Origin of stereoselectivity of the alkylation of cyclohexadienone-derived bicyclic malonates.

The diastereoselectivity of the alkylation of bicyclic malonates has been studied experimentally and computationally. In accordance with previous observations during a total synthesis of sorbicillactone A, alkylations involving methyl iodide proceed from the concave (endo) face of the bicyclo[4.3.0]nonene ring system. In contrast, carbon-based electrophiles larger than methyl iodide approach from the convex (exo) face. Computational studies using M06-2X and B3LYP methods have revealed that the observed stereoselectivity is explained by subtle energetic differences between a staggered transition state with less torsional strain and unfavorable steric interactions with the cyclohexenone ring. Using this model as a guide, hydrogenation of the C-C double bond was used to alter the steric environment of the substrate. As expected, this led to a reversal in the diastereoselectivity during the alkylation with methyl iodide.