Synthesis of Morpholine-Based Analogues of (?)-Zampanolide and Their Biological Activity

Article abstract of DOI:10.1002/chem.202003996

We describe the convergent synthesis of three prototypical examples of a new class of analogues of the complex, cytotoxic marine macrolide (?)-zampanolide that incorporate an embedded N-substituted morpholine moiety in place of the natural tetrahydropyran ring. The final construction of the macrolactone core was based on a high-yielding intramolecular HWE olefination, while the hemiaminal-linked side chain was elaborated through a stereoselective, BINAL-H-mediated addition of (Z,E)-sorbamide to a macrocyclic aldehyde precursor. The synthesis of the common functionalized morpholine building block involved two consecutive epoxide openings with tosylamide and the product of the first opening reaction, respectively, as nucleophiles. Of the three morpholino-zampanolides investigated, the N-acetyl and the N-benzoyl derivatives both exhibited nanomolar antiproliferative activity, thus being essentially equipotent with the natural product. In contrast, the activity of the N-tosyl derivative was significantly reduced.

Full text of DOI:10.1002/chem.202003996

Accepted Article
Accepted Article
Title: Synthesis of Morpholine-Based Analogs of (−)-Zampanolide and
Their Biological Activity
Authors: Karl-Heinz Altmann, Christian Paul Bold, Melanie Gut,
Jasmine Schürmann, Daniel Lucena Agell, Jose Fernando
Diaz, and Juerg Gertsch
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Synthesis of Morpholine-Based Analogs of (−)-Zampanolide and
Their Biological Activity
[
a]
[a]
[a]
[b]
[c]
Christian Paul Bold, Melanie Gut, Jasmine Schürmann, Daniel Lucena-Agell, Jürg Gertsch,
[
c]
[a]
José Fernando Díaz, and Karl-Heinz Altmann*
[
a]
M. Sc. C. P. Bold, M. Sc. M. Gut, J. Schürmann, Prof. Dr. K.-H. Altmann
Department of Chemistry and Applied Biosciences
Institute of Pharmaceutical Sciences
Swiss Federal Institute of Technology
HCI H405
Vladimir-Prelog-Weg 4, 8093 Zürich (Switzerland)
E-mail: karl-heinz.altmann@pharma.ethz.ch
Homepage: https://pharmaceutical-biology.ethz.ch/
D. Lucena-Agell, Dr. J. F. Díaz
Centro de Investigaciones Biolόgicas Margarita Salas
Consejo Superior de Investigaciones Científicas
Ramiro de Maeztu 9, 28040 Madrid (Spain)
E-mail: fer@cib.csic.es
[
b]
c]
[
Prof. Dr. J. Gertsch
Department of Chemistry and Applied Biosciences
Institute of Biochemistry and Molecular Medicine
University of Bern
Bühlstrasse 28, 3012 Bern
E-mail: juerg.gertsch@ibmm.unibe.ch
Supporting information for this article is given via a link at the end of the document.
Abstract: We describe the convergent synthesis of three prototypical
examples of a new class of analogs of the complex, cytotoxic marine
macrolide (−)-zampanolide, which incorporate an embedded N-
substituted morpholine moiety in place of the natural tetrahydropyran
ring. The final construction of the macrolactone core was based on a
high-yielding intramolecular HWE olefination, while the hemiaminal-
linked side chain was elaborated through a stereoselective, BINAL-H-
mediated addition of (Z,E)-sorbamide to a macrocyclic aldehyde
precursor. The synthesis of the common functionalized morpholine
building block involved two consecutive epoxide openings with
tosylamide and the product of the first opening reaction, respectively,
as nucleophiles. Of the three morpholino-zampanolides investigated,
the N-acetyl and the N-benzoyl derivative both exhibited nanomolar
antiproliferative activity, thus being essentially equipotent with the
natural product. In contrast, the activity of the N-tosyl derivative was
significantly reduced.
chemical biology.[5] While these compounds are generally more
difficult to access than terrestrial natural products and often can
only be obtained in very small quantities, marine natural products
have been argued to show a higher incidence of significant
bioactivity, which is often associated with a high degree of
structural novelty.[6] At this point in time, 13 marine natural
products have been developed into FDA-approved drugs
(including 3 antibody-drug conjugates), while 23 compounds are
undergoing Phase I, II or III clinical trials (including 15 antibody-
drug conjugates).[4a],[7] While these numbers may appear relatively
small, they have to be judged in relation to the total number of
marine natural products currently known (around 30'000),[
which is much smaller than the number of terrestrial natural
products (>300'000).[8]
4c]
(−)-Zampanolide (1) is a macrolide that was first isolated in 1996
by Tanaka and Higa from the marine sponge Fasciospongia
rimosa.[ The compound was found to inhibit the proliferation of
several cancer cell lines in vitro with nanomolar potency, although
the underlying mechanism of action was not investigated.
9]
Introduction
Natural products are a highly prolific source of lead structures for
drug discovery and development, owing to their coverage of
structural space that is only sparsely populated by common
synthetic organic molecules.[1] Thus, a substantial fraction of
currently employed prescription drugs worldwide are either
natural products themselves or can be considered as being
natural product-derived.[ Historically, the vast majority of
bioactive natural products have been obtained from terrestrial
plants, bacteria, or fungi;[3] however, more recently, natural
products originating from marine organisms, such as sponges,
corals, algae, or marine bacteria have attracted significant
attention as potential leads for drug discovery[4] or as probes in
2]
In 2009, (−)-zampanolide (1) was re-isolated from the Togan
sponge Cacospongia mycofijensis by Field et al.[10] who also
demonstrated that 1 was a potent microtubule-stabilizing agent
1
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(
MSA) which arrests cells in the G2/M phase of the cell cycle and
[
10]
induces apoptosis.
Thus, (−)-zampanolide (1) inhibits cancer
cell growth through the same mechanism as the established
anticancer drugs paclitaxel, taxotere, cabazitaxel, and
ixabepilone. [11]
As first demonstrated by biochemical experiments, 1 binds to the
taxol-binding site on ꞵ-tubulin in a covalent fashion.[12] These
conclusions were subsequently confirmed by the high resolution
X-ray crystal structure of the tubulin/zampanolide complex;[13] the
structure also revealed that the covalent attachment of 1 to -
tubulin is based on the 1,4-addition of the imidazole moiety of
Results and Discussion
Synthetic planning. Our overall strategy for the synthesis of
morpholino-zampanolides 4-6 was to build on our previous work
on the total synthesis of (−)-zampanolide (1)[15e] and of

His229 to the enone moiety in the macrocycle.
Structurally, (−)-zampanolide (1) is characterized by a highly
unsaturated 20-membered macrolactone core with an embedded
syn-2,6-disubstituted tetrahydropyran ring carrying an exocyclic
methylene group. Attached to C(19) of this macrolactone core is
a (Z,E)-sorbamide-containing side chain that also includes a
highly unusual hemiaminal group which connects the sorbamide
moiety to the macrocycle. Only few other natural products are
known that incorporate related N-acyl hemiaminal groups.[14]
Given its intriguing structural features and its medically relevant
mode of action, it is unsurprising that (−)-zampanolide (1) has
been the target of several total syntheses,[15] including work from
our own group.[15e] Prior to these efforts, Smith had reported the
synthesis of ent-1;[16] this work established the absolute
configuration of the natural product to be 11S, 15S, 19S, 20S.
Work has also been reported on the synthesis and biological
evaluation of analogs of 1,[15e],[17] but overall, the SAR around the
compound is still underexplored. Importantly, however, it has
been demonstrated that the C(19) side chain is crucial for the
potent biological activity of 1, as both aldehyde 2 and alcohol 3
are significantly less potent than 1.[15e],[18] On the other hand, we
have shown that the removal of the exocyclic methylene group
[
19]
zampanolide analogs to the largest extent possible; including
the usage of building blocks that had served as intermediates in
the synthesis of 1. Thus, the C(19) side chain was to be
elaborated in the last step of the synthesis by an aza-aldol
reaction of aldehydes 8a-c with (Z,E)-sorbamide (7) (Scheme 1),
employing methodology that has recently been developed in our
laboratory for the stereoselective establishment of the hemiaminal
center in zampanolide-type structures.[19]
from either 1[ or 2/3
19]
[15e],[18b]
has no significant effect on biological
potency. This observation is in line with the structural data on the
zampanolide/-tubulin complex, which show no significant
interactions between the methylene group and the protein.[13]
Based on our findings for the 13-desmethylene derivatives of 1, 2,
and 3, we became interested in analogs of 1, where the THP ring
would be replaced by a morpholine moiety. We felt that the
morpholine N-atom could serve as a convenient site for the
attachment of a variety of substituent groups to modulate the
physico-chemical and biological properties of such analogs,
including the attachment of tumor-targeting and/or prodrug
moieties. In light of these considerations, it was the objective of
this work to establish an efficient platform for the synthesis of N-
substituted morpholino-zampanolides and to collect some
preliminary biological data for a select number of derivatives. As
we felt it important to understand the size tolerance for
substituents on the morpholine nitrogen, acetamide 4 and
sulfonamide 5 were chosen as initial target structures for
synthesis and biological evaluation; based on the biological
results with 4 and 5, we subsequently extended our synthetic work
to benzamide 6.
In the following, we describe the successful synthesis of
morpholino-zampanolides 4-6 and the assessment of their
microtubule-binding affinity and antiproliferative activity.
Scheme 1. Retrosynthesis of morpholino-zampanolides 4, 5, and 6. a: R =
C(O)CH ; b: R = Ts; c: R = C(O)Ph.
3
2
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The macrocycle was envisioned to be closed by an intramolecular
could be readily reprotected under standard conditions (TBDPSCl,
imidazole, DMAP in DMF) to furnish 13 in 92% yield, thus bringing
the total yield of 13 to 70% (from 14). The route depicted in
Scheme 3 was highly scalable and provided 13 in multigram
quantities.
Horner–Wadsworth–Emmons
(HWE)
reaction;
the
macrocyclization precursors for the HWE reaction would be
obtained by esterification of the acid 9[15e] with morpholine
derivatives 10a-c. The latter were to be accessed from vinyl iodide
12 via iodine/metal exchange and subsequent reaction of the
vinylmetal species with PMB-protected (R)-glycidol (11). For
amide-based target structures 4 and 6, this was to be followed by
cleavage of the tosyl group and acylation. Vinyl iodide 12 would
be derived from alkyne 13 by a stannylcupration/Sn-iodine
exchange sequence. The construction of the crucial morpholino
core with the appropriately functionalized appendices at positions
2
and 6 (morpholine numbering) was considered one of the major
challenges in the synthesis of 4-6, given the scarcity of methods
that are available for the synthesis of systems similar to 13. We
opted to approach the problem based on methodology that has
been developed by Penso and co-workers for the stereoselective
preparation of 2,6-disubstituted N-tosyl morpholines.[20] The
method entails opening of a first epoxide with tosyl amide (17),
protection of the resulting -hydroxyethyl sulfonamide as a THP-
ether, base-catalyzed opening of a second epoxide with the
sulfonamide as a nucleophile, tosylation of the ensuing free
hydroxy group, cleavage of the THP-ether, and final base-
mediated ring-closure to the morpholine. Based on this protocol,
morpholine derivative 13 was envisioned to be accessible from
tosyl amide (17) and the known epoxides 15[21] and 16[15e] via
sulfonamide 14. The latter would be obtained if the first step of the
above reaction sequence involved opening of 16 with 17. While
this approach inevitably leads to N-tosyl morpholines, we felt
confident that the removal of the tosyl group at the appropriate
stage would be feasible.
Synthesis of alcohol 10b. The synthesis of building block 10b in
a first stage required the preparation of the known epoxides 15[21]
and 16[15e] (Scheme 2). The synthesis of 15 involved the tosylation
of commercially available 3-pentyn-1-ol (18), followed by base-
induced elimination to furnish pent-2-en-4-yne (19) in 84% overall
yield.[22] Ene-yne 19 was then converted into the enantiopure
epoxide 15 by a Jacobsen epoxidation/(salen)cobalt-catalyzed
3
Scheme 2. a) p-Toluenesulfonyl chloride, NEt , dichloromethane, 0 °C → rt, 24
h, 97%; b) KOH, Teepol , H O, 160 °C, 4 h, 86%; c) NaOCl, (R,R)-Jacobsen’s
2
catalyst, dichloromethane, 0 °C, 15 h, 62%, ee, 48%; d) (S,S)-Co-salen complex
TM
hydrolytic kinetic resolution (HKR) sequence in 39% yield.[21]
Epoxide 16 was obtained in 3 steps from D-aspartic acid
according to Zurwerra et al.[15e] Ring-opening of 16 with p-
(
20), H
2
O, Et
2
O, 0 °C → rt, 22 h, 65%, ee ≥ 99.5%; e) p-toluenesulfonamide
CO , dioxane, 90 °C, 24 h, 81%; f)
(17), benzyltriethylammonium chloride, K
2
3
toluenesulfonamide (p-TsNH
2
; 17) under solid-liquid phase-
3,4-dihydropyran, pyridinium p-toluenesulfonate, dichloromethane, rt, 1.5 h,
quant.; g) 21, benzyltriethylammonium chloride, K CO , dioxane, 90 °C, 20 h,
8%; h) p-toluenesulfonyl chloride, NEt 4-(dimethylamino)pyridine,
dichloromethane, 0 °C → rt, 3.5 h, 93%; i) pyridinium p-toluenesulfonate, aq.
EtOH, rt, 40 h, 48% (13), 24% (22); j) tert-butyldiphenylchlorosilane, imidazole,
4-(dimethylamino)pyridine, N,N-dimethylformamide, rt, 2 h, 92%; k) CuCN, n-
transfer catalysis (SL-PTC) conditions[20] and subsequent
protection of the resulting secondary alcohol as a THP-ether then
2
3
5
3
,
gave the fully protected aminodiol 21 as
diastereomers (at C(2) of the THP ring). Reaction of tosyl amide
1 with oxirane 15 under the same phase transfer conditions
employed for the ring-opening of 17 with p-TsNH
,[20] followed by
a mixture of
BuLi, Bu
iodosuccinimide, tetrahydrofuran, −18 °C, 75 min, 74% (94%; see text); l) n-
BuLi, BF •OEt , toluene, −78 °C → −100 °C, 2 h, 76%.
3
SnH, MeOH, tetrahydrofuran, −78 °C → −15 °C, 16.5 h, 96%; l) N-
2
2
3
2
tosylation of the ensuing secondary hydroxy group by reaction
with TsCl in DCM furnished tosylate 14 in 54% overall yield (from
2
1). Initial attempts at the removal of the THP protecting group
With a scalable route to morpholine derivative 13 established, the
alkyne functionality was then converted into the corresponding
under the conditions reported by Penso, i. e. treatment of 14 with
pyridinium p-toluenesulfonate (PPTS) in aqueous ethanol at
vinyl iodide moiety by a stannylcupration/iodination sequence with
5
5 °C,[20] only led to decomposition. Gratifyingly, this problem
23]
Bu
3
Sn(Bu)CuCNLi [
2
and N-iodosuccinimide (NIS) (Scheme 2).
could be overcome by conducting the reaction at room
temperature; in contrast to the examples reported by Penso et al.,
these conditions not only led to cleavage of the THP-ether but
directly gave the ultimately desired morpholine derivative 13 in
The stannylated product was obtained in excellent yield (96%) as
a single isomer. Vinyl iodide 12 was isolated in 74% yield after
column chromatography. In addition to 12, significant amounts of
the stannane precursor were recovered, although TLC analysis
had indicated complete conversion. The recovered stannane was
re-subjected to the iodination conditions, but in contrast to the
previous experiment, the excess NIS was not quenched with
48% isolated yield. In addition, the free alcohol 22 was obtained
in 24% yield, resulting from partial cleavage of the TBDPS-ether
group, either before or after morpholine ring formation. Alcohol 22
3
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Chemistry - A European Journal
10.1002/chem.202003996
FULL PAPER
aqueous sodium thiosulfate. Instead, the reaction mixture was
diluted with hexanes and directly purified by column
chromatography. In doing so, the desired product 12 could be
isolated in 74% yield. (Resubmitting the recovered intermediate
stannane to reaction with N-iodosuccinimide gave additional 12,
brining the total isolated yield to 94%). The subsequent iodine-
lithium exchange followed by reaction of the ensuing vinylmetal
species with PMB-protected (R)-glycidol (11) afforded alcohol
achieve full conversion, 5 portions of 2 equiv. of (S)-BINAL-7
complex solution were added over 10 min. The N-acetyl
morpholine-based zampanolide analog 4 was obtained in 75%
yield after flash column chromatography (FC) (according to 1H-
NMR of the FC-purified material); for reasons that have not been
explored, the isolated yield after preparative NP-HPLC
purification was only 15%. 4 was obtained with a dr of >99:1.[
In an alternative approach towards analog 4, we have also
explored the removal of the N-tosyl group at the stage of alkyne
13 (Scheme 4). Different desulfonation conditions were examined
30]
10b in 76% yield.
Synthesis of morpholino-zampanolide 4. The elaboration of
alcohol 10b into the N-acetyl morpholine-based zampanolide
analog 4 in the first step involved cleavage of the tosyl group from
the morpholine nitrogen with magnesium turnings in methanol,[24]
to afford the free amine 23 in 85% yield (Scheme 3). The latter
could then be acetylated in almost quantitative yield with
isopropenyl acetate;[25] subsequent esterification of this
intermediate with acid 9 under Yamaguchi conditions[15e],[26]
followed by global desilylation afforded diol 24 in 84% yield over
in this context (Na in pentanol[ , TMSCl and NaI in acetonitrile
31]
[32]
and Mg in MeOH[24]), of which only magnesium in methanol
delivered the desired product in a reasonable yield. The free
morpholine 26 was acetylated with acetic anhydride to provide 27
in excellent yield. The latter was converted into vinyl iodide 28 by
[23]
stannylcupration/iodination with Bu
3
Sn(Bu)CuCNLi
2
and NIS.
28 was obtained in 88% yield; similar to the conversion of alkyne
13 into vinyl iodide 12 (cf. Scheme 2), 12% of the intermediate
stannane was also recovered. Unfortunately, attempts at the
elaboration of 28 into alcohol 10a via iodine-lithium exchange
were unsuccessful, using either tBuLi or nBuLi. Only complex
product mixtures were obtained, possibly due to deprotonation of
the acetyl residue.
2
steps. Oxidation of 24 with Dess-Martin periodinane[27] gave a
keto aldehyde that underwent smooth Ba(OH) -mediated HWE
reaction,[15e],[28] to furnish the desired macrolactone in 68% yield
based on 24). Oxidative PMB-cleavage with DDQ then provided
2
(
primary alcohol 25, which was readily oxidized to aldehyde 8a
with Dess-Martin periodinane.[27]
Scheme 4. a) Mg, MeOH, 0 °C → rt, 19 h, 96%; b) acetic anhydride, pyridine,
dichloromethane, -78 °C, 60 min, 91%; c) CuCN, n-BuLi, Bu
tetrahydrofuran, −78 °C −15 °C, 21 h, 99%; d) N-iodosuccinimide,
tetrahydrofuran, −17 °C → −10 °C, 30 min, 88% (+12% Sn-intermediate); e) t-
BuLi or n-BuLi, BF •OEt , toluene, −78 °C → −100 °C, decomposition.
3
SnH, MeOH,
→
3
2
Scheme 3. a) Mg, MeOH, 0 °C → rt, 2.5 h, 85%; b) isopropenylacetate, 60 °C,
3
h, 96%; c) 9, 2,4,6-trichlorobenzoyl chloride, NEt
tetrahydrofuran, rt, 2.5 h, quant.; d) HF•pyridine, tetrahydrofuran, 0 °C → rt, 14
h, 84%; e) Dess-Martin periodinane, dichloromethane, rt, 55 min; f) Ba(OH)
tetrahydrofuran / H O 40:1, 0 °C, 12 min, 68% over 2 steps; g) 2,3-dichloro-5,6-
dicyano-p-benzoquinone, dichloromethane / phosphate buffer (pH 7.2) 40:1, rt,
3
, 4-(dimethylamino)pyridine,
Synthesis of morpholino-zampanolide 5. The esterification of
[
15e]
alcohol 10b with acid
9
was again performed under
and the crude ester was directly
2
,
Yamaguchi conditions[
15e],[26]
2
submitted to silyl-ether cleavage with HF•pyridine, to afford diol
2
7
1
h, 98%; h) Dess-Martin periodinane, NaHCO
3
, dichloromethane, rt, 50 min,
2
9 in 78% overall yield (Scheme 5). Double oxidation of 29 with
8%; i) 7, LiAlH
5%, dr > 99:1.
4
, EtOH, (S)-(−)-1,1’-bi(2-naphthol), tetrahydrofuran, rt, 18 min,
DMP,[ followed by a Ba(OH) -mediated macrocylization[15e] then
27]
2
provided macrolactone 30 in 69% overall yield (from 29). Removal
of the PMB-protecting group with DDQ followed by oxidation of
the liberated hydroxy group with buffered DMP[27] gave aldehyde
The addition of (Z,E)-sorbamide (7) to aldehyde 8a was then
performed using methodology that we have recently developed
8b in 89% overall yield.
The elaboration of 8b into morpholino-zampanolide analog 5 was
again accomplished via the putative amide transfer reagent that
is likely formed upon addition of (Z,E)-sorbamide (7) to a THF
solution of (S)-BINAL-H.[19],[29] Again, 5 portions of 2 equiv. of the
for the stereoselective establishment of the hemiaminal center in
zampanolide-type structures.[19] Thus, 7 was added to a solution
of (S)-BINAL-H[29] to form a putative amide transfer complex; this
solution was then added immediately to a THF solution of 8a and
(
S)-BINAL-7 complex solution had to be added to aldehyde 8b
the mixture was stirred for 18 min. In contrast to other examples
investigated,[
19]
over 10 min, in order to achieve full conversion. None of the
the addition of only 2 equiv of the putative (S)-
1
C(20)R epimer of 5 could be observed in the H-NMR spectrum
BINAL-7 complex resulted in almost no conversion of 8a. To
4
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Chemistry - A European Journal
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FULL PAPER
benzoquinone, dichloromethane / phosphate buffer (pH 7.2) 40:1, rt, 90 min,
of the crude reaction mixture. After purification by flash column
chromatography, 5 was obtained in 81% yield. The aza-aldol
reaction between 8b and 7 was also carried out with (S)-TRIP as
a chiral Lewis acid catalyst, as described by Gosh et al. for the
synthesis of (−)-zampanolide (1) from 2.[15c],[15d] In line with the
observations reported by Gosh, the reaction produced a ca. 2:1
4
93%; (g) Dess-Martin periodinane, dichloromethane, rt, 90 min; h) 7, LiAlH ,
EtOH, (S)-(−)-1,1’-bi(2-naphthol), tetrahydrofuran, rt, 20 min, 32% over 2 steps,
dr > 99:1.
Thus, benzoylation of 23 with benzoyl chloride followed by
Yamaguchi esterification of the ensuing benzamide 10c with acid
diastereomeric mixture of 5 and its C(20)R-epimer in 68% yield
1
(
based on H-NMR). The diastereomers were separable by
9[15e]
and cleavage of both silyl-ethers with HF•pyridine furnished
preparative NP-HPLC.[30]
diol 31 in 78% overall yield. Double oxidation of 31 and Ba(OH)
mediated HWE ring-closure then gave the desired macrolactone
2 in 65% yield from 31. While all other intermediates described
2
-
3
in this study were oils, 32, gratifyingly, proved to be crystalline;
this allowed its skeletal structure and its absolute and relative
configuration to be confirmed by X-ray crystallography (Fig. 1).
This confirmation was particularly comforting as the presence of
slowly interconverting rotamers around the tertiary amide bond
led to extensive line broadening in the 1H-NMR spectra of all
intermediates as well as the final product 6, to an extent that the
signals for one proton of each of the CH groups  to the amide
2
nitrogen were virtually invisible; this was also the case for the
corresponding 13C-signals (see the SI for details). DDQ-mediated
cleavage of the PMB group in 32, DMP oxidation of the ensuing
free hydroxy group, to give aldehyde 8c, and (S)- BINAL-H-
mediated aza-aldol reaction with (Z,E)-sorbamide (7) afforded the
desired morpholino-zampanolide 6 in 30% overall yield after
purification by preparative NP-HPLC.[
30]
Scheme
5.
a)
9,
2,4,6-trichlorobenzoyl
chloride,
NEt
3
,
4-
(dimethylamino)pyridine, tetrahydrofuran, rt, 2 h; b) HF·pyridine, tetrahydrofuran,
0
°C → rt, 23 h, 78% over 2 steps; c) Dess-Martin periodinane, dichloromethane,
, tetrahydrofuran / H O 40:1, 0 °C, 15 min, 69% over 2
2,3-dichloro-5,6-dicyano-p-benzoquinone, dichloromethane /
phosphate buffer (pH 7.2) 5:1, rt, 74 min, quant.; f) Dess-Martin periodinane,
NaHCO , dichloromethane, rt, 70 min, 89%; g) 7, LiAlH , EtOH, (S)-(−)-1,1’-bi(2-
naphthol), tetrahydrofuran, rt, 20 min, 81%, dr > 95:5.
rt, 25 min, d) Ba(OH)
steps; e)
2
2
3
4
Synthesis of morpholino-zampanolide 6. The synthesis of
morpholino-zampanolide 6 from 23 followed the same sequence
of transformations as described above for the synthesis of analog
4
(Scheme 6).
Figure 1. X-Ray structure of macrolactone 32.
Biological assessment of morpholino-zampanolides 4-6. In
order to establish a basic understanding of the biological
consequences of the replacement of the methylenated
tetrahydropyran ring in 1 by N-substituted morpholine moieties,
the antiproliferative activity of morpholino-zampanolides 4-6 was
assessed against different cancer cell lines in vitro. As can be
seen from the data summarized in Table 1, both carboxamide
derivatives 4 and 6 retain nanomolar antiproliferative potency and,
thus, exhibit comparable activity as the natural product 1.[33]
In comparison, and rather surprisingly, tosylamide 5 is ca. 2-3
orders of magnitude less potent than 4 or 6. These activity
differences are also evident at the level of the dactylolide analogs
8a and 8b, although to a lesser extent. The differences in
antiproliferative activity between 4, 5, and 6 are in agreement with
the differences in apparent microtubule-binding affinity as
determined by the ability of the compounds to displace the
Scheme 6. a) PhC(O)Cl, NEt
trichlorobenzoyl chloride, NEt
h, 90%; c) HF·pyridine, tetrahydrofuran, 0 °C → rt, 21 h, 89%; d) Dess-Martin
periodinane, dichloromethane, rt, 90 min; (e) Ba(OH) , tetrahydrofuran / H
0:1, °C, 14 min, 65% over steps; (f) 2,3-dichloro-5,6-dicyano-p-
3
, dichloromethane, rt, 75 min, 98%; b) 9, 2,4,6-
, 4-(dimethylamino)pyridine, tetrahydrofuran, rt, 3
3
2
2
O
fluroescent
microtubules.[ Thus, the apparent binding constants K at 35 °C
taxol
34]
derivative
Flutax-2
from
stabilized
4
0
2
b
5
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Chemistry - A European Journal
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FULL PAPER
7
-1
7
-1
for 4 and 6 were determined as 6.16 x 10 M and 0.80 x 10 M ,
respectively (vs. 21.4 x 107 M-1 for 1).[12] There is no strict
correlation between microtubule-binding affinity and the
antiproferative activity of microtubule-stabilzing agents[35] and
these numbers are even less informative for covalent tubulin
binders such as 1; however, it is still significant that sulfonamide
containing side chain relied on a new method recently developed
in our laboratory for the stereoselective establishment of the
hemiaminal center in zampanolide-type structures; the desired
target structures were obtained in good yields and with high
selectivity (even if the yield of 4 after NP-HPLC purification was
only 15% (vide supra)). As a crucial element of our approach
towards morpholino-zampanolides, the common morpholine
building block 10b could be accessed from epoxides 15 and 16 in
26% overall yield. Importantly, the removal of the tosyl group from
the morpholine nitrogen in 10b could be achieved without difficulty,
thus offering the opportunity for the incorporation of a multitude of
substituents late in the synthesis. While it would have been even
more advantageous to deprotect the morpholine nitrogen only
after macrocyclization, preliminary attempts to remove the tosyl
group from macrolactone 30 were unsuccessful.
5
was not able to displace taxol from microtubules even at a
concentration of 25 M, which is in line with its profoundly reduced
growth inhibitory activity.
The synthesis of benzamide 6 was only embarked on after the
cellular data for 4 and 5 had become available. The compound
was meant to determine if the reduced potency of 5, compared to
, was caused by the increased steric bulk of the nitrogen
substituent or might be related to its different mode of attachment
sulfonamide vs. carboxamide). The data suggest that an amide
4
(
linkage may indeed be a more preferred mode of attachment of
substituent groups to the morpholine nitrogen. However, more
examples need to be investigated, in order to confirm (or refute)
this conclusion. Independent of this, and more important in the
context of the current study, even our limited data set shows that
the morpholino-zampanolide scaffold offers the potential for the
development of new, highly potent antiproliferative agents.
Although our sample size at this point is very small, the available
data on cancer cell growth inhibition in vitro indicate that acyl
substituents on the morpholine nitrogen in morpholino-
zampanolides are well tolerated, even for the case of a relatively
bulky benzoyl group. It will be important to determine if potent
activity is still retained for even (much) larger groups, as this has
important implications for the design of tumor-targeted conjugates,
including antibody-drug conjugates with non-cleavable linkers.
Somewhat surprisingly, sulfonamide 5 was substantially less
potent than carboxamides 4 and 6. It remains to be determined if
the activity difference between 5 and 6 is caused by the para-
methyl group in 5 (which is absent in 6) or if it may result from the
different mode of attachment of the (para-methyl) phenyl moiety
to the morpholine nitrogen (sulfonamide vs. carboxamide).
In summary, we have established a reliable platform for the
synthesis of N-substituted morpholino-zampanolides that should
enable the synthesis of a broad variety of analogs of this type.
The preliminary biological data obtained for 4 and 6 clearly
suggest that a broader exploration of this new class of
zampanolide congeners should be an interesting and promising
endeavor. Work along these lines is ongoing in our laboratory.
Table 1. Antiproliferative activity of morpholino-zampanolides 4-6 and of
a]
(
‒)-dactylolide analogs 8a, 8b against human cancer cells. (IC50 values [nM]).[
A549
lung)
PC-3
(prostate)
MC-7
(breast)
HeLa
(cervical)
Compound
(
4
5
ND
1.7 ± 0.11
1703 ± 258
ND
2.4 ± 0.13
1995 ± 205
ND
1.3 ± 0.09
1673 ± 155
ND
ND
6
6.7 ± 0.9
3.2 ± 0.4
ND
1 [b]
2.9 ± 0.4
229 ± 16
6.5 ± 0.7
290 ± 43
NA
8
a
b
189 ± 24
8
ND
2440 ± 236
751 ± 69
3095 ± 273
247 ± 2.6
2990 ± 300
NA
2 [b]
Experimental Section
301 ± 4.3
[
a] Cells were exposed to compounds for 72 h. Protein content of fixed cells was
For all experimental details see the Supporting Information. Deposition
Number 2026328 contains the supplementary crystallographic data for this
paper.
then determined by methylene blue staining. For further experimental details,
see ref. [36]. IC50 values are the mean of at least 3 independent experiments ±
SD. ND = not determined; NA = not available from ref.[15e]. [b] Data from ref.[15e]
Acknowledgements
Conclusion
We gratefully acknowledge financial support by the Swiss
National Science Foundation (projects 200021_149253 and
200020_175744 (KHA)), Funding was also obtained from
Ministerio de Ciencia e Innovación PID2019-104545RB-I00 and
Fondo de Investigaciones Sanitarias COV20/01007 (JFD) and
H2020-MSCA-ITN-2019 860070 TUBINTRAIN (JFD). We are
indebted to Dr. Bernhard Pfeiffer and Philipp Waser (ETHZ) for
NMR support, to Kurt Hauenstein (ETHZ) for technical advice and
to Oswald Greter, Louis Bertschi, Michael Meier, and Daniel Wirz
We have established an efficient route for the synthesis of
morpholine-based analogs of the marine macrolide (−)-
zampanolide (1), which is a potent microtubule-stabilizing agent
and as such represents a potential lead for anticancer drug
discovery. Building on the chemistry that we had developed in the
context of our total synthesis of 1, the final construction of the core
macrolactone was based on macrocyclization by an
intramolecular HWE reaction. Ring-closure proceeded very
efficiently in all three cases investigated here. The successful
transfer of this macrocyclization approach to a modified scaffold
speaks to the robustness of the methodology (which has also
been adopted by others).[17] The elaboration of the sorbamide-
(
ETHZ) for HRMS spectra acquisition. We are grateful to Nils
Trapp (ETHZ) for providing the X-ray crystal structures of
compound 32 and Daniel Zindel (ETHZ) for determination of the
enantiomeric excess of compound 15 by chiral GC. We thank
Matthias Rubin, Tatiana Hofer (University of Bern, Switzerland)
6
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FULL PAPER
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7
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Entry for the Table of Contents
A synthetic approach has been developed towards a new class of analogs of the marine macrolide (–)-zampanolide, with a
morpholine moiety replacing the tetrahydropyran ring. The morpholine core was elaborated from epoxides 15 and 16 and tosyl amide
(
17). The macrocycle was closed by a HWE reaction, and a stereoselective aza-aldol reaction installed the full sorbamide-derived
side chain. Analogs 4 and 6 showed nM antiproliferative activity.
Institute and/or researcher Twitter usernames: D-CHAB@ETH_DCHAB
8
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