Total synthesis of the thiazolyl peptide GE2270 A

Article abstract of DOI:10.1002/anie.200700684

(Chemical Equation Presented) When one door closes, another opens: In the synthesis of the thiazolyl peptide GE2270 A (1), the bonds labeled I and II at the pyridine core were established by two consecutive cross-coupling reactions. Amide bond formation (

Full text of DOI:10.1002/anie.200700684

Angewandte
Chemie
DOI: 10.1002/anie.200700684
Total Synthesis
Total Synthesis of the Thiazolyl Peptide GE2270 A**
H. Martin Müller, Oscar Delgado, and Thorsten Bach*
Dedicated to Professor George A. Olah on the ocassion of his 80th birthday
GE2270 A (1) is the prototypical
member of the GE2270 thiazolyl pep-
tides,[1] a family of antibiotics produced
by Planobispora rosea.[2,3] The initially
proposed structure[4] was revised in
1995[5] when the stereochemical con-
figuration was shown to be derived
from naturally occurring amino acids.
The absolute and relative configura-
tion of the phenylserine fragment was
elucidated in 2005.[6] The structural
assignment of GE2270 A has been
recently confirmed by a high-resolu-
tion (1.6 Š) X-ray crystal structure
analysis of the complex of GE2270 A
and the bacterial elongation factor EF-
Tu[7] and by a total synthesis.[8] Indeed,
interest in the synthesis of the GE2270
thiazolyl peptides[8,9] is mainly related
to their potent activity as inhibitors of
Scheme 1. Synthetic strategy for the construction of the thiazolyl peptide GE2270 A (1) by
regioselective cross-coupling reactions. Fmoc=9-Fluorenylmethoxycarbonyl, PG=protecting
group, TBDMS=tert-butyldimethylsilyl, TBDPS=tert-Butyldiphenylsilyl.
bacterial protein biosynthesis.[7,10] Their unique mode of
action at a bacteria-specific enzyme makes the GE2270
thiazolyl peptides important lead structures for the discovery
of new antiinfective agents. We have therefore directed our
attention to a synthetic route to this compound class, and we
herein report on our total synthesis of GE2270 A.
Our synthetic strategy (Scheme 1) relied on the consec-
utive introduction of three advanced subunits to a central
pyridine core by regioselective cross-coupling reactions.[11]
The zincated pyridine 2 was envisioned to react with the
“southern” subunit 3 (Negishi cross-coupling,[12,13] step I),
before the regioselective introduction of the “northern”
subunit 4 at C6 (Negishi cross-coupling, step II) and of the
“eastern” fragment 5 at C2 (Stille cross-coupling,[14] step III).
The completion of the synthesis should then involve a
macrolactamization (step IV) and protecting-group removal.
Alternatively, the amide coupling (step IV) could be carried
out prior to the last cross-coupling (step III), which should
then serve to realize the ring closure. The latter strategy
ultimately emerged as the more successful one.
The disconnection of the “southern” subunit 3 at the
amide junctions leads back to three thiazole-containing
fragments (Scheme 1, steps V and VI). While 2-iodothiazole-
4-carboxylic acid could be readily synthesized in analogy to
reported compounds,[15] the synthesis of the more complex
chiral thiazoles required longer reaction sequences. For the
preparation of thiazole 9 (Scheme 2) we applied the alkyla-
tion method recently reported by Deng und Taunton.[16] This
strategy allowed the late introduction of the methoxymethyl
moiety after the formation of the thiazole ring by Hantzsch
synthesis. In this manner, instead of preparing a complex
ketocarboxylate,[5] we could use the commercially available
ethyl bromopyruvate. The thioamide 7 was obtained from N-
tert-butoxycarbonyl(Boc)-protected valine (6), and the sub-
sequent elaboration to the chiral thiazole 8 proceeded in
excellent yield. Minor racemization in the alkylation of 8
(89% ee)[17] could be avoided by simply switching the
protecting group from Boc to trityl (Tr) (> 95% ee). Despite
the protecting-group manipulations the overall yield for the
synthesis of fragment 9 (55%) was more than acceptable.
The Gabriel synthesis[18] was chosen for the assembly of
the asparagine-derived thiazole 13 (Scheme 3). Although a
[*] Dipl.-Chem. H. M. Müller,[+] Dr. O. Delgado,[+] Prof. Dr. T. Bach
Lehrstuhl für Organische Chemie I
Technische Universität München
Lichtenbergstrasse 4, 85747 Garching (Germany)
Fax: (+49)89-289-13315
E-mail: thorsten.bach@ch.tum.de
Homepage: http://www.ch.tum.de/oc1/tbach/
[+] Both authors contributedequally to the project.
[**] This project was supportedby the Deutsche Forschungsgemein-
schaft (Ba 1372-9), by the Alexander von Humboldt Foundation
(research scholarship to O.D.), by the Universität Bayern e.V.
(predoctoral scholarship to H.M.M.), and by the Fonds der
Chemischen Industrie. We thank Wacker-Chemie (Munich), DSM
Fine Chemicals (Linz), andUmicore (Hanau) for the donation of
chemicals. The help of Dipl.-Chem. Jochen Klages in recording the
NMR spectra is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 2. Synthesis of the valine-derived thiazole 9. Reaction condi-
tions: a) IBC (1.05 equiv), NMM (1.1 equiv), 25% NH3 (aq), THF,
À208C to 258C, 16 h, 96%; b) Lawesson reagent (0.5 equiv), THF,
258C, 16 h, 82%; c) KHCO3 (8 equiv), BrCH2COCO2Et (3 equiv), DME,
À158C, 24 h; d) TFAA (4.1 equiv), pyridine (8.8 equiv), DME, À158C,
2 h, 96% yieldover two steps; e) TFA/CH 2Cl2 (1:5); f) TrCl (1 equiv),
NEt3 (2.6 equiv), DME, 258C, 16 h, 98% yieldover two steps; g) LDA
(1.1 equiv), THF, À788C, 1 min, then ICH2OCH3 (3 equiv), À788C,
5 min, 74%; h) TFA/CH2Cl2 (1:5). DME=dimethoxyethane, IBC=iso-
butyl chloroformate, LDA=lithium diisopropylamide, NMM=N-meth-
ylmorpholine, py=pyridine, TFAA=trifluoroacetic anhydride, TFA=tri-
fluoroacetic acid, TrCl=trityl chloride,.
method for the preparation of precursor 11, based on the
insertion of a carbene into an N H bond, was available,[19] we
Scheme 3. Synthesis of the asparagine-derived thiazole 13 andcou-
pling to the “southern” fragment 3. Reaction conditions: a) H-Thr-
OMe (1.2 equiv), HBTU (1.2 equiv), HOBt (1.2 equiv), NEt3
(3.6 equiv), DMF, À258C to 258C, 16 h, 86%; b) IBX (2.2 equiv),
MeCN, reflux, 3.5 h; c) Lawesson reagent (1.5 equiv), THF, reflux, 5 h,
À
followed the more conventional route.[20] The CO NH bond
À
was established by the peptide coupling of the carboxylic acid
10 with threonine methyl ester. The oxidation of 11[21] yielded
the corresponding ketoamide, which was treated with Law-
esson reagent[22] in order to effect the heterocyclization. The
hydrogenolysis of benzyl ester 12 was subsequently carried
out in the presence of Pearlmanꢀs catalyst Pd(OH)2/C, and the
coupling of the resulting carboxylic acid with methylamine
yielded the enantiomerically pure fragment 13 (> 95% ee)[17]
in very high yield. Saponification of 13 and coupling with
fragment 9[23] proceeded without loss of stereochemical
integrity. To complete the synthesis of subunit 3, 2-iodothia-
zole-4-carboxylic acid was conveniently attached in excellent
yield. The order of events in the peptide coupling sequence
(step V before VI) proved to be crucial to avoid a possible
epimerization.
45% yieldover two steps; d) H (1 atm), 20% Pd(OH)2/C (7 mol%),
2
MeOH, 608C, 16 h; e) IBC (1 equiv), NMM (1 equiv), 40% H2NMe
(aq) (1.2 equiv), THF, À258C to 258C, 1 h, 83% yieldover two steps;
f) LiOH (3.5 equiv), MeOH, 08C to 258C, 16 h; g) EDC (1.2 equiv),
HOBt (2 equiv), DMF, À108C to 258C, 16 h, 92% yieldover two
steps; h) AcCl (10 equiv), EtOH, 08C to 258C, 16 h; i) EDC (1.2 equiv),
HOBt (3 equiv), DMF, À108C to 258C, 16 h, 99% yieldover two steps.
EDC=N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride,
HBTU=O-(benzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluor-
ophosphate, HOBt=1-hydroxybenzotriazole, H-Thr-OMe=(S)-threo-
nine methyl ester, IBX=2-iodoxybenzoic acid.
tives we contemplated the use of an analogue of 4 tritylated at
the proline amide N atom as well as zinc reagent 16, which
could be conveniently prepared from the corresponding
bromide.[28] Whereas in the first case no successful coupling
The organozinc reagent 2 was generated by reductive
metalation of the corresponding halide using N,N-dimethyla-
cetamide (DMA) as solvent.[24] Extensive optimization
showed that for the success of the subsequent Negishi cross-
coupling of 3 the amount of DMA had to be kept as low as
possible. Since the metalation of 2,6-dibromo-3-iodopyri-
dine[25] could be carried out in a DMA/THF mixture—instead
of pure DMA—and this pyridine was more conveniently
obtained[26] than 2,3,6-tribromopyridine,[27] 2b was the
reagent of choice for the first Negishi cross-coupling (step I,
Scheme 1). The yield of product 15 was remarkably high
(Scheme 4). However, the second cross-coupling entailed
more serious difficulties. While the organozinc 4 could be
readily obtained from the corresponding bromide, the follow-
ing coupling with 15 failed in all cases, even when a
considerable excess of 4 (10 equiv) was employed. We
reasoned that the failure could be caused by the presence of
several acidic protons in our substrate. As possible alterna-
À
was observed, the desired C C bond formation was secured in
the second case. Despite the moderate yield, the reaction
proceeded with high levels of regiocontrol and the introduc-
tion of the serine–proline subunit appeared unproblematic.
In the third consecutive cross-coupling event (step III) the
“eastern” fragment 5a was to be introduced at C2 in 17 by a
Stille coupling. The bithiazole 5a was synthesized in analogy
to our reported procedure[29] from 2,4-dibromothiazole,
TBDMS-protected (S)-methyl mandelate, and Fmoc-pro-
tected glycine (9 steps, 32% overall yield). The following
macrocyclization (step IV) turned out to be troublesome. In
accordance with previous experience by Nicolaou et al.,[8] the
yield for the sequence involving the deprotection of the
amine, saponification of the ester, and macrolactamization
was in all cases below 30%. While there was no obvious
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order. If the amide forma-
tion (step IV) was per-
formed in an intermolecu-
lar fashion, the Stille cou-
pling (step III) could serve
to close the macrocycle. To
validate this plan, removed
the Fmoc group of 5a
(piperidine in DMF, 258C,
70%), and the resulting
amine 5b was coupled
with the carboxylic acid
obtained from the saponi-
fication of ester 17. To ou r
delight, the formation of
the macrocycle 19 with the
Pd-catalyzed
cross-cou-
pling method proceeded in
75% yield (Scheme 4).
To complete the syn-
thesis, the tert-butyl ester
19 was hydrolyzed reveal-
ing the corresponding car-
boxylic acid, an intermedi-
ate that had been previ-
ously
converted
into
Scheme 4. Regioselective introduction of the pyridine substituents by cross-coupling and formation of
macrolactam 19. Reaction conditions: a) 2b (3 equiv), [Pd2(dba)3] (6 mol%), TFP (12 mol%), THF/DMA
(5:1), 458C, 16 h, 87%; b) 16 (8 equiv), [PdCl2(PPh3)2] (30 mol%), DMA, 458C, 3.5 h, 48%; c) 1m LiOH/tert-
BuOH/THF (1:2:1), 258C, 1 h; d) 5b (1 equiv), DPPA (1.7 equiv), iPr2NEt (3.4 equiv), DMF, 258C, 16 h, 87%
yieldover two steps; e) [Pd(PPh 3)4] (20 mol%), toluene (1 mLmmolÀ1), 858C, 75%. dba=Dibenzylideneace-
tone, DPPA=diphenylphosphoryl azide, TFP=tri(2-furyl)phosphane.
GE2270 A by the stepwise
coupling of serine and pro-
line amide (5 steps, 22%
yield).[8] We opted for the
direct coupling of the free
acid with the complete di-
peptide 20, a step that pro-
alternative macrocyclization method in the Nicolaousyn-
thesis, our strategy offered—as already indicated in the
introduction—the possibility of reverting the coupling
ceeded in very good yield (Scheme 5). The dipeptide was
conveniently prepared from l-proline in six steps and 49%
overall yield.[30] Formation of the undesired diketopiperazine
could be avoided when the ammonium salt of 20, obtained
upon deprotection of the N-Boc precursor, was mixed with 19
and TOTU prior to the addition of the base (iPr2NEt). Finally,
the formation of the oxazoline ring was achieved by exposure
of 21 to an excess N,N-(diethylamino)sulfur trifluoride
(DAST),[31] leading, upon cleavage of the silyl ether from
the phenylserine, to GE2270 A. The obtained material was
identical in all respects to the natural product.[4,5]
In summary, the thiazolyl peptide GE2270 A has been
prepared in a short and convergent fashion. The synthesis
proceeds in 20 steps and 4.8% overall yield (longest linear
sequence) starting from the N-Boc-protected valine (6). Our
strategy allows the facile modification of all building blocks
and hence should be applicable to the preparation of
analogous thiazolyl peptides.
Received: February 14, 2007
Published online: May 14, 2007
Scheme 5. Completion of the total synthesis of GE2270 A (1). Reaction
conditions: a) TFA/CH2Cl2 (1:9), 258C, 2 h; b) 20 (4 equiv), TOTU
(1.5 equiv), iPr2NEt (10 equiv), DMF, 258C, 4 h, 65% yieldover two
steps; c) DAST (26 equiv), CH2Cl2, À788C, 1 h; d) TBAF (2.5 equiv),
THF, 258C, 2 h, 55% yieldover two steps. DAST =(diethylamino)sulfur
trifluoride, TBAF=tetrabutylammonium fluoride, TOTU=O-[(ethoxy-
carbonyl)cyanomethyleneamino]-N,N,N’,N’-tetramethyluronium tetra-
fluoroborate.
Keywords: antibiotics · cross-coupling · macrocycles ·
.
natural products · total synthesis
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Communications
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