Synthesis of the pyoverdin chromophore by a biomimetic oxidative cyclization

Article abstract of DOI:10.1021/ol802804c

The fluorescent dihydropyrimido[1,2-a]quinoline chromophore of the pyoverdin siderophores has been synthesized by a biomimetic oxidative cyclization using an iodine (III) reagent, followed by elimination and dehydrogenation.

Full text of DOI:10.1021/ol802804c

ORGANIC
LETTERS
2009
Vol. 11, No. 7
1519-1522
Synthesis of the Pyoverdin
Chromophore by a Biomimetic Oxidative
Cyclization
Raymond C. F. Jones,*,† Sze Chak Yau,† James N. Iley,‡ Janet E. Smith,‡
James Dickson,‡ Mark R. J. Elsegood,‡ Vickie McKee,‡ and Simon J. Coles§
Chemistry Department, Loughborough UniVersity, Loughborough, Leicestershire LE11
3TU, U.K., Chemistry Department, Open UniVersity, Walton Hall, Milton Keynes MK7
6AA, U.K., and EPSRC National Crystallography SerVice, School of Chemistry,
UniVersity of Southampton, Highfield, Southampton SO17 1BJ, U.K.
r.c.f.jones@lboro.ac.uk
Received December 4, 2008
ABSTRACT
The fluorescent dihydropyrimido[1,2-a]quinoline chromophore of the pyoverdin siderophores has been synthesized by a biomimetic oxidative
cyclization using an iodine(III) reagent, followed by elimination and dehydrogenation.
The pyoverdins are siderophores secreted by fluorescent
Pseudomonas sp. including the pathogenic P. aeruginosa
under conditions of iron starvation.1 Structurally they
comprise a species-specific peptide of 6-19 residues con-
taining D-amino acids and hydroxamates and a characteristic
2,3-dihydro-1H-pyrimido[1,2-a]quinoline chromophore, as
exemplified by pyoverdin Pf CCM2798 1 from P. fluorescens
(Figure 1).2 This example also illustrates the occurrence of
the tetrahydropyrimidine amino acid unit in some pyoverdins.
Our interest in cyclic amidine amino acids as dipeptide
mimics3 led us to prepare some tetrahydropyrimidine pseudo-
dipeptides 2 related to pyoverdins (Figure 2).4 The chro-
† Loughborough University.
Figure 1. Structure of pyoverdin Pf CCM2798. In subsequent
‡ Open University.
structures, substituents on the chromophore are represented as R
groups.
§ University of Southampton.
(1) (a) Fuchs, R.; Budziekewicz, H. Curr. Org. Chem. 2001, 5, 265–
288. (b) Budziekewicz, H. FEMS Microbiol. ReV. 1993, 104, 209–228.
(2) Demange, P.; Bateman, A.; Macleod, J. K.; Dell, A.; Abdallah, M. A.
Tetrahedron Lett. 1990, 31, 7611–7614.
mophore is believed to be derived biogenetically by oxidative
cyclization of tetrahydropyrimidine amino acid residue 3, a
proposal supported by incorporation studies with tyrosine
and 2,4-diaminobutyric acid5 and by the co-occurrence of
(3) (a) Jones, R. C. F.; Dickson, J. J. Peptide Sci 2001, 42, 220–223,
and references cited therein. (b) Jones, R. C. F.; Gilbert, I. H.; Rees, D. C.;
Crockett, A. K. Tetrahedron 1995, 51, 6315–6336.
(4) Jones, R. C. F.; Crockett, A. K. Tetrahedron Lett. 1993, 34, 7459–
7462.
10.1021/ol802804c CCC: $40.75
Published on Web 03/04/2009
2009 American Chemical Society

chromophore and of homologues in the cyclic amidine ring
using phenolic oxidation by hypervalent iodine.11
The required substrates for oxidation were the phenolic
cyclic amidines 7-9. These were all prepared from 2-(4-
hydroxyphenyl)propanoic acid 10 by the sequence outlined
in Scheme 1. Treatment with benzyl bromide (K2CO3,
Figure 2. Tetrahydropyrimidine pseudodipeptides.
other related metabolites such as ferribactins,6 which lack
the chromophore but contain the tyrosine-based tetrahydro-
pyrimidine 3, isopyoverdins 4,7 and the corresponding 5,6-
dihydro metabolites 5 and 6 (Figure 3).8
Scheme 1. Synthesis of Cyclic Amidine Oxidation Substrates
acetone reflux) and subsequent ester hydrolysis (NaOH,
MeOH aq) afforded the benzyl ether (98%) which was
converted into amide 11 via the acid chloride (oxalyl
chloride, THF, DMF cat.; 96%) and ammonolysis (NH3 aq,
d 0.88, THF; 76%). After less reliable attempts at O-
alkylation with Meerwein’s salt, our preferred protocol for
carboxamide activation was treatment with methyl trifluo-
romethanesulfonate (CH2Cl2, reflux) to afford the imidate
salt 12 (we have also successfully employed S-alkylation of
piperidine thioamides to achieve this carboxyl activation4).
The crude imidate was treated directly with the appropriate
diamine (EtOH, reflux) to form the required cyclic amidine.
Thus, 1,3-diaminopropane led to the tetrahydropyrimidine
13 (n ) 1) (83%)12 as its trifluoromethanesulfonate salt
which was debenzylated by hydrogenolysis (Pd-C, 1 atm
H2, EtOH) to afford oxidation substrate 7 (99%). Likewise,
reaction of 12 with 1,2-diaminoethane or 1,4-diaminobutane,
and subsequent hydrogenolysis, led to the corresponding
imidazoline 8 (75 and 99%)12 and 1,3-diazepine 9 (24 and
97%), respectively, again as the trifluoromethanesulfonate
salts.
Figure 3. Cometabolites biogenetically related to pyoverdins.
The recent demonstration of enzymic oxidation of the
simple analogue 7 of tetrahydropyrimidine 3 to form a model
for the dihydropyrimido[1,2-a]quinoline chromophore, albeit
in very low yield,9,10 prompts us to report here our biomi-
metic chemical synthesis of a model for the pyoverdin
(5) (a) Thompson, B. N.; Gould, S. J. Tetrahedron 1994, 50, 9865–
9872. (b) Bo¨ckmann, M.; Taraz, K.; Budziekewicz, K. Z. Naturforsch. C:
Biosci. 1997, 52, 319–324.
(6) Hohlneicher, U.; Hartmann, R.; Taraz, K.; Budziekewicz, H. Z.
Naturforsch. 1995, 50C, 337–344.
(7) Jacques, Ph.; Ongena, M.; Gwose, I.; Seinsche, D.; Schro¨der, H.;
Delfosse, Ph.; Thonart, Ph.; Taraz, K.; Budziekewicz, H. Z. Naturforsch.
1995, 50C, 622–629.
(8) (a) Gwose, I.; Taraz, K. Z. Naturforsch. 1992, 47C, 487–502. (b)
Michalke, R.; Taraz, K.; Budziekewicz, H.; Thonart, Ph.; Jacques, Ph. Z.
Naturforsch. 1995, 50C, 855–857.
(9) Dorrestein, P. C.; Poole, K.; Begley, T. P. Org. Lett. 2003, 5, 2215–
2217
.
Oxidative cyclization of tetrahydropyrimidine 7 to the
pyoverdin chromophore ring system requires closure via a
nitrogen atom. Our own studies and the work of others have
clearly demonstrated that primary amides such as 11 or the
corresponding piperidine amides cyclize via the carbonyl
oxygen atom on iodine(III) oxidation.13 Ring closure via
nitrogen has been demonstrated for acyl hydrazides,14
sulfonamides,15 or cyclic imidates (oxazines, dihydroox-
(10) (a) Routes to cyclic amidine [1,2-a]-fused quinolines have also been
reported by closure of the amidine ring as the final step using conventional
or electrochemical protocols. See, for example: Jones, R. C. F.; Smallridge,
M. J.; Chapleo, C. B. J. Chem. Soc., Perkin Trans. 1 1990, 38, 5–391. (b)
Okimoto, M.; Yoshida, T.; Hoshi, M.; Hattori, K.; Komata, M.; Numata,
K.; Tomozawa, K. Aust. J. Chem. 2007, 60, 236–242
.
(11) (a) Wirth, T. Angew. Chem., Int. Ed. 2005, 44, 3656–3665. (b)
Wirth, T. HyperValent Iodine Chemistry: Modern DeVelopments in Organic
Synthesis; Wirth, T., Ed.; Springer: Berlin, 2003; Vol. 224, pp 185-208.
(c) Tohma, H.; Kita, Y. HyperValent Iodine Chemistry: Modern DeVelop-
ments in Organic Synthesis; Wirth, T., Ed.; Springer: Berlin, 2003; Vol.
224, pp 209-248.
(12) Some recovered amide 11 was isolated along with tetrahydro-
pyrimidine 13 (n ) 1) and imidazoline 13 (n ) 0) (15 and 4%,
respectively).
(13) (a) Smith, J. E. M.Phil. Thesis, The Open University, 2004. (b)
Kita, Y.; Tohma, H.; Kikuchi, K.; Inagaki, M.; Yakura, T. J. Org. Chem.
1991, 56, 435–438.
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Org. Lett., Vol. 11, No. 7, 2009

azoles) although in the latter case the imidate then undergoes
cleavage.16 Since our initial attempts at direct oxidative
cyclization of the cyclic amidines 7-9 were unpromising,
we elected to oxidize in the presence of nucleophilic alcohol
solvents.17 Thus, treatment of tetrahydropyrimidine 7 with
bis(trifluoroacetoxy)iodobenzene (BTIB) in MeOH (20 °C,
5 min) afforded a dienone intermediate 14a (Table 1) that
than originally anticipated, presumably because the quino-
linones were already protonated at the amidine function,
hindering alkoxy-group protonation.
Treatment of 15a with trifluoromethanesulfonic acid
afforded a mixture of the methoxy- and hydroxytetrahydro-
pyrimidoquinoline salts 16a and 16b. We propose that
O-alkylation arises by formation of the methylating agent
methyl trifluoromethanesulfonate from the eliminated metha-
nol in the reaction medium. In support of this, addition of
MeOH to the acidic reaction medium afforded methyl ether
16a as the major product (57%) (Scheme 2). In contrast,
Table 1. Oxidative Cyclization of Pyrimidine 7
Scheme 2. Completion of the Tetrahydropyrimidoquinolines
(()-dienone salt (()-pyrimidoquinolinone
entry
R1
OMe
14a
salt 15 (yield, %)
1
2
3
4
5
6
7
14a
14b
14c
14d
14e
14f
15a (46)
15b (34)
OEt
OCHMe2
OCMe3
OCH2Ph
NHCOMe
OH
15f (41)
15g (4)
14g
a Dienones 14 were not fully characterized.
elimination from ethoxy compound 15b afforded solely the
phenolic dihydropyrimidinoquinoline 16b (38%), as presum-
ably any ethyl trifluoromethanesulfonate formed is either less
effective as an alkylating agent, or undergoes elimination.
The structure of methyl ether 16a was confirmed by an X-ray
crystal structure.19 Completion of the pyoverdin chromophore
model was achieved by dehydrogenation of 16a using DDQ
(1,4-dioxane, reflux) to afford tricyclic salt 17a (38%),
purified by reverse-phase HPLC; the structure of 17a was
also confirmed by a crystal structure determination. Dehy-
drogenation of 16b could also be achieved in low yield (10%)
to give a highly polar product 17b that was not fully
characterized. These dehydrogenations could also be achieved
on silica by microwave irradiation but gave less pure
products.
could not be fully characterized but when chromatographed
on basic alumina afforded tetrahydropyrimidoquinolinone
trifluoromethanesulfonate 15a (46%) as product of the
desired oxidative cyclization through a nitrogen atom.18 It
is apparent that the intramolecular conjugate addition is
alumina-mediated; indeed, stirring the dienone in solution
with alumina also led to cyclization, but chromatographic
conversion was the more efficient protocol. When the BTIB
oxidation and alumina treatment sequence was repeated with
EtOH as oxidation solvent, the corresponding tricyclic salt
15b was formed (34%) via dienone 14b. Likewise, 2-pro-
panol, tert-butyl alcohol, and benzyl alcohol afforded di-
enones 14c-e that were not completely characterized. From
an oxidation in moist acetonitrile, the cyclization product
15f was isolated (41%) via acetamide adduct 14f, and the
hydroxyl adduct 15g was observed in low yield (ap-
proximately 4%) from a reaction in water, via 14g. The
structures of the methoxy derivative 15a and ethoxy deriva-
tive 15b were confirmed by X-ray crystallographic analysis,
which also revealed cis-6,6-ring fusion in the crystalline
material.19,20
The oxidative cyclization sequence was also performed
on the imidazoline 8 and 1,3-diazepine 9 (Scheme 3). Thus,
BTIB oxidation using MeOH or EtOH as solvent, and
subsequent alumina chromatography, afforded reduced imi-
dazoquinolinone trifluoromethanesufonates 18a and 18b from
imidazoline 8 (39% and 16%, respectively). Diazepine 9
(17) Tohma, H.; Morioka, H.; Takizawa, S.; Yarisawa, M.; Kita, Y.
Tetrahedron 2001, 57, 345–352, and references cited therein.
(18) (a) Pouysegu, L.; Avella, A.-V.; Quideau, S. J. Org. Chem. 2002,
61, 3425–3436, and references cited therein. (b) Wipf, P.; Methot, J.-L.
Org. Lett. 2000, 2, 4213.
Aromatization of the carbocyclic ring by elimination of
alcohol from 15a,b under acidic conditions was more difficult
(14) Clemente, D.-T. V.; Lobo, A. M.; Prabhakar, S. Tetrahedron Lett.
1994, 35, 2043–2046.
(19) Crystal structures of 15a, 15b, 16a, and 17a were refined to
convergence with R factors of 0.061, 0.054, 0.042 and 0.097, respectively.
Full details are presented in the Supporting Information.
(15) Ciufolini, M. A.; Canesi, S.; Ousmer, M.; Braun, N. A. Tetrahedron
2006, 62, 5318–5337.
(16) Braun, N. A.; Bray, J. D.; Ousmer, M.; Peters, K.; Peters, E. M.;
Bouchou, D.; Ciufolini, M. A. J. Org. Chem. 2000, 65, 4397–4408.
(20) A cis-ring junction is expected from attack of the amidine onto the
dienone from the face opposite the alkoxy group.
Org. Lett., Vol. 11, No. 7, 2009
1521

The dihydropyrimidoquinoline 17a showed strong fluo-
rescence,21 as do the natural pyoverdins; the seven-ring
analogue 21 showed a much weaker fluorescence (ap-
proximately 25% of that of 17a). The UV spectra of
chromophores 17 and 21 compared acceptably with those
of the natural pyoverdins,22 recognizing that they have an
incomplete substituent set.
Scheme 3
.
Oxidative Cyclizsations of Imidazoline 8 and
Diazepine 9
We have thus chemically generated models for the
dihydropyrimido[1,2-a]quinoline chromophore of the py-
overdin siderophores via a biomimetic route involving
oxidative cyclization. Efforts continue toward synthesis of
the pyoverdins.
Acknowledgment. We acknowledge the financial support
of Loughborough University (S.C.Y.), the Open University
(J.D.), and EPSRC (J.E.S.), and the EPSRC National
Crystallography Service at Southampton for the structure
determination of 15a.
Supporting Information Available: Procedures and
spectral data for the synthesis of 7-9 and oxidative cycliza-
tions to 17a, 16b, 18a, and 21; X-ray crystal data including
CIF files for 15a, 15b, 16a, and 17a. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL802804C
similarly afforded the reduced alkoxyazepinoquinolinone
salts 19a and 19b (44% and 29%, respectively). Intermediates
18 have not been taken further, but treatment of both 19a
and 19b in trifluoromethanesulfonic acid-MeOH gave the
methoxyhexahydroazepinoquinoline salt 20 (66% and 50%,
respectively). DDQ dehydrogenation of 20 as previously led
to the homologue 21 of the pyoverdin chromophore model
(35%).
(21) 17a: fluorescence em (MeOH-H2O 1:1 v/v, pH 6.5, ex 338, 353,
λ
max) 372 (ε 5.09 × 106), and 382 (4.80 × 106), respectively; UV-vis
(MeOH-H2O 1:1 v/v, pH 6.5, λ) 219 (ε 6.3 × 104), 338 (2.3 × 104), and
353 (2.0 × 104). No significant change over pH range 6.5-9.
(22) For example: Demange, P.; Wendenbaum, S.; Linget, C.; Mertz,
C.; Cung, M. T.; Dell, A.; Abdallah, M. A. Biol. Met. 1990, 3, 155–170.
For pyoverdin Pa from P. aeruginosa ATCC 15692: UV-vis (pH 4.2, λ)
365 nm (ε l.4 × l04) and 380 (1.4 × 104).
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Org. Lett., Vol. 11, No. 7, 2009