Development of a scalable synthesis of a common eastern tricyclic lactone for construction of the nodulisporic acids

Article abstract of DOI:10.1021/op060204l

A scalable, second-generation synthesis of the densely functionalized eastern tricyclic lactone (+)-6, a common intermediate, for construction of the nodulisporic acids has been achieved. Modifications to the first-generation route now permit access to (+

Full text of DOI:10.1021/op060204l

Organic Process Research & Development 2007, 11, 19−24
Development of a Scalable Synthesis of a Common Eastern Tricyclic Lactone
for Construction of the Nodulisporic Acids
Amos B. Smith, III,* La´szlo´ Ku¨rti, Akin H. Davulcu, and Young Shin Cho
Department of Chemistry, Monell Chemical Senses Center and Laboratory for Research on the Structure of Matter,
UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, U.S.A.
Abstract:
A scalable, second-generation synthesis of the densely func-
tionalized eastern tricyclic lactone (+)-6, a common intermedi-
ate, for construction of the nodulisporic acids has been achieved.
Modifications to the first-generation route now permit access
to (+)-6 in 17 steps with an overall 16.5% yield. Key carbon-
carbon bond constructions include a Kirk-Petrow (phenylthio)-
methylation, a Sc(OTf)3-catalyzed hydroxymethylation, a Stille
carbonylation, and a Koga three-component, conjugate addi-
tion-alkylation sequence.
Introduction
(+)-Nodulisporic acid A (1), the most complex member
of the nodulisporane class of indole diterpenoids, was first
isolated by Ondeyka and co-workers at the Merck Research
Laboratories as part of an extensive screening program to
identify structurally unique, biologically active natural
products (Figure 1).1 The nodulisporic acids comprise a
Figure 1. Representative nodulisporic acid congeners.
thus launched at Merck & Co. to develop (+)-1 as a novel
anti-flea therapeutic agent for companion animals. To date
family of potent insecticides particularly effective against
flea and tick infestations in dogs and cats.2 Nodulisporic acid
A [(+)-1] was found to be the most potent nodulisporane,
displaying a 10-fold greater systemic adulticidal efficacy than
ivermectin against the Ctenocephalides felis (common flea),
a commercially relevant flea target.
over 1000 synthetic analogues have been prepared.4,5
Given our long-standing interest in the total synthesis of
indole diterpenoids, in conjunction with the architectural
complexity and remarkable antiparasitic activity of (+)-
nodulisporic acid A (1), we initiated a synthetic program to
devise a modular synthetic strategy that would permit
construction not only of the parent nodulisporic acid A [(+)-
1] but also a number of analogues not readily accessible by
chemical modification of the naturally occurring indole
diterpenoids (i.e., the nodulisporanes). At the outset of this
program, the cornerstone of our modular synthetic strategy
was envisioned to involve the indole synthetic protocol6,7
developed in our laboratory and successfully employed to
construct a number of related indole diterpenoid natural
products, including penitrem D8 and 21-isopentenylpaxilline.9
From the retrosynthetic perspective (Scheme 1), application
The closely related nodulisporic acids B, D, and F (cf. 2,
3, and 4) proved to be less active by 5 to >100-fold.
Importantly, (+)-nodulisporic acid A (1) displays no apparent
toxicity to the host animal while effectively killing fleas
subsequent to their ingestion of a blood meal. The mode of
action of the nodulisporic acids involves modulation of the
invertebrate-specific, glutamate-gated, chloride ion channels
not present in mammals.3 However, extensive biological
evaluation revealed that, although (+)-1 possessed good in
vivo and in vitro activity, the stability and pharmacokinetic
profile were not optimal. A medicinal chemistry campaign,
in conjunction with a chemical mutagenesis program, was
(4) Chakravarty, P. K.; Shih, T. L.; Colletti, S. L.; Ayer, M. B.; Snedden, C.;
Kuo, H.; Tyagarajan, S.; Gregory, L.; Zakson-Aiken, M.; Shoop, W. L.;
Schmatz, D. M.; Wyvratt, M.; Fisher, M. H.; Meinke, P. T. Bioorg. Med.
Chem. Lett. 2003, 13, 147-150.
(5) Ok, D.; Li, C.; Shih, T. L.; Salva, S.; Ayer, M. B.; Colletti, S. L.;
Chakravarty, P. K.; Wyvratt, M. J.; Fisher, M. H.; Gregory, L.; Zakson-
Aiken, M.; Shoop, W. L.; Schmatz, D. M.; Meinke, P. T. Bioorg. Med.
Chem. Lett. 2002, 12, 1751-1754.
* To whom correspondence should be addressed. E-mail: smithab@
sas.upenn.edu.
(1) Ondeyka, J. G.; Helms, G. L.; Hensens, O. D.; Goetz, M. A.; Zink, D. L.;
Tsipouras, A.; Shoop, W. L.; Slayton, L.; Dombrowski, A. W.; Polishook,
J. D.; Ostlind, D. A.; Tsou, N. N.; Ball, R. G.; Singh, S. B. J. Am. Chem.
Soc. 1997, 119, 8809-8816.
(2) Sings, H.; Singh, S. In The Alkaloids: Chemistry and Biology; Cordell, G.
A., Ed.; Elsevier Academic Press: San Diego, CA, United States, 2003;
Vol. 60, pp 51-163.
(6) Smith, A. B., III; Visnick, M.; Haseltine, J. N.; Sprengeler, P. A. Tetrahedron
1986, 42, 2957-2969.
(3) Ludmerer, S. W.; Warren, V. A.; Williams, B. S.; Zheng, Y.; Hunt, D. C.;
Ayer, M. B.; Wallace, M. A.; Chaudhary, A. G.; Egan, M. A.; Meinke, P.
T.; Dean, D. C.; Garcia, M. L.; Cully, D. F.; Smith, M. M. Biochemistry
(Mosc). 2002, 41, 6548-6560.
(7) Smith, A. B., III; Visnick, M. Tetrahedron Lett. 1985, 26, 3757-3760.
(8) Smith, A. B., III; Kanoh, N.; Ishiyama, H.; Hartz, R. A. J. Am. Chem. Soc.
2000, 122, 11254-11255.
(9) Smith, A. B., III; Cui, H. Org. Lett. 2003, 5, 587-590.
10.1021/op060204l CCC: $37.00 © 2007 American Chemical Society
Published on Web 12/23/2006
Vol. 11, No. 1, 2007 / Organic Process Research & Development
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Scheme 1. Retrosynthetic analysis of (+)-nodulisporic acid
A (1)
Scheme 2. Retrosynthetic analysis of (+)-6
of our indole retron, after removal of ring D from (+)-1 leads
to a heptacyclic core that would arise from the union of
western and eastern hemisphere subtargets (-)-5 and (+)-6,
respectively. That all of the nodulisporic acids possess the
identical stereogenicity in the eastern hemisphere suggested
that (+)-6 would serve as a viable common building block
for this class of indole diterpenoids.
In 2001, we disclosed a first-generation 17-step synthesis
of (+)-6 that proceeded in 9% overall yield.10 Several
transformations in the original sequence, however, limited
the efficiency, which in turn did not permit ready scale-up.
Thus, before we could contemplate a serious campaign to
construct the nodulisporic acids and derivatives thereof, a
scalable synthesis of (+)-6 was required. Described herein
is a modified synthetic sequence now capable of providing
multigram quantities (ca. 5-10 g) of the requisite advanced
eastern lactone (+)-6.
Construction of lactone (+)-6 began with the preparation
of enantiomerically pure (>95% ee) Wieland-Miescher
ketone (+)-9, employing the well-known L-proline-catalyzed
Robinson annulation.16-18 Chemoselective protection of the
saturated carbonyl as the ethylene ketal (+)-10 was achieved
via the conditions of Demnitz et al. (Scheme 3).19 In our
first-generation approach the transketalization protocol20
required an impractically long reaction time (cf. 1 week);
purification also required a time-consuming flash chromato-
graphic separation. The Demnitz transformation which called
for the treatment of (+)-9 with a full equivalent of p-
toluenesulfonic acid in ethylene glycol scaled well, allowing
for the conversion of 50-g batches of (+)-9 in ca. 20-25 min.
Importantly, crystallization from hexanes/Et2O (20:1) fur-
nished (+)-10 in 89-91% yield, without the need for
chromatographic purification. Small amounts of the bis-ketal
product and unreacted Wieland-Miescher ketone remained
in solution during the crystallization even at low tempera-
tures. In practice, 215 g of (+)-10 can be prepared in 1 day.
That a highly selective reaction was combined with an
efficient crystallization protocol enabled us to increase
considerably the material throughput.
One-carbon homologation of (+)-10 at the R-carbon was
next achieved via the Kirk-Petrow protocol.11 This trans-
formation is inherently slow (4 days), and as such, the
thiophenol employed in slight excess undergoes partial air
oxidation to give Ph2S2 that coelutes with (+)-8, making
purification in our first-generation sequence difficult. Imple-
mentation of a protective Ar atmosphere to exclude air
completely prevented formation of Ph2S2. Purification of
(+)-8 could thus be achieved by base extraction of PhSH
and crystallization; a total of ca. 260 g was prepared.
Results and Discussion
Tricyclic lactone (+)-6 possesses a number of synthetic
challenges, given the array of functionality including the
following: the trans-fused lactone at C(3) and C(12) (nod-
ulisporic acid numbering), the trans-fused decalin with
vicinal quaternary stereocenters at C(3) and C(4), and the
quaternary stereocenter at C(8). To access this array of
stereogenicity, our first-generation synthesis employed to
good advantage the following carbon-carbon bond construc-
tion steps beginning with (+)-Wieland-Miescher ketone
(9): a Kirk-Petrow (phenylthio)methylation11 and a dias-
tereoselective Sc(OTf)3-catalyzed hydroxymethylation to
construct the C(8) quaternary stereocenter,12 a Birch reduc-
tion13 directed by the C(4) quaternary stereocenter to
introduce the trans-fused decalin system, and a Stille
carbonylation followed by a Koga alkylation to generate the
C(3) and C(12) stereocenters in a tandem fashion.14,15
(10) Smith, A. B., III; Cho, Y. S.; Ishiyama, H. Org. Lett. 2001, 3, 3971-3974.
(11) Kirk, D. N.; Petrow, V. J. Chem. Soc. 1962, 1091-1096.
(12) Kobayashi, S. Chem. Lett. 1991, 2187-2190.
(13) Stork, G.; Rosen, P.; Goldman, N.; Coombs, R. V.; Tsuji, J. J. Am. Chem.
Soc. 1965, 87, 275-286.
(14) Hashimoto, S. I.; Yamada, S. I.; Koga, K. Chem. Pharm. Bull. 1979, 27,
771-782.
(15) Kogen, H.; Tomioka, K.; Hashimoto, S.; Koga, K. Tetrahedron 1981, 37,
3951-3956.
(16) Buchschacher, P.; Fuerst, A. Org. Synth. 1985, 63, 37-43.
(17) Harada, N.; Sugioka, T.; Uda, H.; Kuriki, T. Synthesis 1990, 53-56.
(18) Gutzwiller, J.; Buchschacher, P.; Fuerst, A. Synthesis 1977, 167-168.
(19) Ciceri, P.; Demnitz, F. W. J. Tetrahedron Lett. 1997, 38, 389-390.
(20) Bauduin, G.; Pietrasanta, Y. Tetrahedron 1973, 29, 4225-4231.
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Scheme 3. Preparation of aldehyde (+)-18
Figure 2. Side- and byproducts.
along with several corresponding solutions: (1) given that
lithium metal dissolves slowly in liquid ammonia, consider-
able time was required to generate the dissolved metal
solution (ca. 1 h); (2) the t-BuOH must be freshly distilled
prior to use21 to diminish the formation of ketone 19 (Figure
2); (3) efficient capture of the lithium enolate (11) required
the use of freshly distilled TMSCl and Et3N; and (4) the
presence of residual thiophenol upon workup was found to
inhibit the subsequent Sc(OTf)3-promoted hydroxymethyla-
tion. Pleasingly, the residual thiophenol could be readily
removed without silyl ether hydrolysis via passage of the
product through a pad of basic alumina. Thus by employing
standard academic glassware, 25-g batches of (-)-13 could
be readily prepared. Quenching the excess lithium metal was
achieved with isoprene. With these improvements, the two-
step preparation of (-)-13 could be readily accomplished in
moderate to good yield (56-72%).
The three-step conversion of (-)-13 to (-)-16 (Scheme
3) was next performed without need for purification of
(-)-14 and (-)-15. To this end diastereoselective hydride
reduction of â-hydroxyketone (-)-13 was achieved with
tetramethylammonium triacetoxyborohydride, a reagent in-
troduced by Gribble22,23 and further developed by Evans and
co-workers.24 When the reducing agent was added to a cold
(-40 °C) solution of (-)-13, an inseparable mixture of
diastereomeric 1,3-diols (5.5:1) was formed, favoring
(-)-14. The lack of stereoselectivity may have been due to
significant heat evolution above -40 °C. At these temper-
atures, the reduction was less selective. This problem
becomes worse as the scale is increased. However, when
the reaction was conducted via inverse addition of the
substrate (-)-13 to a cooled (-40 °C) solution of the
reducing agent, the diastereoselectivity improved dramatically
to 15:1. Presumably, with inverse addition (i.e., addition to
a slurry of the reducing agent at -40 °C) control of the
temperature is easier. Prior to this modification, the minor
diastereomer could only be removed at the stage of the bis-
TBS ether (-)-21. Removal of the dioxolane protecting
group was also problematic in the first-generation synthesis,
The trans-decalin stereochemistry required for (+)-6 was
next installed by Birch reduction of (+)-8, employing the
intermediacy of lithium enolate 11, the latter captured as the
corresponding trimethylsilyl enol ether 12 (Scheme 3).
Hydroxymethylation was then achieved under aqueous
conditions, employing Sc(OTf)3 to furnish â-hydroxyketone,
(-)-13, possessing the correct configuration at the C(8)
quaternary center. In our first-generation synthesis this
reaction sequence proved problematic, particularly when the
Birch reduction was conducted on intermediate scale (5-
10 g). Extensive experimentation revealed several issues,
(21) With this change the formation of 19 was decreased to 5% from as high as
40% in the first-generation procedure.
(22) Gribble, G. W.; Nutaitis, C. F. Tetrahedron Lett. 1983, 24, 4287.
(23) Gribble, G. W.; Ferguson, D. C. Chem. Commun. 1975, 535.
(24) Evans, D. A.; Chapman, K. T. Tetrahedron Lett. 1986, 27, 5939-5942.
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Scheme 4. Preparation of tricyclic lactone (+)-6
given that the lengthy reaction time (6 h) permitted the THF
to undergo partial oxidation in the aqueous 2 N HCl solution,
leading to up to 10% of acetal (-)-20. A simple purge of
the aqueous HCl solution with Ar prior to the hydrolysis
completely eliminated formation of (-)-20. An improvement
in the bis-TBS protection of (-)-15 was also introduced.
Reduction in the amount of TBSOTf from 2.8 to 2.0 equiv
led to a much cleaner reaction, thereby simplifying chro-
matographic purification of (-)-16. Presumably the strong
Lewis acidity of TBSOTf results in some decomposition of
the substrate and/or product. With these improvements, the
three-step sequence now proceeds in 82% overall yield,
requiring only chromatographic purification of ketone
(-)-16.
Having installed four of the requisite six stereogenic
centers in (+)-6, we turned to the installation of the γ,δ-
unsaturated aldehyde functionality, that would set the
remaining two centers (Scheme 3). Treatment of ketone
(-)-16 with 2 equiv of LiHMDS at -78 °C, followed by
capture of the resulting lithium enolate with the Comins’
reagent,25 furnished enol triflate (-)-17 in excellent yield
after purification by flash chromatography. On large scale,
the subsequent Stille Pd-catalyzed carbonylation26 initially
proved problematic due to the quality of the Pd(PPh3)4. Best
results were obtained with Pd(PPh3)4 purchased from Strem
Chemical Co. Purging the reaction mixture with carbon
monoxide prior to the addition of the catalyst also permitted
both a lower catalyst load (cf. from 30 to 10 mol %) and a
shorter reaction time (4 vs 2 h) compared to the first-
generation protocol. Under these conditions, carbonylation
could be conveniently achieved on 10 g scale; a total of
31 g of (+)-18 was prepared.
With large quantities of the vinyl aldehyde (+)-18 in hand,
the stage was now set for the tandem introduction of the
remaining two stereocenters [C(3) and C(12)] employing the
Koga reaction.14,15 Based on the first-generation sequence,
we faced two serious problems. First, the required chiral
auxiliary (+)-23 was available in only moderate yield,
requiring an impractically long period to produce (ca.
4 weeks). Second, the reaction time required to prepare
aldimine (+)-24 was excessively long. After extensive
experimentation, both procedures were markedly improved
(Scheme 4 and Scheme 5). First, preparation of the chiral
amine auxiliary (+)-23 (Scheme 5) was accelerated by
loading the components at a low temperature (-40 °C) and
running the reaction at room temperature in a sealed tube to
prevent loss of the volatile isobutylene; under these condi-
tions consumption of the starting material was complete in
only 23 h. Preparation of (+)-23 (ca. 30 g) could thus be
achieved in a just a few days.
Scheme 5. Preparation of chiral auxiliary (+)-23
unacceptably long (ca. 33 days), affording a 10:1 mixture
of product and unreacted aldehyde (+)-18. Initially, we
experimented with azeotropic removal of the water with
benzene via the Dean-Stark tactic. Under these conditions
(80 °C), decomposition of (+)-18 proved to be a significant
problem. Gratifyingly, by adding 15 mol % of the HCl salt
of (+)-23 and using toluene as solvent, followed by
successive vacuum azeotropic distillation, we obtained imine
(+)-24 in near quantitative yield (Scheme 4). Purification
was then best achieved via flash column chromatography
Turning to the preparation of the R,â-unsaturated imine
(+)-24, the first generation protocol called for the use of
4 Å molecular sieves as the dehydrating agent. Although a
high yield of (+)-24 was obtained, the reaction time was
(25) Comins, D. L.; Dehghani, A. Tetrahedron Lett. 1992, 33, 6299-6302.
(26) (a) Scott, W. J.; Crisp, G. T.; Stille, J. K. J. Am. Chem. Soc. 1984, 106,
4630-4632. (b) Cacchi, S.; Morera, E.; Ortar, G. Tetrahedron Lett. 1985,
26, 1109-1112.
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Figure 3. Explanation of the observed stereochemistry in
(+)-25.
using basic alumina. The alternative use of buffered silica
gel resulted in modest hydrolysis (∼5-10%) of the imine.
Turning to the Koga alkylation,14,15 which entails a tandem
1,4-addition followed by in situ alkylation, imine (+)-24 was
treated with vinylmagnesium bromide to furnish an inter-
mediate magnesium aza-enolate that was then captured with
excess MeI in the presence of HMPA. Removal of the chiral
auxiliary was next achieved in high yield by using oxalic
acid. In the first-generation synthesis, citric acid had been
employed. The reaction proved slow (3 days) and suffered
from the formation of an unidentified byproduct. By replac-
ing the citric acid (pKa ) 3.15) with the more acidic oxalic
acid (pKa ) 1.23), the rate of imine hydrolysis increased
significantly to furnish cleanly (+)-25 in 12 h without
formation of side products.
The stereochemical outcome of the Koga alkylation can be
understood employing the model illustrated in Figure 3. Axial
delivery of the vinyl group and subsequent axial alkylation
with methyl iodide are the direct result of the stereoelectronic
preference for axial delivery and enolate-like alkylations.
Optimization of the Koga reaction required considerable
effort. The composition and origin of the vinylmagnesium
bromide reagent are critical factors. Freshly prepared vinyl-
magnesium bromide works considerably better than aged
reagent. Nonetheless, on occasion even freshly prepared
reagent affords a poor yield. As a result, we recommend a
small-scale (100 mg) test reaction with the vinylmagnesium
bromide at hand before committing to larger scale.
Pinnick oxidation of aldehyde (+)-25 next furnished
carboxylic acid (+)-26.27 Particularly important here is the
use of distilled 2-methyl-2-butene and not the technical grade
(90%) reagent. Methyl ester formation is then achieved
without purification by treating (+)-26 with a slight excess
of (trimethylsilyl)diazomethane; the yield of (+)-27 was 83%
for the two steps.28
Figure 4. Single-crystal X-ray image of (+)-6. (Generated from
the PDB file using Jmol Viewer, version 10).
While the four steps to convert aldehyde (+)-25 to ester
(+)-29 did not require major modification, vis-a`-vis the first-
generation synthetic sequence, significant changes were made
to the final two steps, that is the conversion of aldehyde
(+)-29 to lactone (+)-6, the eastern hemisphere subtarget.
First, lowering the temperature of the chemoselective NaBH4
reduction of the C(12) aldehyde moiety in (+)-29 to 0 °C
furnished the corresponding hydroxyester (+)-30 in near
quantitative yield, without over-reduction of the ester as was
observed at higher temperatures in the original procedure.
Second, the acid-promoted lactonization was abandoned in
favor of a strong base protocol, thereby eliminating the
modest decomposition, presumably arising via loss of the
TBS groups in both the starting material and the lactone.
Best results involved use of LiHMDS at 0 °C. These
conditions permitted facile lactonization to furnish the eastern
hemisphere lactone (+)-6 in 95% yield after chromatographic
purification. The structure of (+)-6 was confirmed by single-
crystal X-ray analysis (Figure 4).
Conclusion
With ester (+)-27 in hand, the vinyl group was next
converted to the aldehyde moiety via ozonolysis, employing
reduction of the ozonide with dimethylsulfide or PPh3,
followed by treatment with DBU in THF to effect epimer-
ization; aldehyde (+)-29 was produced in a combined yield
of 65% for the two steps. Due to the hindered nature of the
vinyl group in (+)-27 3 days were required to reduce the
ozonide; fortunately, the reaction progress can be monitored
easily by TLC.
A scalable synthesis of the common eastern hemisphere
lactone (+)-6 for construction of the nodulisporic acids has
been achieved. The improvements in the second-generation
sequence, compared to the first-generation route, entailed
elimination of eight chromatographic purifications, the
modification and amplification of several reaction conditions,
and significant reduction in the overall reaction times for
several transformations. The second-generation synthesis now
proceeds with an overall yield of 16.5%, corresponding to
an average of 90% yield for each step. Importantly, the
efficiency of the sequence will now enable us to proceed
with our synthetic campaign to construct the nodulisporic
acids.
(27) Bal, B. S.; Childers, W. E., Jr.; Pinnick, H. W. Tetrahedron 1981, 37, 2091-
2096.
(28) Hashimoto, N.; Aoyama, T.; Shioiri, T. Chem. Pharm. Bull. 1981, 29, 1475-
1478.
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Acknowledgment
obtaining NMR spectra, high-resolution mass spectra, and
X-ray crystallographic data, respectively.
Financial support was provided by the National Institutes
of Health (National Institute of General Medical Sciences)
through Grant GM-29028 and by the Bristol-Myers Squibb
Pharmaceutical Research Institute (for stipend support of
A.H.D.). We are also grateful to Eli Lilly and Company for
the graduate fellowship to L.K. and to Dr. P.T. Meinke and
the Merck Research Laboratories for generous financial
support of this research program. Finally, we thank Dr. G.
Furst, Dr. R. K. Kohli, and Dr. P. Carroll for assistance in
Supporting Information Available
Experimental details and selected spectra. This material
is available free of charge via the Internet at http://
pubs.acs.org.
Received for review October 4, 2006.
OP060204L
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