Resveratrol Diastereomers
887
hypochromic (" ¼ 30400 to " ¼ 14200 dm cmꢁ 1 molꢁ 1) shift of the (Z)-diastereo-
mer and rather large Stokes shifts (99 and 81 nm for (Z)- and (E)-1) of the emission
spectra. The fluorescence quantum yields of the two diastereomers (Ff(E) ¼ 0.003)
and (Ff(Z) ¼ 0.02) differ by nearly one order of magnitude, which may be ratio-
nalized from the higher inflexibility of the sterically congested (Z)-diastereomer as
derived from the semiempirical calculations mentioned above. Upon monodepro-
tonation a bathochromic shift of the main absorption band of (E)-1 to 319 nm and
of its fluorescence band to 450 nm was observed. The absorption of (Z)-1 was
shifted to 299 and its fluorescence to 402 nm. Such shifts are typical for the depro-
tonation of phenols [9]. Interestingly enough, the fluorescence quantum yields of
the two monodeprotonated species became equal (Ff(E) ¼ Ff(Z) ¼ 0.0015).
Experimental
(E)-Resveratrol ((E)-1) was of commercial origin (Sigma; ꢂ 99%). Photodiastereomerizations were
executed by means of a Hanau TQ 150 Z2 UV-lamp under an Ar atmosphere (MeOH or DMSO
solution). For preparative HPLC separations a C8-reversed-phase column (250ꢄ 4.6 mm) on a Dionex
P680=ASI100=PDA100 HPLC instrument applying a gradient from solvent A (H2O=MeCN ¼ 95=5,
acidified with acetic acid to pH¼ 3) to B (H2O=MeCN¼ 5=95), with a flow rate of 4 cm3 minꢁ 1, per
cycle 200 mm3 of a 1.75 g dmꢁ 3 solution of 1 in solvent A were used. 1H and 13C NMR spectra were
recorded by means of a Bruker Avance 200 MHz instrument. UV-spectra were measured on a Hewlett
Packard HP 8453 and fluorescence spectra on a Hitachi F 4010 spectrometer. Fluorescence quantum
yields were determined using quinineÁ HCl as the standard (Ff ¼ 0.546 [11]). Thermal isomerizations
were followed using a Varian CARY 100 BIO instrument with a Peltier cell thermostat allowing to set
temperatures within ꢅ 0.01ꢃC. AM1 calculations [12a–12c] were executed using the program package
MOPAC [12d] on the SGI Origin computer of the LIZENS of the Johannes Kepler University Linz.
Acknowledgments
We are grateful to DI B. Schwarzinger and DI B. Lackner for discussions, HPLC settings, and
recordings of spectra.
References
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