Stereoselectivity of model C22-23 aldol coupling for spirangiens A & B

A model system was prepared to investigate the diastereoselectivity of the key C22-23 aldol coupling for the synthesis of spirangiens A & B. The lithium enolate of model ketone 3 was coupled with the differently protected aldehydes 4 (acetonide) and 5 (silyl) giving 3:1 and 3.5:1 dr, respectively, in favour of the unnatural (S) isomer in both cases. The lack of any significant effect on the aldol stereoselectivity induced by the aldehyde protecting groups contrasts with previous literature reports.