A reductive coupling strategy towards ripostatin A

Article abstract of DOI:10.3762/bjoc.9.175

Synthetic studies on the antibiotic natural product ripostatin A have been carried out with the aim to construct the C9-C10 bond by a nickel(0)-catalyzed coupling reaction of an enyne and an epoxide, followed by rearrangement of the resulting dienylcyclopropane intermediate to afford the skipped 1,4,7-triene. A cyclopropyl enyne fragment corresponding to C1-C9 has been synthesized in high yield and demonstrated to be a competent substrate for the nickel(0)-catalyzed coupling with a model epoxide. Several synthetic approaches toward the C10-C26 epoxide have been pursued. The C13 stereocenter can be set by allylation and reductive decyanation of a cyanohydrin acetonide. A mild, fluoride-promoted decarboxylation enables construction of the C15-C16 bond by an aldol reaction. The product of this transformation is of the correct oxidation state and potentially three steps removed from the targeted epoxide fragment.

Full text of DOI:10.3762/bjoc.9.175

A reductive coupling strategy towards ripostatin A
Kristin D. Schleicher and Timothy F. Jamison*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1533–1550.
Department of Chemistry, Massachusetts Institute of Technology, 77
Massachusetts Ave., Cambridge, MA 02139, United States
doi:10.3762/bjoc.9.175
Received: 01 May 2013
Accepted: 04 July 2013
Published: 31 July 2013
Email:
Timothy F. Jamison* - tfj@mit.edu
* Corresponding author
This article is part of the Thematic Series "Transition-metal and
organocatalysis in natural product synthesis".
Keywords:
catalysis; natural product; nickel; reductive coupling; ripostatin A;
synthesis
Guest Editors: D. Y.-K. Chen and D. Ma
© 2013 Schleicher and Jamison; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Synthetic studies on the antibiotic natural product ripostatin A have been carried out with the aim to construct the C9−C10 bond by
a nickel(0)-catalyzed coupling reaction of an enyne and an epoxide, followed by rearrangement of the resulting dienylcyclopropane
intermediate to afford the skipped 1,4,7-triene. A cyclopropyl enyne fragment corresponding to C1−C9 has been synthesized in
high yield and demonstrated to be a competent substrate for the nickel(0)-catalyzed coupling with a model epoxide. Several syn-
thetic approaches toward the C10−C26 epoxide have been pursued. The C13 stereocenter can be set by allylation and reductive
decyanation of a cyanohydrin acetonide. A mild, fluoride-promoted decarboxylation enables construction of the C15−C16 bond by
an aldol reaction. The product of this transformation is of the correct oxidation state and potentially three steps removed from the
targeted epoxide fragment.
Introduction
The ripostatins (A, B, and C) are a family of antibiotic natural vancomycin and methicillin [3]. Ripostatin A exists as an equi-
products, isolated in 1995 by Höfle and colleagues from librium mixture of ketone and hemiketal forms, the ratio of
cultures of the myxobacterium Sorangium cellulosum (Figure 1) which is reported to be 55:45 in methanolic solution. In the
[1,2]. Ripostatins A and B are active against Gram-positive hemiketal form, the bicyclic framework of ripostatin A features
bacteria due to their inhibition of bacterial ribonucleic acid an unusual in/out connectivity. The 14-membered macrocyclic
polymerase. These compounds inhibit chain initiation of RNA core of ripostatin A contains three double bonds, arranged in a
synthesis in Staphylococcus aureus, a particularly infectious rare 1,4,7-skipped triene. The alkene geometry was determined
bacterial strain with reported drug resistance to the antibiotics to be (2E,5E,8E) by measurement of NOE enhancements [4].
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Ripostatin B and its C15 epimer can be obtained from use of ring-closing metathesis to form the 14-membered macro-
ripostatin A by reduction with sodium borohydride, while ripo- cycle, preceded by one or more Stille couplings to generate the
statin C can be formed from ripostatin A by a mild base-medi- double 1,4-diene (Figure 2).
ated elimination. Consequently, ripostatin A was selected as the
primary target for synthesis.
Notwithstanding the successful syntheses of ripostatins, prepar-
ation of configurationally defined skipped polyenes (1,4-dienes
At the outset of our efforts, the only published synthetic study and higher homologues) remains a significant challenge in
on the ripostatins was Kirschning’s synthesis of C1−C5 and organic chemistry. The doubly allylic protons found in these
C6−C24 fragments of ripostatin B [5]. However, these frag- structures may be sensitive to strong base as well as hydrogen
ments could not be connected by esterification due to steric abstraction [10,11]. While classical methods for the preparation
hindrance from bulky protecting groups, as well as suscepti- of 1,4-dienes include partial reduction of alkynes and carbonyl
bility of the skipped dienes therein to double bond isomeriza- olefination, a variety of transition-metal-mediated processes
tion and migration under basic conditions. However, the ripo- have been developed for the synthesis of skipped dienes of
statins intrigued others, and three total syntheses of ripostatin B varied substitution patterns [12-15]. Most recently, Sigman and
were published in succession in 2012 [6-8]. Tang and Prusov colleagues have reported a palladium-catalyzed 1,4-difunction-
extended their synthetic method to syntheses of alization of 1,4-butadiene with vinylboronic acid and vinyl
15-deoxyripostatin A, and later ripostatin A itself [9]. All of triflate that can be used to rapidly access the skipped triene of
these approaches to the ripostatins share several key features: ripostatin A [16].
Figure 1: Structures of the ripostatins.
Figure 2: Retrosynthesis of ripostatin A.
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We recognized that the C11 stereocenter and the C8–C9 trisub- portion via a 1,5-hydrogen rearrangement (Figure 3). This
stituted olefin of ripostatin A mapped onto a nickel-catalyzed strategy would allow us to take advantage of the high regiose-
coupling reaction of alkynes and epoxides developed in our lectivity in enyne–epoxide reductive coupling reactions.
laboratory (Scheme 1) [17]. In the intermolecular reaction, Furthermore, the proposed rearrangement would serve to differ-
stereospecific cis addition across the alkyne is observed, and the entiate the ester groups, as hydrogen would migrate from adja-
stereochemistry at the epoxide is preserved in the transforma- cent to the ester cis to the dienyl chain only.
tion. Aliphatic epoxides are opened selectively (>95:5) at the
terminal position. Although very high regioselectivity with While offering a unique approach to the skipped triene portion
respect to the alkyne is observed when R1 = Ph and R2 = Me, of ripostatin, this route was not without significant uncertainty.
attempts to differentiate between aliphatic alkyne substituents First, substrates containing a vinyl cyclopropane unit had not
lead to mixture of regioisomers. When a 1,3-enyne is coupled been previously tested under nickel-catalyzed reductive
with simple epoxides, however, >95:5 regioselectivity is coupling conditions. Nickel(0) is known to catalyze the
observed for C–C bond formation distal to the pendant alkene rearrangement of vinylcyclopropanes to cyclopentenes;
[18].
however, activating substituents are commonly required [19-
21]. Furthermore, application of a proposed 1,5-hydrogen
In conjunction with the nickel-catalyzed fragment coupling, we rearrangement to ripostatin A would require that the
wished to investigate whether it would be possible to delay reaction proceed to give the triene with E,E,E configuration
introduction of the potentially sensitive 1,4,7-triene by masking selectively out of four possible configurational outcomes
it as a cyclopropyldiene, then unveiling the skipped triene (Scheme 2).
Scheme 1: Nickel-catalyzed reductive coupling of alkynes and epoxides.
Figure 3: Proposed retrosynthesis of ripostatin A featuring enyne–epoxide reductive coupling and rearrangement.
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Scheme 2: Potential transition states and stereochemical outcomes for a concerted 1,5-hydrogen rearrangement.
Rearrangement of vinylcyclopropanes has been used to prepare trated by the adjacent alkoxide, which is believed to direct for-
1,4-skipped dienes of varying geometry (Scheme 3). It has long mation of a tricyclic titanacyclopentane that subsequently frag-
been known that cis-disubstituted vinylcyclopropanes can ments in a stereospecific manner [31].
undergo 1,5-hydrogen migration under thermal conditions to
deliver acyclic 1,4-dienes [22]. In this reaction manifold, the We were intrigued by the apparent difference in selectivity
new “acceptor-derived” double bond is formed via an endo observed in cyclopropane rearrangements proceeding via a
transition state, leading to a cis olefinic configuration in the neutral pathway versus those proceeding by more polar or
product (Scheme 3, reaction 1). Berson has quantified the ener- directed mechanisms. Although the E geometry of the central
getic preference in this transformation, while Turos has shown olefin in the ripostatin A triene is more consistent with a polar
that the presence of a silicon substituent on the “donor carbon” mode of reactivity than a neutral 1,5-hydrogen migration, we
facilitates the hydrogen migration [23-26].
still wished to investigate the outcome of rearrangement under
thermal conditions. To the best of our knowledge, such a
In contrast, Wilson has reported formation of the trans double rearrangement has not been explored for structures containing
bond in the opening of a cyclopropane with an adjacent hydroxy an additional alkenyl substituent in conjugation, or with elec-
or mesylate leaving group, regardless of the initial configur- tron-withdrawing groups adjacent to the site of hydrogen migra-
ation of the cyclopropane (Scheme 3, reaction 2) [27]. tion. In particular, the latter’s ability to facilitate the buildup of
Braddock has demonstrated that the internal 3,4-E olefin is negative charge at an adjacent carbon might play an important
obtained exclusively in Prins reactions terminated by cyclo- role in the stereoelectronic course of the reaction.
propylmethylsilane (Scheme 3, reaction 3), which may be
Results and Discussion
explained by the participation of a carbocation that is stabilized
Synthesis of cyclopropylenyne and reductive
coupling with model epoxide
Diethyl 1,3-acetonedicarboxylate (Scheme 4, 16) was rapidly
identified as an inexpensive five-carbon fragment possessing
the appropriate oxygenation pattern for preparation of the
C1–C9 enyne fragment. However, due to keto–enol tautomer-
by the adjacent cyclopropane ring in the bisected conformation
where (CHOR)R′ is oriented anti to the cyclopropane [28-30].
Finally, Micalizio has described a titanium-mediated, alkoxide-
directed fragment coupling reaction between vinylcylopropanes
and vinyldimethylchlorosilane (Scheme 3, reaction 4) in which
the stereochemical outcome of the rearrangement is orches-
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Scheme 3: Rearrangements of vinylcyclopropanes to acylic 1,4-dienes.
Scheme 4: Synthesis of cyclopropyl enyne.
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ization, carbonyl olefination methods are of limited utility for gous to one reported by Smith [38]. Allylation of 24 with (+)-B-
this substrate. Instead, the ketone was protected as the mixed allyldiisopinocampheylborane generated the alcohol 25 in high
S,O-ketal and reduced to the diol 17. Protection of the hydroxy yield and enantioselectivity. Directed epoxidation using
groups and removal of the ketal afforded ketone 19. A number VO(acac)2 and tert-butyl hydroperoxide was initially performed
of alternative promoters were investigated to avoid the use of in order to furnish 27 directly; however, this proceeded in only
mercury(II) salts in the ketal deprotection (including MeI, modest yield and diastereoselectivity (53%, 2.8:1 syn/anti).
H2O2, AgClO4/I2); however, these generally led to concomi- Although the ratio of syn/anti epoxide diastereomers could be
tant removal of the TBS groups.
enhanced by subjecting the mixture to hydrolytic kinetic resolu-
tion [39], greater throughput could be obtained by converting 25
Ketone 19 was converted to the α,β-unsaturated ester 20 using to the tert-butyl carbonate, performing an iodocyclization, then
the Peterson olefination [32]. Treatment of 20 with the sulfur cleaving the iodocarbonate and closing the epoxide under basic
ylide derived from trimethylsulfoxonium iodide [33,34] led to conditions. Silyl protection of the secondary alcohol afforded
recovery of starting material at room temperature, but decompo- the desired model compound 28.
sition at elevated temperatures. Instead, the enone was smoothly
reduced to the allylic alcohol, and a Furukawa-modified Enyne 6 and epoxide 28 were subjected to standard nickel-
Simmons–Smith reaction [35] afforded the cyclopropyl alcohol catalyzed reductive coupling conditions, and reductive coupling
22 in high yield.
proceeded in good yield, leaving the cyclopropane ring intact
(Scheme 6). However, the desired diene 29 was isolated along
Oxidation to the cyclopropyl aldehyde 23 offered a branching with the regioisomeric product 30 in approximately a 3:1 ratio.
point from which either the E- or Z-substituted enyne could be The desired product could be partially separated from the
synthesized, should we wish to study the rearrangement of both regioisomer by careful silica gel chromatography.
diene geometries. To access our initial target, the E-enyne 6, a
Takai olefination [36] was used to generate the E-vinyl iodide. It may be instructive at this stage to consider the proposed
The vinyl iodide was sufficiently stable for purification by silica mechanism of the nickel-catalyzed alkyne or enyne–epoxide
gel chromatography but, following purification, was immedi- reductive coupling reaction (Scheme 7). In contrast to the mech-
ately carried forward to a Sonogashira reaction with propyne anistic framework proposed [40,41] for reductive coupling reac-
[37]. The enyne could be obtained in 15:1 E/Z selectivity, in tions of alkynes and aldehydes developed in our laboratory
10 steps and 35% overall yield. It was found that the use of [42,43], it is believed that epoxide oxidative addition precedes
freshly distilled THF in the Takai olefination and careful alkyne addition, as opposed to concerted oxidative coupling. At
temperature control in the subsequent cross coupling were crit- least when dimethylphenylphosphine is used as ligand, this may
ical to the preservation of high E/Z selectivity over the course of proceed via the intermediacy of a betaine species. In the reduc-
these transformations. Once isolated, however, enyne 6 proved tive coupling reaction of enynes and epoxides, the olefin coordi-
to be quite stable and could be stored for extended periods at nates to nickel and directs alkyne insertion.
0–5 °C without appreciable isomerization or decomposition.
Because of this directing effect, formation of the regioisomeric
With a suitable cyclopropylenyne in hand, a model epoxide sub- diene product is atypical for reductive coupling reactions of
strate containing the 1,3-oxygenation pattern found in enynes and epoxides. However, in reactions of 1-phenyl-1-
ripostatin A (Scheme 5) was prepared by using a route analo- propyne and epoxides with oxygenation in the 3-position
Scheme 5: Synthesis of model epoxide for investigation of the nickel-catalyzed coupling reaction.
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Scheme 6: Nickel-catalyzed enyne–epoxide reductive coupling reaction.
Scheme 7: Proposed mechanism for the nickel-catalyzed coupling reaction of alkynes or enynes with epoxides.
(Scheme 8), it was found that while epoxides containing adja- 3:1 regioselectivity observed in the “normal” direction is
cent silyl ethers afforded mainly the expected regioisomer (7:1 consistent with chelation playing a diminished role relative to
37a/37b), epoxides with sulfonate esters (e.g., tosyl, 32) and the seven-membered chelates invoked for coupling of
esters (e.g., acetyl, 33) afforded a regioisomeric mixture of 3-oxygenated epoxides. We attempted to discern whether this
opposite (albeit poor) selectivity relative to that normally interaction was the reason for the observed regioselectivity by
observed for unfunctionalized epoxides [44]. This is proposed performing the nickel-catalyzed coupling reaction with 1,2-
to be an effect of the coordinating ability of Lewis basic oxygen epoxyoctane, which lacks potentially chelating functional
atoms in tosylates and esters, which may disrupt the binding groups (Scheme 9). Although the coupling product 39 appears
and directing effect of phenyl or alkenyl groups.
to be formed in the reaction with either Bu3P or PhMe2P as
ligand, the non-polar nature of this molecule complicates
In the case of the ripostatin A model system, the most likely chromatographic purification, and mixtures of what appears to
candidate for chelation is the oxygen protected as the PMB be 39 along with one or more other products were obtained.
ether. Although an eight-membered chelate might seem too Based on these results, regioisomer formation cannot be
large to play an important role in directing regioselectivity, the excluded.
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Scheme 8: Regioselectivity changes in reductive couplings of alkynes and 3-oxygenated epoxides.
Scheme 9: Enyne reductive coupling with 1,2-epoxyoctane.
Several aspects of the enyne synthesis and the nickel-catalyzed native, differentiation of the alcohols allowing for sequential
coupling reaction require further investigation. As 6 does not oxidation may be necessary.
itself appear to undergo thermal rearrangement, it seems advan-
tageous to convert this compound to the corresponding diester. Dithiane approach to epoxide
Preliminary investigation indicates that oxidation of the diol With respect to the synthesis of the proposed epoxide fragment
derived from 6 is complicated by the 1,5-relationship of the 5, we initially envisioned using the reaction of lithiated dithi-
alcohols. Despite the greater complexity inherent to this alter- anes with epoxides (Figure 4) [45-49]. We reasoned that 41
Figure 4: Initial retrosynthesis of the epoxide fragment by using dithiane coupling.
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could be expediently accessed from the corresponding alde- generated and decided to investigate this step of the reaction
hyde by making use of a Claisen rearrangement to set the geom- independently of reaction with the epoxide electrophile. To this
etry of the γ,δ-unsaturated double bond.
end, 41 was treated with tert-butyllithium at –78 °C in a 90:10
THF/HMPA mixture, referred to as the method of first choice
In the forward direction, the allylic alcohol 44 was obtained for lithiation of complex dithianes (Scheme 11). Following
from reaction of the alkenyllithium reagent derived from warming to –42 °C, the reaction was quenched with deuterated
2-bromopropene with phenylacetaldehyde (Scheme 10). In our methanol. Analysis of the product by 1H NMR revealed that no
hands, the organolithium afforded significantly higher and more deuterium incorporation (to the sensitivity of integration) had
reproducible yields than either the commercially available occurred at the desired dithiane site, while approximately 80%
Grignard reagent or the organocerium. Johnson–Claisen deuterium incorporation had occurred at the allylic/benzylic
rearrangement [50] of 44 proceeded smoothly to give the γ,δ- site.
unsaturated ester 45. Conducting the reaction without added
solvent in a microwave reactor at 170 °C allowed the reaction to These results indicated that the presence of the trisubstituted
proceed in just 30 minutes; a significant improvement over C18–C19 olefin would interfere with dithiane coupling.
heating the reaction mixture in toluene under reflux, which typi- However, given the suitability of the Claisen rearrangement for
cally required 48 hours to obtain a comparable yield. Reduction formation of this bond, we wished to preserve that transforma-
and oxidation afforded the aldehyde 47, which could then be tion. Accordingly, an alternate route that would capitalize on
converted to dithiane 41.
the electrophilic nature of aldehyde 47 to form the bond corres-
ponding to C14–C15 of ripostatin A instead was sought.
Unfortunately, attempts to couple 41 with an oxygenated
epoxide fragment under a variety of conditions reported by Oxy-Michael approach to epoxide
Smith for lithiation and electrophilic trapping [51] were unsuc- We were intrigued by a recent report by Falck describing an
cessful. We suspected that the lithiated dithiane was not being organocatalytic oxy-Michael addition to achiral δ-hydroxy-α,β-
Scheme 10: Synthesis of dithiane by Claisen rearrangement.
Scheme 11: Deuterium labeling reveals that the allylic/benzylic site is most acidic.
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enones (Scheme 12) [52]. The hydroxy group is delivered in a epoxide of 52 with the ethylenediamine complex of lithium
directed fashion from the boronic acid hemiester generated in acetylide in a 1:1 THF/DMSO solvent mixture at 0 °C allowed
situ from the substrate and phenylboronic acid. It is proposed the terminal alkyne to be accessed in 84% yield without
that complexation of the tertiary nitrogen to boron and coordi- rearrangement to the internal alkyne. Silyl protection afforded
nation of the carbonyl act in a push/pull fashion, simultane- alkyne 50, which was prone to decomposition upon extended
ously enhancing the nucleophilicity of the boronate oxygen as storage, even at −20 °C.
well as imposing a chiral environment around the enone. Ali-
phatic enones react more sluggishly in this transformation; Hydrozirconation of alkyne 50 with Schwartz’s reagent [54,55]
however, 3,4,5-trimethoxyphenylboronic acid may be used was followed by transmetallation to zinc and nonselective addi-
as a more efficient nucleophilic partner to circumvent this limi- tion into aldehyde 47. Oxidation of the resulting allylic alcohol
tation.
mixture afforded the enone 53. Prior to the key oxy-Michael ad-
dition, it was necessary to remove the tert-butyldimethylsilyl
Application of this transformation to the ripostatin A epoxide protecting group. Use of TBAF under buffered (acetic acid) or
fragment 5 allows installation of the C13 hydroxy group via unbuffered conditions proved sluggish and somewhat low-
conjugate addition to the δ-hydroxy-α,β-enone 49 (Figure 5). yielding, typically ~50%. Various other promoters led to
Although the diastereoselectivity of this reaction using decomposition (BF3·OEt2) or low conversion (CsF); however, a
substrates with a chiral center at the hydroxy group had not modest improvement in yield was noted with the use of 1% HCl
been investigated in the published study, substitution at the in MeOH (with a small amount of THF to dissolve the starting
carbinol position was reportedly well tolerated for the reaction material).
using γ-hydroxy-α,β-enones. Although the presence of chirality
at the δ-position allows for diastereoselective intramolecular The oxy-Michael addition with chiral thiourea and phenyl-
oxy-Michael addition of hemiacetal-derived alkoxides into α,β- boronic acid proceeded to give a single diastereomer; however,
unsaturated esters, the extension of this reaction to ketones was after 48 h at 50 °C, only 17% of the syn diol was formed, and
not successful [53]. In turn, we intended to prepare 49 by 56% of the starting material was re-isolated. Under the modi-
hydrometalation of 50 and addition into aldehyde 47.
fied conditions for less reactive aliphatic aldehydes using 3,4,5-
trimethoxyphenylboronic acid, we were unable to isolate the
To this end, (R)-glycidol was protected as the para-methoxy- desired diol from the reaction mixture. Since the oxy-Michael
benzoate ether to give the PMB glycidol 52 (Scheme 13), in substrate derived from 53 contains an existing stereocenter at
which the configuration is now assigned as (S). Opening the the directing hydroxy group, we also attempted to carry out the
Scheme 12: Oxy-Michael addition to δ-hydroxy-α,β-enones.
Figure 5: Revised retrosynthesis of epoxide 5.
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Scheme 13: Synthesis of functionalized ketone by oxy-Michael addition.
reaction with diisopropylamine as a substitute for the thiourea Iodocyclization approaches to epoxide
catalyst. This modification afforded both the syn and anti diols In the preparation of model epoxide 28, iodocyclization was
in roughly a 1:1 ratio and a combined yield of 40–45%, albeit used to introduce oxygenation in a stereoselective fashion from
without recovery of starting material. The syn and anti dia- a chiral homoallylic alcohol. Applying this disconnection, we
stereomers could be separated by repeated silica gel chromatog- hypothesized that we might be able to introduce the epoxide
raphy, and the desired syn diol was converted to the bis-sily- functional group of 5 by iodocyclization of the tert-butyl
lated compound 54.
carbonate 55 (Figure 6). Although the additional double bond in
this substrate presents a potential site for competing reaction
However, attempted oxidative deprotection of the PMB ether of pathways, we were encouraged by a report by Bartlett in which
54 with DDQ led to destruction of the material. It seems likely the iodocyclization of the tert-butyl carbonate derivative of 1,7-
that this is again due to the presence of the allylic/benzylic site octadien-4-ol afforded exclusively the 6-membered carbonate
in the molecule, although no individual decomposition prod- derivative, with product arising from cyclization onto the more
ucts could be identified. Although it is possible that further distant double bond not detected [56]. We set out to access 55
screening of deprotection conditions might have allowed us to from deprotonation of methyl ketone 57 on the less hindered
move forward with this route, the modest yields and long reac- side and alkylation with bromide 56.
tion time of the oxy-Michael addition severely limited material
throughput. In order to proceed with the synthesis, we needed a The E-allylic bromide was prepared rapidly, albeit in modest
more robust route, with the following criteria: (1) no protecting yield, with an Appel reaction [57] of the known allylic alcohol
groups requiring oxidative cleavage, and (2) introduction of the [58]. To synthesize ketone 57, we opted to utilize an asym-
C13 hydroxy group at an early stage.
metric aldol reaction to set the stereochemistry of the β-hydroxy
Figure 6: Retrosynthesis by using iodocylization to introduce the epoxide.
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group. Since the report of Evans’s diastereoselective asym- considered potential routes arising from retrosynthetic discon-
metric aldol reaction using the boron enolates of N-acyloxazol- nection of the C15−C16 bond (Figure 7). It was recognized that
idinones [59], numerous chiral-auxiliary-based methods have reaction of the enolate of ester 45, a compound previously
been developed for the synthesis of syn- or anti-propionate synthesized in just two steps, and subsequent oxidation could
aldol units. However, many of these auxiliaries, including the give the β-ketoester 61. Decarboxylation of this compound
Evans oxazolidinones, fail to give high stereoselectivities when would provide rapid access to the key iodocyclization substrate
employed in acetate aldol reactions [60]. Of the methods avail- 55.
able, we selected Sammakia’s boron enolate-based strategy
using the N-acetylthiazolidinethione 58 (Scheme 14) for its high Aldehyde 62 was prepared by reduction of the thiazolidine-
reported diastereoselectivity with aliphatic aldehydes and its thione 60 with DIBAL-H (Scheme 15). Treatment of the ester
avoidance of toxic tin reagents [61].
with LDA, followed by trapping with the aldehyde, afforded the
aldol adduct as a mixture of up to four possible diastereomers.
The reaction of the brightly colored 58 with but-3-enal This was then oxidized under Ley’s conditions [62] to the
proceeded in moderate yield, with an initial diastereoselectivity β-ketoester 61, itself a mixture of two diastereomers.
around 10:1, with further enhancement following silyl protec-
tion and purification. Although the silylated compound 60 Initially, we attempted to induce decarboxylation of 61 by treat-
proved to be reluctant to form the Weinreb amide, microwave ment with LiOH in a 1:1 water/THF mixture. No reaction was
irradiation allowed this process to proceed on a reasonable time observed at room temperature, but heating to 70 °C resulted in
scale. Grignard addition to the Weinreb amide afforded ketone elimination of the β-siloxy group. The Krapcho reaction offers
57.
an essentially neutral method for the decarboxylation of base-
and acid-sensitive substrates [63]. Under these conditions
Unfortunately, attempts to unite ketone 57 and bromide 56 via (sodium chloride in wet DMSO at elevated temperatures) the
alkylation were unsuccessful. Although deprotection at the less- desired decarboxylation reaction proceeded, although only in
substituted site of the methyl ketone using LDA was verified in modest yield (Scheme 16). Reduction of the temperature from
a deuterium quench experiment, the alkylation did not proceed 180 °C to 120 °C led to much lower conversion, as expected,
at temperatures from −78 °C to 0 °C. While the ketone was but did not improve mass recovery in the reaction. Somewhat
re-isolated cleanly following the reaction, the bromide was more surprisingly, silyl deprotection with TBAF was also low
converted to a mixture of olefinic compounds. Faced with the yielding. Interestingly, when the order of these operations was
difficulty of forming the C16−C17 bond by alkylation, we reversed, the Krapcho conditions led to a complex product mix-
Scheme 14: Synthesis of ketone 57 using thiazolidinethione chiral auxiliary.
Figure 7: Retrosynthesis involving decarboxylation of a β-ketoester.
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Scheme 15: Synthesis of β-ketoester 61.
Scheme 16: Decarboxylation of 61 under Krapcho conditions.
ture, the major component of which appeared to be the dienone, removed first, the free hydroxy group would undergo elimina-
formed by elimination and isomerization of the terminal olefin tion, it seemed likely that both deprotections would proceed at
into conjugation.
ambient temperature, which might circumvent this issue. Trans-
esterification of methyl ester 45 to TMSE ester 64 proceeded in
We reasoned that the decarboxylation and silyl deprotection good yield, and following an analogous procedure for aldol
steps could be coupled into one operation by switching from the reaction and oxidation the TMSE β-ketoester was obtained
methyl ester to the 2-trimethylsilylethyl (TMSE) ester. (Scheme 17). Treatment with an excess of TBAF in THF at
Although there was concern that if the TBS group were room temperature overnight resulted in formation of the
Scheme 17: Improved synthesis of 63 and attempted iodocyclization.
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Beilstein J. Org. Chem. 2013, 9, 1533–1550.
β-hydroxyketone 63. Although the yield for this transformation Acetonide approach to epoxide
remained moderate, it was higher than that obtained for To obtain the key C10−C11 epoxide in 5 in stereoselective
either of the individual steps from the methyl series that it fashion from displacement of a leaving group at C10, a means
replaced.
for the selective formation of a syn-1,3-diol at C11 and C13 is
required. Rychnovsky has demonstrated that alkylation of
The cleavage of TMSE β-ketoesters with TBAF·3H2O has been 4-cyano-1,3-dioxanes (cyanohydrin acetonides) constitutes a
described in the literature as a chemoselective method for decar- practical and valuable approach to syn-1,3-diol synthesis [66].
boxylation in the presence of other types of β-ketoesters [64]. The lithiated cyanohydrin acetonides react as nucleophiles with
Comparable yields for the decarboxylation to form 63 were alkyl, allyl, and propargyl halides, as well as with epoxides.
obtained with this reagent as with the anhydrous solution, or Although the alkylation itself is highly stereoselective in favor
when the reaction was run in DMF instead of THF. The use of of the axial nitrile, the syn-1,3-diol stereochemistry is ulti-
TAS-F (tris(dimethylamino)sulfonium trifluoromethylsilicate) mately set in a subsequent reductive decyanation step. We
was clearly inferior, leading to incomplete conversion and elim- planned to synthesize 5 by reaction of the cyanohydrin
ination. With TBAF, partial elimination could sometimes be acetonide 67 with the epoxide electrophile 66 (Figure 8) [67].
observed; however, this typically occurred in less than 10%.
Given these results, it seems that the fluoride-mediated depro- The dimethyl derivative of L-malic acid was chemoselectively
tection of TMSE β-ketoesters is deserving of further explo- reduced with borane-dimethylsulfide and sodium borohydride
ration and utilization as a method for the decarboxylation of to afford diol 69 (Scheme 18) [68]. The primary alcohol was
sensitive synthetic intermediates.
protected as the TIPS ether, and the secondary alcohol subse-
quently converted to the TMS ether. Reduction with DIBAL-H
Alcohol 63 was derivatized as the Boc carbonate, a reaction afforded the aldehyde 71 without over-reduction to the alcohol.
plagued by the formation of the carbonate arising from two Acetonide formation proceeded smoothly to give 67 as an
molecules of 63. The ratio of Boc derivative to symmetrical inconsequential mixture of diastereomers. However, attemps to
carbonates is dependent on the acidity of the alcohol and not ne- alkylate the lithiated anion of 67 with epoxide 66 led only to
cessarily improved by increasing the stoichiometry of Boc2O recovery of starting material. Interestingly, although alkyla-
[65]. Disappointingly, treatment of 55 with IBr or I2 led to tions of the acetonide are known to be stereoselective, protona-
exhaustive decomposition of the material. Similarly, attempts to tion does not appear to be, as a cis and trans mixture was
convert the homoallylic olefin of 55 into the epoxide via obtained from the attempted reaction of a single acetonide
directed oxidation with VO(acac)2 and TBHP again resulted in diastereomer.
an intractable mixture of products.
In the course of investigating why 66 and 67 failed to react,
While investigations into installing the epoxide via iodocycliza- attempts were made to trap the anion of 67 with a more reac-
tion were ultimately not fruitful, in the course of this route an tive electrophile. Allyl bromide reacted rapidly, affording the
expeditious and perhaps underappreciated disconnection was product 73 as a single diastereomer (Scheme 19). The configur-
identified in the construction of the C15−C16 bond via a simple ation of this compound, as well as subsequent compounds along
aldol reaction, followed by TBAF-promoted decarboxylation to this route, could be assigned as the 1,3-syn acetonide by
remove the ester. We concluded that selective reaction of the analysis of the 13C chemical shifts of the acetonide methyl
terminal olefin in the presence of the trisubstituted olefin was groups [69]. It was recognized that conversion of this olefin to
not a feasible proposition. Therefore, a substrate with oxygena- the aldehyde would provide an ideal electrophile for a revised
tion present at C10 and C11 from an early stage was needed as β-ketoester decarboxylation strategy. To this end, reduction of
well.
the nitrile proceeded with the expected selectivity; this arises
Figure 8: Retrosynthesis utilizing Rychnovsky’s cyanohydrin acetonide methodology.
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Beilstein J. Org. Chem. 2013, 9, 1533–1550.
Scheme 18: Synthesis of cyanohydrin acetonide and attempted alkylation with epoxide.
Scheme 19: Allylation of acetonide and conversion to aldehyde.
from equilibration of the intermediate radical to the more stable The alcohol 77 and tosylate 78 contain all of the carbon atoms
axial radical. It was essential to allow the reaction to warm of epoxide fragment 5 in the correct oxidation state. The
under reflux due to the insolubility of the starting material in remaining steps required to access the epoxide consist of
liquid ammonia. Ozonolysis of the pendant olefin afforded alde- acetonide deprotection, displacement of the tosylate or another
hyde 75 in high yield.
appropriate leaving group to obtain the terminal epoxide, and
silyl protection.
Following the prior procedure, the lithium enolate of TMS ester
64 was reacted with aldehyde 75, and the mixture of diastereo- Conclusion
meric alcohols was oxidized to the β-ketoester 76 (Scheme 20). Nickel-catalyzed reductive coupling methodologies are an
This substrate did not appear to be prone to elimination, and attractive fragment coupling strategy for the synthesis of com-
treatment of the β-ketoester with TBAF in THF provided the plex natural products. The formation of sensitive skipped diene
decarboxylated and deprotected alcohol 77. The primary units in this context remains largely an unsolved problem for
alcohol could be converted to the tosylate 78 in good yield with organic chemistry, but reactions for the rearrangement of vinyl-
tosyl chloride, triethylamine, and trimethylamine hydrochloride cyclopropanes present an intriguing avenue for exploration. To
as the catalyst.
facilitate future studies in this vein, a cyclopropylenyne corres-
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Beilstein J. Org. Chem. 2013, 9, 1533–1550.
Scheme 20: Synthesis of the epoxide precursor by an aldol−decarboxylation sequence.
ponding to the C1−C9 carbons of ripostatin A was synthesized Dr. Jeff Simpson for NMR assistance and Li Li for obtaining
in ten steps and up to 35% yield, and was shown to be a viable HRMS data, and Dr. Norman Lee of the Boston University
substrate in a nickel-catalyzed coupling reaction. The lack of Chemical Instrumentation Center for obtaining HRMS data.
complete regioselectivity in this transformation highlights that The National Institute of General Medical Sciences and
the factors governing selectivity in this reaction are more com- Novartis provided funding for the research described.
plex and more substrate dependent than initially anticipated.
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