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.

Graphical abstractDownload full-size image