Preparation of 2,5-Anhydrohexitols (Part I). Silicon-Directed Stereocontrolled Cyclization

ABSTRACT Stereoselective chain-extension of carbohydrate aldehydes with the hydroxymethylating reagent (dimethylphenylsilyl)methylmagnesium chloride (1) followed by acid-mediated cyclization gives access to 2,5-anhydro-hexitols. The stereoselectivity of the ring closure depends on the nature of the acid, i.e., treatment with excess BF3.Et2O or catalytic H2SO4 leads to tetrahydrofurans with 2,3-cis or 2,3-trans configuration, respectively. Concomitant elimination is effectively suppressed in case of cyclisation of the more sterically hindered isopropyl substituted silanes.