Synthetic studies on the ingenane diterpenes. An improved entry into a trans-intrabridgehead system.

[reaction: see text] The efficient construction of an ingenol intermediate exhibiting "insideminus signoutside" intrabridgehead stereochemistry is reported. The sequence features the net conversion of a cis-intrabridgehead compound into a highly strained trans-species via palladium-mediated isomerization of an allylic epoxide followed by a low-temperature alkoxide-accelerated 1,5-hydrogen migration.