Stereoselective synthesis of the γ-lactam hydrolysate of the thiopeptide cyclothiazomycin

Article abstract of DOI:10.1021/ol0485870

(Chemical Equation Presented) Bohlmann-Rahtz pyridine synthesis of a chiral nonracemic enamine and thiazolylpropynone gives a terminal-protected pyridine-containing γ-amino acid in high optical purity in a sequential one-pot multicomponent reaction that proceeds with total control of regiochemistry and with minimal racemization. Further elaboration has established the synthesis of the γ-lactam acidic hydrolysate of the macrocyclic thiopeptide antibiotic cyclothiazomycin, a selective renin inhibitor, in only four steps and 30% overall yield and has confirmed its structure.

Full text of DOI:10.1021/ol0485870

ORGANIC
LETTERS
2004
Vol. 6, No. 19
3401-3404
Stereoselective Synthesis of the
γ-Lactam Hydrolysate of the
Thiopeptide Cyclothiazomycin
Mark C. Bagley* and Xin Xiong
School of Chemistry, Cardiff UniVersity, P.O. Box 912, Cardiff, CF10 3TB, UK
bagleymc@cf.ac.uk
Received July 21, 2004
ABSTRACT
Bohlmann−Rahtz pyridine synthesis of a chiral nonracemic enamine and thiazolylpropynone gives a terminal-protected pyridine-containing
γ-amino acid in high optical purity in a sequential one-pot multicomponent reaction that proceeds with total control of regiochemistry and
with minimal racemization. Further elaboration has established the synthesis of the γ-lactam acidic hydrolysate of the macrocyclic thiopeptide
antibiotic cyclothiazomycin, a selective renin inhibitor, in only four steps and 30% overall yield and has confirmed its structure.
The thiopeptide or thiazolylpeptide antibiotics, according to
Be´rdy’s structural classification,1 comprise at least 29
different families of sulfur-containing macrocyclic peptide
secondary metabolites produced by actinomycetes, Gram-
positive mycelial sporulating bacteria. They possess a wide
range of biological properties, most inhibiting bacterial
protein synthesis by impeding conformational changes in the
bacterial ribosome, binding to the complex of 23S rRNA
and protein L11,2 or by preventing the formation of the
aminoacyl-tRNA‚Ef-Tu‚GTP ternary complex.3 Cyclothi-
azomycin (1), one of 76 structurally distinct actinomycete
thiopeptide natural products, was isolated from the fermenta-
tion broth of Streptomyces sp. NR0516 obtained from a soil
sample collected at Kanagawa, Japan, and is a selective
inhibitor of human plasma renin at an IC50 of 1.7 µM.4 Its
unique structure, consisting of a (1-amino-1-ethyl)pyridine
heterocyclic domain embedded in two macrocyclic peptide
loops,5 and absolute stereochemistry were determined using
a combination of spectroscopic and analytical methods,
supported by chemical degradation studies that isolated a
heterocyclic amino acid as γ-lactam 2 and saramycetic acid
I (3) from the acid hydrolysate of the natural product
(Scheme 1).6 The preparation of some of the unusual
structural motifs present in cyclothiazomycin has attracted
synthetic interest,7,8 but the stereospecific synthesis of its
γ-amino acid central heterocyclic domain has not been
addressed. Heterocyclic amino acids are of particular interest
as modified components of peptides, proteins, peptide nucleic
acids (PNAs), and peptidoglycans and can function as
mediators of neuronal signal transduction.9 Although the
stereospecific synthesis of pyrimidyl, isoxazolyl, pyrazolyl,
and pyridyl R-amino acids9 and conformationally constrained
(5) Aoki, M.; Ohtsuka, T.; Itezono, Y.; Yokose, K.; Furihata, K.; Seto,
H. Tetrahedron Lett. 1991, 32, 221.
(1) Be´rdy, J. AdV. Appl. Microbiol. 1974, 18, 309.
(2) Porse, B. T.; Leviev, I.; Mankin, A. S.; Garrett, R. A. J. Mol. Biol.
1998, 276, 391.
(3) Anborgh, P. H.; Parmeggiani, A. EMBO J. 1991, 10, 779.
(4) Aoki, M.; Ohtsuka, T.; Yamada, M.; Ohba, Y.; Yoshizaki, H.;
Yasuno, H.; Sano, T.; Watanabe, J.; Yokose, K. J. Antibiot. 1991, 44, 582.
(6) Aoki, M.; Ohtsuka, T.; Itezono, Y.; Yokose, K.; Furihata, K.; Seto,
H. Tetrahedron Lett. 1991, 32, 217.
(7) Shin, C.; Okabe, A.; Ito, A.; Ito, A.; Yonezawa, Y. Bull. Chem. Soc.
Jpn. 2002, 75, 1583.
(8) Endoh, N.; Yonezawa, Y.; Shin, C. Bull. Chem. Soc. Jpn. 2003, 76,
643.
10.1021/ol0485870 CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/26/2004

Scheme 1. Cyclothiazomycin (1) and Its Hydrolysates
Scheme 2. Racemization in Heteroannulation Reactionsa
a Top: Racemization in the Hantzsch synthesis of thiazole-
containing δ-amino acid 5. Bottom: Proposed Bohlmann-Rahtz
synthesis of pyridine-containing (R)-γ-amino acid 8.
without racemization (Scheme 2). The synthesis of a related
heterocyclic δ-amino acid, alanine-derived thiazole 5, by
Hantzsch cyclocondensation of the corresponding thioamide
and ethyl bromopyruvate, results in considerable racemiza-
tion due to the acid instability of hydroxythiazoline 4
(Scheme 2).10a The significant challenge in a stereospecific
approach to the cyclothiazomycin lactam 2 concerns how to
prevent the loss of optical purity in an analogous process by
epimerization of 6-hydroxy-5,6-dihydropyridine 7 when the
double-bond isomerization/cyclodehydration Bohlmann-
Rahtz sequence inherent in the reaction requires high
temperatures or an acid catalyst in order to facilitate pyridine
8 heteroannulation.15
To this end, reacting the known (R)-ketoester 916 with
ammonium acetate gave enamine 6; however, during its
formation or purification, this chiral intermediate racemized
on exposure to heat (ethanol at reflux), Brønsted acids (5:1
toluene-acetic acid), or silica gel and could only be isolated
in 70% yield and 92% ee by carrying out the reaction at
room temperature in ethanol and using the crude material
without purification (Scheme 3).
azole-containing δ-amino acids has received considerable
attention,10 the preparation of the corresponding γ-amino
acids has been somewhat neglected. This communication
describes the stereoselective synthesis of conformationally
constrained pyridine-containing γ-amino acids and prepares
both the cyclothiazomycin domain and its lactam hydrolysate
2 in order to verify natural product structure and facilitate
its future synthesis.
Our approach utilizes a Bohlmann-Rahtz reaction11 to
assemble the pyridine domain from acyclic precursors
starting from the corresponding amino acid, a highly suc-
cessful heteroannulation method used in the synthesis of
promothiocin A,12 dimethyl sulfomycinamate,13 and the tris-
(thiazolyl)pyridine domain of the amythiamicins.14 However,
the effectiveness of a Bohlmann-Rahtz strategy, in this case
hitherto unreported, relies upon the ready availability of chiral
enamine 6, which must proceed through the heteroannulation
(9) For lead references and recent examples of the synthesis of
heterocyclic R-amino acids, see: (a) Dondoni, A.; Massi, A.; Minghini,
E.; Sabbatini, S.; Bertolasi, V. J. Org. Chem. 2003, 68, 6172. (b) Adlington,
R. M.; Baldwin, J. E.; Catterick, D.; Pritchard, G. J.; Tang, L. T. J. Chem.
Soc., Perkin Trans. 1 2000, 303. (c) Adlington, R. M.; Baldwin, J. E.;
Catterick, D.; Pritchard, G. J.; Tang, L. T. J. Chem. Soc., Perkin Trans. 1
2000, 2311.
(10) (a) Bredenkamp, M. W.; Holzapfel, C. W.; Zyl, W. J. Synth.
Commun. 1990, 20, 2235. (b) Aguilar, E.; Meyers, A. I. Tetrahedron Lett.
1994, 35, 2473. (c) Bertram, A.; Hannam, J. S.; Jolliffe, K. A.; Gonzalez-
Lopez de Turiso, F.; Pattenden, G. Synlett 1999, 1723. (d) Bertram, A.;
Blake, A. J.; Gonzalez-Lopez de Turiso, F.; Hannam, J. S.; Jolliffe, K. A.;
Pattenden, G.; Skae, M. Tetrahedron 2003, 59, 6979.
The Bohlmann-Rahtz reaction of enamine 6 and readily
available propynone 1014 under traditional heteroannulation
conditions, Michael addition at 50 °C for 10 min followed
by cyclodehydration at 135 °C (entry 1), gave pyridine 8a
as a single regioisomer in 73% yield, albeit only in 14% ee.
The microwave-assisted reaction17 resulted in appreciable loss
(11) Bohlmann, F.; Rahtz, D. Chem. Ber. 1957, 90, 2265.
(12) Bagley, M. C.; Bashford, K. E.; Hesketh, C. L.; Moody, C. J. J.
Am. Chem. Soc. 2000, 122, 3301.
(13) Bagley, M. C.; Dale, J. W.; Xiong, X.; Bower, J. Org. Lett. 2003,
5, 4421.
(14) Bagley, M. C.; Dale, J. W.; Jenkins, R. L.; Bower, J. Chem.
Commun. 2004, 102.
(15) 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.
3402
Org. Lett., Vol. 6, No. 19, 2004

N-terminal-protected amino acid 8a was isolated directly in
55% yield and 96% ee from this one-pot reaction, demon-
strating a facile stereoselective route to pyridyl γ-amino acids
from the corresponding â-ketoester that avoids isolation of
chiral enamine intermediates.
Scheme 3. Synthesis of the Cyclothiazomycin Domain
From chiral nonracemic pyridine 8a it was anticipated that
the total synthesis of the cyclothiazomycin lactam 2 would
be realized simply by hydrolysis in accordance with the
degradation studies on the natural product (Scheme 1).6
To our surprise, when pyridine 8a was heated to 110 °C
in 6 N hydrochloric acid, a complex mixture of products
was obtained. Under milder conditions, when the reaction
was repeated at room temperature, the N- and C-terminal
protecting groups were cleaved but the ethyl thiazole-4-
carboxylate group remained untouched to give γ-lactam 12
in 94% ee (Scheme 4). Base-catalyzed hydrolysis at room
Scheme 4. Synthesis of the Cyclothiazomycin Hydrolysate,
γ-Lactam 2
of material but did improve the optical purity somewhat
(entry 2). As predicted, the one-pot acid-catalyzed heteroan-
nulation process18 was much more efficient but did little to
prevent racemization throughout the process (entry 3),
presumably as a consequence of the acid instability of
hydroxydihydropyridine 7. However, in combination with a
Michael addition under traditional Bohlmann-Rahtz condi-
tions, the acid-catalyzed cyclodehydration of the diamino-
dienone intermediate 11 at 60 °C caused a significant increase
in the optical activity of pyridine 8a (entry 4).
Changing the cyclodehydrating agent from a Brønsted acid
to N-iodosuccinimide (NIS), a reagent effective in this
transformation at 0 °C,19 further improved the stereoselec-
tivity of the process (entry 5), giving pyridine 8a in 92%
ee. Although an excellent result, the optical purity of the
product was still limited by the stereochemical instability of
enamine 6 following isolation of this Bohlmann-Rahtz
precursor.
To overcome the limiting racemization of enamine 6, a
new sequential one-pot process was investigated for the
stereoselective synthesis of the cyclothiazomycin domain.
Ammonium acetate was added to a solution of â-ketoester
9 (>99% ee) in ethanol. After 4 h at room temperature,
thiazolylpropynone 10 was added and the mixture was stirred
to complete the Michael addition. The mixture was then
cooled to 0 °C, and N-iodosuccinimide was added (Scheme
3, entry 6). Pleasingly, after column chromatography, C- and
temperature, with acidic workup, was expected to complete
the synthesis of the cyclothiazomycin hydrolysate 2, but the
UV and 1H and 13C NMR spectroscopic data of the material
isolated from the natural product,6 which was identified as
hydrochloride 13, failed to match that of the colorless
precipitate obtained from the reaction. Even more surprising
was the observation that elution on a solid-phase extraction
column, a procedure used to isolate hydrolysate 2 from the
natural product,20,21 returned only the pyridinium salt.22
However, when hydrochloride 13 was stirred with 1 equiv
of polymer-supported 4-(dimethylamino)pyridine (PS-DMAP)
(16) Honore´, T.; Hjeds, H.; Krogsgaard-Larsen, P.; Christiansen, T. R.
Eur. J. Med. Chem. 1978, 13, 429.
(17) Bagley, M. C.; Lunn, R.; Xiong, X. Tetrahedron Lett. 2002, 43,
8331.
(18) Bagley, M. C.; Dale, J. W.; Bower, J. Synlett 2001, 1149.
(19) Bagley, M. C.; Glover, C.; Merritt, E. A.; Xiong, X. Synlett 2004,
811.
(20) We are grateful to Masahiro Aoki (Chugai Pharmaceutical Co., Ltd.,
Japan) and Prof. Haruo Seto (Institute of Applied Microbiology, The
University of Tokyo, Japan) for further details.
Org. Lett., Vol. 6, No. 19, 2004
3403

in dry methanol for 18 h, filtered, and evaporated, chiral
nonracemic 2-(2-pyridyl)thiazole-4-carboxylic acid 2 was
obtained in 80% yield and 88% ee, the spectroscopic
characteristics and chromatographic behavior of which were
in agreement with reported literature data.6,20,23 The entire
synthetic sequence gives the C- and N-terminal-protected
heterocyclic domain of cyclothiazomycin in 96% ee and 55%
yield in only one step from the known (R)-â-ketoester 9 and
generates the γ-lactam hydrolysate of the natural product in
88% ee and 30% overall yield in only four steps.
In summary, we have described the stereoselective syn-
thesis of a chiral diaminoalkenoate, diaminodienone, and the
γ-lactam form of a heterocyclic amino acid, corresponding
to Bohlmann-Rahtz enamine, dienamine intermediate, and
pyridine product, respectively, in high optical purity, good
yield, and very few synthetic steps from a chiral pool
precursor with total regiocontrol. The utility of our facile
approach, centered on a sequential one-pot, three-component
process, has been demonstrated in the first successful
synthesis of the cyclothiazomycin γ-lactam hydrolysate,
which verifies its structure, with minimum racemization and
now is expected to be used in the total synthesis of the
actinomycete thiopeptide secondary metabolite.
Acknowledgment. We thank the Great Britain-China
Educational Trust (award to X. Xiong), Henry Lester Trust,
Ltd. (award to X. Xiong), and Royal Society for their
generous support and the E.P.S.R.C. Mass Spectrometry
Service, Swansea, for high-resolution spectra.
Supporting Information Available: Experimental pro-
cedures and characterization data for all compounds, includ-
ing methods used for the determination of optical purity. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(21) Hydrolysis of 1 (refs 6 and 20) in aq HCl (6 N) at 110 °C under
nitrogen gave a residue that was suspended in water and transferred to a
solid-phase extraction column (Sep-pak C18), gradient eluting with water
and methanol. Purification by column chromatography on Sephadex LH20,
eluting with methanol, gave hydrolysate 2 as a film, Rf 0.64 on silica, eluting
with nBuOH-AcOH-water (3:1:1).
OL0485870
(22) Pyridinium hydrochloride 13 was obtained as a colorless solid, Rf
0.58 on silica, eluting with nBuOH-AcOH-water (3:1:1), mp 287 °C dec
(from aqueous methanol): [R] +30.8 (c 0.26, MeOH); UV λmax (MeOH)
234 nm (ꢀ 12 400), 314 nm (ꢀ 17 000). See Supporting Information for
more experimental details and spectroscopic data.
(23) Dihydropyrrolo[3,4-b]pyridine 2 was obtained as a colorless solid,
mp 185 °C (dec): [R] +22.7 (c 0.12, MeOH); UV λmax (MeOH) 230 nm
(ꢀ 12 200), 321 nm (ꢀ 14 400). See Supporting Information for more
experimental details and spectroscopic data.
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Org. Lett., Vol. 6, No. 19, 2004