Preparation of 2,5-anhydrohexitols - (Part I). Silicon-directed stereocontrolled cyclization

Stereoselective chain-extension of carbohydrate aldehydes with the hydroxym ethylating reagent (dimethylphenylsilyl)methylmagnesium chloride (1) follow ed by acid-mediated cyclization gives access to 2,5-anhydro-hexitols. The s tereoselectivity of the ring closure depends on the nature of the acid, i.e ., treatment with excess BF3. Et2O or catalytic H2SO4 leads to tetrahydrofu rans with 2,3-cis or 2,3-trans configuration, respectively. Concomitant eli mination is effectively suppressed in case of cyclisation of the more steri cally hindered isopropyl substituted silanes.