Homo-Roche Ester Derivatives by Asymmetric Hydrogenation and Organocatalysis

Abstract

Asymmetric hydrogenation routes to homologues of The Roche ester tend to be restricted to hydrogenations of itaconic acid derivatives, that is, substrates that contain a relatively unhindered, 1,1-disubstituted alkene. This is because in hydrogenations mediated by RhP₂ complexes, the typical catalysts, it is difficult to obtain high conversions using the alternative substrate for the same product, the isomeric trisubstituted alkenes (<b>D</b> in the text). However, chemoselective modification of the identical functional groups in itaconic acid derivatives are difficult; hence, it would be favorable to use the trisubstituted alkene. Trisubstituted alkene substrates can be hydrogenated with high conversions using chiral analogs of Crabtree’s catalyst of the type IrN­(carbene). This paper demonstrates that such reactions are scalable (tens of grams) and can be manipulated to give optically pure homo-Roche ester chirons. Organocatalytic fluorination, chlorination, and amination of the homo-Roche building blocks was performed to demonstrate that they could easily be transformed into functionalized materials with two chiral centers and α,ω-groups that provide extensive scope for modifications. A synthesis of (<i>S,S</i>)- and (<i>R,S</i>)-γ-hydroxyvaline was performed to illustrate one application of the amination product