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 can be catalytic.
The two new hydroxyl groups are therefore syn. For the drawn substrate, approach opposite the two existing solid-wedge substituents is less crowded: draw both new C-OH bonds dashed, while retaining the solid-wedge COBn 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.
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