Isolation, Synthesis, and Radical-Scavenging Activity of Rhodomelin A, a Ureidobromophenol from the Marine Red Alga Rhodomela confervoides

Article abstract of DOI:10.1021/acs.orglett.7b03716

A novel ureidobromophenol, rhodomelin A (1), was characterized from Rhodomela confervoides. Its structure was elucidated by spectroscopic analysis. Both enantiomers of 1 were synthesized using a convergent strategy starting from d/l-pyroglutamic acids, respectively, allowing assignment of the R-configuration for the naturally occurring isomer by chiral HPLC analysis. Rhodomelin A represents the first example of a naturally occurring ureidopyrrolidone alkaloid incorporating a γ-aminobutyric acid unit. The scavenging activity of 1 toward DPPH (1,1-diphenyl-2-picrylhydrazyl) and ABTS (2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)) radicals was assayed.

Full text of DOI:10.1021/acs.orglett.7b03716

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Isolation, Synthesis, and Radical-Scavenging Activity of Rhodomelin
A, a Ureidobromophenol from the Marine Red Alga Rhodomela
confervoides
Ke Li,†,∥ Ya-Fei Wang,‡,∥ Xiao-Ming Li,† Wei-Jia Wang,‡ Xu-Ze Ai,‡ Xin Li,† Sui-Qun Yang,†
James B. Gloer,§ Bin-Gui Wang,*,† and Tao Xu*,‡
†Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, and Laboratory of Marine
Biology and Biotechnology of Qingdao National Laboratory for Marine Science and Technology, Nanhai Road 7, Qingdao 266071,
China
‡Key Laboratory of Marine Drugs, Ministry of Education, School of Medicine and Pharmacy, Ocean University of China, and
Laboratory for Marine Drugs and Bioproducts of Qingdao National Laboratory for Marine Science and Technology, Yushan Road
5, Qingdao 266003, China
§Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States
S
* Supporting Information
ABSTRACT: A novel ureidobromophenol, rhodomelin A (1), was characterized from Rhodomela
confervoides. Its structure was elucidated by spectroscopic analysis. Both enantiomers of 1 were
synthesized using a convergent strategy starting from D/L-pyroglutamic acids, respectively, allowing
assignment of the R-configuration for the naturally occurring isomer by chiral HPLC analysis.
Rhodomelin A represents the first example of a naturally occurring ureidopyrrolidone alkaloid
incorporating a γ-aminobutyric acid unit. The scavenging activity of 1 toward DPPH (1,1-diphenyl-
2-picrylhydrazyl) and ABTS (2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)) radicals was
assayed.
he marine red algal species of the genus Rhodomela
of 1. The ability to scavenge DPPH and ABTS radicals was
assayed as well.
T
(family Rhodomelaceae, order Ceramiales) have been
described as a rich source of bromophenolic compounds.1 We
have previously reported several bromophenols from Rhodo-
mela confervoides.2 Further investigation of the polar fraction of
the algal extract has now resulted in the isolation of a novel
ureidobromophenol, namely, rhodomelin A (1, Figure 1),
An air-dried and ground sample of the marine red alga R.
confervoides was extracted with 95% EtOH at room temper-
ature. After the solvent was removed under reduced pressure at
<40 °C, the residue was suspended in H2O and then
successively partitioned with petroleum ether, EtOAc, and n-
butanol. The EtOAc extract was purified using a combination
of silica gel and Sephadex LH-20 column chromatography
steps to yield compound 1.
Rhodomelin A (1), obtained as a brown powder, displayed a
characteristic dibrominated pseudomolecular ion cluster at m/
z 510/508/506 (1:2:1) [M − H]− in the negative ion ESI mass
spectrum. The molecular formula was determined to be
C16H19Br2N3O6 by HRFABMS, indicating eight degrees of
unsaturation. The IR spectrum exhibited absorption bands for
NH/OH group(s) at 3747 and 3649 cm−1, for carboxy and
amide carbonyl groups at 1733 and 1640 cm−1, and for
aromatic unit(s) at 1534 and 1448 cm−1.
Figure 1. Chemical structure of rhodomelin A (1).
which represents the first example of a ureidopyrrolidone
alkaloid incorporating γ-aminobutyric acid and 2,3-dibromo-
4,5-dihydroxybenzyl units. Compound 1 has a chiral center at
C-1″ which is far from the 2,3-dibromo-4,5-dihydroxybenzyl
chromophore, thus precluding assignment of its absolute
configuration with confidence by ECD methods. Attempts to
obtain quality crystals of 1 for X-ray crystallographic analysis
were not successful.
In examining the 1H NMR spectrum, signals for two
methines (H-6′ and H-1″) and six methylenes (H2-2 ∼ H2-4,
H2-7′, H2-2″, and H2-3″) as well as two NH protons (NH-2
and NH-3) were observed (Table 1). The 13C NMR data
revealed the presence of 16 carbon resonances, which were
grouped by DEPT and HSQC spectra into categories of one
We report herein the isolation and structure elucidation of 1,
the synthesis of both possible enantiomers, and subsequent
chiral HPLC analysis to determine the absolute configuration
Received: November 30, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03716
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Letter
a
1
Table 1. H and 13C NMR Data for Rhodomelin A (1)
is far from the 2,3-dibromo-4,5-dihydroxybenzyl chromophore
and thus ECD calculation was not useful in determining its
absolute configuration. Attempts to get quality crystals for X-
ray analysis were not successful. We therefore concentrated our
efforts toward a total synthesis of both enantiomers of 1.
Rhodomelin A (1) contains an acid/base-sensitive 5-
ureidopyrrolidone unit and a dibromo-substituted phenolic
moiety. Retrosynthetically speaking, compound 1 could be
disconnected to amino ester 2 and pyroglutamic isocyanate 3
through a nucleophilic 1,2-addition. The former (2) could
further be derived from lanosolaldehyde (4), which could be
synthesized from vanillin in two steps,3 while the latter (3)
could be generated through Curtius rearrangement4 from
commercially available chiral pyroglutamic acid in a one-pot
transformation. This convergent strategy would allow both
enantiomers of 1 be accessed through one unified route
starting with D/L-pyroglutamic acids (Scheme 1).
no.
δH mult (J, Hz)
δC
1
2
3
4
5
1′
2′
3′
4′
5′
6′
7′
174.5, C
30.8, CH2
23.2, CH2
45.3, CH2
156.5, C
129.2, C
113.5, C
112.8, C
143.5, C
145.4, C
113.3, CH
50.4, CH2
2.20, t (7.3)
1.68, m
3.13, m
6.63, s
4.39, d (16.7)
4.32, d (16.7)
5.37, m
1″
2″
62.0, CH
27.5, CH2
2.29, m
1.84, m
2.30, m
2.05, m
Scheme 1. Retrosynthetic Analysis of Rhodomelin A (1)
3″
29.1, CH2
176.1, C
4″
NH-2
NH-3
6.98, d (8.1)
7.88, br s
a
1
Recorded in DMSO-d6 at 500 MHz for H and 125 MHz for 13C.
aliphatic and one aromatic methine, six aliphatic methylenes,
and eight nonprotonated (five aromatic and three carbonyl)
carbon atoms (Table 1). COSY data disclosed two spin
systems corresponding to −NHCH(NH)CH2CH2− and
−CH2CH2CH2− substructures (Figure 2). Through iterative
Our synthesis started with regioselective protection of the
known compound lanosolaldehyde 4,3 followed by a reductive
amination with amino ester salt 6 in 72% yield. The
multisubstituted phenolic fragment 2 was obtained readily
upon quantitative removal of Boc protecting group (Scheme
2).
Figure 2. Key HMBC (red arrows) and COSY (blue bold lines)
correlations for rhodomelin A (1).
Scheme 2. Synthesis of Compound 2
analyses of the battery of 1D and 2D NMR spectroscopic data,
compound 1 was deduced to have four substructures including
a 2,3-dibromo-4,5-dihydroxybenzyl unit,2 a 4-substituted
butanoic acid moiety, a ureido group,1b and a pyrrolidone
residue. These fragments accounted for all components of the
molecular formula and unsaturations. The linkage of one
nitrogen atom (N-1) in the ureido unit to C-4 of the butanoic
acid moiety and to C-7′ of the 2,3-dibromo-4,5-dihydrox-
ybenzyl unit was supported by HMBC correlations from H-4
to C-5 and from H2-7′ to C-2′, C-6′, and C-5, respectively,
whereas the linkage of another nitrogen atom in the ureido
unit (N-2) to the pyrrolidone residue was evidenced by a
COSY interaction from NH-2 to H-1″ and by an HMBC
correlation from NH-2 to C-2″ (Figure 2). These key
correlations enabled assignment of the planar structure of 1
as N-(2,3-dibromo-4,5-dihydroxybenzyl)-N′-(5-oxopyrrolidin-
2-yl)-γ-ureidobutyric acid, which was named rhodomelin A. A
hypothetical biosynthetic pathway for 1 is proposed in Scheme
S1 (Supporting Information).
The second required fragment, L-pyroglutamic isocyanate
(3), was formed diastereoselectively in situ through a modified
one-pot Curtius rearrangement4 and was then coupled with
fragment 2 to provide a protected version of the conserved
chiral fragment 3 in good yield (75%). The Bn- and Cbz-
protecting groups were then removed in one step under
Compound 1 has one chiral center at C-1″ and assignment
of its absolute configuration was challenging. This chiral center
B
DOI: 10.1021/acs.orglett.7b03716
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
hydrogenation, and an additional LiOH-mediated hydrolysis
yielded (R)-configured rhodomelin A (Scheme 3).
Scheme 3. Synthesis of (R)-Rhodomelin A
Figure 3. Chiral HPLC profiles of (R)-1, (S)-1, natural 1, the mixture
of (S)-1 and natural 1, and the mixture of (R)-1 and natural 1 (from
top to bottom) over a CHIRALPAK AD-H column (eluent: n-
hexane/2-propanol 75:25, flow rate 1 mL/min), detected at 235 nm.
TEAC value of 4.37 mM (Table 2). The synthetic (R)- and
(S)-1 were also tested for the activity against DPPH radicals,
a
Table 2. DPPH and ABTS Radical-Scavenging Activity of 1
compd
DPPH (IC50, μM)
TEAC (mM)
1
BHT
ascorbic acid
3.82 0.01
82.13 0.20
20.07 0.15
4.37 0.24
n.t.
1.02 0.01
a
Each value is presented as the means SD (n = 3). n.t.: not tested.
with the (R)-isomer having stronger activity (IC50 = 4.60 μM)
than that of the (S)-isomer (IC50 = 8.90 μM).
The same synthetic sequence was followed starting from D-
pyroglutamic acid and yielded (S)-rhodomelin A (Scheme 4).
In conclusion, a novel ureidobromophenol (rhodomelin A,
1) was isolated and identified from the marine red alga
Rhodomela confervoides. Its planar structure was determined by
analysis of spectroscopic data, and the absolute configuration
was unambiguously assigned by total synthesis and chiral
HPLC analysis. Rhodomelin A (1) represents the first example
of a naturally occurring ureidopyrrolidone alkaloid incorporat-
ing a γ-aminobutyric unit and showed potent scavenging
activity against DPPH and ABTS radicals.
Scheme 4. Synthesis of (S)-Rhodomelin A
ASSOCIATED CONTENT
* Supporting Information
■
S
The NMR data, optical rotations, and chiral HPLC profiles
of the natural rhodomelin A (1) were compared with those of
synthesized (R)- and (S)-1. As shown in Table S1 (Supporting
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03716.
1
Information), the H and 13C NMR data for natural 1 were
Experimental details and spectra for natural and
synthesized 1 (PDF)
essentially identical to those of synthesized (R)-1 and (S)-1. As
for specific optical rotation, the value of rhodomelin A ([α]25
D
+18.5 (c 0.09, MeOH)) was in good agreement with that of
the synthesized (R)-1 ([α]25 +21.1 (c 0.19, MeOH)) and
AUTHOR INFORMATION
Corresponding Authors
■
D
opposite to that of the synthesized (S)-1 ([α]25 −17.9 (c
D
0.28, MeOH)), indicating that the absolute configuration at C-
1″ of natural rhodomelin A (1) was R.
*E-mail: wangbg@ms.qdio.ac.cn.
*E-mail: xutao@ouc.edu.cn.
ORCID
The results from chiral HPLC analysis showed that natural 1
had same retention time and identical UV spectrum as that of
synthesized (R)-1. As expected, natural 1 and synthesized (S)-
1 had the same UV profile, but different retention times
(Figure 3), further confirming that the absolute configuration
at C-1″ of natural 1 is R.
James B. Gloer: 0000-0002-9261-7571
Bin-Gui Wang: 0000-0003-0116-6195
Author Contributions
∥K.L. and Y.-F.W. contributed equally to this work.
Notes
The radical-scavenging activities of natural 1 were evaluated
using DPPH5 and TEAC6 assays following previously reported
methods. Compound 1 displayed significant scavenging
activity against DPPH with IC50 value of 3.82 μM, which is
21.5-fold more potent than that of the positive control BHT
(IC50 = 82.13 μM). In addition, this compound also exhibited
moderate scavenging activity against ABTS radicals with a
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Natural Science Foundation
of China (NSFC Grant No. 31330009 and 81502913) and by
■
C
DOI: 10.1021/acs.orglett.7b03716
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the NSFC-Shandong Joint Fund for Marine Science Research
Centers (U1606403 and U1706213). T.X. thanks the
Thousand Talents Plan for Young Professionals and the
Scientific and Technological Innovation project from
QNLMST (2015ASTP-ES14). B.-G.W. and T.X. appreciate
the support of Taishan Scholar Project from Shandong
Province.
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DOI: 10.1021/acs.orglett.7b03716
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