Scheme 2
separated by column or preparative thin-layer chromatog-
In the solid state, cinnamoyldopamines (E)-3b, (E)-3c, and
(E)-5c showed photoreactivity to give dimers 6, 7, and 8,
respectively, while other compounds (E)-3a, (E)-5a, and (E)-
5b were virtually photostable (Scheme 2, Table 1).5,7 The
raphy on silica gel (benzene/acetone) or by preparative HPLC
on Asahipak GS-320 (MeOH). Isolation yields were reason-
able (Scheme 1).5
Solution photolyses of cinnamoyldopamines (E)-3a-3c
and (E)-5a-5c in methanol resulted in E-Z isomerization
(Scheme 2) and the E/Z ratio at the photostationary state
was around 50/50 in each case.7 The irradiation time required
to reach the photostationary state was much shorter for 3,4-
methylenedioxy derivatives (E)-3c and (E)-5c than for other
ones. In fact, the photoisomerization quantum yield (esti-
mated at 276 nm8) for (E)-3c (0.45) was much higher than
that for (E)-3a (0.086) or (E)-3b (0.095). Control experiments
on (E)-cinnamamide, (E)-4-chlorocinnamamide, and (E)-3,4-
methylenedioxycinnamamide have revealed that the photoi-
somerization quantum yields for these three amides are not
very different (0.24, 0.32, and 0.65, respectively). These
results suggest that, when the substituent R2 is hydrogen ((E)-
3a, -5a) or chlorine ((E)-3b, -5b), the excited states were
quenched probably by an intramolecular electron transfer
from the catechol moiety to the cinnamoyl moiety.
Table 1. Products from the Solid-State Photolysis
reactant
% conversion
products, % yield
(E)-3a
(E)-3b
(E)-3c
(E)-5a
(E)-5b
(E)-5c
0
100
51
∼0
∼0
66
6, 100 (Φ ) 0.75)
7, 55
8, 100
successful reactivities of the former three compounds appear
to support a claimed strategy in crystal engineering, where
chloro or methylenedioxy substitution is utilized to steer the
crystal packing of cinnamic acids to a parallel array of double
bonds.10 The dimerization of N-[(E)-4-chlorocinnamoyl]-
O,O′-dimethyldopamine ((E)-3b) into the R-truxillic dimer
6 is a very good solid-state photoreaction, because both the
conversion and the chemical yield were 100%, the crystalline
appearance was intact throughout the reaction,11 and the
quantum yield was very high, i.e., Φ ) 0.75, which was
estimated by a merry-go-round apparatus at 313 nm.12 This
quantum yield is higher than that of the efficient photo-
dimerization of (E)-cinnamic acid, Φ ) 0.59.12 The solid-
(3) (a) Austin, N. E.; Avenell, K. Y.; Boyfield, I.; Branch, C. L.; Hadley,
M. S.; Jeffrey, P.; Johnson, C. N.; Macdonald, G. J.; Nash, D. J.; Riley, G.
J.; Smith, A. B.; Stemp, G.; Thewlis, K. M.; Vong, A. K. K.; Wood, M.
Bioorg. Med. Chem. Lett. 2000, 10, 2553-2555. (b) Stemp, G.; Ashmeade,
T.; Branch, C. L.; Hadley, M. S.; Hunter, J. A.; Johnson, C. N.; Nash, D.
J.; Thewlis, K. M.; Vong, A. K. K.; Austin, N. E.; Jeffrey, P.; Avenell, K.
Y.; Boyfield, I.; Hagan, J. J.; Middlemiss, D. N.; Reavill, C.; Riley, G. J.;
Routledge, C.; Wood, M. J. Med. Chem. 2000, 43, 1878-1885.
(4) For the preparation of 3a-3c, O,O′-dimethyldopamine 1 (2 equiv)
and acid chloride 2 (1 equiv) were mixed together. For the preparation of
5a-5c, dopamine hydrochloride 4‚HCl (2 equiv), NaOH (2 equiv), and
acid chloride 2 (1 equiv) were mixed together.
(5) All new compounds (E)-3b, (E)-3c, (E)-5a, (E)-5b, (E)-5c, 6, 7, and
8 gave satisfactory NMR, IR, MS, and HRMS spectra. 6: colorless plates,
mp 209-210.5 °C (benzene/acetone). 7: colorless prisms, mp 189-192
°C (benzene). 8: white solid, mp 122-127 °C (preparative HPLC). The
spectral data for 6-8 are given in Supporting Information.
(8) An E-Z isomerization of methyl cinnamate (Φ ) 0.32) was used as
a standard: Lewis, F. D.; Quillen, S. L.; Elbert, J. E. J. Photochem.
Photobiol., A 1989, 47, 173-179.
(9) Ito, Y. Synthesis 1998, 1-32. Ito, Y. Mol. Supramol. Photochem.
1999, 3, 1-70.
(10) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids;
Materials Science Monographs, Vol. 54; Elsevier: New York, 1989.
(11) The reaction (E)-3b f 6 has been found to be a perfect single-
crystal-to-single-crystal transformation up to 100% conversion. Ohba, S.;
Ito, Y. Unpublished work.
(6) Literature mp 165-166 °C: Tseng, C.-F.; Iwakami, S.; Mikajiri, A.;
Shibuya, M.; Hanaoka, F.; Ebizuka, Y.; Padmawinata, K.; Sankawa, U.
Chem. Pharm. Bull. 1992, 40, 396-400.
(7) Irradiations were carried out under an argon atmosphere with a 400-W
high-pressure mercury lamp (Pyrex) for several hours (solution) or 20 h
(solid), as described previously.9
(12) Ito, Y.; Matsuura, T. J. Photochem. Photobiol., A 1989, 50, 141-
145.
2412
Org. Lett., Vol. 3, No. 15, 2001