Four-step route to novel bisflavylium dications - First synthesis of phloroglucinol-type derivatives

Article abstract of DOI:10.1055/s-0032-1316681

Various bisflavylium dications have been prepared in an efficient four-step manner utilizing the acid-mediated condensation between bis(arylethynylketones) and activated phenols as the key step. Of special note is that phloroglucinol-type bisflavylium sal

Full text of DOI:10.1055/s-0032-1316681

LETTER
▌2053
letter
Four-Step Route to Novel Bisflavylium Dications – First Synthesis of
Phloroglucinol-Type Derivatives
Synthesis of Phloroglucinol-Type Derivatives
Marie Kueny-Stotz,*a Raymond Brouillard,a Stefan Chassaing*b,c
a
Laboratoire de Chimie des Polyphénols, Université de Strasbourg, 4 rue Blaise Pascal, 67008 Strasbourg, France
E-mail: kuenym@gmail.com
b
LSPCMIB (Laboratoire de Synthèse et Physico-Chimie de Molécules d’Intérêt Biologique), Université de Toulouse, UMR-CNRS 5068,
118 route de Narbonne, 31062 Toulouse Cedex 9, France
c
ITAV (Institut des Technologies Avancées en sciences du Vivant), Centre Pierre Potier, 1 Place Pierre Potier, BP 50624, 31106 Toulouse
Cedex 1, France
Fax +33(5)82991001; E-mail: stefan.chassaing@itav-recherche.fr
Received: 09.05.2012; Accepted after revision: 08.06.2012
ing been identified as authentic bioactive species11 and
more recently as in vitro precursors of bioactive species.12
Abstract: Various bisflavylium dications have been prepared in an
efficient four-step manner utilizing the acid-mediated condensation
between bis(arylethynylketones) and activated phenols as the key
step. Of special note is that phloroglucinol-type bisflavylium salts
have been obtained for the first time thanks to this procedure.
Being involved in flavylium chemistry,1m,2d,12,13 we were
aware of the possibility of designing novel flavylium-con-
taining compounds that could further improve their prop-
erties. We thus became interested in non-natural
biflavylium dications that are surprisingly scarce in the lit-
erature (Figure 1). Indeed, only a handful of reports deal
with such dicationic species despite their attractiveness
both in terms of structures and applications.
Key words: chromophores, condensation, flavylium, phenols,
polycations
The flavylium skeleton 1 is an organic chromophore of
particular relevance because of the characteristic of antho-
cyanins, the most widespread class of natural pigments af-
ter chlorophylls (Figure 1).1 Natural flavylium-containing
pigments are indeed responsible for a wide range of colors
(from shades of red to blue) of flowers, fruits and man-
made red wine.2 Their aptitude for imprinting colors of al-
most the whole visible spectrum is related to diverse pa-
rameters such as pH, complexation with either metal ions
(metalloanthocyanins) or aromatic compounds (copig-
mentation), as well as the structure of the pigments them-
selves (i.e., the nature and the position of the functional
groups attached to 1).1,3 In addition, flavylium-based pig-
ments 1 possess an inherent chemical reactivity that push-
es these native cationic structures into a well-established
network of chemical equilibria giving birth to mixtures of
structurally and thus color-modified species.3d,4 The posi-
tion of the resulting and reversible multistate/multifunc-
tional system is of course governed by previously
mentioned parameters (i.e., pH, complexation, and pig-
ments’ structure).3d
To date, various bisflavylium ions connected via their C4
positions, in a direct manner (i.e., compound 2) or via a
O
Ph
O
Ph
3'
2'
1'
4'
5'
2 ClO4
B
4
4
8
2 ClO4
O
4
7
6
2
6'
A
4
3
4
O
Ph
5
1
2
O
Ph
flavylium (or 2-
phenylbenzopyrilium)
chromophorea
3
Reynolds and van
Allan 1969b
R
R
2 ClO4
4
4
n
O
O
O
OH
4'
N
4 ClO4
R'
R'
Katritzky et al. 1998c
Profiting from this palette of attractive physico-chemical
properties, research on flavylium-based pigments has un-
dergone great advances in the last 30 years.1,3d Thus, nat-
ural and synthetic derivatives have been suggested as food
colorants,5 hair6 and laser7 dyes, sensitizers for solar
cells,8 water/humidity sensors,9 molecular-level optical
memories and logic gates.10 Interestingly, flavylium salts
are also attracting keen interest due to their potent benefi-
cial effects on human health, various flavylium salts hav-
4
methylviologen
bridge
N
4'
HO
O
5
Pina et al. 2011d
Figure 1 Structure of the flavylium chromophore 1 and summary of
the bisflavylium skeletons 2–5 reported so far in the literature. a The
SYNLETT 2012, 23, 2053–2058
Advanced online publication: 08.08.2012
DOI: 10.1055/s-0032-1316681; Art ID: ST-2012-D0402-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
b
numbering is indicated according to flavonoid nomenclature; see
ref. 14; c see ref. 15; d see ref. 16.

2054
M. Kueny-Stotz et al.
LETTER
carbon chain unit (i.e., compounds 3 and 4), have been re- lyzed condensation between phenolic derivatives 7 and
ported in the literature and proved to exhibit promising bis(arylethynylketones) 8, a well-known method for pre-
UV/Vis absorption/fluorescence features.14,15 From a syn- paring monocationic flavylium salts recently revisited by
thetic point of view, the preparation of dications 2–4 relies us.13a,b Worth mentioning here is that we also concentrated
on a common approach, that is, the functionalization/di- on phloroglucinol-type bisflavylium salts whose synthesis
merization of a precursor already featuring the C6–C3–C6 and physicochemical properties have never been reported
structure peculiar to flavonoids. Very recently, Pina et al. so far in the literature (Scheme 1, with R5 = R7 = OH).
described the synthesis of the symmetric bisflavylium 5
Table 1 Preparation of Bis(arylethynylketones) 8a–e
linked via their C4′ positions by a methyl viologen
bridge.16 The authors succeeded in the synthesis of this
O
unique tetracationic framework through standard acid-
catalyzed Robinson condensation.17 Interestingly enough,
the multistate/multifunctional behavior of 5 was investi-
gated and proved to be influenced by a cooperative effect
of the two covalently linked photochromic systems. Of
particular note is that all reported methods are generally
low-to-modest yielding and display a restricted substrate
scope regarding the A-ring substitution, especially in the
case of phloroglucinol-type A-ring which is the most fre-
quent motif encountered in natural flavylium-based
pigments.
O
O
2
O
O
OH
O
O
(i)a
10
(ii), (iii)a
+
Br
Br
O
9a–e
O
8a–e
Overall
In this letter, we describe the synthesis of a series of novel
bisflavylium dications 6 linked via their C4′ positions by
more or less flexible aliphatic/aromatic spacers (Scheme 1).
In order to construct such dicationic species, we planned
to evaluate for the first time the potential of the acid-cata-
Br
Br
Entry
Product 8
isolated
yield (%)b
O
O
O
1
2
3
8a 74
8b 81
8c 82
O
4'
O
4'
n
Br
Br
n
R7
O
7
O
R7
7
O
A
8a n = 3
8b n = 4
8c n = 5
2 PF6
5
5
R5
6
R5
O
R7
O
O
R7
OH
HO
O
O
O
8
4
91
7
7
R5
R5
O
Br
Br
O
O
O
O
O
8d
9
O
O
O
5
81
O
O
Br
Br
H
H
Br
Br
O
O
O
10
10
8e
a Reaction conditions: (i) 10 (2.4 equiv), K2CO3 (3.0 equiv), DMF,
100 °C, 12 h (84–97% yield); (ii) ≡–MgBr (2.6 equiv), THF, 0 °C
to r.t. (84–99% yield); (iii) IBX (4.0 equiv), EtOAc, 80 °C, 12 h
(85–99% yield).
,
=
,
n
n = 3, 4, 5
b See Supporting Information for the characterization data of all com-
pounds.
Scheme 1 Retrosynthetic analysis
Synlett 2012, 23, 2053–2058
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Phloroglucinol-Type Derivatives
2055
Starting from commercially available p-hydroxybenzal- step proposed to furnish the targeted bisflavylium cations.
dehyde (10, Scheme 1), we first prepared via a three-step To this end, we considered three phenolic derivatives 7a–c
sequence a series of five bis(arylethynylketones) 8a–e as condensation partners [i.e., resorcinol (7a), 3,5-dime-
featuring spacers of diverse length and nature (Table 1). thoxyphenol (7b), and phloroglucinol (7c)] and submitted
The sequence began with a known procedure consisting in all 7/8 combinations to the condensation conditions we re-
coupling two equivalents of 10 with diverse dibrominated ported recently (acetic acid as solvent in the presence of
alkylating agents.18 This coupling step via O-alkylation of aqueous hexafluorophosphoric acid, Table 2).13a,b After
10 allowed the efficient formation of bis(benzaldehydes) 48 hours stirring at room temperature, each of the 15 pos-
9a–e in which native phenolic moieties are separated by sible bisflavylium salts 6a–o precipitated out in the medi-
3-, 4-, and 5-carbon aliphatic units (9a–c) as well as 4- and um and were obtained in almost quantitative yields, no
5-carbon units possessing an aromatic residue (9d,e). trace of product resulting from a monocondensation event
Bis(benzaldehydes) 9a–e were then submitted to a nu- being detected in the crude material.
cleophilic addition–oxidation sequence to furnish the ex-
pected bis(arylethynylketones) 8a–e. The nucleophilic
addition of ethynylmagnesium bromide to both aldehydic
Interestingly enough, all pigments 6a–o were only washed
with a little volume of diethyl ether, thus meeting most
criteria of a ‘click chemistry’ reaction.20 Furthermore, the
functions of 9a–e resulted in bis(prop-2-yn-1-ol) interme-
biscondensation of resorcinol (7a) with each of the keton-
diates which were smoothly oxidized by IBX to the corre-
ic partners 8a–e occurred in a highly regioselective man-
ner (Table 2, entries 1–3, 10, and 13). Under these
conditions, the only dicationic regioisomer to be formed
sponding bis(ynones) 8a–e.19 Noteworthy is that this
three-step sequence enabled the access to the desired
bis(arylethynylketones) in a straightforward and efficient
exhibited two 7-hydroxyflavylium moieties, with no trace
manner, 8a–e being obtained in good to excellent overall
of a 5-hydroxyflavylium moiety being detected by NMR
spectroscopic analysis of the crude products. This regio-
yields (i.e., from a 74% yield for 8a to 91% for 8d).
With these five bis(arylethynylketone) building blocks chemical outcome is in full agreement with our recent
8a–e in hand, we then investigated the key condensation monocondensation experiments.13a
Table 2 Synthesis of Bisflavylium Dications via the Acid-Mediated Condensation of Bis(arylethynylketones) 8a–e and Activated Phenols
7a–c
R2
R1
OH
R1
O
R2
O
7a–ca
O
O
O
O
(2.0 equiv)
HPF6 (50% aq)
2 PF6
AcOH, r.t., 48 h
R1
O
O
R2
8a–e
6a–o
Entry
Phenol 7
Product 6
Isolated yield (%)b
HO
O
O
6a 95
6b 99
6c 99
1–3
7a
HO
O
n
n
O
2 PF6
6a n = 3
6b n = 4
6c n = 5
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2053–2058

2056
M. Kueny-Stotz et al.
LETTER
Table 2 Synthesis of Bisflavylium Dications via the Acid-Mediated Condensation of Bis(arylethynylketones) 8a–e and Activated Phenols
7a–c (continued)
R2
R1
OH
R1
O
R2
O
7a–ca
O
O
O
O
(2.0 equiv)
HPF6 (50% aq)
2 PF6
AcOH, r.t., 48 h
R1
O
O
R2
8a–e
6a–o
Entry
Phenol 7
Product 6
Isolated yield (%)b
OMe
MeO
O
OMe
O
6d 95
6e 98
6f 94
4–6
7b
n
MeO
O
n
O
2 PF6
6d n = 3
6e n = 4
6f n = 5
OH
HO
O
OH
O
6g 95
6h 99
6i 99
7–9
7c
n
HO
O
n
O
2 PF6
6g n = 3
6h n = 4
6i n = 5
R2
R1
O
O
2 PF6
O
6j 95
6k 98
6l 99
O
10–12
7a–c
R1
R2
6j R1 = OH; R2 = H
6k R1 = R2 = OMe
6l R1 = R2 = OH
Synlett 2012, 23, 2053–2058
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Phloroglucinol-Type Derivatives
2057
Table 2 Synthesis of Bisflavylium Dications via the Acid-Mediated Condensation of Bis(arylethynylketones) 8a–e and Activated Phenols
7a–c (continued)
R2
R1
OH
R1
O
R2
O
7a–ca
O
O
O
O
(2.0 equiv)
HPF6 (50% aq)
2 PF6
AcOH, r.t., 48 h
R1
O
O
R2
8a–e
6a–o
Entry
Phenol 7
Product 6
Isolated yield (%)b
R2
R2
R1
O
O
O
6m 94
6n 99
6o 92
R1
O
13–15
7a–c
2 PF6
6m R1 = OH; R2 = H
6n R1 = R2 = OMe
6o R1 = R2 = OH
a Compound 7a: R1 = OH, R2 = H; compound 7b: R1 = R2 = OMe; compound 7b: R1 = R2 = OH.
b See Supporting Information for the characterization data of all dications, spectroscopic data for 6a,g are being given in ref.21,22 as illustrative
examples.
In conclusion, we have developed an expeditious and
highly efficient procedure (i.e., overall yields of 70–90%)
Typical Procedure for the Acid-Mediated Condensation of Phe-
for the synthesis of bisflavylium skeletons. The key final
step of the sequence consists in the facile acid-mediated
biscondensation of activated phenols and easy-to-prepare
bis(arylethynylketones), thus expanding the scope of this
‘click chemistry’-like protocol. Of particular interest is
that this procedure allowed the successful access for the
nolic Derivatives with Bis(arylethynylketones)
To a solution of phenol 7 (1.0 mmol, 2.0 equiv) and bis(arylethynyl-
ketone) 8 (0.5 mmol, 1.0 equiv) in AcOH (2 mL) was added aque-
ous HPF6 (0.5 mL, 50% in H2O). The solution, becoming
immediately dark red, was stirred at r.t. for 48 h. The resulting mix-
ture was then poured into Et2O (20 mL) when the bisflavylium di-
hexafluorophosphate precipitated. The orange to red solid was
first time to bisflavylium pigments composed of two phlo- recovered by filtration, washed with Et2O, and finally dried under
vacuum to give the required dicationic species 6 in pure form. Re-
crystallization from AcOH was performed when necessary.
roglucinol-type (i.e., 5,7-dihydroxylated) flavylium moi-
eties. This four-step route is thus alternative but, above all,
complementary to the routes reported so far in the litera-
ture for accessing to such dicationic patterns.14–16 These
dicationic pigments represent novel and attractive bichro-
mophoric materials whose physicochemical properties
and potential applications are currently explored.
Acknowledgment
The authors thank the CNRS and the French Ministry of Research
for financial supports.
While the present work only focused on B-ring 4′,4′-
Supporting Information for this article is available online at
linked bisflavylium dications, one can easily conceive the
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
application of the sequence for preparing a wider range of
derivatives, going from regioisomeric B-ring linked bis-
flavylium to triflavylium or even oligoflavylium cations.
Further work is now under way in those respects and will
be published in due time.
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2053–2058

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M. Kueny-Stotz et al.
LETTER
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(21) Resorcinol-Type Dication 6a
Yellow powder; yield 95%. IR (KBr): 3380 (OH), 1635
(C=O), 855 (PF) cm–1. UV/Vis (MeCN–10% 1 N HCl):
λmax (ε) = 262 (15700), 466 (38600 M–1 cm–1) nm. 1H NMR
(500 MHz, CD3CN–1% TFA-d1): δ = 2.37 (quin, J = 5.9 Hz,
2 H), 4.40 (t, J = 5.9 Hz, 4 H), 7.23 (m, 4 H), 7.40 (dd, J =
8.8, 2.2 Hz, 2 H), 7.47 (dd, J = 2.2, 0.7 Hz, 2 H), 8.06 (d, J =
8.8 Hz, 2 H), 8.15 (d, J = 8.8 Hz, 2 H), 8.38 (m, 4 H), 8.95
(dd, J = 8.8, 0.7 Hz, 2 H). 13C NMR (125 MHz, CD3CN–1%
TFA-d1): δ = 29.9, 66.9, 104.3, 114.2, 117.9, 120.7, 122.6,
122.8, 133.9, 134.2, 155.4, 160.5, 168.2, 169.5, 174.0. ESI-
MS (positive mode): m/z (%) = 259 (100) [M2+]. ESI-
HRMS: m/z calcd for C33H26O6 [M2+]: 259.0859; found:
259.0847.
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(7) Czerney, P.; Graness, G.; Birckner, E.; Vollmer, F.; Rettig,
W. J. Photochem. Photobiol. A 1995, 89, 31.
(8) (a) Cherepy, N. J.; Smestad, G. P.; Grätzel, M.; Zhang, J. Z.
J. Phys. Chem. B 1997, 101, 9342. (b) Sirimanne, P. M.;
Senevirathna, M. K. I.; Premalal, P. K. D.; Pitigala, P. K. D.
D. P. Semicond. Sci. Technol. 2006, 21, 818. (c) Sirimanne,
P. M.; Senevirathna, M. K. I.; Pitigala, P. K. D. D. P.;
Sivakumar, V.; Tennakone, K. J. Photochem. Photobiol. A
2006, 177, 324.
(22) Phloroglucinol-Type Dication 6g
Orange powder; yield 95%. IR (KBr): 3400 (OH), 1635
(C=O), 835 (PF) cm–1. UV/Vis (45% EtOH–45% MeCN–
10% 1 N HCl): λmax (ε) = 278 (14500), 326 (6200), 416 (sh),
476 (20800 M–1 cm–1) nm. 1H NMR (500 MHz, CD3CN–1%
TFA-d1): δ = 2.35 (quin, J = 6.2 Hz, 2 H), 4.38 (t, J = 6.2 Hz,
4 H), 6.72 (d, J = 2.2 Hz, 2 H), 6.96 (dd, J = 2.2, 0.7 Hz, 2
H), 7.19 (m, 4 H), 7.94 (d, J = 8.8 Hz, 2 H), 8.29 (m, 4 H),
9.04 (dd, J = 8.8, 0.7 Hz, 2 H). 13C NMR (125 MHz,
CD3CN–1% TFA-d1): δ = 29.9, 66.7, 96.9, 104.0, 111.5,
114.2, 117.7, 122.6, 133.2, 150.4, 159.6, 160.1, 167.7,
171.5, 173.2. ESI-MS (positive mode): m/z (%) = 275 (100)
[M2+]. ESI-HRMS: m/z calcd for C33H26O8 [M2+]: 275.0808;
found: 275.0798.
(9) Galindo, F.; Lima, J. C.; Luis, S. V.; Melo, M. J.; Parola, A.
J.; Pina, F. J. Mater. Chem. 2005, 15, 2840.
Synlett 2012, 23, 2053–2058
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