fluorescence is quenched at neutral or basic pH. Upon
protonation of the aniline nitrogen fluorescence intensity
increases drastically.
Scheme 1. Synthesis of Nonfunctionalized, Monofunctional,
and Bifunctional BODIPY Dyes
In a recent paper by Urano et al., the development of a
series of pH-activatable dyes with varying pKa values is
described.4 These can be easily transformed into bifunc-
tional dyes, but for monoconjugation they are less con-
venient due to their symmetry.5 Other groups have
investigated the tetramethyl-BODIPY scaffold with a
meso-aniline substituent.3,7a,8 The methyl groups adjacent
to the nitrogens are susceptible to a Knoevenagel-type
condensation with benzaldehydes, yielding dyes with red-
shifted absorption and emission maxima.2b,c,e,9À11 The
potential of the Knoevenagel reaction to extend the con-
jugated system is widely acknowledged, and the reaction has
been used to construct large libraries of (pH-independent)
BODIPY dyes.12 To our knowledge, use of the Knoevena-
gel reaction to incorporate ligation handles in the BODIPY
dye has not been employed, although that would allow
covalent attachment to other (bioactive) molecules, which is
vital for applications in chemical biology. Often used liga-
tion reactions comprise the StaudingerÀBertozzi ligation
(involving azides),13 the copper(I) catalyzed Huisgen 1,3-
cycloaddition (involving azides or alkynes),14 and, in a more
traditional fashion, amide/ester formation (involving car-
boxylic acids). We here show that by using different N,N-
alkylated aniline derived tetramethyl-BODIPYs as a scaf-
fold in combination with functionalized (azide, alkyne,
methyl ester) benzaldehydes for the Knoevenagel reaction,
a series of tunable pH-activatable dyes can be prepared that
have the further advantage of being red-shifted compared to
the currently known pH-dependent BODIPYs. In addition,
we reveal that by applying the Knoevenagel reaction twice,
the synthesis of asymmetric bifunctional dyes with either an
extra conjugation handle or two sulfonic acids to increase
the water solubility can be achieved.
Starting from commercially available N,N-dialkylamino
benzaldehydes (or benzaldehyde in case of 1a, the “always
on” dye) and 2,4-dimethylpyrrole synthesis of nonfunction-
alized BODIPY dyes 1aÀe proved straightforward with
(8) Qin, W.; Baruah, M.; Van der Auweraer, M.; De Schrijver, F. C.;
Boens, N. J. Phys. Chem. A 2005, 109, 7371.
(9) (a) Yu, Y.-H.; Descalzo, A. B.; Shen, Z.; Rohr, H.; Liu, Q.; Wang,
€
Y.-W.; Spieles, M.; Li, Y.-Z.; Rurack, K.; You, X.-Z. Chem.;Asian. J
2006, 1, 176. (b) He, H.; Lo, P.-C.; Yeung, S.-L.; Fong, W.-P.; Ng,
D. K. P. Chem. Commun. 2011, 47, 4748. (c) Dost, Z.; Atilgan, S.;
Akkaya, E. U. Tetrahedron 2006, 62, 8484.
(10) (a) Bura, T.; Retailleau, P.; Ulrich, G.; Ziessel, R. J. Org. Chem.
2011, 76, 1109. (b) Zhu, S.; Zhang, J.; Vegesna, G.; Luo, F.-T.; Green,
S. A.; Liu, H. Org. Lett. 2011, 13, 483.
is usually accomplished by incorporation of a p-(N,N-
dialkyl)aniline moietyatthe meso-positionofthe BODIPY
core.3À5,5,6,7a Due to photoinduced electron transfer (PeT)7
(3) Ying, L.-Q.; Branchaud, B. P. Bioorg. Med. Chem. Lett. 2011, 21,
3546.
(4) Urano, Y.; Asanuma, D.; Hama, Y.; Koyama, Y.; Barrett, T.;
Kamiya, M.; Nagano, T.; Watanabe, T.; Hasegawa, A.; Choyke, P. L.;
Kobayashi, H. Nat. Med. 2009, 15, 104.
(5) Hoogendoorn, S.; Habets, K. L.; Passemard, S.; Kuiper, J.; van
der Marel, G. A.; Florea, B. I.; Overkleeft, H. S. Chem. Commun. 2011,
47, 9363.
(6) Rurack, K.; Kollmannsberger, M.; Daub, J. New J. Chem. 2001,
25, 289.
(7) (a) Kollmannsberger, M.; Rurack, K.; Resch-Genger, U.; Daub, J.
J. Phys. Chem. A 1998, 102, 10211. (b) de Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.;
Rice, T. E. Chem. Rev. 1997, 97, 1515. (c) Sunahara, H.; Urano, Y.; Kojima,
H.; Nagano, T. J. Am. Chem. Soc. 2007, 129, 5597.
(11) Buyukcakir, O.; Bozdemir, O. A.; Kolemen, S.; Erbas, S.;
Akkaya, E. U. Org. Lett. 2009, 11, 4644.
€
(12) (a) Schuller, A.; Goh, G. B.; Kim, H.; Lee, J.-S.; Chang, Y.-T.
Mol. Inf. 2010, 29, 717. (b) Lee, J.-S.; Kang, N.-Y.; Kim, Y. K.; Samanta,
A.; Feng, S.; Kim, H. K.; Vendrell, M.; Park, J. H.; Chang, Y.-T. J. Am.
Chem. Soc. 2009, 131, 10077. (c) Lee, J.-S.; Kim, H. K.; Feng, S.;
Vendrell, M.; Chang, Y.-T. Chem. Commun. 2011, 47, 2339. (d) Vendrell,
M.; Krishna, G. G.; Ghosh, K. K.; Zhai, D.; Lee, J.-S.; Zhu, Q.; Yau,
Y. H.; Shochat, S. G.; Kim, H.; Chung, J.; Chang, Y.-T. Chem. Commun.
2011, 47, 8424.
(13) Saxon, E.; Bertozzi, C. R. Science 2000, 287, 2007.
(14) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless,
K. B. Angew. Chem., Int. Ed. 2002, 41, 2596. (b) Tornoe, C. W.;
Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057.
Org. Lett., Vol. 13, No. 20, 2011
5657