= Solution
<Osmium tetroxide> adds to the alkene through a concerted cyclic addition, giving a five-membered osmate <ester> with both C-O bonds on the same face. Hydrolysis releases the vicinal diol; NMO reoxidizes reduced osmium so that OsO$_4$ can be catalytic.
The two new hydroxyl groups are therefore \b[syn]. For the drawn substrate, approach opposite the two existing solid-wedge substituents is less crowded: draw \b[both new C-OH bonds dashed], while retaining the solid-wedge CO$_2$Bn and NHBoc bonds. The major diol is cis at its two new centres and anti to the pre-existing substituted face.
Syn delivery is the <stereospecificity> of cyclic osmylation; preference for one of the two diastereotopic alkene faces is substrate-controlled <diastereoselectivity>. Attack from the other face would still be syn but would give the minor diastereomer.
Back to article page
