Elaboration of 8,9-Dehydro Precursors

Fully as expected from the inspection of molecular models, the generation of the oxetane ring has the effect of heightening the structural curvature in compounds such as 85 and 86. Reagent approach from the exterior should materialize under kinetically controlled conditions. These features are reflected in the catalytic hydrogenation of 86, which results in saturation of the double bond from the (3 surface along with reductive debromination to furnish 87. The lesson learned here is that the oxetane ring should be released or not formed at all if an opportunity to approach C9 from the a direction has any chance to take place. 

This line of thinking prompted the treatment of 86 with zinc in refluxing methanol for the purpose of generating 88. When this ketone was reduced with diisobutylaluminum hydride in tetrahydrofuran at low temperature, the cis diol 89 was produced with a stereoelectivity in excess of 10 1. Monomesylation of the 

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