Total synthesis and complete structural assignment of thiocillin i

Article abstract of DOI:10.1021/ja110166x

The total synthesis of the thiopeptide antibiotic, thiocillin I, is described. This work unequivocally defines the full structure (constitution and configuration) of the natural product as 1.

Full text of DOI:10.1021/ja110166x

ARTICLE
pubs.acs.org/JACS
Total Synthesis and Complete Structural Assignment of Thiocillin I
Virender S. Aulakh§ and Marco A. Ciufolini*
Department of Chemistry, The University of British Columbia, 2036 Main Mall Vancouver, BC V6T 1Z1, Canada
S Supporting Information
b
ABSTRACT: The total synthesis of the thiopeptide antibiotic, thiocillin I, is
described. This work unequivocally defines the full structure (constitution and
configuration) of the natural product as 1.
’ INTRODUCTION
Thiocillin I (1, Figure 1) and its congeners are thiopeptide
antibiotics1 isolated from Bacillus cereus.2 This substance has
become of special significance in recent times, notably through
the work of Walsh,3 in that biosynthetic investigations4 have
enabled the production of analogues displaying useful biological
properties through the genetic manipulation of producing organ-
isms. Such efforts could lead to novel anti-infective agents with
improved therapeutic properties, thereby alleviating the ongoing
“antibiotic crisis.”5
A parallel approach based on chemical synthesis may become
competitive with the recombinant strategy, if improved methods for
Figure 1. Structure of Thiocillin I.
the construction of key molecular subunits were to resolve at least
some of the problems associated with the “formidable challenge”3 of a
de novo assembly of thiopeptide architectures. A nettlesome aspect of
Scheme 1. One-Step BohlmannÀRahtz Synthesis of Thio-
such endeavors is the preparation of the pyridineÀthiazole cluster
found at the core of those antibiotics, noteworthy solutions for which
have materialized over the past few years. For instance, the construc-
tion of the core of thiostrepton and related compounds incorporating
a reduced pyridine inspired the development of remarkable biomi-
metic cycloadditions of azadienes;6 efforts aiming to produce fully
aromatic cores by organometallic functionalization of a preexisting
pyridine have engendered important advances in Pd-mediated cou-
pling reactions,7 while approaches to a de novo construction of a
pyridine nucleus fully decorated with the requisite complement of
thiazolyl substituents, or at least with as many such thiazoles as
possible, have produced significant refinements of the Hantzsch8 and
the BohlmannÀRahtz9 reactions. The foregoing notwithstanding, the
construction of thiopeptide cores remains a lengthy proposition,
generally requiring more than 10 linear steps. This difficulty continues
to stimulate research aiming to simplify the task.
peptide Cores
findings, we devised a method to prepare pyridines 4through the one-
step, three-component condensation of fragments 2 and 3 in the
presence of NH4OAc (Scheme 1).11 Heterocyclic clusters such as 4
embody the core structures of a number of thiopeptides incorporating
a fully aromatic pyridine. In this article, we illustrate an application of
In that connection, the work of Bagley has shown that a Bohlmann-
Rahtz avenue to the pyridine nucleus provides much opportunity for
the development of more concise pathways.10 Expanding upon these
Received: November 11, 2010
Published: March 29, 2011
r
2011 American Chemical Society
5900
dx.doi.org/10.1021/ja110166x J. Am. Chem. Soc. 2011, 133, 5900–5904
|

Journal of the American Chemical Society
ARTICLE
Scheme 2. Oxidation of 2-Methylthiazoles to
2-Formylthiazoles
Scheme 3. Preparation of Ynones Required for the
BohlmannÀRahtz Reaction
Scheme 4. The Bagley One-Step Variant of the Bohl-
mannÀRahtz Reaction
the new technique in what appears to be the first total synthesis12 of 1.
In addition to resolving a number of technical difficulties inherent to
past synthetic schemes, this exercise also provides full structural proof
for the natural product. Indeed, the constitution of 1 was reasonably
secure at the onset of the present work; however, its configuration
remained to be ascertained. The similarity of thiocillin to micrococcin
P113 suggested the configuration depicted in Figure 1: a surmise that
was ultimately confirmed as detailed herein.
’ RESULTS AND DISCUSSION
Current methods for the preparation thiopeptide cores via a
BohlmannÀRahtz pyridine synthesis could be improved in two
respects. First, ynones 3 required for this chemistry derive from
2-formylthiazoles such as 5 and 6 (Scheme 2). Past routes to
these materials have evolved from glycolonitrile (7)14 or diethox-
yacetonitrile (8),15 the elaboration of which to the desired
subgoals, especially to 6, requires up to 8 steps. A more concise
avenue to 5 and 6 is surely desirable. A good solution in that sense
appeared to be the SeO2 oxidation of 2-methylthiazoles such as 9
and 10: a transformation that was undocumented at the onset of
our endeavor. Indeed, substrates 9 and 10 resisted the action of
SeO2 in refluxing ethanol or dioxane. However, oxidation to the
corresponding 2-formylthiazoles occurred easily in refluxing
acetic acid over 12 h.11 The reaction of 9 was best carried out
at a concentration of 0.5 M, whereupon the desired 5 was
obtained in 55% yield after purification. A significant byproduct
of the reaction was 4-carbethoxythiazole (21% yield), which was
readily removed by chromatography. This compound presum-
ably arose through overoxidation of 5 to the corresponding acid,
followed by decarboxylation. Higher concentrations (1 M or
greater) caused the formation of unacceptably large quantities of
4-carbethoxythiazole, while more dilute conditions slowed down
the rate of the reaction to a considerable degree. The oxidation of
10 under the same conditions also proceeded in 55% yield (after
chromatography), but no discrete byproducts could be detected
in this case.16 The formylthiazoles thus obtained were smoothly
advanced to ynones 13 and 15 by addition of ethynylmagnesium
bromide and oxidation of the resultant carbinols with IBX17 or
with DessÀMartin reagent, respectively (Scheme 3).11
Second, a conventional BohlmannÀRahtz reaction requires
the prior conversion of the enolizable component (2 in the
present case) into an enamine, which subsequently undergoes
cyclocondensation with the ynone in a second step of the
sequence. A variant of this chemistry devised by Bagley10a
circumvents the need for a two-step synthesis by causing an
enolizable ketone, 16, an ynone, 17, and NH4OAc to combine in
refluxing ethanol, leading to the expected pyridine 20 in excellent
yield (Scheme 4). The transformation presumably proceeds via
in situ formation of enamine 18, which then enters the actual
BohlmanÀRahtz pathway. We have utilized this procedure for
various synthetic objectives, and indeed we have found that
the Bagley method generally affords better results than the
traditional, stepwise protocol. However, the chemistry appears
to perform well only if the enolizable component is part of a
1,3-dicarbonyl system, for example, if R2 = COOEt; otherwise,
the enamine must still be prepared separately, and then com-
bined with the ynone in refluxing EtOH. To illustrate, the Bagley
synthesis of the amythiamicin core18 required the conversion of a
ketone structurally related to 2 into the corresponding enamine,
which then reacted with a congener of 3 to afford a pyridine of the
type 4. A method for the direct merger of 2, 3, and NH4OAc
would clearly be advantageous.
Initial attempts to combine 13 or 15 with the known 218 under
Bagley conditions met with failure. Analogy with the difficulties
encountered in the Ugi reaction of R-amino acids with aromatic
aldehydes19 suggested that the problem was attributable to an
insufficiently rapid rate of formation of an enamine such as 18.
5901
dx.doi.org/10.1021/ja110166x |J. Am. Chem. Soc. 2011, 133, 5900–5904

Journal of the American Chemical Society
ARTICLE
Scheme 5. Preparation of the Core of Thiocillin I
Scheme 6. Synthesis of the “Western” Segment of Thiocillin Ia
While in the Ugi context the appropriate remedy proved to be
Bronsted acid-assisted Lewis acid (“BLA”) catalysis,20 plain
Bronsted acid catalysis afforded a perfectly adequate solution in
the present case. Indeed, Bagley had established that particular
BohlmannÀRahtz reactionsproceed more efficiently in mixtures of
toluene and acetic acid (typically 5:1) as the solvent.21 Experiments
aiming to induce the combination of 13 and 15 with 21 and
NH4OAc in media of progressively greater acidity revealed that the
transformation proceeded best in refluxing acetic acid,11 that is, in a
manner analogous to the EidenÀHerdeis pyridine synthesis.22
Pyridines 22 and 23 were thus obtained in 63 and 52% yield,
respectively, after chromatographic purification (Scheme 5). It
should be noted that under the present conditions the TBS group
originally present in 21 was replaced by an acetyl unit, presumably
through acid-promoted desilylation and subsequent Fischer ester-
ification of the liberated alcohol. Compound 23 is recognized as a
protected form of the core cluster of micrococcin P1ÀP2,13
thiocillin I,2 and YM266183.23 Its preparation by the present
method (4 linear steps from 10; 20% overall yield) compares
favorably with earlier routes. On the other hand, commercial 10 is
costly; so, subsequent work relied on synthetic 10 prepared from 9
in 3 steps.24 This notwithstanding, the route to 23 from 9 (7 linear
steps, 16% overall yield) was still preferable to other known
approaches.
a (a) SO3 pyr, DMSO, Et3N, CH2Cl2 0 °C to rt; (b) methyl cysteinate
3
HCl, NaHCO3, 1:1 MeOH, H2O, rt, 6 h; (c) MnO2, MeCN, reflux, 6 h,
3
30% (a)À(c); (d) 1:4 TFA/CH2Cl2, rt, 2 h; (e) EDCI, HOBt, CH2Cl2,
rt, 12 h, 65% of 30 from 27 (d)À(e); (f) MsCl, Et3N, then DBU; 32% of
34 from 30 (d)À(f); (g) TFA, MeOH.
the dehydroaminoacid motif) followed by deblocking of the serinyl
segment.
The total synthesis of thiocillin I continued with ParekhÀDoering
oxidation of the known (R)-alcohol 2425 (Scheme 6). Im-
mediate condensation of the sensitive aldehyde 25 with methyl
cysteinate hydrochloride, under the conditions of the Shioiri thiazole
synthesis,26 gave a mixture of diastereomers of the expected thiazo-
line 26.27 In crude form, the latter was immediately aromatized to
thiazole 27. As thoroughly detailed in the literature, only so-called
At this juncture, we faced a minor dilemma. In principle, the
synthesis of 1 could be completed through the union of
fragments 23 and 35 by methodology analogous to that devised
earlier for the synthesis of micrococcin.13 However, the modified
BohlmannÀRahtz reaction outlined in Scheme 5 had afforded
compound 23 as an acetate ester. Using this compound as such in
subsequent steps would have forced either a revision of late-stage
protecting group tactics (Scheme 7: the appropriate hydroxyl
groups in fragments 38 and 39 must be orthogonally protected),
or a reoptimization of the final sequence with an acetyl-protected
pyridine segment. In the interest of time, we opted to convert 23
into 36, so that thiocillin I could be reached by retracing the
chemistry devised earlier (Scheme 7). Thus, ester saponifi-
cation and N-BOC activation of the oxazolidinone prepared
the molecule for coupling with the known 39.13,14 The emerging
28
“chemical” MnO2 performed adequately in this oxidative step.
The BOC group in 27 was then released without incident by the
customary TFA treatment. No evidence of acid-promoted elimina-
tion of the tertiary alcohol, either in 27 or in later synthetic
intermediates incorporating such a fragment, was ever garnered as
a consequence of TFA treatment. Amine 28 was then coupled with
the previously described acid 29.14 The resultant 30 was again
N-deblocked and amine 31 was coupled with the known29 acid 32,
resulting in formation of 33. This material was advanced to amine
trifluoroacetate salt 35 by mesylation/dehydration (introduction of
5902
dx.doi.org/10.1021/ja110166x |J. Am. Chem. Soc. 2011, 133, 5900–5904

Journal of the American Chemical Society
ARTICLE
Scheme 7. Elaboration of the Complete “Eastern” Portion of
Scheme 8. Total Synthesis of Thiocillin Ia
Thiocillin Ia
a (a) BOP-Cl, 35, Et3N, MeCN; (b) LiOH, 1:1 THF-H2O; (c) TFA,
CH2Cl2, 4 h; (d) DPPA, Et3N, DMF, 5 h, 12% from 43.
a (a) K2CO3, 1:1 CH2Cl2-EtOH, 48 h; (b) TBS-Cl, imidazole DMF, rt,
12 h; (c) LiOH, 1:1 THF-H2O, rt, 4 h; (d) BOC2O, Et3N, CH2Cl2; (e)
BOP-Cl, Et3N, MeCN, rt, 6 h, 20% (a)À(e); (f) MsCl, Et3N, CH2Cl2, 2
h, then DBU, 3 h; (g) TBAF, THF, 3 h; (h) DessÀMartin periodinane,
CH2Cl2, rt, 5 h, 84% (f)À(h); (i) NaClO2, 2-methyl-2-butene, NaH2-
PO4, aq. THF, rt, 4 h.
Present Addresses
§Novartis, Inc., Cambridge, MA.
’ ACKNOWLEDGMENT
We thank the University of British Columbia, the Canada
Research Chair program, NSERC, CIHR, and MerckFrosst
Canada, for support of our program. We are grateful to Dr.
Dan Sorensen, of MerckFrosst Canada, for the HPLC purifica-
tion and the spectroscopy of synthetic 1, and especially to Dr.
Hiroki Satou, of the Shionogi Pharmaceutical Company, Japan,
for a gift of authentic thiocillin I.
40 underwent smooth dehydration to 41 upon reaction with
methanesulfonyl chloride in the presence of DBU, as a prelude to
deblocking of the primary alcohol and stepwise oxidation thereof
to carboxylic acid 44. This material was very polar and difficult to
purify; so it was best advanced directly to the final steps of the
synthesis. Crude 44 was thus coupled with fragment 35 to yield
45 (Scheme 8). Global deblocking followed by macrocyclization
of monoseco intermediate 46 with DPPA30 afforded 1, which
proved to be identical31 to natural thiocillin I, kindly provided by
the Shionogi Co. Japan.
The modular nature of the synthesis just described enables the
preparation of natural thiopeptides and their analogues in a
straightforward manner, thus, bringing medicinal chemistry re-
search on such antibiotics within the realm of realistic possibility.
Results in this area will be reported in due course.
’ REFERENCES
(1) Reviews: (a) Bagley, M. C.; Dale, J. W.; Merritt, E. A.; Xiong, X.
Chem. Rev. 2005, 105, 685. (b) Hughes, R. A.; Moody, C. J. Angew.
Chem., Int. Ed. 2007, 46, 7930. (c) Arndt, H.-D.; Schoof, S.; Lu, J.-Y.
Angew. Chem., Int. Ed. 2009, 48, 6770.
(2) (a) Shoji, J.; Hinoo, H.; Wakisaka, Y.; Koizumi, K.; Mayama, M.;
Matsuura, S.; Matsumoto, K. J. Antibiot. 1976, 24, 366. (b) Shoji, J.; Kato,
T.; Yoshimura, Y.; Tori, K. J. Antibiot. 1981, 29, 1126.
(3) (a) Acker, M. G.; Bowers, A. A.; Walsh, C. J. J. Am. Chem. Soc.
2009, 131, 17563. (b) Bowers, A. A.; Walsh, C. J.; Acker, M. G. J. Am.
Chem. Soc. 2010, 132, 12182. (c) Bowers, A. A.; Acker, M. G.; Koglin, A.;
Walsh, C. J. J. Am. Chem. Soc. 2010, 132, 7519. (d) Walsh, C. J.; Acker,
M. G.; Bowers, A. A. J. Biol. Chem. 2010, 286, 27525.
(4) (a) Wieland Brown, L. C.; Acker, M. G.; Clardy, J.; Walsh, C. T.;
Fischbach, M. A. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 2559. (b) Liao,
R.; Duan, L.; Lei, C.; Pan, H.; Ding, Y.; Zhang, Q.; Chen, D.; Shen, B.;
Yu., Y. Chem. Biol. 2009, 16, 141.
’ ASSOCIATED CONTENT
S
Supporting Information. Full experimental details and
b
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
(5) (a) von Nussbaum, F.; Brands, M.; Hinzen, B.; Weigand, S.;
H€abich, D. Angew. Chem., Int. Ed. 2006, 45, 5072. (b) Isaacs, D. Indian
Pediat. 2005, 42, 9 and references cited therein. (c) Hausler, T. Viruses vs
Corresponding Author
ciufi@chem.ubc.ca
5903
dx.doi.org/10.1021/ja110166x |J. Am. Chem. Soc. 2011, 133, 5900–5904

Journal of the American Chemical Society
ARTICLE
Superbugs: A Solution to the Antibiotic Crisis? Palgrave Macmillan:
Houndsmill, U.K., 2006.
(6) (a) Nicolaou, K. C.; Nevalainen, M.; Safina, B. S.; Zak, M.; Bulat,
S. Angew. Chem., Int. Ed. 2002, 41, 1941. (b) Moody, C. J; Hughes, R. A.;
Thompson, S. P.; Alcaraz, L. Chem. Commun. 2002, 1760. See also
ref 1b.
26, 5155. (c) Hamada, Y.; Shibata, M.; Shioiri, T. Tetrahedron Lett. 1985,
26, 5159. (d) Sugiyama, H.; Yokokawa, F.; Shioiri, T. Org. Lett. 2000,
2, 2149.
(27) For a related construction see ref 12a.
(28) This special grade of MnO2 is available at modest cost from the
Wako Chemical Co., Japan.
(7) Noteworthy in this regard is the work of Bach: (a) Muller, H. M.;
Delgado, O.; Bach, T. Angew. Chem., Int. Ed. 2007, 46, 4771. (b)
Delgado, O.; Muller, H. M.; Bach, T. Chem.—Eur. J. 2008, 14, 2322.
See also: (c) Martin, T.; Laguerre, C.; Hoarau, C.; Marsais, F. Org. Lett.
2009, 11, 3690. (d) Martin, T.; Verrier, C.; Hoarau, C.; Marsais, F. Org.
Lett. 2008, 10, 2909.
(8) Ciufolini, M. A.; Shen, Y. C. J. Org. Chem. 1997, 62, 3804.
(9) (a) Bohlmann, F.; Rahtz, D. Chem. Ber. 1957, 90, 2265. Review:
(b) Bagley, M. C.; Glover, C.; Merritt, E. A. Synlett 2007, 2459. Moody
introduced the reaction into current synthetic practice during his
pioneering synthesis of promothiocin: (c) Moody, C. J.; Bagley, M. C.
Chem. Commun. 1998, 2049. (d) Bagley, M. C.; Bashford, K. E.;
Hesketh, C. L.; Moody, C. J. J. Am. Chem. Soc. 2000, 122, 3301.
(10) (a) Bagley, M. C.; Dale, J. W.; Bower, J. Chem. Commun.
2002, 1682. (b) Bagley, M. C.; Chapaneri, K.; Dale, J. W.; Xiong, X.;
Bower, J. J. Org. Chem. 2005, 70, 1389. See also: (c) Bagley, M. C.; Brace,
C.; Dale, J. W.; Ohnesorge, M.; Phillips, N. G.; Xiong, X.; Bower, J.
J. Chem. Soc., Perkin Trans. 1 2002, 1663.
(29) Nakamura, Y.; Shin, C.-G.; Umemura, K.; Yoshimura, J. Chem.
Lett. 1992, 1005.
(30) (a) Shioiri, T.; Ninomiya, K.; Yamada, S. J. Am. Chem. Soc.
1972, 94, 6203. Use in macrolactamization reactions: (b) Hamada, Y.;
Shioiri, T. Chem. Rev. 2005, 105, 4441. Reviews: (d) Shioiri, T. TCIMail
2008, 134, 1 (http://www.tci-asiapacific.com/tcimail/backnumber/
article/134drE.pdf). (c) Liang, H. Synlett 2008, 2554.
(31) One of the referees recommended that an NMR spectrum of a
mixture of natural and synthetic thiocillin I be provided. Unfortunately,
the sample of synthetic 1 had deteriorated by the time that the
suggestion was made, preventing us from obtaining said spectrum. On
the other hand, stacked plots of the 1H NMR spectrum of natural and
synthetic thiocillin I (provided as Supporting Information) reveal that all
chemical shifts of nonexchangeable protons match to within (0.01
ppm and coupling constants likewise match. This leaves no doubt that
synthetic 1 is structurally identical to authentic thiocilin I.
(11) Aulakh, V. S.; Ciufolini, M. A. J. Org. Chem. 2009, 74, 5750.
(12) Synthetic studies: (a) Yonezawa, Y.; Saito, H.; Suzuki, S.; Shin,
C.-G. Heterocycles 2002, 57, 903. (b) Suzuki, S.; Yonezawa, Y.; Shin,
C.-G. Chem. Lett. 2004, 33, 814.
(13) Review: (a) Ciufolini, M. A.; Lefranc, D. Nat. Prod. Rep. 2010,
27, 330. Total synthesis and full structural elucidation: (b) Lefranc, D.;
Ciufolini, M. A. Angew. Chem., Int. Ed. 2009, 48, 4198.
(14) See refs 8 and,13 as well as: Ciufolini, M. A.; Shen, Y.-C. Org.
Lett. 1999, 1, 1843.
(15) Merritt, E. A.; Bagley, M. C. Synlett 2007, 954.
(16) This reaction forms a black precipitate of reduced forms of Se. It
is certainly possible that said precipitate entrains byproducts formed
during the reaction.
(17) The use of IBX for the oxidation of such carbinols was
introduced by Moody (ref 9c,9d). We found that the DessÀMartin
periodinane performed better than IBX in the oxidation of 14.
(18) Bagley, M. C.; Dale, J. W.; Jenkins, R. L.; Bower, J. Chem.
Commun. 2004, 102.
(19) Godet, T.; Bonvin, Y.; Vincent, G.; Ciufolini, M. A. Org. Lett.
2004, 6, 3281.
(20) (a) Ishihara, K.; Yamamoto, H. J. Am. Chem. Soc. 1994,
116, 1561. (b) Ishihara, K.; Kaneeda, M.; Yamamoto, H. J. Am. Chem.
Soc. 1994, 116, 11179. Review: (c) Yamamoto, H.; Cheon, C. H. In
Catalytic Asymmetric Synthesis, 3rd ed.; Ojima, I., Ed.; John Wiley &
Sons: Hoboken, NJ, 2010; Chapter 3, pp 119ff. Early examples: (d)
Deaton, M. V.; Ciufolini, M. A. Tetrahedron Lett. 1993, 34, 2409.
See also: (e) Ciufolini, M. A.; Zhu, S.; Deaton, M. V. J. Org. Chem.
1997, 62, 7806.
(21) (a) Bagley, M. C.; Chapaneri, K.; Dale, J. W.; Xiong, X.; Bower,
J. J. Org. Chem. 2005, 70, 1389. See also: (b) Bagley, M. C.; Brace, C.;
Dale, J. W.; Ohnesorge, M.; Phillips, N. G.; Xiong, X.; Bower, J. J. Chem.
Soc., Perkin Trans. 1 2002, 1663.
(22) Eiden, F.; Herdeis, C. Arch. Pharm. 1977, 310, 744. A similar
reaction may be carried out in ionic liquids: Karthikeyan, G.; Perumal,
P. T. Can. J. Chem. 2005, 83, 1746.
(23) Nagai, K.; Kamigiri, K.; Arao, N.; Suzumura, K.-I.; Kawano, Y.;
Yamaoka, M.; Zhang, H.; Watanabe, M.; Suzuki, K. J. Antibiot. 2003,
56, 123.
(24) Ref 11 as well as Huang, L.; Quada, J. C., Jr.; Lown, J. W. Het.
Commun. 1995, 1, 335.
(25) Ciufolini, M. A; Valognes, D.; Xi, N. Angew. Chem., Int. Ed.
2000, 39, 2493 and references cited therein.
(26) (a) Hamada, Y.; Kohda, K.; Shioiri, T. Tetrahedron Lett. 1984,
25, 5303. (b) Hamada, Y.; Shibata, M.; Shioiri, T. Tetrahedron Lett. 1985,
5904
dx.doi.org/10.1021/ja110166x |J. Am. Chem. Soc. 2011, 133, 5900–5904