Total Synthesis of (+)-Prunustatin A: Utility of Organotrifluoroborate-Mediated Prenylation and Shiina MNBA Esterification and Macrolactonization to Avoid a Competing Thorpe-Ingold Effect Accelerated Transesterification

Article abstract of DOI:10.1021/acs.orglett.8b02396

A convergent total synthesis of (+)-prunustatin A is described through the assembly of two key fragments and a macrolactonization. Shiina MNBA couplings were used for the formation of each of the four ester bonds in the tetralactone ring, including the key macrocyclization which was essential to minimize competing Thorpe-Ingold accelerated transesterification. Other key steps included an organoboron-based prenylation using potassium prenyltrifluoroborate and a carbonyldiimidazole-mediated coupling to form the salicylamide.

Full text of DOI:10.1021/acs.orglett.8b02396

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Total Synthesis of (+)-Prunustatin A: Utility of
Organotrifluoroborate-Mediated Prenylation and Shiina MNBA
Esterification and Macrolactonization To Avoid a Competing
Thorpe−Ingold Effect Accelerated Transesterification
Maja W. Chojnacka and Robert A. Batey*
Davenport Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto ON, Canada
M5S 3H6
S
* Supporting Information
ABSTRACT: A convergent total synthesis of (+)-prunustatin A is described through the assembly of two key fragments and a
macrolactonization. Shiina MNBA couplings were used for the formation of each of the four ester bonds in the tetralactone ring,
including the key macrocyclization which was essential to minimize competing Thorpe−Ingold accelerated transesterification.
Other key steps included an organoboron-based prenylation using potassium prenyltrifluoroborate and a carbonyldiimidazole-
mediated coupling to form the salicylamide.
runustatin A (1) is a natural product isolated in 2005 from
Pa soil sample collected on the volcanic island of Kumajima
in Okinawa, Japan, and determined by Shin-ya to be a potent
anticancer agent against hypoxic fibrosarcoma cells.1 It is
produced by the bacterial species Streptomyces violaceoniger,
which belongs to a family of biocontrol agents used against
phytopathogenic fungi.2 Structurally, 1 is a member of the
antimycin depsipeptide class of compounds3 that includes
(+)-SW-163A 2,4 JBIR-06,1e JBIR-52,5 neoantimycin 3,6 the
parent antimycin family of compounds 4,6b,7 and respirantin 58
(Figure 1). They share an aromatic 3-formamidosalicylate
headgroup attached via an amide bond to an L-threonine unit
embedded within a di-, tri-, or tetralactone macrocyclic ring
(9-, 12-, 15-, or 18-membered). Other antimycin depsipeptides
incorporate different aromatic head groups, with the entire
family related through a nonribosomal peptide synthetase
(NRPS)-polyketide synthase (PKS) biosynthetic pathway.
Over the years, there has been significant interest in this
class of depsipeptides due to their anticancer, antifungal, and
Figure 1. Structures of (+)-prunustatin A (1) and related natural
immunosuppressant properties. The anticancer properties of
products 2−5 belonging to the antimycin class of compounds.
(+)-prunustatin A are believed to stem from its ability to
downregulate the expression of GRP78, which is a glucose-
products has included the synthesis of antimycin A1b (2014)10
regulated molecular chaperone that plays an important role in
as well as respirantin and kitastatin (2014).11 In the latter case,
the presence of a lactam group permitted the use of a
macrolactamization approach using HATU, whereas Shiina’s
the survival of tumors subjected to chemotherapy. The
inhibition of GRP78 expression may be a secondary effect of
blocking the mitochondrial ATP production.9
The structural motifs present in these molecules pose
numerous synthetic challenges, especially the formation of the
macrocyclic rings. Our previous work in this class of natural
Received: July 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02396
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Letter
Scheme 1. Retrosynthesis of (+)-Prunustatin A (1)
reagent (MNBA: 2-methyl-6-nitrobenzoic anhydride)12 was
required for the challenging macrolactonization step needed to
form the 9-membered dilactone ring present in antimycin. We
now report a total synthesis of (+)-prunustatin A that
addresses some of the problems encountered by other groups
in the total synthesis of 1.
In summary, Kawanishi employed a linear sequence of ester
bond c, d, and b formation and macrocyclization of bond a,
Hale employed a linear sequence of ester bond b, c, and d
formation and macrocyclization of bond a, while Usuki
employed a linear sequence involving ester bond a, d, and b
formation, macrocyclization of bond c, and subsequent C11
gem-dimethyl group introduction. Based upon this precedent,
we were particularly interested in seeing whether a fully
convergent synthesis of 1 might be possible and whether the
troublesome gem-dimethyl-promoted butyrolactone formation
could be avoided using an approach based upon a bond b
macrocyclization of a seco acid substrate that incorporated a
pre-existing C11 gem-dimethyl group. The requisite fragments
would also need to be synthesized from readily available
precursors using a convergent strategy and allow for the late-
stage incorporation of different aromatic headgroups, pref-
erably avoiding the use of a preformed O-protected 3-
formamidosalicylic acid as a coupling partner.
The quasi-symmetric nature of the tetralactone ring in 1 is
such that there are four possible convergent strategies for its
formation. An approach involving initial C6−C12 and C14−
C4 fragment synthesis through ester bond b and d formation
would require subsequent ester bond a and c formation. This is
undesirable due to the very hindered nature of the ester bond c
and the possibility of competitive decarboxylation of any C9−
C12 β-keto-α-gem-dimethyl ester precursor fragment. Alter-
natively, the convergent synthetic approach involving a late-
stage combination of bond d and b formation would allow for
the inherently more challenging ester bond c formation to be
incorporated more straightforwardly for the formation of the
“southern” fragment 9. Therefore, we chose a strategy in which
the key seco acid intermediate 7 required for the lactone bond
b formation would be formed through an intermolecular ester
bond d disconnection between two similar size fragments, 8
(C2−C7) and 9 (C9−C15) which, in turn, would be accessed
through ester bond a and c formation, respectively, and use a
prenyl trifluoroborate salt 11 to introduce the C11 gem-
dimethyl group in 9 (Scheme 1).
The major challenges associated with the synthesis of
(+)-prunustatin A (1) are the formation of the 15-membered
tetralactone ring, the requisite seco acid precursor, and the β-
keto-α-gem-dimethyl ester functionality, which is also found in
JBIR-06, respirantin, kitastatin, and the subsequent attachment
of the 3-formamidosalicylate headgroup. Each of the existing
approaches to 1 by Kawanishi,13 Usuki,14 and Hale15 have
involved linear strategies for the formation of triester
precursors, followed by macrolactonization through bond a
or c formation (Scheme 1). In 2014, Kawanishi achieved the
first total synthesis of 1 using a macrocyclization to form bond
a in 60% yield using MNBA/DMAP at 50 °C. Attempts to
remove a benzyl group from the O8 position of a C9−C15
ester precursor revealed a propensity for an undesired
transesterification at the C12 position with concomitant
fragmentation to a butyrolactone as a result of the Thorpe−
Ingold effect of the C11 gem-dimethyl group. Usuki and Hale
encountered problems in their attempts to achieve macro-
lactonization through bond b or d formation, respectively
(Scheme 1). Hale’s initial approaches utilizing a bond d
macrocyclization led only to diastereomers, thus necessitating a
bond a macrocyclization strategy. Hale also reported that
application of Kawanishi’s high-temperature conditions
occurred with problems of lower yields and reproducibility
and, consequently, resorted to a reoptimization of the MNBA/
DMAP conditions at room temperature and at “ultrahigh
dilution” to form the desired macrolactone in 42% yield along
with some dimerized products. Usuki’s initial approach
utilizing a bond b macrocyclization strategy was thwarted
because of undesired butyrolactone formation similar to that
encountered by Kawanishi. As a result, Usuki employed a
strategy employed in Pettit’s respirantin 5 synthesis8b involving
macrocyclization of a less sterically encumbered substrate
lacking the gem-dimethyl group. Thus, macrolactonization was
achieved through transesterification of a β-ketoester leading to
bond c formation in 88% yield using 20 equiv of CuSO4 in
toluene at reflux. This protocol requires a subsequent α-
methylation step to yield the gem dimethyl substituted group
in 62% yield.
The “northern” fragment 8 synthesis started with the
formation of 15 using a modified three-step protocol of
Kirschning (Scheme 2).16 Acetylation of L-lactic acid to give 13
followed by an EDC-mediated tert-butyl protection to give 14
in 84% yield (EDC offered yield and reaction time improve-
ment over the traditional Steglich esterification used in the
original sequence). Cleavage of the acetyl group under basic
B
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issues, 12 was used immediately or stored at −78 °C for up to
a week with enantiomeric purity confirmed by [α]D
determinations. The prior success that we had achieved
using organotrifluoroborate salts in the synthesis of 5
encouraged us to attempt an analogous approach for the
introduction of the gem-dimethyl group. Reaction of the freshly
prepared aldehyde 12 with 3.0 equiv of prenyltrifluoroborate
salt 1111 for 3 days at room temperature using Montmor-
illonite K10 clay based activation afforded 10 as a 5:1 mixture
of anti and syn diastereomers (10a and 10b) with a total yield
of 80%. The salt 11 is a stable, white solid that can be stored
for a prolonged time with minimal decomposition offering a
convenient alternative to organometallic prenylations of
aldehydes and ketones under mild conditions.11 The reaction
time and the amount of the salt 11 required could be reduced
with the use of 15 mol % BF3·OEt2 as a catalyst, giving 10 as a
4:1 mixture of diastereomers in 83% total yield.22 The
structure of the major diastereomer was confirmed by X-ray
crystallography as having an anti relationship consistent with a
Cornforth transition-state model.10
Scheme 2. Synthesis of “Northern” Fragment 8
Even though both diastereomers of 10 converge onto the
same product 28 during the later oxidation step, the alcohols
were separated by column chromatography (Scheme 4). TBS
conditions yielded alcohol 15 in 93% yield. The synthesis of
acid 16 was achieved by TBS protection of commercially
available Z-Thr.17 The ester coupling between 16 and 15 using
MNBA, DMAP, and triethylamime afforded 17 in 90% yield.
Finally, TBAF deprotection of the TBS group yielded 8 in
quantitative yield.18
Scheme 4. Synthesis of “Southern” Fragment 9
The synthesis of the “southern” fragment required the use of
the MOM-protected α-hydroxy aldehyde 12 (Scheme 3). A
diazotization of D-phenylalanine in acetic acid occurs with
overall retention of stereochemistry19 to give the acetate 18,
which upon prolonged treatment with thionyl chloride in
methanol led to 19. MOM protection to yield 2020 followed by
ester group reduction with DIBAL and Swern oxidation gave
aldehyde 12 in high yield.21 Given the possible epimerization
Scheme 3. Synthesis of Aldehyde 12 and Prenylation
Reaction
protection of the major anti diastereomer 10a with TBS-OTf
afforded 22a, which upon dihydroxylation with OsO4,
oxidative cleavage with NaIO4, and Pinnick oxidation yielded
carboxylic acid 24a (94% over three steps). The benzyl ester
protected coupling partner 25 was also synthesized using a
diazotization approach from L-isoleucine in two steps. The
subsequent ester coupling was particularly challenging due to
the steric hindrance of the carboxylic acid 24a, and the use of
Shiina conditions using 3.0 equiv of the secondary alcohol
coupling partner 25 was necessary to afford ester 26a in good
yields. After TBAF deprotection of the TBS group, oxidation
with DMP gave ketone 28. An analogous sequence using the
C
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syn diastereomer 10b also gave 28 in similar yields. Removal of
the benzyl ester protecting group from 28 using H2 and Pd/C
occurred in quantitative yield to give acid 9.
The coupling of the two fragments was once again achieved
using Shiina MNBA coupling conditions to give 29 in
quantitative yield (Scheme 5). The choice of acid-labile
EDCI, and a deprotection step.6−8,13−15 In order to avoid
these problems, we used an indirect approach in which
unprotected 3-nitrosalicylic acid 32 was first activated with
CDI (1,1′-carbonyldiimidazole) under microwave conditions
for 6 min at 85 °C, cooled to room temperature, and then
coupled with the macrocycle 31 in 40 min to afford 33 in 99%
yield using a simple silica plug purification (Scheme 6).
Scheme 5. Fragment Coupling and Double Deprotection/
Macrocyclization
Scheme 6. Completion of the Synthesis of (+)-Prunustatin
A (1)
Reduction of the nitro group yielded aniline 32 in 86% yield
and immediate, mild formylation using N-formylsaccharin 35
(Cossy’s protocol)24 yielded (+)-prunustatin A (1) in 70%
yield. This efficient three-step protocol potentially allows for
late-stage functionalization of either the phenol or aniline
functionalities on the headgroup.
In conclusion, consideration of the quasi-symmetric nature
of the tetralactone ring of (+)-prunustatin A, one of the most
complex cyclic polylactones known, allowed for a convergent
approach to its total synthesis using Shiina MNBA coupling
conditions for each of the ester bonds, a CDI-activated amide
bond formation, and an N-formylsaccharin-mediated formyla-
tion. The requisite α-hydroxy acid fragments were synthesized
through α-amino acid diazotization chemistry, and the C11
gem-dimethyl group of the β-keto-α-gem-dimethyl ester motif
installed using an organoboron-based approach using a stable
prenyl trifluoroborate salt. An efficient double-deprotection/
macrolactonization allowed for ring closure to occur at an ester
bond that had previously been reported to be unsuccessful.14
Further studies on this class of molecules and analogues will be
reported in due course.
protecting groups in compound 29 allowed for the sequential
removal of the MOM and t-Bu ester groups using TFA in a
“one-pot” fashion.23 After thorough drying of the reaction
mixture, the seco acid 7 was obtained in 82−90% purity
(NMR). The remainder of the mixture was the butyrolactone
30 and associated degraded seco acid. This degradation is
facilitated by the Thorpe−Ingold effect as noted in related,
more problematic, reactions in the Kawanishi13 and Usuki
syntheses of 1,14 as well as in Pettit’s respirantin synthesis in
which a pyrrolidinone equivalent of the lactone was formed.8b
Seco acid 7, obtained from 29 with TFA, was subjected to
macrocyclization under slow-addition, high-dilution conditions
(1.2 mM) with MNBA/DMAP to afford the macrocycle 6 in
59% yield over two steps. To minimize the formation of 30
during the double deprotection, TFA was removed using cold
CHCl3 (11−14 °C), and the reaction mixture was dried and
used immediately without exposure to temperature >20 °C to
prevent further degradation to 30. This and proper handling of
the reagents allows for macrocyclization through disconnection
b for the first time with improved efficiency and in good yield.
In comparison previous macrocycle formations were per-
formed over ester bond a (Hale: 42% yield, Kawanishi: 60%
yield) and ester bond c (Usuki: 88% yield) followed by
alkylation (62% yield for α-methylation step; 55% yield over
two steps). The key amine macrocycle 31 was then obtained in
quantitative yield after Pd/C deprotection.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02396.
Full experimental details and characterization data for all
1
compounds including H and 13C NMR spectral data
The 3-formamidosalicylate headgroup is typically coupled to
the amine group of the macrocycle in syntheses of 1 and other
antimycin analogues using a 3-formamidosalicyic acid
protected at the phenolic OH. The synthesis of these
precursors usually requires a few steps including a particularly
harsh N-formylation reaction with sometimes difficult
purification,10 followed by benzyl protection of the phenolic
oxygen, amide coupling with the macrocycle using HATU or
(PDF)
Accession Codes
CCDC 1858839 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
D
DOI: 10.1021/acs.orglett.8b02396
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Organic Letters
Letter
(12) (a) Shiina, I.; Kubota, M.; Oshiumi, H.; Hashizume, M. J. Org.
Chem. 2004, 69, 1822−1830. (b) Shiina, I. Bull. Chem. Soc. Jpn. 2014,
87, 196−233.
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(13) Yamakoshi, S.; Kawanishi, E. Tetrahedron Lett. 2014, 55, 1175−
AUTHOR INFORMATION
■
1177.
Corresponding Author
* E-mail: rob.batey@utoronto.ca.
ORCID
(14) Usuki, Y.; Ogawa, H.; Yoshida, K.-I.; Inaoka, T.; Iio, H. Asian J.
Org. Chem. 2015, 4, 737−740.
(15) Manaviazar, S.; Nockemann, P.; Hale, K. J. Org. Lett. 2016, 18,
2902−2905.
(16) Schmidt, T.; Kirschning, A. Angew. Chem., Int. Ed. 2012, 51,
1063−1066.
Robert A. Batey: 0000-0001-8808-7646
Notes
(17) Woulfe, S. R.; Miller, M. J. J. Org. Chem. 1986, 51, 3133−3139.
(18) The yield was significantly lower when an older bottle of TBAF
reagent was used.
The authors declare no competing financial interest.
(19) (a) Kolasa, T.; Miller, M. J. J. Org. Chem. 1987, 52, 4978−4984.
(b) Larissegger-Schnell, B.; Glueck, S. M.; Kroutil, W.; Faber, K.
Tetrahedron 2006, 62, 2912−2916.
ACKNOWLEDGMENTS
■
The authors thank the University of Toronto and the Natural
Science and Engineering Research Council of Canada
(NSERC) for financial support. M.W.C is grateful to
NSERC for an Alexander Graham Bell Scholarship (CGS-M
and CGS-D3) and to the Ontario Ministry of Education for an
Ontario Graduate Scholarship. We thank Dr. Matthew Forbes
(Toronto) for MS analysis and Dr. Alan Lough (Toronto) for
the X-ray structure determination of 10a.
(20) Hanessian, S.; Tremblay, M.; Petersen, J. F. W. J. Am. Chem.
Soc. 2004, 126, 6064−6071.
(21) Fanning, K. N.; Jamieson, A. G.; Sutherland, A. Org. Biomol.
Chem. 2005, 3, 3749−3756.
(22) Nowrouzi, F.; Thadani, A. N.; Batey, R. A. Org. Lett. 2009, 11,
2631−2634.
(23) Removal of the MOM group occurred very rapidly using excess
TFA, while removal of the tert-butyl group required stirring for 45
min at room temperature.
(24) Cochet, T.; Bellosta, V.; Greiner, A.; Roche, D.; Cossy, J. Synlett
2011, 2011, 1920−1922.
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