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.