Regio- and stereochemical aspects in the synthesis of homoallylic alcohols from benzoins and their iodocyclisation to 2,3-diphenyltetrahydrofurans

Indium mediated allylation, crotylation and cinnamylation of benzoins and its substituted derivatives in THF-H₂O (2/1) provide a range of homoallylic alcohols. In general, the benzoins undergo allylation and crotylation in a sluggish manner compared to those observed earlier in the case of α-hydroxy aldehydes and are significantly affected by the electronic features of both the benzoin and indium organometallic reagent. The reactions exhibit higher order of diastereoselectivities than those observed for α-hydroxy aldehydes. The cinnamylation though proceeds in a highly diastereoselective manner but is restricted to only benzoin and 4,4′-dichlorobenzoin. The homoallylic alcohols undergo I₂ mediated intramolecular diastereoselective cyclization to provide 2,3-diphenyltetrahydrofuran derivatives. The relative stereochemistries in tetrahydrofurans and homoallylic alcohols have been assigned by coupling constants, NOE experiments and in one case by X-ray crystallography.