Biosynthesis inspired Diels-Alder route to pyridines: synthesis of the 2,3-dithiazolylpyridine core of the thiopeptide antibiotics.

Article abstract of DOI:10.1039/b204868j

Reaction of serine derived 1-alkoxy-2-azadienes with dehydroalanine derived dienophiles results in Diels-Alder reaction and aromatisation to give 2,3,6-trisubstituted pyridines, thereby establishing the viability of the proposed biosynthetic route to the

Full text of DOI:10.1039/b204868j

Biosynthesis inspired Diels–Alder route to pyridines: synthesis of the
2,3-dithiazolylpyridine core of the thiopeptide antibiotics
Christopher J. Moody,*a Rachael A. Hughes,a Stewart P. Thompsona and Lilian Alcarazb
a School of Chemistry, University of Exeter, Exeter, UK EX4 4QD. E-mail: c.j.moody@ex.ac.uk
b Medicinal Chemistry Department, AstraZeneca R&D Charnwood, Bakewell Road, Loughborough, UK
LE11 5RH
Received (in Cambridge, UK) 20th May 2002, Accepted 18th June 2002
First published as an Advance Article on the web 12th July 2002
Reaction of serine derived 1-alkoxy-2-azadienes with dehy-
droalanine derived dienophiles results in Diels–Alder reac-
tion and aromatisation to give 2,3,6-trisubstituted pyridines,
thereby establishing the viability of the proposed bio-
synthetic route to the pyridine ring of the thiopeptide
antibiotics originally proposed by Bycroft and Gowland.
The thiopeptide (or thiostrepton) antibiotics are a class of sulfur
containing highly modified cyclic peptides characterised by
several common structural features: the presence of thiazole
and, in some cases, oxazole rings, unusual and dehydro amino
acids, and a heterocyclic centrepiece of a tri- or tetra-substituted
pyridine all in a macrocyclic array. Many of the compounds
such as thiostrepton 1 itself,1 nosiheptide,2 and the micro-
coccins3 have been known for some time. Most of the
thiopeptide antibiotics inhibit protein synthesis in bacteria, and
share a common mode of action. They act by binding to the
complex of 23SrRNA with ribosomal protein L11, inhibiting
the action of GTP-dependent elongation factors.4 Despite the
fascinating biological activity of the thiopeptide antibiotics,
relatively little synthetic work has been carried out to date, and
the only reported total syntheses are that of micrococcin P1 2,5,6
and our own work on promothiocin A.7 However, the syntheses
of various fragments of other thiopeptides have been reported,
for example the pyridine fragments of sulfomycin I,8 nosihep-
tide,9 A10255,10 and GE2270A.11
In a 1978 Chemical Communication Bycroft and Gowland, as
well as reporting the structure of micrococcin P1 2,3b,5
suggested that its pyridine ring (as well as the tetrahydropyr-
idine in thiostrepton 1) could be biogenetically derived ‘from
the interaction of two dehydroalanine units.’ This interesting
proposal for the biosynthesis of the pyridine ring in thiopeptides
was subsequently supported by isotopic labelling experiments
by Floss and coworkers.12,13 Floss also viewed the Bycroft
proposal for the biosynthesis of the pyridine ring as a formal
cycloaddition (not necessarily concerted) followed by ar-
omatisation as shown in Scheme 1, the intermediate tetra-
hydropyridine being clearly related to the corresponding
structural unit in thiostrepton. We now report the realisation of
Bycroft’s original biosynthesis proposal in a biomimetic
cycloaddition route to 2,3,6-trisubstituted pyridines, involving
the Diels–Alder reaction of serine-derived 1-alkoxy-2-aza-
dienes with dehydroalanine derivatives.
Scheme 1).14 The dienes were prepared from O-acetylserine
methyl ester 3 by reaction with the corresponding imidate 4,
commercially available (R2 = Ph) or obtained by reaction of the
carboxamide with triethyloxonium hexafluorophosphate,15
using a procedure based on the literature route to 2-azadiene 5
(R2 = Ph).16 Initial Diels–Alder reactions of the 2-azadiene 5
(R2 = Ph) were carried out using the serine derived N-
acetyldehydroalanine methyl ester 6 (R3
= CO2Me) as
dienophile,17 the NHAc group mimicking the N-terminus
peptide chain in the proposed biosynthetic route (cf. Scheme 1).
Prolonged heating of the two components in xylene resulted in
a 42% yield of the pyridine 7a after chromatography (Scheme
2), thereby establishing for the first time the viability of the
The dienes chosen for study were the 1-alkoxy-2-azadienes 5
which mimic the dehydroalanine dipeptide diene proposed by
Bycroft and Floss, by fixing it in the required ‘enol’ form (cf.
Scheme 1
This journal is © The Royal Society of Chemistry 2002
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CHEM. COMMUN., 2002, 1760–1761

Scheme 3
We thank the EPSRC and AstraZeneca for financial sup-
port.
Scheme 2
Notes and references
† The structure of methyl 2-phenyl-3-(2-thiazolyl)pyridine-6-carboxylate
7f was confirmed by X-ray crystallography; we thank Dr Alex Slawin
(University of St Andrews) for carrying out this structure determination,
details of which will be published separately.
‘biomimetic’ Diels–Alder-aromatisation sequence, albeit under
thermal rather than ‘biological’ conditions. Interestingly, the
pyridine 7a (79%) was also formed slowly on heating the diene
5 (R2 = Ph) alone. That the dehydroalanine derivative ester 6
(R3 = CO2Me) was actually involved in the cycloaddition was
1 Isolation: J. F. Pagano, M. J. Weinstein, H. A. Stout and R. Donovick,
Antibiot. Annu., 1955–1956, 554. Structure: B. Anderson, D. Crowfoot-
Hodgkin and M. A. Viswamitra, Nature, 1970, 225, 233.
2 C. Pascard, A. Ducroix, J. Lunel and T. Prange, J. Am. Chem. Soc.,
1977, 99, 6418.
3 (a) J. Walker, A. Olesker, L. Valente, R. Rabanal and G. Lukacs, J.
Chem. Soc., Chem. Commun., 1977, 706; (b) B. W. Bycroft and M. S.
Gowland, J. Chem. Soc., Chem. Commun., 1978, 256.
4 B. T. Porse, L. Leviev, A. S. Mankin and R. A. Garrett, J. Mol. Biol.,
1998, 276, 391; B. T. Porse, E. Cundliffe and R. A. Garrett, J. Mol. Biol.,
1999, 287, 33.
shown by the use of the corresponding ethyl ester 6 (R3
=
CO2Et) that gave pyridine 7b, together with 7a, formed by
competing ‘dimerisation’ of the azadiene. The reaction was also
investigated under microwave irradiation, and these conditions
were applied to a range of other 1-ethoxy-2-azadienes 5 (R2 =
4-chlorophenyl, 2-thienyl, 2-pyridyl prepared in 38, 65 and 38%
yield respectively from 3). These dienes gave the corresponding
pyridines 7c–e upon heating with N-acetyldehydroalanine ethyl
ester 6 (R3 = CO2Et) as dienophile (Table 1).
5 (a) The structure of micrococcin P1 2 is in doubt since the synthetic
material is different from the natural product: M. A. Ciufolini and Y.-C.
Shen, Org. Lett., 1999, 1, 1843; (b) recent studies have confirmed that
the overall Bycroft–Gowland structure is correct, and the only
remaining issue is the stereochemistry at three centres: B. Fenet, F.
Pierre, E. Cundliffe and M. A. Ciufolini, Tetrahedron Lett., 2002, 43,
2367. In view of these results, there is some doubt about the earlier
reported synthesis of micrococcin P1 (ref. 6).
6 K. Okumura, A. Ito, D. Yoshioka and C.-G. Shin, Heterocycles, 1998,
48, 1319; K. Okumura, T. Suzuki, Y. Nakamura and C.-G. Shin, Bull.
Chem. Soc. Jpn., 1999, 72, 2483.
7 M. C. Bagley, K. E. Bashford, C. L. Hesketh and C. J. Moody, J. Am.
Chem. Soc., 2000, 122, 3301.
Table 1 Preparation of 2,3,6-trisubsituted pyridines 7 by Diels–Alder
reaction of 1-alkoxy-2-azadienes 5 with dienophiles 6
Yieldb
(%)
7
R2
R3
Conditions a
a
b
c
d
e
f
Ph
Ph
4-ClC6H4
2-Thienyl
2-Pyridyl
Ph
CO2Me
CO2Et
CO2Et
CO2Et
CO2Et
Xylene, 48 h
Xylene, 48 h
DCB, MW, 180 °C, 4 h
DCB, MW, 180 °C, 2.5 h
DCB, MW, 180 °C, 4 h
42
25
60
58
28
64
50
2-Thiazolyl DCB, 48 h
2-Thiazolyl DCB, MW, 180 °C, 4 h
f
Ph
8 T. R. Kelly and F. Lang, J. Org. Chem., 1996, 61, 4623.
9 K. Umemura, H. Noda, J. Yoshimura, A. Konn, Y. Yonezawa and C.-G.
Shin, Bull. Chem. Soc. Jpn., 1998, 71, 1391.
10 K. Umemura, S. Ikeda, J. Yoshimura, K. Okumura, H. Saito and C.-G.
Shin, Chem. Lett., 1997, 1203.
a Reactions carried out in the specified solvent at reflux. DCB
=
1,2-dichlorobenzene; MW = microwave (all reactions carried out in a CEM
Focused Synthesiser at the temperature specified). b In some cases, the
pyridine formed by competing ‘dimerisation’ of the azadiene 5 is also
isolated.
11 K. Okumura, H. Saito, C. -G Shin, K. Umemura and J. Yoshimura, Bull.
Chem. Soc. Jpn., 1998, 71, 1863.
Given the presence of a 2-thiazolyl substituent at the
3-position of the pyridine ring in all thiopeptide antibiotics, the
2-thiazolyl dienophile 6 (R3 = 2-thiazolyl), prepared in 49%
yield by reduction of 2-acetylthiazole oxime using iron with
acetic anhydride–acetic acid in toluene,18 was investigated next.
Diels–Alder reaction under conventional or microwave heating
gave the pyridine 7f in reasonable yield,† together with pyridine
7a (26 or 50%) formed by competing ‘dimerisation’ of the
azadiene. The reaction was then extended to the thiazole
containing 2-azadiene 8, prepared from 2-methylthiazole-
4-carboxamide via the corresponding imidate in 72% yield from
serine derivative 3. Diels–Alder reaction (microwave heating,
neat, 180 °C, 15 min) with the 2-thiazolyl dienophile 6 (R3 =
2-thiazolyl) gave the 2,3-dithiazolylpyridine 9a in 29% yield.
Reaction of the corresponding 4-ethoxycarbonylthiazole dieno-
phile gave the 2,3-dithiazolylpyridine 9b (64%) containing an
additional ester group for further elaboration (Scheme 3).
Thus we have established for the first time the viability of the
long-standing proposal for the biosynthesis of the pyridine ring
in the thiopeptide antibiotics involving a cycloaddition of two
serine derived dehydroalanine type fragments.19
12 U. Mocek, A. R. Knaggs, R. Tsuchiya, T. Nguyen, J. M. Beale and H.
G. Floss, J. Am. Chem. Soc., 1993, 115, 7557.
13 U. Mocek, Z. Zeng, D. OAHagan, P. Zhou, L.-D. G. Fan, J. M. Beale and
H. G. Floss, J. Am. Chem. Soc., 1993, 115, 7992.
14 For reviews of azadienes, see: P. Buonora, J. C. Olsen and T. Oh,
Tetrahedron, 2001, 57, 6099; S. Jayakumar, M. P. S. Ishar and M. P.
Mahajan, Tetrahedron, 2002, 58, 379.
15 G. Pattenden and S. M. Thom, J. Chem. Soc., Perkin Trans. 1, 1993,
1629.
16 C. Balsamini, A. Bedini, R. Galarini, G. Spadoni, G. Tarzia and M.
Hamdan, Tetrahedron, 1994, 50, 12375.
17 Although not commonly used as dienophiles, dehydroalanine deriva-
tives do participate in Diels–Alder reactions; for examples, see: A.
Avenoza, C. Cativiela, M. A. Fernandez-Rico and J. M. Peregrina, J.
Chem. Soc., Perkin Trans. 1, 1999, 3375; B. A. Burkett and C. L. L.
Chai, Tetrahedron Lett., 2000, 41, 6661.
18 M. J. Burk, G. Casy and N. B. Johnson, J. Org. Chem., 1998, 63,
6084.
19 Since this manuscript was submitted, Nicolaou and coworkers have
described a related biosynthesis inspired approach to the tetra-
hydropyridine core of thiostrepton: K. C. Nicolaou, M. Nevalainen, B.
S. Safina, M. Zak and S. Bulat, Angew. Chem., Int. Ed., 2002, 41,
1941.
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