When drawing the chair conformation of the cyclohexane next to the pentene in molecule 15, it becomes apparent that there is a larger angle above the more substituted carbon of the double bond than there is below it. Even though that carbon may be more sterically hindered due to the proximity of the axial hydrogen and methyl group, there is more area on the top face of the molecule for the cobalt/oxygen complex to attack. The other diastereomer, with the hydroxyl group on the bottom face of the molecule, also forms, but does so in a lesser amount since the attacking complex is somewhat "blocked" by the smaller angle.