title: Exciton coupling circular dichroism of an allylic N-imidazolyl group in amaranzole A, a marine natural product from Phorbas amaranthus.
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, USA.
authors.forename: Brandon I
authors.initials: BI
authors.lastname: Morinaka
authors.forename: Tadeusz F
authors.initials: TF
authors.lastname: Molinski
mesh_terms: D000818:Animals; D001688:Biological Products / Q000737:chemistry*; D002942:Circular Dichroism; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009792:Oceans and Seas; D011161:Porifera / Q000737:chemistry*; D013256:Steroids / Q000737:chemistry*
chemical_list: D001688:Biological Products; D007093:Imidazoles; D013256:Steroids; C526092:amaranzole A; C029899:imidazole