Mechanism

1) Alkene double bond attacks OsO4 forming a Osmium Tetroxide complex.

2) Water attacks Osmium atom forming the first hydroxyl group.

3) Second water molecule attacks forming the final hydroxyl group.

---------------------------------------------------------------------------------------------------------------------------------------

Cyclic Acetal Formation

4) Methoxy group on 2,2-dimethoxypropane is protonated

5) Protonated Methoxy group leaves as electrons from other methoxy group form a carbonyl

6) Hydroxyl group from Molecule (+)-28 attacks carbonyl carbon.

7) Methoxy group is protonated, making it a good leaving group

8) Oxygen “pushes” off the protonated methoxy group, forming a carbonyl.

9) The second hydroxyl group on Molecule (+)-28 then attacks the newly-formed carbonyl carbon, forming an acetal. This molecule is Molecule (+)-29