Studies in marine macrolide synthesis: stereocontrolled synthesis of a C21–C34 subunit of the aplyronines

The C21–C34 subunit 27 of the aplyronines, containing eight stereocentres and a terminal N-methyl-N-vinylformamide moiety, was prepared using the Horner–Wadsworth–Emmons coupling of β-ketophosphonate 5 with aldehyde 19. The two stereotetrad sequences were constructed by chiral ketone aldol reactions, while the N-methyl-N-vinylformamide was introduced using a novel Wittig olefination.