Suzuki cross-coupling approaches to the synthesis of bioactive 3-substituted and 5-substituted-4-methoxy-6-methyl-2-pyrones

Article abstract of DOI:10.1016/S0960-894X(03)00546-8

Suzuki cross-coupling has been used to access a wide range of 3- and 5-substituted 2-pyrones, which show remarkable inhibitory activity against bacteria, yeasts and fungi. 3-Octenyl and 5-octenyl 2-pyrones inhibit human ovarium carcinoma (A2780) and human chronic myelogenous leukaemia (K562) cell lines at the micromolar level.

Full text of DOI:10.1016/S0960-894X(03)00546-8

Bioorganic & Medicinal Chemistry Letters 13 (2003) 2667–2671
Suzuki Cross-coupling Approaches to the Synthesis of Bioactive 3-
Substituted and 5-Substituted-4-methoxy-6-methyl-2-pyrones
Lester R. Marrison,a Julia M. Dickinsona and Ian J. S. Fairlambb,*
aDepartment of Chemistry and Materials, John Dalton Building, The Manchester Metropolitan University, Chester Street,
Manchester M1 5GD, UK
bDepartment of Chemistry, University of York, Heslington, York YO10 5DD, UK
Received 9 April 2003; revised 8 May 2003; accepted 28 May 2003
Abstract—Suzuki cross-coupling has been used to access a wide range of 3- and 5-substituted 2-pyrones, which show remarkable
inhibitory activity against bacteria, yeasts and fungi. 3-Octenyl and 5-octenyl 2-pyrones inhibit human ovarium carcinoma (A2780)
and human chronic myelogenous leukaemia (K562) cell lines at the micromolar level.
# 2003 Elsevier Ltd. All rights reserved.
The 2-pyrone moiety is found in a large number of
naturally-occurring biologically active compounds.1
Simple changes in the substitution pattern on the
2-pyrone ring often leads to incredible diverse biological
activity. For example, 4-hydroxy-2-pyrone moieties
have been shown to be key components in non-peptidic
HIV protease inhibitors,2 whereas 4-alkenyl/ alkynyl/
aryl/ alkyl substituted-6-methyl-2-pyrones (1–4, Fig. 1),
recently reported by us, demonstrate remarkable anti-
microbial activities, human ovarium carcinoma (A2780)
and human chronic myelogenous leukaemia (K562)
inhibitory properties.3 From a synthetic standpoint the
pyrone ring system can be very useful, finding applica-
tion as a cyclobutane precursor,4 a diene component in
Diels–Alder reactions,5 and as a precursor to other het-
erocyclic systems.6 Until very recently the construction
of substituted 2-pyrones was fraught with difficulties,
often requiring a multi-step synthesis via an acyclic
precursor. Organometallic couplings (Suzuki, Sonoga-
shira and Stille) onto the pyrone ring system have gone
some way to overcoming these limitations, enjoying
recent successes from the groups of Pleixats,7 Cho8 and
ourselves.9 Our continued drive towards identifying new
biologically active substituted 2-pyrones, in particular 3-
and 5-substituted-4-methoxy-6-methyl-2-pyrones, promp-
ted our investigations on the Suzuki cross-couplings of
3-halo- and 5-halo-4-methoxy-6-methyl-2-pyrones. Herein
we report synthetic details and some quite outstanding
antimicrobial activities for 3- and 5-substituted-4-meth-
oxy-6-methyl-2-pyrones.
Several Suzuki cross-couplings of 3-iodo- and 5-bromo-
4-methoxy-6-methyl-2-pyrone with arylboronic acids
have recently been reported.7 However, we were some-
what surprised that 3-bromo-4-methoxy-6-methyl-2-
pyrone could not be coupled under seemingly standard
conditions. Moreover the reaction times presented were
.
incredibly prolonged (19 h to 6 days!) using [Pd2dba3 dba]
(dba=dibenzylidene acetone) in refluxing toluene or
Pd(PPh3)4 in refluxing dimethoxyethane. We decided to
investigate the Suzuki cross-couplings of 3-bromo- and
5-bromo-4-methoxy-6-methyl-2-pyrone using our opti-
mised conditions9 for the synthesis of 3-alkenyl and
5-alkenyl-6-methyl-2-pyrones.
Suzuki Cross-Couplings
3-Bromo-4-methoxy-6-methyl-2-pyrone 5 was synthe-
sised in two steps from commercially available triacetic
acid lactone in 55% overall yield (Fig. 2).10
*Corresponding author. Tel.:+44-1904-434091; fax:+44-1904-432516;
e-mail: ijsf1@york.ac.uk
Figure 1. Bioactive 4-substituted-6-methyl-2-pyrones 1–4.
0960-894X/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00546-8

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L. R. Marrison et al. / Bioorg. Med. Chem. Lett. 13 (2003) 2667–2671
Table 3. Coupling of various arylboronic acids with 5a
Figure 2. Substrates 5 and 6 for Suzuki cross-coupling.
Table 1. Coupling of various arylboronic acids with 5a
Entry
Pyrone (R)
Yieldb
1
2
3
4
5
6
H-, 17
p-CH3-, 19
p-Cl-, 20
p-CH3O-, 21
m-NO2-, 22
p-CHO-, 23
79 (3)
73 (3)
69 (3)
65 (28)
38 (9)
49 (3)
Entry
Pyrone (R)
Yield from
boronic acid
(%)b
Yield from
benzodioxaboroles
(%)b
aConditions: (i) Pd(OAc)2 (6 mol %), PPh3 (18 mol %), arylboronic
acid, Na2CO3, benzene, Á, 6 h.
bIsolated yields after flash chromatography (%). Yield of reduced
product 18 in brackets (%).
1
2
3
4
5
CH3(CH2)2, 7
CH3(CH2)3, 8
CH3(CH2)4, 9
CH3(CH2)5, 10
Ph, 11
42
64
42
43
29
32
30
30
34
23
aConditions: (i), Pd(OAc)2 (6 mol %), PPh3 (18 mol %), alkenyl-
boronic acid, Na2CO3, benzene, Á, 6 h.
Higher reaction yields were obtained from reactions
where boronic acids were employed. This is advanta-
geous, as the boronic acids are not air sensitive, unlike
the benzodioxaboroles, which is presumably a con-
tributing factor to the lower yields for the latter. For
this reason, only the alkenylboronic acids were cross-
coupled with 6 (Table 2). The yields demonstrate that it
is easier to couple groups onto the 3-position as opposed
to the 5-position.
bIsolated yields after flash chromatography (%).
Bromination of commercially available dehydroacetic
acid at 0 ꢀC, followed by deacetylation with 90% sul-
phuric acid11 and then methylation provides 5-bromo-
methoxy-6-methyl-2-pyrone 6 starting material in 46%
yield.
With a view to optimizing the yield, it was predicted
that alteration of the ligand would be of great benefit.
Recent work in Pd-catalysed coupling reactions reveals
that the steric (cone angle, y) and electronic (s and p
donor/acceptor strengths) properties of the ligand for
palladium play a significant role in the yields of cross-
coupling reactions employing aryl and alkenyl halides/
psuedo halides. Little is known about the effect of dif-
ferent ligands on the Pd-catalysed cross-coupling reac-
tions of halogenated pyrone substrates.9 Two electron-
rich and sterically demanding ligands were chosen for
evaluation in the Suzuki cross-coupling of 6, namely
PCy3 and P(t-Bu)3.13 The yields of the cross-coupled
products 12–16 increased substantially for each alkenyl
We have previously established for 4-bromo-6-methyl-
2-pyrone that NaOEt and NaOH are far too strong for
the 2-pyrone unit to withstand due to the ease of basic
hydrolysis.9 Indeed, rapid pyrone decomposition for 5 is
observed in min. A mandatory base is Na2CO3 (2M),
although K2CO3 can be used albeit less effectively.
Using Na2CO3 and Pd(OAc)2 (6 mol %) with PPh3
(ligand for Pd) as the catalytic system in benzene/ethanol
at reflux gave good reproducible results.12 Thus com-
pound 5 (Table 1) was coupled with alkenylboronic
acids/alkenylbenzodioxaboroles in reaction times of less
than 6 h! Substantially less time than originally reported
for the coupling of arylboronic acids.7
Table 2. Coupling of various arylboronic acids with 6
Entry
Pyrone (R)
Isolated yields
a
b
b
Pd(OAc)2/PPh3
Pd(dba)2/PCy3
Pd(dba)2/P(t-Bu)3
1
2
3
4
5
CH3(CH2)2, 12
CH3(CH2)3, 13
CH3(CH2)4, 14
CH3(CH2)5, 15
Ph, 16
34
38
42
43
53
64
58
67
56
73
54
62
65
69
82
aConditions: (i) Pd(OAc)2 (6 mol %), L (18 mol %), alkenylboronic acid, Na2CO3, benzene, Á, 6 h. Isolated yields after flash chromatography (%).
b
t
.
Conditions: (i) Pd2dba3 dba (2.5 mol %), L (PCy3 or P Bu3) (5 mol %), alkenylboronic acid, Na2CO3, benzene, Á, 6 h.

L. R. Marrison et al. / Bioorg. Med. Chem. Lett. 13 (2003) 2667–2671
2669
Table 4. Coupling of various arylboronic acids with 6a
acids and substrates containing sp2 carbon–halide
bonds. Moreover, they demonstrate that 5 and 6 can be
used as benchmark substrates for testing new catalyst
systems.
Initially we investigated both P(t-Bu)3 and PPh3 as
ligands for Suzuki cross-coupling of 5 and 6 with aryl
boronic acids. We believed that an improvement on the
conditions previously reported could be made (2.5
Entry
Pyrone (R)
Yieldb
.
mol % Pd2dba3 dba or 5 mol % Pd(PPh3)4, K2CO3,
toluene or DME, rflx.).7 For phenylboronic acid, we
found that coupling to 5 proceeded equally well using
P(t-Bu)3 and PPh3, providing 17 in 79 and 81% yields,
respectively. We also encountered the formation of an
unusual dehalogenated product 18 in minor quantities
(3 and 5% for each ligand, respectively). Dehalogenated
side-products have been recently observed in Suzuki
cross-couplings of 4-bromopyrrole-2-carboxylates.14
Given the small difference in yields observed for each
ligand, we focused on the employment of Ph3P for cou-
pling arylboronic acids to both 5 and 6, as it is relatively
air and moisture stable and more easily handled than
t-Bu3P (Tables 3 and 4). The results clearly demonstrate
that a wide variety of arylboronic acids can be coupled
to 5 (entries 2–6, Table 3). Surprisingly, 18 was formed
in 28% yield when p-methoxyphenyl boronic acid was
employed, at the expense of the coupled product 21
(entry 4, Table 3). The less activated arylboronic acids
coupled less well, although modest yields for 22 and 23
were still obtained (entries 5 and 6, Table 3), providing a
range of candidates for biological testing.
1
2
3
4
5
6
H-, 24
p-CH3-, 25
p-Cl-, 26
p-CH3O-, 27
m-NO2-, 28
p-CHO-, 29
76
75
77
71
7
9
aConditions: (i) Pd(OAc)2 (6 mol %), PPh3 (18 mol %), arylboronic
acid, Na2CO3, benzene, Á, 6 h.
bIsolated yields after flash chromatography (%).
boronic acid. It has been suggested by Fu and co-
workers13b that a palladium monophosphine [Pd(PR3)]
might be the active catalyst. The steric bulk and the
electron-richness (s-donor) of the phosphine ligands
are proposed as being important factors for increased
.
reactivity. Compound 5 reacts with Pd2dba3 CHCl3 and
P(t-Bu)3 to give either a T-shaped or square planar
(O-methoxy coordination) complex. 31P NMR (202MHz)
shows a singlet at d 61.47, indicating the presence of one
phosphine ligand coordinated to Pd.
The aforementioned results extend on the trends
observed in the cross-coupling reaction of aryl boronic
For substrate 6, the coupled products 24–27 could
be obtained in useful yields (entries 1–4, Table 4).
Table 5. Antimicrobial activities of 3-, 4- and 5-substituted-6-methyl-2-pyronesa
Compd
Bacillus
subtilisb
Escherichia
colib
Staphylococcus
aureusb
Candida
albicansc
Saccharomyces
cerevisiaec
Schizosaccharomyces
pombec
3-pent, 7
3-hex, 8
3-hept, 9
3-oct, 10
18
19
020000
18
0
16
18
18
18
18
16
17
19
16
6
21
2021
000 13 (15)
00
0
0
0 000
0
0
3-pheth, 11
4-pent, 30
4-hex, 31
4-hept, 32
4-oct, 33
4-pheth, 34
5-pent, 12
5-hex, 13
5-hept, 14
5-oct, 15
5-pheth, 16
H-, 17
p-CH3-, 19
p-Cl-, 20
p-CH3O-, 21
m-NO2-, 22
p-CHO-, 23
H-, 24
0
0
0
0
0
0
19
18
18
2018
17
17
18
2016
14
6
2016
024
17
28
40
21
034
200
000
011
15
29
18
10
18
20
15
31
12
16
0
12
26
18
022
0
0
0
0
6
0
0
0
0
0
0
16
17
0
6
6
20 000
18
0
0
6
0
0030
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
20
6
6
p-CH3-, 25
p-CH3O-, 27
8
14
6
5
0
13
11
018
aEach cellulose disk contained 200 mg of the test compound. NT=not tested. Radii/mm of the inhibition zone. EtOH control (no inhibition).
bAfter 96 h incubation.
cAfter 48 h incubation. Control inhibitors, including squalestatin S1, as for ref 3 and 15a.

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L. R. Marrison et al. / Bioorg. Med. Chem. Lett. 13 (2003) 2667–2671
Table 6. Antimicrobial activities of 3-, 4- and 5-substituted-6-methyl-2-pyronesa
Compd
Aspergillus
nigerb
Fusarium
oxysporumb,c
Pythium
ultimumb,d
Pythium
mammillatumb
Rhizoctonia
solanib,e
Botrytis
cinereab,f
3-pent, 7
3-hex, 8
3-hept, 9
3-oct, 10
93 (>95)
77 (91)
90 (95)
8 (48)
8 (48)
11 (56)
11 (60)
11 (60)
5 (45)
2 (25)
2 (24)
5 (30)
NT
8 (35)
7 (35)
7 (24)
10 (46)
12 (51)
7.4 (60)
11 (60)
8 (60)
7 (60)
12 (60)
11 (60)
12 (51)
10 (54)
11 (61)
67 (80)
37 (57)
61 (75)
43 (100)
35 (100)
42 (100)
25 (100)
NT
44 (100)
43 (100)
47 (100)
53 (100)
92 (100)
49 (100)
36 (100)
38 (100)
44 (100)
47 (100)
50 (100)
41 (93.4)
33 (100)
40 (100)
67 (100)
75 (100)
55 (100)
51 (100)
81 (100)
77 (100)
79 (100)
67 (100)
61 (100)
69 (100)
56 (100)
51 (100)
53 (100)
60 (100)
58 (100)
47 (100)
44 (100)
45 (100)
42 (100)
59 (100)
39 (100)
33 (100)
12 (100)
29 (94)
47 (100)
63 (100)
58 (100)
58 (100)
62 (100)
63 (100)
65 (100)
47 (100)
49 (100)
53 (100)
89 (95)
100 (100)
100 (100)
100 (100)
96 (100)
95 (100)
100 (100)
100 (100)
100 (100)
100 (100)
100 (100)
100 (100)
NT
100 (100)
100 (100)
100 (100)
100 (100)
100 (100)
100 (100)
NT
3-pheth, 11
4-pent, 30
4-hex, 31
4-hept, 32
4-oct, 33
4-pheth, 34
5-pent, 12
5-hex, 13
5-hept, 14
5-oct, 15
5-pheth, 16
H-, 17
p-CH3-, 19
p-Cl-, 20
p-CH3O-, 21
m-NO2-, 22
p-CHO-, 23
H-, 24
p-CH3-, 25
p-CH3O-, 27
>95 (100)
>95 (100)
92 (95)
89 (94)
89 (94)
100 (100)
82 (90)
68 (>95)
82 (95)
90 (93)
NT
30 (100)
31 (93)
NT
NT
43 (100)
34 (92)
24 (94)
22 (90)
NT
NT
NT
NT
NT
NT
NT
NT
NT
71 (90)
>95 (100)
>95 (100)
>95 (95)
70 (82)
100 (100)
83 (>95)
71 (>90)
>90( >95)
85 (90)
NT
NT
NT
aEach test plate was inoculated by pippetting 20 mL of a 10mg/mL solution of the pyrone in absolute EtOH, 2 cm offset from the center of the plate
containing the organism.18 Bioassays were performed using either plate or well methods.19
bNumbers represent percentage growth of the organism after 72 h incubation (NT=not tested). Numbers in brackets represent % growth after 7
days incubation.
cEtOH control (15% growth after 72 h, 65% after 7 days).
dEtOH control (100% growth after 72 h).
eEtOH control (>95% after 72 h).
fEtOH control (65% growth after 72 h, 100% after 7 days).
However, poor yields were obtained for 28 and 29
(entries 5 and 6, Table 4). Dehalogenation did not occur
in reactions of 6, suggesting that the 3-position is more
susceptible to reduction, but also more reactive to cross-
coupling. Similar differences in reactivity have been
observed for 3,5-dibromo-2-pyrone, which allows for
regioselective cross-coupling at the 3-position.8
cinerea. The 5-substituted analogues as a class show
inhibitory activity against this organism.
The growth inhibitory activities of selected 2-pyrones
were determined in A2780human ovarium carcinoma
and K562 human chronic myelogenous leukaemia cell
lines using an in vitro cell culture system (MTT assay).17
The 3-octenyl (10) and 5-octenyl derivatives (15)
demonstrated promising activity against both cell lines
(IC50 (A2780) 30.3 mM; (K562) 30.1 mM; and (A2780)
26.9 mM; (K562) 33.3 mM, respectively.
Biological Activity
The 3-alkenyl (7–11) and 5-alkenyl (12–16) pyrones were
screened for promising antimicrobial activity against
bacteria, fungi and yeasts using reported assays.15 For
comparison, our previously reported 4-alkenyl pyrones
(30–34) are given.16 The screening results of the 3-aryl
(17, 19–23) and 5-aryl pyrones (24–29) are included. All
anti-microbial activities are shown in Tables 5 and 6.
These results clearly illustrate that a plethora of analo-
gues show inhibitory activity. Notable examples, with
potent inhibition, are given here. Analogue 20 inhibits
S. cerevisiae (the only compound in the aryl class that is
active). Analogue 31 shows excellent inhibitory activity
against S. pombe, comparable to that of squalestatin S1!
an extremely potent inhibitor of this organism.15b Inhib-
itory activity towards P. ultimum is seen for 8, which
retains potent inhibitory activity at 20 mg (39% inhibi-
tion), although activity is lost at 2 mg. An exceptional
result is seen for 14 (5-heptenyl analogue) against B.
In conclusion, we have synthesised a range of 3-alkenyl/
aryl- and 5-alkenyl/aryl-6-methyl-2-pyrones using Suzuki
cross-coupling. These studies represent the first compre-
hensive investigations into such coupling processes for 3-
bromo- and 5-bromo-4-methoxy-6-methyl-2-pyrones
and important reaction conditions are given, which
improve substantially on those previously reported.7
The 3-octenyl and 5-octenyl-4-methoxy-6-methyl-2-pyr-
ones (10 and 15) have been identified as promising new
cytotoxic agents against both A2780and K562 cell
lines. A significant study into the antimicrobial activities
of the 2-pyrone system has been conducted. The results
highlight remarkable inhibitory activity against a range
of bacteria, yeasts and fungi. Substituted pyrones are
clearly bioactive against a range of species and in the near
future we hope to exploit their application. The complete
results of these studies will be reported in due course.

L. R. Marrison et al. / Bioorg. Med. Chem. Lett. 13 (2003) 2667–2671
2671
Acknowledgements
9. Marrison, L. R.; Dickinson, J. M.; Ahmed, R.; Fairlamb,
I. J. S. Tetrahedron Lett. 2002, 43, 8853.
10. De March, P.; Moreno-manas, M.; Pi, R.; Ripoll, I.;
Sanchex-Fernando, F. J. Heterocyclic Chem. 1985, 22, 1537.
11. De March, P.; Moreno-manas, M.; Ripoll, I. Synth.
Comm. 1984, 14, 521.
We thank the Patterson Institute for Cancer Research
(Manchester, UK) for cell line testing, in particular Dr.
Tim Ward of the PICR Cell Culture Unit. The EPSRC
(PhD studentship to L.R.M.) and MMU for funding
are gratefully acknowledged. I.J.S.F. thanks the U.o.Y
for financial support in the form of an IRPF grant. We
gratefully acknowledge a generous loan of palladium
salts from Johnson Matthey PLC.
12. Representative procedure: Pd(OAc)2 (6 mol %), PPh3 (18
mol %) and 2 (1 mmol) in benzene (2 mL) and aq Na2CO3 (1
mL, 2M) were magnetically stirred under N2 for 0.5 h. To this
was added a solution of the boronic acid (1.1 mmol) in EtOH
(1 mL) and the solution was refluxed for 6 h, then allowed to
cool to room temperature. The mixture was extracted into
hexane (2Â10mL) and the combined extracts washed with
saturated aq NaCl (2Â10mL), dried (MgSO 4) and con-
centrated in vacuo to give the crude product, which was pur-
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17. See ref 3 for full details of the bioassay used to determine
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18. For complete assay procedures for the 2-pyrones, see:
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Metropolitan University (UK), 1998; Dickinson, J. M. PhD
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