The synthesis and biological evaluation of a range of novel functionalised benzopyrans as potential potassium channel activators

Article abstract of DOI:10.1016/j.bmcl.2007.11.135

A range of novel benzopyrans have been synthesised and biologically evaluated for K_ATP channel activity employing cromakalim 1 as a benchmark K_ATP channel opener. Although the compounds that were evaluated demonstrated a reduced ability to relax phenylephrine stimulated rat thoracic tissue, we provide evidence that benzopyrans 7a-h may be operating via an alternative mechanism than ATP-sensitive K⁺ channel activity.

Full text of DOI:10.1016/j.bmcl.2007.11.135

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Bioorganic & Medicinal Chemistry Letters 18 (2008) 1237–1240
The synthesis and biological evaluation of a range of
novel functionalised benzopyrans as potential potassium
channel activators
Elizabeth Tyrrell,a,* Kibur Hunie Tesfa,a Iain Greenwoodb and Alistair Manna
aSchool of Pharmacy & Chemistry, Kingston University, Penrhyn Road, Kingston, Surrey KT1 2EE, UK
bSt. Georges, University of London, CranmerTerrace, London SW17 0RE, UK
Received 27 September 2007; revised 22 November 2007; accepted 26 November 2007
Available online 23 December 2007
Abstract—A range of novel benzopyrans have been synthesised and biologically evaluated for KATP channel activity employing cro-
makalim 1 as a benchmark KATP channel opener. Although the compounds that were evaluated demonstrated a reduced ability to
relax phenylephrine stimulated rat thoracic tissue, we provide evidence that benzopyrans 7a–h may be operating via an alternative
mechanism than ATP-sensitive K+ channel activity.
Ó 2008 Published by Elsevier Ltd.
Historically the benzopyran cromakalim 1 represents
one of the first reported anti-hypertensive agents to act
solely via potassium channel modulation.1 The synthe-
ses of analogues to 1 have increased in intensity over
the past few years and several reviews have been pub-
lished.2 Therapeutic agents, that function via the open-
ing of potassium channels, have become topical for
treating conditions such as hypertension, angina and
epilepsy however and more recently urinary
incontinence.3
possessed components of the key pharmacophores pres-
ent in cromakalim 1. In particular the benzopyran nu-
cleus, a trans arrangement at the C-3, C-4 chiral
centres and an activated aromatic ring.5 Benzopyrans,
2 and 3, were synthesised using a novel variation of an
intramolecular Nicholas reaction6 that was developed
in our laboratories. These were obtained in good yield
and with extremely high levels of trans-diastereoselectiv-
ity (Scheme 1).7
More recently we have focused our attention upon the
development of asymmetric Nicholas cyclisation reac-
tions analogous to those shown above. Our aims were
twofold the first was an attempt to synthesise homochi-
ral variants of compound 4 and then to investigate the
levels of asymmetric induction during the subsequent
cyclisation reaction.8 Our second aim was to screen
these new molecules for potassium channel activity.
The asymmetric synthesis of the propargyl alcohols,
based upon 4, was successfully accomplished using an
asymmetric alkylation reaction described by Carreira.9
This procedure involves the addition of a zinc acetylide
to aldehyde 5 in the presence of a chiral ligand such as
N-methylephedrine. This provided access to homochiral
propargyl alcohols 6 in high yield and excellent enanti-
oselectivities.10 Exposure of the dicobalt hexacarbonyl
derivatives of the propargyl alcohols 6 to Lewis acid
led to a smooth cyclisation reaction. Oxidative decom-
plexation provided the optically active benzopyrans,
7a–h, in very high yield (Scheme 2).
Our interest in cromakalim 1 was first highlighted in
1998 when we revealed the results obtained from the
screening of a small library of benzopyrans as potential
potassium channel activators (Fig. 1).4
Our study compared the potential of our compounds,
against cromakalim 1, to inhibit oxytocin induced uter-
ine spasms. The results from these studies showed that
cromakalim 1 induced complete inhibition of oxytocin
induced uterine spasm at a 5 lM level, whereas analogue
2 produced a 98% reduction at 35 lM and 3 induced a
71% reduction at 8 lM. Despite the relative simplicity
of our analogues we concluded that they nevertheless
Keywords: Benzopyrans; Nicholas reaction; Chiral; Potassium channel
activators; Cromakalim; Rat thoracic tissue; Smooth muscle
relaxation.
*
Corresponding author. Tel.: +44 208 547 8810; fax: +44 208 547
7562; e-mail: e.tyrrell@kingston.ac.uk
0960-894X/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2007.11.135

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E. Tyrrell et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1237–1240
H
N
O
H
F
H
H
F
Br
NC
OH
CH3
CH3
O
O
O
OMe
2
Br
3
1
Figure 1. Cromakalim 1 and the most active analogues 2 and 3.
Co2(CO)6
HBF4
OH
Co2(CO)8
OH
O
4
O
Co2(CO)6
CAN
H
H
H
H
F
+
O
O
O
Scheme 1. Synthesis of benzopyrans.
Ph
O
i. Zn(OTf)2,
(+)-N-methylephedrine
TEA then
H
R1
R2
R1
H
OH
ii. Phenylacetylene
R2
O
O
Ph
R3
5
ee up to 99%
6
Ph
R3
7a R1 = R2 = R3 = H
7b R1 = R2 = H, R3 = OMe
7c R1 = F, R2 = R3 = H
7d R1 = R3 = F, R2 = H
7e R1 = R2 = H, R3 = F
7f R1 = R3 = Cl, R2 = H
7g R1 = NO2, R2 = R3 = H
H
H
R1
R2
i. Co2(CO)8
ii. BF3OEt2
iii CAN
F
O
R3
7a-h
7h R1 = Ph, R2 = R3 = H
Scheme 2. Benzopyrans from homochiral propargyl alcohols.
In contrast to our earlier studies4 the presence of a phen-
ylalkynyl moiety in 6 exerted a significant influence
upon the formation of the new stereogenic centres to af-
ford compounds 7a–h with a cis relative stereochemistry
C-3 and C-4.11 We were therefore very keen to ascertain
any similarity in activity between 7a–h and the corre-
sponding trans-isomers 2 and 3.
From Figure 2 it may be seen that cromakalim 1 produced
a marked and rapid relaxation which plateaued within
5 min and amounted to a 56% 3% at 1 lM and
72% 2% at 10 lM (n = 8). Although none of the benzo-
pyrans 7a–h proved as effective as cromakalim 1 for
inducing vasorelaxation, we were encouraged by the re-
sults obtained with compounds 7c–e and 7g (Fig. 3).
Isometric tension recordings on segments of mouse
thoracic aorta were undertaken to assay the biologi-
cal activity of benzopyrans 7a–h. After an initial per-
iod of equilibration the tissues were contracted by
application of the a-1 adenoceptor stimulant phenyl-
ephrine (1 lM). Once the contraction had stabilized
cromakalim 1 or one of the compounds under assay
7a–h at 1 and 10 lM was added to the bath for
10 min (Fig. 2).
This first series of experiments established that compound
7g was the most potent benzopyran however it was clear
that the vasorelaxant effect was significantly slower than
that of cromakalim 1. Consequently, the maximum relax-
ant effect of these compounds has been underestimated
using the isochronal protocol. An amended protocol
was therefore employed in order to determine the full
relaxant efficacy of compounds 7a–h. In these experi-
ments the tissue was contracted with phenylephrine and

E. Tyrrell et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1237–1240
1239
Figure 4. The effect of cromakalim 1 upon contracted MTA tissue in
the presence of glibenclamide.
Figure 2. The effect of cromakalim (1 lM) on contracted MTA.
Figure 3. The effects of time and concentration upon the relaxation of
MTA using 7c (above) and 7g (below).
Figure 5. The effect of benzopyrans 7c (above) and 7e (below) upon
contracted MTA tissue in the presence of glibenclamide.
then concentrations of the relevant benzopyran (1–
40 lM) were applied until the relaxation had stabilized.
Under these conditions 7c–e and 7g produced signifi-
cantly greater relaxations at 10 lM than in the first proto-
col. The mean relaxations produced by these agents and
the time taken are shown (Table 1).
order to ascertain whether they exerted their activity
by an analogous mechanism to cromakalim 1. Tissue
segments were initially incubated with 10 lM gliben-
clamide, a selective blocker of ATP-sensitive K+ chan-
nels, before being contracted by 1 lM of
phenylephrine. Not unexpectedly the addition of cro-
makalim (20 lM) failed to elicit any relaxation of
the tissue tension (n = 6) (Fig. 4).
The mode of action of cromakalim 1 has been well
documented12 and involves the activation of ATP-sen-
sitive K+ channels formed from the combination of
Kir 6.1/6.2 channel proteins and SUR2 accessory pro-
teins.12,13 In a series of further experiments we inves-
tigated the mode of action of benzopyrans 7a–h in
In contrast compounds 7c, 7e and 7g (20 lM) produced
a mean relaxation of 44 10, 30 12 and 30 18% in
the presence of glibenclamide (Fig. 5).

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E. Tyrrell et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1237–1240
3. Andersson, K.-E. Pharmacol. Toxicol. 1992, 70, 244;
Table 1.
Drizin, I.; Holladay, M. W.; Lin, Y.; Zhang, H. Q.;
Gopalakrishnan, S.; Gopalakrishnan, M.; Whiteaker, K.
L.; Buckner, S. A.; Sullivan, J. P.; Carroll, W. A. Bioorg.
Med. Chem. Lett. 2002, 12, 1481.
Compound
Relaxationa
(%)
Time
(Min.)
Relaxationb
(%)
Time
(Min.)
7c
7d
7e
7g
36
20
28
43
1
8
5
3
55
41
43
40
3
3
2
2
83
65
92
88
2
2
2
3
33
24
21
30
3
2
2
6
4. Mann, A.; Muller, C.; Tyrrell, E. J. Chem. Soc., Perkin
Trans. 1998, 1427.
5. As well as SAR data in Ref. 3 see Russell, K. Bioorg. Med.
Chem. Lett. 1996, 6, 913.
a Conc. = 10 lM (n = 3).
b Conc. = 40 lM (n = 3).
6. Nicholas, K. M. Acc. Chem. Res. 1987, 20, 207.
7. Tyrrell, E.; Skinner, G. A.; Bashir, T. Synlett 2001, 12,
1929; Tyrrell, E.; Tillett, C. Tetrahedron Lett 1998, 39,
9535; Berge, J.; Claridge, S.; Mann, A.; Muller, C.;
Tyrrell, E. Tetrahedron Lett. 1997, 38, 685.
8. Tyrrell, E.; Millet, J.; Hunie Tesfa, K.; Williams, N.;
Mann, A., et al. Tetrahedron 2007, 63, 12769.
9. Frantz, D. E.; Carriera, E. M. J. Am. Chem. Soc. 2000,
122, 1806; Anand, N. K.; Carreira, E. M. J. Am. Chem.
Soc. 2001, 123, 9687; Boyall, D.; Frantz, D.; Carreira, E.
M. Org. Lett. 2002, 4, 2605.
10. Tyrrell, E.; Hunie Tesfa, K.; Millet, J.; Muller, C.
Synthesis 2006, 18, 3099; Tyrrell, E.; Hunie Tesfa, K.;
Mann, A.; Singh, K. Synthesis 2007, 10, 1491.
11. The cis stereochemistry for compounds 7a–h was based
upon the coupling constant J for the methine protons
which was found to be between 5.1 and 5.8 Hz. In contrast
the coupling constants derived from the 1H NMR
spectrum for the corresponding methine protons in
compounds 2 and 3 were found to be 10 and 11 Hz,
respectively.
Whilst the effect of these agents was attenuated in the
presence of glibenclamide there was still nevertheless
marked relaxation. This suggests that these substi-
tuted benzopyrans can relax smooth muscle by an-
other mechanism unrelated to opening of ATP-
sensitive K+ channels. Our investigations into this
outcome and the activity of these benzopyrans are
on-going and the outcomes will be disseminated in
due course.
References and notes
1. Hamilton, T. C.; Weir, S. W.; Weston, A. H. Br. J.
Pharmacol. 1986, 88, 103.
2. Emfield, J. R.; Russell, K. Annu. Rep. Med. Chem. 1995,
81; Roxburgh, C. J. Synthesis 1996, 307; Cho, H.; Katoh,
S.; Sayama, S.; Murakami, K.; Nakanishi, H.; Kajimoto,
Y.; Ueno, H.; Kawasaki, H.; Aisaka, K.; Uchida, I. J.
Med. Chem. 1996, 39, 3797.
12. Aschcroft, F. M.; Gribble, F. M. Trends Pharm. Sci. 2005,
21, 439.
13. Cechetti, V.; Tabarrini, O.; Sabatini, S. Curr. Top. Med.
Chem. 2006, 6, 1049.