Explain the origin of the observed diastereoselective alkylation going from 11 to 12.

The observed diastereoselective alkylation results from the lowest energy conformation. This lowest energy conformation results from the hydrogen bonding and the matching of the p-orbitals. The oxygens from the bulky LHMDS can hydrogen bond with the alcohol on molecule 11, thus stabilizing the conformation. In addition, the p-orbitals of the alkyne on molecule 11 can also interact with the p-orbitals of the carbonyl (C=O bond), thus stabilizing the conformation even more. Therefore, the favored diastereomer is where the LHMDS is sticking up from the plane, on the same side as the methyl.

In the other diastereomer, the LHMDS group would be under the plane. This means that the oxygens in the LHMDS would not be able to hydrogen bond with the hydrogen on the alcohol because they are too far away. Also, in the other projection, the two largest groups are neighboring and causes great steric hindrance which is unstable.