Vol. 64, No. 8
Chem. Pharm. Bull. 64, 1239–1241 (2016)
1239
Note
Antiproliferative Activity of Amathaspiramide Alkaloids and Analogs
,a
Jun Shimokawa,a Koji Chiyoda,b Hirotatsu Umihara,b and Tohru Fukuyama*
a Graduate School of Pharmaceutical Sciences, Nagoya University; Furo-cho, Chikusa-ku, Nagoya 464–8601,
b
Japan: and Graduate School of Pharmaceutical Sciences, University of Tokyo; 7–3–1 Hongo, Bunkyo-ku, Tokyo
113–0033, Japan.
Received March 14, 2016; accepted March 30, 2016; advance publication released online May 10, 2016
Assisted by the total syntheses of all the amathaspiramides, six natural products and four synthetic in-
termediates with partially fluctuating structures were prepared and subjected to a growth inhibition assay in
four human cancer cell lines. The results showed amathaspiramides A, C, and E had moderate antiprolifera-
tive activity. Examination of the structure–activity relationship revealed the importance of the amine or
imine substructure on the pyrrolidine moiety and the 8R stereochemistry on the N-acyl hemiaminal moiety
for the antiproliferative activity of amathaspiramide alkaloids.
Key words amathaspiramide; divergent synthesis; antiproliferative activity; cell viability assay; natural prod-
uct synthesis
Amathaspiramide alkaloids (1–6) were isolated by Morris
and Prinsep in 1999 from a New Zealand collection of marine
Results and Discussion
The outline of the synthesis was reported in our previous
bryozoan Amathia wilsoni.1) The structural characteristics publication9) and is to be introduced briefly in this manu-
of these compounds ranges from the highly functionalized script (Chart 1). The synthesis started with the asymmetric
diazaspiro[3.3]nonane framework, an intriguingly stable N- hydrogenation of butenolide 7 to give the lactone 8. Sequential
acyl hemiaminal, a dibrominated methoxyphenyl group, and a acylation and alkylation diastereoselectively gave the quater-
variably oxidized pyrrolidine moiety connected through a C-5 nary intermediate 9. Removal of benzyl group and Curtius
tetrasubstituted spiro center. Bioactivities of these molecules rearrangement followed by the acidic hydrolysis afforded a
have been reported only partially. Among them, antiviral γ-lactam 10. Bromination under the forced reaction conditions
activity against Poliovirus type 1, antimicrobial activity to- gave the dibromide 11 and this was converted to the cyclic
ward Bacillus subtilis, and cytotoxicity to BSC-1 cells were imide 12 in two steps. Regio-, chemo-, and diastereoselec-
reported for amathaspiramides A (1) and E (5).1) These α-tert- tive reduction of the cyclic imide moiety at the 8-position
alkylamino acids in general are known to be enzyme inhibitor proceeded only by the use of diisobutylaluminum hydride
themselves,2) as well as to add conformational twist or rigidity (DIBAL) as the reagent, which represented the total synthesis
to peptides.3) A recent report also proposed structurally simi- of amathaspiramide D (4). This molecule could be considered
lar compounds as β-turn mimetics.4) Despite these promis- the common precursor to all the other members. Amathaspi-
ing reports, the limited availability of amathaspiramides has ramide D (4) was initially treated with Schwartz reagent that
hampered further biological investigations on these molecules mediated the selective reduction of secondary lactam in the
and their analogs. Of the six natural congeners, Hughes and presence of the N-acyl hemiaminal and tertiary lactam, which
Trauner,5) Ohfune and colleagues,6,7) and Soheili and Tambar8) gave amathaspiramide E (5). Reductive methylation of 5 gave
reported total synthesis of amathaspiramide F, an only 8S amathaspiramide A (1) and simple reduction of imine gave
member. Eventually, our group has reported the only com- amathaspiramide C (3). The stereochemistry of the C8 N-
prehensive syntheses of all the amathaspiramides,9) and Lee acyl hemiaminal moiety was surprisingly stable and did not
and colleagues reported the efficient formal synthesis through epimerize during the transformations. This stable stereochem-
our common intermediate.10) From the outset of our synthetic istry was epimerized only under the basic reaction conditions
study, we envisioned establishing a scalable and versatile to give amathaspiramide F (6). Finally, amathaspiramide B (2)
synthetic route to all the amathaspiramides whose pyrrolidine was synthesized by the protection of hemiaminal of 4 with
moieties range over cyclic imine, secondary or tertiary amine ethoxyethyl group under acidic conditions and methylation
as well as unsubstituted or N-methyl γ-lactam. Therefore, we under basic conditions followed by the acidic hydrolysis of the
employed a systematic access for the spirocyclic core and the protecting group.
variable oxidation states of the spiro pyrrolidine units from
As the potential anti-proliferative activity of these natural
a late-stage common intermediate,11) which could enable the products as well as the synthetic intermediates on human
preparation of the diverse substructure within a uniform core carcinoma cells remained virtually unexplored, we were in-
skeleton for structure–activity relationship (SAR) study. The terested in comparing the biological effects of these structur-
purpose of the current work was to acquire the novel data ally closely related compounds on human cancer cell lines.
on the bioactivity of these natural products as well as the in- The antiproliferative activity was assessed in vitro using a
formation on the SAR, facilitated by the assay of a group of panel of four human cancer cell lines: HCT116 (colon cancer),
structurally similar molecules bearing the same core structure. PC-3 (prostate cancer), MV4-11 (acute myeloid leukemia),
MiaPaCa-2 (pancreas cancer). The assays were conducted over
three day incubation period at 37°C and were run in dupli-
*To whom correspondence should be addressed. e-mail: fukuyama@ps.nagoya-u.ac.jp
© 2016 The Pharmaceutical Society of Japan
Shimokawa, Jun
