Structure revision of poecillastrin c and the absolute configuration of the β-hydroxyaspartic acid residue

Article abstract of DOI:10.1021/acs.orglett.7b02835

The planar structure of poecillastrin C (1) was revised through selective reduction of the ester carbon. The absolute configuration of the β-hydroxyaspartic acid (OHAsp) residue was determined to be D-threo by Marfey's analysis. The acid hydrolysate of the reduction product of 1 liberated (2R, 3R)-2-amino-3, 4-dihydroxybutanoic acid, demonstrating that the β-carboxyl group in poecillastrin C was esterified. The structures of poecillastrins B-D and 73-deoxychondropsin A were also revised.

Full text of DOI:10.1021/acs.orglett.7b02835

Letter
pubs.acs.org/OrgLett
Structure Revision of Poecillastrin C and the Absolute Configuration
of the β‑Hydroxyaspartic Acid Residue
†
,†
‡
§
†
⊥
Raku Irie, Kentaro Takada,* Yuji Ise, Susumu Ohtsuka, Shigeru Okada, Kirk R. Gustafson,
,
†
and Shigeki Matsunaga*
†Laboratory of Aquatic Natural Products Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo,
Bunkyo-ku, Tokyo 113-8657, Japan
‡
Sugashima Marine Biological Laboratory, Nagoya University, Toba, Mie 517-0004, Japan
§
Takehara Marine Station, Hiroshima University, Takehara, Hiroshima 725-0024, Japan
⊥
Molecular Targets Laboratory, Center for Cancer Research, National Cancer Institute, Frederick, Maryland 21702, United States
*
S Supporting Information
ABSTRACT: The planar structure of poecillastrin C (1) was
revised through selective reduction of the ester carbon. The
absolute configuration of the β-hydroxyaspartic acid (OHAsp)
residue was determined to be D-threo by Marfey’s analysis. The
acid hydrolysate of the reduction product of 1 liberated
(
2R,3R)-2-amino-3,4-dihydroxybutanoic acid, demonstrating
that the β-carboxyl group in poecillastrin C was esterified.
The structures of poecillastrins B−D and 73-deoxychondrop-
sin A were also revised.
hondropsins and poecillastrins are unique marine natural
C
products. They were isolated only in small amounts from
taxonomically diverse deep-sea sponges; chondropsins A, B,
1
,2
and D from Chondropsis sp., 73-deoxychondropsin A and
3
chondropsin C from Ircinia sp., chondropsin A and 73-
4
deoxychondropsin A from Psammoclemma sp., poecillastrins
5
,6
A−C from Poecillastra sp., poecillastrin C (1) and D from
7
8
Jaspis sp., and mirabalin from Siliquariaspongia sp. Chon-
dropsins and poecillastrins were discovered as potent cytotoxic
compounds with characteristic mean-graph profiles in the
National Cancer Institute’s 60-cell antitumor screen and later
+
found to inhibit fungal and mammalian vacuolar H -ATPases
9
(
V-ATPases). Due to their potent activity and unique
structural features, this class of compounds is considered as
promising leads for new therapeutic agents, especially as
anticancer agents. However, further developmental studies have
been hampered by restricted supply of the compounds from
their natural sources. Total synthesis was unfeasible due to
limited stereochemical information; only the relative stereo-
chemistry of the THP ring has been reported, and none of the
absolute configurations of 26 stereogenic centers in chondrop-
attached to the nitrogen-bearing methine formed the ester
linkage. Although the distance between the carbomethoxy
protons and the α-methine proton is shorter than that between
the carbomethoxy protons and the β-methine proton, both
distances fall within the range that can give observable NOEs.
Because both carboxyl groups in the OHAsp residues are
located within three bonds from either of the methine protons,
it is not possible to assign the two carboxyl carbons by HMBC
data, unless a HMBC cross-peak is observed from the amide
proton (H-3) to one of the carboxyl carbons, which was not
10
sin A has been reported.
The most difficult problem in the structure elucidation of
chondropsin A was to assign which of the two carboxylic acid
moieties in the OHAsp residue formed the ester linkage and
which was free. The authors of ref 1 converted the free
carboxylic acid group to a methyl ester and observed an NOE
between the O-methyl signal and the oxymethine proton. From
this analysis, they concluded that the carboxylic acid group
Received: September 11, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02835
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
observed in chondropsin A. In the structure elucidation of
other chondropsin congeners, the same mode of ester
formation was proposed without firm evidence.
In the course of our search for bioactive marine metabolites,
which cause morphological changes in rat embryonic fibroblast
the acid hydrolysate of the reduction product of poecillastrin C
would afford (2R,3S)-3-amino-2,4-dihydroxybutanoic acid (3)
if the structure of poecillastrin C is 1; the compound with
structure 2 would give (2R,3R)-2-amino-3,4-dihydroxybutanoic
acid (4).
3
Y1 cells, we found activity in the extract of a marine sponge
The protected forms of compounds 3 and 4 were prepared as
follows. The carboxyl group of monoethylfumarate (5) was
reduced to the allyl alcohol (Scheme 2), which was protected
Poecillastra sp. collected at Oshima-shinsone. The combined
EtOH and CHCl /MeOH (1:1) extract of the sponge (70 g,
3
wet weight) was partitioned between CHCl and H O, and the
3
2
H O layer was further extracted with n-BuOH. The CHCl and
Scheme 2. Synthesis of 9, 10, and Their Enantiomers
2
3
n-BuOH fractions were combined and fractionated by ODS
flash chromatography and RP-HPLC to give 3.2 mg of
poecillastrin C as the main active constituent (Supporting
Information, SI). The structure of poecillastrin C had been
assigned as 1. We examined the NMR data of poecillastrin C
and found that the spectroscopic data could not exclude the
possibility of structure 2. Therefore, we set out to obtain secure
chemical evidence for the structure around the OHAsp moiety.
We took advantage of the different reactivity of the ester and
carboxylic acid moieties toward hydride reduction (Scheme 1).
by the TBS group. The resultant TBS ether (6) was subjected
13
to the Sharpless asymmetric aminohydroxylation to give a
mixture of 7, 8, and their enantiomers. The enantiomeric
mixtures were obtained after HPLC separation. The relative
configurations of all the products were shown to be syn by
Scheme 1. Differentiation of the Alternate Modes of Lactone
Formation by Chemical Modification
conversion to the cyclic carbamates (9, 10, and their
3
enantiomers), in which a J
for each.
value of 4.8 Hz was observed
H2,H3
14
The optical resolution of the enantiomers and assignment of
their absolute configurations were achieved by the modified
15
Mosher’s method (Scheme 3). The mixture of 7 and its
enantiomer was derivatized with (R)-α-methoxy-α-trifluorome-
thylphenylacetyl chloride (MTPACl) to give 11 and 12, which
1
were separated by HPLC. Their H NMR data showed that the
absolute configuration of 11 was (2R,3S). The mixture of 8 and
Scheme 3. Preparation of (S)-MTPA Esters 11−14
In structure 1, reduction with NaBH would convert the C-1
4
ester carbonyl carbon to a hydroxymethyl, while the C-34
carboxylic acid would stay intact, thereby affording 3-amino-
11
2
,4-dihydroxybutanoic acid following acid hydrolysis. On the
other hand, if a compound with structure 2 is reduced with
NaBH and then hydrolyzed, it would afford 2-amino-3,4-
4
dihydroxybutanoic acid.
As a prelude, we determined the absolute configuration of
two chiral centers in the OHAsp residue. The acid hydrolysate
12
of poecillastrin C was subjected to Marfey’s analysis, which
showed that the OHAsp residue was D-threo (SI). Therefore,
B
DOI: 10.1021/acs.orglett.7b02835
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
its enantiomer was examined in the same way to give 13 and
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
14, and the configuration of 13 was established as (2R,3R).
Then, 11, 13, and the reduction product of poecillastrin C were
hydrolyzed and subjected to Marfey’s analysis, which
demonstrated that the hydrolysate of 13 and the reduction
product of poecillastrin C were identical (Figure 1, SI).
Therefore, the structure of poecillastrin C was reassigned as 2.
REFERENCES
■
(1) Cantrell, C. L.; Gustafson, K. R.; Cecere, M. R.; Pannell, L. K.;
Boyd, M. R. J. Am. Chem. Soc. 2000, 122, 8825−8829.
2) Rashid, M. A.; Cantrell, C. L.; Gustafson, K. R.; Boyd, M. R. J.
(
Nat. Prod. 2001, 64, 1341−1344.
3) Rashid, M. A.; Gustafson, K. R.; Boyd, M. R. Tetrahedron Lett.
001, 42, 1623−1626.
4) Chevallier, C.; Laprevote, O.; Bignon, J.; Debitus, C.; Guenard,
D.; Sevenet, T. Nat. Prod. Res. 2004, 18, 479−484.
5) Rashid, M. A.; Gustafson, K. R.; Crouch, R. C.; Groweiss, A.;
Pannell, L. K.; Van, Q. N.; Boyd, M. R. Org. Lett. 2002, 4, 3293−3296.
6) Takada, K.; Choi, B. W.; Rashid, M. A.; Gamble, W. R.;
(
2
(
(
(
Cardellina, J. H., II; Van, Q. N.; Lloyd, J. R.; McMahon, J. B.;
Gustafson, K. R. J. Nat. Prod. 2007, 70, 428−431.
(7) Takemoto, D.; Takekawa, Y.; van Soest, R. W. M.; Fusetani, N.;
Matsunaga, S. Biosci., Biotechnol., Biochem. 2007, 71, 2697−2700.
8) (a) Plaza, A.; Baker, H. L.; Bewley, C. A. J. Nat. Prod. 2008, 71,
73−477. (b) Correction: Plaza, A.; Baker, H. L.; Bewley, C. A. J. Nat.
Prod. 2009, 72, 324−324.
9) Bowman, E. J.; Gustafson, K. R.; Bowman, B. J.; Boyd, M. R. J.
Biol. Chem. 2003, 278, 44147−44152.
10) Attempts were made to synthesize mirabalin without the
information on the absolute configuration: (a) Echeverria, P.-G.;
Prevost, S.; Cornil, J.; Ferard, C.; Reymond, S.; Guerinot, A.; Cossy, J.;
(
4
Figure 1. LC−MS chromatograms of Marfey’s derivatives: (a) acid
hydrolysate of 11; (b) acid hydrolysate of 13; (c) acid hydrolysate of
the reduction product of poecillastrin C.
(
(
The same set of experiments run with authentic samples of
poecillastrins B, D, and 73-deoxychondropsin A liberated 4,
demonstrating that the mode of ester formation in these
compounds had to be revised (SI).
́
́
́
Ratovelomanana-Vidal, V.; Phansavath, P. Org. Lett. 2014, 16, 2390−
2393. (b) Cornil, J.; Echeverria, P.-G.; Reymond, S.; Phansavath, P.;
Ratovelomanana-Vidal, V.; Guerinot, A.; Cossy, J. Org. Lett. 2016, 18,
4534−4537.
A combination of microscale reduction, hydrolysis, and
derivatization reactions was performed on four different
poecillastrin and chondropsin macrolides, and the resulting
products were compared with appropriate synthetic standards.
The natural products all provided derivatives of the OHAsp
residue that are only consistent with macrolactonization
occurring via the side chain carbonyl and not the α-amino
acid carbonyl. In this work, we showed that the planar
structures of the chondropsin/poecillastrin class of metabolites
need to be revised, setting the stage for further stereochemical
assignment of this class of bioactive compounds.
(11) Hirosawa, S.; Takahashi, Y.; Hashizume, H.; Miyake, T.;
Akamatsu, Y. J. Antibiot. 2014, 67, 265−268.
(12) Marfey, P. Carlsberg Res. Commun. 1984, 49, 591−596.
(13) Reddy, K. L.; Sharpless, K. B. J. Am. Chem. Soc. 1998, 120,
1
(
207−1217.
14) (a) Esgulian, M.; Belot, V.; Guillot, R.; Deloisy, S.; Aitken, D. J.
Org. Biomol. Chem. 2017, 15, 1453−1462. (b) Gutierrez, M. L.;
Garrabou, X.; Agosta, E.; Servi, S.; Parella, T.; Joglar, J.; Clapes, P.
Chem. - Eur. J. 2008, 14, 4647−4656.
(15) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem.
Soc. 1991, 113, 4092−4096.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02835.
Description of experimental procedure, LC−MS chro-
matograms, and NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*
E-mail: atakada@mail.ecc.u-tokyo.ac.jp.
E-mail: assmats@mail.ecc.u-tokyo.ac.jp.
*
ORCID
Kirk R. Gustafson: 0000-0001-6821-4943
Shigeki Matsunaga: 0000-0002-8360-2386
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was partly supported by JSPS KAKENHI Grant Nos.
5252037, 16H04980, 17J08775, and 17H06403 from The
■
2
C
DOI: 10.1021/acs.orglett.7b02835
Org. Lett. XXXX, XXX, XXX−XXX