Stereochemical requirements of oxidative cyclizations in extended iterative organoiron-mediated routes to alkaloids

Oxidative cyclization by reaction of benzylic and phenolic OH groups on tricarbonyl(η(4)-cyclohexa-1,3-diene)iron(0) complexes has been achieved with the hypervalent iodine oxidant PIFA which was shown to be compatible with the tricarbonyliron complex. The reaction proceeds with substrates with the nucleophilic substituent on the opposite face of the ligand to the iron. IBX gives efficient oxidation of the benzyl alcohol to the aldehyde in the presence of the Fe(CO)(3) group. Reduction of 1-arylcyclohexadienyliron(1+) complexes with sodium borohydride to access the endo series also gave a novel rearranged 2-aryl reduction product with a 5-endo OMe group. The cis relative stereochemistry of the oxidative cyclization product, the exo delivery of hydride to the 1-arylcyclohexadienyliron(1+) complex, and the 2-aryl-5-endo-methoxy relative stereochemistry of the rearranged product were proved by X-ray crystallography.

A novel, facile approach to frondosin B and 5-epi-liphagal via a new [4 + 3]-cycloaddition

A new [4 + 3]-cycloaddition between benzofuran allylic alcohols and dienes, promoted by camphorsulfonic acid, has been identified. A novel strategy which used this cycloaddition as a key step has been developed for the synthesis of 6,7,5-tricyclic skeleta, and syntheses toward frondosin B (1) and 5-epi-liphagal (2) have been achieved via short routes in good yields.