1,1'-Bis(thymine)ferrocene Nucleoside: Synthesis and Study of Its Stereoselective Formation.

The synthesis of 1,1'-bis(thymine)ferrocene nucleoside is reported. This nucleoside was obtained in a two-step synthetic methodology including a Michael addition reaction of 1,1'-bis(3-chloropropionyl)ferrocene with thymine to afford the bis(thymine) adduct in 44 % yield. In the second step, the two prochiral carbonyl functionalities in the Michael adduct were reduced to hydroxyl groups with sodium borohydride. This apparently straightforward reaction proceeds in a highly stereoselective fashion to yield the title ferrocenyl nucleoside as a racemic mixture that consists of the R,R and the S,S isomers. The absolute configuration of the chiral carbon atoms in the nucleoside was assigned on the basis of single-crystal X-ray diffraction analysis of the methyl derivative. Furthermore, the mechanism of reduction of the bis(thymine) adduct was investigated by using DFT calculations. The two critical minima, pre-reactive complex, and semi-reduced intermediate, as well as two corresponding transition states were located to support the observed stereoselectivity. The redox properties of 1,1'-bis(thymine)ferrocene nucleoside, its precursor, and congeners were investigated using cyclic voltammetry. For the title compound a reversible redox process was found at a low potential of -30 mV versus FcH/FcH+ (FcH=Fe(η5 -C5 H5 )2 ) as the reference redox couple.