O
O
O
O
O
OH
O
OH O
H
H
H
HO
N
HO
N
HO
N
H
N
N
N
HN
HN
HN
HN
H
O
O
O
O
O
1
3
2
4
4
Figure 2. Key 1H–1H COSY (bold) and HMBCs (?) of 1, 2, and 4.
Figure 3. Key NOESY correlations of 1–4.

2440
S. Lee et al. / Bioorg. Med. Chem. Lett. 26 (2016) 2438–2441
Table 2
The known compounds were identified as cyclo(
(5),19,20 cyclo( -Pro-Gly) (6),21 cyclo( -Ala) (7),22 and cyclo
-Pro-
-Pro-
-Ala) (8),23 by comparing their spectroscopic and physical
L-Pro-L-Asp)
1H and 13C NMR data of compounds 4 and 5 in D2Oa
D
L
L
(L
D
Position
4
5
data with reported values. Their absolute configurations were
established using a combination of NOESY and Marfey’s method.
dH
dC
dH
dC
1
2
3
4
5
6
7
168.6
171.9
On the other hand, cyclo(L-Pro-L-Asp) (5) has been previously
reported but only as a synthetic product.20 The NMR data assign-
ment of 5 as a natural product is reported here (Table 2).
4.16 dd (7.5, 7.0)
61.7
172.1
4.22 dd (7.5, 7.0)
61.7
170.5
The isolated diketopiperazines (1–8) were tested for antifungal
and antimicrobial properties in triplicate against standardized bac-
terial and yeast strains from the American Type Culture Collection
(ATCC) [Gram-negative bacteria; Escherichia coli (ATCC 25922),
Gram-positive bacteria; Bacillus subtilis (ATCC 6633) and Staphylo-
coccus aureus (BAA-2313) and yeasts; Saccharomyces cerevisiae
(ATCC 9763) and Candida albicans (ATCC 10231)]. In this study,
the minimum inhibitory concentration and the minimum fungici-
dal concentration values of compounds 1–8 were determined
using broth-dilution techniques. Unfortunately none of the isolates
4.01 br s
4.32 qd (6.5, 1.5)
62.8
68.7
4.38 dd (5.0, 4.5)
2.72 dd (19.5, 5.0)
2.77 dd (19.5, 4.5)
51.8
38.2
8
9
10
11
1.19 d (6.5)
21.3
31.5
24.4
48.1
177.3
30.8
24.7
48.2
2.21 m; 1.78 m
1.93 m; 1.84 m
3.51 m; 3.37 m
2.21 m; 1.82 m
1.96 m; 1.83 m
3.43 m; 3.41 m
1H and 13C NMR data were recorded at 600 and 150 MHz, respectively. Coupling
constants (in Hz) are given in parentheses.
a
displayed antimicrobial activities at <200 lg/mL.
elucidated by the HMBC correlations of each
a proton with the
amide carbonyl carbon. The stereochemistry of 2 and 3 was estab-
lished based on Marfey’s method and a NOESY analysis.11–13 The
relative configuration of the two stereogenic centers in 2 was
assigned 3S⁄ and 6R⁄ by the NOESY correlations combined with
molecular modeling, whereas the two stereogenic centers of 3
were deduced to be 3R⁄ and 6R⁄ in the relative configuration based
on the NOESY correlations (Fig. 3). An LC–MS analysis of the Mar-
fey’s derivative derived from 2 allowed us to determine that com-
Acknowledgement
This paper was supported by SEOK CHUN Research Fund, Sung-
kyunkwan University, 2014.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2016.03.
115.
pound 2 possessed
L-proline, indicating the absolute configuration
of the proline -position as 3S. This result determined a 6R config-
a
uration of 2 based on the relative stereochemistry previously
established in 2. On the other hand, the LC–MS analysis of the Mar-
fey’s derivative derived from 3 allowed determination of the abso-
References and notes
lute configuration of its
6R configuration based on the assigned relative configuration of
a-carbon as 3R, which also determined a
1. Newman, D. J.; Cragg, G. M. J. Nat. Prod. 2012, 75, 311.
2. Flores-Bustamante, Z. R.; Rivera-Orduña, F. N.; Martínes-Cárdenas, A.; Flores-
Cotera, L. B. J. Antibiot. 2010, 63, 460.
3. Jiang, Z. D.; An, Z. Stud. Nat. Prod. Chem. 2000, 22, 245.
4. Henkel, T.; Brunne, R. M.; Müller, H.; Reichel, F. Angew. Chem., Int. Ed. 1999, 38,
643.
5. Pimm, S. L.; Russell, G. J.; Gittleman, J. L.; Brooks, T. M. Science 1995, 269, 347.
6. Cao, S.; Ross, L.; Tamayo, G.; Clardy, J. Org. Lett. 2010, 12, 4661.
7. Cao, S.; Clardy, J. Tetrahedron Lett. 2011, 52, 2206.
8. Cao, S.; McMillin, D. W.; Tamayo, G.; Delmore, J.; Mitsiades, C. S.; Clardy, J. J.
Nat. Prod. 2012, 75, 793.
3. Thus, the structure of 2 was characterized as a cyclic dipeptide
called cyclo(
dipeptide called cyclo(
Compound 4 was isolated as a white amorphous powder with a
L-Pro-D
-Glu)15 and the structure of 3 was a cyclic
D
-Pro-D
-Glu).16
25
negative specific rotation value, [
a
]
À11.0 (c 0.05, H2O). Its
D
molecular formula was established to be C9H14N2O3 from the [M
+H]+ peak at m/z 199.1091 (calcd. for C9H15N2O3, 199.1083) in
the positive-ion HR-ESI-MS spectrum. The 1H and 13C NMR spectra
(Table 2) of 4 and the detailed analysis of the 2D NMR data (COSY,
HSQC, and HMBC data) of 4 suggested the presence of threonine
and proline, whose diketopiperazine structure was further estab-
lished by COSY and HMBC experiments (Fig. 2). The absolute con-
figuration of 4 was determined by the modified Mosher’s method
(see Supplementary Data) and the Marfey’s method in combination
9. Kim, K. H.; Beemelmanns, C.; Murillo, C.; Guillen, A.; Umana, L.; Tamayo-
Castillo, G.; Clardy, J.; Cao, S. J. Chem. Res. 2014, 38, 722.
10. The EtOH extract of liquid cultures was fractionated by the HP-20 column
chromatography eluting with a step gradient of two column volumes of 100%
H2O, 20% MeOH/H2O, 40% MeOH/H2O, 60% MeOH/H2O, 80% MeOH/H2O, and
100% MeOH to give six fractions (fractions A–F). Fraction C was fractionated by
preparative reversed-phase HPLC (C18 column, Phenomenex Luna,
250 Â 21.2 mm, 5
lm) using 8% CH3CN/H2O (+0.1% formic acid) for 20 min,
then to 100% CH3CN (+0.1% formic acid) in the next 10 min, and 100% CH3CN
(+0.1% formic acid) for the next 5 min (flow rate: 10 mL/min) to give 35
fractions (fractions C1–C35). Fractions C11, C12, C13 and C14 were further
purified using semi-preparative HPLC (Agilent 1200 Series HPLC system,
with a NOESY analysis.11–13 Treating 4 with (S)-(+)-
a-methoxy-a-
(trifluoromethyl)phenylacetyl chloride [(S)-MTPA-Cl] and DMAP
in the pyridine produced the (R)-MTPA esters 4r. Similarly, treating
4 with (R)-(-)-MTPA-Cl afforded the (S)-MTPA ester 4s. Analysis of
Phenyl-hexyl column, Phenomenex Luna, 250 Â 10.0 mm, 5
lm, flow rate:
2 mL/min). Fraction C11 was purified with 3.3% CH3CN/H2O (+0.1% formic acid)
to give compounds 4 (0.5 mg, tR 24.8 min), 5 (0.6 mg, tR 25.5 min) and 6
(0.8 mg, tR 21.2 min), and fraction C12 with 4.0% CH3CN/H2O (+0.1% formic
acid) to give compounds 1 (0.5 mg, tR 31.8 min) and 8 (0.4 mg, tR 23.5 min).
Fraction C13 was purified using 4.0% CH3CN/H2O (+0.1% formic acid) to give 7
(1.4 mg, tR 30.3 min), and fraction C14 using 5.7% CH3CN/H2O (+0.1% formic
acid) to give compounds 2 (1.1 mg, tR 26.7 min) and 3 (0.7 mg, tR 28.1 min)
11. Marfey, P. Carlsberg Res. Commun. 1984, 49, 591.
the 1H NMR chemical shift differences (
DdS-R) (see Supplementary
Data) of the two MTPA esters allowed assignment of the absolute
configuration of C-7 as 7R. In addition, the LC–MS analysis of the
Marfey’s derivative derived from 4 allowed assignment of the
absolute configuration of C-3 as 3S. Finally, the NOESY correlation
between H-3 and H-7, but the lack of a NOESY correlation between
H-3 and H-6, determined a 6R configuration of 4 (Fig. 3). Thus, the
12. Noh, H. J.; Hwang, D.; Lee, E. S.; Hyun, J. W.; Yi, P. H.; Kim, G. S.; Lee, S. E.; Pang,
C.; Park, Y. J.; Chung, K. H.; Kim, G. D.; Kim, K. H. J. Ethnopharmacol. 2015, 163,
106.
13. Small samples of compounds 1–4 (1: 0.2 mg; 2: 0.1 mg; 3: 0.1 mg; 4: 0.1 mg)
were individually hydrolyzed with 6 N HCl (1 mL) at 110 °C for 15 h. Each
reaction was allowed to cool to room temperature, dried in vacuo. A solution of
structure of 4 was elucidated as a cyclic dipeptide called cyclo(
Pro-
-allo-Thr).17 A literature survey revealed that compound 4,
cyclo( -Pro-D-allo-Thr), was recently isolated from the South China
L-
D
L
Marfey’s reagent (20
aqueous NaHCO3 (100
cooled to room temperature, and acidified with 2 N HCl (50
l
L, 10 mg/mL in acetone) was added, followed by 1 N
L). Each reaction was heated to 80 °C for 10 min,
L). The reaction
Sea marine sponge Callyspongia sp.,18 but its NMR spectroscopic
l
l
data were not reported. This is the first NMR data assignment of
cyclo(L-Pro-D-allo-Thr).
mixture was filtered and analyzed by LC–MS using the following gradient; 0–
30 min, linear gradient from 10% to 70% CH3CN/H2O + 0.1% formic acid; 30–

S. Lee et al. / Bioorg. Med. Chem. Lett. 26 (2016) 2438–2441
2441
40 min, linear gradient from 70% to 100% CH3CN/H2O + 0.1% formic acid.
Standards were prepared from the appropriate authentic or -prolines
(0.2 mg) by derivatizing them with Marfey’s reagent using the above
procedure. The retention times for Marfey’s derivatives were as follows:
1219, 1030 cmÀ1
;
1H (600 MHz) and 13C (150 MHz) NMR data, see Table 1;
D
-
L
ESIMS (positive-ion mode) m/z: 227 [M+H]+. HR-ESIMS (positive-ion mode) m/
z: 227.1041 [M+H]+ (Calcd for C10H15N2O4, 227.1032).
25
17. Cyclo(
L-Pro-
D-allo-Thr) (4): White amorphous powder; [
a]
À11.0 (c = 0.05,
D
proline
compounds 1–4 was also co-injected with standards to confirm their
assignment which proved that compounds 1, 2, and 4 contain -proline (1:
15.98 min; 2: 15.95 min; 4: 15.96 min) while compound 3 contains -proline
(
L-Pro, 15.95 min and
D-Pro, 16.41 min). Samples prepared from
H2O); UV (MeOH) kmax (loge) 202 nm; IR (KBr) mmax 3385, 2920, 1673, 1452,
1220, 1030 cmÀ1
;
1H (600 MHz) and 13C (150 MHz) NMR data, see Table 2;
L
ESIMS (positive-ion mode) m/z: 199 [M+H]+. HR-ESIMS (positive-ion mode) m/
D
z: 199.1091 [M+H]+ (Calcd for C9H15N2O3, 199.1083).
(16.40 min). The other known compounds were analyzed using the same
18. Chen, Y.; Peng, Y.; Gao, C.; Huang, R. Nat. Prod. Res. 2014, 28, 1010.
25
procedure.
19. Cyclo(
UV (MeOH) kmax (log
1222, 1032 cmÀ1 1H (600 MHz) and 13C (150 MHz) NMR data, see Table 2;
;
L-Pro-
L
-Asp) (5): White amorphous powder; [
a
]
À25.5 (c 0.06, H2O);
D
25
14. Cyclo(
L-Pro-
D-trans-Hyp) (1): White amorphous powder; [
a]
À30.8 (c 0.05,
e) 205 nm; IR (KBr) mmax 3358, 2948, 1710, 1670, 1456,
D
H2O); UV (MeOH) kmax (log
e
) 204 nm; IR (KBr) mmax 3387, 2925, 1670, 1450,
1221, 1032 cmÀ1
;
1H (600 MHz) and 13C (150 MHz) NMR data, see Table 1;
ESIMS (positive-ion mode) m/z: 213 [M+H]+. HR-ESIMS (positive-ion mode) m/
z: 213.0872 [M+H]+ (Calcd for C9H13N2O4, 213.0875).
ESIMS (positive-ion mode) m/z: 211 [M+H]+. HR-ESIMS (positive-ion mode) m/
z: 211.1087 [M+H]+ (Calcd for C10H15N2O3, 211.1083).
20. Challa, C.; Kumar, N.; John, M.; Lankalapalli, R. S. Med. Chem. Res. 2014, 23,
2377.
21. Kolyasnikova, K. N.; Vichuzhanin, M. V.; Konstantinopol’skii, M. A.; Trofimov, S.
S.; Gudasheva, T. A. Pharm. Chem. J. 2012, 46, 96.
22. Selvakumar, S.; Sivasankaran, D.; Singh, V. K. Org. Biomol. Chem. 2009, 7, 3156.
23. Lenman, M. M.; Lewis, A.; Gani, D. J. Chem. Soc., Perkin Trans. 1 1997, 16, 2297.
25
15. Cyclo(
L-Pro-
D-Glu) (2): White amorphous powder; [
a]
À5.0 (c 0.11, H2O); UV
D
(MeOH) kmax (log
e
) 200 nm; IR (KBr)
m
max 3365, 2941, 1708, 1672, 1459, 1215,
1030 cmÀ1
;
1H (600 MHz) and 13C (150 MHz) NMR data, see Table 1; ESIMS
(positive-ion mode) m/z: 227 [M+H]+. HR-ESIMS (positive-ion mode) m/z:
227.1034 [M+H]+ (Calcd for C10H15N2O4, 227.1032).
25
16. Cyclo(
D-Pro-
D-Glu) (3): White amorphous powder; [
a]
+10.5 (c 0.07, H2O);
D
UV (MeOH) kmax (log
e) 208 nm; IR (KBr) mmax 3366, 2940, 1705, 1670, 1450,