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The five-membered enone-like carbon framework can be obtained by an intramolecular aldol condensation ofEnolization at an internal methylene next to one ketone followed by attack on the opposite ketone closes a five-membered ring. dehydration leaves one acetyl group attached to one alkene carbon and one methyl group on the other. The two nitrile-bearing carbons remain adjacent and their original cis relationship is retained if epimerization is avoided.

Now reverse a reductive ozonolysis: the diketone arises by opening a six-membered cyclohexene whose alkene carbons each carry Me. That cyclohexene is the Diels-Alder reaction product of 2,3-dimethylbuta-1,3-diene and maleonitrile. The latter is the supplied cis dinitrile. The stereospecificity of the cycloaddition puts the two CN groups on the same face.

Thus the forward plan is Diels–Alder addition, reductive ozonolysis, then five-membered aldol ring closure and dehydration. Use a reductive ozone work-up that stops at the diketone. During the final step, avoid prolonged strongly basic heating because the carbons bearing CN can be deprotonated and epimerized. Achiral reagents give the racemic pair with the required cis relationship; resolution would be necessary if only the depicted enantiomer were required. The diene supplies six carbons and maleonitrile supplies four, accounting for all ten carbons without an unexplained carbon deletion.

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 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 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.