Zincophorin C1-C9 fragment and seven tetrahydropyran analogues
were prepared diastereoselectively by sequential iodoetherification
and radical hydrogen-transfer reactions. Stereoselective formation
of 3,7-trans or 3,7-cis rings
was rationalized through minimization of allylic-1,3 strain in chair-like
transition states. Subsequent hydrogen-transfer provided 7,8-anti or 7,8-syn isomers
under acyclic stereocontrol or endocyclic control respectively.
The latter approach relies on the formation of a [4.4.0] bicyclic
complexes resulting from the chelation of the oxygen of the tetrahydropyran
ring and the ester by a bidentate Lewis acid.
ionophore - iodoetherification - radical reduction - Lewis acid - substituted tetrahydropyrans