Synthesis of the heterocyclic core of the GE2270 antibiotics and structure elucidation of a major degradation product

Article abstract of DOI:10.1002/anie.200461715

Two birds were killed with one stone by the synthesis of the trisubstituted pyridine (R,R)-1 and its diastereoisomer. In a short synthetic sequence for the construction of the GE2270 thiazolyl peptides, 2,3,6-tribromopyridine was converted into pyridine 1

Full text of DOI:10.1002/anie.200461715

Angewandte
Chemie
Natural Products
GE2270 thiazolyl peptides and the synthesis of a degradation
product that proves the absolute configuration of GE2270A.
The degradation product 2 is depicted in Scheme 1. It was
obtained from GE2270A by Tavecchia et al. after hydrolysis
with formic acid, treatment with Na2CO3/MeOH, aminolysis
with benzylamine, and subsequent acetylation.[9] The com-
pound was characterized by UV/Vis, IR, and 1H NMR
spectroscopy as well as by its specific rotation (Scheme 1).
Synthesis of the Heterocyclic Core of the GE2270
Antibiotics and Structure Elucidation of a Major
Degradation Product**
Golo Heckmann and Thorsten Bach*
The thiazolyl peptide GE2270A (1) was isolated in 1991 from
Planobispora rosea ATCC53733.[1] The compound, like many
other closely related GE2270 thiazolyl peptides,[2] shows
Scheme 1. Retrosynthetic disconnection of the GE2270A degradation
product 2 into the fragments 3–6.
antibiotic activity by inhibiting the bacterial elongation factor
(EF) Tu (GE2270A: IC50 = 5 nm).[3] EF-Tu is a protein in
bacteria cells that catalyzes the binding of aminoacyl-tRNA
to the A site of the ribosome.[4] In eukaryotes this function is
carried out by the elongation factor EF-1 alpha, which is not
inhibited by GE2270A.[5] The remarkable antibiotic activity
and the demanding molecular architecture of the GE2270
thiazolyl peptides make them interesting synthetic targets.[6–8]
A total synthesis of these compounds has not yet been
reported.
According to our strategy the central 2,3,6-trisubstituted
pyridine moiety of the GE2270 thiazolyl peptides should be
synthesized from easily accessible 2,3,6-tribromopyridine
(3)[10] by three consecutive C C bond-formation reactions.
ꢀ
Pyridine 2 was used as a model system to establish our plan
for the construction of the GE2270 core. In the present case
we hoped to address the 3-position of the pyridine by
employing a regioselective bromine–lithium exchange reac-
tion.[11] The transmetalation step should be followed by a
cross-coupling with a 2-bromothiazole (e.g. 4). In subsequent
cross-coupling reactions, positions 2 and 6 of the pyridine ring
should be attacked nucleophilically.[12,13] It was our opinion
that the more easily accessible position 6 would be prone to
regioselective substitution by a suitable zinc reagent (e.g. 5).
The final cross-coupling reaction should be performed with a
zinc, tin, or boron reagent prepared from bromide 6.
The structure of GE2270A was established in 1991[1b] but
corrected in 1995.[9] The configurations of four of the six
stereogenic centers could be elucidated by degradation
reactions and by structure comparison with synthetic mate-
rial.[9] Only the configuration of the 1,2-amino alcohol in the
eastern part of the molecule is not known. Herein we report a
concise synthetic approach to the core structure of the
To elucidate the structure of the degradation product 2 it
was necessary to use both possible diastereomeric bromides
threo- and erythro-6 in enantiomerically pure form. We
started with the bithiazoles threo-(S,R)-7 (7a) and erythro-
(R,R)-7 (7b), which were synthesized from enantiomerically
pure (R)-mandelonitrile (Scheme 2). Bithiazole erythro-
(R,R)-7 (7b) was generated from the corresponding erythro
amino alcohol in full analogy to the procedure used for the
synthesis of the known compound 7a.[14] Cleavage of the tert-
butyldimethylsilyl (TBDMS) protecting group with tetrabu-
[*] Dipl.-Chem. G. Heckmann, Prof. Dr. T. Bach
Lehrstuhl fꢀr Organische Chemie I
Technische Universitꢁt Mꢀnchen
Lichtenbergstrasse 4, 85747 Garching (Germany)
Fax: (+49)89-2891-3315
E-mail: thorsten.bach@ch.tum.de
[**] This project was supported by the Deutsche Forschungsgemein-
schaft (Ba 1372-9/1) and by the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. 2005, 44, 1199 –1201
DOI: 10.1002/anie.200461715
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1199

Communications
proceeded more smoothly than the cross-coupling with the
analogous 2-bromothiazole N-benzylamide which was also
attempted. In an additional step the amide was generated in
good yield by treating ester 9 with N-benzylamine/diisobutyl-
aluminum hydride (Dibal-H).[16] The next cross-coupling
reaction was preceded by reductive metalation of methyl 2-
bromothiazole-4-carboxylate with zinc.[17] The resulting zinc
compound reacted with 2,6-dibromopyridine 10 in a Negishi
cross-coupling. The regioselectivity was not completely
perfect, and besides the desired 6-substituted 2-bromo-
pyridine 11 the 2-substituted 6-bromopyridine was formed.
The regioisomeric ratio (r.r.) was 6.5:1. It was necessary to
apply an excess of the zinc compound to achieve a quanti-
tative turnover. When we used 1.3, 1.8, and 2.2 equivalents of
the zinc regent, the yield of 11 increased under otherwise
identical conditions from 41% to 52% to 62%. In the last
case 20% of the starting material could be recovered. Finally,
the inseparable mixture of regioisomers underwent a Stille
cross-coupling with the stannanes 8a and 8b.[18] The reaction
proceeded well without complications at 808C in dioxane.
The mixture of regioisomers was separated by semiprepar-
ative HPLC.[18]
Scheme 2. Synthesis of the diastereomeric stannylated bithiazoles 8a
and 8b from the precursors 7a and 7b.
tylammonium fluoride (TBAF) and removal of the tert-
butoxycarbonyl (Boc) protecting group with trifluoroacetic
acid (TFA) furnished the free amino alcohol, which was then
doubly acetylated. The overall yields of these three steps for
both diacetates 6a (81%) and 6b (74%) were satisfactory.
The introduction of a metal in the 4-position of the bithiazole
was best achieved by Pd-catalyzed stannylation with hexa-
methylditin at 808C.[8a] Attempts to zincate after bromine–
lithium exchange at the 4-position were unsuccessful. The
prepared stannanes 8a and 8b were used directly in the next
reaction.
The 1H NMR spectra of the degradation product[9] and of
the erythro isomer 2b[18] match perfectly, whereas the spectra
of the threo isomer 2a exhibit significant differences. The
differences are found mainly at the spin system of the N,O-
diacetylated amino alcohol. The chemical shift of the
CH(OAc)-proton of the degradation product was reported
as d = 6.20 ppm and its coupling constant as 3J = 8.0 Hz[9] (2a:
6.32, 3J = 5.2 Hz; 2b: 6.19, 3J = 8.0 Hz). The CH(NHAc)
proton gives rise to a virtual triplet at d = 5.60 ppm and
3Jffi8.1 Hz (2a: 5.66, dd, 3J = 8.8, 5.2 Hz; 2b: 5.58, virt. t,
3Jffi8.4 Hz). The synthetic (R,R) enantiomer 2b exhibited a
positive specific rotation (Scheme 3, [a]2D0 = + 30.7 (c = 0.31,
CHCl3)), and consequently a (S,S) configuration could be
assigned to the degradation product and also to the thiazolyl
peptides GE2270. The absolute values of the specific rotation
for 2b (Scheme 3) and for the degradation product ent-2b
(Scheme 1) are nearly identical, whereas for 2a a specific
rotation of [a]2D0 = ꢀ2.9 (c = 0.56, CHCl3) was determined.
The construction of the core fragment of the thiazolyl
peptides GE2270 described herein should also be applicable
to the synthesis of the natural product. The cross-coupling
reactions allow a high degree of functionality within the
different reaction partners and should facilitate the linkage of
more complex building blocks. Studies in this direction and
further optimization of the cross-coupling chemistry are
currently being carried out.
As we had hoped, tribromopyridine 3 was converted
regioselectively into the 3-lithium compound and transmeta-
lated to the zinc compound, which underwent a Negishi cross-
coupling with the ethyl ester 4[15] (Scheme 3). This reaction
Received: August 19, 2004
Published online: January 14, 2005
Keywords: asymmetric synthesis · cross-coupling · heterocycles ·
.
metalation · natural products
Scheme 3. Synthesis of the trisubstituted pyridines 2a and 2b from
2,3,6-tribromopyridine (3) and structure comparison with the known
degradation product, which is assigned the structure ent-2b.
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Angewandte
Chemie
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[8] Synthetic routes to the central pyridine core of related thiazoyl
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Gu, Tetrahedron Lett. 1991, 32, 4263 – 4266; b) from a 1,5-
diketone: M. A. Ciufolini, Y. C. Shen, J. Org. Chem. 1997, 62,
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[18] A solution of bromopyridine 11 (27.4 mg, 53.0 mmol; r.r. = 6.5:1),
stannane 8b (35.2 mg, 64.0 mmol), and tetrakis(triphenylphos-
phine)palladium(0) (6.1 mg, 10 mol%) in 1.5 mL of dioxane was
stirred under an argon atmosphere for 16 h at 808C. Saturated
ammonium chloride solution (15 mL) was added to the dark
brown suspension, and the aqueous layer was extracted with
CH2Cl2 (2 ꢂ 20 mL). The combined organic layers were dried
(Na2SO4) and filtered. A small portion of silica gel was added,
and the solvent was removed in vacuo. The residue was purified
by flash chromatography (silica gel 60, 2 ꢂ 18 cm, solvent: ethyl
acetate) and 26.5 mg (61%) of the desired product (r.r. = 6.5:1)
was obtained. To separate the regioisomers a portion of the
product was purified by semipreparative HPLC (YMC, ODS-A,
250 ꢂ 20; MeCN/H2O 20:80!100:0 in 30 min; flow =
15.0 mLminꢀ1). The desired product 2b was isolated as a
colorless solid. Rf = 0.43 (EtOAc); [a]2D0 = + 30.7 (c = 0.31,
CHCl3); UV (MeCN/H2O = 89/20): lmax = 302 nm; 1H NMR
([D6]DMSO, 360 MHz): d = 1.75 (s, 3H), 2.00 (s, 3H), 3.90 (s,
3H), 4.46 (d, 3J = 6.1 Hz, 2H) 5.58 (virt. t, 3Jffi8.4 Hz, 1H), 6.19
3
(d, J = 8.0 Hz, 1H), 7.21–7.40 (m, 10H), 7.90 (s, 1H), 8.29 (s,
3
1H), 8.34 (d, 3J = 8.2 Hz, 1H), 8.37 (s, 1H), 8.75 (d, J = 8.2 Hz,
3
3
1H), 8.75 (s, 1H), 8.84 (d, J = 8.6 Hz , 1H), 9.10 ppm (t, J =
6.2 Hz, 1H); 13C NMR ([D6]DMSO, 90 MHz): d = 21.0 (q), 22.5
(q), 42.4 (t), 52.4 (q), 54.4 (d), 74.8 (d), 118.1 (d), 119.2 (d), 123.3
(d), 126.9 (d), 127.3 (d), 127.4 (d), 128.4 (d), 128.6 (d), 129.6 (s),
132.5 (d), 137.1 (s), 139.7 (s), 140.1 (d), 147.4 (s), 147.5 (s), 149.8
(s), 150.1 (s), 151.0 (s), 152.8 (s), 160.6 (s), 161.3 (s), 162.1 (s),
163.6 (s), 167.9 (s), 169.2 (s), 169.3 (s), 170.5 ppm (s), two
aromatic CH signals overlap; HRMS (FAB): C39H32N7O6S4
[(M+H)+] calcd: 822.1297, found: 822.1289.
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[13] Selected monographs on cross-coupling reactions: a) Metal-
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