Determination of the absolute configuration of rubroflavin by comparison of measured and calculated CD spectra of its thermolysis product 3-methanesulfinyl-5-methylmercaptophenol.

The absolute configuration of rubroflavin has been determined indirectly by comparison of the measured and the calculated CD spectrum of its thermal decomposition product 3-methanesulfinyl-5-methylmercaptophenol. Performing geometry optimizations at the HF/6-31+G* level we found fifteen local minima for the (R)-isomer of 3-methanesulfinyl-5-methylmercaptophenol. The CD spectrum of the compound was then obtained as a superposition of the Boltzmann-weighted spectra for each structure calculated with the non-empirical CIS method. The corresponding Boltzmann factors have been calculated employing the relative energies of these minima determined at the ZPE + MP2/6-31+G*//HF/6-31+G* level of ab initio theory. Comparing the signs of the observed and calculated longest wave length Cotton effect we assign an absolute configuration to the thermolysis product. Since additional calculations revealed that the tricoordinate sulfur atom in rubroflavin and in its decomposition product is configurationally stable under the conditions of thermolysis we conclude that the absolute configuation at the corresponding sulfur atom of rubroflavin is the same.