A Short stereoselective synthesis of prepiscibactin using a SmI2-mediated reformatsky reaction and zn2+-induced asymmetric thiazolidine formation

Article abstract of DOI:10.1021/ol502958u

Prepiscibactin (1) is a possible intermediate in the biosynthesis of piscibactin, the siderophore responsible for the iron uptake of the bacterium Photobacterium damselae subsp. piscicida, the aethiological agent of fish pasteurellosis. Compound 1 was syn

Full text of DOI:10.1021/ol502958u

Letter
pubs.acs.org/OrgLett
A Short Stereoselective Synthesis of Prepiscibactin Using a
SmI2‑Mediated Reformatsky Reaction and Zn2+-Induced Asymmetric
Thiazolidine Formation
Yuri Segade, Marcos A. Montaos, Jaime Rodríguez,* and Carlos Jimenez*
́
Departamento de Química Fundamental, Facultade de Ciencias e Centro de Investigacion
́
s de Ciencias Avanzadas (CICA),
Universidade da Coruna, A Coruna E-15071, Spain
̃
̃
S
* Supporting Information
ABSTRACT: Prepiscibactin (1) is a possible intermediate in the biosynthesis of piscibactin, the siderophore responsible for the
iron uptake of the bacterium Photobacterium damselae subsp. piscicida, the aethiological agent of fish pasteurellosis. Compound 1
was synthesized by a convergent approach starting from L-/D-cysteine and 2-hydroxybenzonitrile. The key steps were a highly
diastereoselective SmI2-mediated Reformatsky reaction and Zn2+-induced asymmetric thiazolidine formation followed by
lactamization. The absolute configuration 9R,10S,12R,13S was established for 1, and this confirmed the previous relative
stereochemistry proposed on the basis of NOE and computational methods.
asteurellosis is a fish disease that causes significant
Peconomic losses in marine aquaculture worldwide, and
since 1990 it has been the major pathological problem in the
culture of sea bream and sea bass in Mediterranean countries,
including Spain.1 Piscibactin (Pcb) is the siderophore
responsible for the iron uptake of the bacterium Photobacterium
damselae subsp. piscicida, the aethiological agent of pasteur-
ellosis. During the isolation of Pcb from iron-deficient cultures
in CM9 medium, a smaller metabolite, namely prepiscibactin
(prePcb, 1), was isolated and characterized.2 The partial
structures of prePcb and Pcb were predicted by analysis of the
gene cluster involved in the siderophore biosynthesis. From the
proposed biosynthetic pathway for Pcb, we envisaged that
prePcb, 1, could be an intermediate in the biosynthesis of Pcb.
The relative stereochemistry of prePcb, 1, was proposed on the
basis of a combination of NOE correlations and DFT studies.2
Since siderophores3 are critical for the growth and virulence of
the producer pathogens, the bacterial iron acquisition systems
are promising targets for the design of new antibiotic
strategies.4 Consequently, the synthesis of prePcb, 1, would
be very helpful in elucidating the biosynthetic pathway of Pcb
and thus for the development of new rationally designed
antibacterials against pasteurellosis. Herein, we present the first
total stereoselective synthesis of prePcb, 1, in a process that
features two key diastereoselective steps: a SmI2-mediated
Reformatsky reaction (dr > 99:1) of an α-chloroacetyl-2-
oxazolidinone with a thiazolidinic aldehyde and Zn2+-induced
asymmetric thiazolidine formation followed by lactamization.
The structure of prePcb, 1, is closely related to those of the
siderophores pycohelin5a and yersiniabactin,5b the reported
syntheses6 of which were very useful in planning our synthetic
strategy (Scheme 1). Our retrosynthetic analysis of prePcb, 1,
involved the preparation of key synthetic intermediates A and B
from D- and L-cysteine, respectively, as chiral sources for C9S
and C12S. The secondary alcohol with the S configuration at
C13 would be generated in a stereoselective manner using a
SmI2-mediated Reformatsky reaction, while the chiral center at
Scheme 1. Retrosynthetic Analysis for Prepiscibactin (1)
Received: October 7, 2014
Published: October 24, 2014
© 2014 American Chemical Society
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C10S would be constructed through the diastereoselective
formation of the thiazolidine ring in a metal-chelation/
lactamization sequence.
Scheme 2. Samarium Iodide Mediated Reformatsky
Synthesis of Alcohols 4 and 6a−b
Although the classical Reformatsky reaction was introduced
over 125 years ago,7 the stereoselective version is a more recent
development.8 One of the great advantages of this approach is
that the enolate is formed under neutral conditions and
therefore base or acid is not required to generate the enolate or
to activate the electrophile, respectively.9 Metal-catalyzed
enantio- and diastereoselective Reformatsky reactions between
aldehydes or ketones and various electrophiles are now
possible.10 In particular, the asymmetric samarium iodide
mediated Reformatsky11 process developed by Fukuzawa et
al.12 seems to be very useful for the diastereoselective
preparation of β-hydroxy derivatives from aldehydes and α-
bromoacetyl-2-oxazolidinones. This process provides an
alternative approach to asymmetric aldol reactions in the
synthesis of this kind of compound. However, very few
examples of this reaction have been reported in the literature.13
One of the few examples of diastereoselective Reformatsky
reactions to give β-hydroxy-γ-amino acids without the double-
differentiating effect of α-substituents was reported by Burke’s
group. They used this coupling reaction for the synthesis of the
β-hydroxy-γ-amino acids N-Boc-isostatine and N-Boc-dolaiso-
leucine with complete diastereoselectivity and in good yields.14
However, additions of α-haloacetyl-2-oxazolidinones to N-Boc-
(S)-prolinal gave diastereoselectivities that ranged from low (dr
5:1 for syn) to moderate (dr 14:1 for anti). Validation of this
methodology for the construction of β-hydroxy-γ-amino acid
derivatives requires more examples to cover a range of different
substrates. Moreover, examples involving the use of other
heterocycles have not been reported, e.g., thiazolidines, in
which the presence of an additional sulfur atom could affect the
outcome of the reaction.
gave a mixture of diastereoisomers (6S)-6a/(6R)-6b (Scheme
2). HPLC separation and analysis of the NMR 1H−1H
homonuclear and 13C−1H heteronuclear coupling constants
indicated a 1:1.1 ratio for the syn-6a/anti-6b compounds (see
SI). These results again confirm that the stereochemical
outcome of the SmII-mediated Reformatsky coupling is highly
dependent on the chirality of the auxiliary employed in the
reaction.
The high diastereoselectivity of the syn product obtained
using α-chloroacetyl-2-oxazolidinone (R)-3, in relation to the
results reported by Burke and co-workers for the additions of α-
haloacetyl-2-oxazolidinones to N-Boc-(S)-prolinal, is probably
due to the more sterically hindered environment created by the
two methyl groups and the sulfur atom in the heterocyclic ring.
However, a 1:1.1 syn/anti mixture was obtained with (S)-α-
chloroacetyl-2-oxazolidinone [(S)-3)]. In this case, the result
can be explained by the similar steric environments in the
chelated transition states for the samarium enolate attack at the
re or si face of the aldehyde.
Our synthetic efforts started with the preparation of the
thiazolidinic aldehyde 2 from commercially available L-cysteine
according to the procedure reported by Duthaler et al. with
appropriate modifications (see Supporting Information (SI)).15
At the same time, (R)-α-chloroacetyl-2-oxazolidinone [(R)-3)]
was prepared using the methodology developed by Roush and
Brown.16
With these products in hand, the next step was the crucial
SmI2-induced Reformatsky reaction (Scheme 2). Reaction of a
mixture of the freshly prepared thiazolidinic aldehyde 2 and α-
chloroacetyl-2-oxazolidinone (R)-3 with an excess (3 equiv) of
SmI2 resulted in the formation of secondary alcohol 4 as a
single diastereoisomer (dr >99%, see Scheme 2), as
demonstrated by HPLC and NMR (see SI). Standard oxidative
removal of the chiral auxiliary of 4 with H2O2/LiOH gave 5a, in
which the sulfur had been oxidized to sulfoxide. In order to
avoid this oxidation LiOH alone was used to obtain the acid 5b.
The syn stereochemistry of 4 was determined by applying J-
based configurational analysis to the hydrolyzed product 5b
(see SI).17 An HSQC-HECADE experiment allowed the values
of the 2JCH and 3JCH coupling constants of 5b to be determined.
The measurement gave a set of coupling constant values
On following the procedure of Mislin et al.,18 condensation
of D-cysteine hydrochoride and 2-hydroxybenzonitrile gave a
thiazolinic carboxylic acid, which was treated without
purification with N,O-dimethylhydroxylamine in the presence
of EDCI and DIPEA to yield the corresponding Weinreb amide
7a.
Reduction of the resulting hydroxamic ester with LiAlH4
afforded the thiazolinic aldehyde 8a. In order to study the
influence of the free phenolic OH group in the condensation
with the thiol amine, the TBDPS-protected 7b was also
obtained from 7a and reduced to 8b using a procedure similar
to that described above (Scheme 3).
Treatment of thiazolidine 4 with 75% TFA in CH2Cl2 led to
the concomitant removal of the acetonide and Boc protecting
moieties to give 9. Subsequent condensation of 9 with
thiazolinic aldehyde 8b, in the presence of potassium acetate
in MeOH, produced the unexpected bis-thiazoline 10. The
3
3
[3J(H3H4) = 6.4 Hz, J(C5H3) = 4.5 Hz, J(C2H4) = 4.7 Hz, and
2J(C3H4) = −3.6 Hz, medium, large/medium, large/medium, and
large, respectively] that are consistent with the presence of a
mixture of two staggered conformers of the syn configuration
(see SI).
The reaction of 2 was repeated with α-chloroacetyl-2-
oxazolidinone (S)-3 under similar reaction conditions, and this
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prepiscibactin, 1, along with its epimer at C10, 10-epi-
prepiscibactin, 1′, in a 1:1 ratio (Scheme 6). The synthetic
Scheme 3. Synthesis of Fragment A
Scheme 6. Final Steps of the Synthesis of 1 and Key NOE
Correlations Found in 1′
cyclization reaction between the deprotected thiol amine 9 and
thiazolinic aldehyde 8a yielded a mixture of two diaster-
eoisomers at C10 (11) in a 1:1.5 ratio, as deduced by 1H NMR
spectroscopy. Removal of the chiral auxiliary with a methanolic
solution of sodium methoxide at 0 °C gave compound 12. This
proved to be very unstable, and all attempts at cyclization were
unsuccessful (Scheme 4). It was therefore decided to reverse
the order of the reactions.
Scheme 4. First Approach for the Synthesis of 1
product 1 was spectroscopically indistinguishable from the
natural product, and it also had a very similar CD spectrum (see
SI). 10-epi-Prepiscibactin, 1′, showed an NOE between H12
and H10, clearly indicating a cis disposition (see Scheme 6).
The absence of an NOE in the similar pair of protons in
prepiscibactin, 1, was key in the structural elucidation of the
natural compound, as mentioned in the discussion of its
isolation.2
Once the conditions to access 1 had been established,
modifications to improve the diastereoselective formation of
the product were examined. It is well-known that 2-substituted
thiazolidine rings are rapidly equilibrated into an approximately
1:1 mixture of the C2 epimers due to ring opening−reclosure.19
However, once the nitrogen is acylated such epimerization is
not observed. Thiazolidine formation was tested using several
catalysts, including scandium trifluoromethanesulfonate,20 with-
out success. Gratifyingly, when the intermediate 15 was treated
with ZnCl221 followed by lactamization under reflux in toluene,
formation of the required isomer prepiscibactin, 1, in relation to
10-epi-prepiscibactin, 1′, was enhanced to give a 4:1 ratio. This
result can be understood by considering that complexation of
15 with Zn2+ would shift the equilibrium to the required trans-
thiazolidine ring, which cyclizes to give 1 as the major
diastereoisomer.
In summary, we have accomplished the first total synthesis of
prepiscibactin, 1, a possible intermediate in the biosynthesis of
piscibactin, the siderophore responsible for the iron uptake of
the bacterium Photobacterium damselae subsp. piscicida. A highly
diastereoselective SmI2-mediated Reformatsky reaction (dr >
99:1) between an α-chloroacetyl-2-oxazolidinone and a
thiazolidinic aldehyde was employed to obtain the chiral center
at C13S. Furthermore, diastereoselective formation of the
required isomeric thiazolidine ring after treatment of 15 with
ZnCl2 followed by lactamization diastereoselectively provided
the chiral center at C10S. The synthetic product allowed us to
establish the absolute configuration 9R,10S,12R,13S for 1. The
knowledge acquired in the synthesis of 1 is being exploited
further in our ongoing studies into the synthesis of piscibactin.
Removal of the chiral auxiliary from thiazolidine 4 with an
ethanolic solution of sodium ethoxide at 0 °C gave the
thiazolidine ethyl ester 13. Treatment of 13 with 75% TFA in
CH2Cl2 to remove the acetonide and Boc protecting groups
afforded 14. The dimer of 14 was formed through a disulfide
bridge, and the product was therefore treated with PPh3 and
HCl(c) in dioxane/water (Scheme 5).
Condensation of thiazolinic aldehyde 8a with 14 in the
presence of potassium acetate in MeOH at 0 °C gave 15, which
was immediately heated under reflux in toluene for 12 h to give
Scheme 5. Synthesis of Fragment B
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Letter
(12) Fukuzawa, S.; Matsuzawa, H.; Yoshimitsu, S. J. Org. Chem. 2000,
65, 1702−1706.
Siderophores are critical for the growth and virulence of the
producer pathogens, and the synthesis of prePcb can therefore
be useful to confirm the proposed biosynthesis pathway of Pcb
and enable its total synthesis. This knowledge could be applied
in the development of novel treatments against pasteurellosis
based on the iron uptake mechanism of these bacteria.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and spectroscopic data for all intermediates and
(14) Nelson, C. G.; Burke, T. R. J. Org. Chem. 2012, 77, 733−738.
(15) Duthaler, R. O.; Wyss, B. Eur. J. Org. Chem. 2011, 4667−4680.
(16) Roush, W. R.; Brown, B. B. J. Org. Chem. 1993, 58, 2162−2172.
(17) Matsumori, N.; Kaneno, D.; Murata, M.; Nakamura, H.;
Tachibana, K. J. Org. Chem. 1999, 64, 866−876.
1
copies of H and 13C NMR spectra details; HRESIMS and
HPLC chromatograms. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
́
(18) Rivault, F.; Schons, V.; Liebert, C.; Burger, A.; Sakr, E.;
■
Abdallah, M. A.; Schalk, I. J.; Mislin, G. L. A. Tetrahedron 2006, 62,
2247−2254.
*E-mail: jaime.rodriguez@udc.es.
*E-mail: carlos.jimenez@udc.es.
Notes
́ ́
(19) (a) Szilagyi, L.; Gyorgydeak, Z. J. Am. Chem. Soc. 1979, 101,
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427−432. (b) Schlegel, K.; Taraz, K.; Budzikiewicz, H. BioMetals 2004,
17, 409−414.
(20) (a) Hirama, M.; Oishi, T.; Uehara, H.; Inoue, M.; Maruyama,
M.; Oguri, H.; Satake, M. Science 2001, 294, 1904−1907. (b) Yu, R.
H.; Polniaszek, R. P.; Becker, M. W.; Cook, C. M.; Yu, L. H. L. Org.
Process Res. Dev. 2007, 11, 972−980.
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(b) Zamri, A.; Schalk, I. J.; Pattus, F.; Abdallah, I. J. Bioorg. Med. Chem.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Ministerio de
Economia y Competitividad (MINECO) (AGL2012-39274-
C02). We also thank the CESGA-ITS program for computa-
tional facilities and Servizos de Apoio a Investigacion
́
from the
University of A Coruna (SAI-UDC) for analytical support.
̃
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