Diastereoselective Synthesis of Steroid-[60]Fullerene Hybrids and Theoretical Underpinning

The reaction of C60 with pregnen–20–carboxaldehyde, a biologically active synthetic steroid, by using a 1,3–dipolar cycloaddition reaction (Prato’s protocol) results in the formation of pyrrolidine rings bearing a new stereogenic center on the C2 of the five-membered ring. The formation of the fullerene-steroid hybrids proceeds with preference for the Re face of the 1,3−dipole, with formation of a diastereomeric mixture in 73:15 ratio. The investigation of the chiroptical properties of these conjugates allowed determining the absolute configuration of the new fulleropyrrolidines. In addition, a thorough spectroscopical study permitted to determine the structure of the two mono–cycloadducts. The electrochemical properties of the new hybrids were also evaluated by cyclic voltammetry, both systems exhibit three quasi-reversible reduction waves which are cathodically shifted in regard to the parent C60. Theoretical calculations help supporting the experimental data. A conformational study combining semiempirica...