Stereoconvergent Synthesis of Cyclopentenyl Nucleosides by Palladium-Assisted Allylic Reaction

Abstract

The Tsuji-Trost reaction of cyclopentenyl carbonates was explored by experimental studies and computational calculations, in search for the synthetic conditions enabling the stereoconvergent synthesis of cyclopentenyl nucleosides. Under common conditions, cyclopentenyl reagents provide Tsuji-Trost products only in a weakly stereoconvergent manner. However, changes in the amount of catalyst (up to 1 eq) affect either the regio- and stereoselectivity of the reaction, selectively leading to cis configured cyclopentenyl N9-linked purine nucleosides, regardless the relative configuration (cis or trans) of the starting substrates. DFT studies clarified that this could be due to the low activation energy related to the crucial, Pd-dependent π-inversion step.