Synthesis of the Spiroiminal Moiety and Approaches to the Synthesis of Marineosins A and B

A short
and efficient synthesis of model spiroiminals that have the same stereochemistry
as marineosins A and B, but different conformations, was carried out
in six or seven steps from 6-methyl­tetrahydropyran-2-one. These
spiroiminals were also prepared biomimetically by reduction of an
enol ether. A more highly substituted spiroiminal with the same stereochemistry
and conformation as marineosin A was prepared in 11 steps from parasorbic
acid. A macrocyclic pyrrole lactone was prepared stereospecifically
in 10 steps. A five-step sequence converted the lactone to a late
hemi-iminal intermediate that has resisted the methylation and spiroiminal
formation that would lead to marineosin A.