Stereoselective synthesis of the C13-32 spiroacetal fragment of spirangien A

Stereoselective synthesis of an advanced intermediate towards the highly cytotoxic polyketide metabolite spirangien A was achieved. A key step in the synthesis was installation of the C23 stereocentre by a substrate controlled C22-23 aldol reaction. Comparison of this result with previous literature reports suggests that the facial preference of the aldehyde controls the outcome of the C22-23 aldol coupling and depends on the O27/25 protecting groups when reacting with complex ketones, which contain the C17-15 stereochemistry.