Brandon I Morinaka1, Tadeusz F Molinski. 1. Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, USA.
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. (c) 2008 Wiley-Liss, Inc.
A new steroidal alkaloidn class="Chemical">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.
Authors: John W Blunt; Brent R Copp; Wan-Ping Hu; Murray H G Munro; Peter T Northcote; Michèle R Prinsep Journal: Nat Prod Rep Date: 2007-01-11 Impact factor: 13.423
Authors: Colin K Skepper; John B Macmillan; Guang-Xiong Zhou; Makoto N Masuno; Tadeusz F Molinski Journal: J Am Chem Soc Date: 2007-03-21 Impact factor: 15.419