Morinaka Brandon I, Molinski Tadeusz F
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, USA.
Chirality. 2008 Sep;20(9):1066-70. doi: 10.1002/chir.20636.
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.
一种带有C24-咪唑基的新型甾体生物碱amaranzole A（10）在短波长处显示出异常大的裂分圆二色光谱，我们将其归因于C25 C=C双键与咪唑基的极化π-π跃迁之间的激子耦合圆二色性（ECCD）。由光学纯的（R）-（-）-2-氨基丁醇制备的模型4,5-二取代咪唑11表现出类似的ECCD以及与咪唑环质子化状态变化一致的溶剂和pH依赖性。对12（11的二氢衍生物）的计算和圆二色性测量表明，在10中4-羟基苯基与咪唑基的共轭不强，观察到的ECCD完全由C=C双键与孤立的咪唑π-π跃迁之间的耦合引起。
