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A new steroidal alkaloid amaranzole A (10) with a C24-imidazolyl group displays an unusually large split-CD spectrum at short wavelengths that we assign to exciton coupled circular dichroism (ECCD) between the polarized pi-pi* transitions of the C25 C=C double bond and the imidazolyl group. A model 4,5-disubstituted imidazole 11, prepared from optically pure (R)-(-)-2-aminobutanol, exhibited similar ECCD and solvent and pH-dependence consistent with changes in the protonation state of the imidazole ring. Calculations and CD measurement of 12 (the dihydro-derivative of 11) suggest that the 4-hydroxyphenyl group is not strongly conjugated to the imidazole group in 10, and the observed ECCD is entirely accounted for by coupling between the C=C double bond and isolated imidazole pi-pi* transitions. (c) 2008 Wiley-Liss, Inc.


Brandon I Morinaka, Tadeusz F Molinski. Exciton coupling circular dichroism of an allylic N-imidazolyl group in amaranzole A, a marine natural product from Phorbas amaranthus. Chirality. 2008 Sep;20(9):1066-70

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PMID: 18690668

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