The application of a different cross metathesis to the formal
synthesis of halichlorine (2) was then explored. While we
were able to access structures of the general type 6,
20
cyclizations of amines obtained upon N-deprotection of such
compounds via intramolecular 1,6-conjugate addition were
problematic, perhaps owing to competing elimination pathways.
Although such conversions are still being explored,
an alternative route to 4 was developed.
We discovered that the cross metathesis reaction of 10
with crotonaldehyde proceeded in 89% yield with excellent
diastereoselectivity (>20:1 E/Z ratio) (Scheme 3)