The half‐sandwich titanocene CpTi III Cl 2 as efficient system for the preparation of 2,5‐dihydrofurans via α‐allenols

The half-sandwich titanocene reagent (CpTiCl2)-Cl-III, obtained by in situ reduction of commercial CpTiCl3 with manganese, is an excellent system for the Barbier-type reaction between aldehydes and propargylic halides, leading to homopropargylic alcohols and alpha-allenols. An efficient and straightforward methodology for the conversion of aldehydes into 2,5-dihydrofurans involving a two-step sequence (Ti-III addition-Ag-I cyclization) is presented. The usefulness of the method is proved by the preparation of a Natural Product: a dihydrofuranic labdane, isolated from the leaves of Mikania sp. nov.