A new approach to phorbol by [4 + 3] oxyallyl cycloaddition and intramolecular Heck reaction.

[formula: see text] A new synthetic strategy for a functionalized tricyclic core of phorbol has been developed by means of a [4 + 3] oxyallyl cycloaddition and subsequent intramolecular Heck reaction. The [4 + 3] oxyallyl cycloadduct 7 was chosen as the B-ring precursor of phorbol. Subsequent elaboration took advantage of its well-defined diastereofacial bias to afford the tricycle 5. This method should be of general value in the construction of 6,7- or 5,7-fused bicyclic systems.