The effects of nonpolar and polar solvents on the Q, and Q, energies of bacteriochlorophyll (BChl) a and bacteriopheophytin (BPhe) a were examined by electronic absorption spectroscopy. All of the four different energies exhibited a linear dependence on R(n) = (nZ -l)/(nz + 2), where n is the refractive index of the solvent, in both nonpolar and polar solvents. The energy of each state of both pigments could be expressed as v = -dR(n) + e (in cm-I) where coefficient d was related to the dispersive interaction between the solute and the solvent molecules. A theory developed by Nagae showed that coefficient d originates from the quantum-mechanical fluctuation of the multipole moments of the solute, in terms of which the following characteristics of the observed d values were explained: (1) In all of the four cases of the Q, and Q, energies of both BChl a and BPhe a, the d values for the polar solvents were smaller than those for the nonpolar solvents. (2) In both nonpolar and polar solvents, the d value of BChl a was larger than that of BPhe a in the Q, energy, whereas the d value of BPhe a was larger than that of BChl a in the Q, energy. (3) The d value of the Q, energy was larger than that of the Q, energy for either case of BChl a or BPhe a.

ABSTRACT:13 C-enriched bacteriochlorophyll c (R[E, E] BChl c F ) was suspended in chloroform to form an aggregate showing the Q y absorption at 705 nm. (1) The aggregate exhibited several largely split 13 C-NMR signals suggesting the presence of nonequivalent BChl c molecules in the form of the piggyback dimer. (2) Changes in the 13 C chemical shifts were traced when methanol was titrated to dissolve the aggregate, and the aggregation shifts (in reference to the monomeric state) were determined as a function of the amount of methanol titrated, and they were analyzed empirically. (3) The ring-current effects were calculated based on the loop-current approximation, and the results were compared with the observed aggregation shifts for 13 C and 1 H nuclei (the 1 H aggregation shifts were determined by extrapolation of the data taken from Mizoguchi, T.; Limantara, L.; Matsuura, K.; Shimada, K.; Koyama, Y. J Mol Structure 1996, 379, 249 -265). The results showed that the assembly of two straight columns consisting of the piggyback dimer stacked in the antiparallel orientation is the best choice as a model for the B705 aggregate. (4) Three-dimensional F1 13 C-edited F3 13 C-filtered heteronuclear single-quantum nuclear-Overhauser-effect spectroscopy was applied to the aggregate consisting of a 1 : 1 mixture of 13 C-labeled and unlabeled BChl c in order to selectively detect the intermolecular 1 H-1 H NOE correlations. The NOE correlations were explained in terms of a straight column, supporting the above model.

Epothilone D, a microtubule-stabilizing macrolide, is an attractive synthetic target molecule as a potential anti-cancer drug candidate. As part of our ongoing synthetic studies of this natural product, this paper describes the synthesis of the thiazole-containing northern segment of epothilone D via an E-selective bromomethylenation and a Ni/Cr-mediated cross-coupling reaction.

forrnation will be di$cussed in the light of im.form ation imm X-ray crystaltography.