Selectivity in the Robinson Annulation

Many modifications of the fundamenuil Robinson annulation sequence have appeared over the years, one of which uses silyl derivatives such as 559. An important modification uses chiral precursors or chiral 

The metal also plays a role. The presence of a highly covalent 0—Li bond favors the trans product, but a more ionic O—Na or O—K bond, in alcoholic solvents, favors the cis.- The lithium species is less bulky, due to poorer solvation in the aprotic solvent. The protic solvent hydrogen bonds extensively with the ionic sodium species, leading to a large increase in the relative size of the alkoxide moiety. This bulky group influences the geometry of the intermediate and leads to an increase in the cis product.-  

---